The cell membrane surrounds and protects the cytoplasm. It finely controls what moves in and out of the cell. But how?
The cell membrane (plasma membrane) has a hydrophobic inner layer. This means “water-hating”, although oddly enough water can pass through it because it’s a tiny molecule (this is called osmosis – movement of water across a semi-permeable membrane). So can small uncharged molecules like oxygen and carbon dioxide.
Water-hating equates to fat-friendly (or “lipophilic”), so lipids can also pass through biological membranes easily.
Molecules that pass freely through (diffuse through) the bilipid layer move down the concentration gradient (i.e. from higher to lower concentration) more than they do in the other direction.
Bozeman Science has a really good summary of the topic.
Water-soluble molecules can’t pass through the water-hating bilipid layer. But they can move in and out of the cell with the help of proteins that are embedded in the cell membrane. I’m going to focus on these here.
There are three types of membrane transport protein. They each help a specific molecule through the plasma membrane. The first two allow facilitated diffusion. This means there’s a net (overall) movement down the concentration gradient – i.e. from the more concentrated side to the more dilute side until the concentration on both sides is in equilibrium (balanced). Diffusion, including facilitated diffusion, doesn’t need any energy input. The third type of membrane transport protein moves substances against (up) the concentration gradient – one side becomes more concentrated and the other becomes less concentrated. This does need energy because it’s working against what would occur naturally. This is called active transport.
Here they are:
(The example videos are worth looking at because you’ll come across some of these systems later. Don’t try and remember all the details – just get an idea of how the different types of proteins work)
1. Channel proteins. The molecules go through these. These may have a gate so that the molecule only goes through if the gate is open.
2. Transporters bind the molecule, changing shape, and let them through the membrane to the other side. When they release the molecule on the other side of the membrane these proteins change back to the original shape. The speed of movement through transporters is limited, because once the transporters have changed shape they can’t take up other molecules until they release the molecule and change back to the original shape. The links in the RESOURCES section below have good diagrams that show this.
(The following video has lots of ads but is quite good because it also shows the role of exocytosis and endocytosis.)
3. ATP-powered pumps are the membrane transport proteins responsible for active transport – they can move a molecule from the side with a lower concentration to the side with a higher concentration (i.e. against the concentration gradient). Active transport needs energy from somewhere. This energy is derived from a molecule called ATP. This is an important molecule that’s used to power lots of biological processes.
To recap, you’ll notice ion channels and transporters let the molecules down the concentration gradient (to a lower concentration) so NO energy is needed. They can be used to help a molecule that can diffuse across the membrane to get through faster (facilitated diffusion).
Proteins are made by cells. They can be used by the cell that made them, in which case they stay inside the cells. But if they’re needed somewhere else they can get through the cell membrane by a different process called exocytosis.
As always, let me know if anything’s not clear so I can improve this site.
Check the excellent Bozeman Science video above.
BBC Bitezise has a good review of this topic. http://www.bbc.co.uk/education/guides/zc9tyrd/revision
This website has a good summary of the three different types of these membrane transport proteins: http://www.creative-biolabs.com/Membrane-Transport-Proteins.html?gclid=Cj0KEQiA6IC2BRDcjPrjm_istoUBEiQASrLz1g0wdkxUqjZOM7duczcLZP-medfQDz_D9LEn9l2Y8kwaAkc38P8HAQ.
This site’s also good. http://www.ncbi.nlm.nih.gov/books/NBK21592/. It’s quite detailed but the image caption and summary are pretty good for your purposes.