Every cell is bounded by a plasma membrane that controls what enters and leaves. The CLEP exam tests both the membrane's structure and the several ways materials cross it, often through short data or scenario sets. This is core Molecular and Cellular content.
The plasma membrane is described by the fluid mosaic model: a phospholipid bilayer ("fluid") studded with a "mosaic" of proteins that can drift laterally.
[GRAPH: Cross-section of a membrane. Two rows of lollipop-shaped phospholipids face tail-to-tail: round hydrophilic heads point outward toward water on both sides, wavy hydrophobic tails meet in the center. Several globular proteins span or dot the bilayer; a chain of sugars (glycoprotein) sticks out from the top surface.]
Membrane proteins do many jobs: - Channel and carrier proteins move substances across (transport). - Receptor proteins bind signaling molecules. - Recognition/marker proteins (often glycoproteins) let cells identify one another.
The membrane is selectively permeable: small nonpolar molecules (O₂, CO₂) slip through the tail region easily, while ions and large polar molecules need proteins.
Passive transport moves substances down their concentration gradient (high → low); no ATP is spent.
| Process | What moves | How |
|---|---|---|
| Diffusion | Small nonpolar molecules (O₂, CO₂) | Directly through the bilayer, high → low |
| Osmosis | Water | Across a selectively permeable membrane, from higher to lower water concentration |
| Facilitated diffusion | Ions, glucose, large polar molecules | Through channel/carrier proteins, still high → low, no ATP |
Osmosis is simply the diffusion of water across a selectively permeable membrane. Water moves toward the side with more solute (less free water).
Tonicity compares solute concentration outside vs. inside the cell:
| Solution | Solute outside vs. inside | Water net movement | Animal cell | Plant cell |
|---|---|---|---|---|
| Hypertonic | More solute outside | Out of cell | Shrinks (crenates) | Plasmolyzes (wilts) |
| Hypotonic | Less solute outside | Into cell | Swells, may burst (lyse) | Becomes turgid (wall prevents bursting) |
| Isotonic | Equal | No net movement | Stays normal | Stays flaccid |
The plant cell wall is the key difference: in a hypotonic solution an animal cell can burst, but a plant cell only becomes firm (turgid) because the wall resists further expansion.
Active transport moves substances against their gradient (low → high), which costs energy (ATP). It uses carrier proteins called pumps.
The sodium-potassium (Na⁺/K⁺) pump is the classic example: using one ATP, it pumps 3 Na⁺ out of the cell and 2 K⁺ in, both against their gradients. This maintains the ion imbalances essential for nerve and muscle function.
Very large particles cross the membrane in vesicles, which always requires energy: - Endocytosis: the membrane folds inward to bring material in. Engulfing solids (e.g., a white blood cell taking in a bacterium) is phagocytosis; taking in fluid is pinocytosis. - Exocytosis: a vesicle fuses with the membrane to release material out (e.g., secreting hormones or enzymes).
Water potential is a way to predict which direction water moves: water flows from higher water potential to lower water potential. Pure water has the highest potential; adding solute lowers it. On CLEP you only need the qualitative idea — no formula or calculation. So if a cell's interior has lower water potential than its surroundings (more solute inside), water moves into the cell.
Q1 — E (Phospholipid bilayer with embedded proteins). - Correct: The fluid mosaic model describes a two-layer sheet of phospholipids ("fluid") dotted with proteins ("mosaic"). - A) Single protein layer: proteins are embedded, not the main sheet. B) Cellulose wall: that is a plant cell wall, outside the membrane. C) Solid cholesterol sheet: cholesterol only tucks between phospholipids. D) Nucleic acid layers: DNA/RNA are not membrane structure. - Fix rule: "Fluid mosaic" = phospholipid bilayer + floating proteins.
Q2 — C (Water across a selectively permeable membrane, high → low water). - Correct: Osmosis is specifically the diffusion of water across a selectively permeable membrane toward lower water concentration (more solute). - A) Solutes with ATP: that is active transport of solutes, not water. B) Proteins moving: unrelated. D) Ions against gradient: active transport. E) Large particles in vesicles: endocytosis. - Fix rule: Osmosis = water only, moving toward the saltier (higher-solute) side.
Q3 — C (Facilitated diffusion down the gradient). - Correct: Facilitated diffusion uses proteins but spends no ATP because molecules move down their gradient — it is passive. - A) Na⁺/K⁺ pump, E) active transport into a root: both against the gradient, requiring ATP. B) Endocytosis and D) exocytosis: vesicle processes that require energy. - Fix rule: Down the gradient = passive (no ATP), even when a protein helps.
Q4 — B (Loses water and shrinks). - Correct: In a hypertonic solution (more solute outside), water leaves the cell, so an animal cell shrinks/crenates. - A) Swell and burst: that is the hypotonic outcome. C) No change: describes isotonic. D) Gain water/rigid: opposite direction of water flow. E) Pump in solutes: cells do not respond this way to prevent shrinking here. - Fix rule: Hypertonic outside → water exits → animal cell shrinks.
