Biological membranes Flashcards
Role of membranes at surface of the cell:
Separate cell contents from the outside environment
Cell recognition and signalling
Regulating the transport of materials into or out of cells
Role of membranes within cells:
form organelles ;
isolate the contents of an organelle and so metabolic pathways
site for attachment of enzymes and ribosomes
provide selective permeability – control what goes in and out of an organelle
creation of concentration gradients or specific environments (pH)
What is cell signalling?
Process that leads to communication and coordination between cells, e.g. hormones binding to their receptors on the cell surface membrane to trigger a response or reaction inside the cell. It allows for cell recognition and the coordination of the action of different cells
What is the role of membrane bound receptors where hormones and drugs can bind?
Hormones are used in cell signalling. The Target Cells have a receptor which is complementary to the hormone, meaning that it can bind to the receptor cells, triggering the desired internal response.
Drugs have also been developed which bind to the receptor molecules on cells. Beta-blockers are used to prevent a muscle from increasing the heart rate to a dangerous level, and some drugs used to treat schizophrenia mimic a natural neurotransmitter which some individuals cannot produce.
What is the fluid mosaic model?
Used to describe arrangement of molecules in the membrane.
phospholipid molecules form a continuous, double bilayer.
bilayer = fluid as phospholipids are constantly moving.
cholesterol molecules present throughout, and protein molecules are scattered through bilayer. Some proteins have polysaccharide chain attached - glycoproteins. Some lipids have polysaccharide chains attached - glycolipids.
What are the role of phospholipids?
Have a hydrophobic head and a fatty acid tail. They form a bilayer separating the cell from the outside.
They are fluid so components can move around freely. They act as a barrier, selecting what goes in/out of the cell. They are permeable to small and/or non-polar molecules, but impermeable to large molecules and ions.
What is the role of cholesterol?
Gives the membranes stability by sitting between fatty acid tails and therefore making the barrier more complete, preventing molecules like water and ions from passing through the membrane.
What is the role of glycolipids?
Phosopholipid molecules that have a carbohydrate part attached. They are used for cell signalling, cell surface antigens and cell adhesion.
What are the roles of channel proteins?
Channel proteins allow the movement of SMALL POLAR substances, such as IONS. .
Carrier proteins actively move substances across the cell surface membrane. large molecule sugar, into and out of the cell as they can’t travel directly through the cell surface membrane.
What are the roles of carrier proteins?
Carrier proteins actively move substances across the cell surface membrane. large molecule sugar, into and out of the cell as they can’t travel directly through the cell surface membrane.
What are the roles of glycoproteins?
Act as antigens
Enable the identification of cells as self or non-self
Used in cell signalling
Act as receptors or binding sites for hormones. They have a specific shape that is complementary to shape of the communicating molecule which binds to the receptor
Act as receptors on transport proteins to trigger movement
Allow cell adhesion to hold cells together in a tissue
Attach to water molecules to stabilise the membrane
What is the effect of temperature on membrane fluidity?
Increasing the temperature means that the molecules have more kinetic energy. This increased movement makes the membrane leaky, so molecules which would not normally do so can move into and out of the cell.
The membrane structure is disrupted as the phospholipid bilayer melts. Membrane proteins and carrier proteins are denatured and unable to function. The membrane becomes more permeable
What is simple (passive) diffusion?
the net movement of molecules from a region of high concentration of the molecule to an area of lower concentration of the molecule down a concentration gradient.
Small, non-polar substances diffuse through the phospholipid bilayer in between the phospholipid molecules. Water can do this because although it is polar it is small
What is facilitated diffusion?
Large and charged molecules need to be transported across the phospholipid bilayer, they can’t just diffuse across.
Channel proteins form pores in the bilayer which are shaped to allow only one type of ion through. Aquaporins are channel proteins allowing water through. Channel proteins are often gated (can be opened and closed)
Carrier proteins are shaped to fit a specific molecule (like glucose). Once this fits in the carrier changes shape to allow the molecule through to the other side of the membrane
What is active transport?
The movement of molecules or ions across membranes, using ATP to drive ‘protein pumps’ within the membrane. Moves molecules and ions against a concentration gradient.
