Cell Structure and Function
The cell is the fundamental structural and functional unit of all known living organisms. The AAMC MCAT content outline expects you to compare prokaryotic and eukaryotic cells, identify every major cytoplasmic organelle and its role, and describe chromosome structure. This is a high-yield area for the Bio/Biochem section.
Cell Structure
A cell is bounded by a plasma membrane, contains cytoplasm with organelles suspended in it, and houses genetic material in a nucleus or nucleoid region. The basic structural plan is conserved across all living organisms, but cells differ in size, shape, and internal organization depending on function.
Plasma membrane
A selectively permeable phospholipid bilayer that regulates what enters and leaves the cell. Each phospholipid is amphipathic — hydrophilic heads face the watery interior and exterior, hydrophobic tails point inward. Embedded integral (transmembrane) proteins and surface-associated peripheral proteins act as transporters, channels, receptors, and enzymes; cholesterol buffers fluidity in animal cells; external glycolipids and glycoproteins form the glycocalyx for cell recognition. This arrangement is the fluid mosaic model (Singer and Nicolson, 1972): the membrane is a two-dimensional fluid in which components drift laterally.
Cytoplasm
A semi-fluid medium (cytosol) plus suspended organelles. Cytoplasm is the site of glycolysis, protein synthesis (free ribosomes), and many metabolic intermediate reactions. In eukaryotes, the cytoskeleton (microfilaments, microtubules, intermediate filaments) maintains shape and drives intracellular transport.
Nucleus / nucleoid
Eukaryotes have a true membrane-bound nucleus containing linear chromosomes; prokaryotes have a nucleoid — an unbound region with a single circular DNA molecule. The eukaryotic nucleus also contains the nucleolus, the site of rRNA synthesis and ribosome subunit assembly.
Membrane Transport
The selectively permeable membrane controls traffic in and out of the cell. Transport is split into passive (no ATP; moves down a gradient) and active (requires energy; moves against a gradient).
- Simple diffusion — small nonpolar molecules (O2, CO2) and lipids cross the bilayer directly, high → low concentration.
- Facilitated diffusion — polar/charged solutes (ions, glucose) move down their gradient through channel or carrier proteins.
- Osmosis — net diffusion of water across the membrane (often via aquaporins) toward the region of higher solute concentration.
Describes how a solution affects cell volume by water movement: in a hypotonic solution water enters and the cell swells (may lyse); in a hypertonic solution water leaves and the cell shrinks (crenation / plasmolysis); in an isotonic solution there is no net movement.
Primary active transport uses ATP directly — e.g., the Na+/K+-ATPase pumps 3 Na+ out and 2 K+ in per ATP, maintaining the resting membrane potential. Secondary active transport (co-transport) uses the gradient set up by primary pumps to drive a second solute (e.g., the Na+/glucose symporter in the gut) — symport (same direction) or antiport (opposite).
Endocytosis brings material in by engulfing it in vesicles — phagocytosis (solids/"cell eating"), pinocytosis (fluids), and receptor-mediated endocytosis (specific, e.g., LDL uptake). Exocytosis fuses vesicles with the membrane to release contents (neurotransmitters, hormones, enzymes). Both require ATP.
Cytoplasmic Organelles
Eukaryotic cells contain a suite of membrane-bound organelles, each specialized for a step of cellular metabolism. The MCAT expects you to know structure, function, and common diseases or quirks of each.
Double-membrane organelle. The inner membrane is folded into cristae which house the electron transport chain and ATP synthase. Site of the Krebs cycle (matrix) and oxidative phosphorylation. Contains its own circular DNA (mtDNA) and 70S ribosomes — evidence for endosymbiotic origin.
Rough ER is studded with ribosomes — site of synthesis of secretory and membrane proteins. Smooth ER lacks ribosomes — site of lipid synthesis, steroid hormone production, and detoxification (heavy in liver hepatocytes).
Stack of flattened cisternae. Modifies, sorts and packages proteins arriving from the rough ER. Adds carbohydrate side-chains (glycosylation) and dispatches vesicles to the membrane, lysosomes, or for secretion.
Membrane-bound sacs of hydrolytic enzymes active at acidic pH (~5). Digest worn-out organelles (autophagy), engulfed pathogens (phagocytosis) and macromolecules. Found mainly in animal cells; Tay-Sachs disease is caused by a missing lysosomal enzyme (hexosaminidase A).
Small single-membrane organelles that carry out oxidative reactions. They break down very-long-chain fatty acids by beta-oxidation and detoxify substances (including alcohol in the liver). Oxidase enzymes generate hydrogen peroxide (H2O2), which the enzyme catalase then converts to water and O2. Do not confuse them with lysosomes (which do acid hydrolysis).
Sites of protein synthesis. Composed of rRNA + protein. Two subunits join on an mRNA. 80S in eukaryotic cytoplasm (60S + 40S); 70S in prokaryotes, mitochondria, and chloroplasts (50S + 30S). Ribosomes have no membrane.
Double-membrane organelle of photosynthesis. Contains stacks of thylakoids (grana) suspended in stroma. Light reactions occur on thylakoid membranes; the Calvin cycle in the stroma. Like mitochondria, has its own DNA and 70S ribosomes.
Plant cells have a single large central vacuole that stores water, ions, pigments, and waste products — it also generates turgor pressure. Animal cells have small, transient vacuoles (food, contractile, etc.).
Centrioles are paired hollow cylinders (9 triplets of microtubules) that organize the mitotic spindle in animal cells. Plant cells lack centrioles. The cytoskeleton itself comprises microtubules, microfilaments (actin), and intermediate filaments.
