Transport across cell membranes notes

In a nutshell

The cell-surface membrane, and the membranes around organelles, control which substances move into and out of a cell.

This subtopic is about how the membrane is built (the fluid-mosaic model) and the five ways substances cross it: simple diffusion, facilitated diffusion, osmosis, active transport and co-transport.

Assumed knowledge: Lipids, Proteins, ATP.

Core content

The fluid-mosaic model

All cell membranes have the same basic structure: a phospholipid bilayer with proteins and other molecules embedded in it. This includes the cell-surface membrane and the membranes around organelles.

It is called fluid because the phospholipids can move sideways within their layer, and mosaic because the proteins are scattered through the bilayer in a patchwork.

Component	Role in the membrane
Phospholipid bilayer	The basic barrier. Its hydrophobic core lets small, non-polar (lipid-soluble) molecules through but blocks large, polar or charged ones.
Channel proteins	Water-filled pores that let specific polar molecules and ions pass by facilitated diffusion.
Carrier proteins	Change shape to move specific molecules or ions across, in facilitated diffusion and active transport.
Glycoproteins	Proteins with a carbohydrate chain; act as receptors and in cell recognition.
Glycolipids	Phospholipids with a carbohydrate chain; act in cell recognition and cell adhesion.
Cholesterol	Sits between the phospholipid tails and restricts the movement of the other molecules making up the membrane, so the membrane is less fluid and more stable.

The phospholipid bilayer makes the membrane partially permeable: what can cross, and how, depends on a molecule's size, polarity and charge. That is what the rest of this note is about.

Simple diffusion

Simple diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is passive: it does not use ATP.

The phospholipid bilayer sets a hard limit on what can diffuse straight through it:

Small, non-polar (lipid-soluble) molecules (for example O₂ and CO₂) dissolve in the bilayer and cross freely.
Large, polar or charged particles (for example glucose, amino acids and ions) cannot cross the hydrophobic core, so they need transport proteins (see below).

The rate of simple diffusion increases with:

a steeper concentration gradient (bigger difference across the membrane),
a larger surface area of membrane,
a shorter diffusion distance (thinner membrane or exchange surface),
a higher temperature (more kinetic energy).

Facilitated diffusion

Facilitated diffusion is the passive movement of molecules or ions across a membrane down a concentration gradient, through channel or carrier proteins. Like simple diffusion it does not use ATP; unlike simple diffusion it needs proteins, because the particles are too large, polar or charged to cross the bilayer.

Channel proteins form a pore for specific ions or polar molecules.
Carrier proteins bind the molecule, then change shape to release it on the other side.

The key contrast with simple diffusion is what happens as the gradient gets steeper: simple diffusion keeps rising, but facilitated diffusion levels off once all the channel or carrier proteins are in use (saturated).

Active transport

Active transport is the movement of molecules or ions across a membrane against a concentration gradient (from a lower to a higher concentration), using carrier proteins and ATP.

It uses only carrier proteins, never channel proteins.
The energy comes from the hydrolysis of ATP (ATP is made in respiration), so anything that reduces respiration (such as a lack of oxygen or a low temperature) reduces the rate of active transport.
Because it can move substances the "wrong" way up a gradient, it is how cells take up substances that are already more concentrated inside than outside.

Osmosis and water potential

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

Water potential (Ψ) measures the tendency of water molecules to move out of a solution. It is measured in kilopascals (kPa):

Pure water has a water potential of 0 kPa, the highest possible value.
Adding solute lowers the water potential, making it more negative (for example −200 kPa).

So water always moves from the less negative (higher) water potential to the more negative (lower) water potential, down the water potential gradient, until the two sides are equal (equilibrium, no net movement).

Still don't get it? · water potential as a negative number

Think of water potential like the balance on a pre-paid card that starts at zero and can only go into the red. Pure water is a card with exactly £0 on it, and that is as full as it ever gets. Every scoop of solute you dissolve is a small charge to the card, pushing the balance further below zero.

Now put two cards next to a gate that only water can cross. Water always drains from the card that is closer to zero (say £0, or −£100) towards the more overdrawn card (say −£400), trying to even them out.

The exam version: pure water has the highest water potential (0 kPa); adding solute makes the water potential lower, meaning more negative. Water moves from higher (less negative) water potential to lower (more negative) water potential, down the water potential gradient. The trap is language: −100 kPa is a higher water potential than −400 kPa, so pick either "higher and lower" or "more and less negative" and stay with it, because mixing the two contradicts itself.

The effect on cells follows directly from the gradient:

Animal cell in a solution of higher water potential: water enters by osmosis and, with no cell wall, the cell may burst (lysis). In a lower water potential it loses water and shrivels (crenation).
Plant cell in a solution of higher water potential: water enters, the cell becomes turgid (the cell wall resists further entry). In a lower water potential it loses water and the membrane pulls away from the wall (plasmolysis).

Co-transport: absorbing glucose and sodium ions in the ileum

Co-transport couples the movement of two substances through the same carrier protein: the movement of one substance down its gradient drags the other against its gradient. AQA illustrates this with the absorption of glucose and sodium ions by the cells lining the mammalian ileum (small intestine).

The sequence, in order:

Sodium ions are actively transported out of the epithelial cell into the blood, by a carrier protein using ATP. Potassium ions are transported into the cell at the same time.
This keeps the sodium ion concentration low inside the cell, so a sodium ion concentration gradient exists between the gut lumen and the cell.
Sodium ions then diffuse from the lumen into the cell through a co-transporter protein, bringing glucose in with them. The glucose is carried against its own concentration gradient.
Glucose then leaves the cell and enters the blood by facilitated diffusion.

