MCAT Biology Cell Biology Review

Success on the MCAT Biology and Biochemical Foundations section demands a robust and integrated understanding of cell biology. This knowledge forms the bedrock for comprehending physiology, genetics, and pathology. Mastering these concepts allows you to move beyond memorization to predict cellular behavior, interpret experimental data, and answer complex, passage-based questions with confidence.

The Plasma Membrane: Gateway and Identity

The plasma membrane is a selectively permeable barrier that defines the cell. Its fluid mosaic model describes a dynamic sea of phospholipids with embedded proteins and cholesterol. Phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails, forming a bilayer. Cholesterol modulates fluidity: at high temperatures, it stabilizes the membrane, while at low temperatures, it prevents tight packing and maintains flexibility.

Transport across this membrane is categorized as passive or active. Simple diffusion moves small, nonpolar molecules (e.g., $O_2$, $C{O}_2$) down their concentration gradient. Facilitated diffusion uses channel or carrier proteins to move ions or polar molecules (e.g., glucose via GLUT transporters) without energy. Osmosis is the diffusion of water across a semipermeable membrane toward higher solute concentration; tonicity (hypotonic, isotonic, hypertonic) dictates cell volume changes.

Active transport requires ATP to move substances against their gradient. The sodium-potassium pump (Na+/K+ ATPase) is quintessential, exporting 3 Na+ and importing 2 K+ per ATP, establishing vital electrochemical gradients. Secondary active transport (co-transport) uses these pre-established gradients: symporters move two substances in the same direction, while antiporters move them in opposite directions. Large particles cross via endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis.

Intracellular Architecture: Organelles and Cytoskeleton

Eukaryotic cells compartmentalize functions within organelles. The nucleus, surrounded by a double membrane with nuclear pores, houses DNA and the nucleolus for rRNA synthesis. The endoplasmic reticulum (ER) includes the rough ER (studded with ribosomes for protein synthesis and modification) and smooth ER (lipid synthesis, detoxification). Synthesized proteins are packaged in the Golgi apparatus for modification, sorting, and vesicular shipment.

Lysosomes contain hydrolytic enzymes for intracellular digestion, while peroxisomes contain oxidases for fatty acid breakdown and detoxifying hydrogen peroxide. Mitochondria, the powerhouses, conduct aerobic respiration via the citric acid cycle and oxidative phosphorylation to produce ATP; they have their own DNA and double membrane. Ribosomes, composed of rRNA and protein, are the sites of translation and exist free in the cytosol or bound to the ER.

The cytoskeleton provides structure, enables motility, and facilitates intracellular transport. It consists of:

• Microfilaments (actin): Smallest diameter; involved in muscle contraction (with myosin), cytokinesis, and amoeboid movement.
• Intermediate filaments: Diverse proteins (e.g., keratin, vimentin) providing mechanical strength and anchoring organelles.
• Microtubules: Largest diameter; composed of tubulin; form highways for motor proteins (kinesin and dynein); constitute the mitotic spindle, cilia, and flagella (9+2 structure).

Communication, Adhesion, and the Cell Cycle

Cells communicate via signal transduction pathways. A ligand binds to a membrane receptor, initiating a cascade. Common pathways include:

• G-protein coupled receptors (GPCRs): Ligand binding activates a G-protein, which then activates an effector (e.g., adenylate cyclase to produce cAMP, a second messenger).
• Receptor tyrosine kinases (RTKs): Ligand binding causes dimerization and autophosphorylation, activating intracellular kinase domains that trigger pathways like MAPK.
• Intracellular receptors: For hydrophobic ligands (e.g., steroids) that diffuse through the membrane.

Cell adhesion molecules (CAMs) enable cells to stick to each other and the extracellular matrix. Key types are cadherins (calcium-dependent, homophilic binding for cell-cell adhesion), integrins (link the cytoskeleton to the extracellular matrix, facilitating signaling and migration), and selectins (bind carbohydrates, important in immune cell rolling).

The cell cycle is tightly regulated. Interphase consists of G1 (growth), S (DNA synthesis), and G2 (preparation). Mitosis (prophase, metaphase, anaphase, telophase) ensures nuclear division, followed by cytokinesis. Cyclins and cyclin-dependent kinases (CDKs) drive the cycle forward at checkpoints (G1/S, G2/M, spindle). Apoptosis, or programmed cell death, is a regulated, non-inflammatory process involving cell shrinkage, membrane blebbing, and DNA fragmentation, often initiated by caspases. This contrasts with necrosis, which is traumatic and inflammatory.

Stem Cells, Cancer, and Experimental Methods

Stem cells are characterized by self-renewal and potency. Totipotent cells can become any cell, including placental (e.g., zygote). Pluripotent cells (e.g., embryonic stem cells) can become any body cell. Multipotent cells (e.g., hematopoietic stem cells) can become a subset of related cells. Understanding stem cells is key for developmental biology and regenerative medicine.

Cancer biology integrates many cell biology concepts. Cancer arises from accumulated mutations that lead to uncontrolled cell proliferation, evasion of growth suppressors, resistance to apoptosis, enabled replicative immortality, induced angiogenesis, and activation of invasion/metastasis. Oncogenes are mutated versions of proto-oncogenes that promote growth (e.g., Ras), while tumor suppressor genes (e.g., p53, Rb) inhibit cell cycle progression; both alleles must be inactivated for loss of function.

MCAT passages often describe experimental techniques. Be prepared to interpret:

• Cell fractionation: Separates organelles by centrifugation for functional study.
• Microscopy: Light vs. electron (transmission vs. scanning) for visualization.
• Fluorescence tagging: Tracking protein localization.
• Knockout/knockdown models: Assessing gene function.
• Western blot (protein), Southern blot (DNA), Northern blot (RNA), and PCR.

Common Pitfalls

1. Confusing Osmosis and Diffusion: Remember, osmosis specifically refers to water movement. A common trap is thinking solutes "want" to dilute themselves. Water moves to equalize solute concentration. In a hypertonic solution, the cell loses water and shrinks (crenates).
2. Misunderstanding Energy Requirements: Facilitated diffusion is still passive. The protein provides a pathway but does not use ATP; movement is still down a gradient. Active transport always requires energy, either directly (primary) or indirectly (secondary).
3. Overlooking Signal Transduction Specificity: Not all ligands work on all receptors. Know the classic pairs: hydrophilic ligands (peptides, amines) bind surface receptors (GPCRs, RTKs). Hydrophobic ligands (steroids, thyroid hormone) bind intracellular receptors.
4. Conflating Apoptosis and Necrosis: Apoptosis is a neat, programmed "suicide" beneficial to the organism. Necrosis is messy, accidental "murder" causing inflammation. The MCAT may test the morphological or physiological differences.

Summary

• The fluid mosaic model describes the dynamic plasma membrane, which regulates transport via passive (diffusion, osmosis, facilitated) and active (primary, secondary) mechanisms.
• Organelles compartmentalize functions, while the cytoskeleton (microfilaments, intermediate filaments, microtubules) provides structure, transport, and motility.
• Signal transduction (e.g., GPCR, RTK pathways) allows cellular communication, and cell adhesion molecules (cadherins, integrins) enable structural organization.
• The cell cycle is tightly regulated by cyclins and CDKs, and apoptosis is a controlled death pathway. Dysregulation of these processes underpins cancer biology.
• Stem cell potency (totipotent, pluripotent, multipotent) and common experimental methods (blots, microscopy, fractionation) are high-yield topics for data interpretation on the exam.