2- Diffusion and facilitated diffusion

What is diffusion?

• Movement of molecules rom an area of high to low concentration

• Passive: does not use ATP

• May or may not move through a membrane

• O2, CO2 and some water

Diffusion and kinetic energy

• All molecules have kinetic energy and can move freely in a gas or liquid

• If there’s a high concentration of a molecule in an area then molecules will randomly collide as they move and will spread out

• More will move into an area where they’re in lower concentration

• When molecules have moved down their concentration gradient and are evenly dispersed they have reached equilibrium

Small and fatty soluble molecules

• Small molecules like O2 and CO2 pass directly through the membrane

• Fat soluble molecules such as steroid hormones can diffuse directly across as they dissolve in the bilayer

• Flat soluble molecules still move down their concentration gradient

Factors affecting rate of diffusion

Distance (thickness of membranes):

• The ticker the membrane the molecules have to cross, the slower the rate of diffusion

Temperature:

• Temp increase= more kinetic energy= molecules move faster= faster rate of diffusion

Surface area:

• Folded membranes and microvilli increase surface area

• More diffusion can occur over a larger surface area as there’s more space for it to move across

• Cells specialised for absorption have extensions to the CSM called microvilli

Size of diffusing molecule:

• Smaller ions or molecules diffuse more rapidly than larger ones as they have more kinetic energy so can move faster

Concentration gradient:

• The steeper the concentration gradient, the faster the rate of diffusion

• Maintaining gradients allows for very fast diffusion

Maintaining concentration gradients

• Molecules entering cells pass into organelles, which maintains the concentration gradient

• Oxygen diffuses into the cell and then into the mitochondria and is used for aerobic respiration

• Carbon dioxide diffusing into the palisade mesophyll cells in a leaf and then into chloroplast

Facilitated diffusion

• Large, polar or charge molecules that don’t act with the non-polar bilayer

• Can’t interact with the fatty acid tails of PBL

• Proteins in membrane transport these molecules

• Down the concentration gradient

What must happen in order for proteins to carry out their function?

• They must fold into a particular 3D shape (known as its conformation)

• Molecules/ ions must be able to bind to the shape of the protein in order to be transported (complimentary to the protein channel)

• This makes transport proteins specific to the ion they move

• If their shape changes, the molecule/ion will no longer be complementary to the protein- so transport stops

• High temperatures outside of protein optimum disrupt the 3D shape of protein by breaking H-bonds holding it together. This shape change is irreversible- denaturing

• Once denatured, gaps/holes appear in the membrane which makes the cell prone to leaking outs contents-n as the physical barrier is now imperfect

How do channel proteins do facilitated diffusion?

• Provide crossing point for polar molecules/ ions

• Neurone plasma membranes have channels deco sodium and potassium ions

• Membranes with protein channels are selectively permeable

• Epithelial cells do the respiratory system contain chloride ions which regulate the composition of mucus

How do carrier proteins do facilitated diffusion?

• Water lines centre that allow polar molecules to interact with, or the amino acids lining the channel have polar R groups. R groups determine the amino acid that is present- we use a pool of 20 different amino acids in our protein chains

• Glucose molecules bind to a carrier protein

• The carrier protein then opens to allow the glucose into the cel

Factors effecting facilitated diffusion:

• Same as regular diffusion: temperature, concentration gradient, membrane surface area, membrane thickness

• Specific to facilitated diffusion: Number of channels present and whether or not they’re open, less proteins= slower rate

Facilitated diffusion: water and osmosis

Water:

• Polar and insoluble to the PLBL

• Some diffusion does occur

• Specific water channel proteins (aquaporins)

Osmosis:

• Passive: doesn’t require ATP

• Movement of water from an area of high water potential to low water potential through and partically permeable membrane