Membrane Transport: Study Guide (Biology 2.5)

Why cells keep gradients

A cell is not a bag that slowly comes to match its surroundings. Inside a typical animal cell, potassium ions (K⁺) are many times more concentrated than in the fluid outside, while sodium ions (Na⁺) are much more concentrated outside. A root hair cell holds mineral ions at higher concentrations than the soil water around it. These differences are concentration gradients: differences in the concentration of a solute across a membrane.

Gradients are possible because the plasma membrane is selectively permeable (Topic 2.4). Its hydrophobic interior lets small nonpolar molecules such as O₂ and CO₂ through, but blocks ions and large polar molecules unless a protein provides a route. A cell can therefore control which substances cross, in which direction and how fast. That control lets it keep a steady internal environment that differs from the outside. This is how organisms keep their solute and water balance.

This topic sorts the ways substances cross a membrane into three groups:

1. Passive transport: movement down a gradient, with no direct use of metabolic energy.
2. Active transport: movement that needs a direct input of energy, often against a gradient.
3. Bulk transport (endocytosis and exocytosis): large substances or large amounts carried inside membrane vesicles, also using energy.

Concentration gradients and diffusion

Molecules in a liquid or gas move constantly and randomly. If there are more molecules of a solute on one side of a membrane, more of them will hit the membrane and cross from that side, simply because there are more of them. Fewer cross back the other way. The result is a net movement from high to low concentration. This is diffusion.

Two points matter for exam answers:

• Molecules do not “want” to spread out. Net movement is a result of random motion and unequal numbers.
• When the concentrations become equal, movement does not stop. Molecules keep crossing in both directions at equal rates, so there is no net movement. This is a dynamic equilibrium.

The steeper the gradient (the bigger the difference in concentration), the faster the net movement, other things being equal.

Passive transport

Passive transport is the net movement of a substance from a region of higher concentration to a region of lower concentration, without a direct input of metabolic energy. The energy comes from the random motion of the molecules themselves, not from ATP.

There are three types, and you meet two of them in more detail soon:

• Simple diffusion through the phospholipid bilayer, for small nonpolar molecules such as O₂, CO₂ and N₂. Oxygen diffuses into a respiring cell because the cell uses it up, keeping the inside concentration low.
• Facilitated diffusion through a channel or carrier protein, for ions and large polar molecules such as glucose. A protein is involved, but the process is still passive: it only works down the gradient (Topic 2.6).
• Osmosis, the passive movement of water across a membrane (Topic 2.7).

For an uncharged solute, passive transport can never build up the substance above its concentration on the other side. At best it brings the two sides to equal concentration. (For ions, the charge across the membrane also matters; see Topic 2.6.)

Active transport

Active transport moves substances across a membrane using a direct input of energy, usually from the hydrolysis of ATP. It always needs a specific membrane protein, often called a pump. In many cases, active transport moves a substance from a region of low concentration to a region of high concentration: against its gradient. This is how a cell can accumulate a nutrient it is short of, or remove a substance it must keep low.

Examples you should recognise:

• Root hair cells take up mineral ions such as nitrate from soil water in which they are already less concentrated than in the cell.
• Animal cells use a pump that moves Na⁺ out and K⁺ in, keeping the gradients described at the start of this guide. You study how it works, and how it affects the charge across the membrane, in Topic 2.8.

Because active transport depends on ATP, anything that stops ATP production (lack of oxygen for an aerobic cell, a metabolic poison, low temperature that slows respiration) slows or stops it. Passive transport carries on, as long as a gradient exists.

Bulk transport: endocytosis and exocytosis

Some substances are far too large for any channel or pump: whole bacteria, droplets of fluid, large proteins. Cells also sometimes need to move a very large amount of material at once. For these jobs they use vesicles, small sacs bounded by membrane.

