Chapter 4: The Structure and Function of the Cell

Chapter 4: The Structure and Function of the Cell

Introduction to Cell Biology

Cells are the fundamental units of life, and all living organisms are composed of one or more cells. Understanding cell structure and function is essential for studying biology, as it provides insight into how life operates at the microscopic level.

• Cell Theory: All living things are made of cells; cells are the basic units of structure and function in living organisms; all cells arise from pre-existing cells.

• Types of Cells: Cells are classified as prokaryotic (bacteria and archaea) or eukaryotic (protists, fungi, plants, animals).

Microscopy and the Discovery of Cells

Microscopes are essential tools for studying cells, as most cells are too small to be seen with the unaided eye. The development of microscopy revolutionized biology by allowing scientists to observe cell structure and function.

• Robert Hooke: First recorded person to observe cell walls in cork using a microscope.

• Anton van Leeuwenhoek: Built his own microscopes and observed 'animalcules' (microorganisms).

Types of Microscopes

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses to magnify images up to ~1,000x life-size.

• Electron Microscope (EM): Uses beams of electrons for much higher resolution, allowing visualization of subcellular structures.

Microscopy Techniques

• Brightfield (stained specimen): Enhances contrast by staining cells.

• Phase-contrast: Amplifies differences in refractive index.

• Differential-interference contrast (Nomarski): Uses optical modifications to exaggerate differences in density.

Cell Size and Surface Area-to-Volume Ratio

Cell size is limited by the surface area-to-volume ratio, which affects the ability of the cell to exchange materials with its environment.

• Surface Area: Determines the rate at which materials can enter or leave the cell.

• Volume: Determines the amount of metabolic activity the cell can support.

• Formula:

$\text{Surface Area of a Cube} = 6s^2$ $\text{Volume of a Cube} = s^3$ $\text{Surface Area-to-Volume Ratio} = \frac{6s^2}{s^3} = \frac{6}{s}$

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells

Prokaryotic cells are simpler and smaller than eukaryotic cells. They lack membrane-bound organelles and a nucleus.

• Domains: Bacteria and Archaea

• Key Features:

  • Nucleoid: Region where DNA is located (not membrane-bound)

  • Ribosomes: Sites of protein synthesis

  • Plasma Membrane: Encloses the cell

  • Cell Wall: Provides structural support

  • Flagella: Used for movement

  • Fimbriae: Attachment structures

Eukaryotic Cells

Eukaryotic cells are larger and more complex, containing membrane-bound organelles and a nucleus.

• Domains: Protists, Fungi, Plants, Animals

• Key Features:

  • Nucleus: Contains DNA, surrounded by a double membrane

  • Membrane-bound Organelles: Specialized structures (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus)

  • Cytoskeleton: Network of fibers for support and movement

  • Plasma Membrane: Controls entry and exit of substances

Cell Organelles and Their Functions

Nucleus

The nucleus is the control center of the cell, containing most of the cell's genetic material.

• Chromosomes: DNA molecules associated with proteins (histones)

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis

• Nuclear Envelope: Double membrane with pores for transport

Ribosomes

Ribosomes are the sites of protein synthesis, translating genetic information from mRNA into polypeptides.

• Free Ribosomes: Suspended in cytosol, produce proteins for use within the cell

• Bound Ribosomes: Attached to the endoplasmic reticulum or nuclear envelope, produce proteins for export or membranes

Endomembrane System

The endomembrane system is a group of organelles involved in synthesis, modification, and transport of cellular materials.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, plasma membrane

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins and membranes

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs, stores calcium ions

Golgi Apparatus

• Function: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell

• Structure: Flattened membranous sacs (cisternae)

Lysosomes

• Function: Digestive compartments containing hydrolytic enzymes to break down macromolecules

• Phagocytosis: Engulfing food particles and breaking them down

• Autophagy: Recycling the cell's own organelles and macromolecules

Vacuoles

• Function: Storage, waste disposal, protection, and hydrolysis

• Central Vacuole (plants): Maintains cell shape, stores nutrients, and degrades waste products

Energy Conversion Organelles

Mitochondria

Mitochondria are the sites of cellular respiration, generating ATP from organic molecules using oxygen.

• Structure: Double membrane, inner membrane folded into cristae, contains its own DNA and ribosomes

• Function: ATP production via oxidative phosphorylation

Chloroplasts

Chloroplasts are the sites of photosynthesis in plant cells, converting light energy into chemical energy.

• Structure: Double membrane, stacks of thylakoids (grana), stroma containing DNA and ribosomes

• Function: Production of sugars from carbon dioxide and water using light energy

Endosymbiotic Theory

This theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, ribosomes, reproduce independently within cells

Peroxisomes

Peroxisomes are oxidative organelles that break down fatty acids and detoxify harmful substances.

• Function: Produce hydrogen peroxide and convert it to water

Cytoskeleton and Cell Structure

Cytoskeleton

The cytoskeleton is a network of fibers that provides structural support, motility, and organization within the cell.

• Microtubules: Largest diameter; made of tubulin; involved in cell shape, organelle movement, chromosome separation

• Microfilaments (Actin Filaments): Smallest diameter; made of actin; involved in cell shape, muscle contraction, cytoplasmic streaming

• Intermediate Filaments: Medium diameter; provide mechanical strength and maintain cell shape

Cell Walls and Extracellular Matrix

Cell walls provide structural support in plants, fungi, and some protists, while the extracellular matrix (ECM) supports animal cells.

• Plant Cell Wall: Composed of cellulose microfibrils, other polysaccharides, and proteins

• ECM (Animal Cells): Composed of glycoproteins (e.g., collagen), proteoglycans, and fibronectin; provides support and regulates cell behavior

Cell Junctions

Cell junctions connect cells and facilitate communication and adhesion.

• Plasmodesmata (plants): Channels that allow transport of materials between plant cells

• Tight Junctions (animals): Prevent leakage of extracellular fluid

• Desmosomes (animals): Anchor cells together for mechanical stability

• Gap Junctions (animals): Allow passage of ions and small molecules between cells

Comparing Cell Organelles

Different cell types contain unique combinations of organelles, reflecting their specialized functions.

Organelle	Plant Cell	Animal Cell	Bacterial Cell
Nucleus	Yes	Yes	No
Cell Wall	Yes (cellulose)	No	Yes (peptidoglycan)
Chloroplasts	Yes	No	No
Mitochondria	Yes	Yes	No
Ribosomes	Yes	Yes	Yes
Plasma Membrane	Yes	Yes	Yes
Vacuole	Large central	Small or absent	No

Summary

• Cells are the basic units of life, classified as prokaryotic or eukaryotic.

• Microscopy is essential for studying cell structure and function.

• Cell organelles perform specialized functions necessary for life.

• Cell structure reflects function, and differences exist between plant, animal, and bacterial cells.

Additional info: Some context and definitions were expanded for clarity and completeness.