Comprehensive Notes on Cell Structure and Function
The Fundamental Units of Life: Cells and Their Organization
Cells: Basic Building Blocks
Cells serve as the fundamental building blocks for all organisms.
In single-celled organisms, the cell itself constitutes the entire organism, performing all necessary life functions.
Hierarchy of Multicellular Organisms
Multicellular organisms exhibit a hierarchical organization:
Cells: The basic unit of life.
Tissues: Composed of interconnected cells that share a common function.
Organs: Formed by the combination of several different tissues working together.
Organ Systems: Created when multiple organs function collaboratively to achieve a broader physiological role.
Organism: The complete entity formed by the cooperative functioning of multiple organ systems.
Cell Size and Visualization
Cell size varies significantly, with most cells being too small to be observed by the naked eye.
Microscopes are essential tools that enable the visualization and study of small cells.
Parameters in Microscopy
Magnification and Resolving Power are the two most critical parameters in microscopy.
Magnification
Magnification is defined as the process of enlarging an object in its apparent size.
Resolution (Resolving Power)
Resolving power is the ability of a microscope to distinguish between two adjacent structures as separate entities.
A higher resolution directly corresponds to better clarity and detail in the resulting image.
Types of Microscopes
Microscopes equipped with different optical systems are used for various types of studies.
Compound Light Microscopes:
Utilize visible light, which is bent through a lens system, to provide magnification.
Transparent objects, such as many types of cells, often require treatment with chemical stains to differentiate and distinguish their various internal parts.
Electron Microscopes:
Achieve significantly higher magnification and resolution by employing beams of electrons instead of light.
Transmission Electron Microscopes (TEM): Capable of revealing fine internal details within cells, providing cross-sectional views.
Scanning Electron Microscopes (SEM): Provide detailed three-dimensional exterior views of cell surfaces.
Cell Theory: A Biological Cornerstone
Cell theory is a foundational principle underlying all of biology, comprising three main tenets:
Cells are the basic units of life.
All living organisms are composed of one or more cells.
All cells originate from pre-existing cells.
Fundamental Components of All Cells
All cells, regardless of type, share four common components:
Plasma Membrane: An enclosing membrane that acts as a barrier, separating the cell's interior from its external environment.
Cytoplasm: Consists of a gel-like substance called cytosol, within which other cellular components are suspended.
DNA: The genetic material of the cell, carrying hereditary instructions.
Ribosomes: Cellular structures responsible for synthesizing proteins.
Prokaryotic Cells
Characteristics of Prokaryotes
Prokaryotic cells lack membrane-enclosed internal compartments, most notably a true nucleus.
The majority of prokaryotes possess a cell wall, which typically contains peptidoglycan.
It is widely believed that prokaryotes closely resemble the earliest forms of life on Earth.
Organisms classified under the domains Archaea and Bacteria are prokaryotes.
Generalized Structure of a Prokaryotic Cell
Chromosomal DNA is localized in a region within the cytoplasm called the nucleoid, not enclosed by a membrane.
Ribosomes are freely distributed throughout the cytoplasm.
The cell membrane is externally surrounded by a protective cell wall.
Other structures, such as capsules or flagella, may be present in some, but not all, bacterial species.
Small Size of Prokaryotic Cells
Prokaryotic cells are generally considerably smaller than eukaryotic cells.
Reasons for their small size include:
A more favorable surface area to volume ratio, which enhances the efficiency of material transport (moving nutrients in and waste out of the cell).
The absence of specialized internal transport modifications found in eukaryotic cells, necessitating a smaller size for efficient diffusion.
The logistics of carrying out cellular metabolism impose limits on cell size, emphasizing the critical role of the surface area to volume ratio. Smaller cells naturally possess a greater surface area relative to their overall volume.
Eukaryotic Cells
Components of a Eukaryotic Cell
(Based on the provided diagram, key components of a eukaryotic cell include: nucleolus, nuclear envelope, nuclear pore, nucleus, plasma membrane, flagellum, cytoplasm, centrosome, microfilament, ribosome, mitochondrion, microtubule, rough endoplasmic reticulum, smooth endoplasmic reticulum, peroxisome, Golgi complex, cilia, lysosome.)
Eukaryotic Plasma Membrane
The eukaryotic plasma membrane is fundamentally a phospholipid bilayer with numerous embedded and associated proteins.
Key components include:
Phospholipid Bilayer: The fundamental structure.
Integral Membrane Proteins: Proteins embedded directly within the bilayer.
Peripheral Membrane Proteins: Proteins loosely associated with the surface of the membrane.
Cholesterol: A lipid that regulates membrane fluidity.
Protein Channel: Specific integral proteins that allow passage of certain molecules.
Glycoprotein: A protein with an attached carbohydrate chain, often involved in cell recognition.
Glycolipid: A lipid with an attached carbohydrate chain, also important for cell recognition.
Filaments of the Cytoskeleton: Provide structural support and anchor the membrane.
Cytoplasm
The cytoplasm is the region situated between the plasma membrane and the nuclear envelope.
It comprises organelles suspended within a gel-like fluid called cytosol, along with the cytoskeleton.
The cytoplasm is approximately 70-80\% water but maintains a semi-solid consistency due to the presence of dissolved proteins.
Nucleus
Typically, a eukaryotic cell contains only one nucleus, though some are multinucleated.
The nucleus is usually the largest organelle within the cell, often larger than most prokaryotic cells themselves.
Key nuclear components include:
DNA and Proteins: These combine to form chromatin, a complex material during most of the cell's life cycle.
Chromatin Condensation: During cell division, chromatin condenses further to form distinct chromosomes.
Nucleolus: A prominent region within the nucleus that is the primary site of ribosomal RNA (rRNA) synthesis.
Nuclear Envelope:
This is a double membrane that encloses the nucleus.
Its primary functions include separating the DNA from the cytoplasm, thereby segregating the processes of transcription (in the nucleus) from translation (in the cytoplasm).
It is perforated by numerous nuclear pores.
Nuclear pores act as channels that connect the nucleoplasm (the interior of the nucleus) with the cytoplasm.
They actively regulate the flow of molecules, both large and small, in and out of the nucleus.
Large molecules, such as proteins, require a specific Nuclear Localization Signal (NLS) to successfully pass through the nuclear pores.
Ribosomes
Ribosomes are composed of two distinct subunits of different sizes; those in eukaryotes are slightly larger than in prokaryotes.
They are primarily made of specialized RNA (rRNA) and proteins.
During the process of protein synthesis, ribosomes are responsible for assembling amino acids into functional proteins.
Mitochondrion
The mitochondrion is the primary site within the cell for the conversion of stored energy (from macromolecular molecular bonds) into a more biologically usable form: adenosine triphosphate (ATP).
Its inner membrane is highly folded, creating structures known as cristae.
The area enclosed by the inner membrane is called the mitochondrial matrix, which contains enzymes for cellular respiration, mitochondrial DNA, and ribosomes.
Peroxisomes
Peroxisomes are small, rounded organelles enclosed by a single membrane.
They are the sites where reactions that break down fatty acids and amino acids occur.
Additionally, peroxisomes play a role in detoxifying various poisons within the cell.
