Chapter 4: Structure and Function of the Cell

Chapter 4: Structure and Function of the Cell

Introduction to Cell Biology

Cells are the fundamental units of life, and understanding their structure and function is essential in biology. This chapter explores the diversity of cell types, their organelles, and the tools used to study them.

Microscopy and the Study of Cells

Historical Use of Microscopes

• Robert Hooke: First recorded person to observe cells in cork and coined the term "cell" after seeing cell walls.

• Antonie van Leeuwenhoek: Improved microscope design and observed living cells, which he called "animalcules." He made his own microscopes and was the first to see bacteria and protozoa.

Microscopy revolutionized cell biology by allowing scientists to visualize structures too small for the unaided eye.

Types of Microscopes

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses. Lenses refract (bend) the light, magnifying the image up to ~1,000x life-size.

• Electron Microscopes (EM): Use beams of electrons for much higher resolution. Includes Transmission Electron Microscopes (TEM) for internal structures and Scanning Electron Microscopes (SEM) for surface details.

Parameters and Limitations of Microscopy

• Magnification: Ratio of an object's image size to its real size.

• Resolution: Minimum distance two points can be separated and still be distinguished as separate points.

• Contrast: Difference in brightness between parts of the image.

Microscopy Techniques

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

• Phase-contrast: Amplifies differences in density.

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

• Fluorescence microscopy: Uses fluorescent markers to label molecules or structures.

• Confocal microscopy: Uses lasers and optical sectioning for sharper images.

Cell Fractionation

Cell fractionation is a technique that separates cell components based on size and density using centrifugation. This allows scientists to study the functions of organelles in isolation.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells

• Domains: Bacteria and Archaea

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

• No membrane-bound organelles

• Smaller size: 1-5 μm

• Cell wall: Provides structural support

• Other structures: Fimbriae, flagella, ribosomes

Eukaryotic Cells

• Domains: Protists, fungi, animals, plants

• Nucleus: Contains DNA, surrounded by a double membrane

• Membrane-bound organelles: Includes mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.

• Larger size: 10-100 μm

• Cytoskeleton: Provides structural support and motility

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
DNA Location	Nucleoid (not membrane-bound)	Nucleus (membrane-bound)
Organelles	None	Membrane-bound organelles
Size	1-5 μm	10-100 μm
Cell Wall	Present (peptidoglycan)	Present in plants/fungi (cellulose/chitin)

Cell Membranes and Surface Area

The plasma membrane surrounds the cell, controlling the movement of substances in and out. It consists of a phospholipid bilayer with hydrophilic and hydrophobic regions.

• Surface area-to-volume ratio: As a cell increases in size, its volume grows faster than its surface area, limiting the efficiency of diffusion and transport.

Formula:

• Total surface area: $(height \times width \text{ of 1 side}) \times 6 \times \text{number of cells}$

• Total volume: $(height \times width \times length \text{ of cell}) \times \text{number of cells}$

• Surface area-to-volume ratio: $\frac{\text{surface area}}{\text{volume}}$

Internal Membranes and Compartmentalization

Eukaryotic cells have internal membranes that compartmentalize functions, allowing specialized organelles to carry out distinct processes.

Nucleus

• Contains most of the cell's DNA

• Surrounded by nuclear envelope (double membrane with pores)

• Chromosomes: DNA organized with proteins (histones)

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis

Ribosomes

• Protein factories composed of rRNA and proteins

• Free ribosomes: Suspended in cytosol

• Bound ribosomes: Attached to endoplasmic reticulum or nuclear envelope

Endomembrane System

The endomembrane system is involved in protein and lipid synthesis, trafficking, and metabolic functions.

• Nuclear envelope

• Endoplasmic reticulum (ER): Rough ER (with ribosomes) and Smooth ER (no ribosomes)

• Golgi apparatus: Shipping and receiving center

• Lysosomes: Digestive compartments

• Vacuoles: Diverse functions in storage and hydrolysis

Endoplasmic Reticulum (ER)

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

• Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions

Golgi Apparatus

• Modifies, sorts, and packages products from ER

• Produces glycoproteins and glycolipids

• Ships materials via transport vesicles

Lysosomes

• Membranous sacs containing hydrolytic enzymes

• Digest macromolecules via phagocytosis and autophagy

Vacuoles

• Plant cells: Central vacuole stores water, ions, and organic molecules

• Protists: Contractile vacuoles pump excess water

• Fungi: Vacuoles for hydrolysis

Energy Conversion Organelles

Mitochondria

• Site of cellular respiration (uses oxygen to generate ATP)

• Double membrane: Outer and inner membranes; inner membrane folded into cristae

• Matrix: Contains mitochondrial DNA and ribosomes

Chloroplasts

• Site of photosynthesis (converts light energy to chemical energy)

• Found in plants and algae

• Structure: Double membrane, thylakoid stacks (grana), stroma (fluid with DNA and ribosomes)

Endosymbiotic Theory

• Mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells

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

Peroxisomes

• Oxidative organelles that break down fatty acids and detoxify harmful substances

• Produce hydrogen peroxide ($H_2O_2$), which is converted to water

Cytoskeleton

The cytoskeleton is a network of fibers that organizes structures and activities in the cell, providing mechanical support, maintaining cell shape, and enabling motility.

Components of the Cytoskeleton

Component	Structure	Function
Microtubules	Hollow tubes (tubulin)	Cell shape, organelle movement, chromosome separation
Microfilaments	Actin filaments	Cell shape, muscle contraction, cytoplasmic streaming
Intermediate Filaments	Fibrous proteins	Cell shape, anchorage of organelles

Motor Proteins

• Interact with cytoskeleton to produce motility

• Use ATP to "walk" vesicles and organelles along fibers

Cell Walls and Extracellular Structures

Plant Cell Walls

• Composed of cellulose microfibrils embedded in other polysaccharides and proteins

• Primary cell wall: Thin and flexible

• Middle lamella: Sticky layer rich in pectins between cells

• Secondary cell wall: Strong and durable, deposited in some cells

Extracellular Matrix (ECM) of Animal Cells

• Composed of glycoproteins (e.g., collagen, fibronectin)

• Functions: Support, adhesion, movement, regulation

• Integrins connect ECM to cell's cytoskeleton

Cell Junctions

• Plasmodesmata (plants): Channels that perforate cell walls, allowing transport of solutes between cells

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

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

• Gap junctions (animals): Allow passage of materials between cells for communication

Integration of Cell Components

The cell's properties and functions arise from the integration of its components, including the cytoskeleton, organelles, and membranes. This coordination enables complex activities such as metabolism, signaling, and movement.

Additional info: Some explanations and definitions have been expanded for clarity and completeness.