Skip to content
Marlbridge
Free trial

Flashcards · AP® Biology · Unit 2

Unit 2: Cells flashcards

64 cards on topics 2.1 to 2.10. Pick a topic or study the whole unit.

Study mode needs JavaScript. Every card is listed below, so you can still cover the answers and test yourself.

All 64 cards

For a quick read-through or printing. Cover the right-hand side to test yourself.

2.1 Cell Structure and Function

What do ribosomes do, and where are they found?
Ribosomes (rRNA and protein, no membrane) make proteins by reading mRNA and are found in all cells. Free ribosomes make proteins for the cytosol; ribosomes on rough ER make proteins for export, membranes and lysosomes.
How do rough ER and smooth ER differ in function?
Rough ER has bound ribosomes: protein synthesis, with new export proteins going straight into a separate compartment. Smooth ER: lipid synthesis and detoxification. Both give mechanical support and a transport route.
What does the Golgi complex do?
A stack of flattened sacs that checks folding, chemically modifies products (for example adding sugar chains: glycosylation) and packages them in vesicles for their destinations. It does not make proteins.
What do lysosomes do?
Membrane sacs of hydrolytic enzymes that digest and recycle material and help carry out apoptosis (programmed cell death). The membrane keeps digestion from harming the rest of the cell.
How is the mitochondrion's structure linked to its function?
A double membrane (smooth outer, folded inner called cristae) makes two compartments for aerobic respiration. Cristae add membrane area for ATP-making proteins, not more fluid.
What is the role of the plant central vacuole?
It stores water and nutrients and keeps turgor pressure that supports the plant. A wilting cell has a smaller vacuole because it has lost water. Animal cells have smaller, more numerous storage vacuoles.
What path does a secreted protein take through the cell?
Rough ER (made and labelled first) → transport vesicle → Golgi → secretory vesicle → exocytosis outside the cell. Vesicles carry proteins between ER and Golgi; proteins do not cross those membranes.
Do plant cells have mitochondria?
Yes. Plant cells have mitochondria as well as chloroplasts; chloroplasts (double membrane) are the site of photosynthesis.

2.2 Cell Size

Why does surface area-to-volume ratio limit cell size?
Everything a cell exchanges crosses its surface, but every part of its volume creates demand. As a cell grows, volume rises faster than surface area, so SA/V falls and demand per unit surface rises.
What are the SA/V formulas for a cube and a sphere?
Cube of side s: SA = 6s², V = s³, SA/V = 6/s. Sphere of radius r: SA = 4πr², V = (4/3)πr³, SA/V = 3/r. Cube of side 5 µm: 1.2 µm⁻¹. Use radius, not diameter, and keep the units.
What happens to SA, V and SA/V when length is multiplied by k?
SA is multiplied by k², V by k³, and SA/V is divided by k.
How do cells and organisms increase surface area for exchange?
Flat or long shapes, and membrane folds and projections that add area for almost no volume: microvilli in the gut lining, root hairs, cristae. Leaves exchange gases through stomata controlled by guard cells.
Why does a mouse need more food per gram than a horse?
Its higher SA/V means it loses heat faster per gram of body, so it needs a higher metabolic rate per unit mass. Smaller organisms usually have the higher rate per unit mass.
How do you find the SA of a cylinder?
SA = 2πrh + 2πr², and V = πr²h. A common mistake is leaving out the two ends (2πr²).

2.3 Plasma Membrane

How are phospholipids arranged in the plasma membrane?
In a bilayer: polar, hydrophilic phosphate heads face the watery fluid inside and outside; nonpolar, hydrophobic fatty acid tails face each other in the middle. It forms by itself as the most stable arrangement, with no energy needed.
What is the fluid mosaic model?
The membrane is phospholipids with proteins, steroids such as cholesterol (vertebrate animals), glycoproteins and glycolipids set into them. All can move sideways; "fluid" means components drift, not that the membrane has no shape.
How do integral and peripheral proteins differ?
Integral proteins pass through the bilayer (channels, carriers, receptors). Peripheral proteins sit on one face (enzymes, signalling, links to the cytoskeleton).
How are membrane proteins positioned by their R groups?
Hydrophobic regions (nonpolar R groups) touch the fatty acid tails. Hydrophilic regions (polar and charged R groups) face the cytosol or outside fluid, or line a pore. A whole membrane protein is not hydrophobic.
How can you tell the outside face of a membrane in a diagram?
Carbohydrate chains of glycoproteins and glycolipids point outward; they work in cell recognition and adhesion.
How long must a membrane-crossing α-helix be?
Helix length = number of amino acids × 0.15 nm. About 20 or more nonpolar amino acids are needed to cross the 3 to 4 nm hydrophobic interior; 18 give 2.7 nm.
What does cholesterol do in animal membranes?
It sits among the tails and keeps the membrane stable as temperature changes.

