Adaptations for Rapid Transport (AQA AS Biology): Revision Note
Exam code: 7401
Adaptations for rapid transport
The rate at which substances are transported across cell membranes varies depending on the type of transport involved (e.g. diffusion, facilitated diffusion, or active transport)
Some cells are specialised to allow rapid transport of molecules across their internal or external membranes to support key functions such as absorption, secretion, or exchange of gases
The rate of transport depends on several factors, including:
temperature
surface area of the exchange surface
concentration gradient across the membrane
thickness (or diffusion distance) of the exchange surface
number of protein channels or carrier proteins
availability of ATP (for active transport)
Adaptation | How it Increases Transport | Example |
|---|---|---|
Increased surface area | More membrane surface allows more substances to cross simultaneously | Microvilli on epithelial cells in the small intestine |
More channel proteins | Allows faster facilitated diffusion of specific ions or polar molecules | Na⁺/K⁺ channels in neurones |
More carrier proteins | Speeds up facilitated diffusion and active transport of larger molecules | Glucose carriers in kidney tubules and intestinal epithelium |
Thin exchange surface | Reduces diffusion distance, speeding up the rate of diffusion | Alveolar and capillary walls are one cell thick |
Rich blood supply | Maintains a steep concentration gradient by constantly removing or supplying substances | Capillary networks in alveoli and villi |
Ventilation or flow of the surrounding medium | Replaces substances to maintain high/low external concentrations, sustaining a gradient | Ventilation in lungs maintains O₂/CO₂ gradients |
Many mitochondria | Provides more ATP for active transport, supporting uptake against a concentration gradient | Root hair cells for ion uptake from the soil |
Examples of transport in specialised cells
Root hair cells
Adapted for the absorption of water and mineral ions from the soil
They have long ‘hair-like’ projections
This increases the surface area, boosting the rate of osmosis and active transport
A thin cell wall
This gives a shorter diffusion distance for water
The permanent vacuole stores water and mineral ions as they enter the cell
This helps to maintain a steep water potential gradient
Epithelial cells of the small intestine
Adapted for the absorption of digested food molecules
They have microvilli on the surface
This provides a large surface area for increased diffusion
A rich capillary network continually transports the products of digestion away from the epithelial cells
This ensures a steep concentration gradient
Many co-transport proteins
This facilitates active uptake of glucose and amino acids
Cells in the collecting duct of the kidney
Adapted for the uptake of water
These cells have membranes that contain a very high number of aquaporins
Aquaporins are special channel proteins that allow the facilitated diffusion of water through cell membranes
This allows these kidney cells to reabsorb water
Neurones and muscle cells
Adapted for the transport of sodium, potassium and calcium across the membrane
necessary for the transmission of electrical impulses around the body
Cell membranes in these cells have channel proteins for sodium, potassium and calcium ions
The opening and closing of ion channel proteins, and the number of channels present, affect how quickly ions move by facilitated diffusion
This directly influences the speed of electrical transmission:
Along neurone membranes during nerve impulses
Across muscle cell membranes during muscle contraction
Examiner Tips and Tricks
In the case of the kidney cells described above, water is transported across the cell membrane via facilitated diffusion through channel proteins. Don’t forget, however, water can also diffuse through cell membranes (this can occur even though it is a polar molecule because it is a relatively small molecule).
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