Membrane Transport: How Substances Cross Cell Membranes

Parent hub: Cell Biology, Genetics & Evolution | Prerequisites: Cell Organelles: Overview & How Organelles Work Together, Microscopy and Cell Size: How Microscopes Reveal Cells, Cell Membrane Structure | Next logical topic (separate, not yet written): Cell Division

Quick Definition

Membrane transport is the movement of substances into, out of, or across a cell membrane. Some substances cross on their own, down a concentration gradient (a difference in concentration between two places). Others need a protein to help them, and some are pushed against the gradient using energy from ATP (adenosine triphosphate, the cell's main energy-carrying molecule). Very large packages are carried in vesicles (small membrane sacs) instead. Together, these mechanisms let a cell take in what it needs, remove waste, and keep its internal conditions different from its surroundings — which is essential, because a cell that simply matched its surroundings could not do controlled chemistry.

Why It Matters

Every function of a cell depends on transport. A muscle cell needs calcium ions released at the right moment; a root hair cell needs mineral ions from soil water that is far more dilute than the cell sap; a gut cell needs glucose even after a meal has made blood glucose high; a nerve cell needs a precise difference in sodium and potassium across its membrane before it can fire. If you understand transport, you can predict what happens when a plant wilts, why salt kills slugs, why a red blood cell bursts in pure water, and why a kidney can produce concentrated urine. If you do not, those are just facts to memorise.

Learning Objectives

• Distinguish passive transport (down a gradient, no ATP) from active transport (against a gradient, ATP used).
• Explain simple diffusion, facilitated diffusion and osmosis, and state what moves in each case.
• Use water potential to predict the direction of osmosis, and explain turgid, flaccid and plasmolysed plant cells.
• Distinguish channels from carriers, and explain why carrier transport saturates.
• Explain the sodium–potassium pump, including its 3 Na+ out / 2 K+ in stoichiometry, and why it is electrogenic.
• Explain primary and secondary active transport, including cotransport in the gut and kidney.
• Distinguish endocytosis (phagocytosis, pinocytosis, receptor-mediated) from exocytosis, and give examples.
• Predict, from a new scenario, which mechanism is involved and in which direction a substance moves.

Key Terms

Concentration gradient — a difference in concentration of a substance between two regions; substances tend to spread from high to low concentration. Diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. Facilitated diffusion — diffusion across a membrane with the help of a channel or carrier protein, still down the gradient and without ATP. Osmosis — the diffusion of water across a selectively permeable membrane, from higher water potential to lower water potential. Water potential — a measure of the tendency of water to move; pure water has the highest water potential, and adding solute lowers it. Active transport — movement against a gradient using energy, usually directly from ATP or indirectly from another gradient. Channel — a protein pore through which specific ions or water pass without the protein changing shape. Carrier — a protein that binds a substance, changes shape, and releases it on the other side. Aquaporin — a water channel protein that lets water cross rapidly while excluding ions. Cotransport — transport in which the downhill movement of one substance drives the uphill movement of another. Endocytosis — taking material into a cell in a vesicle formed from the membrane. Exocytosis — releasing material from a cell when a vesicle fuses with the membrane. Vesicle — a small membrane-bound sac used to carry material inside a cell or to the membrane. Turgor pressure — the pressure of the cell contents against the cell wall in a swollen plant cell.

Core Concept

There are only two questions to ask about any transport event, and they answer most exam questions. First: is the substance moving down its gradient or against it? Down the gradient is passive and free; against it costs energy. Second: does it cross the lipid bilayer directly, through a protein, or in a vesicle? Small non-polar molecules cross the bilayer directly. Ions and large polar molecules cannot — the hydrophobic core of the bilayer (explained in Article 2, Cell Membrane Structure) blocks them — so they need a channel or carrier. Very large items need a vesicle. Everything in this lesson is a version of those two questions, applied to different substances.