Q5 — C (Takes in water and becomes turgid). - Correct: In a hypotonic solution water enters the plant cell, but the rigid cell wall stops bursting, so the cell becomes firm (turgid). - A) Bursts immediately: the wall prevents this — the key trap. B) Plasmolysis: happens in hypertonic, not hypotonic. D) Loses water/wilts: wrong water direction. E) No change: water does enter; the cell swells to turgid. - Fix rule: Plant cell in hypotonic → water in, but wall holds → turgid, not burst.
Q6 — B (3 Na⁺ out, 2 K⁺ in, using ATP). - Correct: The pump uses ATP to export 3 sodium ions and import 2 potassium ions per cycle, both against their gradients. - A) 2 Na⁺ in / 3 K⁺ out without ATP: reverses direction and drops the ATP requirement. C) Water by osmosis: the pump moves ions, not water. D) Glucose down a gradient: that is facilitated diffusion. E) Simple diffusion: a pump is active, not diffusion. - Fix rule: Remember "3 out, 2 in, ATP burned" for Na⁺/K⁺.
Q7 — D (Endocytosis). - Correct: Engulfing a whole bacterium in a vesicle is endocytosis (specifically phagocytosis). - A) Exocytosis: releases material out, the opposite. B) Osmosis: water movement. C) Facilitated diffusion: for small solutes, not whole cells. E) Active ion transport: moves single ions, not a bacterium. - Fix rule: Taking a large item in = endocytosis; pushing it out = exocytosis.
Q8 — A (Hypotonic solution). - Correct: Swelling and bursting means water rushed in, which happens when the outside is hypotonic (less solute than the cell). - B) Hypertonic: would shrink the cell. C) Isotonic and E) equal water potential: no net water movement, no swelling. D) Saturated with solutes: that is effectively hypertonic, causing shrinkage. - Fix rule: Cell swells/bursts → surroundings were hypotonic (water floods in).
Q9 — A (Hydrophobic core repels charged ions). - Correct: The nonpolar fatty-acid interior of the bilayer blocks charged ions, so they need channel or carrier proteins. - B) Too large to fit between proteins: size is not the barrier; charge is. C) Dissolve the membrane: ions do not dissolve it. D) Fully permeable: false — the membrane is selectively permeable. E) Covalent bonds with heads: ions do not bond covalently to phospholipids. - Fix rule: Charged/polar particles can't cross the hydrophobic core — they need protein help.
Q10 — D (Moves into the cell). - Correct: Water flows toward lower water potential; if the cell's interior is lower, water enters the cell. - A) and E) Move out: wrong direction — water goes toward lower potential, which is inside here. B) Stationary: a gradient exists, so water moves. C) Only with ATP: osmosis is passive, no ATP. - Fix rule: Water goes high → low water potential; lower inside means water moves in.
Q11 — D (Uses proteins but no energy; moves down the gradient). - Correct: Facilitated diffusion is passive because molecules move down their concentration gradient; the protein just provides a path, no ATP needed. - A) Against the gradient: that would make it active. B) Requires ATP: false for facilitated diffusion. C) Only in dead cells: untrue. E) Water exclusively: facilitated diffusion moves ions and solutes, not only water. - Fix rule: "Passive" is about direction (down-gradient), not whether a protein is involved.
Q12 — E (Tubing gains mass as water enters). - Correct: Water moves by osmosis into the higher-solute sugar solution, so the tubing gains mass — direct evidence of osmosis. - A) Loses mass: implies water left, the wrong direction. B) Water turns sweet immediately: sugar is too large to cross the tubing; this would suggest leakage, not osmosis. C) No change: contradicts osmosis occurring. D) Sugar exits freely: the membrane retains sugar; osmosis moves water, not sugar. - Fix rule: In osmosis, water moves toward the solute — the high-solute side gains mass.
Q1 — E (Phospholipid bilayer with embedded proteins). - Correct: The fluid mosaic model describes a two-layer sheet of phospholipids ("fluid") dotted with proteins ("mosaic"). - A) Single protein layer: proteins are embedded, not the main sheet. B) Cellulose wall: that is a plant cell wall, outside the membrane. C) Solid cholesterol sheet: cholesterol only tucks between phospholipids. D) Nucleic acid layers: DNA/RNA are not membrane structure. - Fix rule: "Fluid mosaic" = phospholipid bilayer + floating proteins.
Q2 — C (Water across a selectively permeable membrane, high → low water). - Correct: Osmosis is specifically the diffusion of water across a selectively permeable membrane toward lower water concentration (more solute). - A) Solutes with ATP: that is active transport of solutes, not water. B) Proteins moving: unrelated. D) Ions against gradient: active transport. E) Large particles in vesicles: endocytosis. - Fix rule: Osmosis = water only, moving toward the saltier (higher-solute) side.
