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Cell membranes (Fluid mosaic model (Cholesterol (In eukaryotic cell…
Cell membranes
Fluid mosaic model
Explains how cell membranes could be more dynamic and interact more with the cell's environment
Singer and Nicolson
Membrane consists of a phospholipid bilayer
Proteins float in it
Some have pores
Act as channels to allow ions to pass through
Some are carriers
Can carry specific molecules across the membrane by changing shape
Some proteins may be attached to carrier proteins
Function as
Enzymes
Antigens
Receptor sites
For complementary-shaped signalling chemicals such as hormones
Lipid molecules can change places with each other
Giving fluidity
Two layers of phospholipid molecules
Hydrophilic heads
In contact with watery exterior or interior
Hydrophobic tails
Centre of the membrane
Cholesterol
In eukaryotic cell membranes
Regulates membrane fluidity
Maintain mechanical stability
Resist effects of temperature changes on the structure of the membrane
Glycocalyx
Outside membrane
Formed from the carbohydrate chains attached to either
Lipids
Glycolipids
Proteins
Glycoproteins
Different cell membrane compositions
Neurones
Protein channels
Allow entry and exit of ions to bring about the conduction of electrical impulses
Protein carriers
Myelin sheath
Formed by flattened cells wrapped around them several times
Gives several layers of the cell membrane
White blood cells
Special protein receptors
Enable them to recognise the antigens on foreign cells
Root hair cells
Carrier proteins
Actively transport nitrate ions
Roles of membranes
Surface of cells
Separates cell's components from its external environment
Regulates transport of materials in and out of cell
Has antigens
Organism's immune system can recognise the cell as being 'self' and not attack
May release chemicals to signal to other cells
Contains receptors to do so
Site of cell communication
Hormones and drugs may bind to the receptors
Site of chemical reactions
Within cells
Mitochondria
Cristae
Large surface area for reactions
Localise some of the enzymes needed for respiration to occur
Chloroplasts
Thylakoid
House chlorophyll
Some photosynthesis reactions occur
Plasma membranes of epithelial cells
Digestive enzymes
Line of small intestines
Catalyse some of the final stages in the breakdown of certain types of sugars
Permeability
Refers to the ability to let substances pass through
Very small molecules can diffuse through the cell membrane
Some substances dissolve in the lipid layer and pass through
Some substances pass through special protein channels
Some are carried by carrier proteins
Factors affecting membrane permeability
Temperature
Increase
Phospholipids acquire more kinetic energy and move around more in a random way
Increases membrane fluidity
May affect infolding of plasma membrane during phagocytosis
Can also affect cell signalling
Cholesterol can act as a buffer
Reduces the increase in membrane permeability
Membrane permeability increases
Membrane embedded proteins' positions can be altered
Proteins that act as enzymes can drift aside
Can alter the rate of reactions they catalyse
Decrease
Saturated fatty acids become compressed
Unsaturated fatty acids become compressed and push adjacent phospholipid molecules away
Maintains membrane fluidity
Proportion of unsaturated and saturated fatty acids within the cell membrane determine membrane's fluidity
Cholesterol
Buffers effect of lowered temperature to prevent reduction in membrane's fluidity
Prevents phospholipid molecules from packing too closely