The Starting Point: Selective Permeability

Article 2 established the rule that predicts most of this lesson. The hydrophobic interior of the phospholipid bilayer lets small, non-polar molecules such as oxygen and carbon dioxide dissolve in it and cross directly, by simple diffusion. Small, uncharged polar molecules such as water cross slowly on their own, and much faster through aquaporins. Ions such as Na+, K+ and Ca2+, and large polar molecules such as glucose and amino acids, cannot cross the hydrophobic core unaided. They need help: a channel, a carrier, a pump, or a vesicle. When a question asks "can this cross unaided?", apply that rule before anything else.

Figure 1. Passive transport follows the gradient and needs no ATP. Active transport goes against the gradient and costs energy — for example, the sodium-potassium pump.

Passive Transport 1: Simple Diffusion

Diffusion happens because particles move randomly. In a region of high concentration, more particles wander out than wander in, so there is a net movement from high to low concentration, even though no individual particle is "trying" to go anywhere. At equilibrium (when concentrations are equal on both sides), particles still move, but there is no net movement.

Four factors control the rate of diffusion: the size of the concentration gradient (a steeper gradient gives faster net diffusion), the surface area available (more area, more crossing), the distance to be crossed (a shorter distance is faster — this is why cells are small, as Article 1 showed with surface area to volume ratio), and temperature (warmer particles move faster). This is why a single-celled organism can rely on diffusion alone, while a large multicellular organism cannot: diffusion over millimetres is far too slow to supply a whole body, so animals and plants use transport systems — blood, xylem and phloem — to bring substances close to cells, and diffusion only has to cover the last, short distance.

In a cell, simple diffusion across the membrane is limited to the substances that can dissolve in the lipid core: oxygen into respiring cells, carbon dioxide out of them, and small non-polar molecules generally. A common error is to say that ions diffuse "through the membrane" unaided. They do not: they diffuse through channels, which is facilitated diffusion, not simple diffusion.

Passive Transport 2: Facilitated Diffusion

Facilitated diffusion is still passive — it goes down the gradient and uses no ATP — but it uses a protein. There are two kinds of protein, and exams test the difference relentlessly (Figure 3).

A channel is a pore. Ions or water pass through it without the protein changing shape. Channels are selective: a potassium channel lets K+ through but largely excludes Na+, because the pore's size and the arrangement of charges inside it fit one ion better than another. Many channels are gated: they open or close in response to a signal, such as a voltage change or the binding of a molecule. Aquaporins are a special case: they are channels for water, letting water cross up to ten times faster than it would through the lipid alone, while excluding ions — including protons (H+), which is important because it keeps the cell's proton gradients intact.

A carrier works differently: it binds the substance on one side, changes shape, and releases it on the other side. Glucose entering a red blood cell uses a carrier (the GLUT family of proteins). Because a carrier must bind, change shape and release, one carrier at a time, carrier transport has a maximum rate: when all carriers are busy, adding more substance cannot speed transport further. This is called saturation, and it is the diagnostic difference between channel and carrier transport in a graph question. Channel transport also levels off eventually, but for a different reason (the pore's capacity), and it is typically much faster per protein than carrier transport.

A useful check: if a question says transport stops when ATP is removed, it is not facilitated diffusion — facilitated diffusion needs no ATP. If it says transport continues without ATP but stops when a specific protein is blocked, it is facilitated diffusion.

Figure 2. A channel is a pore; a carrier binds and changes shape; a pump uses ATP; a cotransporter uses one substance's downhill gradient to drive another uphill.

Osmosis and Water Potential

Osmosis is a special case of diffusion, and it causes more exam errors than any other transport topic, because students state it in terms of concentration when they should state it in terms of water potential. The precise definition is: osmosis is the diffusion of water across a selectively permeable membrane from a region of higher water potential to a region of lower water potential (Figure 2).

Water potential is a measure of how free water is to move. Pure water has the highest water potential (defined as zero). Adding solute lowers water potential, because solute particles bind water and reduce the number of free water molecules. A concentrated sugar solution therefore has a lower water potential than pure water, and water moves by osmosis towards the solution — into the concentrated side, not out of it. Saying "water moves from low concentration to high concentration" is only correct if you mean low solute concentration to high solute concentration, and it is safer to always say "high water potential to low water potential".

