Cellular Transport — Print · Grade 10 Science
Chapter: Cells, Membranes & ExchangeCellular Transport
Learning goals
- I can distinguish passive transport (diffusion, osmosis, facilitated diffusion) from active transport and bulk transport.
- I can predict how a cell responds in hypertonic, hypotonic, and isotonic solutions.
Readings for this unit
Before this unit
Compare concentrations and distinguish solute from solvent.
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain. Trace the explained visual parts and write a first model, then attempt the illustrated and changed independent cases. In the third session explain a correction and revise the opening account using evidence.
Start with something to explain
Why can a sample gain mass in one solution and lose it in another?
Illustrative evidence case. Matched tissue models show opposite signed mass changes; permeability and nonpenetrating solutes determine the expected water response.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Why can a sample gain mass in one solution and lose it in another?
Your first model
Label internal/external solution, water direction and the mass measurements.
Build the language from the idea
- diffusion
- The net movement of particles from where they are more concentrated to where they are less concentrated, down the concentration gradient.
- osmosis
- The diffusion of water across a selectively permeable membrane, from higher water concentration (lower solute) to lower water concentration (higher solute).
- passive transport
- Movement of substances across the membrane down their concentration gradient, requiring no energy from the cell.
- facilitated diffusion
- Passive transport in which channel or carrier proteins help specific molecules (like glucose or ions) cross the membrane.
- active transport
- Movement of substances against their concentration gradient, requiring energy (ATP) and carrier proteins.
- endocytosis
- Bulk transport in which the membrane wraps around material and pulls it into the cell inside a vesicle.
- exocytosis
- Bulk transport in which a vesicle fuses with the membrane to release its contents outside the cell.
- tonicity
- A solution’s effect on cell volume through net water movement, depending on effectively nonpenetrating solutes, membrane permeability and the stated conditions.
The cell membrane is selectively permeable, so getting materials across it is a managed operation, not a free-for-all. Two useful categories describe transport across a membrane; large cargo can also move in vesicles. In passive transport, substances drift across on their own, moving down a concentration gradient — from where they are crowded to where they are sparse — and no additional cellular energy supply is required for that passive transfer. In active transport, the cell burns energy to push substances the 'wrong' way, against the relevant electrochemical gradient. Which camp a molecule uses depends on the direction it needs to go and whether it can slip through the membrane unaided.
The simplest passive process is diffusion: particles in constant random motion naturally spread out until they are evenly mixed. Drop dye into still water and it fans out until the color is uniform; that endpoint, with no net movement in any direction, is equilibrium. Because small nonpolar molecules like oxygen and carbon dioxide dissolve right through the phospholipid core, they diffuse across the membrane directly — oxygen flooding into a cell that is busy using it up, carbon dioxide leaking out as waste piles up.
Water gets its own name for this: osmosis, the diffusion of water across a selectively permeable membrane. Under equal pressure and with impermeant solutes in the simple model, net water movement is toward higher effective solute concentration. Molecules do not try to balance a solution; water potential, permeability and pressure determine the outcome. Molecules that are too big or too charged to cross the lipid on their own, such as glucose and ions, take a different passive route called facilitated diffusion, slipping through channel or carrier proteins. It is still passive — the molecules ride their gradient downhill — but they need a protein doorway to get through.
Sometimes a cell must hoard a substance, holding far more of it inside than the surroundings contain — think of a nerve cell stockpiling ions to fire, or your gut absorbing the last traces of nutrients from digested food. Passive movement of ions depends on electrical as well as concentration differences; maintaining a gradient generally requires energy and regulated pathways. So the cell pays for active transport, using an energy source, directly through ATP-driven pumps or indirectly through a coupled gradient, that drive molecules against their gradient. The famous example is the sodium-potassium pump, which spends energy to shove sodium out and haul potassium in, maintaining the imbalance that your nerves and muscles depend on.
