AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Cells may be adapted for rapid transport across their internal or external…
Full specification wording
Transport across cell membranes: Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes.
The rate at which a cell can move substances across a membrane depends on how much membrane there is and on how many transport proteins are set into it.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 5 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The rate at which a cell can move substances across a membrane depends on how much membrane there is and on how many transport proteins are set into it. Cells specialised for absorption increase surface area by folding the membrane: the epithelial cells lining the ileum bear microvilli, finger-like projections of the cell-surface membrane, and so do the cells of the proximal convoluted tubule in the kidney. The same strategy appears on internal membranes, where the inner mitochondrial membrane is folded into cristae so that more electron carriers and ATP synthase can be accommodated, and thylakoids stack into grana to hold more photosystems.
What earns the marks
- An increased surface area of membrane, produced by microvilli or by folding such as cristae.
- An increased number of channel or carrier proteins in the membrane.
- Linking either adaptation to a faster rate of movement across the membrane.
- The converse: a reduced surface area, or fewer co-transport, carrier or channel proteins, reduces the rate of absorption.
- Relating reduced absorption to a change in water potential and so to water moving by osmosis in the opposite direction.
Common mistakes and how to put them right
Writing villi when the cell-level adaptation required is microvilli.
Offering more mitochondria as the answer when the question specifies surface area or number of proteins.
Saying more channel proteins speed up simple diffusion, which does not use proteins at all.
Claiming that increasing surface area makes the concentration gradient steeper.
Describing the adaptation without saying what it achieves, so the rate of transport is never mentioned.
Exam tips
- Match the adaptation to the transport process named in the stem; proteins matter for facilitated diffusion, active transport and co-transport, surface area for all of them.
- Internal membranes count too, so cristae and grana are valid answers when the question says internal.
- When damage to a gut lining is described, follow the chain through: fewer proteins or less surface area, then water potential, then osmosis.
Try the lesson
The start of the Transport across cell membranes: Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes. lesson, made from checked teaching notes for this specification point.
Have a go first
Which adaptation of a cell increases the rate of transport across its external membrane?
Listen
The rate at which a cell can move substances across a membrane depends on how much membrane there is and on how many transport proteins are set into it. Cells specialised for absorption increase surface area by folding the membrane: the epithelial cells lining the ileum bear microvilli, finger-like projections of the cell-surface membrane, and so do the cells of the proximal convoluted tubule in the kidney. The same strategy appears on internal membranes, where the inner mitochondrial membrane is folded into cristae so that more electron carriers and ATP synthase can be accommodated, and thylakoids stack into grana to hold more photosystems. A strong answer earns marks for these points. an increased surface area of membrane, produced by microvilli or by folding such as cristae. an increased number of channel or carrier proteins in the membrane. linking either adaptation to a faster rate of movement across the membrane. the converse: a reduced surface area, or fewer co-transport, carrier or channel proteins, reduces the rate of absorption. Exam tip: Match the adaptation to the transport process named in the stem; proteins matter for facilitated diffusion, active transport and co-transport, surface area for all of them.
Key words
Say what each word means before you turn the card.
microvilli
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cristae
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number of transport proteins
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rate of transport
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See every definition
- microvilli
- Finger-like projections of the cell-surface membrane that increase the surface area of cells specialised for absorption, such as the epithelial cells lining the ileum.
- cristae
- Folds of the inner mitochondrial membrane that increase surface area so that more electron carriers and ATP synthase can be accommodated.
- number of transport proteins
- An increase in the protein channels and carrier molecules set into a membrane raises the rate at which substances are moved across it.
- rate of transport
- The speed at which a cell can move substances across a membrane depends on how much membrane there is and on how many transport proteins are set into it.
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