Cell Structure and Function: Comprehensive Study Notes

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The discovery of cells marked a foundational moment in biology, leading to the development of cell theory and modern cell biology.

Robert Hooke (1665): Observed cork under a microscope and coined the term cellulae (cells).
Anton van Leeuwenhoek: Improved microscope technology (270x magnification) and observed protozoa and bacteria, calling them animalcules.
Mathias Schleiden & Theodor Schwann (1838–1839): Concluded that all plants and animals are made of cells.

Cell theory is a central concept in biology, describing the properties and significance of cells.

Three Tenets of Cell Theory:
1. All organisms are composed of one or more cells.
2. The cell is the unit of life.
3. Cells arise only by division from preexisting cells.
Key Contributors: Matthias Schleiden, Theodor Schwann, Rudolf Virchow.

Cells are the smallest units of life, exhibiting diverse forms and functions.

Size: Typically measured in micrometers (μm); 1 μm = 10-6 m.
Number: Organisms may consist of one to billions of cells (e.g., human body).
Chemical Composition: DNA, RNA, proteins, H2O, C6H12O6, Na+, Mg2+, Ca2+, etc.
Genetic Information: Stored in nucleic acids.
Structure-Function Relationship: Cell structure determines organ function.

Cells are classified based on their structural features into two major types.

Prokaryotic Cells: Bacteria and Archaea; lack a nucleus.
Eukaryotic Cells: Yeasts, plants, animals, fungi; possess a nucleus and organelles.

Prokaryotic cells are simpler and lack membrane-bound organelles.

Common Morphologies:
- Rods (bacilli): e.g., Mycobacterium tuberculosis
- Spheres (cocci): e.g., Staphylococcus aureus
- Spirals (vibrios, spirilla, spirochaetes): e.g., Vibrio cholerae, Helicobacter pylori, Treponema pallidum
Cell Structures:
- Capsule: Polysaccharide layer for protection and attachment.
- Cell Wall: Peptidoglycan structure; Gram-positive (thick, purple stain), Gram-negative (thin, pink stain).
- Plasma Membrane: Phospholipid, protein, carbohydrate; separates cell from environment.
- Cytoplasm: Gel-like matrix with water, proteins, carbohydrates, lipids, ions.
- Nucleoid: Region containing circular, naked DNA (bacterial chromosome).
- Plasmid: Small, circular DNA; transferable via conjugation or transformation.
- Pili (Fimbriae): Surface structures for attachment and DNA transfer.
- Flagellum: Protein structure for motility.

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

Organisms: Protists, algae, yeasts, fungi, plants, animals.
Size: 5–100 μm.
Nucleus: Enclosed by a nuclear envelope; contains DNA.
Organelles:
- Ribosomes: Free (soluble proteins) or attached to ER (secretory/membrane proteins).
- Endoplasmic Reticulum (ER): Rough ER (ribosomes, protein synthesis), Smooth ER (lipid, cholesterol, steroid synthesis).
- Golgi Apparatus: Modifies, manufactures, and transports macromolecules.
- Lysosome: Hydrolytic enzymes for macromolecule degradation and recycling.
- Mitochondria: ATP generation via cellular respiration; contains circular mtDNA; origin via endosymbiosis.
- Vacuole: Storage (large in plants, small/few in animals).
- Peroxisome: Catalase enzyme converts H2O2 to H2O and O2;
- Centrosome: Mitotic spindle formation in cell division.
- Microvilli: Increase surface area for absorption.
- Flagella: Motility.
- Cytoskeleton: Microtubules, microfilaments, intermediate filaments; cell shape and movement.
- Nucleus: Contains DNA, nucleolus (ribosome synthesis), nuclear pores (molecule exchange), chromatin (DNA + histone proteins).

The plasma membrane is a dynamic barrier regulating cell-environment interactions.

Composition: Phospholipid bilayer, proteins, carbohydrates.
Properties:
- Self-closure
- Flexibility, fluidity
- Asymmetry
- Semi-permeability
Lipid Bilayer:
- Phospholipids: ~55%; four types, phosphatidylcholine most common.
- Cholesterol: 25–35%; maintains fluidity, limits permeability.
- Glycolipids: 10–15%; contain oligosaccharides, regulate membrane activities.
Membrane Proteins:
- Integral (Transmembrane): ~70%; amphipathic, span membrane.
- Peripheral: ~30%; attached to integral proteins.
- Functions: Channels, transporters, receptors, cell recognition, joining, enzymes, cytoskeleton/ECM attachment.
Glycocalyx (Carbohydrates):
- Oligosaccharides on cell surface; glycosylation forms glycoproteins/glycolipids.
- Functions: negative charge, cell recognition/interaction, antigenic properties.
Functions of Plasma Membrane:
- Envelop cell
- Semi-permeable barrier
- Recognition and interaction
- Signal transduction
- Adherence

Transport across the plasma membrane is essential for cellular homeostasis and function.

Diffusion: Movement from high to low concentration; no energy required.
Facilitated Diffusion: Passive transport aided by membrane proteins (channels/carriers).
Affecting Factors:
- Solute size
- Lipid solubility
- Concentration gradient
- Ionization
- Temperature
- Cellular activity

Requires energy (ATP)
Moves substances against concentration gradient
Types:
- ATP-driven (e.g., Na+/K+ pump)
- Ion gradient-driven (coupled transport)
Example Equation:

Endocytosis: Uptake of substances via vesicles (pinocytosis, receptor-mediated, phagocytosis).
Exocytosis: Export of bulky molecules via secretory vesicles.

These notes cover foundational topics in cell biology, including cell structure, function, and membrane transport, which are essential for understanding more advanced topics in genetics and molecular biology.