Three words describe solutions relative to a cell. A solution is isotonic if it has the same water potential as the cell, so there is no net water movement. It is hypotonic if it has a higher water potential than the cell (more dilute), so water enters the cell. It is hypertonic if it has a lower water potential than the cell (more concentrated), so water leaves the cell. These words are relative: the same solution can be hypotonic to one cell and hypertonic to another, depending on the cell's own water potential.

Figure 3. Water moves by osmosis from higher water potential to lower water potential. A plant cell in water becomes turgid; in a concentrated solution it becomes flaccid, then plasmolysed.

What happens to plant cells

A plant cell in pure water (hypotonic surroundings) takes in water by osmosis. The cell swells, but it does not burst, because the rigid cell wall pushes back. The pressure of the contents against the wall is turgor pressure, and a cell in this state is turgid. Turgor is what keeps a lettuce leaf crisp and a young stem upright — a plant that loses turgor wilts.

A plant cell in a concentrated solution (hypertonic surroundings) loses water. First it becomes flaccid: soft, with no turgor pressure, but the membrane still touches the wall. If it loses more water, the membrane pulls away from the wall, and the cell is plasmolysed. Plasmolysis can be reversed if the cell is returned to water in time, but prolonged plasmolysis kills the cell — which is why salt spread on a slug, or on a weed, draws water out of its cells by osmosis and kills it, and why over-fertilising soil can damage roots.

What happens to animal cells

Animal cells have no cell wall, so the same rules have worse consequences. A red blood cell in pure water swells and bursts — haemolysis. A red blood cell in a concentrated salt solution shrivels — crenation. This is why intravenous fluids must be isotonic with blood: a hypotonic drip would burst red blood cells, and a hypertonic drip would shrivel them.

Worked example: predicting osmosis

Question. A plant cell with a water potential of −0.6 MPa (megapascals, the unit of water potential) is placed in a solution of −0.2 MPa. In which direction does water move, and what happens to the cell?

Answer. Water moves from higher water potential (−0.2 MPa, the solution) to lower water potential (−0.6 MPa, the cell), so water enters the cell. The cell becomes more turgid. Common trap. Students often pick the "more negative" number as the source. Remember: water moves towards the more negative value, not away from it. The more negative value is the destination.

Active Transport: Paying to Go Uphill

Active transport moves a substance against its gradient, so it needs energy. In most cases the energy comes directly from ATP, in a process called primary active transport. The best-known example is the sodium–potassium pump (Figure 1, Figure 3).

The sodium–potassium pump is a protein in the plasma membrane of animal cells that, for each ATP molecule broken down, pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell. Two consequences follow, and both are examined. First, the pump keeps intracellular Na+ low and K+ high, which is the gradient that nerve cells and muscle cells later use: when a nerve impulse travels, Na+ rushes in down the gradient the pump created. Second, the pump is electrogenic: because it moves 3 positive charges out and only 2 in, it makes the inside of the cell slightly more negative, contributing to the membrane potential (the voltage difference across the membrane). Stopping the pump — for example with the poison ouabain, or by removing ATP — collapses both the concentration gradients and, over time, the membrane potential.

Plants and fungi use a similar primary pump: a proton pump (H+-ATPase) that pushes protons out of the cell, creating a proton gradient across the membrane. That gradient is then used for secondary active transport, as below.

A useful diagnostic: active transport is the only kind that can build and maintain a concentration difference against the gradient, and it stops when ATP is removed. If a substance accumulates inside a cell to a higher concentration than outside, some active process must be involved somewhere in its uptake, even if the final step is passive.

Secondary Active Transport and Cotransport

Not all uphill transport uses ATP directly. In secondary active transport, the energy comes from a gradient that was itself created by a primary pump. The classic example is in the gut and kidney: glucose is absorbed from the intestine into gut cells against its gradient, carried by a cotransporter that moves Na+ and glucose together in the same direction (symport). Na+ moves down its gradient (into the cell, where Na+ is kept low by the sodium–potassium pump), and that downhill movement provides the energy to drag glucose uphill with it. Glucose then leaves the gut cell into the blood by facilitated diffusion, down its gradient, through a carrier.

