Cell membrane and the movement of substances Flashcards
Describe the structure of a phospholipid bilayer
The hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane with water on both sides
The hydrophobic tails of both phospolipid layers point in to the cell surface membrane repelled by the water on both sides
Describe the functions of membranes within a cell
They act as barriers for example between the internal contents of the cell or between the contents of an organelle and a cytoplasm, they can separate one part of an organelle from another part, and chemical reactions can also take place in membranes, and they also play a role in cell signalling
What are the functions of phospholipid molecules inside a cell membrane?
Allow lipid soluble substances to enter and leave the cell ( hydrophobic molecules) as the inside of the bilayer is hydrophobic so these molecules can easily pass through the membrane e.g steroid hormones
Prevents water soluble molecules entering and leaving the cell ( hydrophilic molecules), because these molecules are polar so they cannot easily pass through the non polar region of the membrane. Water moleules ( which are polar) can still pass through as they are small
Describe the structure of cholesterol in the cell membrane
They have a polar hydrophilic group at one end which can attract the polar head molecule on a phospholipid, and the rest of cholesterol is non polar and hydrophobic which can attract the non polar fatty acids in phospholipids and pull them together
Describe the functions of cholesterol in the cell surface membrane
Because cholesterol interacts with phospholipids it increases the strength of the cell surface membrane making it more stable and less likely to be damaged
Cholesterol also reduces the sideways movement of the phospholipds and other molecules in the membrane helping to control the fluidity of the membrane, which prevents the membrane from becoming too fluid under warm conditions and too ridgid under cool conditions
It also packs the spaces between the phospholipids which reduces the movement of water soluble chemicals across the membrane
Why is the cell surface membrane described as a fluid mosaic?
Fluid refers to how the phospholipid molecules can move around in each layer meaning the membrane is flexible and can change shape
Mosaic is used because proteins are studded in the membrane, and the arrangement of these proteins vary like a mosaic.
What is an intrinsic protein in a cell membrane?
Proteins that are fully embedded in the membrane from one side to the other and because they pass straight through the lipid bilayer, they have hydrophobic amino acids on the outside surface of that protein, and these can interact with the hydrophobic fatty acid tails in the phospholipid bilayer
Describe the two categories of membrane proteins that are intrinsic
Protein channels have channels lined with amino acids that are hydrophilic and are filled with water molecules, and they allow water soluble molecules and ions to diffuse through.
Carrier proteins change shape or position in order to transfer molecules or ions from one side of the membrane to the other
Describe the function of extrinsic proteins in the cell membrane
They are proteins which do not span the membrane and they are found on one side or the other of membranes, and sometimes they may attach to intrinsic proteins. Some act as structural role in the membrane whereas some act as enzymes and some are receptors for other molecules such as hormones
What is a glycoprotein?
The carbohydrate molecule attached to membrane extrinsic proteins.
What are the functions of glycoproteins?
They allow cells to attach to each other to form tissues, they act as recgocnition sites and allow cells to recognise each other
What are glycolipids?
Carbohydrates that are found attached to phospholipid molecules
What are the functions of glycolipids?
They act as recognition sites, help cells to attach to each other and form tissues and they help to maintain stability of the membrane
What may cause a molecule to not be able to freely diffuse across the partially permeable cell membrane?
-Not soluble in lipids (hydrophilic molecules) so they cannot pass through the phospholipid layer
-If they are too large to pass through channels in the membrane
-If they are the same charge as the charge on the protein molecules, even if they are small enough to pass through they will be repelled
-Polar molecules may have difficulty passing through the non polar hydrophobic tails in the phospholipid bilayer
