Membrane structure, osmosis and transport

What you’ll learn

• Use the fluid mosaic model to explain membrane properties.
• Compare diffusion, facilitated diffusion, active transport and vesicle transport.
• Predict water movement from water potential and evaluate permeability evidence.

Understand the idea

The fluid mosaic model describes a phospholipid bilayer containing mobile proteins and other components. A typical phospholipid has glycerol joined to two fatty-acid tails and a phosphate-containing head. Hydrophilic heads face aqueous fluids and hydrophobic tails face inward. The non-polar interior restricts many ions and polar solutes; small non-polar molecules can cross more readily. Channel and carrier proteins provide selective routes. Cholesterol affects packing and fluidity, while glycoproteins and glycolipids contribute to recognition. The model is an interpretation of evidence and can be refined when new observations require it.

Diffusion is net movement down a concentration gradient. Facilitated diffusion uses proteins but still moves down the relevant gradient without a direct ATP supply. Carrier proteins can also carry out active transport, coupling energy use to movement against a gradient. ATP hydrolysis is an immediate energy source for many pumps. Endocytosis brings material into vesicles formed from the membrane; exocytosis releases material when vesicles fuse with it. Neither should be confused with a single solute passing through a channel.

Osmosis is the net movement of water through a partially permeable membrane from higher to lower water potential. Adding solute generally lowers water potential. Animal cells may swell or shrink, whereas a plant cell wall limits expansion and allows turgor pressure to develop. Core practical 3 uses released pigment as evidence of membrane damage. Higher absorbance can indicate greater permeability if pigment amount, tissue area, rinsing and exposure time are controlled; temperature and alcohol can affect both the bilayer and membrane proteins.

Key formulae

$$\%\text{ mass change}=\frac{m_{\mathrm{final}}-m_{\mathrm{initial}}}{m_{\mathrm{initial}}}\times100$$

$$\mathrm{ATP+H_2O\rightarrow ADP+P_i}\quad\text{energy is transferred}$$

Worked example

A plant-tissue sample increases from 2.40 g to 2.58 g in a solution. Calculate percentage mass change and state the direction of net water movement.

1. Mass change is 2.58 − 2.40 = 0.18 g.
2. Percentage change is (0.18/2.40) × 100 = 7.5%.
3. Under the stated interpretation that mass change reflects water movement, water entered the tissue down a water-potential gradient.

Answer: A 7.5% increase; net water movement into the tissue.