COLLEGE PHYSICS - Chapter 4 CELL STRUCTURE

Cell Structure: The Fundamental Units of Life

4.1 Studying Cells

The Fundamental Units of Life

• Cells are the basic building blocks of all organisms.
• In single-celled organisms, the cell itself constitutes the entire organism.
• Being alive implies being comprised of one or more cells.

Hierarchical Organization of Multicellular Organisms

• Cells: The fundamental unit of structure and function.
• Tissues: Composed of interconnected cells working together for a common function.
• Organs: Formed by the combination of several tissues.
• Organ Systems: Multiple organs working in concert.
• Organism: Multiple systems functioning together to form a complete living being.

Cell Size and Microscopy

• Most cells are too small to be seen with the naked eye, necessitating the use of microscopes.
• The term "microscope" comes from "micro" (small) and "scope" (to look at).
• Two primary types of microscopes are discussed: light microscopes and electron microscopes.

Important Parameters in Microscopy

• Magnification: The process of enlarging an object's appearance.
  • Example ratios: 1:2, 1:1, 5:1.
• Resolving Power (Resolution): The ability of a microscope to distinguish two adjacent structures as separate entities.
  • Higher resolution leads to better clarity and detail in the image.

Optical Systems: Light Microscopy

• Compound Light Microscopes: Utilize visible light, bending it to provide magnification.
  • Typical college biology lab light microscopes can magnify up to approximately 400 times and have a resolution of about 200 nanometers (nm).
  • Transparent objects, like most cells, often require chemical stains to differentiate their various parts.

Optical Systems: Electron Microscopy

• Electron microscopes achieve significantly higher magnification and resolution by using beams of electrons instead of light.
  • They can provide a magnification of up to 100,000x and a resolution of 50 picometers (pm).
• Transmission Electron Microscopes (TEMs): Designed to show fine details within cells.
• Scanning Electron Microscopes (SEMs): Provide three-dimensional (3-D) exterior views of cell surfaces.

Cell Theory: An Underlying Principle of Biology

• Cells are the basic units of life.
• All living organisms are made of cells.
• All cells come from pre-existing cells.

4.2 Prokaryotic Cells

Four Common Components of All Cells

1. Plasma Membrane: An enclosing membrane that separates the cell's interior from its external environment.
2. Cytoplasm: Made of a gel-like substance called cytosol, within which other cellular components are found.
3. DNA: The genetic material of the cell.
4. Ribosomes: Cellular structures responsible for synthesizing proteins.

Characteristics of Prokaryotes

• Prokaryotes lack a membrane-enclosed internal nucleus and other membrane-bound organelles.
• Most possess a cell wall, which typically contains peptidoglycan.
• They are thought to be structurally similar to the earliest forms of cells on Earth.
• Organisms classified in the domains Archaea and Bacteria are prokaryotes.

Generalized Structure of a Prokaryotic Cell

• Chromosomal DNA: Localized in a specific area within the cytoplasm called the nucleoid region.
• Ribosomes: Located in the cytoplasm.
• Cell Membrane: Enclosed by a cell wall.
• Other structures, such as capsules or flagella, may be present in some, but not all, bacteria.

Reasons for the Small Size of Prokaryotic Cells

• Prokaryotic cells are generally smaller than eukaryotic cells.
• Surface Area to Volume Ratio: This ratio is more favorable for efficient movement of materials into and out of the cell when the cell is small.
  • As cells increase in size, their volume increases faster than their surface area, limiting the rate at which materials can be exchanged.
  • This surface area-to-volume ratio is a fundamental limiting factor for all cells.
• They lack the specialized modifications found in eukaryotes that facilitate internal transport over longer distances.

4.3 Eukaryotic Cells

Overview of Eukaryotic Cells

• Eukaryotic cells are typically larger and more complex than prokaryotic cells, characterized by the presence of a membrane-bound nucleus and other membrane-bound organelles.
• Visual examples: Animal Cell (Page 19) and Plant Cell (Page 20) diagrams illustrate the distinct organelles present.

