Topic 1 : Cells
1.3 Membrane structure
Membrane Structure
Properties of cell membrane
- Cell membranes are composed of phospholipids that consist of a hydrophilic (attracted to water) head and a hydrophobic (repelled by water)
tail - The phospholipid head contains a negatively charged phosphate group which because of its charge is attracted water because of its polarity
- The fatty acid hydrocarbon tail has no charge and is therefore repelled by water
- When placed in water, the phospholipids naturally form a double layer with the heads facing outwards towards the water and the tails facing each
other inwards (micelle or liposome) - This forms a very stable structure that surrounds the cell because of the attractions and bonds that are formed between the heads to the water
and to each other, and the hydrophobic interactions between the tails - Even though it is a very stable structure, it is still fluid, as the phospholipids can move along the horizontal plane
- To increase stability, many cells have cholesterol embedded between the phospholipids
Phospholipids
- A bilayer produced from huge numbers of molecules
- Composed of a three-carbon compound called glycerol
- Two of the glycerol carbons have fatty acids
- The third carbon is attached to a highly polar organic alcohol that includes a bond to
a phosphate group
Cholesterol
- Allow membrane to function effectively at a wider range of temperatures
- Plant cells do not have cholesterol molecules
- Cholesterol is a lipid that belongs in the steroid group and is also a component of the cell membrane
- Most of the cholesterol molecule is hydrophobic and therefore embeds within the tails of the bilayer. A small portion (hydroxyl –OH group) is
hydrophilic and is attracted to the phospholipid head - Cholesterol embedded in the membrane will reduce the fluidity making the membrane more stable by the hydrophilic interactions with the
phospholipid heads - While cholesterol adds firmness and integrity to the plasma membrane and prevents it from becoming overly fluid, it also helps maintain its fluidity
by disrupting the regular packing of the hydrocarbon tails. - Therefore, cholesterol helps prevent extremes– whether too fluid, or too firm– in the consistency of the cell membran
Proteins
- Embedded in the fluid matrix of the phospholipids bilayer
Membrane protein functions
- Transport: Protein channels (facilitated) and protein pumps (active)
- Receptors: Peptide-based hormones (insulin, glucagon, etc.) binding——relay the message
- Anchorage: permanent or temporary junctions
- Cell recognition: MHC proteins and antigens
- Intercellular (between cells) joinings: Tight junctions, an identification label
- Enzymatic activity: Metabolic pathways (e.g. electron transport chain)