Endocytosis brings material into the cell. The plasma membrane folds inward around the material, and the fold pinches off to form a new vesicle inside the cell. Examples:

• Phagocytosis (“cell eating”): a white blood cell engulfs a bacterium. A single-celled amoeba takes in food particles the same way.
• Pinocytosis (“cell drinking”): the cell takes in small droplets of the surrounding fluid with whatever is dissolved in it.
• Some cells take in specific substances that first bind to receptor proteins on the surface (receptor-mediated endocytosis). You only need the general idea here.

Exocytosis moves material out of the cell. A vesicle, often made by the Golgi apparatus, moves to the plasma membrane, fuses with it and releases its contents outside. Examples include cells of the pancreas releasing digestive enzymes and the hormone insulin, and nerve cells releasing neurotransmitters.

Both processes need energy. The cell uses ATP to reshape the membrane and to move vesicles along the cytoskeleton. Notice also that the cargo never passes through the hydrophobic interior of the membrane: it stays wrapped in membrane the whole time.

Membrane bookkeeping. Exocytosis adds membrane to the cell surface; endocytosis removes it. A vesicle 100 nm (0.10 µm) across has a surface area of 4πr² = 4π × (0.05 µm)² ≈ 0.0314 µm². A spherical cell 20 µm across has a surface area of about 1257 µm². So about 4000 vesicles fusing would add 10% to the cell surface. Secreting cells balance this by taking membrane back in by endocytosis.

Comparing the mechanisms

Feature	Passive transport	Active transport	Endocytosis / exocytosis
Direction	high → low concentration only	can be low → high	in (endo) or out (exo), regardless of gradient
Direct metabolic energy (ATP)?	no	yes	yes
Membrane protein needed?	only for facilitated diffusion	yes, a specific pump	vesicle formation uses proteins; cargo is not carried by a pump
What moves	small molecules, ions, water	ions and small molecules	large molecules, particles, large amounts of fluid
Example	O₂ into a respiring cell	mineral ions into root hair cells	white blood cell engulfing a bacterium; release of insulin

Worked example 1: is uptake passive or active?

Question. Cultured cells are placed in a large volume of medium containing an amino acid with no overall charge at 2.0 mmol L⁻¹. Because the volume is large, the outside concentration stays at 2.0 mmol L⁻¹. One batch of cells is untreated. A second batch is given a metabolic inhibitor that blocks ATP production. The concentration of the amino acid inside the cells is measured over 60 minutes (fictional data).

Time / min	0	5	10	20	40	60
Untreated, inside / mmol L⁻¹	0	2.4	4.2	6.4	8.2	8.8
With inhibitor, inside / mmol L⁻¹	0	0.8	1.3	1.8	2.0	2.0

(a) Calculate the mean rate of uptake in the first 5 minutes for each batch. (b) Calculate the ratio of inside to outside concentration at 60 minutes for each batch. (c) Use the data to decide how the amino acid enters, and justify your answer.

(a) Initial rates.

1. Untreated: 2.4 mmol L⁻¹ ÷ 5 min = 0.48 mmol L⁻¹ min⁻¹.
2. With inhibitor: 0.8 mmol L⁻¹ ÷ 5 min = 0.16 mmol L⁻¹ min⁻¹.
3. The untreated cells take up the amino acid 0.48 ÷ 0.16 = 3.0 times as fast at the start.

(b) Inside : outside ratios at 60 minutes.

1. Untreated: 8.8 ÷ 2.0 = 4.4. The inside is more than four times as concentrated as the outside.
2. With inhibitor: 2.0 ÷ 2.0 = 1.0. The two sides have reached equal concentration.

(c) Interpretation.

• By 5 minutes, the untreated cells already hold more amino acid (2.4 mmol L⁻¹) than the medium (2.0 mmol L⁻¹). From then on, every further rise is movement from low to high concentration. Passive transport cannot do this, so the untreated cells must use active transport.
• When ATP production is blocked, uptake is slower and stops when inside equals outside. That is the pattern expected for passive transport: net movement down the gradient until equilibrium.
• Together, the two batches suggest the cells have both a passive route and an ATP-dependent route for this amino acid. The inhibited cells reach only 2.0 ÷ 8.8 ≈ 23% of the untreated level.

