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Credit Sprint's Guide to Biology Unit 2: Cell Structure and Function

AP® weight 10–13%CLEP® weight 10–14%

Cells are the basic unit of life. Organelles divide up the work in eukaryotic cells, the plasma membrane controls what enters and leaves, and surface area-to-volume ratio limits how big a cell can be.

Phospholipid bilayer and transport Channel Pump (ATP) O₂, CO₂: simple diffusion facilitated (high → low) active (low → high) outsideinside

What you need to know

  • Prokaryotes (bacteria, archaea) have no nucleus or membrane-bound organelles; eukaryotes have both. Both have ribosomes, a plasma membrane, cytoplasm and DNA.
  • Ribosomes make proteins. Rough ER is studded with ribosomes and makes proteins for secretion or membranes; smooth ER makes lipids and detoxifies.
  • The Golgi complex modifies, sorts and packages proteins into vesicles. Lysosomes digest with hydrolytic enzymes.
  • Mitochondria (cellular respiration) and chloroplasts (photosynthesis) have double membranes, their own DNA and ribosomes: evidence for endosymbiosis.
  • As a cell grows, volume increases faster than surface area, so the SA:V ratio falls and exchange becomes less efficient. Folds and flat shapes raise SA:V.
  • The fluid mosaic model: a phospholipid bilayer with embedded proteins, cholesterol (in animals) and carbohydrate markers.
  • Small nonpolar molecules (O₂, CO₂) diffuse straight through the bilayer; ions and large polar molecules need transport proteins.
  • Passive transport (diffusion, facilitated diffusion, osmosis) moves substances down their gradient with no ATP. Active transport moves them against the gradient using ATP.
  • Water moves toward the more negative water potential (higher solute concentration). In a hypertonic solution a cell loses water; in a hypotonic one it gains water.
  • Endocytosis and exocytosis move large substances in and out with vesicles.

Equations and rules

Surface area of a cube = 6s²; volume = s³; SA:V = 6/s
Water potential: Ψ = Ψs + Ψp
Solute potential: Ψs = −iCRT (R = 0.0831 L·bar/mol·K, T in kelvin)

Exam traps

Watch forHypertonic describes the solution with MORE solute. Water moves toward it.
Watch forFacilitated diffusion uses a protein but no ATP. Protein ≠ active.
Watch forA smaller cell has a LARGER SA:V ratio.

Key terms

  • Prokaryote: A cell with no nucleus or membrane-bound organelles, such as a bacterium.
  • Ribosome: Makes proteins by translating mRNA. Found in every cell.
  • Rough ER: ER with ribosomes. Makes proteins for secretion or for membranes.
  • Golgi complex: Modifies, sorts and ships proteins in vesicles.
  • Lysosome: A vesicle of digestive (hydrolytic) enzymes that breaks down molecules and old organelles.
  • Endosymbiotic theory: Mitochondria and chloroplasts came from bacteria engulfed by an ancestral cell. Evidence: double membranes, own circular DNA and ribosomes.
  • Surface area-to-volume ratio: Smaller cells have more surface per unit of volume, so they exchange materials more efficiently.
  • Fluid mosaic model: The membrane is a fluid phospholipid bilayer with proteins embedded like tiles in a mosaic.
  • Facilitated diffusion: Passive movement down a gradient through a channel or carrier protein. No ATP.
  • Active transport: Moving a substance against its gradient using energy, such as the sodium-potassium pump.
  • Osmosis: Diffusion of water across a selectively permeable membrane toward lower water potential.
  • Hypertonic: A solution with more solute than the cell. The cell loses water.
  • Water potential (Ψ): The tendency of water to move. Water flows from high Ψ to low Ψ.

Practice questions

Five of the 21 Unit 2 questions in Credit Sprint. Every choice is explained in the app.

1. A cell has ribosomes, a plasma membrane and DNA but no nucleus or mitochondria. It is most likely
  1. a virus
  2. a plant cell
  3. an animal cell
  4. a prokaryote, such as a bacterium
Show answer

a prokaryote, such as a bacterium. Prokaryotes lack membrane-bound organelles.

2. Cube A has 1-cm sides and cube B has 3-cm sides. Compared with A, cube B has a
  1. larger surface area-to-volume ratio
  2. smaller surface area
  3. identical SA:V ratio
  4. smaller surface area-to-volume ratio
Show answer

smaller surface area-to-volume ratio. A: 6:1; B: 54:27 = 2:1.

3. A red blood cell is placed in a solution with a higher solute concentration than its cytoplasm. The cell will
  1. swell and burst
  2. take in solutes by active transport until it bursts
  3. shrink as water leaves by osmosis
  4. stay the same size
Show answer

shrink as water leaves by osmosis. The solution is hypertonic.

4. Cholesterol in animal cell membranes helps
  1. make the membrane fully permeable to ions
  2. transport glucose
  3. store genetic information
  4. keep membrane fluidity stable across temperatures
Show answer

keep membrane fluidity stable across temperatures. It prevents both stiffening and excess fluidity.

5. A protist that lives in fresh water constantly pumps out water with a contractile vacuole. Its cytoplasm must be
  1. isotonic to the pond water
  2. hypertonic to the pond water
  3. impermeable to water
  4. hypotonic to the pond water
Show answer

hypertonic to the pond water. Water keeps entering by osmosis.