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 are glycoproteins

a protein with a carbohydrate/lipid

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