2.4 Membrane Permeability

Why is the plasma membrane selectively permeable?
Its interior is hydrophobic. The nonpolar tails block ions and polar molecules; an ion would have to leave its water shell to enter the tails. Charge, not size, blocks ions.
Which molecules cross the phospholipid bilayer directly?
Small nonpolar molecules (N₂, O₂, CO₂) cross freely. Small polar, uncharged molecules (H₂O, NH₃) cross in small amounts. Glucose and ions such as Na⁺, K⁺, Cl⁻ and H⁺ need channel or transport proteins.
What are cell walls made of in different groups?
Bacteria: peptidoglycan. Archaea: varied, not peptidoglycan. Fungi: chitin. Plants: cellulose. Animal cells have no wall.
What does a cell wall do?
It is a structural boundary, a barrier to some substances, and protection from osmotic lysis. It stops the cell expanding and bursting; the membrane, not the wall, does the fine selecting.
How do you calculate and use standard error in membrane experiments?
SE = s ÷ √n. Mean ± 2 SE is roughly a 95% confidence interval. Bars that do not overlap: reject H₀. Bars that overlap: fail to reject H₀. Values 10, 12, 14: s = 2, SE = 2 ÷ √3 ≈ 1.15.
How should you word a conclusion from experimental data?
Data support a hypothesis, or lead you to reject or fail to reject H₀. Never write "accept the null hypothesis" or "prove the hypothesis".

2.5 Membrane Transport

What is diffusion, and what happens at equilibrium?
Net movement from high to low concentration caused by random motion. At equal concentrations, molecules keep moving both ways but there is no net movement (dynamic equilibrium); diffusion does not stop.
How do passive and active transport differ?
Passive (simple diffusion, facilitated diffusion, osmosis) moves substances down their gradient with no direct metabolic energy. Active transport needs direct energy input (usually ATP) and a specific membrane protein, and can move substances from low to high concentration.
What are endocytosis and exocytosis?
Endocytosis folds the membrane inward to form a vesicle that takes material in (phagocytosis of bacteria). Exocytosis releases material when a vesicle fuses with the membrane (insulin release). Both need energy; cargo travels in vesicles, not through the bilayer.
How can concentration data show active uptake?
Find inside ÷ outside concentration. For an uncharged solute still entering, a ratio above 1 suggests active transport: 6.0 ÷ 1.5 = 4.0. Merely entering a cell is not evidence of active transport.
Is active transport defined as movement against the gradient?
No. It is defined by direct energy input through a membrane protein; it can go against the gradient. Facilitated diffusion uses a protein but is passive.
Why is changing temperature a poor test of ATP dependence?
Temperature affects diffusion and membrane fluidity as well as ATP production, so it is not a clean test. An inhibitor that only blocks ATP production is better.

2.6 Facilitated Diffusion

What is facilitated diffusion?
Movement of ions and large polar molecules through transport proteins, down their concentration gradient, with no energy input.
How do channel proteins and carrier proteins differ?
Channels form a hydrophilic pore and are fast, specific and often gated. Carriers bind a specific molecule and change shape, so they are slower. Channels do not pump ions.
What are aquaporins?
Channel proteins that move large quantities of water quickly (kidney collecting ducts, root cells). Water still crosses the bilayer slowly without them, and movement through them is passive.
How do you tell simple from facilitated diffusion on a rate graph?
Rate proportional to concentration (straight line): simple diffusion, nothing to saturate. Rate levels off at high concentration: carriers or channels saturated, so facilitated diffusion (or active transport).
Why can ion movement polarize a membrane?
Ions carry charge. When K⁺ leaves through K⁺ channels it carries positive charge out, and the negative ions it balanced cannot follow, so the inside becomes negative.
What happens to facilitated diffusion if more transport proteins are added?
The maximum rate rises; the direction is unchanged. If only the protein route doubles, do not double the whole rate, since part may come through the bilayer.

2.7 Tonicity and Osmoregulation

What is osmosis?
The passive net movement of water across a selectively permeable membrane. Water moves; the solutes that set up the gradient cannot cross.
What do hypotonic, hypertonic and isotonic mean?
They compare two solutions. A solution hypotonic to a cell has fewer solute particles, so water enters. Hypertonic: more solute, water leaves. Isotonic: no net movement. Always say what is being compared.
What happens to animal and plant cells in different solutions?
Animal cells lyse in hypotonic and shrivel in hypertonic solutions. Plant cells become turgid in hypotonic (the wall stops bursting), flaccid in isotonic and plasmolysed in hypertonic.
What is the water potential equation?
Ψ = Ψp + Ψs, in bars; water moves from higher to lower Ψ. In an open container Ψp = 0, so Ψ = Ψs.
How do you calculate solute potential?
Ψs = −iCRT. i = particles per unit (sucrose 1, NaCl 2, CaCl₂ 3), C in mol L⁻¹, R = 0.0831 L bar mol⁻¹ K⁻¹, T in K (°C + 273). 0.10 M sucrose at 27 °C: −(1)(0.10)(0.0831)(300) = −2.49 bar.
How do freshwater protists osmoregulate?
They have no wall and live in hypotonic water, so water keeps entering; a contractile vacuole pumps it out. Plant cells instead rely on the wall and central vacuole to stay turgid.
Why plot percentage mass change in osmosis experiments?
Starting masses differ, so raw mass change is not comparable. Percentage change = (final − initial) ÷ initial × 100; the isotonic point is where the line crosses zero.

