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Transport across cell membranes - Coggle Diagram
Transport across cell membranes
Structure of cell surface membrane
Phospholipids:
hydrophilic heads point to the outside of the cell-surface membrane attracted by water on both sides
hydrophobic tails point to the centre of the membrane, repelled by water on both sides
lipid soluble substances move through the membrane in this section
prevent water soluble substances entering and leaving the cell
make membrane flexible and self-sealing
Proteins:
some occur on the surface if the bilayer and never extend across it. Give mechanical support to the membrane or act as cell receptors
others span across the bilayer. Some are protein channels which allow water-soluble ions to diffuse across. Others are carrier proteins that bind to ions or molecules then change shape to move the molecules across
Cholesterol:
add strength. Very hydrophobic and so prevent water loss and loss of dissolved ions. Pull fatty acids of phospholipids together, limiting their movement without making the membrane too rigid
make membranes less fluid at high temps
Glycolipids:
made of carbs covalently bonded with a lipid. Carb portion extends from bilayer to watery environment outside the cell, where it acts as a receptor for specific chemicals.
Help attach cells together to form tissues
Glycoproteins:
carb chains attached to extrinsic proteins.
Act as recognition sites, attach cells together to form tissues, allow cells to recognise one another e.g lymphocytes can recognise an organism’s own cells
Permeability of membrane:
most molecules don’t freely diffuse across it as many are not lipid-soluble, too large to pass through channels, of he same charge as charge on protein channels and are repelled, electrically charged so can’t pass through non-polar hydrophobic tails in bilayer.
Fluid-mosaic model:
fluid - individual phospholipid molecules can move relative to each other - makes mmebrane flexible that changes shape constantly
mosaic - proteins are embedded in bilayer and vary in shape, size and pattern.
Diffusion
Simple diffusion:
no external energy - passive
The net movement of molecules or ions from a region of high concentration to an area of lower concentration until evenly distributed.
Facilitated diffusion:
passive, down a conc gradient. Involves protein channels and/or carrier proteins
Protein channels:
form water filled hydrophilic channels across the membrane. Selective only open in the presence of a specific ion. Ion binds the proteins causing it to change shape - one side closes and the other opens to allow passage of the ion through the membrane.
Carrier proteins:
span the membrane. Molecules e.g. glucose bind to the protein causing it to change shape allowing it to pass through without external energy.
Osmosis
The passage of water from a region of high water potential to a region with a higher water potential through a selectively permeable membrane.
Water potential - the tendency of water to move out of a solution (kPa)
In plant cells:
if a plant is placed in a dilute solution, water will move into the plant by osmosis. Volume of cell inc as water enters the vacuole. The expanding protoplast pushes against cell wall and pressure inc in the cell. Inc in pressure prevents water entering and bursting the cell. When cell is fully inflated with water it is turgid. Provides support for the plant. Plant wilts if it doesn’t receive enough water.
If plant placed in solution with low water potential, water leaves via osmosis. Decreases volume as water leaves vacuole, protoplast shrinks and pressure decreases. Protoplast pulls away from the cell wall - plasmolysis.
In animals:
If cell placed in low water potential solution, water leaves by osmosis and cell shrivels up.
If placed in cell with higher potential the cell bursts.
Active transport
Movement of molecules or ions into or out of a cell from a region of high to low conc using ATP and proteins. Energy required to make proteins change shape.
Sodium potassium pump:
Na+ ions actively transported out of epithelial cells into blood via a carrier proteins. Maintains a higher conc of Na+ in lumen than in epithelial cells. Na+ diffuse into epithelial cells down conc gradient through different type of carrier protein. As Na+ diffuse through 2nd protein they carry a.a. Ir glucose in the cell with them. Glucose/ a.a. Pass into the blood plasma by facilitated diffusion using another carrier.
Co-transport and absorption of glucose in the ileum
The coupled movement of substances across a cell membrane via carrier proteins. Involves facilitated diffusion and active transport