Cell Structure and Function

Nucleus

• Contains DNA (genetic information) on linear chromosomes
• Nuclear envelope: double membrane with nuclear pores (regulate transport)
• Nucleolus: site of ribosomal RNA (rRNA) synthesis

Ribosomes

• Made of rRNA and protein; site of translation (protein synthesis)
• Free ribosomes: synthesise proteins for the cytoplasm
• Bound ribosomes (on rough ER): synthesise proteins for membranes, export, or lysosomes

Endomembrane System

A network of interconnected membranes that work together:

Rough Endoplasmic Reticulum (RER):

• Studded with ribosomes
• Synthesises and processes proteins (folding, glycosylation)
• Transports proteins in vesicles to the Golgi

Smooth Endoplasmic Reticulum (SER):

• No ribosomes
• Lipid synthesis, steroid hormone production, detoxification (liver), calcium storage (muscle)

Golgi Apparatus:

• Stack of flattened membrane sacs (cisternae)
• Modifies, sorts, and packages proteins and lipids
• Produces vesicles for secretion (exocytosis), lysosomes, or membrane components
• Has a cis face (receiving) and trans face (shipping)

Lysosomes:

• Contain hydrolytic enzymes (acid hydrolases, pH ~5)
• Intracellular digestion: break down macromolecules, worn-out organelles (autophagy), foreign material (phagocytosis)
• Apoptosis: programmed cell death

Vacuoles:

• Central vacuole (plants): stores water, ions, pigments; maintains turgor pressure
• Contractile vacuole (protists): expels excess water
• Food vacuoles: digest ingested material

Mitochondria

• Double membrane: outer membrane + highly folded inner membrane (cristae)
• Matrix: contains enzymes for the Krebs cycle, circular DNA, 70S ribosomes
• Site of cellular respiration (ATP production via oxidative phosphorylation)
• Endosymbiotic theory: mitochondria evolved from engulfed aerobic bacteria

Chloroplasts (Plants and Algae)

• Double membrane + internal thylakoid membranes (stacked into grana)
• Stroma: fluid surrounding thylakoids; site of Calvin cycle
• Thylakoid membranes: site of light reactions (photosystems, ETC, ATP synthase)
• Contain own circular DNA and 70S ribosomes → endosymbiotic origin

Cytoskeleton

• Network of protein fibres providing structure, support, and movement:
  • Microfilaments (actin): cell shape, muscle contraction, cytokinesis, amoeboid movement
  • Intermediate filaments: mechanical strength (keratin)
  • Microtubules (tubulin): chromosome movement (spindle), organelle transport, cilia/flagella
• Centrosome with centrioles: organise microtubules (animal cells)

Cell Wall (Plants)

• Made of cellulose microfibrils
• Provides rigid structural support
• Freely permeable (unlike cell membrane)

As cells grow larger, their volume increases faster than their surface area.

$\text{SA:V ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

For a cube with side length $s$: $\text{SA} = 6s^2, \quad V = s^3, \quad \text{SA:V} = \frac{6}{s}$

Why this matters:

• Cells need sufficient surface area for exchange of nutrients, gases, and wastes
• As cells get larger, SA:V ratio decreases → exchange becomes insufficient
• This limits cell size and explains why cells divide rather than growing indefinitely
• Adaptations: microvilli, folding (cristae), compartmentalisation

Mitochondria and chloroplasts were once free-living prokaryotes engulfed by ancestral eukaryotes:

Evidence:

• Own circular DNA (similar to bacteria)
• Own 70S ribosomes (bacterial-sized)
• Double membrane (inner = original bacterial membrane; outer = host's vesicle membrane)
• Binary fission — replicate independently within the cell
• DNA sequences are similar to α-proteobacteria (mitochondria) and cyanobacteria (chloroplasts)

Question: Explain how the structure of the mitochondrion is related to its function. (4 points)

Solution:

The mitochondrion has a double membrane. The outer membrane is smooth and allows passage of small molecules. The inner membrane is highly folded into cristae, which vastly increases the surface area available for the electron transport chain and ATP synthase — the enzymes of oxidative phosphorylation. The matrix (enclosed by the inner membrane) contains enzymes for the Krebs cycle and the mitochondrion's own circular DNA and ribosomes (70S), allowing it to produce some of its own proteins. The intermembrane space between the two membranes accumulates $H^+$ ions, creating the proton gradient (chemiosmosis) that drives ATP synthesis.

• Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both.
• The endomembrane system (ER → Golgi → vesicles → membrane) processes and transports proteins and lipids.
• Mitochondria (respiration) and chloroplasts (photosynthesis) have double membranes and own DNA — evidence of endosymbiosis.
• SA:V ratio limits cell size; smaller cells exchange materials more efficiently.
• The cytoskeleton provides structure, supports transport, and enables movement.