Cell Structure & Function

What this unit covers

The topics below follow the published Biology course framework for Unit 2. This unit is worth 10–13% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Prokaryotic vs. Eukaryotic Cells13 min · 3 objectivesDistinguish prokaryotic and eukaryotic cells by their defining structural features · Explain how compartmentalization by internal membranes benefits eukaryotic cells · Relate the surface-area-to-volume ratio to limits on cell size
• Eukaryotic Organelles14 min · 3 objectivesIdentify the major eukaryotic organelles and state the function of each · Trace the path of a secreted protein through the endomembrane system · Compare organelles unique to plant cells with those unique to animal cells
• The Membrane & Cellular Transport14 min · 3 objectivesDescribe the fluid-mosaic model and the roles of its components · Distinguish passive transport (diffusion and facilitated diffusion) from active transport · Explain how bulk transport moves large materials by endocytosis and exocytosis
• Osmosis, Tonicity & Water Potential13 min · 3 objectivesDefine osmosis and predict water movement across a selectively permeable membrane · Classify solutions as hypertonic, hypotonic, or isotonic and predict effects on cells · Use the water-potential equation to determine the direction of water movement
• Endosymbiotic Theory & the Origin of Eukaryotic Cells14 min · 3 objectivesMarshal the four independent lines of evidence — double membranes, circular DNA, bacteria-like ribosomes, and binary fission — that support the endosymbiotic origin of mitochondria and chloroplasts · Reconstruct the sequence by which an ancestral prokaryote gave rise to a compartmentalized eukaryotic cell, distinguishing autogenous origins from endosymbiotic ones · Interpret experimental observations (e.g., antibiotic sensitivity of organelle ribosomes) as tests of the endosymbiotic hypothesis
• Membrane Transport, Quantified15 min · 3 objectivesContrast the rate-versus-concentration behavior of simple diffusion, facilitated diffusion, and active transport, explaining why carrier-mediated transport saturates · Calculate water potential using Ψ = Ψp + Ψs and the solute-potential relationship Ψs = −iCRT to predict both the direction and the magnitude (in MPa) of water movement · Compare the responses of walled plant cells and wall-less animal cells to hypertonic, hypotonic, and isotonic solutions, and identify when bulk transport is required

Formulas in Unit 2

Every term in Unit 2

All 37 terms we publish for Cell Structure & Function, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Surface area to volume ratio

Volume grows faster than surface area, so a large cell cannot exchange materials fast enough. This constrains cell size and explains microvilli and flattened shapes.

Endosymbiotic theory

Mitochondria and chloroplasts descend from engulfed prokaryotes. Evidence: double membranes, circular DNA, their own ribosomes, and binary-fission-like division.

Rough ER

Ribosome-studded ER that folds and modifies proteins destined for secretion, membranes or lysosomes.

Smooth ER

Synthesises lipids, stores calcium and detoxifies drugs. Abundant in liver cells and in cells making steroid hormones.

Golgi apparatus

Modifies, sorts and packages proteins from the ER, tagging them for their destination — the cell's postal sorting office.

Lysosome

Acidic vesicle of hydrolytic enzymes that digests macromolecules and worn organelles. Its enzymes work at pH 5, so a leak is buffered by the cytosol.

Vacuole

Storage compartment; the plant central vacuole holds water and generates turgor pressure that keeps the plant upright.

Peroxisome

Breaks down fatty acids and detoxifies, producing hydrogen peroxide that catalase then converts to water and oxygen.

Cell wall

Rigid outer layer — cellulose in plants, chitin in fungi, peptidoglycan in bacteria — that prevents lysis in hypotonic surroundings.

Fluid mosaic model

The membrane is a fluid phospholipid bilayer with proteins drifting laterally within it, not a rigid sandwich.

Cholesterol in membranes

A fluidity buffer: it restrains movement at high temperature and prevents tight packing at low temperature.

Selective permeability

Small nonpolar molecules cross freely; ions and large polar molecules need transport proteins. The bilayer's hydrophobic core is the reason.

Passive transport

Movement down a concentration gradient without ATP — simple diffusion, facilitated diffusion and osmosis.

Facilitated diffusion

Passive movement through a channel or carrier protein. Still down-gradient, but selective and saturable.

Active transport

Movement against a gradient using ATP. The sodium-potassium pump exports 3 Na⁺ for every 2 K⁺ imported, building charge and concentration gradients.

Osmosis

Diffusion of water across a selectively permeable membrane, from lower to higher solute concentration.

Hypotonic, hypertonic, isotonic

Relative solute concentration outside a cell. Hypotonic surroundings make animal cells swell and burst but make plant cells turgid — the useful state.

Water potential

Ψ = Ψp + Ψs. Water moves from higher to lower water potential; solute potential is negative, so adding solute lowers Ψ.