Eukaryotic Plasma Membrane

• Structure: A phospholipid bilayer with various embedded proteins.
  • Cholesterol is found within the interior of the bilayer, contributing to membrane fluidity and stability.
  • Carbohydrate chains are present on the external surface, involved in cell recognition and adhesion.
• Function: Controls the selective passage of molecules into and out of the cell, maintaining intracellular homeostasis.
• Modifications (e.g., Microvilli): Increases surface area, particularly relevant in cells involved in absorption, such as those lining the small intestine.

Cytoplasm

• Location: The region between the plasma membrane and the nuclear envelope.
• Composition: Consists of organelles suspended in a gel-like cytosol, along with the cytoskeleton.
• Consistency: Approximately 70-80\% water, but maintains a semi-solid consistency due to the proteins dissolved within it.

Nucleus

• General: Usually a single, prominent organelle per cell; it is larger than most prokaryotic cells.
  • The plural form is nuclei.
• Function: Primarily stores the cell's genetic material (DNA).
• Nucleolus: A dense structure within the nucleus that directs the synthesis of ribosomes from ribosomal RNA (rRNA) and proteins.

Nuclear Envelope

• Structure: A double membrane that encloses the nucleus.
• Function: Separates the DNA inside the nucleus from the cytoplasm, thereby separating the processes of transcription (DNA to mRNA) and translation (mRNA to protein).
• Nuclear Pores: Perforate the nuclear envelope, connecting the nucleoplasm (interior of the nucleus) with the cytoplasm.
  • These pores regulate the flow of molecules between the nucleus and the cytoplasm.
  • Large molecules require a specific nuclear localization signal (NLS) to pass through the pores.

Ribosomes

• Structure: Composed of two different-sized subunits.
  • Ribosomes in eukaryotes are slightly larger than those in prokaryotes.
  • Made of special RNA (rRNA) and proteins.
• Synthesis: Synthesized in the nucleolus.
• Function: During protein synthesis, ribosomes serve as the platform for the assembly of amino acids into proteins.

Mitochondrion (Plural: Mitochondria)

• Function: The primary site for the conversion of stored energy in molecules (like glucose) into a more usable form (ATP) through cellular respiration.
  • Often referred to as the "powerhouse of the cell."
• Structure: Enclosed by a double membrane.
  • The inner membrane is highly folded into structures called cristae.
  • The area enclosed by the inner membrane is called the mitochondrial matrix.
• Genetic Material: Mitochondria possess their own circular DNA (inherited maternally) and ribosomes, supporting the endosymbiosis hypothesis.

Peroxisomes

• Structure: Small, rounded organelles enclosed by a single membrane.
• Functions:
  • Site for reactions that break down fatty acids and amino acids.
  • Involved in detoxifying various poisons within the cell.
  • Contain the enzyme catalase, which breaks down hydrogen peroxide (H2O2) — a toxic byproduct of many metabolic reactions — into water (H2O) and oxygen (O2).

Vesicles and Vacuoles

• General: Both are membrane-bound sacs used for storage and transport of substances within the cell.
• Size Difference: Vacuoles are generally larger than vesicles.
• Vesicles: Can fuse with the plasma membrane to release contents outside the cell or incorporate membrane components.

Contrasting Animal and Plant Cells

• Animal Cells Have/Plant Cells Do Not:
  • Centrosome: Contains two centrioles (made of nine triplets of microtubules), which act as a microtubule-organizing center (MTOC) near the nucleus.
  • Lysosomes: Act as the cell's "garbage disposal," containing hydrolytic enzymes to digest biomolecules and worn-out organelles.
• Plant Cells Have/Animal Cells Do Not:
  • Cell Wall: A rigid protective structure external to the plasma membrane, composed of cellulose (unlike prokaryotic cell walls which contain peptidoglycan).
  • Chloroplasts: Sites of photosynthesis, containing the pigment chlorophyll to capture solar energy and produce sugars from CO_2, water, and sunlight.
  • Large Central Vacuole: Occupies most of the cell's volume, helping regulate water concentration and contributing to cell expansion by maintaining turgor pressure against the cell wall.

