Cell membranes Flashcards
Describe and explain the fluid mosaic model?
Phospholipids form a continuous bilayer, which is described as fluid because it constantly moving. Cholesterol and proteins are scattered through the bilayer like a mosaic. Some proteins have a polysaccharide chain attached called glycoproteins and some lipids have a polysaccharide chain attached called glycolipids.
Describe the role of the phospholipids?
They are made of two main parts, the phosphate head, which is hydrophilic (attracts water) and the fatty acids tails are hydrophobic (repel water). This means they will always form a bilayer, where the centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances through.
Describe the role of cholesterol in the cell membrane?
Cholesterol is a type of lipid that helps maintain the membranes stability. They fit between the phospholipids
Describe the roles of glycolipids?
The carbohydrate chain extends from the bilayer, where it acts as a cell surface receptor. Their function is to act as a recognition site, help maintain the stability of the membrane and help cells attach to one another to form tissues.
Describe the roles of glycoproteins (peripheral proteins)?
The carbohydrate chain extends from the bilayer and acts as a cell surface receptor for hormones and neurotransmitters. They acts as recognition sites and help attach cells together to form tissues.
Describe the role of integral proteins in the cell membrane?
These help transport substances such as ions, sugars and amino acids that can’t diffuse across the cell membrane. There are different types, which include channel and carrier proteins.
How to test the permeability of a cell membranes?
Cut five equal sized pieces of beetroot with a scalpel and rinse to remove any excess pigment. Add 5cm3 of water using a measuring cylinder into 5 test tubes. Put each piece of beetroot into a different test tube and add each test tube into a water bath at different temperatures (10°C, 20°C, 30°C, 40°C, 50°C) for the same length of time. Remove the beetroot from the test tube and use a colorimeter to test each coloured liquid. It measured the absorbance, so the higher the absorbance, the more pigment released and the higher the permeability of the membrane.
How do temperatures below 0°C effect membrane permeability?
The phospholipids don’t have much energy, so they can’t move much. They’re packed closely together, and the membrane is rigid.
What is diffusion?
The net movement of a substance from a high concentration to a low concentration down a concentration gradient. It is a passive process, so it does not require energy.
What can oxygen and carbon dioxide diffuse by simple diffusion through a cell membrane?
Because they are small, they can pass through the spaces between phospholipids.
They are non-polar, which makes them soluble in lipids so can dissolve in the hydrophobic bi-layer.
How do channel proteins work to allow for facilitated diffusion?
They form pores in the membrane to allow charged substances to diffuse through the membrane down a concentration gradient. Part of the inside surface of the protein can move to open or close the pore to allow them to control the exchange of ions.
How do carriers proteins work to allow for facilitated diffusion?
They move large molecules across membranes, different carrier proteins facilitate the diffusion of different molecules. First, the molecule attaches to the carrier protein, which causes the protein to change shape. This then releases the molecule on the other side of the membrane.
Describe and explain factors that affect simple diffusion?
The concentration gradient, as the higher it is the faster the rate of diffusion. As diffusion takes place the difference in the concentration decreases until equilibrium is reached, so the rate of diffusion slows down over time.
The diffusion distance, the shorter the diffusion distance the faster the rate of diffusion.
The surface area, the larger the surface area the faster the rate of diffusion as more particles can be diffused in the same amount of time.
Describe and explain the factors that affect facilitated diffusion?
The concentration gradient, the higher it is, the faster the rate of diffusion up to a point, when equilibrium is reached and the rate of facilitated diffusion will level off.
The number of channel or carrier proteins, once all the proteins are in use, then facilitated diffusion can’t happen any faster. Therefore the greater number of channel or carrier proteins, the faster the rate of diffusion.
What is osmosis?
It is the net movement of water from a high water potential to a low water potential, through a partially permeable membrane, down a concentration gradient.