Contrasting Animal and Plant Cells
While both animal and plant cells are eukaryotic, they exhibit significant structural differences:
Similarities:
Both possess Microtubule Organizing Centers (MTOCs).
Animal Cells:
Have centrioles associated with the MTOC, and this complex is referred to as the centrosome.
Contain lysosomes.
Lack a cell wall, chloroplasts, other specialized plastids, and a large central vacuole.
Plant Cells:
Possess a rigid cell wall external to the plasma membrane.
Contain chloroplasts (sites of photosynthesis) and other specialized plastids (e.g., amyloplasts for starch storage).
Feature a large central vacuole that occupies most of the cell's volume.
Do not have centrioles or lysosomes.
Animal Cell Components (Diagrammatic Representation)
(A typical animal cell contains: Intermediate filament, Ribosomes, Rough endoplasmic reticulum, Nucleus, Nucleolus, Chromatin, Golgi apparatus, Golgi vesicle, Mitochondria, Plasma membrane, Cytoplasm, Vacuole, Microtubule, Centrosome, Microfilament, Lysosome, Smooth endoplasmic reticulum, Secretory vesicle, Peroxisome.)
Plant Cell Components (Diagrammatic Representation)
(A typical plant cell contains: Plasmodesmata, Endoplasmic reticulum (smooth and rough), Nucleus (with chromatin and nucleolus), Cell wall, Plasma membrane, Cytoplasm, Central vacuole, Cytoskeleton (microtubules, intermediate filaments, microfilaments), Chloroplast, Plastid, Ribosomes, Golgi apparatus, Mitochondria, Peroxisome.)
Specialized Organelles and Structures
Centrosome (Animal Cells)
The centrosome is composed of two centrioles that are oriented at right angles to each other.
Each centriole is a cylindrical structure made up of nine triplets of microtubules.
Non-tubulin proteins provide structural integrity, holding the microtubule triplets together.
Plant Cell Walls
The cell wall is a rigid, protective structure located external to the plasma membrane in plant cells.
A key distinction from prokaryotic cell walls is that plant cell walls are primarily composed of cellulose, whereas most prokaryotic cell walls contain peptidoglycan.
Chloroplasts
Chloroplasts are double-membrane organelles, unique to plant cells and algae.
Similar to mitochondria, they possess their own ribosomes and DNA, supporting the endosymbiotic theory.
The inner membrane encloses an aqueous fluid called the stroma.
Within the stroma are sets of interconnected and stacked fluid-filled membrane sacs known as thylakoids.
Each stack of thylakoids is referred to as a granum (plural: grana).
The Central Vacuole (Plant Cells)
Plant cells typically contain a large central vacuole that can occupy a significant portion of the cell's volume.
This vacuole plays crucial roles in regulating water concentration, particularly under varying environmental conditions, and contributes significantly to cell expansion by maintaining turgor pressure against the cell wall.
Endosymbiosis Theory
The Endosymbiosis Theory hypothesizes that mitochondria and chloroplasts originated as formerly independent prokaryotic organisms.
These free-living prokaryotes are believed to have become endosymbionts (organisms living within another organism) of the prokaryotic ancestors of eukaryotes.
Supporting evidence for this theory includes:
Mitochondria and chloroplasts having their own distinct DNA and ribosomes.
The size of these organelles being remarkably similar to that of independent prokaryotes.
Much of the original research and prominent advocacy for this theory was undertaken by Lynn Margulis.
Examples of Symbiosis
Algae living within a flatworm.
Algae found within salamander eggs.
Lichens, which are a symbiotic association between algae and fungi.
Nitrogen-fixing bacteria living in root nodules of plants.
The Endomembrane System
The endomembrane system is a complex network of internal membranes and organelles present in eukaryotic cells.
Its collective function is to modify, package, and transport lipids and proteins throughout the cell or for secretion.
Components of the endomembrane system include:
The nuclear envelope
Lysosomes and vesicles
The endoplasmic reticulum (ER)
The Golgi apparatus
The plasma membrane
Lysosomes (Animal Cells)
Lysosomes, found predominantly in animal cells, contain potent digestive enzymes.
These enzymes are responsible for breaking down large biomolecules (such as proteins, lipids, and carbohydrates) and even worn-out or damaged organelles within the cell.
They are crucial for processes like phagocytosis, where a food particle is engulfed forming a food vacuole, which then fuses with a lysosome to digest its contents.
Endoplasmic Reticulum (ER)
The ER is an extensive network of interconnected membranous sacs and tubules that extends throughout the cytoplasm.
It plays a central role in modifying proteins (specifically the rough ER) and synthesizing lipids (primarily the smooth ER).
The hollow interior space within the ER tubules is referred to as the lumen or cisternal space.
The membrane of the ER is continuous with the outer membrane of the nuclear envelope.
Rough Endoplasmic Reticulum (RER)
The RER is characterized by the presence of ribosomes attached to its cytoplasmic surface.
These ribosomes synthesize proteins that are destined for secretion, insertion into membranes, or delivery to other organelles.
Newly synthesized proteins undergo modifications, such as folding and the acquisition of side chains, within the lumen of the RER.
Modified proteins are either directly incorporated into cellular membranes or packaged for secretion from the cell (e.g., protein hormones, enzymes).
The RER also contributes to the synthesis of phospholipids, which are essential components of cellular membranes.
Phospholipids or modified proteins that are not intended to remain within the RER are transported to their destinations via transport vesicles that bud off from the RER's membrane.
Smooth Endoplasmic Reticulum (SER)
The SER is continuous with the RER but distinguishes itself by having few or no ribosomes on its cytoplasmic surface.
Key functions of the SER include:
Synthesis of carbohydrates, lipids, and steroid hormones.
Detoxification of various medications and poisons, particularly in liver cells.
Storage of calcium ions (Ca^{++}).
In muscle cells, a specialized form of SER called the sarcoplasmic reticulum is dedicated to storing and releasing Ca^{++} ions, which are vital for coordinated muscle contractions.
Golgi Apparatus (Golgi Body)
The Golgi apparatus is an organelle composed of a series of flattened membrane-bound sacs called cisternae.
Its primary role is to sort, package, and tag lipids and proteins that arrive from the ER via transport vesicles, ensuring they reach their correct destinations.
The Golgi apparatus has two distinct faces:
The cis face is the receiving side, typically oriented towards the ER.
The trans face is the opposite, or shipping, side.
Transport vesicles from the ER fuse with the cis face of the Golgi, releasing their contents into its lumen.
As proteins and lipids traverse through the various cisternae of the Golgi, they undergo further modifications, often involving the addition of short chains of sugar molecules, to facilitate their sorting and targeting.
The Cytoskeleton
The cytoskeleton is an intricate network of protein fibers that provides structural integrity and dynamic capabilities to the eukaryotic cell.
Its fundamental functions include:
Helping to maintain the characteristic shape of the cell.
Holding certain organelles in specific, stable positions within the cytoplasm.
Allowing for the directed movement of cytoplasm and vesicles within the cell, enabling intracellular transport.
Enabling the movement of entire cells within multicellular organisms, crucial for processes like development, tissue repair, and immune response.
