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2.5 Biological Membranes - Coggle Diagram
2.5 Biological Membranes
2.5.1 The structure of cell membranes
Partially permeable
Separates the cell contents from its exterior environment
Allows small molecules to diffuse through between structural molecules
Allows small molecules to dissolve in the lipid layer and pass through
other substances can pass through via carrier proteins and channel proteins
Roles of membranes
Within the cell
Cristae in mitochondria have a large surface area for aerobic respiration and respiration for enzymes
Thylakoid membranes (inner membrane in chloroplasts) house chlorophyll to allow photosynthesis to occur
There are digestive enzymes on the plasma membrane of epithelial cells that line the small intestine, these enzymes catalyse the breakdown of sugars
At the surface of cells
Separates the cell components from its external environment. (e.g. cytoplasm, tissue fluid etc)
Regulate transport of materials into and out of the cell
Can contain enzymes for metabolic pathways
Contains antigens so the immune system can recognise the cell
Can release chemicals to signal other cells
Contain receptors for chemical signaling and cell communication
May be the site of chemical reactions
The fluid mosaic model
Made up of two layers of phospholipid molecules
Hydrophilic heads are in contact with the watery interior or exterior
Hydrophobic tails are in the centre of the membrane away from water
Proteins on the surface on the membrane
Some have pores and act as channels to allow ions to pass through
Protein carriers change shape to carry specific molecules across the membrane
Carrier proteins may have enzymes, antigens or receptor sites for complementary-shaped signaling chemicals such as hormones
Eukaryotic cell membranes contain cholesterol which helps to regulate fluidity of the membrane, maintain chemical stability and resist temperature change
Outside the membrane is the glycocalyx. (Formed from carbohydrate chains attached to either lipids (glycolipid) or proteins (glycoproteins) in the membrane)
2.5.2 Diffusion across membranes
Simple diffusion
Molecules have kinetic energy and can move freely within gas or liquid media. (Relies only on kinetic energy)
If there is high concentration of a type of molecule in an area, the molecules will collide and spread further from each other.
More molecules will move to an area of low concentration until they eventually become dispersed
Some small molecules can pass through cell membranes by simple diffusion
Fat-soluble molecules can diffuse through the cell membrane as they dissolve in the lipid bilayer.
Water is polar and insoluble on the lipid bilayer so the phospholipid bilayer would seem impermeable. Due to water being present in a high concentration, diffusion does occur.
Aquaporins may be present in some membranes when a high rate of water movement is required
How the concentration gradient is maintained
Many molecules pass into organelles within cells and are used for metabolic reactions
Oxygen diffusing into the cytoplasm of respiring cells diffuse into mitochondria and used for aerobic respiration
Carbon dioxide diffusing into the palisade mesophyll of a plant leaf will diffuse into chloroplasts and be used for photosynthesis
Factors that affect the rate of simple diffusion
Temperature - as temperature increases, molecules have more kinetic energy, rate of diffusion increases
Diffusion distance - the thicker the membrane, the slower the rate of diffusion
Surface area - more diffusion cam take place across a larger surface area. (microvilli increase surface area)
Size of diffusing molecule - smaller ions or molecules diffuse faster than larger molecules.
Concentration gradient - The steeper the gradient the faster the diffusion
Facilitated diffusion