4-Transport Across Cell Membranes

Question 1

Cell Surface Membrane

Answer 1

A plasma membrane that surrounds cells and forms the boundary between cell cytoplasm and the environment.

Question 2

Phospholipid Bilayer

Answer 2

Hydrophilic head point outside the cell surface membrane, attracted by water on both sides.
Hydrophobic tails point to the centre of the plasma membrane, repelled by water on both sides.
Allows lipid soluble substances to enter/exit cell.
Prevent water soluble substances from entering/exiting cell.
Make membrane flexible and self sealing.

Question 3

Protein Channels

Answer 3

Span cell surface membrane and are water filled tubes that allow water soluble ions to diffuse across the plasma membrane.

Question 4

Carrier Proteins

Answer 4

Bind to ions/molecules (glucose/amino acids) and change shape in order to move the molecules across the membrane.

Question 5

Function of Proteins in Membrane

Answer 5

Provide structural support
Act as channels allowing water-soluble substances across membrane.
Allow active transport across membrane through carrier proteins.
Form cell surface receptors for identifying cells.
Help cells adhere together.
Act as receptors (hormones).

Question 6

Function of Cholesterol in Membrane

Answer 6

Reduce lateral movement of other molecules (phospholipids).
Make membrane less fluid at high temperatures.
Prevent leakage of dissolved ions/water from the cell.

Question 7

Function of Glycolipids in Membrane

Answer 7

Act as recognition sites
Help maintain stability of membrane
Help cells attach to one another to form tissues

Question 8

Function of Glycoproteins in Membrane

Answer 8

Act as recognition sites
Help cells attach to each other to form tissues
Allow cells to recognise each other, e.g. lymphocytes can recognise an organism’s own cells.

Question 9

Permeability of Cell Surface Membrane

Answer 9

Most molecules can’t freely diffuse across the membrane because they aren’t soluble in lipids, too large to pass through, same charge as protein channels so are repelled, and are electrically charged (polar) so have difficulty passing through non-polar hydrophobic tails in bilayer.

Question 10

Fluid-Mosaic Model

Answer 10

FLUID~ Individual phospholipid molecules can move relative to one another which gives the membrane a flexible structure that is constantly changing shape.
MOSAIC~ Proteins that are embedded in the phospholipid bilayer vary in shape/size/pattern the same way tiles/stones vary in mosaics.

Question 11

Simple Diffusion

Answer 11

The net movement of molecules/ions from an area of high concentration to an area of low concentration down a concentration gradient until they are evenly distributed.

Question 12

Facilitated Diffusion

Answer 12

Diffusion assisted by protein channels and carrier proteins.

Question 13

Osmosis

Answer 13

The passage of water from an area of high water potential to an area of low water potential through a selectively permeable membrane, down the water potential gradient.

Question 14

Active Transport

Answer 14

The movement of molecules/ions into or out of a cell from an area of low concentration to an area of high concentration using ATP and carrier proteins.

Question 15

Co-transport and Absorption of Glucose in Ileum

Answer 15

1) Na+ ions are actively transported out of epithelial cells by Na-K pump.
2) Maintains a higher concentration of Na+ ions in lumen of intestine than epithelial cells.
3) Na+ ions diffuse into epithelial cells and carry amino acid/glucose molecules with them.
4) Glucose/amino acids pass into blood plasma by facilitated diffusion using a different carrier.