5 - Plasma membrane Flashcards
Roles of membranes within and surface of cells?
- partially permeable barriers between cell and its environment, between organelles, and within organelles.
- controls which substances enter and leave cells.
- sites of chemical reactions
- sites of cell communication.
What is compartmentalisation?
formation of separate membrane-bound areas in a cell.
Why is compartmentalisation important?
- metabolism includes many different and often incompatible reactions.
- allows the specific conditions required for cellular reactions.
What model was used to describe the cell surface membrane?
fluid-mosaic model
Why is the fluid-mosaic model used to describe the cell surface membrane?
- the phospholipids are free to move within the layer relative to each other (fluid).
- proteins embedded in the bilayer vary in size, shape, position.
How is the cell surface membrane a … bilayer?
phospholipid bilayer.
- hydrophilic phospholipid heads (polar) face the aqueous environment
- hydrophobic fatty acid tails (non-polar) face inwards away from the aqueous environment.
- both tissue fluid and cytoplasm are aqueous
- molecules arrange themselves to form a bilayer.
What kind of molecules can pass through the cell membrane with ease?
fat-soluble substances.
- due to hydrophobic bilayer centre, the bilayer acts as a barrier to water-soluble substances.
phospholipid
- hydrophilic phosphate ‘head’
- hydrophobic fatty acid ‘tail’
- molecules arrange to form a bilayer.
cholesterol
- lipid with a hydrophilic end and hydrophobic end
- positioned between phospholipids in the bilayer.
- regulates the fluidity of membranes.
- stabilises the membrane
- prevents phospholipid molecules from grouping too closely and crystallise.
what is cell signalling?
Communication between cells to trigger a response inside the cell.
thickness of the cell surface membrane?
7nm
What are intrinsic proteins and some examples?
- proteins that span the whole width of the membrane.
- channel proteins
- carrier proteins
- glycoproteins
What are extrinsic proteins and some examples?
- proteins that are present in one side of the bilayer.
- peripheral proteins
channel proteins?
- intrinsic protein
- hydrophilic channel
- allows passive movement of small, uncharged, or charged/polar particles down a conc gradient.
- e.g oxygen
Carrier proteins?
- intrinsic protein
- transports molecules and ions across membrane by active transport, facilitated diffusion.
- e.g sodium ion
glycoproteins?
- intrinsic protein
- proteins with attached carbohydrate chain.
- act as receptors for cell signalling
- when molecule binds to glycoprotein, chemical reaction is triggered in the cell.
- stabilises membrane by forming hydrogen bonds with surrounding water molecule.
glycolipids?
- lipids with an attached carbohydrate chain.
- act as cell antigens.
- can be recognised by cells of immune system as self or non-self.
What acts as membrane bounds receptors and what is their role?
- glycoprotein
- glycolipids
- membrane-bound receptors are sites where drugs and hormones can bind.
How do drugs work?
- bind to receptors in cell membranes
- triggers a response in the cell
- or blocks the receptor and prevents it from working.
What factors affect membrane structure and permeability?
- temperature
- solvents.
How does temperature affect membrane structure and permeability?
- increase in temperature, phospholipids have more kinetic energy and will move more. Gaps are created.
- membrane becomes more fluid and starts to lose its structure. Permeability increases.
- carrier and channel proteins are denatured, cannot control what enters/leaves cell. Increases permeability.
How do solvents affect membrane structure and permeability?
- many organic solvents like ethanol are non-polar and dissolve membranes.
- strong alcohol solutions are toxic (destroy cells). Lower conc alcohols do not dissolve membranes but still cause damage.
- as solvent concentration increases, membrane permeability also increases.
What happens when a membrane is disrupted?
- becomes more fluid
- becomes more permeable.
How does alcohol affect membrane structure?
- non-polar alcohol molecules enter cell surface membrane and fit between the phospholipids.
- this disrupts the structure, increasing membrane permeability.
What are the two passive transport methods?
- diffusion
- facilitated diffusion
What is the definition of diffusion?
The net movement of particles from an area of higher concentration to an area of lower concentration.
- passive process
- continues until there is a concentration equilibrium between the two areas.
What does it mean by ‘passive process’?
- no external energy is needed for the process to happen.
What does it mean by ‘concentration gradient’?
- path from an area of higher concentration to an area of lower concentration.
Why does diffusion occur?
- particles have kinetic energy
- the random movements of particles.
factors affecting rate of diffusion?
- temperature (higher)
- steepness of concentration gradient (steeper)
- surface area (greater)
- thickness of exchange surface (thinner)
How does surface area affect rate of diffusion?
