3.1.3 Substances are exchanged between organisms and their environment by passive or active transport across exchange surfaces. The structure of plasma membranes enables control of the passage of substances across exchange surfaces.
Plasma membranes
- The arrangement of phospholipids, proteins and carbohydrates in the fluid-mosaic model of membrane structure.
- The role of the microvilli in increasing the surface area of cell-surface membranes.
In GCE AQA Biology you will be asked to know the fluid-mosaic model of the plasma membrane. In this model, the plasma membrane is composed of a flexible phospholipid bilayer, with various proteins, cholesterol, and carbohydrates of different size and shape interspersed within (hence ‘mosaic’).
Phospholipids
- The amphipathic nature of phospholipids makes them naturally form a bilayer in an aqueous environment 🌊
- The hydrophobic, nonpolar fatty acid tails face each other and away from water
- The hydrophilic, polar phosphate heads face water
- This makes them form a bilayer with the phosphate heads facing outwards and a layer of fatty acid tails in between
- The weak hydrophobic interactions between the fatty acid tails mean that the phospholipid bilayer is flexible, as individual phospholipids can move around
- The non-polar layer of fatty acids only allows other nonpolar substances to cross the barrier freely
Embedded in this double layer of phospholipids is a variety of other substances, such as proteins, cholesterol, glycolipids, and glycoproteins
Proteins
- Proteins associated with the plasma membrane can come in two forms
- Integral: embedded in the phospholipid layer
- Peripheral: on the surface of the layer
- Protein functions in the plasma membrane:
- Facilitated diffusion e.g. aquaporins, potassium ion pump in axons
- Active transport e.g. sodium-potassium pump
- Adhesion e.g. skin
- Receptors e.g. insulin hormone receptors
- Immobilised enzymes e.g. maltase in the small intestine
- Provide structural support
Cholesterol
- Cholesterol functions:
- Reduce lateral movement of phospholipids
- Reduce fluidity at high temperatures - restricts molecular motion
- Prevents water from leaking from the cell
Glycolipids
- A glycolipid is a carbohydrate covalently bonded to a lipid
- Lipid portion is embedded in the lipid-soluble fatty acid layer, carbohydrate extends into the aqueous environment outside the cell
- Functions
- Cell-surface receptor for a specific chemical e.g. ABO blood group system is because of different glycolipids
- Increase stability of the cell membrane
- Aid with cell adhesion
Glycoproteins
- A glycoprotein is a carbohydrate attached to the peripheral proteins (outside)
- Functions
- Cell-surface receptor for hormones and neurotransmitters
- Aid with cell adhesion
- Allow for cells to recognise each other as their own
Microvilli
Some special cells also have a feature known as microvilli. Microvilli are finger-like extensions ✌️of the cell-surface membrane.
- Functions
- Increase surface area so the cell has more membrane area across which membrane transport can happen, increasing the rate of these transport methods
- Increase surface area so the cell can have a higher density of protein channels/carrier proteins, to increase the rate of facilitated diffusion/active transport
- Microvilli are found on cells such as the intestinal epithelial cell, which require high rates of membrane transport in order to absorb the products of digestion from the lumen of the small intestine into the blood
And we're all done for today!
References
Toole, G., & Toole, S. (2015). Aqa biology A level. Oxford: Oxford University Press.
Images from
- https://www.quora.com/What-is-the-definition-of-phospholipid-bilayer
- https://www.sciencedirect.com/topics/medicine-and-dentistry/phospholipid-bilayer
- https://vivadifferences.com/structural-difference-between-cilia-and-microvilli/
- https://www.thoughtco.com/cell-membrane-373364