Three Components of the Cytoskeleton
These components differ in size and specific functions:
Microfilaments (Smallest diameter)
Intermediate Filaments (Intermediate diameter)
Microtubules (Largest diameter)
Microfilaments
Primarily involved in cell movement, encompassing both the movement of internal cellular parts and the locomotion of the entire cell.
They play a crucial role in determining and stabilizing cell shape.
Microfilaments are constructed from actin monomers.
Microtubules
Form a rigid internal skeleton in some cells, providing structural support.
They serve as tracks or frameworks along which motor proteins can transport vesicles and other structures within the cell.
Microtubules are assembled from tubulin dimers.
Each microtubule typically comprises 13 chains of tubulin dimers arranged around a central cavity.
Cilia and Flagella
Cilia and flagella are motility structures built upon a characteristic microtubule arrangement, commonly known as the "9+2 array."
Ultrastructure ( 9+2 array ):
Consists of 9 doublet microtubules arranged in a circle on the outside.
Contains 2 unfused single microtubules located in the center.
Spokes connect the outer doublets to the central pair.
Cilia are generally shorter and more numerous than flagella.
They also exhibit different beating patterns, leading to varied forms of cellular movement.
Extracellular Structures
Plant Cell Wall
Provides structural support and acts as a barrier against infection.
Plasmodesmata are specialized channels that connect adjacent plant cells, allowing for direct communication and material transport between their cytoplasms.
Extracellular Matrix (ECM) in Animals
The ECM is a complex network of macromolecules secreted by cells, providing structural and biochemical support to surrounding cells.
It consists of three main components:
Collagens and other fibrous proteins: Provide tensile strength.
Glycoproteins called proteoglycans: Help resist compression and provide hydration.
Linking proteins: Connect ECM components to each other and to cell surface receptors.
Intercellular Junctions
Intercellular junctions are specialized structures that provide direct channels of communication and adhesion between adjacent cells.
The types of junctions differ between plants and animals.
Plasmodesmata (Plants)
These are minute channels that traverse the cell walls of adjacent plant cells.
They directly connect the cytoplasm of one cell to the cytoplasm of another, facilitating the movement of water, ions, small solutes, and even some macromolecules between cells.
Tight Junctions (Animals)
Tight junctions form watertight seals between adjacent animal cells.
Their primary function is to prevent materials from leaking through the spaces between cells, effectively regulating paracellular transport.
They are commonly found in epithelial cells, which line internal organs, cavities, and the outer surfaces of the body, where preventing fluid leakage is critical.
Desmosomes (Animals)
Desmosomes are short, specialized proteins (cadherins) in the plasma membrane that function as "spot welds."
They mechanically join adjacent cells, anchoring them strongly together.
Desmosomes are particularly prevalent in tissues that experience significant mechanical stress and stretching, such as the heart, lungs, and muscles.
They are exclusively found in animal cells.
Gap Junctions (Animals)
Gap junctions closely resemble plasmodesmata in plants in their function, as they form channels that allow ions, nutrients, and other small materials to move directly between adjacent animal cells.
These junctions develop when six proteins, known as connexins, assemble to form an elongated, doughnut-like structure called a connexon within the plasma membrane.
When the connexons of two adjacent cells align perfectly, they complete a continuous channel, enabling direct cytoplasmic communication.
A gap junction essentially functions as a protein-lined pore, allowing the passage of water and small molecules between adjacent animal cells, crucial for rapid communication and coordination.
Simpler Summary of Cells and Their Organization
Cells are the fundamental units of life, forming all living organisms.
In multicellular organisms, cells organize into tissues, organs, and organ systems to form a complete organism.
Most cells are tiny and require microscopes for visualization, which involve magnification (enlarging) and resolution (distinguishing separate structures).
The Cell Theory states: cells are basic life units, all organisms are made of cells, and all cells come from pre-existing cells.
All cells share four basic components: a plasma membrane (outer barrier), cytoplasm (internal jelly-like substance with components), DNA (genetic material), and ribosomes (protein makers).
Prokaryotic cells (like bacteria) are simpler, lack a true nucleus and other membrane-bound internal compartments, and are generally smaller.
Eukaryotic cells (animal, plant, fungi) are larger and more complex, featuring a true nucleus and various membrane-bound organelles.
The nucleus stores DNA as chromatin and is enclosed by the nuclear envelope with pores.
Ribosomes synthesize proteins.
Mitochondria are powerhouses, converting energy into ATP.
The Endoplasmic Reticulum (ER) modifies proteins (rough ER) and synthesizes lipids/detoxifies (smooth ER).
The Golgi apparatus sorts, packages, and tags proteins and lipids.
Lysosomes (animal cells) break down waste and cellular debris.
Peroxisomes break down fatty acids and detoxify substances.
Plant cells have unique features: a rigid cell wall (made of cellulose), chloroplasts (for photosynthesis), and a large central vacuole (for water storage and turgor) but lack centrioles and lysosomes.
Animal cells have centrioles (part of centrosome) and lysosomes but lack a cell wall, chloroplasts, and a large central vacuole.
The Cytoskeleton (microfilaments, intermediate filaments, microtubules) provides structural support, facilitates movement, and helps transport substances within the cell.
Cilia and flagella are structures for cell movement, built from microtubules in a "9+2 array" pattern.
The Endomembrane System (nuclear envelope, ER, Golgi, lysosomes, plasma membrane) collectively modifies, packages, and transports lipids and proteins.
The Endosymbiosis Theory explains that mitochondria and chloroplasts originated from free-living prokaryotes that became internal symbionts.
Intercellular junctions connect cells:
Plasmodesmata (plants) are channels between plant cells.
Tight junctions (animals) create watertight seals.
Desmosomes (animals) are "spot welds" for strong adhesion.
Gap junctions (animals) form channels for communication, similar to plasmodesmata.
The Fundamental Units of Life: Cells and Their Organization
Cells: Basic Building Blocks
Cells serve as the fundamental building blocks for all organisms.
In single-celled organisms, the cell itself constitutes the entire organism, performing all necessary life functions.
Hierarchy of Multicellular Organisms
Multicellular organisms exhibit a hierarchical organization:
Cells: The basic unit of life.
Tissues: Composed of interconnected cells that share a common function.
Organs: Formed by the combination of several different tissues working together.
Organ Systems: Created when multiple organs function collaboratively to achieve a broader physiological role.
Organism: The complete entity formed by the cooperative functioning of multiple organ systems.
Cell Size and Visualization
Cell size varies significantly, with most cells being too small to be observed by the naked eye.
Microscopes are essential tools that enable the visualization and study of small cells.
Parameters in Microscopy
Magnification and Resolving Power are the two most critical parameters in microscopy.
Magnification
Magnification is defined as the process of enlarging an object in its apparent size.
Resolution (Resolving Power)
Resolving power is the ability of a microscope to distinguish between two adjacent structures as separate entities.
A higher resolution directly corresponds to better clarity and detail in the resulting image.
Types of Microscopes
Microscopes equipped with different optical systems are used for various types of studies.
Compound Light Microscopes:
Utilize visible light, which is bent through a lens system, to provide magnification.
Transparent objects, such as many types of cells, often require treatment with chemical stains to differentiate and distinguish their various internal parts.
Electron Microscopes:
Achieve significantly higher magnification and resolution by employing beams of electrons instead of light.
