Mind your membranes
Emma Dixon takes a closer look at the cell membrane
The cell membrane is an essential part of the cell. It separates the inside and outside of the cell, essentially defining its boundary.
First, the cell membrane is a barrier. Because it is semi-permeable, the membrane prevents large molecules diffusing into the cell unless the correct transport proteins are present. This allows the cell to control what goes in or out. Second, the membrane plays a part in signalling and signal recognition. Many membrane-associated proteins (integral or peripheral membrane proteins) are involved in signalling with neighbouring cells or in detecting and relaying signals from outside of the cell.
What makes a membrane?
The basis of the membrane is the phospholipid bilayer, which comprises a double layer of phospholipids. Phospholipids are made up of a phosphate-containing ‘head’ region and two fatty acid ‘tails’. The polar heads are hydrophilic (‘water-loving’) and the non-polar tails are hydrophobic (‘water-hating’) – this causes the phospholipid tails to orient away from the water-containing cytoplasm and extracellular matrix, forming a double layer of phospholipids.
This basic semi-permeable membrane is studded with other components, including proteins and cholesterol. Membrane-associated proteins can be bound to sugar chains (glycoproteins) and are often involved in signal recognition. Cholesterol molecules act as ‘spacing’ molecules between phospholipids to maintain fluidity and stability.
This is especially important with fluctuating temperatures – when hot, the membrane becomes more fluid and cholesterol stabilises the membrane. When cold, the fluidity in the membrane decreases, so cholesterol can separate the phospholipids out to enforce fluidity.
But how does this structure allow molecules to pass in and out of the cell? Hydrophobic, non-polar molecules (including oxygen, carbon dioxide and steroids) can diffuse freely through the membrane because the phospholipid ‘tails’ create a hydrophobic core. The passage of any other molecule is governed by its charge and size.
Small, polar molecules (including water and glycerol) can pass through the membrane. Larger polar molecules (including the sugars glucose and sucrose) and ions (including sodium ions/Na+ and potassium ions/K+) are unable to pass through the membrane directly. These molecules rely on transport proteins embedded in the membrane to be moved across the membrane.
On the move
There are three ways in which a molecule can cross the membrane. Small, polar, hydrophobic molecules that are able to pass directly through the membrane do so by passive simple diffusion – they only need a concentration gradient to move across the membrane.
Larger molecules crossing the membrane down a concentration gradient do so by passive facilitated diffusion. This requires either a channel or a carrier protein, which both allow specific molecules to cross the membrane. Because the molecules are travelling down their concentration gradient, no energy is required for this transport. Channel proteins act like a pore to let their specific target molecule cross the membrane.
Carrier proteins act a bit differently. On the extracellular side, they bind their target molecule, and this binding causes the carrier protein to change shape and allow the target molecule to move into the cell. Both these transport proteins can either always be working or be ‘gated’, meaning they are only allowed to work at certain times. For example, they might only be turned on when the cell needs to import a specific molecule.
The third type of transport is active transport – where large or charged molecules cross the membrane against their concentration gradient. This process requires energy, in the form of ATP made through respiration. One of the most important examples of active transport in the cell is the sodium ion/potassium ion – Na+/K+ – pump (sometimes called the Na+/K+ ATPase), which moves two potassium ions (K+) into the cell for each three sodium ions (Na+) it moves out. This maintains the electrochemical gradient of these ions. Its ‘pumping’ process is somewhat similar to the carrier proteins. First, the intracellular face picks up three sodium ions, then the pump hydrolyses ATP to release energy to power the next step. The pump then undergoes a conformational change – where the pump’s structure changes so the sodium ions are facing the extracellular matrix. These sodium ions are released, and the pump picks up two potassium ions and again changes in shape to let the potassium ions move into the cell.Lead image:
Maurizio De Angelis/Wellcome Images
- Mind your membranes [PDF 486KB]