Check. The answer uses both lines of evidence: the inside rising above the outside, and the effect of removing ATP. Either alone is weaker. The amino acid has no overall charge, so you can compare concentrations directly; for ions, charge also matters (Topic 2.8).

Worked example 2: evaluating an experiment and proposing a better one

Question. A student wants to show that a freshwater amoeba needs energy to take in yeast cells by endocytosis. She mixes amoebae with yeast cells stained with a dye, at 20 °C and at 4 °C. After 30 minutes she counts the stained vesicles in 20 amoebae at each temperature (fictional data). Mean number of yeast-containing vesicles per amoeba: 6.2 at 20 °C and 0.8 at 4 °C. She concludes: “Endocytosis needs ATP.”

(a) Calculate the percentage decrease at 4 °C. (b) Evaluate the conclusion. (c) Propose a new investigation that would test the claim more directly.

(a) Percentage decrease = (6.2 − 0.8) ÷ 6.2 × 100 = 5.4 ÷ 6.2 × 100 = 87% (87.1% before rounding).

(b) Evaluation.

1. The result is consistent with the claim: cold slows respiration, so less ATP is available, and endocytosis falls sharply.
2. But temperature changes many things at once. Cold makes the membrane less fluid, slows the movement of both cells, and slows the random collisions that bring yeast into contact with amoebae. Any of these could reduce uptake without ATP being involved. Temperature is a confounding variable for the claim about ATP.
3. So the data support the idea that endocytosis is temperature-sensitive, but they do not show that ATP specifically is needed.

(c) A better investigation.

• Hypothesis: if endocytosis needs ATP, blocking ATP production will reduce the number of yeast-containing vesicles.
• Independent variable: presence of a metabolic inhibitor that blocks ATP production (for example none, low dose, high dose).
• Dependent variable: mean number of stained vesicles per amoeba after 30 minutes.
• Controlled variables: temperature (all at 20 °C), number of yeast cells per amoeba, time, stain, culture age. Include a control with the inhibitor’s solvent alone.
• Extra check: measure ATP in a sample of amoebae from each group, to confirm the inhibitor really lowers ATP.
• Prediction: if the claim is right, vesicle numbers fall as the dose rises, while temperature is held constant. If numbers stay near 6.2, the claim is not supported.

This is the kind of reasoning the course asks for when it asks you to propose a new investigation: name what was weak in the first design, then change one variable at a time and keep the rest constant.

Common misconceptions

• “Diffusion stops when concentrations are equal.” Molecules keep crossing both ways. Only the net movement stops.
• “Molecules move to low concentration because they want more space.” Net movement comes from random motion and unequal numbers on each side.
• “Passive transport never involves proteins.” Facilitated diffusion uses channel and carrier proteins and is still passive.
• “Passive transport uses no energy at all.” It uses no metabolic energy from the cell. The molecules’ own kinetic energy drives it.
• “Active transport means moving against the gradient.” The defining feature is the direct input of energy. Moving low → high is the usual reason for it.
• “Endocytosis carries molecules through the bilayer.” The cargo stays inside a vesicle and never crosses the hydrophobic interior.
• “Cells take in glucose by endocytosis because it is large.” Glucose is moved by membrane proteins. Endocytosis is for much larger particles or bulk fluid.
• “A cell tries to get rid of its gradients.” Cells spend energy to keep gradients; losing them is a sign of failure, for example when ATP runs out.

Where this leads

Next, Topic 2.6, Facilitated Diffusion, looks closely at the channel and carrier proteins used in passive transport. Topic 2.7 covers osmosis and tonicity, and Topic 2.8 explains how pumps such as the Na⁺/K⁺ pump use ATP. Review the previous topic on membrane permeability if the bilayer rules feel shaky. Then try the practice questions, and consolidate with the revision notes and the checklist.