The general pattern is worth learning, because it appears in many questions: a primary pump creates a gradient; a cotransporter spends that gradient to move something else uphill. Other cotransporters move substances in opposite directions (antiport): the sodium–calcium exchanger, for example, lets Na+ in down its gradient while pushing Ca2+ out against its gradient, which is how heart muscle cells keep intracellular calcium low between beats.

Distinguish the two kinds of active transport clearly in an answer. Primary active transport uses ATP directly. Secondary active transport uses a gradient, and ATP only indirectly — the ATP was spent earlier, by the primary pump. A question that asks "does this need ATP?" about a cotransporter expects "not directly; it needs the Na+ gradient, which is maintained by ATP".

Bulk Transport: Endocytosis and Exocytosis

Some cargo is too large for a channel, carrier or pump: a bacterium, a droplet of fluid, a whole protein, or a batch of hormone molecules. Cells move such cargo in vesicles, in a process called bulk transport (Figure 4). Both directions use ATP, because reshaping membranes and moving vesicles costs energy.

Endocytosis brings material in. The membrane folds around the material, pinches off, and forms a vesicle inside the cell. There are three kinds. Phagocytosis ("cell eating") takes in large particles, such as a bacterium engulfed by a white blood cell. Pinocytosis ("cell drinking") takes in small droplets of fluid, with whatever solutes are dissolved in it — it is non-specific, unlike the next kind. Receptor-mediated endocytosis is specific: receptors on the membrane bind a particular molecule (for example, cholesterol carried in low-density lipoprotein, LDL), and only the bound molecules are taken in. This is how cells take up cholesterol for membrane building, and it is the step that fails in familial hypercholesterolaemia, where LDL receptors are defective and cholesterol accumulates in the blood.

Exocytosis sends material out. A vesicle inside the cell fuses with the plasma membrane and releases its contents outside. Examples include the secretion of hormones from endocrine cells, digestive enzymes from pancreatic cells, neurotransmitters from nerve endings, and cell-wall materials from plant cells. In plants, exocytosis also delivers new membrane and wall material to growing regions, such as the tip of a root hair.

Bulk transport is connected to the endomembrane system described in Article 2 and the Cell Organelles lesson: vesicles shuttle between the endoplasmic reticulum, the Golgi apparatus and the plasma membrane, so a protein made in the rough endoplasmic reticulum can be secreted by exocytosis without ever crossing a membrane directly. That is a frequent exam link: secretion is exocytosis, not diffusion, even though the secreted molecule ends up outside the cell.

Figure 4. Endocytosis folds the membrane in to form a vesicle; exocytosis fuses a vesicle with the membrane to release its contents. Both need ATP.

Putting It Together: One Substance, Many Routes

A single substance can use several routes, depending on the cell and the situation. Glucose is the best example. It enters a red blood cell by facilitated diffusion through a carrier, down its gradient. It enters a gut cell by cotransport with Na+, against its gradient, powered by the sodium–potassium pump. It leaves a liver cell by facilitated diffusion when blood glucose falls. A question that asks "how does glucose cross a membrane?" therefore has no single answer — the correct answer always starts "it depends on the cell and the gradient". The same is true of water (simple diffusion through the lipid, or facilitated through aquaporins, or osmosis — all three descriptions can be correct, at different levels of detail), and of ions (channels for passive flow, pumps for active maintenance, cotransporters for coupled movement).

The decision tree from the Core Concept section is the way to handle such questions: down or against the gradient? Bilayer, protein or vesicle? If you can answer those two questions from the information given, the mechanism follows.

Worked Example: Reading a Transport Graph

Question. A graph shows the rate of uptake of a substance against its external concentration. The rate rises steeply at first, then levels off at a maximum, even though external concentration keeps rising. Which mechanism is most likely, and why?

Answer. Carrier-mediated transport (facilitated diffusion or a carrier-based active process). The levelling off is saturation: when all carrier proteins are occupied, more substance cannot increase the rate. Follow-up. How would you distinguish facilitated diffusion from active transport on this graph alone? You cannot — both saturate. You would need a second test: remove ATP. If uptake stops, it is active; if it continues, it is facilitated diffusion. A channel would show a different pattern: a much faster rate that does not saturate in the same way at ordinary concentrations.