Q3 — C (Facilitated diffusion down the gradient). - Correct: Facilitated diffusion uses proteins but spends no ATP because molecules move down their gradient — it is passive. - A) Na⁺/K⁺ pump, E) active transport into a root: both against the gradient, requiring ATP. B) Endocytosis and D) exocytosis: vesicle processes that require energy. - Fix rule: Down the gradient = passive (no ATP), even when a protein helps.
Q4 — B (Loses water and shrinks). - Correct: In a hypertonic solution (more solute outside), water leaves the cell, so an animal cell shrinks/crenates. - A) Swell and burst: that is the hypotonic outcome. C) No change: describes isotonic. D) Gain water/rigid: opposite direction of water flow. E) Pump in solutes: cells do not respond this way to prevent shrinking here. - Fix rule: Hypertonic outside → water exits → animal cell shrinks.
Q5 — C (Takes in water and becomes turgid). - Correct: In a hypotonic solution water enters the plant cell, but the rigid cell wall stops bursting, so the cell becomes firm (turgid). - A) Bursts immediately: the wall prevents this — the key trap. B) Plasmolysis: happens in hypertonic, not hypotonic. D) Loses water/wilts: wrong water direction. E) No change: water does enter; the cell swells to turgid. - Fix rule: Plant cell in hypotonic → water in, but wall holds → turgid, not burst.
Q6 — B (3 Na⁺ out, 2 K⁺ in, using ATP). - Correct: The pump uses ATP to export 3 sodium ions and import 2 potassium ions per cycle, both against their gradients. - A) 2 Na⁺ in / 3 K⁺ out without ATP: reverses direction and drops the ATP requirement. C) Water by osmosis: the pump moves ions, not water. D) Glucose down a gradient: that is facilitated diffusion. E) Simple diffusion: a pump is active, not diffusion. - Fix rule: Remember "3 out, 2 in, ATP burned" for Na⁺/K⁺.
Q7 — D (Endocytosis). - Correct: Engulfing a whole bacterium in a vesicle is endocytosis (specifically phagocytosis). - A) Exocytosis: releases material out, the opposite. B) Osmosis: water movement. C) Facilitated diffusion: for small solutes, not whole cells. E) Active ion transport: moves single ions, not a bacterium. - Fix rule: Taking a large item in = endocytosis; pushing it out = exocytosis.
Q8 — A (Hypotonic solution). - Correct: Swelling and bursting means water rushed in, which happens when the outside is hypotonic (less solute than the cell). - B) Hypertonic: would shrink the cell. C) Isotonic and E) equal water potential: no net water movement, no swelling. D) Saturated with solutes: that is effectively hypertonic, causing shrinkage. - Fix rule: Cell swells/bursts → surroundings were hypotonic (water floods in).
Q9 — A (Hydrophobic core repels charged ions). - Correct: The nonpolar fatty-acid interior of the bilayer blocks charged ions, so they need channel or carrier proteins. - B) Too large to fit between proteins: size is not the barrier; charge is. C) Dissolve the membrane: ions do not dissolve it. D) Fully permeable: false — the membrane is selectively permeable. E) Covalent bonds with heads: ions do not bond covalently to phospholipids. - Fix rule: Charged/polar particles can't cross the hydrophobic core — they need protein help.
Q10 — D (Moves into the cell). - Correct: Water flows toward lower water potential; if the cell's interior is lower, water enters the cell. - A) and E) Move out: wrong direction — water goes toward lower potential, which is inside here. B) Stationary: a gradient exists, so water moves. C) Only with ATP: osmosis is passive, no ATP. - Fix rule: Water goes high → low water potential; lower inside means water moves in.
Q11 — D (Uses proteins but no energy; moves down the gradient). - Correct: Facilitated diffusion is passive because molecules move down their concentration gradient; the protein just provides a path, no ATP needed. - A) Against the gradient: that would make it active. B) Requires ATP: false for facilitated diffusion. C) Only in dead cells: untrue. E) Water exclusively: facilitated diffusion moves ions and solutes, not only water. - Fix rule: "Passive" is about direction (down-gradient), not whether a protein is involved.
Q12 — E (Tubing gains mass as water enters). - Correct: Water moves by osmosis into the higher-solute sugar solution, so the tubing gains mass — direct evidence of osmosis. - A) Loses mass: implies water left, the wrong direction. B) Water turns sweet immediately: sugar is too large to cross the tubing; this would suggest leakage, not osmosis. C) No change: contradicts osmosis occurring. D) Sugar exits freely: the membrane retains sugar; osmosis moves water, not sugar. - Fix rule: In osmosis, water moves toward the solute — the high-solute side gains mass.