For cargo too large for any protein channel — a whole bacterium, a droplet of fluid, a batch of proteins for export — cells use bulk transport, which also costs energy. In endocytosis, the membrane bulges inward around the material and pinches off a vesicle, swallowing it whole (this is how some white blood cells eat invaders). In exocytosis, the reverse happens: a vesicle travels to the membrane, fuses with it, and dumps its contents outside — the route your cells use to release hormones, neurotransmitters, and digestive enzymes.
Osmosis has dramatic, testable consequences captured by tonicity. In a *hypertonic* solution (greater effective concentration of nonpenetrating solutes outside), water rushes out, so an animal cell shrivels and a plant cell's contents pull away from its wall (plasmolysis). In a *hypotonic* solution (lower effective concentration of nonpenetrating solutes outside), water floods in: an animal cell swells and can burst, while a plant cell becomes firm and turgid — safe, because its rigid wall pushes back. In an *isotonic* solution effective osmotic conditions match, so water moves both ways equally and the cell holds steady. This is exactly why hospital IV drips are made isotonic to blood, why a salted slug shrivels, and why controlling water balance is a front-line job of staying alive — the very definition of homeostasis in the next lesson.
Diffusion is net movement down a concentration gradient due to random motion, while individual particles continue moving both ways. At equilibrium there is no net movement, not an end to molecular motion. Membrane permeability determines which particles can actually cross.
Osmosis is net water movement across a selectively permeable membrane in response to differences in water potential. In a simple equal-pressure model with impermeant solutes, water tends to move toward the higher effective solute concentration. Permeating solutes and pressure complicate real cases.
Active transport can move particles against a gradient using energy, directly or through coupling to another gradient. Distinguish active transport from facilitated diffusion. Vesicle transport handles larger cargo and involves membrane changes rather than passage through a simple channel.
Build your understanding
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain.
Part 1 · Gradients, and what moving down one costs
A concentration gradient is a change in amount per volume across space. Diffusion arises from random molecular motion, producing net transfer down a suitable gradient when a pathway is available. At dynamic equilibrium, molecules still move but opposing average transfers balance. For ions, electrical differences also matter, giving an electrochemical gradient; concentration alone may not predict net direction. Passive transport needs no direct cellular energy input to drive that down-gradient step, although maintaining a prior gradient may require energy elsewhere.
Read the visual model
Read particle count per equal volume and distinguish net transfer from each random movement.
Does zero net flux mean the molecules have stopped moving?
Visual reasoning — after your attempt
No. Opposing transfers can balance while molecular motion continues.
Particles in solution move constantly and randomly. Where a substance is more concentrated in one region than another, that random motion produces a net movement from high to low concentration until the difference disappears. This is diffusion, and it costs the cell nothing: the energy comes from the thermal motion of the particles themselves. Diffusion occurs in a beaker as readily as in a cell.
Equilibrium does not mean stillness. At equal concentrations, particles continue to cross in both directions at equal rates, so there is no net change while exchange continues. Describing an isotonic cell as one where "water stops moving" misstates the mechanism, and the distinction matters when interpreting any experiment where mass or volume stays constant.
Pause and explain
Why does diffusion require no energy from the cell, when the substance clearly moves?
Check your reasoning after answering
The movement comes from the random thermal motion the particles already have. Net movement down a gradient is simply the statistical outcome of that motion, so the cell supplies nothing; only movement against a gradient needs an energy input.
Part 2 · Osmosis and tonicity
Osmosis is net water movement across a selectively permeable membrane under a difference in water chemical potential. A useful simple model compares nonpenetrating-solute concentration: water tends to enter a cell in a dilute, hypotonic external solution and leave in a hypertonic one. Tonicity describes the sustained volume effect of effectively nonpenetrating solutes, not every dissolved particle indiscriminately. Cell walls and pressure can resist swelling, and some solutes cross. State membrane properties and initial conditions before predicting a volume outcome.
Read the visual model
The solute-marker count remains while increased water volume dilutes the interior in the model.
Why might a plant cell resist bursting in a hypotonic solution?
Visual reasoning — after your attempt
Its cell wall can oppose expansion and support pressure buildup; the water movement model must include that mechanical response.
