2dMovement of substances into and out of cells

Three ways substances cross the cell membrane. Learn the definitions word-for-word — marks are awarded for exact phrases like net movement, concentration gradient and partially permeable membrane.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (i.e. down a concentration gradient), as a result of their random movement.

• It is passive — no energy from respiration is needed.
• It moves gases and small soluble molecules: oxygen and carbon dioxide across the alveoli, glucose into cells, urea out of liver cells into the blood.
• Particles keep moving in both directions; "net" means more go one way than the other until the concentrations are equal.

Osmosis

Osmosis is the net movement of water molecules from a dilute solution (high water concentration) to a concentrated solution (low water concentration) through a partially permeable membrane.

• A partially permeable membrane lets small water molecules through but not larger solute molecules such as sucrose.
• Osmosis is really just diffusion of water, so it is also passive.
• Never write "semi-permeable" — the specification says partially permeable.

Effects on cells:

Active transport

Active transport is the movement of particles against a concentration gradient (from a lower to a higher concentration) across a cell membrane, using energy released by respiration.

• Uses carrier proteins in the membrane; needs mitochondria to supply energy.
• Examples you should be able to quote:
  • Root hair cells absorbing mineral ions (e.g. nitrate) from very dilute soil water into a more concentrated cytoplasm.
  • Glucose reabsorbed from the filtrate into the blood at the proximal convoluted tubule (PCT) of the nephron.
  • Salt (sodium ions) absorbed from the large intestine into the blood.

Summary table

Examiner tip. Two specific traps. (1) In kidney questions, examiners repeatedly report that candidates lose marks for not naming the proximal convoluted tubule (PCT) and not naming active transport — name both explicitly. (2) Read the question stem fully: candidates asked about salt absorption in the large intestine wrote answers about the kidney. And when a question is about movement of substances, use the technical word — say "by diffusion", not "it moves across".

Four factors are named on the specification: surface area to volume ratio, distance, temperature and concentration gradient. You must be able to say why each one changes the rate, not just that it does.

Surface area to volume ratio

The larger the surface area to volume ratio (SA:V), the faster substances move in and out relative to the size of the organism, because there is more membrane area per unit of cytoplasm to supply.

Cubes make this obvious:

As an object gets bigger, SA:V falls — so large organisms cannot rely on diffusion alone and need exchange surfaces and a transport system.

Concrete examples: root hair cells have a long extension that increases surface area for absorbing water and mineral ions; villi and microvilli increase the surface area of the small intestine; alveoli give the lungs a huge surface area for gas exchange.

Distance (thickness of the exchange surface)

The shorter the distance, the faster the rate — diffusion is only fast over very short distances.

• Alveolus wall and capillary wall are each one cell thick.
• Villi have a thin, one-cell-thick wall between the gut contents and the blood.

Temperature

The higher the temperature, the faster the rate. Particles have more kinetic energy, so they move faster and randomly spread out more quickly.

Common mistake: examiners see "high temperature kills enzymes" and "denatures bacteria". Enzymes are not alive, so they cannot be killed — they are denatured. Bacteria are alive — they are killed. Here, temperature affects the rate of movement simply through kinetic energy of the particles, so don't drift into an enzyme essay unless the question asks for it.

Concentration gradient

The steeper the concentration gradient (the bigger the difference in concentration between the two regions), the faster the rate of diffusion or osmosis.

Organisms maintain steep gradients:

• Ventilation keeps alveolar oxygen high and carbon dioxide low.
• Blood flow carries absorbed substances away, keeping the concentration in the blood low.

Note: active transport works against the gradient, so a steeper gradient makes it harder and requires more energy from respiration.

Answering rate questions

Always link the factor to a reason:

"Increasing the temperature increases the rate of diffusion because the particles gain more kinetic energy and move faster."

Examiner tip. Watch the command word. 'Describe' = say what happens (e.g. "the rate increases as temperature increases"). 'Explain' = give the reason ("…because particles have more kinetic energy"). Examiners report confusing 'describe' and 'explain' as one of the commonest ways marks are thrown away on this topic.

Two standard non-living investigations. Know the apparatus, what you measure and what you control.

A. Agar cubes — effect of surface area to volume ratio

Apparatus: a block of agar containing phenolphthalein indicator and dilute sodium hydroxide (pink), cut into cubes of different sizes; dilute hydrochloric acid; beaker; ruler; stopwatch.

