What are the functions of cell membranes ?
• Compartmentalisation
• Providing a selectively permeable barrier
• Transporting solutes
• Communication and intracellular interactions
• Scaffold for biochemical pathways and activities
• Scaffold for attachments to cytoskeleton and adjacent cells
Summarise the importance of compartmentalisation
- separates organelles from the cytoplasm so each organelle is doing its own function simultaneously
- increases the efficiency of subcellular processes by confined spaces in cells
- it allows the creation of specific microenvironments
Summarise the importance of the membrane providing a selectively permeable barrier
- it prevent unrestricted exchange of molecules
- allows communication between compartments
Summarise the importance of membranes transporting solutes
Helps organelles to communicate with each other and the external environment
Compare and contrast methods of transporting solutes
1) simple diffusion -
- passive
- no specialised proteins required
- goes down a concentration gradient
2) facilitated diffusion -
- passive
- carrier proteins needed
- goes down a concentration gradient
3) osmosis
- passive
- specific to water
- goes down a concentration gradient
4) active transport
- active (requires energy from ATP)
- goes against a concentration gradient
- requires specialised channel and carrier proteins
Summarise the importance of communication and intracellular interactions
External stimuli is transmitted into cells via specific receptors in the plasma membrane and so cells need to carry out specific responses
Stimuli includes nutrients, ions, hormones, viruses and toxins
Responses include endocytosis, intracellular signalling cascades , mitosis and changes in gene expression
Summarise the importance of membranes acting as a scaffold for biochemical pathways and activities
- Membranes are sites of chemical reactions where enzymes involved are embedded in membrane structure
- Membranes provide scaffolding within which components can be ordered for effective interaction
- Membranes bring enzymes and substrates closer to each other to increase their collisions
- Membranes are sites of energy transduction (chemical gradients are transformed into electrical gradients)
Summarise the importance for membranes acting as a scaffold for attachments to the cytoskeleton and adjacent cells
- the membrane supports the cytoskeleton by providing attachment sites
- the cytoskeleton interacts with the extracellular matrix via transmembrane molecules providing cell shape and strength
Summarise the general properties of membranes
• it is a sheet like bilayer composed of polar amphipathic lipids
• it is 6-10 nm thick
• all bilayers have a cytosolic face (faces the cytoplasm) and exoplasmic face (faces the external environment)
How is the phospholipid bilayer held together ?
It is held together by cooperative non-covalent interactions -
• hydrophobic interactions - these ensure the hydrocarbon tails are isolated from aqueous surroundings
• van der waals forces - attraction between hydrocarbon tails favours close packing
• electrostatic and hydrogen bonding - interactions between polar heads and water molecules keeps the bilayers together
• bilayers 1 - close in on themselves to form compartments
• bilayers 2 - invagination (endocytosis) and evagination (exocytosis)
Summarise the fluid - mosaic model
- proposed by Singer and Nicholson in 1972
- it describes how the phospholipids, cholesterol molecules, proteins and carbohydrates are distributed across membranes
- mosaic : the membrane is fluid which makes it impossible for it to form an impenetrable barrier
- lipids can diffuse laterally, rotate and flip flop
- proteins can only diffuse laterally and rotate but never flip flop
Summarise membrane composition
Membrane composition varies with cell type and it defines a cell’s function.
The bilayer components are distributed unequally between the 2 surfaces to create asymmetry.
- lipids and sterols form the permeable barrier
- proteins carry out majority of the functions
- polysaccharides linked to lipids make glycolipids and polysaccharides linked to proteins make glycoproteins used for signalling
Describe the structure and function of a nerve cell membrane
Structure : lipid 76% - protein 18% - carbohydrate 3%
Function : it insulates axons of peripheral nerves and has a high stability and so a low metabolic activity
Describe the structure and function of a red blood cell membrane
Structure : lipid 43% - protein 49% - carbohydrate 8%
Function : stable and metabolically active so it can transfer oxygen to tissues and remove carbon dioxide
Describe the structure and function of a mitochondrial inner membrane
Structure : lipid 24% - protein 76% - carbohydrate - 0%
Function : high metabolic activity for the electron transport chain in aerobic respiration
What are the general rules for chemical diversity of membranes ?
- higher percentage of lipids = more stable
- greater amounts of protein (enzymes) = high metabolic activity
- polysaccharides are absent from internal membranes
- cholesterols reduce the permeability of the membrane and increase compressibility
Describe the mobility of lipids in the bilayer
Lipids can undergo rapid lateral diffusion and transverse diffusion
- transverse diffusion is the movement of the phospholipid from one face of the bilayer to the other face
- lateral diffusion gives the membrane fluidity
What is the role of the enzymes scramblase and flippase in the mobility of lipids ?
Scramblase enzymes ensure both monolayers remain equally populated
Flippase enzymes move the phospholipids from one face to the other face for asymmetry
Why are there different proportions of lipids on either face of the membrane ?
The asymmetrical disposition of the lipids helps to give symmetry
What happens if there is no cholesterol present in the cell membrane ?
- The phospholipids could separate from each other leaving gaps in the membrane allowing the passage of unwanted substances
- The phospholipids could get closer together and prevent the passage of important substances
- Could result in diseases such as Alzheimer’s
Summarise the distribution of glycolipids
- only found on the exoplasmic leaflet
- unable to show transverse diffusion
- asymmetrically distributed
- protect the cell from mechanical and chemical damage
How are membrane proteins classified ?
They are grouped into 3 classes in regards to their relationship with the phospholipid bilayer
What are the 3 classes of membrane proteins ?
- intrinsic or integral
- extrinsic or peripheral
- lipid anchored
Describe the structure and function of integral membrane proteins
- penetrate hydrophobic core of the bilayer
- single pass or multi pass transmembrane proteins
- totally embedded in the membrane
- function as receptors, channels, transporters and electron transfer agents
- they are amphipathic - have hydrophobic and hydrophilic parts
