Transport across membranes Flashcards
(12 cards)
Describe how an ester bond is formed in a phospholipid molecule. [2]
- Condensation (reaction) OR Loss of water;
- Between of glycerol and fatty acid;
Explain the arrangement of phospholipids in a cell-surface membrane. [2]
- Bilayer OR Water is present inside and outside a cell;
- Hydrophobic (fatty acid) tails point away/are repelled from water OR Hydrophilic (phosphate) heads point to/are in/are attracted to water;
Explain why a cell membrane may be described as a fluid-mosaic? [2]
- Fluid – Each molecule within the membrane is able to move around within the membrane.
- Membrane is made up from a variety of different molecules (e.g, different proteins) arranged into a mosaic.
Name and describe five ways substances can move across the cell-surface membrane into a cell. [5]
- (Simple) diffusion of small/non-polar molecules down a concentration gradient;
- Facilitated diffusion down a concentration gradient via protein carrier/channel;
- Osmosis of water down a water potential gradient (using aquaporins);
- Active transport against a concentration gradient via protein carrier using ATP;
- Co-transport of 2 different substances using a single specific carrier protein;
Many different substances enter and leave a cell by crossing its cell surface membrane.
Describe how substances can cross a cell surface membrane. [7]
- (Simple / facilitated) diffusion from high to low concentration / down concentration gradient;
- Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;
- OR
- Large / polar / water-soluble molecules go through proteins;
- Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential;
- Active transport is movement from low to high concentration / against concentration gradient;
- Active transport / facilitated diffusion involves proteins / carriers; (Reject Both use channels)
- Active transport requires energy / ATP;
- Ref. to Na+ / glucose co-transport
The movement of substances across cell membranes is affected by membrane structure.
Describe how. [7]
- Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
- Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances OR (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
- Carrier proteins allow active transport;
- Channel/carrier proteins allow facilitated diffusion/co-transport;
- Shape/charge of channel / carrier determines which substances move;
- Number of channels/carriers determines how much movement;
- Membrane surface area determines how much diffusion/movement;
- Cholesterol affects fluidity/rigidity/permeability;
Give two similarities in the movement of substances by diffusion and by osmosis. [2]
- (Movement) down a gradient
- Passive / not active processes; OR Do not use energy from respiration / from ATP / from metabolism; OR Use energy from the solution;
Compare and contrast the processes by which water and inorganic ions enter cells. [3]
- Comparison: both move down concentration gradient;
- Comparison: both move through (protein) channels in membrane;
- Accept aquaporins (for water) and ion channels
- Contrast: ions can move against a concentration gradient by active transport (Carrier protein)
Describe how substances move across cell-surface membranes by facilitated diffusion. [3]
- Carrier / channel protein;
- (Protein) specific / complementary to substance;
- Substance moves down concentration gradient;
Describe how you would use a 1.0 mol dm−3 solution of sucrose to produce 30 cm3 of a 0.15 mol dm−3 solution of sucrose. [2]
- Add 4.5 cm3 of (1.0 mol dm–3) solution to 25.5 cm3 (distilled) water.
- Mix
Explain the decrease in mass of potato tissue in the 0.40 mol dm−3 solution of sucrose. [2]
- Water potential of solution is less than / more negative than that of potato tissue; (Ψ as equivalent to water potential)
- Tissue loses water by osmosis.
Describe how you would use the student’s results (dilution series and % change in mass) to find the water potential of the potato tissue. [3]
- Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis;
- Find concentration where curve crosses the x-axis / where percentage change is zero;
- Use (another) resource to find water potential of sucrose concentration (where curve crosses x-axis).