Transmission Electron Microscopes (TEM): Capable of revealing fine internal details within cells, providing cross-sectional views.
Scanning Electron Microscopes (SEM): Provide detailed three-dimensional exterior views of cell surfaces.
Cell Theory: A Biological Cornerstone
Cell theory is a foundational principle underlying all of biology, comprising three main tenets:
Cells are the basic units of life.
All living organisms are composed of one or more cells.
All cells originate from pre-existing cells.
Fundamental Components of All Cells
All cells, regardless of type, share four common components:
Plasma Membrane: An enclosing membrane that acts as a barrier, separating the cell's interior from its external environment.
Cytoplasm: Consists of a gel-like substance called cytosol, within which other cellular components are suspended.
DNA: The genetic material of the cell, carrying hereditary instructions.
Ribosomes: Cellular structures responsible for synthesizing proteins.
Prokaryotic Cells
Characteristics of Prokaryotes
Prokaryotic cells lack membrane-enclosed internal compartments, most notably a true nucleus.
The majority of prokaryotes possess a cell wall, which typically contains peptidoglycan.
It is widely believed that prokaryotes closely resemble the earliest forms of life on Earth.
Organisms classified under the domains Archaea and Bacteria are prokaryotes.
Generalized Structure of a Prokaryotic Cell
Chromosomal DNA is localized in a region within the cytoplasm called the nucleoid, not enclosed by a membrane.
Ribosomes are freely distributed throughout the cytoplasm.
The cell membrane is externally surrounded by a protective cell wall.
Other structures, such as capsules or flagella, may be present in some, but not all, bacterial species.
Small Size of Prokaryotic Cells
Prokaryotic cells are generally considerably smaller than eukaryotic cells.
Reasons for their small size include:
A more favorable surface area to volume ratio, which enhances the efficiency of material transport (moving nutrients in and waste out of the cell).
The absence of specialized internal transport modifications found in eukaryotic cells, necessitating a smaller size for efficient diffusion.
The logistics of carrying out cellular metabolism impose limits on cell size, emphasizing the critical role of the surface area to volume ratio. Smaller cells naturally possess a greater surface area relative to their overall volume.
Eukaryotic Cells
Components of a Eukaryotic Cell
(Based on the provided diagram, key components of a eukaryotic cell include: nucleolus, nuclear envelope, nuclear pore, nucleus, plasma membrane, flagellum, cytoplasm, centrosome, microfilament, ribosome, mitochondrion, microtubule, rough endoplasmic reticulum, smooth endoplasmic reticulum, peroxisome, Golgi complex, cilia, lysosome.)
Eukaryotic Plasma Membrane
The eukaryotic plasma membrane is fundamentally a phospholipid bilayer with numerous embedded and associated proteins.
Key components include:
Phospholipid Bilayer: The fundamental structure.
Integral Membrane Proteins: Proteins embedded directly within the bilayer.
Peripheral Membrane Proteins: Proteins loosely associated with the surface of the membrane.
Cholesterol: A lipid that regulates membrane fluidity.
Protein Channel: Specific integral proteins that allow passage of certain molecules.
Glycoprotein: A protein with an attached carbohydrate chain, often involved in cell recognition.
Glycolipid: A lipid with an attached carbohydrate chain, also important for cell recognition.
Filaments of the Cytoskeleton: Provide structural support and anchor the membrane.
Cytoplasm
The cytoplasm is the region situated between the plasma membrane and the nuclear envelope.
It comprises organelles suspended within a gel-like fluid called cytosol, along with the cytoskeleton.
The cytoplasm is approximately 70-80\% water but maintains a semi-solid consistency due to the presence of dissolved proteins.
Nucleus
Typically, a eukaryotic cell contains only one nucleus, though some are multinucleated.
The nucleus is usually the largest organelle within the cell, often larger than most prokaryotic cells themselves.
Key nuclear components include:
DNA and Proteins: These combine to form chromatin, a complex material during most of the cell's life cycle.
Chromatin Condensation: During cell division, chromatin condenses further to form distinct chromosomes.
Nucleolus: A prominent region within the nucleus that is the primary site of ribosomal RNA (rRNA) synthesis.
Nuclear Envelope:
This is a double membrane that encloses the nucleus.
Its primary functions include separating the DNA from the cytoplasm, thereby segregating the processes of transcription (in the nucleus) from translation (in the cytoplasm).
It is perforated by numerous nuclear pores.
Nuclear pores act as channels that connect the nucleoplasm (the interior of the nucleus) with the cytoplasm.
They actively regulate the flow of molecules, both large and small, in and out of the nucleus.
Large molecules, such as proteins, require a specific Nuclear Localization Signal (NLS) to successfully pass through the nuclear pores.
Ribosomes
Ribosomes are composed of two distinct subunits of different sizes; those in eukaryotes are slightly larger than in prokaryotes.
They are primarily made of specialized RNA (rRNA) and proteins.
During the process of protein synthesis, ribosomes are responsible for assembling amino acids into functional proteins.
Mitochondrion
The mitochondrion is the primary site within the cell for the conversion of stored energy (from macromolecular molecular bonds) into a more biologically usable form: adenosine triphosphate (ATP).
Its inner membrane is highly folded, creating structures known as cristae.
The area enclosed by the inner membrane is called the mitochondrial matrix, which contains enzymes for cellular respiration, mitochondrial DNA, and ribosomes.
Peroxisomes
Peroxisomes are small, rounded organelles enclosed by a single membrane.
They are the sites where reactions that break down fatty acids and amino acids occur.
Additionally, peroxisomes play a role in detoxifying various poisons within the cell.
Contrasting Animal and Plant Cells
While both animal and plant cells are eukaryotic, they exhibit significant structural differences:
Similarities:
Both possess Microtubule Organizing Centers (MTOCs).
Animal Cells:
Have centrioles associated with the MTOC, and this complex is referred to as the centrosome.
Contain lysosomes.
Lack a cell wall, chloroplasts, other specialized plastids, and a large central vacuole.
Plant Cells:
Possess a rigid cell wall external to the plasma membrane.
Contain chloroplasts (sites of photosynthesis) and other specialized plastids (e.g., amyloplasts for starch storage).
Feature a large central vacuole that occupies most of the cell's volume.
Do not have centrioles or lysosomes.
Animal Cell Components (Diagrammatic Representation)
(A typical animal cell contains: Intermediate filament, Ribosomes, Rough endoplasmic reticulum, Nucleus, Nucleolus, Chromatin, Golgi apparatus, Golgi vesicle, Mitochondria, Plasma membrane, Cytoplasm, Vacuole, Microtubule, Centrosome, Microfilament, Lysosome, Smooth endoplasmic reticulum, Secretory vesicle, Peroxisome.)
Plant Cell Components (Diagrammatic Representation)
(A typical plant cell contains: Plasmodesmata, Endoplasmic reticulum (smooth and rough), Nucleus (with chromatin and nucleolus), Cell wall, Plasma membrane, Cytoplasm, Central vacuole, Cytoskeleton (microtubules, intermediate filaments, microfilaments), Chloroplast, Plastid, Ribosomes, Golgi apparatus, Mitochondria, Peroxisome.)
Specialized Organelles and Structures
Centrosome (Animal Cells)
The centrosome is composed of two centrioles that are oriented at right angles to each other.
