Movement into and out of Cells
How substances cross cell membranes: three routes, and the rule that decides which one a substance takes.
Diffusion
Particles in gases and liquids are never still. That constant random motion is enough, on its own, to spread substances out.
What diffusion is
Where particles are crowded, more of them move away than arrive, so the group spreads out until the particles are evenly distributed. The energy for this comes from the kinetic energy of the particles themselves; the cell supplies none.
Diffusion and the cell membrane
Some substances pass into and out of cells by diffusion through the cell membrane. Gases such as oxygen and carbon dioxide move this way, and so do some dissolved substances (solutes). Respiring cells use up oxygen, so oxygen diffuses in; carbon dioxide is made inside, so it diffuses out. Leaf cells take in carbon dioxide for photosynthesis in the same way.
Worked example (direction of diffusion): A cell is using oxygen for respiration, so the oxygen concentration inside the cell is lower than outside. In which direction does oxygen diffuse? Step 1. Diffusion is movement from higher to lower concentration. Step 2. Outside is higher and inside is lower, so oxygen moves inwards. Step 3. No energy from the cell is needed; the oxygen’s own motion does the work. Answer: Into the cell.
Rate of Diffusion
Four factors change how fast diffusion happens. Each can be tested by changing one and keeping the others fixed.
| Factor | Change that speeds it up | Reason |
|---|---|---|
| Surface area | Larger area | More membrane for particles to cross at once |
| Temperature | Higher temperature | Particles have more kinetic energy and move faster |
| Concentration gradient | Steeper gradient | More particles move down the gradient than back up it |
| Distance | Shorter distance | Particles have less far to travel |
Investigating diffusion
To test one factor, change only that one and measure the effect. For temperature, drop the same amount of coloured dye into equal volumes of water at different temperatures and time how long the colour takes to spread through the water. Keep the volume of water and amount of dye the same each time, and repeat to check results. To test surface area or distance, use agar cubes of different sizes that contain an indicator, and time how long the colour change takes to reach the middle.
Worked example (reading results): Dye took 120 s to spread through water at 20 °C and 60 s at 40 °C. What does this show? Step 1. The time at 40 °C is half the time at 20 °C. Step 2. Half the time means about twice the rate of diffusion. Step 3. So a higher temperature speeds diffusion, because the particles move faster. Answer: Faster at 40 °C.
Osmosis
Water is the solvent in living organisms, and water itself crosses cell membranes by a special case of diffusion called osmosis.
Water as a solvent
Many substances dissolve in water. In digestion, the small molecules made from food dissolve in water; in excretion, wastes such as urea leave dissolved in urine; in transport, blood plasma carries dissolved substances around the body.
Osmosis
Water diffuses through a partially permeable membrane by osmosis, and moves into and out of cells this way through the cell membrane. Net movement is towards the side with more dissolved solute.
ExtendedWater potential
Water moves from a region of higher water potential to one of lower water potential. Dissolving a solute lowers water potential, so a dilute solution has a higher water potential than a concentrated one.
Worked example (dialysis tubing): A dialysis tubing bag of concentrated sugar solution is placed in water. Predict what happens to its mass. Step 1. Water passes through the tubing; the sugar particles are too large to. Step 2. Outside is more dilute, so water moves into the bag by osmosis. Step 3. The bag gains water, so its mass increases. Answer: Mass increases.
Osmosis in Plant Tissue
Plant tissue can be used as a model of osmosis: weigh it before and after soaking it in solutions of different concentrations.
Investigating plant tissue
Cut equal potato chips, weigh each, and place one in each of several sugar solutions, including pure water. After a set time, blot dry and weigh again. A chip that gains mass has taken water in by osmosis; one that loses mass has lost water to a more concentrated solution.
The line crosses zero where the solution is about as concentrated as the cell contents, so there is no net movement of water. Here that is close to 0.3 mol per dm³.
Worked example (percentage change in mass): A potato chip of mass 5.0 g is left in distilled water and then weighs 5.6 g. Calculate the percentage change in mass. Step 1. Change in mass = 5.6 − 5.0 = 0.6 g. Step 2. Percentage change = 0.6 ÷ 5.0 × 100. Step 3. 0.6 ÷ 5.0 = 0.12, so the change is +12%, a gain, because water entered by osmosis. Answer: +12%.
Plant Cells in Solutions
Plants are supported by the pressure of water inside their cells pressing outwards on the cell walls. When that pressure falls, the plant droops.
ExtendedEffects of different solutions
In a solution more dilute than the cell contents, water enters by osmosis. The cell swells until the wall resists, and it is turgid; the pressure is the turgor pressure. In a solution about as concentrated as the cell contents there is no net movement of water, little turgor pressure remains, and the cell is flaccid. In a solution more concentrated than the cell contents, water leaves and the membrane pulls away from the wall: the cell is plasmolysed.
ExtendedWhy this matters to organisms
Root hair cells have a lower water potential than the soil water, so water enters them by osmosis and the plant takes up water. Animal cells have no wall: a red blood cell placed in very dilute solution takes in so much water that it can burst, and in a concentrated solution it shrinks.
Worked example (naming the state of a cell): A plant cell in concentrated sugar solution has its membrane pulled away from the cell wall. What is this called? Step 1. Water has left the cell by osmosis, as the solution outside is more concentrated. Step 2. The contents shrink away from the wall. Step 3. Membrane pulled away from the wall is plasmolysis. Answer: Plasmolysis.
Active Transport
Diffusion and osmosis can only move substances down a gradient. Active transport moves them the other way, and the cell must pay for it.
| Process | Direction of movement | Energy from the cell? |
|---|---|---|
| Diffusion | Down a concentration gradient | No |
| Osmosis | Water, from dilute to concentrated solution | No |
| Active transport | Against a concentration gradient | Yes, from respiration |
ExtendedCarriers and root hairs
Protein carriers in the membrane move the molecules or ions across. Root hair cells, for example, take up mineral ions from the soil this way, even when the ions are scarcer outside than inside.
Worked example (which process?): A cell takes in a substance although its concentration is already higher inside the cell. When respiration is blocked, the uptake stops. Which process is this? Step 1. Uptake into a region of higher concentration is against the gradient, so it is not diffusion. Step 2. It stops without respiration, so it depends on energy from respiration. Step 3. Movement against the gradient using energy from respiration is active transport. Answer: Active transport.
Exam advice
Common mistakes
Model answer
Recall checklist
- Define diffusion and name its energy source.
- List four factors that change the rate of diffusion.
- State why water is called the solvent in organisms.
- Describe osmosis through a partially permeable membrane.
- Calculate a percentage change in mass.
- Define active transport and name its energy source.
- Describe osmosis in terms of water potential (Extended)
- Use turgid, flaccid and plasmolysis correctly (Extended)
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