Common Misconceptions

• Osmosis is the movement of solute. It is the movement of water; solute does not need to move at all.
• Water moves from low concentration to high concentration. Water moves from high water potential to low water potential; "concentration" without saying of what is ambiguous and often wrong.
• Facilitated diffusion uses ATP because it uses a protein. It does not: it is passive, down the gradient. The protein only provides a route.
• All transport through proteins is active. Channels and carriers are passive; only pumps (and cotransporters, indirectly) are active.
• Active transport means "fast". Speed is not the test; direction against the gradient, and dependence on ATP, are.
• Endocytosis and exocytosis are the same as diffusion. They are bulk transport in vesicles, need ATP, and move whole packages, not single molecules down a gradient.
• A plant cell in pure water bursts. It does not: the wall prevents bursting, and the cell becomes turgid.
• The sodium–potassium pump moves equal numbers of ions. It moves 3 Na+ out and 2 K+ in, which is why it is electrogenic.

Exam Focus

GCSE / IGCSE: diffusion, osmosis and active transport in outline; turgid, flaccid and plasmolysed plant cells; red blood cells in water and in salt solution. A-Level: water potential and its units, facilitated diffusion through channels and carriers, saturation, the sodium–potassium pump, cotransport in the gut and kidney, and endocytosis and exocytosis with examples. AP Biology: membrane transport as the basis for cell communication and homeostasis, aquaporins, and the link between transport and energy. FSc Class 11 and MDCAT: diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis, with plant cell examples and the sodium–potassium pump. BS Botany and introductory university: water potential components (solute potential and pressure potential), secondary active transport, the proton pump in plants, and bulk transport in secretion and the endomembrane system.

Advanced Understanding

Two ideas take this topic beyond the basic syllabus. First, water potential is formally the sum of solute potential (always zero or negative, lowered by solute) and pressure potential (positive in a turgid cell, zero or negative in a plasmolysed cell). The equation, water potential = solute potential + pressure potential, explains why a fully turgid plant cell stops taking in water: as water enters, pressure potential rises until it exactly balances the negative solute potential, and net water potential reaches zero relative to its surroundings, so net osmosis stops — even though the cell still contains solute. Second, membrane potential is not only a nerve phenomenon: every living cell maintains a voltage across its membrane, largely through the sodium–potassium pump and potassium leak channels, and that voltage is itself a form of stored energy that can drive transport, signalling and, in mitochondria and chloroplasts, ATP synthesis. Transport, energy and signalling are therefore not separate topics: the same gradients do all three jobs.

Quick Summary

• Passive transport goes down a gradient and needs no ATP: simple diffusion through the lipid, facilitated diffusion through channels and carriers, and osmosis for water.
• Osmosis is water moving from higher water potential to lower water potential across a selectively permeable membrane.
• Plant cells in water become turgid; in concentrated solution they become flaccid, then plasmolysed. Animal cells burst in water and shrivel in concentrated solution.
• Channels are pores; carriers bind, change shape and release, and saturate at a maximum rate. Aquaporins are water channels.
• The sodium–potassium pump (3 Na+ out, 2 K+ in, using ATP) creates the gradients that nerve and muscle cells use, and is electrogenic.
• Cotransport uses one substance's downhill gradient to drive another substance uphill; the gut glucose cotransporter is the classic example.
• Bulk transport moves large cargo in vesicles: endocytosis in (phagocytosis, pinocytosis, receptor-mediated), exocytosis out. Both need ATP.

FAQ

Is osmosis a type of diffusion? Yes, in the sense that it is net movement caused by random motion, but it is specifically the diffusion of water across a selectively permeable membrane, and it is always stated in terms of water potential, not solute concentration.

Why does facilitated diffusion not need ATP if it uses a protein? Because the protein does not push the substance; it only provides a route. The energy comes from the gradient itself — the substance is moving downhill.

How can a cell take up glucose when blood glucose is already high? In the gut, glucose is taken up by cotransport with Na+, so the Na+ gradient pays for uphill glucose movement. In most other cells, glucose enters by facilitated diffusion down its gradient, which works because the cell constantly uses glucose and keeps its internal concentration low.

