transport across membranes

Question 1

Explain why a cell membrane may be described as a fluid-mosaic?

Answer 1

• The position of the molecules within the membrane is fluid – they are able to move around within the membrane.
• Membrane is made up from a variety of different molecules arranged into a mosaic.

Question 2

Explain the arrangement of phospholipids in a cell-surface membrane.

Answer 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;

Question 3

Many different substances enter and leave a cell by crossing its cell surface membrane.
Describe how substances can cross a cell surface membrane.

Answer 3

1 (Simple / facilitated) diffusion from high to low concentration / down concentration gradient;
2 Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;
OR
Large / polar / water-soluble molecules go through proteins;
3 Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential;
4 Active transport is movement from low to high concentration / against concentration gradient;
5 Active transport / facilitated diffusion involves proteins / carriers;
6 Active transport requires energy / ATP;
7 Ref. to Na+ / glucose co-transport;

Question 4

The movement of substances across cell membranes is affected by membrane structure.

Describe how.

Answer 4

• 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;