Method

1. Cut cubes of side 1 cm, 2 cm and 3 cm from the pink agar. Measure with a ruler.
2. Place all cubes in the same beaker of dilute hydrochloric acid at the same time so they are covered.
3. The acid diffuses into the agar and turns the pink phenolphthalein colourless.
4. Time how long each cube takes to turn completely colourless, or remove after a fixed time, cut in half and measure the depth the acid has penetrated.

Variables

Result: the smallest cube decolourises first. It has the largest surface area to volume ratio, so acid diffuses to its centre in the shortest time. The diffusion distance to the centre is also shortest.

Variation: use cubes of the same size in acid at different temperatures to show that higher temperature speeds up diffusion, or different acid concentrations to show a steeper concentration gradient speeds up diffusion.

B. Visking tubing — a model partially permeable membrane

Apparatus: Visking tubing, string, boiling tube, distilled water, a mixture of starch solution and glucose solution, iodine solution, Benedict's solution, water bath.

Method

1. Tie one end of the Visking tubing, fill with the starch + glucose mixture, tie the other end and rinse the outside with distilled water.
2. Suspend it in a boiling tube of distilled water. Leave for 20–30 minutes.
3. Test the water outside the tubing at the start and at the end:
   • Iodine solution for starch — stays orange-brown, so starch has not left (molecules too large).
   • Benedict's solution, heated in a water bath — turns brick-red, so glucose has diffused out (molecules small enough to pass through).

Conclusion: Visking tubing acts as a partially permeable membrane, like a cell membrane — small molecules diffuse through down the concentration gradient, large molecules do not.

Practical vocabulary examiners test

• Independent variable — the one you deliberately change (cube size).
• Dependent variable — the one you measure (time to decolourise).
• Control variable — kept the same so the test is fair.
• Reliable — repeat each measurement (e.g. three cubes of each size) and calculate a mean; results that agree closely on repetition are reliable.

Examiner tip. Candidates regularly muddle the independent and dependent variables (and even call a control variable the independent variable) and cannot define 'reliable'. Learn the four terms above. Also, if you are asked to plan an investigation, write a plan with a context — note-form answers like "change temperature" score nothing; you must say what you change, over what range, what you measure and what you keep the same.

Potato cylinders in sucrose solutions

Apparatus: potato, cork borer, scalpel/tile, ruler, balance (to 0.01 g), boiling tubes, sucrose solutions of different concentrations (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³), paper towel, stopwatch.

Method

1. Cut potato cylinders with a cork borer and trim them all to the same length (e.g. 4 cm). Blot dry.

2. Measure and record the initial mass of each cylinder.

3. Place one cylinder in each boiling tube containing the same volume of a different sucrose concentration.

4. Leave for the same time (e.g. 30 minutes) at the same temperature.

5. Remove, blot dry with a paper towel (removes surface solution — otherwise the mass is falsely high), and record the final mass.

6. Calculate percentage change in mass:

   % change = (final mass − initial mass) ÷ initial mass × 100

Why percentage change? The cylinders are not all exactly the same starting mass, so percentages allow fair comparison.

Variables

Results and explanation

Plot % change in mass against concentration. Where the line crosses the x-axis (zero change), the sucrose solution has the same concentration as the cell contents.

Reliability: repeat each concentration at least three times and take a mean.

Beetroot alternative

Beetroot cells contain a red pigment in the vacuole. Cylinders placed in concentrated sucrose solution lose water by osmosis; if the membrane is damaged (e.g. by high temperature or ethanol) the pigment leaks out and the surrounding solution turns red. The colour intensity can be measured with a colorimeter.

Common mistake: when explaining why beetroot or potato tissue shrinks, examiners report candidates writing that "the red pigment moved" or that "the sucrose solution moved into the cells". Neither is osmosis. State clearly that water has left the cells by osmosis because the solution outside is more concentrated (lower water concentration) than the cell contents.

Examiner tip. If the question says 'describe' the results, just state the pattern with data ("mass increased by 8% in distilled water and decreased by 12% in 1.0 mol dm⁻³ sucrose"). Only bring in osmosis, water concentration and the partially permeable membrane when asked to 'explain' — examiners note candidates who lose description marks by launching straight into how osmosis works.