Osmosis is the diffusion of water across a selectively permeable membrane, and the direction depends on solute concentrations on each side. Water moves toward the side with more dissolved solute. A hypotonic surrounding has less solute than the cell, so water enters; a hypertonic surrounding has more, so water leaves; an isotonic surrounding produces no net movement. The terms describe the solution relative to the cell, so they are comparisons, not fixed labels.
Consequences depend on what encloses the cell. An animal cell in a hypotonic solution swells and can burst; in a hypertonic solution it shrivels. A plant cell has a rigid wall, so water entering raises internal pressure until the wall resists further entry and the cell becomes turgid, which is what holds a non-woody plant upright. In a hypertonic solution the plant cell's contents pull away from the wall and the plant wilts.
Pause and explain
Why does distilled water burst an animal cell but leave a plant cell firm?
Check your reasoning after answering
Water enters both by osmosis, but the plant cell's rigid wall builds up pressure that opposes further entry, stopping the process at turgidity. An animal cell has no wall, so water keeps entering until the membrane fails.
Part 3 · When a protein is needed, and when energy is too
Facilitated diffusion uses a channel or carrier for down-gradient transport. Active transport couples energy transfer to movement against the relevant driving gradient, directly through ATP or indirectly through another gradient. Primary active transport uses a direct energy source; secondary transport can use a gradient maintained by other processes. A pump and a passive carrier can both become saturated as capacity is limited. Therefore, protein involvement or a plateau alone does not establish whether the step is active; use direction, gradients, coupling and controlled energy perturbations.
Read the visual model
Follow the shown down-gradient route and identify what additional feature an active comparison would require.
Would a saturating uptake curve alone distinguish facilitated diffusion from active transport?
Visual reasoning — after your attempt
No. Both can have limited protein capacity; gradient direction and energy coupling provide additional tests.
Substances that cannot cross the bilayer need a protein. In facilitated diffusion, a channel or carrier provides a route and the substance still moves down its gradient, so no energy is required; glucose entering a cell works this way. Because the route is a finite set of proteins, uptake rises with concentration and then plateaus when the carriers are saturated — a signature that distinguishes protein-mediated transport from simple diffusion, which does not plateau.
Active transport moves a substance against its gradient and therefore requires energy, usually from ATP hydrolysis coupled to a pump protein. This is how root cells accumulate potassium above the soil's concentration and how nerve cells maintain their sodium and potassium differences. Large quantities move differently again: endocytosis encloses material in a vesicle brought inward, exocytosis fuses a vesicle with the membrane to release contents, and both depend on membrane fluidity and consume energy.
Pause and explain
Uptake of a substance rises with concentration and then levels off. What does the plateau rule out?
Check your reasoning after answering
It rules out simple diffusion through the bilayer, which would keep increasing with the gradient. A plateau indicates a limited number of transport proteins that become saturated, so the substance is crossing through carriers or channels.
Part 4 · Reading a transport experiment
An experimental response needs a defined starting value and consistent handling. In this supplied tissue model, mass rises from 10 to 13 g in one condition and falls to 7 g in another. Percent change is (final−initial)/initial×100, giving +30% and −30%. Surface water, leaked solute, tissue damage and unequal blotting can affect mass, so the inference is stronger when those conditions are controlled. A mass curve alone does not directly measure every molecule crossing or identify the original cytosolic solute concentration.
Read the visual model
Compare signed changes from the same starting mass and preserve the interval.
Does a 3 g mass gain necessarily mean precisely 3 g of pure water crossed an intact membrane?
Visual reasoning — after your attempt
Not without the stated controls; adhered solution, solute transfer or damage may contribute. The simple model assumes those alternatives are limited.
Most classroom evidence about transport is indirect: mass change in potato strips, volume change in dialysis tubing, colour change as an indicator crosses a membrane. Interpreting it requires stating which substance moved, in which direction, and what the membrane allowed. Mass gain in a potato strip means water entered, so the solution was hypotonic to the cells; the strip's cells are the reference, which is why the same solution can be hypotonic to one tissue and hypertonic to another.
