Transport Across the Plasma Membrane

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 - 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 - 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

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 - 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 - 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 - 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