Today we are starting a new series looking at how substances are exchanged across cell membranes and transported around organisms. These processes are really important for getting essential resources such as oxygen and glucose to all the cells in an organism. We will start by taking a closer look at the structure of the cell membrane.
The cell membrane around the outside of a cell is often referred to as the cell-surface membrane or the plasma membrane. In animal cells it is literally on the surface, but in plant cells and prokaryotic cells it is found below the cell wall. It has the very important job of regulating what passes in and out of the cell, meaning that it is a partially permeable membrane (some molecules can pass through, others can’t). It also contains receptors that help the cell respond to its environment.
Components of the cell membrane
The main component of cell membranes is phospholipids. If you remember, phospholipids have a hydrophobic and a hydrophilic region. They form a bilayer because the hydrophobic fatty acid tails point in towards each other, and the hydrophilic heads point outwards towards the cytoplasm and tissue fluid.
Because the center of the membrane is hydrophobic, anything dissolved in water (water soluble) cannot pass straight through (in future articles we will see how these substances are helped across). However, if a molecule is lipid soluble and small enough, it can dissolve in the membrane and pass through by simple diffusion.
The fluid mosaic model of the cell membrane was put forward in 1972, which basically says that the cell membrane is fluid (the phospholipids are constantly moving) and that it has ’tiles’ like a mosaic (other molecules such as proteins amongst the phospholipids which are randomly arranged).
One of the other types of molecule found in the cell membrane is cholesterol. Cholesterol gets a lot of bad press, but it does have important functions, one of which is to increase rigidity of the membrane. This is particularly important for animal cells as they don’t have a cell wall and sometimes aren’t surrounded by any other cells, so they need a bit of support. Cholesterol molecules insert themselves in between phospholipids and bind to the fatty acid tails. The membrane becomes less fluid because the phospholipids become more tightly packed together.
There are also lots of proteins, either fixed or moving around, in the membrane. Some of these, such as channel proteins and carrier proteins, help transport substances across the membrane. We will look at these in more detail when we look at diffusion and active transport. Polysaccharides have a role in the cell membrane as well – they can be bound to proteins to form glycoproteins, or bound to lipids to form glycolipids.
Glycoproteins and glycolipids are often receptors which allow the cell to respond to signals such as hormones. They can also be antigens (important for the immune response) or binding sites for other molecules such as drugs. Glycoproteins and glycolipids have the ability to form hydrogen bonds with the surrounding water molecules which is another mechanism of stabilising the membrane.
Don’t forget that membranes are also found within cells around some organelles, such as mitochondria.
Summary
- The cell-surface membrane is a phospholipid bilayer containing other molecules such as cholesterol and proteins.
- It controls what passes in and out of cells, and helps the cell respond to its environment and interact with other cells.
- The fluid mosaic model describes how the phospholipids are constantly moving and how proteins are randomly dispersed throughout like mosaic tiles.
Next, we will look at diffusion, osmosis, and active transport to see how different molecules can be transported across cell membranes.