Endocytosis and exocytosis

Bulk transport by vesicle. Endocytosis brings material in (phagocytosis, pinocytosis, receptor-mediated); exocytosis fuses vesicles to release contents.

Contractile vacuole

Organelle in freshwater protists that pumps out the water constantly entering by osmosis — active transport solving an osmotic problem.

Prokaryote vs eukaryote

Prokaryotes lack a nucleus and membrane-bound organelles and have a single circular chromosome; eukaryotes have both and linear chromosomes.

Nucleolus

Dense region inside the nucleus where ribosomal RNA is transcribed and ribosome subunits are assembled.

Nuclear envelope and pores

Double membrane separating transcription from translation. Pores control what enters and leaves, which is a regulatory step prokaryotes do not have.

Ribosome

Site of translation, made of rRNA and protein. Found free in the cytosol, on the rough ER, and inside mitochondria and chloroplasts.

Endomembrane system

Nuclear envelope, ER, Golgi, lysosomes, vesicles and plasma membrane, working as one connected protein-processing pathway.

Mitochondrial structure and function

Double membrane with a folded inner membrane (cristae) that increases surface area for the electron transport chain; the matrix holds Krebs cycle enzymes.

Chloroplast structure

Double membrane enclosing thylakoid stacks (grana) where light reactions occur, surrounded by stroma where the Calvin cycle runs.

Cytoskeleton

Microfilaments, intermediate filaments and microtubules give shape, anchor organelles and drive movement and cell division.

Cilia and flagella

Microtubule structures in a 9+2 arrangement that move the cell or move fluid across it.

Tight junctions, desmosomes, gap junctions

Tight junctions seal cells so nothing leaks between them; desmosomes rivet cells together; gap junctions let small molecules pass directly between cytoplasms.

Plasmodesmata

Channels through plant cell walls connecting adjacent cytoplasms — the plant equivalent of gap junctions.

Aquaporins

Channel proteins that let water cross the membrane far faster than simple diffusion allows; important in kidney and root cells.

Sodium-potassium pump

Pumps 3 Na⁺ out and 2 K⁺ in per ATP, creating both a concentration and a charge gradient — the basis of the resting membrane potential.

Cotransport

A gradient built by active transport powers the movement of a second substance. Plants use a proton gradient to drag sucrose into phloem.

Turgor pressure

Outward pressure of the vacuole against the cell wall in a hypotonic environment. Its loss is why an unwatered plant wilts.

Lab: Diffusion and Osmosis

Dialysis tubing or potato cores in sucrose solutions. Percent mass change is plotted against concentration; the x-intercept estimates the tissue's solute concentration.

Lab: Transpiration

A potometer measures water uptake under varied humidity, wind and light. Tests how environmental conditions change the rate of the transpiration stream.

What examiners penalize here

• When an AP question links cell size to function, argue from **SA:V**: small cells and highly folded membranes (microvilli, root hairs, alveoli) maximize surface area per volume for faster exchange. Naming the ratio and its direction earns the point.
• Know the three plant-only structures cold: **cell wall, chloroplast, and central vacuole**. A frequent exam prompt gives an unlabeled cell and asks you to identify it as plant or animal — the wall and chloroplasts are the giveaways.
• For any transport question, run a two-step check: (1) Which way relative to the gradient? Down = passive/no ATP, up = active/ATP. (2) Does it need a protein? Small nonpolar = simple diffusion; ion or polar = protein-mediated. Those two answers name the mechanism.
• On free response, state the rule explicitly — "**water moves from higher Ψ to lower Ψ**" — then plug in the numbers, remembering that a **less negative** value is the higher potential. Adding solute makes Ψs more negative and lowers Ψ, pulling water in.
• Memorize the **four evidences** as a set — **double membrane, circular DNA, 70S (bacteria-like) ribosomes, binary fission** — and be ready to say what each demonstrates. Free-response prompts often show organelle data and ask you to argue *for* endosymbiosis; naming the evidence and tying it to a bacterial ancestor earns the points.
• For a water-potential free-response, show the pipeline explicitly: convert to kelvin, compute **Ψs = −iCRT**, set **Ψp = 0** for an open solution (or solve for it at equilibrium), add to get **Ψ = Ψp + Ψs**, then state the rule — **water moves from higher Ψ to lower Ψ**. Carry the MPa units and the negative signs through every line; graders award the reasoning, not just the final number.

Practice Biology

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 8 units of AP Biology

1. Unit 1 · Chemistry of Life
2. Unit 2 · Cell Structure & Function
3. Unit 3 · Cellular Energetics
4. Unit 4 · Cell Communication & Cycle
5. Unit 5 · Heredity
6. Unit 6 · Gene Expression & Regulation
7. Unit 7 · Natural Selection
8. Unit 8 · Ecology