Transport Across Cell 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