The strongest design brackets the answer. Testing several solute concentrations and finding the one that produces no mass change estimates the cells' internal concentration, and the precision of that estimate is set by how finely the concentrations were spaced. Replicate strips are necessary because tissue varies, and surface area and soaking time must be held constant or they become alternative explanations for the mass change.
Pause and explain
Potato strips in 0.2 M sucrose gain mass and in 0.4 M lose mass. What can you conclude, and how precisely?
Check your reasoning after answering
The cells' internal solute concentration lies between 0.2 and 0.4 M, since water entered in the weaker solution and left in the stronger one. The data bracket the value but do not locate it; testing 0.25, 0.30, and 0.35 M would narrow the estimate.
Apply the whole unit · Interpret a potato osmosis series
Illustrative replicate data. Potato strips of equal size soaked for 60 minutes, percent change in mass, three strips per solution. Distilled water: +8, +9, +10. 0.2 M sucrose: +2, +3, +4. 0.4 M sucrose: −5, −6, −7. 0.6 M sucrose: −11, −12, −13. Temperature, strip dimensions, and soaking time were held constant.
Your task
Calculate the mean percent change for each solution, determine the interval in which the cells' internal solute concentration must lie and justify the bracket, and explain why mass change is a valid measure of water movement here. Then identify the uncontrolled factor most likely to distort the result and the next concentrations you would test.
What your response should show
- Choose the appropriate biological model or relationship and state the conditions it assumes.
- Show the counts, rates, ratios, or intermediate reasoning the question requires.
- Use the supplied evidence to justify the conclusion and separate observation from inference.
- Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt
Means: distilled water +9, 0.2 M +3, 0.4 M −6, 0.6 M −12 percent, with ranges of 2 percentage points in each group.
Find the sign change: mass increases through 0.2 M and decreases from 0.4 M, so the isotonic point lies between 0.2 and 0.4 M.
Justify the bracket: water entered when the external solution was more dilute than the cell contents and left when it was more concentrated, so the crossover is where the two match.
Validate the measure: the strips are the same size and soaked for the same time, and sucrose does not cross the membrane appreciably, so a change in mass reflects water gained or lost rather than solute uptake.
Identify the distortion risk: surface area and blotting technique change how much water clings to the strip; inconsistent blotting would shift masses by a few percent, which is comparable to the 0.2 M result.
Next test: 0.25, 0.30, and 0.35 M sucrose with at least five strips each, blotted by a fixed procedure.
The mean changes are +9, +3, −6, and −12 percent, and because within-group ranges are only about 2 percentage points the trend is well supported. Mass rose in distilled water and 0.2 M sucrose and fell in 0.4 M and above, so the cells' internal solute concentration lies between 0.2 and 0.4 M sucrose equivalent; the data bracket that value but do not pin it down. Mass change is a legitimate proxy for water movement because strip size and soaking time were held constant and sucrose does not cross the membrane appreciably, so the mass that moved was water. The most likely distortion is inconsistent blotting, since surface water of a few percent of strip mass is the same size as the 0.2 M signal. Testing 0.25, 0.30, and 0.35 M with five strips each and a fixed blotting procedure would locate the isotonic point more closely.
Scientific references and adapted source figures
Parent / teacher teaching notes
Insist that hypotonic and hypertonic are stated as comparisons — "hypotonic to the potato cells" — because a solution has no tonicity by itself and the missing reference is the most common source of confusion. The saturation plateau is the other idea worth checking: it is the observable difference between diffusion through the bilayer and transport through proteins, and students who treat all passive movement as identical cannot explain it. The potato series runs well at home over an afternoon. Ask for an explanation of each label, arrow and axis before calculation. Keep first and revised models; attempt the changed case before consulting guidance. Distinguish invented instructional records from named historical sources. Source chromosome symbols are allele-category models, not DNA sequences. These independently authored high-school supplements are not the full OpenSciEd teacher editions or an endorsement.
