2.5 Biological Membranes

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2.5.1 The structure of a cell membrane

The role of membranes at the surface of cells:

- separates the cell's components from its external environment

- regulates transport of materials in and out the cell

- may contain enzymes involved in specific metabolic pathways

- has antigens, so that the organisms immune system can recognise the cell as being 'self'

- may release chemicals to signal other cells

- contains receptors for such chemical signals, so site of cell communication or signalling (hormones can bind)

- may be the site of chemical reactions

The role of membranes within cells:

- separates each organelle so they are a discrete entity and able to perform its function

- mitochondria have filed membrane to increase surface area

- inner membrane of chloroplasts house chlorophyll

- there are some digestive enzymes on the plasma membrane of epithelial cells

The fluid mosaic model:

The theory of the cell membrane structure with proteins embedded in a sea of phospholipids. Spanning the membrane are various proteins:

- some of them have pores and act as channels

- some proteins are carriers and, by changing their shape, carry specific molecules across the membrane

- other proteins may be attached to the carrier proteins and function as enzymes, antigens or receptor sites for complementary-shaped signalling chemicals such as hormones

- outside the membrane is glycoproteins and glycolipids, forming the glycocalyx.

2.5.5 Factors affecting membrane structure and permeability

When temperature drops:

- saturated fatty acids become compressed, however the other many unsaturated fatty acids kink pushing adjacent phospholipid molecules away and maintains fluidity

- cholesterol aso buffers the effect of lowered temperature by preventing reduction in the membranes fluidity

When temperature increases:

- the phospholipids acquire more kinetic energy increasing permeability and fluidity

- it also affects the membrane-embedded proteins making them denatured and cell signalling

- the cholesterol buffers the fluidity to some extent

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2.5.2 Diffusion across membranes

Simple diffusion:

Movement of molecules from an area of high concentration of that molecule to an area of low concentration, across the membrane and does not require ATP. Some molecules such as oxygen and carbon dioxide are small so can just pass through the plasma membrane, or fat-soluble molecules can dissolve.

Factors:

-Temperature: higher temp, more kinetic energy, higher rate of diffusion.

-Diffusion distance: shorter the distance, higher rate of diffusion.

-Surface area: more diffusion can take place over a larger surface area.

-Size of diffusing molecule: smaller ions or molecules diffuse more rapidly.

-Concentration gradient: the steeper the gradient the faster the rate of diffusion.

Facilitated diffusion:

Movement of molecules from an area of high concentration of that molecule to an area of low concentration, across a partially permeable membrane via protein channels or carriers, not involving ATP. It happens to small molecules that have polarity, such as ions as they are insoluble in the lipid bilayer.

2.5.3 Osmosis

Key words:

Osmosis: passage of water molecules down their water potential gradient, across a partially permeable membrane

Water potential: measure of the tendency of water molecules to diffuse from one region to another. Pure water has the highest water potential at 0kPa.

Cells in pure water:

Water moves in by osmosis down the water potential gradient. Plant cell wall prevents bursting and becomes turgid. Animal cell bursts open and is cytolysed.

Cells in concentrated sugar solution:

Water moves out of cells by osmosis down a water potential gradient. Plant cell membrane pulls away from cell wall and is plasmolyse. Animal cell shrinks and appears wrinkled so is crenated.

2.5.4 How substances cross membranes using active processes