Biological Membranes 2.5 Flashcards
What is a passive process?
.Some substances can pass across a membrane without using any of the cell’s metabolic energy
.These are passive processes as they use only the kinetic energy of the molecules and do not use ATP
State 2 passive processes
-diffusion
-facilitated diffusion
Define (simple) diffusion
The net movement of molecules from an area of high concentration to an area of low concentration.Down a concentration gradient. It does not involve metabolic energy (ATP)
Define facilitated diffusion
Movement of molecules from an area of high concentration of that molecule to an area of low concentration , across a partially permeable membrane via protein channels or carriers. It does not involve metabolic energy (ATP)
How does simple diffusion occur ?
-all molecules have kinetic energy
-if there is a high concentration of a certain type of molecule in an area ,the molecules will bump in to each other as they randomly move and will eventually spread further from each other
-more will move to an area where they are in lower concentration until eventually they will become evenly dispersed
-when molecules have moved down their concentration gradient they are still moving randomly but remain evenly dispersed so there is no net diffusion. They have reached equilibrium
What particles can diffuse by simple diffusion across the phospholipid bilayer?
-lipid soluble, small, uncharged molecules
E.g oxygen, carbon dioxide , steroid hormones
How is the concentration gradient maintained ?
-many molecules entering cells then pass into organelles and are used for metabolic reactions
-this maintains the concentration gradient and keeps more of the molecules entering the cell
E.g. oxygen diffuses into the cytoplasm of respiring cells , then diffuses into mitochondria and is used for aerobic respiration
Define osmosis
The net movement of water molecules from a higher water potential to a lower water potential, down a water potential gradient.Across a partially permeable membrane
What is water potential?
A measure of the concentration of water molecules that are able to diffuse
What has the highest water potential?
-pure water has the highest possible water potential(0kPa)
-when solute molecules are added , they lower the water potential of the solution(it becomes more negative)The more solute molecules the lower the water potential of the solution
-if 2 aqueous solutions are separated by a partially permeable membrane -water molecules will move from the solution with the highest water potential to the solution with the lowest water potential
-if the water potential on both sides becomes equal ,there will be no net osmosis but the water molecules will continue to move randomly
What is water potential measured in?
-kPa (kilopascals)
What is cytolysis?
In animal cells if a lot of water molecules enter , the cell will swell and burst so the plasma membrane breaks.This is cytolysis
What prevents plant cells from bursting ?
-the rigid and strong cellulose wall will prevent bursting.
-The cell will swell up to a certain size when its contents push against the cell wall
-which will resist any further swelling
-the swollen cell is described as turgid (turgidity of the plant cell has supports plants)
What is a HYPOtonic solution?
-the solution has a small amount of solute and higher water potential
-it is dilute
What is an ISOtonic solution?
-the solution the cell is in has the same amount of solute and the same water potential as the inside of the cell
What is a HYPERtonic solution ?
-the solution has a large amount of solute
-and a lower water potential
-it is concentrated
What happens when a cell is placed in a hypotonic solution (high water potential )?
-water molecules moves by osmosis , down the water potential gradient , across the plasma membrane ,into the cell
-in animal cells ,if a lot of water molecules enter the cell , it will swell and burst as the plasma membrane breaks. This is called cytolysis / haemolysis
-in plant cells the cell wall prevents the cell from bursting , instead the cell becomes turgid
What happen if a cell is placed in an isotonic solution?
-there is no net osmosis
-the cells neither shrink or swell
-they will not gain or lose any mass
What happens if a cell is placed in a hypertonic solution (low water potential)?
-water leaves the cell by osmosis across the partially permeable plasma membrane
-animal cells shrivel and are described as crenated
-in plant cells the cytoplasm of the cell shrinks and the membrane pulls away from the cellulose cell wall .The cells are described as plasmolysed
-plant tissue with plasmolysed cells are described as flaccid
What happens to plant cells that are plasmolysed ?
-they suffer a degree of dehydration and their metabolism cannot proceed , as enzyme catalysed reactions need to be in solution
Define active transport
The movement of molecules and ions from an area of lower concentration to an area of higher concentration (against the concentration gradient) using ATP and carrier proteins.
How are carrier proteins used in active transport?
-the molecule or ion to be transported attaches to a receptor site on the carrier protein, this takes place on the side of the membrane where the chemical is at a lower concentration
-a molecule of ATP binds to the carrier protein,the molecule of ATP undergoes hydrolysis producing a molecule of ADP and a phosphate (this releases energy)
-the phosphate attaches to the carrier protein and causes it to change conformation (it’s shape) with help from the energy released
-this shape change causes the carrier protein to transport the molecule or ion to the other side of the membrane where it is released
-the phosphate leaves the protein causing it to return to its previous shape
Why are carrier proteins specific?
Each carrier protein will only transport one molecule or ion
Cell membranes are …… barriers
partially permeable barriers
-they do not let all types of molecules pass through them
-the component molecules of the cell membrane determine its permeability
