Cell Structures and Functions Vocabulary

Functions of Vacuoles

• Vacuoles act as storage for nutrients and waste materials, protecting the cell from toxicity.
• They are essential for homeostasis, balancing the cell's pH by regulating the influx and outflow of H^+ ions to the cytoplasm.
• Vacuoles contain enzymes important for various metabolic processes.

Vesicles

• Vesicles are structures within the cell formed naturally through exocytosis, endocytosis, or material transport.
• They can also be formed artificially as liposomes.
• Different types exist, including vacuoles, secretory vesicles, and transport vesicles, categorized by function.

Structure of Vesicles

• A vesicle contains liquid or cytosol enclosed by a lipid bilayer.
• The outer layer, called a lamellar phase, resembles the plasma membrane.
• The lipid bilayer has a hydrophobic end and a hydrophilic end.

Functions of Vesicles

• Vesicles store and transport materials in and out of the cell and facilitate molecule exchange between cells.
• Enclosed by a lipid bilayer, they function in metabolism and enzyme storage.
• They temporarily store food and control cell buoyancy.

Plastids

• Plastids are usually oval or spherical, with an outer and inner membrane separated by an intermembrane space.
• The inner membrane encloses the stroma, containing grana.
• Each granum comprises several sac-like thylakoids connected by stroma lamellae.
• Plastids contain DNA and RNA, enabling them to synthesize necessary proteins.
• Types of plastids include etioplasts, proplastids, chromoplasts, chloroplasts, leucoplasts, amyloplasts, elaioplasts, and proteinoplasts.

Functions of Plastids

• Chloroplasts are central to metabolic activities, including photosynthesis, due to their enzymes and components.
• They store food, primarily starch.

Ribosomes

• Ribosomes are ribonucleoproteins with equal parts RNA and proteins, essential for protein synthesis.
• In prokaryotes, they exist freely; in eukaryotes, they are free or attached to the endoplasmic reticulum.
• Ribosomes synthesize biological proteins in all living organisms.
• They arrange amino acids in the order indicated by tRNA and assist in protein synthesis.

Structure of Ribosomes

• Ribonucleoproteins consist of two subunits.
• Prokaryotic cells have 70S ribosomes, with a 50S larger subunit and a 30S smaller subunit.
• Eukaryotic cells have 80S ribosomes, with a 60S larger subunit and a 40S smaller subunit.
• Ribosomes are short-lived, splitting up after protein synthesis and being reused or broken up.

Storage Granules

• Storage granules are membrane-bound organelles, also known as zymogen granules, storing cells' energy reserves and metabolites.

Structure of Storage Granules

• These granules are surrounded by a lipid bilayer and consist mostly of phosphorus and oxygen.
• Their components vary based on location, sometimes containing degradative enzymes for digestive activities.

Functions of Storage Granules

• Many prokaryotes and eukaryotes store nutrients and reserves as storage granules in the cytoplasm.
• Sulfur granules are characteristic of prokaryotes using hydrogen sulfide as an energy source.

Vacuoles

• Vacuoles are membrane-bound structures that vary in size in cells of different organisms.

Structure of Vacuoles

• Vacuoles are surrounded by a membrane called the tonoplast, enclosing fluid containing inorganic materials like water and organic materials like nutrients and enzymes.
• They are formed by the fusion of various vesicles, making them structurally similar to vesicles.

Mitochondria

• Mitochondria are double membrane-bound cell organelles responsible for energy supply and storage.
• They oxidize substrates to release energy in the form of ATP (Adenosine Triphosphate).
• They are called the 'powerhouse of the cell' as they produce ATP, providing energy for cellular activities.
• Mitochondria have their own DNA and are the site of cellular respiration.

Structure of Mitochondria

• Mitochondria have two membranes: a smooth outer layer and a folded inner layer forming cristae.
• The inner mitochondrial membrane contains enzymes, coenzymes, and components of multiple cycles, along with pores for the transport of substrates, ATP, and phosphate molecules.
• Within the membranes is a matrix that contains various enzymes of metabolic processes, like the Krebs cycle.
• Mitochondria also contain single or double-stranded DNA called mtDNA, capable of producing 10% of the mitochondrial proteins.

