2.1 Cell Structure, Organelles & Membrane Transport

2.1 Cell Structure, Organelles & Membrane Transport

Cell biology forms the foundational core of biological science and is a heavily weighted subject in the Pakistan Army Medical Cadet (AMC) Initial Test. A thorough understanding of cellular organization, organelle function, membrane dynamics, and transport mechanisms is essential for pre-medical candidates.

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1. Prokaryotic vs. Eukaryotic Cell Architecture

All living organisms are composed of cells, categorized into two structural types: prokaryotes (e.g., bacteria, cyanobacteria) and eukaryotes (e.g., protozoans, fungi, plants, animals). The key distinguishing characteristics are summarized below:

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2. Plasma Membrane & The Fluid Mosaic Model

Proposed by S.J. Singer and G.L. Nicolson (1972), the Fluid Mosaic Model explains the structure and behavior of the cell membrane:

1. Phospholipid Bilayer: Amphipathic molecules with hydrophilic (polar) phosphate heads facing aqueous environments and hydrophobic (non-polar) fatty acid tails facing inward, forming a semi-permeable barrier.
2. Membrane Proteins:
   • Integral (Intrinsic) Proteins: Span the lipid bilayer (transmembrane proteins) functioning as ion channels, carriers, receptors, and enzymes.
   • Peripheral (Extrinsic) Proteins: Loosely bound to the inner or outer surface, contributing to cell signaling and cytoskeletal anchorage.
3. Cholesterol: Interspersed among fatty acid tails in animal membranes to regulate fluidity—preventing excessive fluidity at high temperatures and preventing crystallization at low temperatures.
4. Carbohydrates: Glycoproteins and glycolipids on the extracellular matrix form the glycocalyx, providing cell recognition, adhesion, and immune identity (e.g., blood group antigens).

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3. The Endomembrane System & Organelles

Eukaryotic cytoplasm is compartmentalized by an interconnected system of membranes that coordinate synthesis, processing, and transport of biochemical compounds.

A. Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with 80S ribosomes; primary site of protein synthesis, folding, and initial glycosylation. Abundant in secretory cells (e.g., pancreatic acinar cells).
• Smooth ER (SER): Lacks ribosomes; functions in lipid and steroid hormone synthesis (e.g., testosterone, estrogen), carbohydrate metabolism, detoxification of drugs/toxins in liver hepatocytes, and calcium ion ($Ca^{2+}$) storage in muscle cells (sarcoplasmic reticulum).

B. Golgi Apparatus (Dictyosomes)

• Stack of flattened, membrane-bound sacs called cisternae.
• Exhibits structural polarity: the convex cis-face (forming face) receives transport vesicles from the RER; the concave trans-face (maturing face) dispatches secretory vesicles.
• Key functions: Modification (glycation, phosphorylation), sorting, packaging of secretory products, formation of primary lysosomes, and synthesis of cell wall polysaccharides (pectin, hemicellulose) in plants.

C. Lysosomes

• Single membrane-bound vesicles formed by the Golgi apparatus containing acid hydrolases (e.g., phosphatases, lipases, proteases, nucleases) operating at optimal pH ~4.5–5.0.
• Primary Lysosomes: Inactive enzyme storage vesicles.
• Secondary Lysosomes (Phagolysosomes): Formed by fusion of primary lysosomes with endosomes/phagosomes for intracellular digestion.
• Autophagy: Degradation of worn-out cellular organelles.
• Autolysis: Programmed cell self-destruction ("suicide bags") via mass release of hydrolytic enzymes during metamorphosis or necrosis.

D. Peroxisomes & Glyoxysomes (Microbodies)

• Peroxisomes: Contain oxidases that degrade fatty acids and amino acids, generating toxic hydrogen peroxide ($H_2O_2$), which is immediately broken down into $H_2O$ and $O_2$ by catalase.
• Glyoxysomes: Specialized plant microbodies found in germinating fatty seeds (e.g., castor bean); contain enzymes of the glyoxylate cycle to convert stored lipids into carbohydrates (gluconeogenesis).

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4. Semiautonomous Energy Transducers

Mitochondria and chloroplasts are double-membrane semiautonomous organelles containing their own circular DNA, 70S ribosomes, and self-replicating capacity (Endosymbiotic Theory).

• Mitochondria: Site of aerobic respiration. The smooth outer membrane encloses an inner membrane folded into cristae to maximize surface area for electron transport chain (ETC) complexes and $F_0F_1$-ATP synthase. The internal matrix contains Krebs cycle enzymes.
• Chloroplasts: Site of photosynthesis in plant cells. Bounded by a double membrane, containing internal stackable disk-like membranes called thylakoids organized into grana (site of light-dependent reactions) suspended in a fluid stroma (site of Calvin cycle carbon fixation).

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5. Cytoskeleton & Centrioles

• Microtubules: Hollow cylinders composed of $\alpha$- and $\beta$-tubulin dimers ($25\text{ nm}$ diameter). Form spindle fibers during mitosis, cilia, flagella ($9+2$ arrangement), and centrioles ($9+0$ triplet arrangement).
• Microfilaments: Solid rods of actin protein ($7\text{ nm}$ diameter). Responsible for amoeboid movement, cytoplasmic streaming (cyclosis), muscle contraction, and cleavage furrow formation during cytokinesis.
• Intermediate Filaments: Fibrous proteins such as keratin ($8\text{--}12\text{ nm}$ diameter) providing mechanical strength and structural anchorage for the nucleus.
• Centrosome: Contains a pair of perpendicular centrioles in animal cells, acting as the main microtubule organizing center (MTOC) for spindle apparatus assembly.

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6. Plasma Membrane Transport Mechanisms

Transport across cell membranes is classified by energy expenditure and directional concentration gradients:

                    ┌── Passive Transport (No ATP, down gradient)
                    │    ├── Simple Diffusion (small nonpolar: O2, CO2)
                    │    ├── Osmosis (water movement via aquaporins)
                    │    └── Facilitated Diffusion (channels & carriers)
Membrane Transport ─┤
                    └── Active Transport (Requires ATP, against gradient)
                         ├── Primary Active Transport (Na+/K+ Pump)
                         ├── Secondary Active Transport (Symport/Antiport)
                         └── Bulk Transport (Vesicular: Endocytosis/Exocytosis)

A. Osmosis & Plant Cell Turgor Mechanics

• Osmosis: Net movement of water across a selectively permeable membrane from a region of higher water potential ($\Psi_w$) / lower solute concentration to lower water potential / higher solute concentration.
• Hypertonic Solution: Higher solute concentration relative to cytosol. Causes animal cells to crenate and plant cells to undergo plasmolysis (shrinkage of protoplast away from cell wall).
• Hypotonic Solution: Lower solute concentration relative to cytosol. Causes animal cells to lyse; causes plant cells to absorb water until turgor pressure ($P$) balances osmotic pressure ($\pi$), creating a turgid state essential for mechanical support.
• Isotonic Solution: Equal solute concentration; no net movement of water.