Chromosomes
A chromosome is a thread-like structure of DNA tightly packed with histone proteins. Eukaryotes have linear chromosomes housed in the nucleus; prokaryotes have a single circular chromosome in the nucleoid.
Structural components
- Chromatin
- The complex of DNA + histone proteins as it appears in non-dividing cells. Two forms: euchromatin (loosely packed, transcriptionally active) and heterochromatin (densely packed, mostly inactive).
- Nucleosome
- The fundamental packing unit: ~146 bp of DNA wrapped 1.65 times around an octamer of 8 histone proteins (two copies each of H2A, H2B, H3, H4). H1 acts as the linker histone.
- Centromere
- Constricted region where sister chromatids are joined and where the kinetochore assembles to attach spindle fibers during mitosis/meiosis.
- Telomere
- Repetitive non-coding sequence (TTAGGG in vertebrates) at the ends of linear chromosomes; protects against degradation and end-fusion. Shortens with each replication unless extended by telomerase.
Human chromosome number
Humans have 46 chromosomes in somatic cells: 22 pairs of autosomes + 1 pair of sex chromosomes (XX female, XY male). Gametes are haploid (n = 23). Aneuploidies such as trisomy 21 (Down syndrome), XXY (Klinefelter) and XO (Turner) result from non-disjunction during meiosis.
Prokaryotic vs Eukaryotic Cell
The single most important comparison in cell biology. Examiners almost always ask one MCQ on a structural or functional difference between these two cell types.
- Prokaryote
- An organism whose cell lacks a true membrane-bound nucleus and membrane-bound organelles. Domain Bacteria and Archaea. Examples: E. coli, cyanobacteria.
- Eukaryote
- An organism whose cells contain a true nucleus and membrane-bound organelles. Includes all animals, plants, fungi, and protists.
Side-by-side comparison
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Size | 1–10 µm | 10–100 µm |
| Nucleus | Absent — DNA in nucleoid region | Present, membrane-bound, contains nucleolus |
| DNA | Single circular chromosome (+ plasmids) | Multiple linear chromosomes |
| Histones | Absent in bacteria (present in archaea) | Present — DNA wraps around histone octamers |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, Golgi, lysosomes, etc.) |
| Ribosomes | 70S (50S + 30S) | 80S in cytoplasm; 70S in mitochondria/chloroplasts |
| Cell wall | Peptidoglycan (bacteria); pseudopeptidoglycan (archaea) | Cellulose (plants), chitin (fungi); absent in animal cells |
| Cell division | Binary fission | Mitosis / meiosis with spindle apparatus |
| Reproduction | Mostly asexual | Both asexual and sexual |
| Examples | E. coli, cyanobacteria, Streptococcus | Animals, plants, fungi, protists |
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter. Read the explanation even when you get the answer right — it's where the deeper concept lives.
Q1. Which of the following organelles contains its own DNA and 70S ribosomes?
Mitochondria (and chloroplasts) carry circular DNA and 70S ribosomes — strong evidence for the endosymbiotic theory that they descend from engulfed prokaryotes. Other organelles depend entirely on nuclear DNA and 80S cytoplasmic ribosomes.
Q2. The basic packing unit of eukaryotic chromatin, consisting of DNA wrapped around 8 histone proteins, is called a:
A nucleosome is ~146 bp of DNA wrapped 1.65 times around an octamer of 8 histones (2 each of H2A, H2B, H3, H4). The string of nucleosomes forms the "beads-on-a-string" structure that condenses further into chromatin fibers.
Q3. Which of these is NOT a feature of a prokaryotic cell?
Prokaryotes have no membrane-bound nucleus — their DNA sits in a region called the nucleoid. They do have 70S ribosomes, a peptidoglycan cell wall (in bacteria), and a single circular chromosome.
Q4. A red blood cell is placed in a hypotonic solution. What happens, and why?
A hypotonic solution has a lower solute concentration than the cytosol, so water moves by osmosis into the cell (toward higher solute). The cell swells and, lacking a wall, can burst (hemolysis). In a hypertonic solution the reverse happens (crenation); an isotonic solution produces no net movement.
Q5. Lysosomal hydrolytic enzymes function optimally at which pH?
Lysosomes maintain an acidic interior (~pH 5) using H⁺ pumps in their membrane. Hydrolytic enzymes inside (proteases, lipases, nucleases) work best at this pH — a built-in safety mechanism, since accidental leakage into the neutral cytosol largely inactivates them.
Quick Recap
- Prokaryote = no nucleus, 70S ribosomes, single circular DNA, no membrane-bound organelles.
- Eukaryote = true nucleus, 80S cytoplasmic ribosomes (70S in mitochondria/chloroplasts), linear DNA wound on histones.
- Mitochondria + chloroplasts have their own DNA and 70S ribosomes (endosymbiotic theory).
- Nucleosome = ~146 bp DNA + 8 histones (H2A, H2B, H3, H4 ×2 each).
- Lysosomes work at pH ~5; deficiency → storage diseases (Tay-Sachs, Pompe). Peroxisomes use catalase to clear H2O2 and oxidize long-chain fatty acids.
- Membrane = fluid mosaic bilayer; passive transport (diffusion, osmosis, facilitated) needs no ATP; active transport (Na+/K+ pump, endo/exocytosis) does.
- Tonicity: hypotonic → cell swells; hypertonic → cell shrinks; isotonic → no net water movement.
- Humans = 46 chromosomes (22 autosome pairs + XX/XY).