Transport across membranes

Definition of diffusion

Diffiusion is the net movement of particles from an area of a higher concentration to an area of lower concentration

The molecules will move both ways but the net movement will be to the area of lower concentration until the particles are evenly distributed

Diffusion is a passive process

What is the concentration gradient?

The concentration gradient is the path from an area of higher concentration to an area of lower concentration. particles move down the concentration gradient

Whats simple diffusion?

Simple diffusion is when molecules can move directly through a membrane

What is facilitated diffusion? Why is it needed?

Facilitated diffusion is when large or charged particles through carrier proteins or channel proteins in the membrane instead, and this is a passive process which does not use energy

it is needed as some larger molecules would diffuse extremely slowly through the phospholipid bilayer due to their size

What do carrier proteins do?

Carrier proteins move large molecules across membranes down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules

The steps of carrier proteins -

first a large molecule attatches to a carrier protein in the membrane

secondly the protein changes shape

this releases the large molecule on the opposite side of the membrane

What do channel proteins do?

Channel proteins form pores in the membrane for charged particles to diffuse through down their concentration gradient

Different channel proteins facilitate the diffusion of different charged particles

What factors does the rate of diffusion depend on?

The concentration gradient- the higher the concentration gradient the faster the rate of facilitated diffusion

The number of channel or carrier proteins - once all the proteins in a membrane are in use, facilitated diffusion cant happen anymore even if you maximise the number of concentration gradient

What are aquaporins

Aquaporins are special channel proteins that allow facilitated diffusion of water through the cell membrane. some kidney cells are adapted to have lots of aquaporins which allow the cell to reabsorb a lot of water that would otherwise be excreted from the body

Osmosis

Osmosis is diffusion of water molecules across a partially permeable membrane from an area of high water potential to an area of low water potential

What is the water potential

Water potential is the potential of water molecules to diffuse out or into a solution

What does the rate of osmosis depend on

The rate of osmosis depends on the water potential gradient with the higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place the difference in water potential decreases until both sides are the same

How do solutions of different concentrations made up?

Say we want to make 15cm³ of a solution of 0.4M sucrose

we start with a solution of known concentration such as 1M

Find the scale factor by dividing the concentration of the solution by the concentration of the solution you want to make eg. 1M/0.4M = 2.5M

This means you have to make the solution 2.5 times weaker, and you do that by having 2.5 x less of the solution.

so 15 / 2.5 = 6

15-6=9

and then add the remaining 6m³ with distilled water

Osmosis practical

Use a cork borer to cut potatoes into identically sized chips about 1cm³

Divide the chips into groups of three and measure the mass of each group using a mass balance

Place one group into each of your sucrose solutions

Leave the chips on the solution for at least 20 minutes

(making sure that they all are in for the same amount of time

Remove the chips and dry gently with a paper towel

Calculate the percentage change in mass for each group

Use the result to make a calibration curve showing change in mass against sucrose concentration

Find the concentration at this point then look up the water potential for that conentration for sicrose solution at the point the curve crosses the X axis

What is active transport?

Active transport uses energy to move molecules and ions across membranes, usually against a concentration gradient

The process is relatively similar to facilitated diffusion where a molecule attatches to the carrier protein, the carrier protein changes shape and this moves the molecules acro

What is the difference between Active transport and facilitated diffusion?

Active transport requires energy, facilitated diffusion does not

ATP is a common source ofn energy in the cell and is produced by respiration. It undergoes hydrolysis produces ADP and Pi which releases energy so molecules can be transported

What are co-transporters?

Co-transporters bind to two molecules at a time and the concentration gradient of one of the molecules is used to move the other molecule acros the concentration gradient

Co-transport

Glucose is absorbed into tthe bloodstream in the small intestine

In the ileum the concentration of glucose is too low for glucose to diffuse into the blood. So glucose is absorbed from the lumen of the ileum by co-transport

Sodium ions are actively transported out of the ileum epithelial cell into the blood by the sodium potassium pump. This creates a concentration gradient - there’s now a higher concentration of sodium ions in the lumen of the ileum than inside the cell

This causes sodium ions to diffuse from the lumen of the ileum into the epithileal cell down their concentration gradient via the sodium glucose co-transporter proteins

The co-transporter contains glucose into the cell with the sodium

as a result the concentration of glucose inside the cell increases

Glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel