Cell Membranes Flashcards
What is the purpose of a membrane?
- Separating cell contents from the outside environment
- Separating organelles from cytoplasm (e.g. lysosomes containing digestive enzymes where their optimum pH is low (acidic), and the rest of the cell is neutral).
- Cell recognition and signalling
- Regulating transport of material into and out of cells
- Holding the components of some metabolic pathways in place.
What is a phospholipid?
Phospholipids consist of:
- A hydrophilic head, made up of a phosphate group and glycerol. As it contains oxygen, it is polar and so water loving.
- A hydrophobic tail, made up of a hydrocarbon chain. It is non polar and so water hating.
- Phospholipids are what make up the cell membrane.
Why are polar substances hydrophilic?
Anything polar (has a spread of charge) can dissolve in water (which is also polar) so they are water loving.
Why are non-polar substance hydrophobic?
Molecules with charges that are evenly distributed around the molecules do not easily dissolve or mix with water, and in fact repel water molecules.
What happens when phospholipids are mixed with water?
If phospholipid molecules are mixed with water, they form a layer at the water surface. The phosphate heads stick into the water, while the fatty acid tails stick up out of the water.
What is a phospholipid bilayer?
If phospholipid molecules are completely surrounded by water, a bilayer can form. Phosphate heads on each side of the bilayer stick into the water, while the hydrophobic fatty acid tails point towards each other. This means the hydrophobic tails are held away from the water molecules.
-In this state, the phospholipid molecules can move freely, just as fluid molecules do, within the plane of the membrane. Very rarely some may ‘flip-flop’ from one monolayer to the other. The hydrophilic head group cannot easily pass through the hydrophobic region in the middle of the bilayer. This gives the bilayer some stability, despite the fact that the phospholipid molecules are not actually bonded together.
Why do phospholipids form a bilayer in plasma membranes?
- Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutions.
- The fatty acid tails are non-polar and will move away from an aqueous environment.
- As both the outside of the cell and cytoplasm is aqueous, phospholipids form two layers with the hydrophobic tails facing inwards.
- The phosphate group faces outwards interacting with the aqueous environment.
How does the phospholipid bilayer act a membrane?
The phospholipid bilayer is the basic structural component of all biological membranes. Essentially, the hydrophobic layer formed by the phospholipid tails creates a barrier to many molecules and separates the cell contents from the outside world. This thin layer of oil is ideal as a boundary in living systems, where most metabolic reactions take place in a water-based environment.
Why do membranes need to be flexible?
They need to be flexible so are not bonded together.
- Vesicle formation means that membranes to be able to curve and pinch.
- Membranes are still sturdy because hydrophobic tails don’t want to touch water so won’t move very much.
How have microscopes allowed the investigation into membranes?
You cannot see membranes through the light microscope, but they can be seen using an electron microscope. Here, the membranes appear as two dark ‘tramlines’ (the phospholipid heads) separated by a pale region (the fatty acid tails).
-The electron microscope have revealed that membranes are about 7-10nm thick.
Why do membranes need other components to make it functional?
A simple phospholipid bilayer would be incapable of performing all the functions of biological membranes. It would also be too fragile to function as a barrier within or around cells. Other components are needed in order to make a functioning biological membrane.
What is the specialisation of membranes?
The specialisation of cell membranes is a part of the process of differentiation. The number and type of these other components varies according to the function of the particular membrane.
-In a multicellular organism, this means the cell membranes of specialised cells have different properties, as do the membranes of specific organelles.
What examples are there of how membranes have specialised in the process of differentiation?
- The plasma membranes (cell surface membranes) of the cells in a growing shoot contain receptors that allow them to detect the molecules that regulate growth.
- Muscle cell membranes contain a large number of the channels that allow rapid uptake of glucose to provide energy for muscle contraction.
- The internal membranes of chloroplasts contain chlorophyll and other molecules needed for photosynthesis.
- The plasma membranes of white blood cells contain special proteins that enable the cells to recognise foreign cells and particles.
Are cell membranes permeable?
(How easy it is for things to pass/diffuse through)
All membranes are permeable to water molecules because water molecules can diffuse through the lipid bilayer. Some membranes are up to 1000 times more permeable to water because they contain aquaporins (protein channels that allow water molecules through them). Cell membranes that are permeable to water and some solutes are described as partially permeable membranes.
What are the features of the fluid mosaic model?
- A bilayer of phospholipid molecules forming the basic structure.
- Various protein molecules floating in the phospholipid bilayer, some completely freely, some bound to other components or to structures within the cell.
- Some (extrinsic) proteins partially embedded in the bilayer on the inside or the outside face; other (intrinsic) proteins completely spanning the bilayer.
