2-4-11-adaptations-for-rapid-transport

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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

Diagram comparing a root hair cell in plants absorbing water from soil and an intestinal epithelial cell in animals absorbing digested food molecules. — **Root hair cells in plants and cells lining the ileum in animals are specialised to maximise transport**

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).

Biology AS AQA