4. CELL MEMBRANES AND TRANSPORT Flashcards
Examples of exocytosis
- secretion of digestive enzymes from the cells of the pancreas
- plants use exocytosis to get their cell wall building materials to the outside of the cell surface membrane
Why is ATP required for active transport
because substances are moved against a concentration gradient
How is ATP required for active transport
- the energy is used to make the carrier protein change shape
- transferring the molecules or ions across the membrane
Ions in high concentration in cells than outside
potassium and chloride ions
Role of the sodium-potassium pump
- for each molecule of ATP used
- three sodium ions are pumped out of the cell and two potassium ions into the cell
Result of the sodium-potassium pump
- the inside of the cell becomes more negative than the outside
- a potential difference is created across the membrane
- significant in nerve cells
Sites where active transport takes place
- reabsorption of certain molecules or ions in the kidneys after filtration
- absorption of some products of digestion in the gut
- in plants, active transport is used to load sugar from photosynthesising cells into the phloem tissue
- in plants, active transport is used to load inorganic ions from the soil into root hairs
Why is it important to maintain a constant water potential inside the bodies of animals
- if the water potential of the solution surrounding the cell is too high, red blood cells bursts (haemolysis)
- if the water potential is too low:
red blood cell gets crenated (shrinks)
plant cell gets plasmolysed (cytoplasm pulls away)
What happens when a plant cell is placed in a hypotonic solution
- water enters the cell by osmosis and the volume of the cell increases
- the cell wall pushes against the expanding protoplast and pressure starts to build up rapidly
- this pressure potential increases until the water potential inside the cell equals the water potential outside the cell and equilibrium is reached
Method of preparing membranes to split open the bilayer
freeze-fracturing
Why is the membrane more fluid when there are more unsaturated fatty acid tails
- the unsaturated fatty acid tails are bent
- so they fit together more loosely
Factors that increase fluidity of the membrane
- more unsaturated fatty acids
- shorter tail
- higher temperature
- more cholesterol
How do organisms respond at a lower temperature to live
- membranes become less fluid
- so bacteria and yeasts increase the proportion of unsaturated fatty acids in their membranes
- so the membrane becomes more fluid
- can also increase the amount of cholesterol in the membrane
- this prevents close packing of the phospholipid tails so increases fluidity
In transmembrane proteins, what are the hydrophobic regions that cross the membrane made of
alpha-helical chains
Why do intrinsic proteins stay in the membrane
- because the hydrophobic regions, made from hydrophobic amino acids, are next to the hydrophobic fatty acid tails
- and are repelled by the watery environment either side of them
- because the hydrophilic regions, made from hydrophilic amino acids, are repelled by the hydrophobic interior of the membrane and therefore face into the aqueous environment inside or outside the cell
- or line hydrophilic pores which pass through the membrane
Types of lipid present in the cell membrane
phospholipids
cholesterol
glycolipids
Types of protein in the cell membrane
protein
glycoprotein
Why can polar (charged) molecules not pass through the cell membrane
- because the fatty acid tails of phospholipids that make up the membrane are non-polar (hydrophobic)
- so they act as a barrier to most water-soluble substances
How can phospholipids act as signalling molecules
- some phospholipids can be modified chemically to act as signalling molecules
- they can move about in the phospholipid bilayer, activating other molecules such as enzymes
- they can be hydrolysed to produce small, water-soluble glycerol related molecules that can diffuse into the cytoplasm and bind to specific receptors
How do cholesterol molecules fit in the phospholipid bilayer
like phospholipids, they have a hydrophilic head and hydrophobic tail so they fit in with their heads at the membrane surface
Presence of cholesterol in cell membranes
- almost as much as phospholipids in animal cells
- much less common in plant cells
- absent from prokaryotic cells (compounds similar to cholesterol serve the same function)
How does more cholesterol increase the fluidity of the membrane
- prevents close packing of the phospholipid tails
Functions of cholesterol
- prevent the membrane from becoming too rigid at low temperatures by preventing close packing of the phospholipid tails
- interactions between phospholipid tails and cholesterol molecules prevent the membrane from becoming too fluid at high temperatures
- important for the mechanical stability of membranes, as without it membranes quickly break and cells brust open
- hydrophobic regions of cholesterol prevent polar molecules or ions fro escaping. particularly useful in the myelin sheath around nerve cells
Functions of glycolipids/glycoproteins
- carbohydrate chains project into the watery fluids surrounding the cell and form hydrogen bonds with water molecules, stabilising the membrane structure
- carbohydrate chains form a sugary coating outside the cell called the glycocalyx
- carbohydrate chains help the glycoproteins and glycolipids to act as receptor molecules, which bind with particular substances at the cell surface membrane
- some glycolipids and glycoproteins act as antigens, allowing cell-cell recognition
