Notes
Organelles
| Structure | Job |
|---|---|
| Ribosomes | Make proteins (free ribosomes for the cytosol; bound to rough ER for secretion and membranes). Found in all cells. |
| Rough ER | Ribosome-studded; folds and modifies proteins headed out of the cell |
| Smooth ER | Makes lipids, detoxifies, stores calcium |
| Golgi complex | Modifies, sorts and packages proteins into vesicles |
| Lysosomes | Hydrolytic enzymes that break down materials (animal cells) |
| Vacuole | Storage; the central vacuole keeps plant cells firm (turgor) |
| Mitochondria | Cellular respiration; folded inner membrane (cristae) increases surface area |
| Chloroplasts | Photosynthesis; thylakoids stacked into grana, surrounded by stroma |
| Nucleus | Holds DNA; site of transcription |
- Prokaryotes (bacteria, archaea) have no nucleus or membrane-bound organelles, but do have ribosomes, DNA, a plasma membrane and usually a cell wall.
- Endosymbiosis: mitochondria and chloroplasts came from engulfed prokaryotes. Evidence: their own circular DNA, their own ribosomes, double membranes, and they divide like bacteria.
Size and surface area
As a cell grows, volume increases faster than surface area, so the surface-area-to-volume ratio falls. Small cells (or folded membranes, villi, root hairs) exchange materials more efficiently.
Membranes and transport
The fluid mosaic model: a phospholipid bilayer with embedded proteins, cholesterol (in animal cells) and carbohydrate chains. Small nonpolar molecules ($\mathrm{O_2}$, $\mathrm{CO_2}$) cross easily; ions and large polar molecules need proteins.
- Passive transport (no energy): diffusion, facilitated diffusion through channels or carriers (like aquaporins), osmosis.
- Active transport (uses ATP): pumps move substances against their gradient, like the $\mathrm{Na^+}/\mathrm{K^+}$ pump.
- Endocytosis and exocytosis move large materials with vesicles.
Water potential and tonicity
Water moves from high water potential to low. $\Psi = \Psi_S + \Psi_P$, with solute potential $\Psi_S = -iCRT$.
- Hypertonic surroundings (more solute outside): the cell loses water. Hypotonic: the cell gains water. Isotonic: no net movement.
Topics in this unit
Each topic has its own short notes and practice.
Practice
All cells make proteins on ribosomes.
Secreted proteins are made on rough ER and packaged by the Golgi.
Circular DNA, their own ribosomes and double membranes all resemble bacteria.
Surface area grows by 4 and volume by 8, so the ratio falls by half.
A protein helps, but no energy is used because it's moving down its gradient.
Water moves toward the higher solute concentration outside.
Both ions move against their gradients using energy.
Small nonpolar molecules dissolve through the hydrophobic core.
Free response
Write your answer first, then open the scoring guide.
Free-response question
Potato cores were placed in sucrose solutions for 24 hours. The percent change in mass was: 0.0 M: +18%, 0.2 M: +6%, 0.4 M: −5%, 0.6 M: −14%.
(a) Estimate the sucrose molarity that is isotonic to the potato cells. Explain.
Show the answer and scoring guide
About 0.3 M, where the percent change in mass would be zero, between +6% at 0.2 M and −5% at 0.4 M.
Check off the points you earned:
(b) Explain the mass change at 0.6 M in terms of water potential.
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The solution has lower water potential than the cells, so water moved out of the cells by osmosis and the cores lost mass.
Check off the points you earned:
(c) Calculate the solute potential of a 0.2 M sucrose solution at 22 °C ($i = 1$, $R = 0.0831$ L·bar/mol·K).
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$\Psi_S = -iCRT = -(1)(0.2)(0.0831)(295) \approx -4.9$ bars.
Check off the points you earned: