Cell membranes and transport Flashcards
Fluid mosaic model
Fluid because both the phospholipids and proteins can move about by diffusion. Mosaic refers to the pattern of the protein molecules when the surface of the membrane is viewed from above, there are lots of different protein molecules
Features of the phospholipid bilayer
The membrane is a double layer (bilayer) of phospholipid molecules, they move about by diffusion within their monolayer. The phospholipid tails face inwards forming a non-polar hydrophobic interior. The heads face the aqueous medium.
How does the structure of the phospholipids affect the membrane
The more saturated they are the more fluid the membrane. The longer the tail the less fluid the membrane. As temperature increases the membrane becomes more fluid.
Phospholipids in membranes
Because the tails of the phospholipid are non-polar it is hard for polar molecules to pass though. Can be modified chemically to act as signalling molecules
Intrinsic proteins
Can be found in the inner or outer layer, or spanning the whole membrane as transmembrane proteins. In transmembrane proteins the hydrophobic regions are made up of alpha helical chains. Intrinsic proteins have hydrophobic and hydrophilic regions. They stay in the membrane because of the hydrophobic regions repelling the water.
Cholesterol
Have a hydrophilic head and a hydrophobic tail. At low temperatures it increases the membranes fluidity. It decreases the membranes fluidity at high temperatures.
Glycolipids and glycoproteins
Some of the lipid and all of the protein molecules have short carbohydrate chains attached to them. The carbohydrate chain forms hydrogen bonds with the water molecule and helps to stabilise the membrane structure.
Carbohydrate chain
Helps the glycolipids and glycoproteins act as receptor molecules which bind with different substances on the cell surface
What do the receptor molecules on the cell surface membrane act as
Signalling receptors - coordinates the activity of the cell
endocytosis
antigens - allowing cell to cell recognition
Transport protein
Provide hydrophilic channels for ions and polar molecules to pass through the membrane. The two types are channel and carrier proteins. Some proteins form part of the cytoskeleton
The process of cell signalling
The signal arrives at the protein receptor on the cell surface membrane. The receptor is a specific shape which recognises the signal. The signal changes the shape of the receptor, as this spans the membrane the message is passed to the inside of the cell (signal transduction). This activates a G protein which acts as a switch to release the “second messenger”, which diffuses through the cell relaying the message. Many second messengers can be made which amplifies the original message. The second messenger activates an enzyme which activates further enzymes increasing the amplification at each stage. Until an enzyme is produced which brings about the required change in cell metabolism
Signalling cascade
The sequence of events triggered by the G protein
Diffusion
The net movement of molecules or ions from a region of high concentration to a region of low concentration down a gradient, as a result of the random movement of particles.
Other ways a receptor can alter the activity of a cell
Opening an ion channel, resulting in a change of membrane potential.
Acting directly as a membrane bound enzyme.
Acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane.
Demonstrating diffusion using Visking tubing
A partially permeable non-living membrane made from cellulose. Large molecules such as starch and sucrose will not pass through but glucose will. This can be demonstrated by filling visking tubing with starch and glucose and testing the water around it for it. You could investigate the amount of glucose over time by using a colorimeter.
Demonstrating diffusion using plant tissue
Pieces of beetroot can be placed in water at different temperatures. Any damage to the cell surface membrane results in a red pigment leaking out. You can test the colour of the water using a colorimeter. You can see how this is affected by temperature or chemicals.
Facilitated diffusion
The diffusion of a substance through transport proteins in a cell membrane. The proteins provide hydrophilic areas that allow the molecule or ion to pass through the membrane which would otherwise not be permeable to them.
Osmosis
The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane as a result of random movement
How the steepness of the concentration gradient affects diffusion
The greater the difference in concentration of molecules, the greater the difference in the number of molecules passing in the two directions and the faster the rate of diffusion
How temperature affects diffusion
At higher temperatures, molecules and ions have more kinetic energy then at low temperatures. They move about faster and thus diffusion takes place faster.
How surface area affects diffusion
The greater the surface area the more molecules or ions which can cross it at any one moment and therefore the faster diffusion can occur. The surface area to volume ratio decreases as the size of the three dimensional object increases.
How the nature of the molecule or ion affects diffusion
Large molecules require more energy to start moving so diffusion is slower. Non-polar objects can diffuse better in non-polar substance like membranes and polar objects can diffuse better in polar substances
Channel proteins
Water filled pores which allowed charged substances to pass through. Most are gated, which means that part of the protein molecule on the inside of the membrane can move to open or close the pore. This allows control of ion exchange.
Carrier proteins
Can flip between two shapes, the binding site is alternately open to one side of the membrane then the other. Direction of movement depends on the relative concentration of molecules on each side of the membrane
What affects the rate of facilitated diffusion
The number of channel or carrier proteins and whether the carrier proteins are open or not.
Water potential
The tendency of water to move out of a solution. Water moves from a region of high water potential to a region of low water potential. Affected by solute potential and pressure potential. 0 in pure water
Solute potential
The extent to which the solute molecules decrease the water potential of the solution. 0 in pure water and becomes more negative the more solute is added
Pressure potential
The pressure exerted on the mixture. It is 0 when no pressure is exerted and becomes more positive the more pressure there is.
What happens when an animal cell is in a dilute solutions
Water enters the animal cell by osmosis and the cell bursts
What happens when an animal cell is in a concentrated solution
Water leaves the animal cell by osmosis and the animal cell shrinks
What happens when a solution has a higher water potential then the plant cell
Water enters the cell via the partially permeable membrane by osmosis. The volume of the cell increase, the cell wall pushes back against the expanding protoplast and pressure begins to build. This increases the water potential of the cell until it equals the water potential outside the cell. The cell wall prevents the plant from bursting. The cell is now turgid.
What happens when a plant cell is placed in a solution of lower water potential
The water leaves the cell by osmosis. The protoplast will shrink until it exerts no pressure on the cell wall. As the protoplast continues to shrink it begins to pull away from the cell wall and the cell is plasmolysed. The point at which pressure potential has just reached zero and plasmolysis is about to occur is known as incipient plasmolysis. The cell is flaccid beforehand
Active transport
The energy consuming transport of molecules or ions across a membrane against a concentration gradient. The energy is supplied by ATP which is produced from respiration in the cell. The energy is used to make carrier proteins change shape.
Sodium-potassium pumps
Found in the cell surface membrane of animal cells. It pumps three sodium ions out of the cell as the same time allowing two potassium ions into the cell for each ATP molecule used. The pump has a receptor cell for ATP, it acts as an ATPase enzyme in bringing about the hydrolysis of ATP to ADP and phosphate in order to release energy.
Endocytosis
The bulk movement of liquids (pinocytosis) or solids (phagocytosis) into the cell, by the infolding of the cell surface membrane to form vesicles containing the substance. It is an active process requiring ATP.
Exocytosis
The bulk movement of liquids or solids out of the cell by the fusion of vesicles containing the substance with the cell surface membrane. It is an active process requiring ATP.
