How do substances move into and out of cells across the selectively permeable membrane?
Passive transport by diffusion and osmosis down concentration gradients, the effects of osmosis on animal and plant cells in hypertonic, hypotonic and isotonic solutions, and active transport against the gradient using energy from respiration.
An SQA National 5 Biology answer on transport across cell membranes, covering the selectively permeable membrane, passive transport by diffusion and osmosis, the effects of osmosis on animal and plant cells in different solutions, and active transport against the concentration gradient using energy from respiration.
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What this dot point is asking
The SQA wants you to explain how substances cross the selectively permeable cell membrane: by diffusion and osmosis (which are passive and need no energy) down a concentration gradient, and by active transport (which needs energy from respiration) against the gradient. You also need to predict what happens to animal and plant cells placed in solutions of different concentration.
The selectively permeable membrane
This control is the whole point of the membrane. It lets useful substances in and waste out while keeping the inside of the cell different from the outside.
Diffusion (passive)
Diffusion is how oxygen and glucose enter cells and how carbon dioxide leaves them. For example, oxygen is at a higher concentration in the blood than inside a respiring cell, so it diffuses into the cell down the gradient. The steeper the concentration gradient, the faster diffusion happens.
Osmosis (passive)
It helps to use the words hypotonic, isotonic and hypertonic to describe the solution outside the cell:
- Hypotonic solution: more water (more dilute) than the cell, so water moves in.
- Hypertonic solution: less water (more concentrated) than the cell, so water moves out.
- Isotonic solution: the same water concentration as the cell, so there is no net movement.
The effect depends on whether the cell has a wall:
| Solution | Animal cell | Plant cell |
|---|---|---|
| Hypotonic | Swells and may burst | Becomes turgid (firm), does not burst |
| Isotonic | Stays the same | Stays the same (flaccid) |
| Hypertonic | Shrinks (shrivels) | Becomes plasmolysed |
A plant cell cannot burst because its cellulose cell wall resists the pressure as water enters, making the cell turgid, which supports the plant. In a hypertonic solution a plant cell loses so much water that the membrane pulls away from the wall, which is called plasmolysis.
Active transport (needs energy)
Cells use active transport when they need to absorb a substance that is already more concentrated inside than outside. For example, root hair cells take up mineral ions from the soil by active transport, even though the soil has a lower ion concentration than the cell.
Examples in context
Example 1. Potato in salt water. A classic experiment places potato strips in salt solutions of different concentration. Strips in concentrated salt lose water by osmosis and become floppy and lighter; strips in pure water gain water, become firm (turgid) and heavier. Measuring the percentage change in mass lets you find the concentration at which there is no net movement, which is isotonic to the potato cells.
Example 2. Mineral uptake in roots. Plants need nitrate ions for making proteins, but the soil often has fewer nitrate ions than the root cell. The plant uses active transport to pump nitrate into the root hair cell against the gradient, using energy from respiration. This is why waterlogged roots, which respire poorly, take up minerals badly.
Try this
Q1. State whether diffusion needs energy, and explain your answer. [1 mark]
- Cue. No, because it moves molecules down the concentration gradient (a passive process).
Q2. Name the process that moves molecules against the concentration gradient. [1 mark]
- Cue. Active transport.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksExplain what happens to a plant cell and an animal cell when each is placed in pure water.Show worked answer →
This compares osmosis in two cell types, so handle each cell separately and use the word osmosis.
Pure water has a higher water concentration than the inside of the cell, so water moves into both cells by osmosis through the selectively permeable membrane.
The animal cell has no cell wall, so as water enters it swells and may burst (lyse).
The plant cell has a cellulose cell wall, so as water enters it swells but cannot burst. It becomes firm, or turgid, and the wall provides support.
Markers reward (1) water entering by osmosis, (2) the animal cell bursting, and (3) the plant cell becoming turgid without bursting.
SQA N5 style2 marksGive two ways in which active transport differs from diffusion.Show worked answer →
Two clear differences are needed, each contrasting both processes.
Difference 1. Diffusion moves molecules down a concentration gradient, from high to low concentration, while active transport moves molecules against the gradient, from low to high concentration.
Difference 2. Diffusion does not require energy, while active transport requires energy from respiration (ATP).
Markers reward each correct contrasting point. Stating only what one process does, without the contrast, scores nothing.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)