Each centriole is a cylindrical structure made up of nine triplets of microtubules.
Non-tubulin proteins provide structural integrity, holding the microtubule triplets together.
Plant Cell Walls
The cell wall is a rigid, protective structure located external to the plasma membrane in plant cells.
A key distinction from prokaryotic cell walls is that plant cell walls are primarily composed of cellulose, whereas most prokaryotic cell walls contain peptidoglycan.
Chloroplasts
Chloroplasts are double-membrane organelles, unique to plant cells and algae.
Similar to mitochondria, they possess their own ribosomes and DNA, supporting the endosymbiotic theory.
The inner membrane encloses an aqueous fluid called the stroma.
Within the stroma are sets of interconnected and stacked fluid-filled membrane sacs known as thylakoids.
Each stack of thylakoids is referred to as a granum (plural: grana).
The Central Vacuole (Plant Cells)
Plant cells typically contain a large central vacuole that can occupy a significant portion of the cell's volume.
This vacuole plays crucial roles in regulating water concentration, particularly under varying environmental conditions, and contributes significantly to cell expansion by maintaining turgor pressure against the cell wall.
Endosymbiosis Theory
The Endosymbiosis Theory hypothesizes that mitochondria and chloroplasts originated as formerly independent prokaryotic organisms.
These free-living prokaryotes are believed to have become endosymbionts (organisms living within another organism) of the prokaryotic ancestors of eukaryotes.
Supporting evidence for this theory includes:
Mitochondria and chloroplasts having their own distinct DNA and ribosomes.
The size of these organelles being remarkably similar to that of independent prokaryotes.
Much of the original research and prominent advocacy for this theory was undertaken by Lynn Margulis.
Examples of Symbiosis
Algae living within a flatworm.
Algae found within salamander eggs.
Lichens, which are a symbiotic association between algae and fungi.
Nitrogen-fixing bacteria living in root nodules of plants.
The Endomembrane System
The endomembrane system is a complex network of internal membranes and organelles present in eukaryotic cells.
Its collective function is to modify, package, and transport lipids and proteins throughout the cell or for secretion.
Components of the endomembrane system include:
The nuclear envelope
Lysosomes and vesicles
The endoplasmic reticulum (ER)
The Golgi apparatus
The plasma membrane
Lysosomes (Animal Cells)
Lysosomes, found predominantly in animal cells, contain potent digestive enzymes.
These enzymes are responsible for breaking down large biomolecules (such as proteins, lipids, and carbohydrates) and even worn-out or damaged organelles within the cell.
They are crucial for processes like phagocytosis, where a food particle is engulfed forming a food vacuole, which then fuses with a lysosome to digest its contents.
Endoplasmic Reticulum (ER)
The ER is an extensive network of interconnected membranous sacs and tubules that extends throughout the cytoplasm.
It plays a central role in modifying proteins (specifically the rough ER) and synthesizing lipids (primarily the smooth ER).
The hollow interior space within the ER tubules is referred to as the lumen or cisternal space.
The membrane of the ER is continuous with the outer membrane of the nuclear envelope.
Rough Endoplasmic Reticulum (RER)
The RER is characterized by the presence of ribosomes attached to its cytoplasmic surface.
These ribosomes synthesize proteins that are destined for secretion, insertion into membranes, or delivery to other organelles.
Newly synthesized proteins undergo modifications, such as folding and the acquisition of side chains, within the lumen of the RER.
Modified proteins are either directly incorporated into cellular membranes or packaged for secretion from the cell (e.g., protein hormones, enzymes).
The RER also contributes to the synthesis of phospholipids, which are essential components of cellular membranes.
Phospholipids or modified proteins that are not intended to remain within the RER are transported to their destinations via transport vesicles that bud off from the RER's membrane.
Smooth Endoplasmic Reticulum (SER)
The SER is continuous with the RER but distinguishes itself by having few or no ribosomes on its cytoplasmic surface.
Key functions of the SER include:
Synthesis of carbohydrates, lipids, and steroid hormones.
Detoxification of various medications and poisons, particularly in liver cells.
Storage of calcium ions (Ca^{++}).
In muscle cells, a specialized form of SER called the sarcoplasmic reticulum is dedicated to storing and releasing Ca^{++} ions, which are vital for coordinated muscle contractions.
Golgi Apparatus (Golgi Body)
The Golgi apparatus is an organelle composed of a series of flattened membrane-bound sacs called cisternae.
Its primary role is to sort, package, and tag lipids and proteins that arrive from the ER via transport vesicles, ensuring they reach their correct destinations.
The Golgi apparatus has two distinct faces:
The cis face is the receiving side, typically oriented towards the ER.
The trans face is the opposite, or shipping, side.
Transport vesicles from the ER fuse with the cis face of the Golgi, releasing their contents into its lumen.
As proteins and lipids traverse through the various cisternae of the Golgi, they undergo further modifications, often involving the addition of short chains of sugar molecules, to facilitate their sorting and targeting.
The Cytoskeleton
The cytoskeleton is an intricate network of protein fibers that provides structural integrity and dynamic capabilities to the eukaryotic cell.
Its fundamental functions include:
Helping to maintain the characteristic shape of the cell.
Holding certain organelles in specific, stable positions within the cytoplasm.
Allowing for the directed movement of cytoplasm and vesicles within the cell, enabling intracellular transport.
Enabling the movement of entire cells within multicellular organisms, crucial for processes like development, tissue repair, and immune response.
Three Components of the Cytoskeleton
These components differ in size and specific functions:
Microfilaments (Smallest diameter)
Intermediate Filaments (Intermediate diameter)
Microtubules (Largest diameter)
Microfilaments
Primarily involved in cell movement, encompassing both the movement of internal cellular parts and the locomotion of the entire cell.
They play a crucial role in determining and stabilizing cell shape.
Microfilaments are constructed from actin monomers.
Microtubules
Form a rigid internal skeleton in some cells, providing structural support.
They serve as tracks or frameworks along which motor proteins can transport vesicles and other structures within the cell.
Microtubules are assembled from tubulin dimers.
Each microtubule typically comprises 13 chains of tubulin dimers arranged around a central cavity.
Cilia and Flagella
Cilia and flagella are motility structures built upon a characteristic microtubule arrangement, commonly known as the "9+2 array."
Ultrastructure ( 9+2 array ):
Consists of 9 doublet microtubules arranged in a circle on the outside.
Contains 2 unfused single microtubules located in the center.
Spokes connect the outer doublets to the central pair.
Cilia are generally shorter and more numerous than flagella.
They also exhibit different beating patterns, leading to varied forms of cellular movement.
Extracellular Structures
Plant Cell Wall
Provides structural support and acts as a barrier against infection.
Plasmodesmata are specialized channels that connect adjacent plant cells, allowing for direct communication and material transport between their cytoplasms.
Extracellular Matrix (ECM) in Animals
The ECM is a complex network of macromolecules secreted by cells, providing structural and biochemical support to surrounding cells.
It consists of three main components:
Collagens and other fibrous proteins: Provide tensile strength.
Glycoproteins called proteoglycans: Help resist compression and provide hydration.
Linking proteins: Connect ECM components to each other and to cell surface receptors.
