3.1.3 Substances are exchanged between organisms and their environment by passive or active transport across exchange surfaces. The structure of plasma membranes enables control of the passage of substances across exchange surfaces.
Diffusion: 

• Diffusion is the passive movement of substances down a concentration gradient. 
• Surface area, difference in concentration and the thickness of the exchange surface affect the rate of diffusion.
• The role of carrier proteins and protein channels in facilitated diffusion.
• Candidates should be able to use the fluid-mosaic model to explain appropriate properties of plasma membranes.

Osmosis: 

• Osmosis is a special case of diffusion in which water moves from a solution of higher water potential to a solution of lower water potential through a partially permeable membrane. 
• Candidates will not be expected to recall the terms hypotonic and hypertonic. Recall of isotonic will be expected.

Active transport: 

• The role of carrier proteins and the transfer of energy in the transport of substances against a concentration gradient.

Absorption: 

• Absorption of the products of carbohydrate digestion. The roles of diffusion, active transport and co-transport involving sodium ions.

There are different types of membrane transport you will need to learn for GCE AQA Biology. 😊

Diffusion 

There are 2 types of diffusion you should be familiar with: 
1. Simple diffusion: passive (using molecules’ inherent kinetic energy), without the help of any channels or proteins 

• Small, non-polar molecules can undergo simple diffusion across plasma membranes 
  • Because they are able to pass through the hydrophobic fatty acid tails of the phospholipid bilayer 
  • Example: oxygen, carbon dioxide 
• Factors affecting diffusion rate
  • Surface area: as surface area increases, the rate of diffusion increases
    • There is more area across which the particles can enter the area of low concentration
  • Difference in concentration: as the difference in concentration between two areas, the rate of diffusion increases 
    • The concentration gradient becomes steeper, which increases the rate
  • Thickness of the exchange surface: as the surface becomes thinner, the rate of diffusion increases 
    • As the surface becomes thinner, there is less resistance and obstruction to the particle’s movement towards the region of lower concentration

2. Facilitated diffusion: diffusion through a protein 

• Charged ions or polar molecules require the help of facilitated diffusion 
  • They are unable to pass through the non-polar of the fatty acids tails in the phospholipid bilayer of the plasma membrane 
  • Example: sodium ions, potassium ions 
• The process is still passive, as ATP is not used to help the molecules pass through the proteins. Rather, the inherent kinetic energy of the moving molecules is used. 
• There are 2 types of facilitated diffusion, classified by the type of protein that helps the molecules to pass through the protein 
• A) Protein channel
  • The channels are specific to the ion that they are meant to carry through, and only open when that ion is present 
  • The ion binds with the protein, causing the protein to close one side of the channel, and open the other side 
  • The proteins create a watery, polar, hydrophilic environment within the channel, allowing charged, water-soluble ions to pass through

• B) Carrier protein 
  • Each carrier protein is also specific to the molecule it transports 
  • The molecule enters the protein and binds to it
  • This changes the shape of the protein so that the molecule is moved into the cell
  • This process is still passive, as the molecules’ inherent kinetic energy (not energy from ATP) is being used 

Osmosis

• Water potential: pressure created by water molecules 
  • Higher water potential = lower solute concentration 
  • Lower water potential = higher solute concentration 
  • When the cell environment and the liquid inside the cell have equal solute concentration and equal water potential, the solution is known as isotonic 
• Osmosis is a special form of diffusion. The process of osmosis is basically the diffusion of water molecules, with the added barrier of a membrane that only allows specific molecules, like water, to pass through: 
  • The water molecules diffuse from high to low water potential across the partially permeable membrane 
  • The membrane prevents solute molecules from diffusing 
  • Dynamic equilibrium is established when the water potentials are equal on both sides, so there is no further net movement of water 

Active transport

•  Active transport can be used to move molecules against their concentration gradient 
  • Example: Sodium-potassium pump 
• Another use of active transport is that the concentration gradient that is set up can then be used to facilitate the passive movement of another molecule - Co-transport 
  • Example: sodium-glucose co-transport 
• Process of active transport 

1. The molecule to be transported binds to specific receptor sites on the carrier protein 
2. ATP binds to the protein, hydrolysing the ATP into ADP + a phosphate molecule 
3. This causes the carrier protein to undergo a conformational change and change shape
4. It subsequently opens up to the other side, releasing the molecule to its destination 
5. The phosphate molecule is released, so the protein returns to its original conformation 
6. The protein is ready to repeat this process again 
7. The phosphate can be added to ADP to make ATP again during cellular respiration  

Absorption

Now that you understand the principles behind these different forms of membrane transport, we can apply them to the example of the absorption of the products of carbohydrate digestion in the small intestine. 🍞

Generally, the goal is to move the products of digestion from the lumen of the ileum into the bloodstream, where the products like glucose, amino acids, and fats can be sent to the places in the body that need it. To do so, the products need to pass through the wall of the small intestine and enter the blood. This process of absorption requires different types of membrane transport: in GCE AQA Biology, you will be asked to examine diffusion, active transport, and co-transport. 

Diffusion 
Diffusion is from high to low concentration. Glucose can diffuse across the intestinal wall as they are very small molecules. 

• Higher concentration inside the lumen - chemical digestion of carbohydrates continually produces glucose 
• Lower concentration in the blood - blood is circulated away from the digestive system by the heart, and glucose is used up by cells during respiration 
• Glucose moves passively from lumen to blood via diffusion 

Active transport and co-transport
However, as more and more glucose diffuses from the intestine to the blood, net movement of glucose stops when the concentrations on either side equalise. To absorb the rest of the glucose, we need a process known as co-transport. This requires ATP because sodium ions are first used to build up a concentration gradient via active transport, which then allows glucose to diffuse along with sodium ions, using a co-transport protein. 

1. Sodium-potassium pump actively transports sodium ions from the intestinal epithelial cells (lining the wall of the ileum) into blood 
2. The lumen of the small intestine now has a higher concentration of sodium ions than the epithelial cells (concentration gradient) 
3. Sodium ions diffuse from the lumen into the epithelial cells, down the concentration gradient 
4. As Na+ ions diffuse, they carry glucose into the cell with them 
5. The glucose can then enter the blood via facilitated diffusion, using another protein carrier. 

Wanna Boost Up Your GCE AQA Biology? Check out our study tips here! 

Written by Justine | Biology Specialist @ Tuttee

 References
Toole, G., & Toole, S. (2015). Aqa biology A level. Oxford: Oxford University Press.
Images from

1. https://www.khanacademy.org/science/biology/membranes-and-transport/passive-transport/a/diffusion-and-passive-transport
2. https://socratic.org/questions/what-is-the-difference-between-simple-diffusion-and-osmosis
3. https://andreabiology.weebly.com/14-membrane-transport.html
4. https://www.awaremed.com/tag/apple-stem-cell/