Biological membranes Flashcards
What is the fluid mosaic model
- this is the theory of cell membrane structures with proteins embedded in a sea of phospholipids
Describe the fluid mosaic model
- Singer and Nicolson proposed a model that allowed the passage of molecules through the membrane
- fabric of the membrane consisted of a phospholipid bilayer with proteins floating in it making a mosaic pattern,
- lipid molecules can change places with each other and some of the proteins move giving fluidity
What do cell membranes do
- the cell membranes form a barrier and separate the cell contents from the cells exterior environment or separate the cells organelles from the cytoplasm
- but they need to allow some molecules to move in and out of the cell and form barriers to prevent some other molecules coming in
How do molecules enter the cell
- some small molecules can simply diffuse through the cell
- some substances can dissolve in the lipid layer and pass through
- other substances pass through special protein channels or are carried by proteins
Why are membranes partially permeable
this is because they do not let all types of molecules pass through them - the component molecules of the cell membrane determine its permeability
What are the roles of membranes at the surface of the cells
- separates the cell components from the outside environment
- regulates transport of materials into and out of the cell
- may contain enzymes involved in specific metabolic pathways
- has antigens so that the organisms immune system can recognise the cell as being self and not attack it
- may release chemicals that signal to other cells
- contains receptors for such chemical signals and is a site for cell communication or signalling
- may be the site for chemical reactions
What are the roles of membranes within cells
In some organelles metabolic processes occur on membranes
for example
- mitochondria have inner membranes called cristae, these give a large surface area for some of the reactions of aerobic respiration and localise some of the enzymes needed for respiration to occur
- inner membranes of chloroplasts called thylakoid membranes house chlorophyll, on these membranes some of the reactions of photosynthesis occur
- some digestive enzymes on plasma membranes of epithelial cells that line the small intestine, these enzymes catalyse some of the final stages in the breakdown of certain types of sugars
describe the fabric of the membrane
- membrane is the lipid bilayer made of up two layers of phospholipid molecules,
- hydrophilic heads are in contact with the watery exterior or watery interior
- the hydrophobic tail regions are in the centre of the membrane away from water
What do the proteins do in the membrane
- some of them have pores and act as channels to allow ions which have an electrical charge and are surrounded by water molecules to pass through
- some are carriers and by changing there shape can carry specific molecules across the membrane
- other proteins are attached to carrier proteins and function as enzymes, antigens or receptor sites for complementary shaped signalling chemicals such as hormones
What does cholesterol do in the membrane
- important for maintaining fluidity of the membrane, maintains mechanical stability and resists the effect of temperature changes on the structure of the membrane
What is outside the membrane
- Glycocalyx - this is formed from the carbohydrate chains attached to either lipids or glycoproteins in the membrane
- they are very hydrophilic and attract water with dissolved solutes and help the cell interact with its watery environment to obtain dissolved substances
How are channel and carrier proteins used in neurones
protein channels and carriers in the plasma membrane covering the long axon allow ions to exit and enter, this causes the conduction of electrical impulses along there length
they have a myelin sheath which is formed by flattened cells wrapped around them several times this means there are several layers of cell membrane
What does the plasma membrane of white blood cells contain
- they contain special protein receptors that enable them to recognise the antigens on foreign cells usually from invading pathogens but also from tissue or organ transplants
what do the carrier proteins do in root hair cells
- they have many carrier proteins to actively transport nitrate ions from the soil into the root hair cells
describe the inner membrane of mitochondria
- they are 76% protein and 24% lipid, this is because the inner membrane contains many electron carriers that are made of protein, and hydrogen ion channels are associated with ATP synthase enzymes
What is diffusion
the movement of molecules from an area of high concentration of that molecule to an area of low concentration, may or may not be across a membrane but it does not involve metabolic energy from ATP
What is facilitated diffusion
the movement of molecules from an area of high concentration of that molecule to an area of low concentration, across a partially permeable membrane via protein channels or carriers it does not involve energy from ATP
why is diffusion and facilitated diffusion described as passive
- they use only the kinetic energy of the molecules and do not use ATP
Describe simple diffusion
- all molecules have kinetic energy so can move freely and randomly
- if there is a high concentration of a certain type of molecule in an area then the molecules will bump into each other as they move
- therefore they eventually spread further from each other
- move to an area where there is a low concentration
what do molecules reach when they move down there concentration gradient
- when they move down there concentration gradient, they are still moving randomly but remain evenly dispersed therefore there is no net diffusion and they reach equilibrium
What type of molecules use simple diffusion
small
non-polar