Intercellular Junctions
Intercellular junctions are specialized structures that provide direct channels of communication and adhesion between adjacent cells.
The types of junctions differ between plants and animals.
Plasmodesmata (Plants)
These are minute channels that traverse the cell walls of adjacent plant cells.
They directly connect the cytoplasm of one cell to the cytoplasm of another, facilitating the movement of water, ions, small solutes, and even some macromolecules between cells.
Tight Junctions (Animals)
Tight junctions form watertight seals between adjacent animal cells.
Their primary function is to prevent materials from leaking through the spaces between cells, effectively regulating paracellular transport.
They are commonly found in epithelial cells, which line internal organs, cavities, and the outer surfaces of the body, where preventing fluid leakage is critical.
Desmosomes (Animals)
Desmosomes are short, specialized proteins (cadherins) in the plasma membrane that function as "spot welds."
They mechanically join adjacent cells, anchoring them strongly together.
Desmosomes are particularly prevalent in tissues that experience significant mechanical stress and stretching, such as the heart, lungs, and muscles.
They are exclusively found in animal cells.
Gap Junctions (Animals)
Gap junctions closely resemble plasmodesmata in plants in their function, as they form channels that allow ions, nutrients, and other small materials to move directly between adjacent animal cells.
These junctions develop when six proteins, known as connexins, assemble to form an elongated, doughnut-like structure called a connexon within the plasma membrane.
When the connexons of two adjacent cells align perfectly, they complete a continuous channel, enabling direct cytoplasmic communication.
A gap junction essentially functions as a protein-lined pore, allowing the passage of water and small molecules between adjacent animal cells, crucial for rapid communication and coordination.
Simpler Summary of Cells and Their Organization
Cells are the fundamental units of life, forming all living organisms.
In multicellular organisms, cells organize into tissues, organs, and organ systems to form a complete organism.
Most cells are tiny and require microscopes for visualization, which involve magnification (enlarging) and resolution (distinguishing separate structures).
The Cell Theory states: cells are basic life units, all organisms are made of cells, and all cells come from pre-existing cells.
All cells share four basic components: a plasma membrane (outer barrier), cytoplasm (internal jelly-like substance with components), DNA (genetic material), and ribosomes (protein makers).
Prokaryotic cells (like bacteria) are simpler, lack a true nucleus and other membrane-bound internal compartments, and are generally smaller.
Eukaryotic cells (animal, plant, fungi) are larger and more complex, featuring a true nucleus and various membrane-bound organelles.
The nucleus stores DNA as chromatin and is enclosed by the nuclear envelope with pores.
Ribosomes synthesize proteins.
Mitochondria are powerhouses, converting energy into ATP.
The Endoplasmic Reticulum (ER) modifies proteins (rough ER) and synthesizes lipids/detoxifies (smooth ER).
The Golgi apparatus sorts, packages, and tags proteins and lipids.
Lysosomes (animal cells) break down waste and cellular debris.
Peroxisomes break down fatty acids and detoxify substances.
Plant cells have unique features: a rigid cell wall (made of cellulose), chloroplasts (for photosynthesis), and a large central vacuole (for water storage and turgor) but lack centrioles and lysosomes.
Animal cells have centrioles (part of centrosome) and lysosomes but lack a cell wall, chloroplasts, and a large central vacuole.
The Cytoskeleton (microfilaments, intermediate filaments, microtubules) provides structural support, facilitates movement, and helps transport substances within the cell.
Cilia and flagella are structures for cell movement, built from microtubules in a "9+2 array" pattern.
The Endomembrane System (nuclear envelope, ER, Golgi, lysosomes, plasma membrane) collectively modifies, packages, and transports lipids and proteins.
The Endosymbiosis Theory explains that mitochondria and chloroplasts originated from free-living prokaryotes that became internal symbionts.
Intercellular junctions connect cells:
Plasmodesmata (plants) are channels between plant cells.
Tight junctions (animals) create watertight seals.
Desmosomes (animals) are "spot welds" for strong adhesion.
Gap junctions (animals) form channels for communication, similar to plasmodesmata.
The Fundamental Units of Life: Cells and Their Organization
Cells: Basic Building Blocks
Cells serve as the fundamental building blocks for all organisms.
In single-celled organisms, the cell itself constitutes the entire organism, performing all necessary life functions.
Hierarchy of Multicellular Organisms
Multicellular organisms exhibit a hierarchical organization:
Cells: The basic unit of life.
Tissues: Composed of interconnected cells that share a common function.
Organs: Formed by the combination of several different tissues working together.
Organ Systems: Created when multiple organs function collaboratively to achieve a broader physiological role.
Organism: The complete entity formed by the cooperative functioning of multiple organ systems.
Cell Size and Visualization
Cell size varies significantly, with most cells being too small to be observed by the naked eye.
Microscopes are essential tools that enable the visualization and study of small cells.
Parameters in Microscopy
Magnification and Resolving Power are the two most critical parameters in microscopy.
Magnification
Magnification is defined as the process of enlarging an object in its apparent size.
Resolution (Resolving Power)
Resolving power is the ability of a microscope to distinguish between two adjacent structures as separate entities.
A higher resolution directly corresponds to better clarity and detail in the resulting image.
Types of Microscopes
Microscopes equipped with different optical systems are used for various types of studies.
Compound Light Microscopes:
Utilize visible light, which is bent through a lens system, to provide magnification.
Transparent objects, such as many types of cells, often require treatment with chemical stains to differentiate and distinguish their various internal parts.
Electron Microscopes:
Achieve significantly higher magnification and resolution by employing beams of electrons instead of light.
Transmission Electron Microscopes (TEM): Capable of revealing fine internal details within cells, providing cross-sectional views.
Scanning Electron Microscopes (SEM): Provide detailed three-dimensional exterior views of cell surfaces.
Cell Theory: A Biological Cornerstone
Cell theory is a foundational principle underlying all of biology, comprising three main tenets:
Cells are the basic units of life.
All living organisms are composed of one or more cells.
All cells originate from pre-existing cells.
Fundamental Components of All Cells
All cells, regardless of type, share four common components:
Plasma Membrane: An enclosing membrane that acts as a barrier, separating the cell's interior from its external environment.
Cytoplasm: Consists of a gel-like substance called cytosol, within which other cellular components are suspended.
DNA: The genetic material of the cell, carrying hereditary instructions.
Ribosomes: Cellular structures responsible for synthesizing proteins.
Prokaryotic Cells
Characteristics of Prokaryotes
Prokaryotic cells lack membrane-enclosed internal compartments, most notably a true nucleus.
The majority of prokaryotes possess a cell wall, which typically contains peptidoglycan.
It is widely believed that prokaryotes closely resemble the earliest forms of life on Earth.
Organisms classified under the domains Archaea and Bacteria are prokaryotes.
Generalized Structure of a Prokaryotic Cell
Chromosomal DNA is localized in a region within the cytoplasm called the nucleoid, not enclosed by a membrane.
Ribosomes are freely distributed throughout the cytoplasm.
The cell membrane is externally surrounded by a protective cell wall.
Other structures, such as capsules or flagella, may be present in some, but not all, bacterial species.
Small Size of Prokaryotic Cells
Prokaryotic cells are generally considerably smaller than eukaryotic cells.