Functions of Mitochondria

• The primary function is the synthesis of energy in the form of ATP, required for cell organelle function.
• Mitochondria help balance the amount of Ca^+ ions within the cell and assist in apoptosis.
• They build different segments of hormones and blood components.
• Liver mitochondria can detoxify ammonia.

Nucleus

• The nucleus is a double-membrane bound cell organelle that carries the cell's genetic information (DNA or RNA) and is where transcription takes place.
• It controls all cellular activities and is the center for genetic material transfer.
• It is often termed the "brain of the cell" as it provides commands for other cell organelles' proper functioning.
• A nucleus is clearly defined in eukaryotic cells but absent in prokaryotic organisms, where genetic material is distributed in the cytoplasm.

Structure of Nucleus

• The nucleus consists of a nuclear envelope, chromatin, and nucleolus.
• The nuclear envelope structure and composition is similar to the cell membrane. It has pores that allow the movement of proteins and RNA in and out of the nucleus, and it enables interaction with other cell organelles while keeping nucleoplasm and chromatin within the envelope.
• The chromatin contains RNA or DNA along with nuclear proteins as genetic material, responsible for carrying genetic information from one generation to another. It may be visible as a chromosome under powerful magnification.
• The nucleolus is like a nucleus within the nucleus, a membrane-less organelle responsible for the synthesis of rRNA and the assembly of ribosomes required for protein synthesis.

Functions of Nucleus

• The nucleus stores and transfers genetic materials in the form of DNA or RNA.
• It aids in transcription by synthesizing mRNA molecules.
• The nucleus controls the activity of all other organelles while facilitating processes like cell growth, cell division, and protein synthesis.
• Chromatin is a complex of genetic material (DNA or RNA) and proteins that participates in cell division.

Peroxisomes

• Peroxisomes are oxidative membrane-bound organelles found in the cytoplasm of all eukaryotes, named for their hydrogen peroxide generating and removing activities.

Structure of Peroxisomes

• Peroxisomes consist of a single membrane and granular matrix scattered in the cytoplasm.
• They exist either as interconnected tubules or individual peroxisomes.
• Compartments within allow optimized conditions for metabolic activities.
• They consist of enzymes like urate oxidase, D-amino acid oxidase, and catalase.
• They are single membrane-bound vesicular structures that contain digestive enzymes that help cells break down and remove toxic materials through oxidation.
• They are involved in biochemical pathways such as β-oxidation of fatty acids, releasing energy as ATP and synthesizing lipids and plasmalogens.

Functions of Peroxisomes

• Peroxisomes produce and eliminate hydrogen peroxide during biochemical processes.
• Oxidation of fatty acids takes place within peroxisomes.
• They are also involved in the synthesis of lipids like cholesterol and plasmalogens.

Plasmodesmata

• Plasmodesmata are tiny passages or channels that allow the transfer of material and communication between different cells.

Structure of Plasmodesmata

• There are 103-105 plasmodesmata connecting two adjacent cells with 50-60 nm in diameter.
• A plasmodesma has three layers:
  • The plasma membrane is continuous with the cell's plasma membrane and has the same phospholipid bilayer.
  • The cytoplasmic sleeve is continuous with the cytosol, allowing the exchange of materials between two cells.
  • The desmotubule is part of the endoplasmic reticulum that provides a network between two cells and allows the transport of some molecules.

Functions of Plasmodesmata

• Plasmodesmata are the primary site for cell communication, allowing the transfer of molecules like proteins, RNA, and viral genomes.

Golgi Apparatus

• The Golgi Complex directs proteins and lipids to their destination, acting as the "traffic police" of the cell.

Functions of Golgi Apparatus

• They are involved in the exocytosis of various products and proteins like zymogen, mucus, lactoprotein, and parts of the thyroid hormone.
• The Golgi Complex synthesizes other cell organelles like the cell membrane and lysosomes.
• They are also involved in the sulfation of various molecules.

