M2C5 - Plasma membranes Flashcards
What are some roles of membranes?
-Site of chemical reactions
-Cell recognition and signalling
-Separating the cell contents from the outside environment
What are some roles of membranes within cells?
-Allow formation of concentration gradients
-Surround organelles, separating them from cytoplasm. - Maintaining different conditions from cytoplasm.
-Sites of chemical reactions
Describe the structure of phospholipids:
-Hydrophillic head
-Hydrophobic tails
-Form the phospholipid bilayer.
How does the structure of phospholipids allow plasma membranes to form?
-phosphate head is hydrophilic
-Bonds with water molecules
-Fatty acid tails are hydrophobic
-Heads orientate towards water and
tails orientate away from water
-So a bilayer forms
What is the structure and function of cholesterol?
-Is a lipid with a hydrophobic end and a hydrophillic end.
-Helps to maintain the fluidity of the membrane, preventing it from becoming too stiff when temperatures are low, or too fluid when temperatures are high.
Describe the structure and function of glycolipids
Glycolipids are lipids with a carbohydrate chain.
They are cell markers or antigens and can be recognised by the immune system.
Describe the structure and function of intrinsic proteins
-Span the whole membrane.
-Have amino acids with hydrophobic R groups
-These interact with the hydrophobic interior of the bilayer, holding the protein in place.
What is the function of channel proteins?
Provide a passage for passive movement (diffusion) of polar molecules through the membrane.
What is the function of carrier proteins?
Allow the movement molecules through the membrane by both passive transport and active transport.
What is cell signalling?
When the chemical binds to the receptor, it ellicits a response from the cell. This may cause a direct response or set off a cascade of events.
Describe the structure and function of Glycoproteins:
-Glycoproteins are proteins with a carbohydrate chain attached.
-Like regular proteins they stabilise membrane structure.
-Glycoproteins can bind cells together in tissues – cell adhesion.
-They can act as receptor molecules (e.g. for hormones and drugs)
-Cell signalling
Describe the structure and function of extrinsic proteins
-Present in only 1 side of the bilayer.
-Have amino acids with hydrophilic R groups on their outer surface which interact with the hydrophilic heads of the layer and/or with intrinsic proteins.
-provide support, can be involved in cell signalling.
How does the fluid mosaic model describe the structure of plasma membranes?
-Forms phospholipid bilayer
-hydrophilic heads facing outwards towards external environment
-hydrophobic tails facing inwards, away from external environment
-Proteins /phospholipids are free to move in membrane.
-Proteins scattered randomly arranged spread throughout between the phospholipids.
How does high temperature effect membrane structure and fluidity?
-The protein molecules vibrate more as the temperature increases.
-If the temperature gets too high the proteins in the membrane will denature.
-This changes the tertiary structure of the protein and so alters its shape. This leaves gaps in the cell membrane.
How does low temperature effect membrane structure and fluidity?
-Very low temperatures, decrease membrane permeability.
-The phospholipids vibrate less, packing together and rarely providing pathways between them.
-Protein channels remain in place, transporter proteins may not work as well, low temperatures make it difficult for the cell to provide ATP needed for active transport.
-At low temperatures all molecules and ions will be moving around less, so few of them will hit the membrane and pass through.
How do solvents affect membrane structure and permeability?
-Organic solvents e.g. Alcohol are less polar than water.
-They dissolve plasma membranes.
-Strong alcohol will destroy cells as a result.
-Alcoholic drinks are less concentrated and so don’t dissolve membranes but still cause damage and disrupt the membrane.
-The non-polar alcohol molecules sit between phospholipids in the membrane, disrupting it.
-When the membrane is disrupted, it becomes more fluid
-More fluid = more permeable.
What is diffusion and how is it a passive process?
Diffusion is the net movement of particles down a concentration gradient: from a region of high concentration to a region of low concentration.
No metabolic energy is expended during diffusion so it is an example of passive transport. The movement relies on the particles kinetic energy.
What is facilitated diffusion?
Diffusion across a plasma membrane through protein channels
How does facilitated diffusion work in channel proteins?
-Channel proteins are water-filled pores
-They allow charged substances (eg. ions) to diffuse through the cell membrane.
-The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore.
-This allows the channel protein to control the exchange of ions.
How does facilitated diffusion work in carrier proteins?
-carrier proteins can switch between two shapes
-This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape.
-The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane.
-Net diffusion of molecules or ions into or out of a cell will occur down a conc gradient so cells move through their own kinetic energy.
What is active transport?
Active transport is the movement of substances across membranes against their concentration gradient from an area of lower concentration to an area of higher concentration.
It requires energy in the form of ATP, a molecule produced by respiration in mitochondria.
What is bulk transport?
Used when extremely large substances need to be moved across a cell membrane.
-Requires energy in the form of ATP. The energy is used to move the membranes around, to form vesicles and to move the vesicles around the cell (using the cytoskeleton).
What is endocytosis?
The bulk transport of material into the cell. E.g. lymphocytes ingesting invading organisms.
What is exocytosis?
The bulk transport of material out of the cell - essentially the reverse of endocytosis. E.g. Release of insulin from pancreatic cells.
Name factors that affect the rate of diffusion:
-Temp
-Diffusion distance
-SA
-Size of molecule
Concentration gradient
Describe cell signalling:
-A signal molecule (e.g. a hormone) is secreted from a cell.
-This binds to a receptor on another cell (e.g. proteins, glycoproteins, glycolipids)
-The receptor – signal molecule binding is specific as their shapes are complementary.
-Attachment of signal molecule causes a change (e.g. a reaction in the cell)
Describe receptors as targets for drug action
Many drugs work by having a shape complementary to receptor molecules. e.g. salbutamol.
-Agonists are drugs that mimic the normal messenger.
-Antagonists block the receptor from the normal messenger. e.g. beta blockers.
Why can’t glucose molecules pass through the membrane through simple diffusion?
-Phospholipids act as a barrier.
-Glucose molecules are too large.