Reasons for their small size include:
A more favorable surface area to volume ratio, which enhances the efficiency of material transport (moving nutrients in and waste out of the cell).
The absence of specialized internal transport modifications found in eukaryotic cells, necessitating a smaller size for efficient diffusion.
The logistics of carrying out cellular metabolism impose limits on cell size, emphasizing the critical role of the surface area to volume ratio. Smaller cells naturally possess a greater surface area relative to their overall volume.
Eukaryotic Cells
Components of a Eukaryotic Cell
(Based on the provided diagram, key components of a eukaryotic cell include: nucleolus, nuclear envelope, nuclear pore, nucleus, plasma membrane, flagellum, cytoplasm, centrosome, microfilament, ribosome, mitochondrion, microtubule, rough endoplasmic reticulum, smooth endoplasmic reticulum, peroxisome, Golgi complex, cilia, lysosome.)
Eukaryotic Plasma Membrane
The eukaryotic plasma membrane is fundamentally a phospholipid bilayer with numerous embedded and associated proteins.
Key components include:
Phospholipid Bilayer: The fundamental structure.
Integral Membrane Proteins: Proteins embedded directly within the bilayer.
Peripheral Membrane Proteins: Proteins loosely associated with the surface of the membrane.
Cholesterol: A lipid that regulates membrane fluidity.
Protein Channel: Specific integral proteins that allow passage of certain molecules.
Glycoprotein: A protein with an attached carbohydrate chain, often involved in cell recognition.
Glycolipid: A lipid with an attached carbohydrate chain, also important for cell recognition.
Filaments of the Cytoskeleton: Provide structural support and anchor the membrane.
Cytoplasm
The cytoplasm is the region situated between the plasma membrane and the nuclear envelope.
It comprises organelles suspended within a gel-like fluid called cytosol, along with the cytoskeleton.
The cytoplasm is approximately 70-80\% water but maintains a semi-solid consistency due to the presence of dissolved proteins.
Nucleus
Typically, a eukaryotic cell contains only one nucleus, though some are multinucleated.
The nucleus is usually the largest organelle within the cell, often larger than most prokaryotic cells themselves.
Key nuclear components include:
DNA and Proteins: These combine to form chromatin, a complex material during most of the cell's life cycle.
Chromatin Condensation: During cell division, chromatin condenses further to form distinct chromosomes.
Nucleolus: A prominent region within the nucleus that is the primary site of ribosomal RNA (rRNA) synthesis.
Nuclear Envelope:
This is a double membrane that encloses the nucleus.
Its primary functions include separating the DNA from the cytoplasm, thereby segregating the processes of transcription (in the nucleus) from translation (in the cytoplasm).
It is perforated by numerous nuclear pores.
Nuclear pores act as channels that connect the nucleoplasm (the interior of the nucleus) with the cytoplasm.
They actively regulate the flow of molecules, both large and small, in and out of the nucleus.
Large molecules, such as proteins, require a specific Nuclear Localization Signal (NLS) to successfully pass through the nuclear pores.
Ribosomes
Ribosomes are composed of two distinct subunits of different sizes; those in eukaryotes are slightly larger than in prokaryotes.
They are primarily made of specialized RNA (rRNA) and proteins.
During the process of protein synthesis, ribosomes are responsible for assembling amino acids into functional proteins.
Mitochondrion
The mitochondrion is the primary site within the cell for the conversion of stored energy (from macromolecular molecular bonds) into a more biologically usable form: adenosine triphosphate (ATP).
Its inner membrane is highly folded, creating structures known as cristae.
The area enclosed by the inner membrane is called the mitochondrial matrix, which contains enzymes for cellular respiration, mitochondrial DNA, and ribosomes.
Peroxisomes
Peroxisomes are small, rounded organelles enclosed by a single membrane.
They are the sites where reactions that break down fatty acids and amino acids occur.
Additionally, peroxisomes play a role in detoxifying various poisons within the cell.
Contrasting Animal and Plant Cells
While both animal and plant cells are eukaryotic, they exhibit significant structural differences:
Similarities:
Both possess Microtubule Organizing Centers (MTOCs).
Animal Cells:
Have centrioles associated with the MTOC, and this complex is referred to as the centrosome.
Contain lysosomes.
Lack a cell wall, chloroplasts, other specialized plastids, and a large central vacuole.
Plant Cells:
Possess a rigid cell wall external to the plasma membrane.
Contain chloroplasts (sites of photosynthesis) and other specialized plastids (e.g., amyloplasts for starch storage).
Feature a large central vacuole that occupies most of the cell's volume.
Do not have centrioles or lysosomes.
Animal Cell Components (Diagrammatic Representation)
(A typical animal cell contains: Intermediate filament, Ribosomes, Rough endoplasmic reticulum, Nucleus, Nucleolus, Chromatin, Golgi apparatus, Golgi vesicle, Mitochondria, Plasma membrane, Cytoplasm, Vacuole, Microtubule, Centrosome, Microfilament, Lysosome, Smooth endoplasmic reticulum, Secretory vesicle, Peroxisome.)
Plant Cell Components (Diagrammatic Representation)
(A typical plant cell contains: Plasmodesmata, Endoplasmic reticulum (smooth and rough), Nucleus (with chromatin and nucleolus), Cell wall, Plasma membrane, Cytoplasm, Central vacuole, Cytoskeleton (microtubules, intermediate filaments, microfilaments), Chloroplast, Plastid, Ribosomes, Golgi apparatus, Mitochondria, Peroxisome.)
Specialized Organelles and Structures
Centrosome (Animal Cells)
The centrosome is composed of two centrioles that are oriented at right angles to each other.
Each centriole is a cylindrical structure made up of nine triplets of microtubules.
Non-tubulin proteins provide structural integrity, holding the microtubule triplets together.
Plant Cell Walls
The cell wall is a rigid, protective structure located external to the plasma membrane in plant cells.
A key distinction from prokaryotic cell walls is that plant cell walls are primarily composed of cellulose, whereas most prokaryotic cell walls contain peptidoglycan.
Chloroplasts
Chloroplasts are double-membrane organelles, unique to plant cells and algae.
Similar to mitochondria, they possess their own ribosomes and DNA, supporting the endosymbiotic theory.
The inner membrane encloses an aqueous fluid called the stroma.
Within the stroma are sets of interconnected and stacked fluid-filled membrane sacs known as thylakoids.
Each stack of thylakoids is referred to as a granum (plural: grana).
The Central Vacuole (Plant Cells)
Plant cells typically contain a large central vacuole that can occupy a significant portion of the cell's volume.
This vacuole plays crucial roles in regulating water concentration, particularly under varying environmental conditions, and contributes significantly to cell expansion by maintaining turgor pressure against the cell wall.
Endosymbiosis Theory
The Endosymbiosis Theory hypothesizes that mitochondria and chloroplasts originated as formerly independent prokaryotic organisms.
These free-living prokaryotes are believed to have become endosymbionts (organisms living within another organism) of the prokaryotic ancestors of eukaryotes.
Supporting evidence for this theory includes:
Mitochondria and chloroplasts having their own distinct DNA and ribosomes.
The size of these organelles being remarkably similar to that of independent prokaryotes.
