Ever wonder how your cells decide what gets in and...
Biology114 views·Updated 12 Jul 2026·3 pagesAQA A-Level Transport Across Cell Membranes Comprehensive Notes
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The Fluid-Mosaic Model
Think of your cell membrane as a flexible sandwich made mostly of phospholipids arranged in a double layer (bilayer). These phospholipids have water-loving heads and water-hating tails, which creates a perfect barrier that's both protective and functional.
Floating within this phospholipid bilayer are various proteins that act like doorways and transporters. Peripheral proteins sit on the surface, whilst integral proteins span right through the membrane, often serving as channels or pumps for moving substances across.
You'll also find glycoproteins and glycolipids decorating the outside surface like name tags - these help cells recognise each other and communicate. Cholesterol molecules scattered throughout act like membrane stabilisers, keeping everything at just the right consistency regardless of temperature changes.
Key Point: The membrane is "fluid" because molecules can move around, and "mosaic" because it's made up of many different components working together!
2
of 3
Transport Types and Diffusion
Cell transport comes in two main flavours: passive transport (no energy required) and active transport (energy needed). Diffusion is the star of passive transport - it's simply particles moving from crowded areas to less crowded ones, like people spreading out in a room.
The membrane's semi-permeability means it's picky about what passes through. Small, nonpolar molecules like oxygen slip through easily, whilst small polar molecules like water need a bit more effort. Large molecules and ions typically require special protein channels to get across.
Osmosis is diffusion's water-focused cousin - it's specifically about water moving through membranes towards areas with higher solute concentration. This creates different scenarios: isotonic (balanced), hypotonic (water rushes in), and hypertonic (water leaves the cell).
Remember: Plant cells handle water pressure better than animal cells because of their rigid cell walls - they can become turgid without bursting!
3
of 3
Active Transport and Diffusion Factors
Sometimes cells need to move substances against the concentration gradient - from low to high concentration areas. This is where active transport comes in, using energy from ATP to power the process through specialised protein pumps.
Co-transport is a clever system where two substances hitchhike together across the membrane. Symport moves them in the same direction, whilst antiport sends them in opposite directions - like the sodium-glucose transport in your intestines that helps absorb nutrients.
Several factors affect how quickly diffusion happens: concentration gradient (the driving force), surface area to volume ratio, distance, temperature, and the number of available channel proteins. Fick's law neatly summarises this - faster diffusion occurs with larger surface areas and concentration differences, but slows down over longer distances.
Exam Tip: Remember that active transport always requires energy and moves substances uphill against their natural flow - think of it as cellular weightlifting!
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Biology114 views·Updated 12 Jul 2026·3 pagesAQA A-Level Transport Across Cell Membranes Comprehensive Notes
cait@caittt
Ever wonder how your cells decide what gets in and what stays out? Cell membranes are like incredibly smart bouncers at a club - they're selective about who passes through and use different methods to control the flow of substances...
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The Fluid-Mosaic Model
Think of your cell membrane as a flexible sandwich made mostly of phospholipids arranged in a double layer (bilayer). These phospholipids have water-loving heads and water-hating tails, which creates a perfect barrier that's both protective and functional.
Floating within this phospholipid bilayer are various proteins that act like doorways and transporters. Peripheral proteins sit on the surface, whilst integral proteins span right through the membrane, often serving as channels or pumps for moving substances across.
You'll also find glycoproteins and glycolipids decorating the outside surface like name tags - these help cells recognise each other and communicate. Cholesterol molecules scattered throughout act like membrane stabilisers, keeping everything at just the right consistency regardless of temperature changes.
Key Point: The membrane is "fluid" because molecules can move around, and "mosaic" because it's made up of many different components working together!
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Transport Types and Diffusion
Cell transport comes in two main flavours: passive transport (no energy required) and active transport (energy needed). Diffusion is the star of passive transport - it's simply particles moving from crowded areas to less crowded ones, like people spreading out in a room.
The membrane's semi-permeability means it's picky about what passes through. Small, nonpolar molecules like oxygen slip through easily, whilst small polar molecules like water need a bit more effort. Large molecules and ions typically require special protein channels to get across.
Osmosis is diffusion's water-focused cousin - it's specifically about water moving through membranes towards areas with higher solute concentration. This creates different scenarios: isotonic (balanced), hypotonic (water rushes in), and hypertonic (water leaves the cell).
Remember: Plant cells handle water pressure better than animal cells because of their rigid cell walls - they can become turgid without bursting!
3
of 3Sign up to see the content. It's free!
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Active Transport and Diffusion Factors
Sometimes cells need to move substances against the concentration gradient - from low to high concentration areas. This is where active transport comes in, using energy from ATP to power the process through specialised protein pumps.
Co-transport is a clever system where two substances hitchhike together across the membrane. Symport moves them in the same direction, whilst antiport sends them in opposite directions - like the sodium-glucose transport in your intestines that helps absorb nutrients.
Several factors affect how quickly diffusion happens: concentration gradient (the driving force), surface area to volume ratio, distance, temperature, and the number of available channel proteins. Fick's law neatly summarises this - faster diffusion occurs with larger surface areas and concentration differences, but slows down over longer distances.
Exam Tip: Remember that active transport always requires energy and moves substances uphill against their natural flow - think of it as cellular weightlifting!
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