OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(a) the roles of membranes within cells and at the surface of cells
Full specification wording
Biological membranes: (a) the roles of membranes within cells and at the surface of cells
Membranes are partially permeable barriers formed from a phospholipid bilayer with proteins, cholesterol and glycoproteins.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 1 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Membranes are partially permeable barriers formed from a phospholipid bilayer with proteins, cholesterol and glycoproteins. At the cell surface, the membrane separates the cytoplasm from the external environment, controls which substances enter and leave, and carries receptor molecules for cell signalling. Within cells, membranes surround organelles such as the nucleus, mitochondria and chloroplasts, compartmentalising reactions so incompatible processes occur in separate regions. Internal membranes also increase surface area for reactions, for example the folded inner membrane (cristae) of mitochondria, and form transport vesicles. Membranes therefore act as selective barriers, sites of signalling and sites of enzyme-catalysed reactions.
What earns the marks
- The cell-surface membrane separates the cytoplasm from the external environment and controls the entry and exit of substances.
- Membranes are partially permeable because of the phospholipid bilayer, allowing selective movement of substances.
- Membranes within cells compartmentalise organelles, keeping different reactions separate.
- Internal membranes such as the cristae increase surface area for enzyme-controlled reactions.
- Membrane proteins act as receptors for cell signalling and as carriers or channels for transport.
Common mistakes and how to put them right
Describing membranes as fully permeable.
Put it right by stating they are partially permeable and control movement.
Ignoring internal membranes and only describing the cell surface.
Put it right by giving roles of organelle membranes such as compartmentalisation.
Treating all membrane proteins as channels.
Put it right by recognising receptor and enzyme roles too.
Exam tips
- Match each option to a specific role: barrier, transport, signalling or compartmentalisation.
- Check whether the option refers to the cell surface or internal membranes, as roles differ.
- Reject options that describe membranes as freely permeable or that omit selective control.
Try the lesson
The start of the Biological membranes: (a) the roles of membranes within cells and at the surface of cells lesson, made from checked teaching notes for this specification point.
Have a go first
Which role is carried out by the cell surface membrane but not by intracellular membranes?
Listen
Membranes are partially permeable barriers formed from a phospholipid bilayer with proteins, cholesterol and glycoproteins. At the cell surface, the membrane separates the cytoplasm from the external environment, controls which substances enter and leave, and carries receptor molecules for cell signalling. Within cells, membranes surround organelles such as the nucleus, mitochondria and chloroplasts, compartmentalising reactions so incompatible processes occur in separate regions. Internal membranes also increase surface area for reactions, for example the folded inner membrane (cristae) of mitochondria, and form transport vesicles. Membranes therefore act as selective barriers, sites of signalling and sites of enzyme-catalysed reactions. A strong answer earns marks for these points. The cell-surface membrane separates the cytoplasm from the external environment and controls the entry and exit of substances. Membranes are partially permeable because of the phospholipid bilayer, allowing selective movement of substances. Membranes within cells compartmentalise organelles, keeping different reactions separate. Internal membranes such as the cristae increase surface area for enzyme-controlled reactions. A common slip: Describing membranes as fully permeable. Instead, stating they are partially permeable and control movement. Exam tip: Match each option to a specific role: barrier, transport, signalling or compartmentalisation.
See it
Watch it explained
What is the one idea the video explains? Say it in a sentence.
Read the described scene instead
Membranes are partially permeable barriers. They are formed from a phospholipid bilayer with proteins, cholesterol and glycoproteins. At the cell surface, a membrane separates the cytoplasm from the external environment. It controls which substances enter and leave the cell, and it carries receptor molecules for cell signalling. Inside cells, membranes surround organelles such as the nucleus, mitochondria and chloroplasts. This compartmentalises reactions, so incompatible processes occur in separate regions. Internal membranes also increase surface area for reactions, and they can form transport vesicles. So membranes can act as selective barriers, signalling sites, and sites of enzyme-catalysed reactions. The phospholipid bilayer makes membranes partially permeable. This means some substances move across while others are restricted, so movement is selective. Membrane proteins also have transport roles: they can act as carriers or channels. They are not all channels, because membrane proteins can also be receptors or enzymes. A common mistake is describing membranes as fully permeable. Correct this by saying they are partially permeable and control movement. Another mistake is ignoring internal membranes and only describing the cell surface. Remember organelle membranes compartmentalise reactions. Also, do not treat all membrane proteins as channels; some are receptors or enzymes. When you consider a membrane role, match it to a specific function: barrier, transport, signalling or compartmentalisation. Check whether the statement refers to the cell surface or to internal membranes, because their roles differ. Reject descriptions that say membranes are freely permeable or that omit selective control. These checks follow the exam tips.
Key words
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Phospholipid bilayer and partial permeability
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Compartmentalisation within cells
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Cell signalling and receptors
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Surface area for reactions
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See every definition
- Phospholipid bilayer and partial permeability
- A partially permeable barrier formed from a phospholipid bilayer with proteins, cholesterol and glycoproteins, allowing selective movement of substances.
- Compartmentalisation within cells
- Membranes surround organelles such as the nucleus, mitochondria and chloroplasts, keeping different reactions separate so incompatible processes occur in separate regions.
- Cell signalling and receptors
- Membrane proteins act as receptors for cell signalling, and the cell-surface membrane carries receptor molecules for cell signalling.
- Surface area for reactions
- Internal membranes increase surface area for reactions, for example the folded inner membrane (cristae) of mitochondria, and form transport vesicles.
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