Structure of Golgi Apparatus

• The structure of the Golgi Complex is pleomorphic; however, it typically exists in three forms: cisternae, vesicles, and tubules.
• The cisternae, which is the smallest unit of the Golgi Complex, has a flattened sac-like structure that is arranged in bundles in a parallel fashion.
• Tubules are present as tubular and branched structures that radiate from the cisternae and are fenestrated at the periphery.

Intermediate Filaments

• Intermediate filaments are a class of filament that makes up the cytoskeleton with an intermediate diameter compared to microfilaments and myosin proteins.

Structure of Intermediate Filaments

• Intermediate filaments contain a family of related proteins.
• The individual filaments are coiled around each other in a helical coiled-coil structure.

Functions of Intermediate Filaments

• Intermediate filaments contribute to the structural integrity of a cell while playing a crucial role in holding tissues of various organs like the skin.

Lysozymes

• Lysozymes are membrane-bound organelles in the cytoplasm of animal cells containing hydrolytic enzymes for the degradation of macromolecules.

Structure of Lysozyme

• Lysozymes have an irregular or pleomorphic shape, mostly found in spherical or granular structures.
• They are surrounded by a lysosomal membrane that contains the enzymes and protects the cytosol with the rest of the cell from harmful enzyme action.
• There are two types of lysozymes:
  • Primary lysozymes containing hydrolytic enzymes like lipases, amylases, proteases, and nucleases.
  • Secondary lysozymes formed by the fusion of primary lysozymes containing engulfed molecules or organelles.

Functions of Lysozyme

• These organelles are responsible for intracellular digestion, degrading larger macromolecules into smaller molecules with the help of enzymes.
• Lysozymes perform autolysis of unwanted organelles within the cytoplasm.
• Besides these, the lysosome is involved in various cellular processes, including secretion, plasma membrane repair, cell signaling, and energy metabolism.

Microfilaments

• Microfilaments are part of the cytoskeleton of a cell made of actin protein in the form of parallel polymers. They are the smallest filaments of the cytoskeleton with high rigidity and flexibility, providing strength and movement to the cell.

Structure of Microfilaments

• The filaments are present either in cross-linked forming networks or as bundles.
• The chains of protein remain twisted around each other in a helical arrangement.

- One of the polar ends of the filament is positively charged and barbed, whereas the other end is negatively charged and pointed.

Functions of Microfilaments

• It generates the strength for the structure and movement of the cell in association with myosin protein.
• They help in cell division and are involved in the products of various cell surface projections.

Microtubules

• Microtubules are also a part of the cytoskeleton differing from microfilaments the presence of tubulin protein. They are long hollow, beaded tubular structures of a diameter of about 24nm.

Structure of Microtubules

• The wall of the microtubules consists of globular subunits present at a helical array of a and b tubulin.
• Similar to microfilaments, the ends of microtubules also have a defined polarity with one end being positively charged while the other being negatively charged.

Functions of Microtubules

• As a part of the cytoskeleton, they provide shape and movement to the cell.
• Microtubules facilitate the movement of other cell organelles within the cell through binding proteins.

Microvilli

• Microvilli are tiny finger-like structures that project on or out of the cells. These exist either on their own or in conjunction with villi.

Structure of Microvilli

• Microvilli are bundles of protuberances loosely arranged on the surface of the cell with little or no cellular organelles.
• These are surrounded by a plasma membrane enclosing cytoplasm and microfilaments.
• These are bundles of actin filaments bound by fimbrin, villin, and epsin.

Functions of Microvilli

• Microvilli increase the surface area of the cell, thus, enhancing the absorption and secretion functions.
• The membrane of microvilli is packed with enzymes that allow the break down of larger molecules into smaller allowing more effective absorption.
• Microvilli act as an anchoring agent in white blood cells and in sperms during fertilization.

Cytoplasm

• Cytoplasm refers to everything present inside the cell except the nucleus.