Endosymbiosis Hypothesis

• It is hypothesized that mitochondria and chloroplasts originated from independent prokaryotic organisms.
• These prokaryotes became endosymbionts (organisms living within another organism) of the prokaryotic ancestors of eukaryotes.
• Evidence supporting this includes:
  • Mitochondria and chloroplasts both have their own circular DNA, similar to prokaryotic DNA.
  • They possess their own ribosomes, which are similar in size and structure to prokaryotic ribosomes.
  • Their size is comparable to that of independent prokaryotes.
  • They replicate by binary fission, similar to prokaryotes.

4.4 The Endomembrane System and Proteins

The Endomembrane System

• Definition: A network of internal membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.
• Components:
  • Nuclear HHH
  • Endoplasmic Reticulum (ER)
  • Golgi Apparatus
  • Lysosomes
  • Vesicles
  • Plasma Membrane

Endoplasmic Reticulum (ER)

• Structure: A complex network of interconnected membranous sacs and tubules.
• The hollow interior of the ER tubules is called the lumen or cisternal space.
• The ER membrane is continuous with the outer nuclear envelope.
• Two Types:
  • Rough Endoplasmic Reticulum (RER): Characterized by the presence of ribosomes attached to its cytoplasmic surface.
    • Functions:
      • Proteins destined for secretion, insertion into membranes, or delivery to other organelles (like lysosomes) are synthesized on these ribosomes.
      • New proteins are modified (e.g., folded, addition of side chains) within the RER lumen.
      • Synthesizes phospholipids for cellular membranes.
      • Modified proteins and phospholipids not staying in the RER are transported to their destinations via transport vesicles that bud off from the RER membrane.
  • Smooth Endoplasmic Reticulum (SER): Continuous with the RER but lacks ribosomes on its surface.
    • Functions:
      • Synthesis of carbohydrates, lipids, and steroid hormones.
      • Detoxification of medications and poisons (e.g., in liver cells).
      • Storage of calcium ions (Ca^{++}). This is particularly important in muscle cells for muscle contraction.

Golgi Apparatus

• Structure: Consists of a series of flattened membrane sacs called cisternae.
• Function: Sorts, packages, and tags lipids and proteins received from the ER so they can reach their correct destinations.
  • Cis Face: The receiving side of the Golgi apparatus, where transport vesicles from the ER fuse and empty their contents into the Golgi lumen.
  • Trans Face: The opposite, exiting side of the Golgi.
• As proteins and lipids travel through the Golgi cisternae, they undergo further modification and are then sorted into new transport vesicles that bud off from the trans face.

Lysosomes (Revisited)

• Location: Primarily found in animal cells.
• Function: Contain powerful digestive (hydrolytic) enzymes.
  • These enzymes break down various large biomolecules (e.g., proteins, nucleic acids, carbohydrates, lipids).
  • They are also responsible for breaking down worn-out organelles and cellular debris, a process called autophagy.

4.5 The Cytoskeleton

The Cytoskeleton: A Network of Protein Fibers

• Definition: A dynamic network of protein filaments and tubules in the cytoplasm of many living cells, giving them shape and coherence.
• Functions:
  • Maintains the overall shape of the cell.
  • Holds certain organelles in specific positions within the cell.
  • Facilitates the movement of cytoplasm and vesicles (intracellular transport) within the cell.
  • Enables cells within multicellular organisms to move, and unicellular organisms to move independently.
• Three Main Components:
  • Microfilaments
  • Intermediate Filaments
  • Microtubules

Intermediate Filaments

• Function: Primarily structural; they have no direct role in cell movement.
  • Bear tension, thereby maintaining the cell's shape.
  • Anchor the nucleus and other organelles in place, creating a supportive scaffolding within the cell.
• Example: Keratin, a fibrous protein common in hair, nails, and the epidermis of the skin, is an intermediate filament.