Still don't get it? · why co-transport is "indirect" active transport

Imagine a revolving door between the street and a shop that only turns when someone walks in from the street. A person who wants to get in from the shop side, against the flow, waits by the door and slips through in the same compartment as a street-side person coming in. They did not push the door themselves; they hitched a ride on someone moving the way the door wants to turn.

Step by step: the cell first spends ATP to pump sodium ions out into the blood, so sodium is now low inside the cell and high in the lumen. Sodium therefore "wants" to flood back into the cell down its gradient. The co-transporter only lets sodium in if a glucose molecule comes through with it, so the inrush of sodium drags glucose in too, even though glucose is going against its own gradient.

The exam version: ATP is used only in step 1 (active transport of sodium ions out of the cell). Glucose itself is not directly pumped; it is carried by facilitated diffusion coupled to sodium moving down its gradient, so glucose moves against its concentration gradient. This is why co-transport is called indirect active transport.

Adaptations for rapid transport

A cell can be adapted to transport substances faster across its membranes by:

Increasing surface area, for example folding the membrane into microvilli (as in ileum epithelial cells), so more transport can happen per cell.
Increasing the number of channel or carrier proteins in the membrane, so more facilitated diffusion, active transport or co-transport can happen at once.

The rate of movement across a membrane also rises with a steeper concentration or water potential gradient. For active or facilitated transport, once the proteins are saturated, adding more gradient no longer speeds things up, so the number of proteins becomes the limiting factor.

Required practical 3: finding the water potential of plant tissue

The aim is to make a dilution series of a solute (usually sucrose) and use it to build a calibration curve that identifies the water potential of a plant tissue (usually potato).

Make up sucrose solutions of known, increasing concentration (the dilution series), plus pure water.
Cut potato pieces of equal size, blot and record the initial mass of each.
Leave one piece in each solution for a set time, then blot and record the final mass.
Calculate the percentage change in mass for each concentration and plot it against concentration.
Where the curve crosses the x-axis (0 % change in mass) there is no net movement of water, so the solution there has the same water potential as the tissue. Read off that concentration and use a supplied resource to find its water potential.

Required practical 4: membrane permeability

This investigates the effect of a named variable (for example temperature, or the concentration of an organic solvent) on the permeability of cell-surface membranes.

A common version uses beetroot, whose cells contain a red pigment. If the membrane is damaged, the pigment leaks out.
The independent variable (for example temperature) is changed, and the amount of pigment released is measured with a colorimeter (higher absorbance means more leakage, so a more permeable or damaged membrane).
Higher temperatures increase permeability, because they increase the movement of phospholipids and can denature membrane proteins, leaving gaps.

Worked examples

Model 4-mark answer: "Describe how glucose is absorbed from the ileum into the blood by co-transport."

Sodium ions are actively transported out of the epithelial cell into the blood, using ATP.
This maintains a lower concentration of sodium ions inside the cell than in the lumen (a sodium ion concentration gradient).
Sodium ions move from the lumen into the cell by facilitated diffusion through a co-transporter protein, carrying glucose in with them against its concentration gradient.
Glucose then moves from the cell into the blood by facilitated diffusion.

Notice a 4-mark "describe" answer needs four distinct, ordered points. Naming "co-transport" earns nothing on its own: you must say what moves, where it moves to, and how.

Data skill: reading the water potential from a calibration curve (Required practical 3).

The graph below shows the percentage change in mass of potato pieces left in different sucrose concentrations.

Read where the line crosses 0 % change in mass: here it crosses at 0.5 mol dm^-3.
At that concentration there is no net movement of water, so the potato tissue has the same water potential as 0.5 mol dm^-3 sucrose.
You would then use a supplied table or calibration graph of water potential against sucrose concentration to convert 0.5 mol dm^-3 into a water potential in kPa.

Common exam mistakes

Saying passive processes "require no energy". They use no ATP (no energy from respiration), but the particles still move using their own kinetic energy. Write "does not use ATP", not "needs no energy".
Describing movement as "along", "across" or "to" a gradient. The mark needs down a concentration gradient (diffusion) or down a water potential gradient (osmosis), or against it (active transport).
Defining osmosis using "concentration of water" instead of water potential, or leaving out the partially permeable membrane. Both are required for the definition marks.
Mixing "higher/lower" with "more/less negative" in the same answer, which contradicts itself. Pick one and stay with it. Remember −100 kPa is a higher water potential than −400 kPa.
Thinking adding solute raises water potential. Pure water is 0 kPa (the highest); adding solute makes it more negative (lower).
Saying active transport uses channel proteins. Active transport uses carrier proteins only (channel proteins are rejected).
In co-transport, saying sodium ions are pumped into the lumen, or confusing the lumen with the epithelial cell. Sodium is actively transported out of the cell into the blood, and glucose enters against its concentration gradient.
Calling chloride ions "chloride molecules", or saying a particle is "soluble" without saying lipid-soluble. For crossing the bilayer, whether a particle is charged or polar matters, not just its size.

Key definitions

Simple diffusion: the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient; a passive process that does not use ATP.
Facilitated diffusion: the passive movement of molecules or ions across a membrane down a concentration gradient, through channel or carrier proteins.
Active transport: the movement of molecules or ions across a membrane against a concentration gradient, using carrier proteins and ATP.
Osmosis: the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Water potential (Ψ): the tendency of water molecules to move out of a solution, measured in kPa; pure water has a water potential of 0 (the highest value) and adding solute makes it more negative.
Co-transport: the coupled movement of two substances across a membrane through the same carrier protein, where one substance moving down its concentration gradient drives the other against its gradient.

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