- greater surface area
- means there is more area for diffusion to occur simultaneously
- rate increases.
Why can small molecules travel across the membrane and what is this called?
- diffusion
- small molecules like oxygen and CO2 are small enough to fit through the phospholipids and diffuse across the membrane.
- even though water molecules are polar, they are small enough to fit through the phospholipids and diffuse across the membrane.
Why are membranes described as partially permeable?
- small, non polar molecules (and water (small)) are able to diffuse through freely.
- large and polar molecules cannot.
What is facilitated diffusion?
- the diffusion across membranes through carrier and channel proteins is called facilitated diffusion.
- movement of particles is down a concentration gradient.
- passive process
How can carrier proteins be involved in facilitated diffusion? (can also be used for active transport)
- when a specific molecule (large) binds to the carrier protein, it causes it to change shape.
- the molecule is then released on the opposite side of the membrane.
How can channel proteins be involved in facilitated diffusion?
- provide hydrophilic channels
- for charged particles to diffuse through
What affects rate of facilitated diffusion?
- temperature
- surface area
- concentration gradient
- thickness of exchange surface (membrane)
- number of channel proteins present in the membrane.
What is active transport?
- movement of molecules or ions from into/out of a cell from an area of lower concentration to an area of higher concentration.
- active process
- energy is supplied by ATP
Which protein in the cell surface membrane is involved with active transport?
- carrier proteins.
steps of active transport?
- molecule/ion binds to carrier protein
- ATP binds to carrier protein on inside of cell and is hydrolysed to ADP and phosphate.
- phosphate ion binds to carrier protein and causes shape of protein to change.
- molecule/ion is released into the cell.
- phosphate molecule is released and recombines with ADP to form ATP.
- carrier protein returns to original shape.
- metabolic energy is supplied by ATP.
define ‘bulk transport’
- another form of active transport.
- large molecules such as proteins are too large to moved by carrier proteins.
steps of endocytosis
- cell surface membrane invaginates when substance approaches.
- membrane fuses to form a vesicle.
- vesicle pinches off and moves into cytoplasm and transports the substances.
- ATP is the energy source.
- eg phagocytosis.
steps of exocytosis
- vesicles containing substances are formed by Golgi apparatus.
- vesicles move towards the cell surface membrane.
- vesicles fuse with the cell surface membrane.
- contents are released outside the cell.
- ATP is the energy source.
Why is ATP energy needed in bulk transport?
- movement of vesicles along cytoplasm
- changing the shape of cells (to engulf)
- fusion of cell membranes with vesicles or when vesicles form.
What is osmosis?
- The movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane.
what is the water potential of pure water?
0kPa
What is the water potential of solutions compared with pure water?
negative
what is the symbol for water potential?
psi, Ψ
define ‘water potential’
- the likelihood of water molecules to diffuse out of or into a solution.
What is a hypotonic solution?
- a solution with more water than solute, compared to another solution.
- has higher water potential than another solution
What is a hypertonic solution?
- a solution with more solute than water, compared to another solution.
- has lower water potential than another solution.
What are isotonic solutions?
two solutions/cells with the same water potential.
What happens to an animal cell in hypotonic solution (higher water potential outside)?
- outside environment of cell has a higher water potential.
- water enters the cell by osmosis via the partially permeable membrane.
- hydrostatic pressure inside the cell increases.
- cell-surface membrane cannot withstand pressure increase.
- animal cell bursts.
- cytolysis.
What happens to an animal cell in hypertonic solution (lower water potential outside)?
- outside environment of cell has lower water potential.
- water leaves the cell by osmosis via the partially permeable membrane.
- hydrostatic pressure inside the cell decreases.
- difference in pressure inside and environment causes cell to shrink and shrivel.
- crenation.
What happens to an plant cell in hypotonic solution (higher water potential outside)?
- outside environment of cell has higher water potential.
- water enters the cell by osmosis via the partially permeable membrane.
- hydrostatic pressure inside cell increases.
- cell surface membrane pushes against the cell wall.
- cell does not burst due to cell wall.
- cell is now turgid.
What happens to an plant cell in hypertonic solution (lower water potential outside)?
- outside environment of cell has lower water potential.
- water leaves the cell by osmosis via the partially permeable membrane.
- hydrostatic pressure inside the cell decreases.
- cell surface membrane pulls away from the cell wall due to difference in pressure.
- cell is now plasmolysed.
What happens if a cell is in an isotonic solution?
The net movement of water molecules into and out of the cell is the same.
factors affecting osmosis?
- water potential gradient
- thickness of exchange surface
- surface area of exchange surface. (more area for water molecules to cross the membrane at the same time).
