Unit 3 – Cell Structure and Function

 1. Cell Theory

Key Principles:

• Classical Cell Theory:
1. All living organisms are composed of cells.
2. The cell is the basic unit of life.
3. All cells arise from pre-existing cells (rejects spontaneous generation).
• Modern Additions:

o    Genetic material (DNA/RNA) is passed during cell division.

o    Energy flow (metabolism) occurs within cells.

o    Cells of similar species share fundamental chemical composition.

Contributors:

• Robert Hooke (1665): Coined the term "cell" after observing cork.
• Matthias Schleiden & Theodor Schwann (1839): Proposed unified cell theory for plants and animals.
• Rudolf Virchow (1855): Stated "Omnis cellula e cellula" (all cells come from cells).

Exceptions: Viruses (non-cellular) challenge the theory but are not considered living.

2. Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Organelles	No membrane-bound organelles	Membrane-bound organelles (e.g., mitochondria)
Size	Smaller (1–10 µm)	Larger (10–100 µm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

3. Plant vs. Animal Cells

Feature	Plant Cell	Animal Cell
Cell Wall	Present (cellulose)	Absent
Chloroplasts	Present (for photosynthesis)	Absent
Vacuoles	Large, central vacuole	Small, temporary vacuoles
Centrioles	Absent	Present (aid in cell division)

4. Cell Organelles & Structures

• Cell Membrane:
• Structure: Phospholipid bilayer with embedded proteins (fluid mosaic model).
• Function: Selectively permeable barrier; regulates transport.
• Cell Wall (Plants):
• Composition: Cellulose, hemicellulose, pectin.
• Function: Provides rigidity and protection.
• Endomembrane System:
• Endoplasmic Reticulum (ER):
• Rough ER: Protein synthesis (ribosomes attached).
• Smooth ER: Lipid synthesis, detoxification.
• Golgi Apparatus: Modifies, sorts, and packages proteins.
• Lysosomes: Contain digestive enzymes (animal cells).
• Vacuoles: Storage (water, ions, waste).
• Mitochondria: Site of cellular respiration (ATP production).
• Ribosomes: Protein synthesis (free or ER-bound).
• Plastids (Plants):
• Chloroplasts (photosynthesis), Chromoplasts (pigments), Leucoplasts (storage).
• Cytoskeleton:
• Microfilaments: Actin; cell movement.
• Microtubules: Tubulin; shape and transport.
• Intermediate Filaments: Structural support.
• Cilia/Flagella: Motor proteins for movement (e.g., cilia in trachea).
• Centrioles: Organize spindle fibers during cell division (animal cells).
• Nucleus:
• Nuclear Membrane: Double membrane with pores.
• Chromatin: DNA-protein complex (condenses into chromosomes).
• Nucleolus: Ribosome synthesis.

5. Biomolecules

1. Proteins:
• Structure: Chains of amino acids (primary, secondary, tertiary, quaternary).
• Function: Enzymes, structural support, transport.
2. Carbohydrates:
• Types: Monosaccharides (glucose), disaccharides (sucrose), polysaccharides (starch).
• Function: Energy storage (glycogen), structural support (cellulose).
3. Lipids:
• Types: Fats (energy storage), phospholipids (membranes), steroids (hormones).
• Function: Insulation, cell signaling.
4. Nucleic Acids:
• DNA: Double helix; genetic blueprint.
• RNA: Single-stranded; protein synthesis.

6. Enzymes

• Properties:
• Biological catalysts; lower activation energy.
• Specific to substrates (lock-and-key or induced-fit model).
• Classification:
• Oxidoreductases: Redox reactions.
• Hydrolases: Hydrolysis (e.g., digestive enzymes).
• Nomenclature: Named after substrate + "-ase" (e.g., lipase).

7. Cell Division

Cell Cycle:

1. Interphase (G1, S, DNA replication; G2).
2. Mitosis:
• Prophase: Chromosomes condense; nuclear envelope breaks.
• Metaphase: Chromosomes align at equator.
• Anaphase: Sister chromatids separate.
• Telophase: Nuclear envelopes reform.
3. Cytokinesis: Cytoplasm divides.

Meiosis:

• Significance: Produces gametes; genetic diversity via crossing over.
• Stages: Meiosis I (homologous chromosomes separate), Meiosis II (sister chromatids separate).

Mitosis vs. Meiosis:

Feature	Mitosis	Meiosis
Daughter Cells	2 diploid cells	4 haploid cells
Genetic Diversity	None (clones)	High (crossing over)

Key Diagrams to Study:

1. Fluid mosaic model of the cell membrane.
2. Structure of mitochondria and chloroplast.
3. Stages of mitosis and meiosis.