AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Movement across membranes occurs by: simple diffusion (involving…
Full specification wording
Transport across cell membranes: Movement across membranes occurs by: simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer) facilitated diffusion (involving the roles of carrier proteins and channel proteins) osmosis (explained in terms of water potential) active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP) co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum).
Simple diffusion is the passive net movement of particles down a concentration gradient.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Simple diffusion is the passive net movement of particles down a concentration gradient. The phospholipid bilayer limits this to small, non-polar molecules like oxygen. Facilitated diffusion is also passive but uses channel proteins for ions or carrier proteins that change shape to transport molecules like glucose. Osmosis is the diffusion of water across a partially permeable membrane from a higher to a lower water potential. Active transport uses carrier proteins and ATP hydrolysis to move substances against their gradient. Co-transport involves moving two substances together; in the mammalian ileum, sodium ions are actively pumped out, creating a gradient that drives sodium and glucose absorption into the cell via a co-transporter protein.
What earns the marks
- Simple diffusion is passive net movement down a concentration gradient, restricted by the bilayer to small non-polar molecules.
- Facilitated diffusion uses channel proteins or carrier proteins to move substances down a gradient without ATP.
- Osmosis is the movement of water across a partially permeable membrane from a higher to a lower water potential.
- Active transport requires carrier proteins and the hydrolysis of ATP to move substances against a concentration gradient.
- Co-transport in the ileum involves sodium ions moving down their gradient, driving the absorption of glucose against its gradient via a co-transporter.
Common mistakes and how to put them right
Describing a water potential as higher when it is more negative.
Put it right: Pure water is 0 kPa and solutions are negative.
Stating that facilitated diffusion uses ATP, or that active transport can use channel proteins.
Describing glucose entering ileum cells directly by active transport, rather than by co-transport with sodium ions.
Exam tips
- Specify the protein involved: channel or carrier for facilitated diffusion, but only carrier for active transport and co-transport.
- For osmosis, always phrase the gradient in water potential terms and include 'partially permeable membrane'.
Try the lesson
The start of the Transport across cell membranes: Movement across membranes occurs by: simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer) facilitated diffusion (involving the roles of carrier proteins and channel proteins) osmosis (explained in terms of water potential) active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP) co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum). lesson, made from checked teaching notes for this specification point.
Have a go first
Which process of movement across membranes is illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum?
Listen
Simple diffusion is the passive net movement of particles down a concentration gradient. The phospholipid bilayer limits this to small, non-polar molecules like oxygen. Facilitated diffusion is also passive but uses channel proteins for ions or carrier proteins that change shape to transport molecules like glucose. Osmosis is the diffusion of water across a partially permeable membrane from a higher to a lower water potential. Active transport uses carrier proteins and ATP hydrolysis to move substances against their gradient. Co-transport involves moving two substances together; in the mammalian ileum, sodium ions are actively pumped out, creating a gradient that drives sodium and glucose absorption into the cell via a co-transporter protein. A strong answer earns marks for these points. Simple diffusion is passive net movement down a concentration gradient, restricted by the bilayer to small non-polar molecules. Facilitated diffusion uses channel proteins or carrier proteins to move substances down a gradient without ATP. Osmosis is the movement of water across a partially permeable membrane from a higher to a lower water potential. Active transport requires carrier proteins and the hydrolysis of ATP to move substances against a concentration gradient. Exam tip: Specify the protein involved: channel or carrier for facilitated diffusion, but only carrier for active transport and co-transport.
See it
Key words
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simple diffusion
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facilitated diffusion
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osmosis
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active transport
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See every definition
- simple diffusion
- Passive net movement of particles down a concentration gradient, restricted by the bilayer to small non-polar molecules.
- facilitated diffusion
- Passive movement of substances down a gradient without ATP, using channel proteins or carrier proteins.
- osmosis
- Movement of water across a partially permeable membrane from a higher to a lower water potential.
- active transport
- Movement of substances against a concentration gradient, requiring carrier proteins and the hydrolysis of ATP.
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Test your understanding of Transport across cell membranes: Movement across membranes occurs by: simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer) facilitated diffusion (involving the roles of carrier proteins and channel proteins) osmosis (explained in terms of water potential) active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP) co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum).
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