4.24 · Cell Membranes and Transport

Movement into & out of Cells — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

20 min read 33 parts Lesson 7 of 20 13 quizzes

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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4.2 Movement into & out of Cells — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe and explain diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis, and the effect of surface area to volume ratio.

Fick's law (idea)
Why it is needed
Proteins used
Channel proteins
Carrier proteins
Rate limit

Part 1 of 12Diffusion

Diffusion = the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient,
- as a result of their random movement.
- It is passive, so no ATP is needed.
- Particles move because of their natural kinetic energy, and at equilibrium movement continues but there is no net change.

Factor	Effect on rate of diffusion
Steeper concentration gradient	More particles cross in one direction than the other, so the rate increases
Higher temperature	Particles have more kinetic energy and move faster
Larger surface area	More particles can cross at any moment; folding (microvilli, cristae) increases area
Shorter distance / thinner membrane	Particles have less far to travel
Smaller, non-polar molecules	They move faster and dissolve in the lipid core, so they cross the bilayer more quickly than large or polar ones

Fick's law (idea)
- rate of diffusion is proportional to ÷ diffusion distance.
  - (surface area × concentration difference)

Part 2 of 12Facilitated diffusion

Why it is needed

Why it is needed ions and polar molecules such as glucose and amino acids cannot cross the hydrophobic core of the bilayer.

Proteins used

Proteins used specific channel or carrier proteins provide the route, and the process is passive and goes down the concentration gradient.

Channel proteins

Channel proteins water-filled pores with a fixed shape that let ions through, and most are gated so that they can open and close.

Carrier proteins

Carrier proteins they bind a specific molecule and switch between two shapes, so the binding site opens first to one side and then to the other.

Rate limit

Rate limit the rate levels off when all the proteins are in use, so it depends on the number of channel and carrier proteins.

Part 3 of 12Osmosis and water potential

Osmosis

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)

Water potential (ψ) the tendency of water molecules to move from one place to another, measured in kPa.
Pure water has ψ = 0 kPa and all solutions have a negative ψ.
Adding solute lowers ψ because solute molecules attract water molecules, so fewer are free to move.

Equation

Equation water potential = solute potential + pressure potential, ψ = ψ_s + ψ_p.

Tonicity

Tonicity a hypotonic solution has a lower solute concentration (higher ψ) than the cell, a hypertonic solution has a higher solute concentration (lower ψ),
- and an isotonic solution is equal.

Part 4 of 12Osmosis in animal and plant cells

External solution	Animal cell (no wall)	Plant cell (cellulose wall)
Higher ψ (more dilute)	Gains water and may burst (haemolysis in red blood cells)	Gains water; the protoplast pushes on the wall, ψ_p rises until ψ inside equals ψ outside, and the cell becomes turgid but does not burst
Equal ψ	No net water movement	No net movement; incipient plasmolysis if ψ_p = 0
Lower ψ (more concentrated)	Loses water, shrinks and becomes crenated	Loses water, becomes flaccid and then plasmolysed

1Plasmolysis

Plasmolysis the protoplast shrinks and the cell surface membrane pulls away from the cell wall, and it occurs only in plant cells.

2Gap in a plasmolysed cell

Gap in a plasmolysed cell the freely permeable cellulose wall lets the external solution fill the gap, so it is not empty or air-filled.

Part 5 of 12Active transport

Active transport

Active transport movement of molecules or ions across a membrane against their concentration gradient, using energy from ATP hydrolysis and carrier proteins (pumps).

Mechanism

Mechanism a carrier protein binds the substance on one side, ATP hydrolysis changes its shape, and the substance is released on the other side.
- (often phosphorylation of the carrier)

Sodium–potassium pump

Sodium–potassium pump each ATP hydrolysed moves 3 Na⁺ out and 2 K⁺ in.

Inhibition

Inhibition respiratory poisons such as cyanide and low oxygen reduce the rate because less ATP is made.

Example

Example uptake of mineral ions by root hair cells.

Part 6 of 12Endocytosis and exocytosis

Bulk transport

Bulk transport larger materials move in vesicles, and both processes need ATP to move vesicles and reshape membranes.

Endocytosis

Endocytosis the cell surface membrane folds inwards around material and forms a vesicle, taking bulk material into the cell.
Phagocytosis takes in solids, for example engulfing of bacteria by neutrophils.
Pinocytosis takes in liquid droplets.

Exocytosis

Exocytosis vesicles from the Golgi body move to the cell surface membrane, fuse with it and release their contents.
Examples are secretion of digestive enzymes, hormones and neurotransmitters.

Part 7 of 12Surface area to volume ratio

1Size and ratio

Size and ratio as a cell or organism grows, volume increases faster than surface area (volume ∝ side³, area ∝ side²), so SA:V falls.

2Consequences

Consequences diffusion distances to the centre increase,
which limits cell size and explains the need for specialised exchange surfaces and transport systems in large organisms.

3Importance of a high ratio

Importance of a high ratio it helps obtain resources, remove wastes and exchange heat and chemicals with the surroundings.

