Movement into and out of cells
Cambridge A-Level Biology (9700) · Unit 4: Cell membranes and transport · 15 flashcards
Movement into and out of cells is topic 4.2 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 4 — Cell membranes and transport , alongside Fluid mosaic membranes. In one line: Simple diffusion is the net movement of molecules from a region of high concentration to a region of low concentration, down a concentration gradient, as a result of their random motion. This process does not require any membrane proteins.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 15 flashcards — 7 definitions and 8 key concepts — covering the precise wording mark schemes reward. Use the 7 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Simple diffusion
Simple diffusion is the net movement of molecules from a region of high concentration to a region of low concentration, down a concentration gradient, as a result of their random motion. This process does not require any membrane proteins.
What the Cambridge 9700 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- investigate the effects of immersing plant tissues in solutions of different water potentials, using the results to estimate the water potential of the tissues
- explain the movement of water between cells and solutions in terms of water potential and explain the different effects of the movement of water on plant cells and animal cells (knowledge of solute potential and pressure potential is not expected)
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Movement into and out of cells
- › Practice calculating surface area to volume ratios for different cube sizes to predict diffusion rates into and out of tissues.
- › State that at equilibrium there is no 'net' movement, but water molecules continue to move in and out of cells in equal amounts.
- › Classify facilitated diffusion as a 'passive' process that uses carrier or channel proteins to move substances down a concentration gradient.
- › Apply the 'fluid' part of the fluid mosaic model; proteins and phospholipids move laterally within the membrane layer.
- › Identify testosterone as a steroid (lipid) hormone; it is fat-soluble and crosses the phospholipid bilayer by simple diffusion.
Define simple diffusion.
Simple diffusion is the net movement of molecules from a region of high concentration to a region of low concentration, down a concentration gradient, as a result of their random motion. This process does not require any membrane proteins.
Describe facilitated diffusion.
Facilitated diffusion is the diffusion of molecules across a membrane through specific protein channels or carriers. It follows the concentration gradient, but requires the assistance of membrane proteins.
Define osmosis.
Osmosis is the net movement of water molecules from a region of high water potential (low solute concentration) to a region of low water potential (high solute concentration), through a partially permeable membrane.
Explain active transport.
Active transport is the movement of molecules across a membrane against their concentration gradient, requiring energy in the form of ATP. This process involves specific carrier proteins.
Describe endocytosis.
Endocytosis is the process by which cells engulf material from their external environment by invagination of the cell membrane, forming vesicles. There are two main types: phagocytosis (solids) and pinocytosis (liquids).
Describe exocytosis.
Exocytosis is the process by which cells release material to their external environment. Vesicles fuse with the cell membrane, releasing their contents outside the cell.
How does increasing the surface area to volume ratio affect the rate of diffusion?
Increasing the surface area to volume ratio increases the rate of diffusion. A larger surface area allows for more molecules to cross the membrane at a given time, while a smaller volume reduces the distance molecules need to travel.
Give an example of a practical to investigate osmosis.
A common practical involves placing potato cylinders in different concentrations of sucrose solution and measuring the change in mass. An increase in mass indicates water uptake (osmosis) and a decrease indicates water loss.
Define water potential.
Water potential is the measure of the relative tendency of water to move from one area to another. Water always moves from an area of high water potential to an area of low water potential.
Describe the expected change in mass of a potato strip immersed in a solution with a higher (less negative) water potential than the potato cells.
The potato strip would gain mass as water moves from the solution into the potato cells via osmosis. Water moves from high to low water potential.
What happens to an animal cell when placed in a solution with a lower (more negative) water potential?
The animal cell will lose water by osmosis and shrivel (crenation). Because animal cells lack a cell wall, they can burst or shrink depending on the direction of water movement.
What happens to a plant cell when placed in a solution with a lower (more negative) water potential?
The plant cell will lose water by osmosis, the cytoplasm shrinks and the cell membrane pulls away from the cell wall (plasmolysis).
Explain why plant cells do not burst when placed in distilled water (a solution with a higher water potential).
Plant cells have a rigid cell wall that provides support and prevents the cell from bursting due to the influx of water. Instead, the cell becomes turgid.
Describe how you could experimentally determine the water potential of potato tissue.
Create a range of sucrose solutions of known water potentials. Measure the mass of several potato cores, immerse each in a different solution, and re-weigh after a set time. Plot a graph of change in mass against water potential. The water potential where the change in mass is zero is the estimated water potential of the potato tissue.
Explain why it is important to ensure all potato cores are the same size/mass at the start of an experiment to determine the water potential of potato tissue.
Starting with the same size/mass allows for a fair comparison between the different solutions. If the cores were different sizes to begin with, it would not be possible to attribute mass changes to differences in water potential alone.
More topics in Unit 4 — Cell membranes and transport
Movement into and out of cells sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Movement into and out of cells deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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