What is the difference between pinocytosis and receptor-mediated endocytosis? Pinocytosis takes in fluid non-specifically — whatever is dissolved in the droplet enters. Receptor-mediated endocytosis is specific: only molecules that bind to receptors on the membrane are taken in.

Why do plant roots need active transport for mineral ions? Soil water is usually more dilute in minerals than the root cell sap, so minerals must be taken up against their gradient, which needs ATP. That is also why waterlogged, oxygen-starved roots take up minerals poorly: without oxygen, respiration falls, ATP falls, and active uptake fails.

Does exocytosis need ATP? Yes. Moving vesicles along the cytoskeleton and fusing membranes costs energy, even though the cargo itself is not being pushed against a concentration gradient.

Related Topics

• Cell Biology, Genetics & Evolution (parent hub)
• Cell Biology (hub)
• Cell Organelles: Overview & How Organelles Work Together (prerequisite)
• Microscopy and Cell Size: How Microscopes Reveal Cells (prerequisite)
• Cell Membrane Structure (prerequisite — the bilayer and proteins that make transport possible)
• Cell Division (next logical topic, to be written separately)

Sources (synthesised, not copied): OpenStax Biology 2e, Chapter 5 (Passive and Active Transport); Biology LibreTexts, Membrane Transport; Campbell Biology, Membrane Transport and Cell Signalling. No historical portrait is used. The sodium–potassium pump mechanism is described from standard textbook accounts; no single discoverer is credited in this lesson because its elucidation was a multi-laboratory effort, and inventing a single credit would be misleading.

Related lesson: Cell Division: Mitosis, Meiosis and the Cell Cycle

20 Explained MCQs — answers hidden until opened

Open each question to reveal its answer and explanation.

1. Which statement best defines diffusion?

A. The movement of water only, using ATP   B. The pumping of ions against a gradient by a protein   C. The movement of whole vesicles into a cell   D. The net movement of particles from higher to lower concentration, caused by random motion
Answer: D. Diffusion is net movement down a gradient driven by random motion; no ATP and no vesicle are involved.

2. Oxygen enters a respiring cell mainly by which route?

A. Exocytosis   B. Active transport using ATP   C. Facilitated diffusion through a carrier   D. Simple diffusion directly through the lipid bilayer
Answer: D. Oxygen is small and non-polar, so it dissolves in the lipid core and crosses directly.

3. Why can a sodium ion not cross the lipid bilayer unaided?

A. It is too large to pass any membrane   B. It is destroyed by the membrane   C. It is charged and surrounded by water, so the hydrophobic core blocks it   D. It is too small to fit between lipids
Answer: C. Charged ions cannot pass the water-fearing hydrophobic core without a protein route.

4. Which feature distinguishes a channel from a carrier?

A. A channel moves substances against the gradient   B. A channel always uses ATP, while a carrier never does   C. A channel carries vesicles into the cell   D. A channel forms a pore and does not change shape as substances pass
Answer: D. Channels are open pores; carriers bind, change shape and release. Both are passive in facilitated diffusion.

5. What is facilitated diffusion?

A. Diffusion across a membrane with the help of a channel or carrier, still down the gradient and without ATP   B. Diffusion of water across a membrane   C. The uptake of large particles in vesicles   D. Movement against a gradient using ATP
Answer: A. Facilitated means helped by a protein, but the movement is still passive and downhill.

6. Aquaporins are best described as:

A. Pumps that push water against its gradient   B. Vesicles that carry water into the cell   C. Channels that let water cross rapidly while excluding ions   D. Carriers that bind glucose and change shape
Answer: C. Aquaporins are water channels; they speed water movement but do not use ATP and exclude ions.

7. Osmosis is defined as the diffusion of water:

A. Against its gradient, using ATP   B. From low water potential to high water potential   C. From high water potential to low water potential, across a selectively permeable membrane   D. From high solute concentration to low solute concentration, without a membrane
Answer: C. Water moves towards the lower (more negative) water potential, across a selectively permeable membrane.

8. Pure water has a water potential of zero. Adding solute to water:

A. Makes osmosis impossible   B. Raises its water potential above zero   C. Lowers its water potential below zero   D. Leaves its water potential unchanged
Answer: C. Solute binds water and reduces free water, so water potential becomes negative.

