Module 2.5 - Biological Membranes Flashcards
Describe the permeability of cell surface membranes.
- Partially permeable membrane
- Some very small molecules simply diffuse through plasma membrane between structural molecules
- Some molecules dissolve in lipid layer + pass through
- Others pass through channel proteins or carrier proteins
What are the roles of the cell surface membrane surrounding cells?
- Separates cell’s components from external environment (e.g. tissue fluid, external surroundings in unicellular)
- Regulates transport of materials into + out of cell
- May contain enzymes involved in specific metabolic pathways
- Has antigens, so immune system can recognise cell as self + not attack it
- May release chemicals that signal to other cells
- Contains receptors for chemical signals + so is a site for cell communication or signalling. Hormones + drugs may bind to membrane bound receptors
- May be the site of chemical reactions
What are the roles of cell surface membranes within cells?
- Give large SA e.g. in mitochondria for some reactions of aerobic respiration + localise some of the enzymes for respiration to occur
- Inner membrane of chloroplasts (thylakoid membranes) house chlorophyll where reactions for photosynthesis occur
- Digestive enzymes on plasma membranes of epithelial cells lining small intestine, that catalyse some of the final stages of breakdown of certain kinds of sugars
Which part of a phospholipid are hydrophobic?
Fatty acid tails
What part of the phospholipid is hydrophilic?
Phosphate head (has a charge)
What is the width of the phospholipid bilayer?
7nm
Describe the structure of channel proteins.
Water filled channel inside the channel protein is lined with hydrophilic amino acids
What is a glycoprotein?
A carbohydrate chain attached to a protein molecule
What might an extrusive protein be used for?
- May act as an enzyme
- Act as a protein receptor site
What is a glycocalyx and what does it do?
- Glycoprotein or glycolipid
- Carbohydrate molecules on outside of membrane are very hydrophilic + attract water with dissolved solutes helping cell interact with its water environment + obtain dissolved substances
What is a glycolipid?
A carbohydrate chain attached to a lipid
What types of intrusive/integral proteins are there?
- Channel proteins
- Carrier proteins
What is the role of channel proteins?
Allow ions (have an electrical charge + surrounded by water molecules) to pass through
What is the role of carrier proteins?
Change shape to carry specific molecules across membrane
What is the role of cholesterol in the phospholipid bilayer?
-Regulate fluidity of membrane
-Maintain mechanical stability
-Resist effects of temperature changes on structure of membrane
(-Reduce permeability of membranes to small, water-soluble molecules)
Describe the plasma membrane of neurones.
- Channel/carrier proteins along long axon allow entry + exit of sodium or potassium ions to allow electrical conductivity of nerve impulses along length
- Myelin sheath formed by flattened cells wrapped around several times, giving several layers of plasma membrane
- Membrane forming myelin sheath is about 20% protein 76% lipid
- At synapses there’s also calcium ions + there may be chloride ion channels
Describe the plasma membrane of white blood cells.
-Contain special protein receptors enabling them to recognise antigens on foreign cells, usually from invading pathogens (but also from tissue/organ transplants)
Describe the plasma membrane of root hair cells.
-Many carrier proteins to actively transport nitrate ions from soil into cells
Describe the plasma membrane of mitochondria.
- 76% protein + 24% lipid
- Inner membrane contains many electrons carriers made of protein + hydrogen ion channels associated with ATP synthase enzymes
What form of energy to passive processes rely on?
Kinetic energy
What kind of molecules cross the phospholipid bilayer by simple diffusion?
O2, CO2, fat soluble molecules (e.g. steroid hormones)
How do water molecules cross the phospholipid bilayer?
- Water is present in such great concentrations simple diffusion does happen
- Unusual case as water is polar + insoluble in lipid
- In membranes where lots of water transport occurs, there may be aquaporins (water-specific channel proteins)
How does the concentration gradient of oxygen get maintained in cells?
Oxygen diffusing into cytoplasm of respiring cells diffuses into mitochondria for aerobic respiration
How does a steep concentration gradient of carbon dioxide get maintained in palisade mesophyll cells of plant leaves?
Carbon dioxide diffuses into chloroplasts for use in photosynthesis
What are factors that affect the rate of simple diffusion across plasma membranes?
- Temperature
- Diffusion distance
- SA
- Size of diffusing molecule
- Concentration gradient
How does temperature affect the rate of simple diffusion across plasma membranes?
-As temperature increases, particles have more KE so rate of diffusion will increase + vice versa
How does the diffusion distance affect the rate of simple diffusion across plasma membranes?
-Thicker the membrane over which the molecule has to diffuse, the slower the rate of diffusion
How does surface area affect the rate of simple diffusion across plasma membranes?
- More diffusion can take place over a larger surface area
- Cells specialised for absorption have extensions to their cell surface membranes called microvilli, increasing SA
How does the size of diffusing molecule affect the rate of simple diffusion across plasma membranes?
Smaller ions/molecules diffuse more rapidly than larger molecules
How does the concentration gradient affect the rate of simple diffusion across plasma membranes?
-Steeper the gradient the faster diffusion to the side where there are fewer molecules will occur (down a concentration gradient)
What is the diameter of channel proteins?
0.8nm
How do ions get through the plasma membrane?
Channel proteins via facilitated diffusion as they’re insoluble in lipids as can’t interact with hydrophobic tail
How does glucose cross the plasma membrane?
- Too large to diffuse through channel proteins so bind to transmembrane carrier proteins which change shape to allow glucose to be released on other side of the membrane
- Specific shape to each molecule
- Via facilitated diffusion
Describe the plasma membrane of epithelial cells that line airways.
- Have chloride ion channels
- Play a role in regulating composition of mucus to trap particles + pathogens
How does a higher solute concentration affect the water potential?
- Higher concentration of solute molecules
- Lower concentration of free water molecules
- Lower water potential
How does a solid not soluble in water affect the water potential?
Not at all as the number of free water molecules is the same
What has the highest water potential?
Pure water
What is water potential measured in?
kPa
What is the water potential of pure water?
0kPa
What has a higher water potential, a dilute solution of concentrated solution?
- Dilute (as water potential around -10kPa0
- Concentrated has a water potential of around -500kPa
What would happen to the movement of water if a cell was put into pure water?
- Lower water potential in cytoplasm of cell
- Water moves into cell by osmosis, down a water potential gradient across the plasma membrane
What happens if lots of water enters animal cells?
- Cell will swell + burst as plasma membrane breaks
- Cytolysis
What happens if lots of water enters plant cells?
- Rigid + strong cellulose cell wall prevents bursting
- Cell will swell up to a certain size, when contents push against cell wall resisting any further swelling
- Cell described as turgid
- Turgidity helps support plants, especially plants that aren’t woody
What happens when the water potential outside animal cells is lower than cytoplasm of cells?
- Water leaves by osmosis across partially permeable membrane
- Cell shrivels + becomes crenated
What happens when the water potential outside plant cells is lower than the cytoplasm of the cell?
- Water leaves via osmosis across partially permeable membrane
- Cell shrinks + membrane pulls away from cellulose cell wall
- Plasmolysed
- Plant tissue with plasmolysed cells is described as flaccid
- Plasmolysed cells suffer a degree of dehydration + metabolism can’t proceed as enzyme catalysed reactions need to be in solution
What happens to ATP in order to produce energy?
Hydrolysed