chapter 5 : plasma membranes

what is the cell surface membrane/plasma membrane

partially permeable barrier between the cell and its environment that regulates the exchange of substances

it’s the site of cell communication and some chemical reactions

what does partially permeable mean

only allows some substances to go through the membranes pores

why are membranes important

compartmentalisation

separate the cell contents from the environment

separate different areas within the cell or within the organelle

what is compartmentalisation

membranes keep the insides of organelles separate from the cytoplasm

why is compartmentalisation important

allows chemical gradients to be maintained at sites of chemical reactions

sites of cell signalling (when cells communicate with each other via receptors on membranes)

what is cell signalling

communication between cells and cell recognition

a molecule released by one cell attached to another and causes a response

how is cell signalling done

molecules leave cells by exocytosis and are picked up by specific complimentary receptor cells to trigger a response

what 3 components are plasma membranes made of

phospholipids

proteins

cholesterol

what is the structure of phospholipids

hydrophilic heads (polar)

hydrophobic tails (non-polar)

how thick is the phospholipid bilayer and what can pass through the gaps in the hydrophilic head

7nm thick

provides a barrier to large polar molecules

fat soluble substances or very small molecules can pass straight through

what are the 2 types of proteins in the phospholipid bilayer

intrinsic: integral proteins, span the entire bilayer

extrinsic: peripheral proteins, smaller proteins on one side of the bilayer only

what do intrinsic proteins do

transport water soluble charged particles as they can’t pass through the hydrophobic heads of the phospholipid bilayer

can act as enzymes

glycoproteins and glycolipids can act as receptors and cell signallers

what are the 2 types of intrinsic proteins

channel proteins

carrier proteins

what is the role of channel proteins

hydrophilic passageway for water and charged/polar substances to diffuse down a concentration gradient

what is the role of carrier proteins

change shape to move substances from one side of the membrane to the other down or against the concentration gradient

what are glycoproteins

intrinsic proteins with carbohydrate side chains attached

what are the 2 roles of glycoproteins

cell adhesion

cell signalling

where are proteins glycosylated and what does it mean

in the golgi apparatus

glycosylated - carbohydrate side chains are added to the protein

what are glycolipids

lipids with glycoproteins attached that act as antigens

the immune system can use them to recognise self and non-self cells

what is cholesterol and what do they do (4)

they are lipids like steroids

has polar and non-polar regions

reduces membrane permeability

increases membrane stability

how are cholesterols arranged

polar regions of cholesterol align with hydrophilic phosphate head and non-polar regions align with hydrophobic fatty acid tails

what can the structure of a cell membrane be described as

a fluid mosaic

what is the effect of temperature on membrane function

kinetic energy of the phospholipids increases and they move around more causing pigment and other molecules to be able to leak out

the membrane becomes more permeable

what happens to the cell membrane as the temperature increases even more (2)

proteins denature so channel and carrier proteins stop working which affects permeability to polar molecules

more gaps are made so more leakage occurs until eventually the membrane will break down completely

why does the water turn red when you cook a beetroot

the membranes of the beetroot cells are damaged and become more leaky releasing the pigment from the cytoplasm

what is the effect of solvents on the function of the cell membrane

many organic solvents are less polar than water or non-polar so they will dissolve phospholipids

it will degrade the membrane and eventually destroy it which allows substances to cross the membrane freely

what is an example of an organic molecule that’s less polar that water and one that is non-polar

less polar - alcohol

non-polar - benzene

what is the effect of alcohol on the cell membrane

it will dissolve / create gaps in the membrane and enter the cells and disrupt the structure

how will different types of alcohol affect cell membranes

very strong / pure alcohols is toxic and will destroy cells

alcoholic drinks (less concentrated alcohol) won’t dissolve membranes but wills till cause damage

how does alcohol affect nerve cells (2)

nerve cells need an intact membrane for nervous impulses to be sent correctly so a damaged membrane will cause delays in the process

continuous damage will destroy the cells

what is the definition of diffusion

net movement of molecules from a higher concentration to a lower concentration across a partially permeable membrane down a concentration gradient due to the random movement of particles which is a passive process

