Transport Across the Plasma Membrane
Entry and exit of materials:
diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis
note: be sure to also learn the diagrams for all these processes and be able to explain them
- Created by: WinchesterD
- Created on: 12-12-14 00:54
Diffusion
Diffusion - the movement of molecules or ions from a high concentration to a low concentration until evenly distributed (dynamic equilibrium)
note: diffusion occurs in cells, between cells or outside of cells
Rate of diffusion depends on:
- Temperature - increased temperature increases movement of molecules (kinetic energy); faster rate of diffusion
- Surface area - the larger surface area, the faster the rate
- Concentration gradient - the greater the difference between high and low concentration, faster the rate
- Thickness of exchange surface - the thinner the surface, the faster the rate (short diffusion pathway)
Example of diffusion: diffusion of oxygen into cells; carbon dioxide out of cells
Fick's law:
Rate is proportional to - Surface Area x Concentration Difference
Thickness
The bigger the number the faster the rate
note: small molecules diffuse faster than large ones
fat soluble molecules diffuse faster than water soluble
Facilitated Diffusion
Facilitated diffusion - the movement of molecules or ions from a high concentration to a low concentration until evenly distributed through carrier or channel proteins
Passive process - no ATP used, relies on kinetic energy of the diffusing molecules
Factors that affect rate:
- Surface area - more channels/carriers increase rate
- Temperature - increased temperature means an increased rate
- Concentration gradient - steeper the gradient faster the rate
- Channel and Carrier proteins - channel proteins cannot change shape; are either open or closed 'gates'. They are selective e.g. channel for Na+; channel for water soluble molecules - - carrier can change shape; e.g. uptake of glucose in gut
Active Transport
Active transport - the movement of molecules from a low to high concentration across a carrier protein and using energy from ATP
Carrier proteins only
ATP is hydrolysed to release energy and the energy allows the protein carrier to change shape
e.g. uptake of amino acids in the gut; sodium/potassium pump
Co-Transport of Sodium and Glucose
Diffusion and facilitated diffusion result in an equilibrium - so the maximum amount of glucose absorbed this way would only be 50% of the concentration
Glucose is also drawn into the cells along with Na+:
- Sodium pumped out of epithelial celss by sodium/potassium (active transport) pump (carrier proteins, ATP) into blood
- Therefore more sodium in the lumen of intestine so it tends to diffuse back into cells from the lumen
- As sodium diffuses back in (along a concentration gradient) it brings glucose with it. It couples with the glucose and they both come in via a co-transport carrier protein
- Glucose then passes into the blood by facilitated diffusion using another carrier
Both glucose and sodium move into the cell, but sodium down the concentration gradient and glucose up the concentration gradient
note: sodium ion concentration powers this movement against the concentration gradient, rather than ATP = indirect form of active transport
Osmosis
Osmosis - is the net movement of water molecules from a higher to a lower water potential across a partially permeable membrane
Water potential - the tendancy of water molecules to move; measured in KPa (negative scale)
Adding solutes to a cell (glucose or ions) lowers water potential
Hypotonic - lots of water Hypertonic - less water Isotonic - equal
The cell wall of a plant prevents the cell from bursting
Protoplast - cell membrane, cytoplasm and nucleus together
Incipient plasmolysis - protoplast beginning to pull away from cell wall
- When the concentration of water outside the cell is high (e.g. in pure water) the water potential is high; protoplast swells; cell is turgid
- When the concentration of water outside the cell is equal the water potential is equal; protoplast doesn't change; cell is in a state of incipient plasmolysis
- When the concentration of water outside the cell is lower (e.g. in concentrated sucrose) the water potential is lower; protoplast shrinks; cell is plasmolysed (flaccid)
note: osmosis is a special kind of diffusion; water only
water passes through special channels called aquqporins
no energy is required
Endocytosis
Endocytosis - is transport into cells and is by the infolding of the cell surface membrane to form a vacuole or vesicle
Active process; using ATP
There are two types:
- Phagocytosis - solids; animals only e.g. white blood cells engulfing bacteria
- Solid particle (e.g. food or bacterium)
- Particle enclosed in vacuole
- Lysosome with digestive enzymes
- Lysosome fuses with vacuole
- Digestion of particle, useful substances absorbed
- Exocytosis of waste
- Pinocytosis - liquids; plants and animals e.g. uptake of fats in the gut
- adsorption of liquid (e.g. fat)
- infolding of membrane
- vacuole formed
- vacuole breaks down, releasing liquid
Exocytosis
Exocytosis - this is the release of substances out of the cell e.g. secretion of enzymes; removal of waste materials
Active process; uses ATP
Occurs in plants and animals
- mitochondira produces ATP
- ATP -> energy -> nucleus
- Rough ER - protein synthesis
- vesicle from rough ER with protein
- golgi body
- golgi vesicle
- secretery vesicle
- exocytosis
- the protein is secreted
note: proteins can be modified at the golgi body e.g. protein + carbohydrate -> glycoprotein
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