What is an extrinsic protein?
Partially embedded in the membrane (on the surface)
What is an intrinsic protein?
Running all the way through the membrane
What components are in the fluid mosaic model of the membrane?
- Phospholipid bilayer
- Cholesterol
- Channel protein
- Carrier protein
- Glycolipid
- Glycoprotein
- Receptor sites
What is a glycolipid?
Where phospholipid molecules have a carbohydrate part attached. It acts as a hormone receptor.
What is a glycoprotein?
Where protein molecules have a carbohydrate part attached.
-It acts as a hormone receptor and can also bind cells together in tissues.
What is the role of cholesterol in a membrane?
Cholesterol gives the membranes of some eukaryotic cells mechanical stability.
The steroid molecule fits between fatty acid tails and helps make the barrier more complete, so substances like water molecules and ions cannot pass easily and directly through the membrane.
What membrane transport systems do membranes have?
- Channel proteins
- Carrier proteins
What is a channel protein?
Channel proteins allow the movement of some substances across the membrane. Molecules of sugar such as glucose are too large and too hydrophilic to pass directly through the phospholipid bilayer. Instead, they enter and leave cells using these protein channels.
They ONLY use diffusion, down a concentration gradient.
What is a carrier protein?
Carrier proteins actively move some substances across the membrane.
They use active transport, against a concentration gradient, and so need energy from ATP.
What is an example of how carrier proteins are used in plants?
- In plant cells, magnesium ions are actively pumped (using ATP energy) into the root hair cells from the surrounding soil, so that the plant ensures a supply of magnesium for the manufacture of chlorophyll.
- When mineral ions are actively transported into root hair cells, they lower the water potential of those cells. This makes water enter by osmosis. Nitrate ions are actively transported into xylem vessels to lower the water potential and cause water uptake from the surrounding root cells.
What is the role of receptor sites in a membrane?
- Some receptor sites allow hormones to bind with the cell so that a cell ‘response’ can be carried out. A cell can respond to a hormone only if it has a receptor for that hormone on its cell surface membrane.
- Cell membrane receptors are also important in allowing drugs to bind, and so affect cell metabolism.
What role do glycoproteins and glycolipids have in cell recognition?
Glycoproteins and glycolipids may be involved in cells signalling that they are ‘self’, to allow recognition by the immune system. Some hormone receptors are glycoproteins and some are glycolipid.
What role do enzymes and coenzymes have in membranes for metabolic processes?
Some reactions in photosynthesis take place in membranes inside chloroplasts. Some stages of respiration take place in membranes of mitochondria. Enzymes and coenzymes may be bound to these membranes. The more membrane there is, the more enzymes and coenzymes it can hold.
-This explains why mitochondrial inner membranes are folded to form cristae, and why chloroplasts contain many stacks of membranes called thylakoids.
What effect does an increase in temperature have on membrane permeability?
Increasing temperature gives molecules more kinetic energy, so they move faster. This increased movement of phospholipids and other components makes membranes leaky, which allows substances that would not normally do so to enter and leave the cell.
What effect does a temperature of below 0°C have on membrane permeability?
Components in the membrane have little energy and so are tightly packed, and rigid.
Channel and carrier protein denature
Membrane become permeable
What effect does a temperature of 0-45°C have on membrane permeability?
Phospholipids have energy to move and aren’t pack tightly.
As temperature increases, so does permeability.
What effect does a temperature of above 45°C have on membrane permeability?
Bilayer breaks down, and melts.
Channel and carrier proteins denature which disrupts membrane, leading to it becoming leaky.
Membrane becomes more and more permeable.
How do organisms living in extremely hot or cold climates adapt to stop the problem of temperature affecting permeability?
Organisms that live in very hot or cold environments need differently adapted molecular components of their membranes, so that their membranes can perform the function needed to maintain life.
-For example, if the organism lived in hot climates it will have a higher cholesterol content to keep it more stable.
Cell Communication and Signalling: Why must amoeba be able to detect nutrients around it?
Amoeba (a single-celled organism) must be able to detect nutrient molecules in the water around it. It must then move towards the nutrient and take the molecules into the cell. If the amoeba cannot detect that there are nutrients present, cannot move towards the nutrients, or cannot take in the nutrients it will be unable to survive.
Cell Communication and Signalling: How does detection of signals play a part in the survival of multicellular organisms?
In multicellular organisms, the survival of the whole organism requires each cell to play its part. So cells must be able to detect the various internal and external signals used to coordinate and carry out the processes involved in growth, development, movement and excretion. Cells must then be able to carry out reaction or functions in response to the signals. The processes involved in communication between the cells of multicellular organisms are extremely complex.