9. A solution is hypertonic to a cell if it has:

A. A lower water potential than the cell   B. A higher water potential than the cell   C. No solute at all   D. The same water potential as the cell
Answer: A. Hypertonic means lower water potential, so water leaves the cell by osmosis.

10. A plant cell placed in pure water becomes turgid because:

A. Water leaves the cell and the wall collapses   B. Active transport pumps water out   C. The membrane dissolves in pure water   D. Water enters by osmosis and the rigid wall pushes back
Answer: D. Water enters down its water-potential gradient; the wall prevents bursting and turgor pressure builds.

11. A plasmolysed plant cell is one in which:

A. The cell wall has dissolved   B. The vacuole has filled completely with water   C. The cell has burst   D. The membrane has pulled away from the cell wall after losing water
Answer: D. Plasmolysis is the membrane pulling away from the wall after extensive water loss.

12. A red blood cell placed in pure water will:

A. Stay unchanged, because animal cells resist osmosis   B. Become plasmolysed   C. Shrivel, because water leaves it   D. Swell and burst, because it has no cell wall
Answer: D. Without a wall, an animal cell cannot resist swelling; in pure water it bursts (haemolysis).

13. The sodium-potassium pump moves, per ATP molecule:

A. 2 Na+ out and 3 K+ in   B. 3 Na+ out and 2 K+ in   C. 3 Na+ in and 2 K+ out   D. Equal numbers of Na+ and K+ in both directions
Answer: B. 3 Na+ out, 2 K+ in is the pump's stoichiometry; the imbalance makes it electrogenic.

14. Why is the sodium-potassium pump described as electrogenic?

A. It generates light energy   B. It uses electricity instead of ATP   C. It moves more positive charges out than in, making the inside more negative   D. It only works in nerve cells
Answer: C. 3 positive charges leave for every 2 that enter, so the cell interior becomes more negative.

15. Carrier-mediated transport saturates because:

A. The membrane becomes impermeable at high concentration   B. ATP runs out at high concentration   C. All carrier proteins become occupied, so the rate cannot rise further   D. Carriers are destroyed at high concentration
Answer: C. Saturation is the diagnostic sign of a carrier: a fixed number of binding sites limits the maximum rate.

16. In the gut, glucose is absorbed against its gradient by:

A. Osmosis through aquaporins   B. A cotransporter that moves Na+ downhill and glucose uphill with it   C. Simple diffusion through the lipid bilayer   D. Phagocytosis of glucose particles
Answer: B. The Na+/glucose cotransporter spends the Na+ gradient, maintained by the sodium-potassium pump, to carry glucose uphill.

17. Secondary active transport differs from primary active transport because it:

A. Does not use any protein   B. Uses a gradient created by a primary pump, rather than ATP directly   C. Moves substances down their gradient only   D. Uses ATP directly at the transporter
Answer: B. Secondary transport spends a gradient; the ATP was used earlier by the primary pump that built that gradient.

18. A white blood cell engulfing a bacterium is an example of:

A. Exocytosis   B. Pinocytosis   C. Facilitated diffusion   D. Phagocytosis
Answer: D. Phagocytosis is endocytosis of large particles; pinocytosis is fluid droplets, and exocytosis is outward.

19. Receptor-mediated endocytosis differs from pinocytosis because it:

A. Uses channels instead of vesicles   B. Takes in fluid non-specifically   C. Releases material from the cell   D. Takes in only molecules that bind to specific receptors
Answer: D. Receptor-mediated uptake is specific; pinocytosis takes in whatever is dissolved in the fluid droplet.

20. Secretion of a hormone from an endocrine cell occurs by:

A. Exocytosis, when a vesicle fuses with the plasma membrane   B. Osmosis   C. Simple diffusion through the lipid bilayer   D. Active transport through a channel
Answer: A. Hormones are packaged in vesicles and released when a vesicle fuses with the membrane.

Separate Interactive Quiz — 20 different questions

These quiz questions are different from the MCQs above. Select an answer for each question. You will not see whether an answer is correct until you submit all 20 questions. Use Reset to try again.