Transport across cells

(Original post by Studentuser131)
How does the membrane affect transport

(Original post by Pengwyn<3)
Assuming this is GCSE, the cell membrane is selectively permeable. It allows small molecules like water, oxygen & carbon dioxide to diffuse through but large molecules like glucose tho pass through. The membrane's like a dumb dictator & requires a specific transport protein to move across the membrane.

(Original post by Studentuser131)
Would the answer be different for as bio, and thanks for replying

(Original post by Pengwyn<3)
Definitely would. Because you're asking how transport is affected, I'll only answer that. This is for WJEC but can also apply across other UK exam boards with some differences. Edexcel is the closest as WJEC is for Wales and Edexcel is changed for England.

You should know how the cell membrane is a phospholipid bilayer with various proteins and knowledge of the fluid mosaic model will help but it's still okay if you haven't learnt it yet.

Now, the following is important to understand why the plasma/cell membrane is as it is and why transportation is done how it is (the functions transport-wise):



This diagram is to help with my explanation:


Phospholipid molecules are amphipathic -- they have a polar part (hydrophobic - fears water) and a non-polar part (hydrophilic - likes water). The hydrophilic phosphate heads 'protect' the hydrophobic hydrocarbon/fatty acid tails by facing the cytoplasm (intracellular environment) and outside (external/extracellular environment).

Diagram of the phospholipids of the phospholipid bilayer illustrating this:

So what does this mean?

It means the properties of molecules passing across the membrane directly affects how they cross it.
Small and non-polar molecules are soluble in lipid (lipid-soluble) allowing them to pass through, for example, carbon dioxide and Vitamin A. They dissolve in the fatty acid/hydrocarbon tails and diffuse through the membrane. However, small polar molecules such as water can pass through via diffusion and osmosis, but this is very slow and depends on the water potential/osmotic pressure. Most of the water quickly is transported through a protein called aquaporin.

Meanwhile, large molecules like glucose, charged molecules like water and ions must pass through the phospholipid bilayer with the help of a membrane protein (carrier/channel protein) - facilitated diffusion (when the concentration of that substance is lower in the intracellular environment than the extracellular). This is because they're too big or unable to dissolve in the fatty acid/hydrocarbon tails or both. These substances are also called water-soluble molecules.

Facilitated diffusion diagram:



When the concentration of the required molecules is higher in the intracellular environment than the outside, cells need active transport. This uses ATP. An example of this being carried out is the sodium-potassium pump which moves ions in the neuron.
- the molecule binds to a transmembrane protein pump and the ATP is hydrolysed.
- this causes a conformational change (the protein's tertiary structure changes) and translocates (moves to another place) the molecule across the membrane.

Sometimes the molecules are passively coupled to an actively transported molecule (also active transport). This is called co-transport. There're two types:
- Symport where both molecules move in the same direction. Think of this like piggy-backing or slipstreaming, not literally but in the sense.
- Antiport where both molecules move in the opposite direction. Two lines in a busy corridor, both moving in the opposite direction, again not literally but in the sense.

Finally, vesicular transport!! Remember the Golgi complex/apparatus/body? Ribosomes from the rough ER need to be delivered to it for completion (like a tertiary structure enzyme) where it's then delivered out the cell past the plasma membrane. These are intracellular vesicles which can move materials between cell organelles, e.g. containing lysosome to only break down a targeted organelle instead of the others - like an old organelle.

There are also certain materials like fluid which requires the cell to package it as it is and this is another example of the use of the phospholipid bilayer's fluidity. This fluidity allows it to break and reform around these certain materials (this requires ATP). There are two types:
- Exocytosis which packages material to be released outside the cell.
- Exo - exit
- Endocytosis which the cell uses inside.
This is also why it's important to not have too much cholesterol. While it helps give the membrane some rigidity, too much of cholesterol restricts this and diffusion.

Hope this helps x

(Original post by Studentuser131)
OMG, thank you so much, you don’t understand how much this has helped me for exam tomorrow

(Original post by Studentuser131)
OMG, thank you so much, you don’t understand how much this has helped me for exam tomorrow