Cell Membranes And Transport Flashcards
Role of fluid mosaic membrane
- allows movement of substances between cells and their surroundings e.g. nutrients and waste products
- allows cells to communicate with each other by cell signalling
Fluid
- Phospholipids can move within their monolayer by diffusion
- cholesterol maintains fluidity
- unsaturated fatty acids form ‘kinks’, increases fluidity as phospholipids don’t fit closely together
- the longer the fatty acid tail, the less fluid membrane
- as temp increases, fluidity of the membrane increases
Mosaic
When viewed through an electron microscope , the proteins (carrier/channel/glycoproteins) are scattered throughout the bilayer.
The cell surface membrane of phospholipids is…
A bilayer
Which direction do the tails of the fatty acids face?
Hydrophobic - inwards, away from aqueous environment
Which direction do the phosphate heads face?
Hydrophillic - faces aqueous environment
Phospholipids
Fatty acid:
- barrier to water soluble compounds/ions
- non-polar, allows passage of lipid soluble substances
- hydrophobic interactions with integral proteins
- structure of fatty acid tails maintains fluidity
Phospholipids can be…
When they…
- Some phospholipids are chemically modified to act as signaling molecules
- when they more between their belayer they activate other molecules e.g. Enzymes
- OR can get hydroysed to release small, water-soluble, glycerol containing molecules which can then diffuse through the cytoplasm and bird to specific receptors
Phosphate head
- Polar
- regulates finality stability
- storage
- restricts movement of phospholipids
- influences permeability of membrane
Cholesterol
- Contain hydrophilic heads and hydrophobic tails
- present in animals, less in plants
- not present in prokaryotes
- maintains finality of membrane
- prevents membrane from rupturing
- prevents membrane from being too rigid or fluid
At low temps cholesterol…
Increases fmidity of the membrane
- bc prevents close-packing of the phospholipid tails
- increased fluidity enables cells to survive cold temps
At higher temps cholesterol…
Prevents membrane from becoming too fluid as there is increased interaction of the phossphelipid tails with the cholesterol molecule
- enables cells to became more stabilised
Cholesterol and stability in cells
Provide mechanical stability
- without cholesterol membranes quickly break and the cells burst open
- What does cholesterol prevent?
prevents polar substances (e.g. ions) from leaking through the membrane due to hydrophobic regions
Prevents membrane being too rigid/fluid
*How are phospholipids able to move within their monolayer
Diffusion
*what does cholesterol do
Maintains fluidity
*what do unsaturated fatty acids form
Kinks —> increase fluidity as phospholipid don’t fit closely together
*what does a longer fatty acid tail mean
Decrease fluidity of the membrane
*how does fluidity change with temperature
Increase temperature = increased fluidity
*what are phospholipids a barrier to
Water soluble compounds (non-polar)
*what do phospholipids allow the passage of
Lipid soluble substances
*what does the structure of fatty acid tail do
Maintains fluidity
*what type of proteins do hydrophobic interactions have
Integral proteins
*phosphaste head polar or non-polar
Polar
*what does a phosphate head do/used for
Regulates fluidity/stability
Storage
*what do phosphate heads restrict
Movement of phospholipids
*what do phosphate heads influence
Permeability of membrane
*what does cholesterol contain
Hydrophilic heads, hydrophobic tails
*what organisms is cholesterol present in
Animals
Less in plants
None in prokaryotes
*what does cholesterol do at lower temperatures
Increase fluidity of membrane —> prevents close packing of phospholipid tails
*what does cholesterol do at higher temperatures
Prevents membrane becoming too fluid —> bc of increased interactions of phospholipid tails with the cholesterol molecule
- cells more stabilised
*what kind of stability is effected by cholesterol
Mechanical stability - without cholesterol, membranes quickly break and burst open
Glycoproteins
- receptors for hormones/neurotransmitters
- cell surface antigens for cell recognition
- stabilise membrane structure by forming H bonds with water molecules
- helps cells adhere together
Glycolipids
- cell surface antigens for recognition
Transport proteins
Channel proteins
- allow facilitated diffusion - high to low concentration
- most are gated
- fixed shape
Carrier proteins
- flip between two shapes
- binding site open at one side of membrane then the other
- facilitated diffusion and active transport - against concentration gradient
How is the rate of facilitated diffusion affected
By the number of channel or carrier proteins in the membrane
Whether proteins are open or closed
Where are peripheral proteins located
E.g.