Much of the original research and prominent advocacy for this theory was undertaken by Lynn Margulis.
Examples of Symbiosis
Algae living within a flatworm.
Algae found within salamander eggs.
Lichens, which are a symbiotic association between algae and fungi.
Nitrogen-fixing bacteria living in root nodules of plants.
The Endomembrane System
The endomembrane system is a complex network of internal membranes and organelles present in eukaryotic cells.
Its collective function is to modify, package, and transport lipids and proteins throughout the cell or for secretion.
Components of the endomembrane system include:
The nuclear envelope
Lysosomes and vesicles
The endoplasmic reticulum (ER)
The Golgi apparatus
The plasma membrane
Lysosomes (Animal Cells)
Lysosomes, found predominantly in animal cells, contain potent digestive enzymes.
These enzymes are responsible for breaking down large biomolecules (such as proteins, lipids, and carbohydrates) and even worn-out or damaged organelles within the cell.
They are crucial for processes like phagocytosis, where a food particle is engulfed forming a food vacuole, which then fuses with a lysosome to digest its contents.
Endoplasmic Reticulum (ER)
The ER is an extensive network of interconnected membranous sacs and tubules that extends throughout the cytoplasm.
It plays a central role in modifying proteins (specifically the rough ER) and synthesizing lipids (primarily the smooth ER).
The hollow interior space within the ER tubules is referred to as the lumen or cisternal space.
The membrane of the ER is continuous with the outer membrane of the nuclear envelope.
Rough Endoplasmic Reticulum (RER)
The RER is characterized by the presence of ribosomes attached to its cytoplasmic surface.
These ribosomes synthesize proteins that are destined for secretion, insertion into membranes, or delivery to other organelles.
Newly synthesized proteins undergo modifications, such as folding and the acquisition of side chains, within the lumen of the RER.
Modified proteins are either directly incorporated into cellular membranes or packaged for secretion from the cell (e.g., protein hormones, enzymes).
The RER also contributes to the synthesis of phospholipids, which are essential components of cellular membranes.
Phospholipids or modified proteins that are not intended to remain within the RER are transported to their destinations via transport vesicles that bud off from the RER's membrane.
Smooth Endoplasmic Reticulum (SER)
The SER is continuous with the RER but distinguishes itself by having few or no ribosomes on its cytoplasmic surface.
Key functions of the SER include:
Synthesis of carbohydrates, lipids, and steroid hormones.
Detoxification of various medications and poisons, particularly in liver cells.
Storage of calcium ions (Ca^{++}).
In muscle cells, a specialized form of SER called the sarcoplasmic reticulum is dedicated to storing and releasing Ca^{++} ions, which are vital for coordinated muscle contractions.
Golgi Apparatus (Golgi Body)
The Golgi apparatus is an organelle composed of a series of flattened membrane-bound sacs called cisternae.
Its primary role is to sort, package, and tag lipids and proteins that arrive from the ER via transport vesicles, ensuring they reach their correct destinations.
The Golgi apparatus has two distinct faces:
The cis face is the receiving side, typically oriented towards the ER.
The trans face is the opposite, or shipping, side.
Transport vesicles from the ER fuse with the cis face of the Golgi, releasing their contents into its lumen.
As proteins and lipids traverse through the various cisternae of the Golgi, they undergo further modifications, often involving the addition of short chains of sugar molecules, to facilitate their sorting and targeting.
The Cytoskeleton
The cytoskeleton is an intricate network of protein fibers that provides structural integrity and dynamic capabilities to the eukaryotic cell.
Its fundamental functions include:
Helping to maintain the characteristic shape of the cell.
Holding certain organelles in specific, stable positions within the cytoplasm.
Allowing for the directed movement of cytoplasm and vesicles within the cell, enabling intracellular transport.
Enabling the movement of entire cells within multicellular organisms, crucial for processes like development, tissue repair, and immune response.
Three Components of the Cytoskeleton
These components differ in size and specific functions:
Microfilaments (Smallest diameter)
Intermediate Filaments (Intermediate diameter)
Microtubules (Largest diameter)
Microfilaments
Primarily involved in cell movement, encompassing both the movement of internal cellular parts and the locomotion of the entire cell.
They play a crucial role in determining and stabilizing cell shape.
Microfilaments are constructed from actin monomers.
Microtubules
Form a rigid internal skeleton in some cells, providing structural support.
They serve as tracks or frameworks along which motor proteins can transport vesicles and other structures within the cell.
Microtubules are assembled from tubulin dimers.
Each microtubule typically comprises 13 chains of tubulin dimers arranged around a central cavity.
Cilia and Flagella
Cilia and flagella are motility structures built upon a characteristic microtubule arrangement, commonly known as the "9+2 array."
Ultrastructure ( 9+2 array ):
Consists of 9 doublet microtubules arranged in a circle on the outside.
Contains 2 unfused single microtubules located in the center.
Spokes connect the outer doublets to the central pair.
Cilia are generally shorter and more numerous than flagella.
They also exhibit different beating patterns, leading to varied forms of cellular movement.
Extracellular Structures
Plant Cell Wall
Provides structural support and acts as a barrier against infection.
Plasmodesmata are specialized channels that connect adjacent plant cells, allowing for direct communication and material transport between their cytoplasms.
Extracellular Matrix (ECM) in Animals
The ECM is a complex network of macromolecules secreted by cells, providing structural and biochemical support to surrounding cells.
It consists of three main components:
Collagens and other fibrous proteins: Provide tensile strength.
Glycoproteins called proteoglycans: Help resist compression and provide hydration.
Linking proteins: Connect ECM components to each other and to cell surface receptors.
Intercellular Junctions
Intercellular junctions are specialized structures that provide direct channels of communication and adhesion between adjacent cells.
The types of junctions differ between plants and animals.
Plasmodesmata (Plants)
These are minute channels that traverse the cell walls of adjacent plant cells.
They directly connect the cytoplasm of one cell to the cytoplasm of another, facilitating the movement of water, ions, small solutes, and even some macromolecules between cells.
Tight Junctions (Animals)
Tight junctions form watertight seals between adjacent animal cells.
Their primary function is to prevent materials from leaking through the spaces between cells, effectively regulating paracellular transport.
They are commonly found in epithelial cells, which line internal organs, cavities, and the outer surfaces of the body, where preventing fluid leakage is critical.
Desmosomes (Animals)
Desmosomes are short, specialized proteins (cadherins) in the plasma membrane that function as "spot welds."
They mechanically join adjacent cells, anchoring them strongly together.
Desmosomes are particularly prevalent in tissues that experience significant mechanical stress and stretching, such as the heart, lungs, and muscles.
They are exclusively found in animal cells.
Gap Junctions (Animals)
Gap junctions closely resemble plasmodesmata in plants in their function, as they form channels that allow ions, nutrients, and other small materials to move directly between adjacent animal cells.
These junctions develop when six proteins, known as connexins, assemble to form an elongated, doughnut-like structure called a connexon within the plasma membrane.
When the connexons of two adjacent cells align perfectly, they complete a continuous channel, enabling direct cytoplasmic communication.
A gap junction essentially functions as a protein-lined pore, allowing the passage of water and small molecules between adjacent animal cells, crucial for rapid communication and coordination.