Cell membranes Module 2 Flashcards
Functions of membranes at the surface of cells (plasma membranes)?
They are barriers between the cell and the environment They’re partially permeable and control what substances enter and leave the cell They allow recognition by other cells They allow cell communication
Functions of membranes within cells?
Compartmentalises organelles, acting as a barrier between the organelle and cytoplasm Can form vesicles to transport substances between different areas of the cell They control which substances enter, and leave the organelle Can be the site of a chemical reaction
What are cell membranes made up of?
Lipids (mainly phospholipids) Proteins Carbohydrates (usually attached to proteins or lipids)
Describe the basic structure of a cell membrane?
Phospholipid molecules form a continuous double layer (bilayer) Bilayer is fluid as phopholipids are continuously moving Cholesterol molecules are present in the bilayer Proteins scattered thoughout the bilayer (like tiles) Some proteins have carbohydrates attached = glycoproten Some lipids also have a polysaccharide chain attached = glycolipid
How do phopholipids form a barrier to dissolved substances?
They have a head and a tail, the head is hydrophillic (attracts water) and the tail is hydrophobic (repels water) They arrange themselves in a bilayer, heads on outside, tails on the inside The centre of the bilayer is hydrophobic, so the membrane doesn’t allow water-soluble substances through it, however does allow fat-soluble substances through
What cholesterol do to the membrane?
Fits between the phospholipids, and bind to the hydrophobic tails, causing them to pack more closely
What do proteins do in the membrane generally?
Control what leaves and enters, and act as receptors
What glycolipids and glycoproteins do in the membrane generally?
Act as receptors for messenger molecules
Explain the general process of cell signalling?
One cell releases a messenger molecule, (eg a hormone) This molecule travels to another cell The messenger molecule is detected by the cell as it binds to a receptor on it’s cell membrane, the receptors will be complementary to the molecule
What is glucagon?
A hormone that’s released when there isn’t enough glucose in the blood, it binds to receptors on liver cells, causing the liver to breakdown stores of glycogen to glucose
Describe a drug that binds to cell receptors?
Cell damage causes the release of histamine, which binds to receptors of other cells and causes inflamation. Antihistamine binds to there receptor cells and blocks histamine binding and causing inflamation
How can membrane permeability be investigated?
Using beetroot cell that contain a coloured pigment that leaks out, the more permeable it is the more pigment will leak out. Leave in solution of water, then measure the solution produced with a calorimeter, so the higher the absorbance, the more permeable it is
Affect of increasing temperature on the cell membrane?
Temperature bellow 0 degrees: Very little energy, so phospholipids packed together very tightly and membrane is rigid. But channel proteins and carrier proteins may deform, increasing the permeability. Ice crystals may form and pierce the membrane, making it highly permeable when it thaws Temperature between 0 and 45 degrees: The phospholipids can move around and aren’t as tightly packed, so the membrane is partially permeable, so as temperature increases they move around more and become more permeable Temperature above 45 degrees: The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable Water inside the cell expands putting pressure on the membrane Channel proteins and carrier proteins deform so they can’t control what enters or leaves the cell, making it even more permeable
How do solvents affect the cell membrane?
Increases the permeability as the solvent dissolves the lipids in the cell membrane, so it loses structure
What’s diffusion?
The net movement of particles from an area of high concentration to an area of low concentration No ATP is required, it is a passive process
What type of molecules can diffuse through the cell membrane?
Small, non polar molecules such as Oxygen and Carbon Dioxide, as can fit in between spaces between phospholipids Water also can because it’s small enough, even though it’s polar
4 factors the rate of diffusion depends upon?
The concentration gradient, the higher it is the faster the rate of diffusion The thickness of the exchange surface, the thinner it is the faster the rate The surface area, the larger the surface area the faster the rate The temperature- higher the temp the faster it is as particles have more kinetic energy so move faster
How can you investigate diffusion in model cells?
Make up some agar jelly, with phenolphthalein (pH indicator which is pink in alkaline conditions, and colourless in acidic conditions), and sodium hydroxide, so the jelly will be a shade of pink Put the agar cubes into dilute hydrochloric acid, and time how long it takes for the HCl to diffuse into the agar cube neutralising it and making it colourless Can use this to investigate surface area, concentration gradient, and temperature
What’s facillitated diffusion?
When larger molecules, Ions and polar molecules don’t diffuse directly through the phospholipid bilayer, so they diffuse through carrier proteins or channel proteins Moves particles down a concentration gradient, and doesn’t require ATP
How do carrier proteins move large molecules into or out of a cell?
A large molecule attaches to a carrier protein in the membrane The protein then changes shape This releases the molecule on the opposite side of the membrane
What do channel proteins do?
Form pores in the membrane for specific charged particles to diffuse through
Describe the process of active transport?
A molecule attaches to a carrier protein, causing it to change shape and allow the molecule across, requires ATP and moves particles from low concentration to a high concentration
Describe cells taking substances in by endocytosis?
Some substances to large to be taken into a cell by carrier proteins So the cell surrounds the substance with a section of it’s plasma membrane, the membrane then pinches off to form a vesicle inside cell containing the ingested substance = endocytosis Process uses ATP for energy
Describe cells secreting substances by exocytosis?
Vesicles containing the substances pinch off from the sacs of the golgi apparatus and move towards the plasma membrane They then fuse with the plasma membrane and release their contents outside the cell Uses ATP as an energy source