The key idea
Passive transport follows the relevant gradient without an added cellular energy input. Active transport can oppose an electrochemical gradient using a coupled energy source. Vesicles transport cargo through membrane changes. Tonicity describes volume effects of effectively nonpenetrating solutes under the stated conditions.
Surface-Area-to-Volume Ratio (cube)
SA:V = 6s² ÷ s³ = 6 / s
s is the side length of a cube-shaped cell; the ratio shrinks as the cell grows, so larger cells exchange materials across their membrane less efficiently.
Surface-Area-to-Volume Ratio (sphere)
SA:V = 4πr² ÷ (4/3)πr³ = 3 / r
r is the radius of a spherical cell; like the cube, the ratio falls as r increases, a key reason cells stay small or divide.
Worked examples
Example 1
- Hypertonic means more solute outside than inside, so water leaves both cells by osmosis.
- The animal cell has no wall, so as it loses water it shrivels (crenates).
- The plant cell also loses water; its membrane and contents pull away from the rigid cell wall — a state called plasmolysis — and the plant wilts.
Example 2 · Worked application
- Identify lower external solute concentration under the simple nonpenetrating-solute model.
- Water moves across a water-permeable membrane toward the higher effective solute concentration.
- The cell may swell, depending on its wall and regulation.
State permeability and effective solute conditions; not every solute is trapped.
Example 3
- Passive transport only moves substances down their gradient, toward equal concentrations.
- Here the cell is building an internal concentration far higher than outside — moving iodine against its gradient.
- Moving against a gradient requires energy (ATP) and pumps, which is the definition of active transport.
Example 4
- The external solution has the higher effective solute concentration.
- Water moves out in the simple osmosis model.
Example 5 · Find and fix the mistake
- Test the claim against the stated conditions: State permeability and effective solute conditions; not every solute is trapped.
- Net osmosis follows differences in water chemical potential; the simple model predicts movement toward higher nonpenetrating solute concentration.
State permeability and effective solute conditions; not every solute is trapped.
Example 6 · Reason from the visual evidence
- Facilitated diffusion uses a channel or carrier for down-gradient transport. Active transport couples energy transfer to movement against the relevant driving gradient, directly through ATP or indirectly through another gradient. Primary active transport uses a direct energy source; secondary transport can use a gradient maintained by other processes. A pump and a passive carrier can both become saturated as capacity is limited. Therefore, protein involvement or a plateau alone does not establish whether the step is active; use direction, gradients, coupling and controlled energy perturbations.
- No. Both can have limited protein capacity; gradient direction and energy coupling provide additional tests.
Original facilitated transport schematic. The protein symbolizes a route, not a measured ion pump, ATP stoichiometry or complete electrical gradient.
Avoid these mistakes
- Equilibrium does not mean that particles stop moving.
Check your understanding
A cell is placed in an equal-pressure solution containing more impermeant solute than its interior. Predict initial water movement.
Discuss the answer
The cell initially loses water; the final behavior also depends on cell structure and pressure.
How does facilitated diffusion differ from active transport?
Discuss the answer
Facilitated diffusion is down the relevant driving gradient through a protein without direct energy input to that step. Active transport couples energy to movement against its driving gradient; protein use alone does not distinguish them.
Return to the phenomenon
Return to the initial case. Why can a sample gain mass in one solution and lose it in another? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Gradients, membrane pathways and mechanical conditions determine transport; observed mass change must be interpreted with handling and solute-transfer controls.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, explain and revise
Complete these tasks after the teaching parts. Preserve your first model and a supported revision; use guidance after a real attempt.
Activity 1 · Return to the opening case
Use the opening question, defined terms, Parts 1–4 and your notebook. The supplied-data route needs no equipment or external account.
Your task
Return to the initial case. Why can a sample gain mass in one solution and lose it in another? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use relevant stated observations.
- Explain the mechanism with a labeled model.
- Distinguish quantities and assumptions.
- Describe a revision and a useful next test.
Hint
Follow objects, materials and information separately. Check what each measured quantity means.
