Essential Cell Biology Exam Notes

Animal Cell: Key Structures

  • Size approx. \times10 larger than prokaryotes; no cell wall

  • Main components: cell membrane, cytoplasm, organelles, cytoskeleton

  • Cell membrane: phospholipid bilayer + sterols (cholesterol); selective permeability

  • Cytoplasm vs cytosol vs protoplasm:

    • Cytoplasm = contents within membrane excluding nucleus

    • Cytosol = fluid portion only

    • Protoplasm = cytoplasm + nucleus

  • Organelles (membrane-bound unless noted):

    • Nucleus: double membrane, nuclear pores, nucleolus (rRNA), chromatin

    • Ribosomes (80S; 40S + 60S) – free (cell use) or bound to RER (export)

    • Endoplasmic Reticulum:
      • RER – protein synthesis, glycoprotein formation
      • SER – lipid synthesis, detoxification

    • Golgi apparatus – modifies (adds S or P), sorts, packages proteins; cis (receiving) & trans (shipping)

    • Lysosomes – digestive enzymes, recycling

    • Peroxisomes – oxidative enzymes (e.g., catalase), H2O2 breakdown

    • Mitochondria – double membrane, cristae (ETC), own DNA & 70S ribosomes; site of aerobic respiration

  • Cytoskeleton: microtubules/filaments; shape, transport, division

Plant Cell Extras

  • Rigid cellulose cell wall; plasmodesmata; synthesized at plasma membrane

  • Layers: middle lamella → primary wall (flexible) → secondary wall (rigid)

  • Central vacuole (tonoplast): storage, turgor, waste, enzymes

  • Plastids: chloroplasts (photosynthesis; stroma, thylakoid→granum), chromoplasts (pigments), leucoplasts (storage)

Fungal Cell Extras

  • Chitin & glucan cell wall; ergosterol in membranes

  • Vacuole functions like plant vacuole & animal lysosome

  • Storage: glycogen; structure: hyphae → mycelia

Cell Membrane Fundamentals

  • Components: phospholipids (amphipathic bilayer), cholesterol (fluidity buffer), proteins (integral / peripheral), carbohydrates (glyco-)

  • Fluid–mosaic model: lateral flex/rotation; selective permeability

  • Protein functions: transport, signal transduction, cell-cell recognition, anchorage

Transport Across Membrane

Passive (no ATP):
• Simple diffusion – small non-polar; rate ↑ with gradient, temp
• Osmosis – water; isotonic, hypertonic (crenation/plasmolysis), hypotonic (lysis/turgid)
• Facilitated diffusion – channel/carrier proteins
Active (ATP):
• Pumps (e.g., \text{Na}^+/\text{K}^+ ATPase)
• Endocytosis / Exocytosis (bulk transport)

Enzymes Overview

  • Biological catalysts (mostly proteins; exceptions: ribozymes)

  • Properties: specific, reused, small amounts, high speed, denatured by \text{pH} & temperature

  • Types (IUB 6): oxidoreductase, transferase, hydrolase, lyase, isomerase, ligase

  • Mechanism: lock-and-key & induced-fit; lower E_a by orientation, micro-environment, strain, temporary covalent bonds

  • Factors: temperature (optimum \approx37^\circC), \text{pH} (most 5-9, pepsin \approx2), substrate/enzyme concentration, inhibitors (competitive, non-competitive, irreversible)

Aerobic Respiration Summary

  • Glycolysis (cytoplasm) → Acetyl-CoA (mitochondrial matrix) → Krebs cycle (matrix) → ETC & chemiosmosis (inner membrane cristae)

  • Yield: \text{NADH} \rightarrow 3\,\text{ATP},\; \text{FADH}_2 \rightarrow 2\,\text{ATP} (theoretical)

Central Dogma & Genetic Code

  • DNA (A,T,G,C) → transcription → mRNA (A,U,G,C) → translation → polypeptide

  • Transcription (RNA polymerase): initiation (promoter), elongation (5'→3'), termination (terminator); template = antisense strand

  • Translation stages: initiation (AUG, Met), elongation (A-P-E sites, peptide bond), termination (UAA/UAG/UGA + release factor); ribosome large+small subunits

  • Genetic code: 64 codons (61 amino acids, 3 stop); degenerate but unambiguous

  • Point mutation example: β-globin \text{GAG} \rightarrow \text{GTG} (Glu→Val) → sickle-cell anemia

Cell Cycle & Mitosis

Interphase (~90%):
• G1: growth, organelles, decision (enter G0 or proceed)
• S: DNA replication (semi-conservative) → sister chromatids
• G2: synthesis of division proteins M phase: 1 Prophase – chromatin condenses, nuclear envelope disassembles, spindle forms 2 Metaphase – chromosomes align at metaphase plate 3 Anaphase – sister chromatids split at centromere, move to poles 4 Telophase – chromosomes decondense, nuclei reform Cytokinesis: • Animals – actin contractile ring → cleavage furrow • Plants – cell plate → new wall Checkpoints: G1 (DNA damage), G_2 (DNA replicated), M (spindle); failure can lead to cancer