- phospholipids
- hydrophilic heads point outward and hydrophobic tails point inward creating a phospholipid bilayer
- lipid soluble material moves through the membrane via this bilayer
- this bilayer:
- allows the movement of lipid-soluble material
- prevents water-soluble substances entering/leaving
- makes the membrane flexible and self-sealing
- proteins
- These are interspersed throughout the cell surface membrane embedded in the phospholipid bilayer in two main ways:
- some occur in the surface of the bilayer and never extend completely across it. These either act to give mechanical support to the membrane or (in conjunction with glycolipids) as cell receptors for molecules such as hormones
- others span the membrane. Protein channels form water-filled tubes to allow water-soluble ions to diffuse across whilst protein carriers bind to ions/molecules and change shape in order to move these molecules across the membrane.
- provide structural support
- allow active transport (carrier proteins)
- act as channels to transport water-soluble substances across (channel proteins)
- form cell-surface receptors for identifying cells
- help cells adhere to each other
- act as receptors (e.g for hormones)
- glycoproteins
- carbohydrate chains attached to many extrinsic proteins on the outer surface of the cell membrane
- act as cell-surface receptors for hormones and neurotransmitters
- help cells attach to one another forming tissues
- allow cells to recognise one another e.g lymphocytes can recognise an organism's own cells
- glycolipids
- made up of carbohydrate covalently bonded with a lipid. The carbohydrate section extends into the phospholipid bilayer where it acts as a cell-surface receptor for specific chemicals
- act as recognition sites
- help to maintain the stability of the membrane
- help cells to attach to one another forming tissues
- cholesterol
- this restricts the movement of other molecules making up the membrane
- the fluid-mosaic model
- the way in which the various molecules are combined into the structure of the cell-surface membrane is known as the fluid-mosaic model:
- fluid because the individual phospholipid molecules can move relative to one another giving the membrane a flexible structure that is constantly changing shape
- mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape/size/pattern similar to the tiles of a mosaic
The permeability of the cell-surface membrane is as follows. Substances can't enter if they are:
- not soluble in lipids (cannot pass through bilayer)
- too large (cannot pass through channels)
- of the same charge (they will be repelled)
- electrically charged (this means they are polar). In this case they have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
• simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer)
• facilitated diffusion (involving the roles of carrier proteins and channel proteins) • osmosis (explained in terms of water potential)
• active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP)
• co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum).
Simple diffusion
This is 'the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed'. It is a passive process
Facilitated diffusion
So we know that only small non-polar molecules can diffuse across the cell-surface membrane. The movement of charged ions and polar molecules is facilitated by protein channels and carrier proteins. It only occurs at specific points along the membrane (where these proteins are situated) and, like simple diffusion, also occurs down a concentration gradient. It is also passive.
- Protein channels form water-filled hydrophilic channels across the membrane that allow specific water-soluble ions to pass through. They are selective and open in the presence of a specific ion (in this way they have control of entry/exit of substances in/out of the membrane). The ion binds to the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side.
- Carrier proteins bind with a specific molecule (e.g glucose) and change their shape in such a way that the molecule is released to the inside of the membrane.
Osmosis
This is 'the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a electively permeable membrane'. It is important to realise that the highest value water potential can be is 0kPa (pure water) and anything not pure water has a negative water potential. Osmosis is essentially the diffusion of water molecules.
In animal cells, when water enters the cell swells and bursts, when water leaves the cell shrinks. In plant cells when water enters the cell becomes turgid, when water leaves the shell becomes plasmolysed (at the same water potential the condition is incipient plasmolysis).
Active transport
This is 'the movement of molecules or ions into or out of a cell from a region of higher concentration to a region of lower concentration using ATP and carrier proteins'. ATP is used to directly move molecules/individually move molecules using a concentration gradient which has already been set up by direct active transport (this is co-transport):
- the carrier proteins that span the membrane bind to a molecule/ion (the molecule/ion binds to the receptor site)
- on the inside of the cell/organelle ATP binds to the protein causing it to split into ADP + Pi. This causes the protein molecule to change shape opening to the opposite side of the membrane
- the molecule/ion is released to the other side of the membrane
- the phosphate molecule is released from the protein which causes the protein to revert to its original shape
Co-transport
Sometimes more than one molecule/ion may be moved in the same direction at the same time by active transport. Occasionally a molecule/ion is removed at the same time one is assed. An example of this is the sodium-potassium pump. Sodium ions are actively removed from the cell/organelle while potassium ions are actively taken in from the surroundings.
Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes.
Differences in.....
- increased surface area = larger area for diffusion
- increased number of channel/carrier protein = more placed for facilitated diffusion/active transport to occur
- a larger difference in concentration gradient/water potential means a faster rate of transport
......and vice versa.
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