Structure of Cytoplasm

• The cytoplasm consists of a cytosol; a gel-like substance that contains other matter:
  • Cell organelles: smaller cell-like bodies bound by separate membranes.
  • Cytoplasmic inclusions: insoluble molecules that store energy and are not surrounded by any layer.
• The cytoplasm is colorless and has about 80% water along with various nutrients required for the cell.
• It is known to have the properties of both viscous matter as well as elastic matter. Under its elasticity, cytoplasm helps in the movement of materials inside the cell by a process termed cytoplasmic streaming.

Functions of Cytoplasm

• Most of the vital cellular and enzymatic reactions like cellular respiration and translation of mRNA into proteins occur in the cytoplasm.
• It acts as a buffer and protects genetic materials as well as other organelles from damage due to collision or change in the pH of the cytosol.
• The process called cytoplasmic streaming helps in the distribution of various nutrients and facilitates the movement of cell organelles within the cell.

Cytoskeleton

• A number of fibrous structures are present in the cytosol that helps give shape to the cell while supporting cellular transport.
• Around three different classes of fibers make up the cytoskeleton which is: microtubules, microfilaments, and intermediate filaments.
  • These are separated based on a protein present in them. They are a complex network of interlinking protein filaments that maintain a cell's shape and internal organization. It also provides a cell with mechanical support. There are three types of cytoskeleton:

Functions of Cytoskeleton

• The most crucial function of the cytoskeleton is to provide shape and mechanical support to the cell against deformation.
• It allows the expansion and contraction of the cell which assists in the movement of the cell.
• It is also involved in the intracellular and extracellular transport of materials.
• Microtubules are hollow protein tubular rods and heterodimers of a- and ß-tubulin. They are located in cilia, flagella, and structures associated with cell movement.
• Intermediate filaments are smaller than microtubules but larger than microfilaments.
• Microfilaments are the thinnest cytoskeleton, made of actin filaments. They are thus strong and flexible and are involved in cell movement.

Cell Membrane (Plasma membrane/ Plasmalemma)

• A plasma membrane is composed of lipids and proteins where the composition might fluctuate based on fluidity, external environment, and the different stages of development of the cell.

Structure of Cell Membrane

• Structurally, it consists of a phospholipid bilayer along with two types of proteins viz. embedded proteins and peripheral proteins that function in providing shape and allowing the movement of particles in and out of the cell.
• The most abundant lipid which is present in the cell membrane is a phospholipid that contains a polar head group attached to two hydrophobic fatty acid tails.
• The embedded proteins act as channels for the transfer of particles across the cell with some proteins acting as receptors for the binding of various components.
• The peripheral proteins function as to provide fluidity as well as mechanical support to the structure of the cell.

Functions of Cell Membrane

• The cell membrane provides mechanical support that facilities the shape of the cell while enclosing the cell and its components from the external environment.
• It regulates what can be allowed to enter and exit the cell through channels, acting as a semi-permeable membrane, which facilities the exchange of essential compounds required for the survival of the cell.
• It generates and distributes signals in and outside of the cell for the proper functioning of the cell and all the organelles.
• It allows the interaction between cells required during tissue formation and cell fusion.

Cell Wall

• An additional non-living layer present outside the cell membrane in some cells that provides structure, protection, and filtering mechanism to the cell is the cell wall.

Structure of Cell Wall

• In a plant cell, the cell wall is made up of cellulose, hemicellulose, and proteins while in a fungal cell, it is composed of chitin.
• A cell wall is multilayered with a middle lamina, a primary cell wall, and a secondary cell wall.
• The middle lamina contains polysaccharides that provide adhesion and allow binding of the cells to one another.
• After the middle lamina is the primary cell wall which is composed of cellulose. The last layer, which is not always present, is the secondary cell wall made of cellulose and hemicellulose.

Functions of Cell Wall

• The critical function of the cell wall is protecting and maintaining the shape of the cell. It also helps the cell withstand the turgor pressure of the cell.
• It initiates cell division by providing signals to the cell and allows the passage of some molecules into the cell while blocking others.

Endoplasmic Reticulum (ER)

• Endoplasmic Reticulum (ER) is present as an interconnection of tubules that are connected to the nuclear membrane in eukaryotic cells.
• There are two types of ER based on the presence or absence of ribosomes on them:
  • Rough ER (RER) with ribosomes attached on the cytosolic face of Endoplasmic Reticulum and thus is involved in protein synthesis
  • Smooth ER (SER) lacks ribosomes and has a function during lipid synthesis.

