Biological membranes Flashcards
1
Q
what is the nature of cell membranes and why
A
- partially permeable- cell membranes form a barrier and separate cell contents from cells exterior environment or separate organelles from cytoplasm- need to allow some molecules through, into or out of cell- some organelles also have inner membranes within them- form barriers too
2
Q
what does permeability refer to
A
the ability to let substances pass through
3
Q
what determines a membrane’s permeability
A
the properties of the component molecules of the cell membrane
4
Q
what are the roles of the plasma membrane
A
- separates cells component from external environment (single-celled = external surroundings, multicellular = tissue fluid or surrounding cells)
- regulates the transport of materials into/out of the cell
- may contain enzymes involved in specific metabolic pathways
- has antigens, so that the organisms immune system can recognise cell as being ‘self’ and not attack it
- may release chemicals to signal to other cells
- contains receptors for chemical signals, and do is a site for cell communication or signalling- hormones and drugs may bind to membrane-bound receptors
- may be site of chemical reactions
5
Q
describe roles of membranes within cells
A
- separates organelle from contents of cells cytoplasm- so each organelle is a discreet entity and able to perform its function
- mitochondria- cristae- large surface area for some of the reactions of aerobic respiration and localise some enzymes need for it to occur
- chloroplasts- thylakoids- house chlorophyll, on membranes some reactions for photosynthesis occur
- some digestive enzymes on plasma membranes of epithelial cells that lien small intestine- catalyse some of final stages in the breakdown of certain types of sugars
6
Q
who proposed the model of cell membranes, what did it compose of
A
- 1972- Singer and Nicolson- Fluid mosaic model
- explained how cell membranes could be more dynamic and interact more with cells environment
- proposed that fabric of membrane consisted of phospholipid bilayer with proteins floating in it- making up mosaic pattern
- lipid molecules can change places with each other, some of proteins may move- fluidity
7
Q
Describe the phospholipid bilayer
A
- fabric of the membrane
- hydrophillic heads- contact with watery in/exterior- polar
- hydrophobic fatty acid tails- non-polar
7nm in width
8
Q
List the components of membranes as per the fluid mosaic model
A
- phospholipid bilayer
- channel protein
- carrier protein
- peripheral protein
- integral proteins
- glycoproteins
- glycolipids
- glycocalyx
- cholesterol
9
Q
Describe channel proteins
A
- water-filled channel lined with hydrophilic amino acids
- integral proteins- span whole membrane- membrane-spanning regions interact with hydrophobic regions of phospholipid bilayer
10
Q
Describe carrier proteins
A
- integral proteins- span whole membrane- membrane-spanning regions interact with hydrophobic regions of phospholipid bilayer
11
Q
Describe peripheral proteins
A
- can be enzymes
- can act as receptor sites for complimentary shaped molecules e.g. hormones, or be antigens
12
Q
Describe glycocalyx
A
- carbohydrate molecules on outside of membrane- very hydrophilic- attract water with dissolved solutes- helps cell interact with its watery environment and obtain dissolved substances
- contains receptors
- glycoprotein- carbohydrate chain attached to protein molecule
- glycolipid- carbohydrate chain attached to lipid
13
Q
Describe cholesterol in cell membranes
A
- gives mechanical strength and fluidity/flexibility
- helps resist effects of temperature change
14
Q
Describe how neurones are specialised in terms of their membrane
A
- protein channels and carriers in plasma membrane covering long axon allow for entry and exit of ions to bring about the conduction of electrical impulses along their length
- myelin sheath formed by flattened cells wrapped around them several times- several layers of membrane- 20% protein, 76% lipid
15
Q
Describe the plasma membranes of white blood cells
A
- contain special protein receptors that enable them to recognise antigens on foreign cells- usually from invading pathogens- also from tissue or organ transplants
16
Q
Describe membranes in root hair cells
A
- Many carrier proteins to actively transport nitrate ions from soil into plant
17
Q
Describe the inner membranes of mitochondria
A
- 76% protein, 24% lipid- inner membranes contain many electron carriers that are made of protein, and hydrogen ion channels associated with ATP synthase enzymes
18
Q
What type of processes are diffusion and facilitated diffusion and why
A
- passive
- don’t use any of cells metabolic energy (ATP)- only use kinetic energy of molecules
19
Q
Describe the process of simple diffusion
A
- all molecules have kinetic energy and can move freely and randomly in gas or liquid media
- will happen even if not mixed by stirring or shaking
- if high concentration of certain type of molecule in area, the molecules will bump into each other as they randomly move- eventually will spread out further from each other
- more will move to an area where there is a lower concentration until they become evenly dispersed
- when molecules move down their concentration gradient, they are still moving randomly but remain evenly dispersed so there is no net diffusion- reached equillibrium
20
Q
Which molecules can diffuse simply
A
- oxygen and carbon dioxide- small enough to pass through cell membranes
- lipid solube molecules- such as steroid hormones- dissolve in the lipid bilayer- still move down concentration gradient
- water- even through polar and insoluble in lipid bilayer, however present in such great concentrations that significant direct diffusion happens. Where high rate of water movement, special water channel proteins (aquaporins) may be needed to allow water to cross membrane without challenge of lipid layer
21
Q
Describe the ways in which a concentration gradient is maintained, give 2 examples
A
- molecules pass into cells then into organelles then used for metabolic reactions
- oxygen diffusing into cytoplasm of respiring cells then diffuses into mitochondria and is used for aerobic respiration
- carbon dioxide diffusing into palisade mesophyll cells of a plant leaf will then diffuse into chloroplasts and be used for photosynthesis
22
Q
Explain the factors which affect rate of simple diffusion
A
- temperature- higher= more kinetic energy- higher rate of diffusion
- diffusion distance- thicker = slower
- surface area- larger = more diffusion can take place- e.g. microvilli
- size of diffusing molecule- smaller = faster
- concentration gradient- steeper = faster movement down gradient
23
Q
When is facilitated diffusion needed
A
- small molecules that have polarity- such as ions that have electrical charge- insoluble in lipid as cant interact with hydrophobic tails- need to diffuse through channel proteins embedded in membrane (0.8nm diameter)
- cholesterol molecules within membranes reduce permeability to small water-soluble molecules
- glucose- too large for protein channel- need carrier protein- opens to allow molecule out
24
Q
Describe how different cell types have different membranes, give examples
A
- different proportion of transmembrane protein channels and carriers- allows call to control what types of molecule pass in/out
- neurone plasma membranes- many channels specific to sodium/pottasium ions- diffusion of these in/out axon is essential for conduction of nerve impulses, also calcium ion and maybe chloride ion channels at synapses
- epithelial cells that line airways- chloride ion channels- crucial role in regualting composition of mucus to trap particles and pathogens
