Unit 2 asks you to connect the structure of cells and their parts to the functions they perform. The big idea is that cells are organized into compartments separated by selectively permeable membranes, and that this organization — together with size constraints like surface-area-to-volume ratio — lets cells exchange materials, regulate their interior, and capture or release energy. You should be able to describe organelles, explain how the fluid-mosaic membrane moves substances by passive and active means, calculate and reason about water potential, and use the endosymbiotic theory to explain the origin of mitochondria and chloroplasts.
Eukaryotic cells partition jobs among membrane-bound organelles. The nucleus stores DNA and is the site of transcription. Ribosomes (made in the nucleolus) build proteins; free ribosomes make cytosolic proteins, while ribosomes on the rough endoplasmic reticulum (RER) make proteins destined for membranes, secretion, or organelles. The smooth ER synthesizes lipids and detoxifies compounds. The Golgi apparatus modifies, sorts, and packages proteins and lipids into vesicles. Mitochondria carry out aerobic cellular respiration (ATP production); chloroplasts (in plants and algae) perform photosynthesis. Lysosomes contain hydrolytic enzymes for intracellular digestion, and vacuoles store water, ions, and wastes (the large central vacuole maintains turgor in plant cells).
Prokaryotes lack these membrane-bound organelles but still carry out the same essential life functions, showing that compartmentalization is one solution — not the only one — to organizing metabolism.
As a cell grows, its volume increases faster than its surface area (volume scales with the cube of length, surface area with the square). A high surface-area-to-volume (SA:V) ratio is favorable because it allows efficient exchange of nutrients, gases, and wastes across the membrane relative to the cell's metabolic demands. When the ratio gets too low, the membrane can no longer support the interior's needs.
Cells and organisms get around this limit with adaptations that increase surface area: staying small and dividing, flattening their shape, or folding membranes (microvilli, the cristae of mitochondria, root hairs). On the AP exam you may be asked to calculate SA:V for cubes or spheres and interpret what a changing ratio means for exchange.
Cell membranes follow the fluid-mosaic model: a phospholipid bilayer in which proteins, cholesterol, and carbohydrates are embedded and can move laterally. Phospholipids are amphipathic — hydrophilic phosphate heads face the aqueous interior and exterior, while hydrophobic fatty-acid tails face inward, away from water. This arrangement makes the membrane selectively permeable: small nonpolar molecules (O2, CO2) cross easily, while large or charged molecules need transport proteins.
Cholesterol (in animal cells) buffers fluidity, keeping the membrane stable across temperature changes. Membrane proteins serve as channels, carriers, receptors, enzymes, and attachment points, while surface glycoproteins and glycolipids function in cell recognition. The membrane's fluidity and protein composition let it regulate what enters and leaves the cell.
Passive transport moves substances down a concentration gradient (high to low) without using cellular energy; the energy comes from the gradient itself and from the entropy-driven random motion of particles. Simple diffusion moves small nonpolar and uncharged molecules directly through the bilayer. Osmosis is the diffusion of water across a selectively permeable membrane.
Facilitated diffusion uses transport proteins to move polar or charged substances (ions, glucose) that cannot cross the bilayer on their own. Channel proteins form pores (including aquaporins for water), and carrier proteins bind a solute and change shape to shuttle it across. Both are still passive — they only speed movement down the existing gradient and do not require ATP.
Active transport moves substances against their concentration gradient (low to high) and therefore requires energy, usually from ATP. The classic example is the sodium-potassium pump, which pumps 3 Na+ out and 2 K+ in per ATP, building the electrochemical gradient cells use for signaling and secondary active transport. Maintaining concentration gradients is itself an energy-requiring process.
Large particles and bulk quantities move by vesicle transport, which also requires energy. In endocytosis the membrane folds inward to bring material into the cell (phagocytosis = solids, pinocytosis = fluids, and receptor-mediated endocytosis = specific molecules). In exocytosis vesicles fuse with the membrane to release contents, such as secreted proteins or neurotransmitters.
Tonicity describes how a solution affects net water movement into or out of a cell. In a hypotonic solution (lower solute outside) water enters and the cell swells or, in plants, becomes turgid. In a hypertonic solution (higher solute outside) water leaves and the cell shrinks (plasmolysis in plants). In an isotonic solution there is no net water movement.
Water moves toward lower (more negative) water potential. Water potential is calculated as:
Ψ = ΨS + ΨP
where ΨS is the solute (osmotic) potential and ΨP is the pressure potential. Pure water at standard conditions has Ψ = 0. Adding solute lowers ΨS (always negative or zero), making Ψ more negative. Solute potential can be found with ΨS = -iCRT (ionization constant × molar concentration × pressure constant × temperature in Kelvin). Water always flows from higher Ψ to lower Ψ.
Compartmentalization — using internal membranes to create separate spaces — lets eukaryotic cells run many incompatible reactions at once. Membranes allow each compartment to maintain a distinct chemical environment (pH, enzyme set, ion concentration) optimized for its function. For example, lysosomes keep digestive enzymes at an acidic pH isolated from the cytosol, and the inner mitochondrial membrane separates the proton gradient that drives ATP synthesis.
This organization increases efficiency by concentrating reactants and enzymes, protecting the rest of the cell from harmful intermediates, and increasing the available membrane surface area for membrane-bound processes. Internal membranes thus support far higher metabolic complexity than would be possible in a single open space.
The endosymbiotic theory explains the origin of mitochondria and chloroplasts: they arose when a larger host cell engulfed free-living prokaryotes that were not digested but instead became permanent residents, providing energy in exchange for protection and resources. Over time the relationship became obligate, and these organelles evolved into the energy-converting structures of eukaryotic cells.
The evidence is strong: mitochondria and chloroplasts have their own circular DNA, their own ribosomes resembling those of prokaryotes, a double membrane (the inner from the original prokaryote, the outer from the host's engulfing membrane), and they reproduce by binary fission independent of the cell. Their size is also similar to that of bacteria. These features mirror prokaryotic traits and support the idea that they were once independent cells.
Practise exam-style questions on this topic.