Shape	Surface area	Volume
Cube	6 × side², so SA:V = 6 ÷ side	side³
Cuboid	2(lw + lh + wh)	l × w × h
Cylinder	2πrh + 2πr²	πr²h

Part 8 of 12Practical: agar cubes and SA:V

Make agar blocks containing phenolphthalein and dilute sodium hydroxide (pink) and cut cubes of side 0.5, 1, 2 and 3 cm.

Calculate surface area, volume and SA:V for each cube.

Immerse all cubes at the same time in equal volumes of dilute hydrochloric acid of the same concentration at constant temperature.

Time how long each cube takes to turn completely colourless as acid diffuses in and neutralises the alkali.

Repeat, calculate means, and plot rate of colour change (1 ÷ time) against SA:V.

Conclusion
- smaller cubes with a higher SA:V decolourise faster, showing diffusion alone is adequate only over short distances.

Part 9 of 12Practicals: Visking tubing, plasmolysis and water potential

Visking tubing

Visking tubing a non-living, partially permeable cellulose membrane.
Fill it with starch and glucose solution and stand it in water.
Test the water at intervalsglucose passes out, but starch is too large to pass through the pores.

Plasmolysis

Plasmolysis mount epidermis strips from coloured tissue such as red onion in a range of sucrose or salt solutions and view under a light microscope.
Count the proportion of cells whose protoplast has pulled away from the wall.

To estimate the water potential of plant tissue, cut equal cylinders of tissue, blot and weigh each.

Place them in a range of sucrose concentrations for a set time at constant temperature.

Blot and reweigh, then calculate percentage change in mass = (final mass − initial mass) ÷ initial mass × 100.

Plot percentage change against sucrose concentration.

Read the concentration where the line crosses 0%, because the tissue has the same ψ as that solution.

Part 10 of 12Comparing the transport mechanisms

Mechanism	Gradient	ATP?	Proteins?	Example
Diffusion	Down	No	No	O₂ and CO₂ across alveoli
Facilitated diffusion	Down	No	Channel or carrier	Glucose through carrier proteins
Osmosis	Down the ψ gradient (water)	No	Aquaporins can help	Water uptake by root hair cells
Active transport	Against	Yes	Carrier (pump)	Na⁺/K⁺ pump
Endocytosis / exocytosis	Bulk	Yes	Vesicles	Phagocytosis of bacteria; secretion of insulin

Part 11 of 12More from the specification

Osmosis

Osmosis the net movement of water across a partially permeable membrane from higher to lower water potential.

Selectively permeable membrane

Selectively permeable membrane a phospholipid bilayer with proteins lets water pass but restricts many solutes.

Potato cylinder investigation

Potato cylinder investigation place cylinders in different sucrose solutions and measure mass or length change to find the isotonic point.

Plant cells

Plant cells they become turgid in dilute solutions and plasmolysed in concentrated solutions.

Investigating diffusion

Investigating diffusion use agar blocks with indicator to compare rates for different surface area to volume ratios.

Indicator

A chemical that changes colour depending on pH.

Selectively permeable

Allowing only some substances through.

Phospholipid bilayer

Two layers of phospholipids with hydrophobic tails inwards

Part 12 of 12More on this topic

Explanation

Explanation as a cube gets larger, volume (cubed) increases faster than surface area (squared), so diffusion takes longer and is less effective.

Trend

Trend the rate of diffusion increases as SA:V increases, because a larger relative surface lets more acid enter per unit volume of agar.

Meaning

Meaning small organisms can rely on diffusion but large ones cannot.

Weigh the cylinders before and after, drying the surface first.

Process	ATP?	Direction	Proteins	Example
Simple diffusion	No	Down gradient	None	Gas exchange in alveoli
Facilitated diffusion	No	Down gradient	Channel or carrier	Glucose into red blood cells
Osmosis	No	High ψ to low ψ	Aquaporins (some)	Water into root hair cells
Active transport	Yes	Against gradient	Carrier (pump)	Na⁺/K⁺ pump
Endocytosis	Yes	Into cell	Vesicles	Phagocytosis
Exocytosis	Yes	Out of cell	Vesicles fuse with membrane	Insulin secretion

Key terms

Term	Meaning
Diffusion	Net movement of particles down a concentration gradient due to random movement; passive.
Facilitated diffusion	Diffusion of ions or polar molecules through channel or carrier proteins; passive.
Osmosis	Net movement of water from higher to lower water potential through a partially permeable membrane.
Water potential (ψ)	The tendency of water to move from one place to another; pure water is 0 kPa and solutions are negative.
Active transport	Movement against a concentration gradient using carrier proteins and ATP.
Endocytosis	Bulk transport into a cell by a vesicle formed from the cell surface membrane.
Exocytosis	Bulk transport out of a cell when vesicles fuse with the cell surface membrane.
Plasmolysis	Shrinking of the protoplast of a plant cell away from the cell wall.
Surface area to volume ratio	Surface area divided by volume; it falls as size increases.

Avoid these mistakes

Saying active transport moves substances down a concentration gradient — it moves them against it, using ATP.

Using the term plasmolysis for animal cells — they shrink (crenate) because they have no wall.

Saying plant cells burst in dilute solutions — the wall resists, so they become turgid.

Confusing channel proteins (fixed pores) with carrier proteins (change shape); active transport uses carriers.

Writing bulk transport instead of endocytosis or exocytosis.

Last updated: 18 October 2026