when does diffusion stop

when the concentration gradient reaches equilibrium

what particles are able to diffuse across a membrane

non-polar molecules (e.g. oxygen or carbon dioxide)

the hydrophobic interior of membranes doesn’t allow charged molecules to diffuse

how fast can different molecules diffuse across a membrane

polar molecules (e.g. water) can diffuse but only slowly

smaller polar molecules pass through easier than larger

what are the 3 types of diffusion

simple diffusion

facilitated diffusion with a protein channel

facilitated diffusion with a carrier protein

what is simple diffusion

small non-polar particles pass through directly through the phospholipid bilayer

what are 2 examples of simple diffusion

oxygen entering a red blood cell

nicotine and caffeine as they are lipid soluble

what is facilitated diffusion (2)

passive diffusion for large or polar molecules that need the aid of a protein to pass through the cell surface membrane

proteins are specific to the molecule that they carry

what is facilitated diffusion with channel proteins

passive transport where the protein acts like a pore in the membrane that can be opened and closed to change the flow

how does diffusion with carrier proteins work

molecule binds to the carrier protein which then changes shape to allow the molecule to cross the membrane

what are protein channels and carrier proteins for

ions and small molecules

larger molecules

what is active transport (2)

transport from an area of low to high concentration against the concentration gradient that requires ATP

active transport protein carriers can only move specific ions or molecules

what are the 2 types of bulk transport

endocytosis - into cells

exocytosis - out of cells

what are 2 types of endocytosis

phagocytosis - transporting solids (e.g. bacteria)

pinocytosis - liquids

how does a sodium potassium pump work (7)

1. sodium ions bind to complimentary shaped binding sites on carrier protein

2. ATP is hydrolysed and phosphate group binds to carrier protein

3. causes confrontational change in shape of carrier protein so it flips

4. potassium ions leave carrier protein and process ready to repeat

5. carrier protein flips back to original shape

6. binding sites for potassium ions are correct complementary shape for ions to bind to carrier protein

7. phosphate group released from protein and recombines with ADP to form ATP

how does temperature affect the rate of diffusion (3)

the higher the temperature the more kinetic energy the particles have so they move faster

the phospholipids also have more kinetic energy so have more gaps between

therefore particles are more likely to pass through the membrane faster

5 factors affecting the rate of diffusion

temperature

concentration gradient

membrane surface area

membrane thickness

number of protein channels

what is osmosis

net passive movement of water molecules from an area of higher to lower water potential down a water potential gradient through a partially permeable membrane due to the random movement of the molecules

what is water potential

the tendency of water molecules to move from one region to another

(water molecules used to dissolve solute aren’t free to move)

what is water potential in terms of Pa

water molecules collide with membrane which applies pressure to it

this pressure is called water potential and is measured in Pa

what is the water potential of pure water like

pure water has the highest water potential

the water potential of water is 0kPa

how does water potential change as the amount of solute increases

water potential becomes more negative as more water molecules are used to dissolve the solute and are therefore not free to move

what happens when a plant cell is placed in pure water

there’s a higher water potential outside the cell (hypotonic solution) so water moved into the cell

the cell swells and becomes turgid but cellulose cell wall stops it from bursting

what happens when you put an animal cell in pure water

there’s a higher water potential outside the cell (hypotonic solution) so water moves into the cell

the cell swells and bursts (cytolysis)

what happens if you put a plant cell in concentrated sugar solution

lower water potential outside the cell (hypertonic) so water moves out of the cells

the cells lose turgid pressure and eventually the plasma membrane pulls away from the cell wall (plasmolysis)

what happens if you put an animal cell in concentrated sugar solution

lower water potential outside the cell (hypertonic solution) so water moves out of cell

cell volume decreases and the cell shrivels (crenation)

what is a flaccid plant cell

a plant cell that has lost water and is not turgid but hasn’t lost enough water to be plasmolysed

what happens to plasmolysed lysed or crenated cells

they die

why is a positive water potential not possible

because pure distilled water has a water potential of 0 so nothing can have a greater water potential than that

what is the formula you can use to complete a serial dilution

m1 x v1 = m2 x v2