Response guidance — discuss after your attempt
Gradients, membrane pathways and mechanical conditions determine transport; observed mass change must be interpreted with handling and solute-transfer controls.
Activity 2 · Transfer to a changed case
Cover the worked guidance and use the unit explanations/figures to solve a changed problem.
Your task
A tissue sample changes from 12 g to 9 g in a stated nonpenetrating-solute model. Find percentage change and explain a suitable osmotic account plus one measurement control.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name the stated conditions, select the relevant mechanism and show intermediate reasoning.
Response guidance — discuss after your attempt
The change is −3/12×100 = −25%. Net water loss is consistent with a hypertonic external solution under the model. Comparable blotting, membrane integrity and limits on solute loss are needed before assigning all mass change to water.
Words to know
- diffusion
- The net movement of particles from where they are more concentrated to where they are less concentrated, down the concentration gradient.
- osmosis
- The diffusion of water across a selectively permeable membrane, from higher water concentration (lower solute) to lower water concentration (higher solute).
- passive transport
- Movement of substances across the membrane down their concentration gradient, requiring no energy from the cell.
- facilitated diffusion
- Passive transport in which channel or carrier proteins help specific molecules (like glucose or ions) cross the membrane.
- active transport
- Movement of substances against their concentration gradient, requiring energy (ATP) and carrier proteins.
- endocytosis
- Bulk transport in which the membrane wraps around material and pulls it into the cell inside a vesicle.
- exocytosis
- Bulk transport in which a vesicle fuses with the membrane to release its contents outside the cell.
- tonicity
- A solution’s effect on cell volume through net water movement, depending on effectively nonpenetrating solutes, membrane permeability and the stated conditions.
Surface-area-to-volume ratio: why cells stay small
A cell can only survive if its membrane can exchange materials fast enough to serve its whole interior — and geometry works against size. As a cell grows, its volume (the demand) climbs faster than its surface area (the supply): for a cube the SA:V ratio is 6/s, and for a sphere it's 3/r, both shrinking as the cell gets bigger. That is why cells stay microscopic or divide instead of ballooning, and why cells that must absorb a lot — like those lining your intestine — sprout thousands of tiny folds called microvilli to multiply their surface area.
Osmoregulation in extreme places
Living in the wrong tonicity is a constant threat, and animals have astonishing fixes. A freshwater fish is hypertonic to its surroundings, so water floods in and it must produce huge volumes of dilute urine to bail it out; a saltwater fish faces the opposite problem and constantly drinks seawater while pumping salt out through its gills. The single-celled Paramecium uses a contractile vacuole that spends energy to squeeze out the water osmosis keeps forcing in — a tiny bilge pump keeping the cell from bursting.
The pump that powers your thoughts
The sodium-potassium pump works nonstop in every one of your cells, using ATP to force sodium out and potassium in against their gradients. Maintaining that imbalance is expensive — a large share, roughly a quarter, of the energy your body burns at rest goes to running these pumps. The payoff is enormous: the charge difference they build across the membrane is the stored voltage that nerve cells discharge to fire signals and muscles use to contract. Every thought and heartbeat rides on active transport.
Facilitated diffusion, glucose, and diabetes
Glucose is too large and polar to cross the bilayer alone, so it rides through carrier proteins called glucose transporters (GLUTs) by facilitated diffusion. In muscle and fat cells, these transporters mostly sit in storage until insulin signals them to move to the membrane and let glucose in. In type 2 diabetes, cells stop responding well to insulin, the transporters stay hidden, and glucose builds up in the blood — a direct link between a membrane-transport protein and one of the world's most common diseases.
Misconception: cells don't 'suck' things in
It's easy to imagine a cell actively pulling in nutrients, but diffusion and osmosis are passive and blind: particles simply move by random motion until concentrations even out, with no effort or 'choice' by the cell. The cell only spends energy when it needs to move something against its gradient, through active transport. Keeping this straight matters — mislabeling passive processes as active is one of the most common errors students make on transport questions.
