The cell membrane Flashcards
Phospholipids
Form a bilayer
Lipid soluble material move through the membrane
The functions of phospholipids in the membrane are to
Allow lipid soluble substances to enter and leave
Prevent water soluble substances entering and leaving the cell
Makes the membrane flexible and self sealing
Proteins
Some proteins occur in the surface of the bilayer and never extend completely across it, they give mechanical support or in conjunction with glycolipids act as cell receptors for molecules such as hormones
Others completely span across the phospholipid bilayer
Channel proteins are water filled tubes to allow water soluble ions to diffuse across the membrane
Carrier proteins bind to ions or molecules like glucose and amino acids, and change shape in order to transfer these across the membrane
Cholesterol
Cholesterol adds strength to the membranes
Very hydrophobic and play an important role in preventing loss of water and dissolved ions from the cell
They reduce lateral movement of other molecules including phospholipids
Make the membrane less fluid at high temps
Prevent leakage of water and dissolved ions from the cell
Glycolipids
Made up of a carbohydrate covalently bonded with a lipid
Extends from phospholipid bilayer into watery environment outside the cell, acts as a cell surface receptor for chemicals (Ie human ABO blood system)
Act as recognition sites
Help maintain stability of membrane
Helps cells to attach to one another and form tissues
Glycoproteins
Act as cell surface receptors for hormones and neurotransmitters
also act as recognition sites and help cells attach to one another to form tisues
Allow cells to recognise one another for example lymphocytes can recognise an organisms own cells
Permeability of the cell surface membrane
Many molecules do not freely cross it because :
Not soluble in lipids and therefore cannot pass phospholipid bilayer
Too large to pass through channels in the membrane
Of the same cherge as the charge on the protein so they are repelled
Polar
Fluid mosaic model of cell surface membrane
Fluid as individual phospholipid molecules can move giving the membrane a flexible structure which is constantly changing in shape
Mosaic as the proteins that are embedded in the bilayer vary in shape size and pattern
Simple diffusion
Net movement of particles from an area of high to low concentration
Facilitated diffusion
Only small, non polar molecules (ie oxygen) can easily cross plasma membranes
It is much harder for charged ions and polar molecules due to the hydrophobic nature of the fatty acid tails on the phospholipid
This movement is made easier by transmembrane channels
Passive process - only relies on kinetic energy
Channel proteins
They allow specific water soluble ions to pass through
The channels are selective. If a particular ion is not present they remain closed
This helps them to have control over the entry and exit of ions
Co-transport
1) Sodium ions are actively transported out of epithelial cells by the sodium potassium pump into the blood. This takes place in a carrier protein.
2) This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells
3) Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of carrier protein
As the sodium ions diffuse in through the second carrier protein they carry either amino acids or glucose molecule with them
4) Therefore the glucose or amino acids pass into the blood plasma
Explain why phospholipids arrange in a bilayer
The phosphate group has a negative charge (polar), whereas the fatty acid tails do not have a charge (non-polar)
This makes the phospho-glycerol head hydrophilic and the fatty acids hydrophobic