Inner or outer surface of membrane
E.g. enzymes, ATP synthesis in chloroplasts and mitochondria
What are proteins on the inside of the CSM attached to
What do they do
cytoskeleton —> System of protein filaments
Help maintain and decide the shape of the cell, may be involved in changes of shape when cell moves
What do the proteins on the inside of the CSM act as
Energy transducers
Cell signalling
The molecular mechanism by which cells detect and respond to external stimuli, including communication between cells
Why does cell signalling occur
So the organism is able to respond appropriately to its environment
Cell signalling process
- Stimulus/signal is present
- Cell secretes a specific chemical - ligand
- Ligand transported by blood then binds to specific receptor of target cell
- Receptor goes through a *conformational change**
- Series of chemical reactions inside cell are triggered
- response occurs
How to calculate SA:V
SA/V (unit 3)
Facilitated diffusion
- diffusion taking place though carrier/channel proteins
- movement of polar molecules/ions moving from a region of high to low concentration
- passive and random process
What type of process is facilitated diffusion
passive and random process
Active transport
Osmosis
The movement of water molecules form a region of high to low water potential through a semi-permeable membrane, down a water potential/concentration gradient
Why does osmosis occur
Due to random movement of water molecules
Water potential equation
Water potential = solute potential + pressure potential
What is water potential
The pressure created by water molecules to move
Water potential of pure water
Zero - hypotonic solution
What happens to water potential when a solute is added to pure water
More negative water potential - hypertonic
(Solute potential always has a negative value)
Pressure potential is usually
0
When will pressure potential have a positive value
When a plant cell is turgid and water molecules are exerting pressure on the cell wall
Isotonic solution
When both solutions have the same concentration of solutes
Describe the movement of water from a hypotonic solution to a hypertonic solution
Net movement of water
osmosis in animal cells
What happens when:
1. Water potential of the surrounding solution is higher than that of the RBC
2. Water potential of the surrounding solution is lower than that of the RBC
- Water will enter the RBC and it will burst
- Water will leave the RBC and it will shrink - crinkled, cell solution becomes more concentrated
How is the round shape of a RBC maintained
When in an isotonic solution
Describe the net flow of water when:
1. The external water potential is less negative than the wp of the cell
2. The external wp is the sam as the wp of the cell
3. The external wp is more negative than the wp of the cell
- Net inflow of water
- No net water movement
- Net outflow of water
Describe osmosis in plant cells
- if wp is higher than the plant - water enters plant so become turgid good due to presence of cellulose cell wall (pressure potential positive)
- if wp of surrounding solution is lower than the plant cell - plant becomes plasmolysed
How to determine the wp of an unknown solution
- place onion cells in a range of sucrose solutions (e.g. every 0.5)
- count plasmolysed cells using microscope
- plot graph of no. plasmolysed cells against concentration of solutions
- place onion tissue in unknown solution
- count no. plasmolysed cells
- use graph p to determine concentration of unknown solution by extrapolation
Bulk transport
Movement of large molecules e.g. proteins/polysaccarides, in and out of cells
(endo and exocytosis)
ATP is needed
Exocytosis process
- vesicle/vacuole moves towards CSM
- fusion of vesicle with membrane - both contain phospholipids
- contents secreted/released
- active process - need ATP
Endocytosis process
- Vesicle containing molecule/molecule moves towards CSM
- membrane engulfs molecule inwards
- membrane is pinched off
Pinocytosis
What is needed
The uptake of bulk liquids
ATP
Importance of the cell surface membrane