In the real world
Tonicity is life-or-death in medicine and nature: IV fluids must be isotonic so blood cells don't burst or shrivel, saltwater fish must constantly pump out salt to keep from dehydrating, and salting or sugaring food preserves it by making the surroundings so hypertonic that invading microbes lose their water and die.
Exit check
Compare net diffusion and active transport.
Check your explanation after answering
Net diffusion follows an appropriate gradient without direct metabolic energy input; active transport uses an energy source to move substances against their electrochemical gradient.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
- protein
- water
- oxygen
- glucose
Answer and explanation
water
Osmosis specifically refers to the movement of water across a selectively permeable membrane toward the side with more solute.
Problem 2
- It never uses proteins
- It only moves water
- It requires energy (ATP) and can move substances against their concentration gradient
- It moves substances down their concentration gradient
Answer and explanation
It requires energy (ATP) and can move substances against their concentration gradient
Active transport spends energy to move substances against the gradient; passive transport moves them down the gradient for free.
Problem 3
- Water leaves and the cell shrivels
- Nothing changes
- The cell builds a cell wall
- Water enters by osmosis and the cell swells and may burst
Answer and explanation
Water enters by osmosis and the cell swells and may burst
Pure water is hypotonic, so water rushes in by osmosis; with no wall to resist, the animal cell swells and can burst (lysis).
Problem 4
Answer and explanation
In endocytosis, the membrane wraps around material and pulls it into the cell inside a vesicle (bringing things in). In exocytosis, a vesicle fuses with the membrane and releases its contents outside the cell (sending things out). Both move large amounts of material and require energy.
Problem 5
Answer and explanation
1 charges
3 positive charges leave and 2 positive charges enter, so the net is 3 − 2 = 1 positive charge moved out.
Problem 6
- So there is no net osmosis — the blood cells neither swell and burst nor shrivel
- So the cells absorb as much water as possible
- So the cells lose water quickly
- Because isotonic fluids contain no water
Answer and explanation
So there is no net osmosis — the blood cells neither swell and burst nor shrivel
An isotonic fluid matches the solute concentration of blood, so water moves in and out equally and red blood cells keep their normal shape.
Problem 7
Answer and explanation
The cell initially loses water; the final behavior also depends on cell structure and pressure.
The external solution has the higher effective solute concentration.
Water moves out in the simple osmosis model.
Response criteria
- Use the stated conditions.
- Show the evidence or calculation.
- Explain the conclusion and any stated limitation.
Problem 8
Hints
State permeability and effective solute conditions; not every solute is trapped.
Answer and explanation
The pump uses coupled energy; facilitated diffusion moves particles down their electrochemical gradient.
Problem 9
Hints
State what the supplied observations or conditions establish, and separate that from any assumption.
Answer and explanation
Its cell wall resists expansion, allowing turgor pressure to balance water entry.
Problem 10
Answer and explanation
No. Both can have limited protein capacity; gradient direction and energy coupling provide additional tests.
Facilitated diffusion uses a channel or carrier for down-gradient transport. Active transport couples energy transfer to movement against the relevant driving gradient, directly through ATP or indirectly through another gradient. Primary active transport uses a direct energy source; secondary transport can use a gradient maintained by other processes. A pump and a passive carrier can both become saturated as capacity is limited. Therefore, protein involvement or a plateau alone does not establish whether the step is active; use direction, gradients, coupling and controlled energy perturbations.
No. Both can have limited protein capacity; gradient direction and energy coupling provide additional tests.
Response criteria
- Read the structures, axes, directions and units correctly.
- Connect the evidence with the biological mechanism.
- Respect stated model assumptions and distinguish an observation from a further inference.
Problem 11
Answer and explanation
The change is −3/12×100 = −25%. Net water loss is consistent with a hypertonic external solution under the model. Comparable blotting, membrane integrity and limits on solute loss are needed before assigning all mass change to water.
The change is −3/12×100 = −25%. Net water loss is consistent with a hypertonic external solution under the model. Comparable blotting, membrane integrity and limits on solute loss are needed before assigning all mass change to water.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.