2.8 Mechanisms of Transport

What does active transport require?
A membrane protein (pump) and a direct input of metabolic energy, usually ATP. Pumps are specific (binding sites fit particular ions) and saturable (maximum rate when all pumps are busy).
How does the sodium–potassium pump work?
3 Na⁺ bind inside → ATP phosphorylates the pump → shape change releases Na⁺ outside → 2 K⁺ bind → phosphate released → shape returns and K⁺ is released inside. One ATP per cycle.
Why does the Na⁺/K⁺ pump help keep the inside of the cell negative?
Each cycle moves 3 positive charges out but only 2 in, so one net positive charge leaves. It is not a 1 : 1 exchange.
What is an electrochemical gradient?
A combination of a concentration difference and a charge difference across a membrane. Many resting animal cells have a membrane potential of about −70 mV (inside negative).
Why must ion pumps keep working?
Ions leak back through channels. If ATP runs out, pumping stops, Na⁺ leaks in and the gradient gradually runs down.
How many ions does the Na⁺/K⁺ pump move for a given ATP use?
3 Na⁺ out and 2 K⁺ in per ATP. 9.0 × 10⁵ ATP per second moves 2.7 × 10⁶ Na⁺ out and 1.8 × 10⁶ K⁺ in.

2.9 Cell Compartmentalization

What is compartmentalization, and why does it help eukaryotic cells?
Membrane-bound organelles each hold their own enzymes, substrates and conditions, so the cell can run many reactions at once and keep opposing pathways apart.
How do compartments minimize competing interactions?
Opposing pathways run in different places, each space keeps its own pH (lysosome about 4.5–5, cytosol about 7.2), harmful enzymes stay contained, and enzymes and substrates are concentrated in a small volume.
How do internal membrane folds help cells?
Many reactions happen on membrane proteins, so folds such as cristae, stacked thylakoids and the ER network add area without adding volume. Folding adds area, not organelle size.
How does pH relate to H⁺ concentration?
Each 1 pH unit is a factor of 10 in H⁺; higher pH means fewer H⁺. pH 5 has 10² = 100 times more H⁺ than pH 7.
How do you total the area of a folded membrane?
Unfolded area plus the area added by all folds. 2.0 µm² plus 10 folds of 0.30 µm² gives 2.0 + 3.0 = 5.0 µm².
What happens if lysosome proton pumps are blocked?
Lysosome pH rises, the digestive enzymes slow down and undigested material builds up.

2.10 Origins of Cell Compartmentalization

Do prokaryotes have internal organization?
Yes, though typically no membrane-bound organelles: a nucleoid (DNA, no membrane), smaller ribosomes, a plasma membrane holding aerobic respiration proteins, and in some species thylakoids (cyanobacteria) or carboxysomes.
What is the endosymbiotic theory?
A host cell took in an aerobic prokaryote that became the mitochondrion. Later, a cell that already had mitochondria took in a cyanobacterium that became the chloroplast. So plants have both organelles and animals only mitochondria.
What evidence supports endosymbiosis?
Mitochondria and chloroplasts have two membranes, their own small circular DNA, bacteria-like ribosomes, divide by splitting like binary fission, and have DNA closest to alphaproteobacteria or cyanobacteria. The strength is in several lines agreeing.
Why can modern mitochondria not live on their own?
Over time most of the guests' genes moved to the nucleus or were lost, so the organelles now depend on the cell.
How do you calculate percentage sequence identity?
Identical positions ÷ positions compared × 100. 1,020 of 1,500 positions: 68.0%. Higher identity suggests a more recent shared ancestor.
Why do almost all eukaryotes have mitochondria but only some have chloroplasts?
Mitochondria were acquired first, by the common ancestor of eukaryotes; chloroplasts were acquired later, by one lineage.

Study Unit 2 further

AP® and Advanced Placement® are trademarks registered by the College Board, which is not affiliated with, and does not endorse, this website. Marlbridge is an independent tuition provider. It is not an authorized AP course provider, an AP school or an exam centre, and these resources are not official College Board materials. Reading them or doing simulations does not meet the laboratory requirement of an AP science course.