Structure of Endoplasmic Reticulum (ER)

• Endoplasmic Reticulum exists in three forms viz. cisternae, vesicles, and tubules.
  • Cisternae are sac-like flattened, unbranched structures that remain stacked one on top of another.
  • Vesicles are spherical structures that carry proteins throughout the cell.
  • Tubules are tubular branched structures forming a connection between cisternae and vesicles.

Functions of Endoplasmic Reticulum (ER)

• ER contains many of the enzymes required for several metabolic processes, and the surface of the ER is essential for other operations like diffusion, osmosis, and active transport.
• One of the crucial functions of ER is the synthesis of lipids like cholesterol and steroids.
• Rough ER allows for the modification of polypeptides emerging out of the ribosomes to prepare secondary and tertiary structures of the protein.
• ER also synthesizes various membrane proteins and has a crucial role in preparing the nuclear envelope after cell division.

Centrioles

• Centrioles are tubular structures mostly found in eukaryotic cells which are composed mainly of the protein tubulin.

Structure of Centriole

• A centriole consists of a cylindrical structure made with nine triplets microtubules that surround the periphery of the centriole while the center has a Y-shaped linker and a barrel-like structure that stabilizes the centriole.
• Another structure called cartwheel is present in a centriole which is made up of a central hub with nine spokes/filaments radiating from it. Each of these filaments/spokes is connected to the microtubules through a pinhead.

Functions of Centriole

• During cell division, centrioles have a crucial role in forming spindle fibers which assist the movement of chromatids towards their respective sides.
• They are involved in the formation of cilia and flagella.

Cilia and Flagella

• Cilia and Flagella are tiny hair-like projections from the cell made of microtubules and covered by the plasma membrane.

Structure of Cilia and Flagella

• Cilia are hair-like projections that have a 9+2 arrangement of microtubules with a radial pattern of 9 outer microtubule doublet that surrounds two singlet microtubules. This arrangement is attached to the bottom with a basal body.
• Flagella is a filamentous organelle, the structure of which, is different in prokaryotes and eukaryotes.
  • In prokaryotes, it is made up of the protein called flagellin wrapped around in a helical manner creating a hollow structure at the center throughout the length.
  • In eukaryotes, however, the protein is absent and the structure is replaced with microtubules.

Functions of Cilia and Flagella

• The most critical role of cilia and flagella is movement. These are responsible for the movement of the organisms as well as for the movement of various particles present around the organisms.
• Some cilia present in some particular organs may have the function of sense. The cilium in the blood vessels, which helps in controlling the flow of blood is an example.

Endosomes

• Endosomes are membrane-bound compartments within a cell originating from the Golgi network

Structure of Endosomes

• There are different types of endosomes based on morphology and the time it takes for the endocytosed materials to reach them.
  • The early endosomes are made with the tubular-vesicular network while the late endosomes lack tubules but contain many close-packed intraluminal vesicles. The recycling endosomes are found with microtubules and are mainly composed of tubular structures.

Functions of Endosomes

• Endosomes allow the sorting and delivery of internalized materials from the cell surface and transport of materials to the Golgi or the lysosomes.

Chloroplast

• A chloroplast is a type of plastic that is involved in photosynthesis in plants and algae. Chloroplast contains an essential pigment called chlorophyll necessary to trap sunlight for the production of glucose.

Structure of Chloroplast

• It is a double-membraned structure with its own DNA which is inherited from the previous chloroplast.
• These are usually lens-shaped with shape and number varying according to cells. They have an outer membrane, an inner membrane, and a thylakoid membrane that enclosed the gel-like matric called the stroma.
• The outer and inner membrane is porous and allows transport of materials while the stroma contains DNA, chloroplast ribosomes, proteins, and starch granules.

Functions of Chloroplast

• The chloroplast is the primary center for light-dependent and light-independent reactions during photosynthesis.
• Different proteins present in chlorophyll are involved in the regulation of photorespiration.