water molecules - although they are polar, water is in such great concentration that direct diffusion does happen - may need special water protein channels where a high rate of water movement is required
How is the concentration gradient maintained
- molecules enter cells then pass into organelles these are used for metabolic reactions and maintains the concentration gradient and keeps more of the molecules entering the cell
- oxygen diffusing into the cytoplasm of respiring cells then diffuses into mitochondria and is used for aerobic respiration
Give examples of how the concentration gradient is maintained
- oxygen diffusing into the cytoplasm of respiring cells then diffuses into mitochondria and is used for aerobic respiration
- carbon dioxide diffusing into the palisade mesophyll cells of a plant leaf will then diffuse into the chloroplasts and be used for photosynthesis
What are the factors that effect the rate of simple diffusion
- temperature - the higher the temperature the more kinetic energy molecules have so the rate of diffusion increases
- diffusion distance - the thicker the membrane that the molecules have to go across the slower the rate of diffusion
- surface area - more diffusion can take place across a larger surface area - e.g. villi in small intestine
- size of diffusing molecule - smaller ions or molecules diffuse more rapidly than larger molecules
- concentration gradient - the steep the concentration gradient the faster the diffusion to the side with the least molecules
What molecules are used for facilitated diffusion
- polar
- small
what happens in facilitated diffusion
- small molecules which are polar are insoluble in lipid because they cannot interact with the hydrophobic tails of the lipid bilayer this means that they diffuse through water filled protein channels embedded in the membrane so they can interact with the polar water molecules
What reduces the permeability to small water soluble membranes
- cholesterol molecules
Describe how glucose gets through the membrane
- glucose molecules are too large to diffuse through the water filled protein channel in the membrane but they bind to a transmembrane carrier protein, this opens to allow the glucose to pass out on the other side of the membrane
- there are specific carrier proteins for different types of molecules
What do neurone plasma membranes have many channels specific to
they have channels specific to either sodium or potassium ions,
- the diffusion of these ions into and out of the neurone axon is crucial for the conduction of nerve impulses
- at synapses there are also calcium ion channels and there may be chloride ions
What type of channels do the plasma membrane of epithelial tissues have
have chloride ion channels which play a crucial role in regulating the composition of mucus to trap particles and pathogens
Describe osmosis
passage of water molecules down their water potential gradient across a partially permeable membrane
Describe water potential
measure of the tendency of water molecules to diffuse from one region to another
what is the solvent
the liquid in which the solute molecules are dissolved in is called the solvent
what do some membranes have
have protein channels known as aquaprotiens
- this allows water molecules to cross the membrane more rapidly
Describe osmosis process
water moves from an area of high water potential to an area of low water potential down the water potential gradient and across a partially permeable memrbaen
What happens when solute molecules are added to water
- the relative number of water molecules is changed
- if the solute molecules dissociated into charged ions such as sodium chloride dissociating into sodium ions and chloride ions they exert more effect on the relative number of water molecules than larger non polar molecules
this is because as sodium chloride molecules dissociate into sodium and chloride ions the number of particles in the solution doubles
What has the highest water potential
pure water 0kPa
What happens to the water potential when solutes are added
they lower the water potential of the solution
what happens when the water potential on both sides of the membrane is equal
there is no net osmosis although the water molecules will continue to move randomly
What is water potential measured in
kPa
The water potential in cells is lower than
that of pure water as there are solutes in solution, in the cytoplasm and inside the vacuoles of plant cells
what happens when cells are placed in a solution of high water potential
the water molecules move from osmosis across a partially permeable membrane from a high water potential in the solvent outside the solution to a low water potential inside the cell
What happens when animal cells are placed in pure water
-if a lot of water molecules enter the cell it swells and bursts as the plasma membrane breaks this is called cytolysis
what happens when water is placed in in a solution of lower water potential
- the water leaves the cells by osmosis across a partially permeable plasma membrane from a high water potential to a low water potential, the animal cells shrivel and are described as crenated
what happens when plant cells are placed in pure water
the rigid a strong cellulose cell wall will prevent bursting, the cell will swell up to a certain size when its contents is push against the cell wall this resists any further swelling, this swollen up is described as turgid
- plant cells help support plants especially those that are not woody
what happens when plant cells are placed in lower water potential
- the cytoplasm of plant cells shrinks and the membrane pulls away from the cellulose cell wall, the cells are described as plasmolysed, plant tissue with plasmolysed cells is described as flaccid
what happens to cells that are plasmolysed
- suffer a degree of dehydration and their metabolism cannot proceed as enzyme catalysed reactions need to be in solution