Microfilaments (Actin Filaments)

• Structure: The narrowest components of the cytoskeleton, giving rise to the name "microfilaments."
  • Made from actin monomers.
• Functions:
  • Involved in the movement of entire cells (e.g., amoeboid movement) or internal parts of the cell (e.g., cytoplasmic streaming).
  • Determine and stabilize cell shape.
  • Together with myosin, actin microfilaments are crucial for muscle contraction in animal cells.

Microtubules

• Structure: Made of tubulin dimers (alpha-tubulin and beta-tubulin).
• Functions:
  • Form a rigid internal skeleton for certain cells, providing structural support.
  • Serve as a framework along which motor proteins (e.g., kinesin, dynein) can move structures (like vesicles and organelles) within the cell.

Cilia and Flagella

• Combined Structure: Both consist of microtubules arranged in a particular 9+2 array.
• Function: Primarily used for motility of the cell itself or for moving substances across the cell's surface.
• Differences:
  • Cilia: Shorter and typically more numerous on the cell surface.
  • Flagellum (plural: flagella): Longer and usually fewer in number per cell.
  • They exhibit different beating patterns (cilia often beat in a coordinated wave, while flagella typically have a whip-like motion).

4.6 Connections between Cells and Cellular Activities

Extracellular Structures

• Every cell is encompassed by a plasma membrane.
• In addition, cells have structures external to the plasma membrane that provide support, protection, or facilitate communication.

Plant Cell Wall (Revisited)

• Location: A rigid protective structure found external to the plasma membrane of plant cells.
• Composition: Primarily made of cellulose (unlike the peptidoglycan in prokaryotic cell walls).
• Functions:
  • Provides structural support and maintains cell shape.
  • Acts as a barrier against mechanical stress and infection.

Intercellular Junctions: Direct Communication Between Cells

• Intercellular junctions are specialized structures that provide direct channels of communication or adhesion between adjacent cells.
• Plants and animals utilize different types of junctions.

Plasmodesmata (Plant Cells)

• Structure: Channels that pass through the cell walls of adjacent plant cells.
• Function: Connect the cytoplasm of neighboring plant cells, allowing for the movement of water, small solutes, and even some macromolecules (like proteins and RNA) from cell to cell.

Tight Junctions (Animal Cells)

• Structure: Form watertight seals between adjacent animal cells.
• Function: Prevent materials from leaking between cells, effectively blocking the passage of fluids.
• Location: Commonly found in epithelial cells that line internal organs and cavities (e.g., in the bladder, intestines, and kidneys) where fluid leakage must be prevented.

Desmosomes (Animal Cells)

• Structure: Short proteins (called cadherins) in the plasma membrane act as "spot welds" or rivets.
• Function: Join adjacent cells in tissues that experience mechanical stress or stretching.
• Location: Abundant in tissues that need to withstand significant forces, such as the heart, lungs, and muscles.

Gap Junctions (Animal Cells)

• Structure: Resemble plasmodesmata in plants; they form channels that allow direct communication between adjacent animal cells.
  • Develop when six proteins, called connexins, form an elongated, doughnut-like structure called a connexon in the plasma membrane of one cell.
  • When connexons of adjacent cells align, they complete a channel between the two cells.
• Function: Allow the passage of ions, nutrients, and other small signaling molecules to move directly between cells.

Extracellular Matrix (ECM) (Animal Cells Only)

• Location: Found only in animal cells.
• Function: Holds cells together in tissues and facilitates communication between cells.
• Three Primary Components:
  • Collagens & other fibrous proteins: Provide structural support and tensile strength.
  • Glycoproteins called proteoglycans: Consist of a core protein with many carbohydrate chains; provide hydration and resist compression.
  • Linking proteins: Connect the fibrous components and proteoglycans to each other and to cell surface receptors (e.g., integrins), thereby anchoring cells within the matrix.