Transport across cell membranes

Question 1

Why are phospholipids important in cell membranes?

Answer 1

— hydrophilic heads point outwards
— hydrophobic tails point inwards (repel water)

Question 2

what are the functions of phospholipids in membranes?

Answer 2

— allow lipid-soluble substances to enter and leave the cell
— prevent water-soluble substances entering and leaving the cell
— make the membrane flexible and self-sealing

Question 3

What are the two main ways proteins are embedded into the bilayer?

Answer 3

— channel proteins
— carrier proteins

Question 4

what are the main functions of the proteins in the membrane?

Answer 4

— provide structural support
— act as channel proteins transporting water-soluble substances
— allow active transport across the membrane
— form cell-surface receptors for identifying cells
— act as receptors

Question 5

What are the functions of cholesterol in the membrane?

Answer 5

— reduce lateral movement of other molecules including phospholipids
— make membrane less fluid at high temps
— prevent leakage of water and dissolve ions

Question 6

what are the function of glycolipids in the membrane?

Answer 6

— act as recognition sites
— help maintain the stability of the membrane
— help cells to attach to one another and so form tissues

Question 7

what are the function of glycoproteins in the membrane?

Answer 7

— act as recognition sites
— help cells to attach to one another and so form tissues
— allows cells to recognise one another

Question 8

Why can’t most molecules freely pass across the membrane?

Answer 8

— not soluble in lipids
— too large
— electrically charged

Question 9

What is simple diffusion?

Answer 9

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed

Question 10

What is facilitated diffusion?

Answer 10

Like simple diffusion but occurs at specific points down the plasma membrane by channel proteins or carrier proteins

Question 11

What are protein channels?

Answer 11

Channels that allow water-soluble ions to pass through.

Question 12

what are Carrier proteins?

Answer 12

Changes shape when binding to a molecule and then releases it, no energy is needed.

Question 13

what is osmosis?

Answer 13

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane

Question 14

What is active transport?

Answer 14

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

Question 15

What is ATP used for in active transport?

Answer 15

— directly move molecules
— individually move molecules using a concentration gradient which had already been set up by active transport

Question 16

In what ways is active transport different to diffusion?

Answer 16

— metabolic energy in the form of ATP
— substances are moved against a concentration gradient
— carrier protein molecules which act as pumps are involved
— the process is very selective

Question 17

Describe the process of the sodium-potassium pump

Answer 17

– sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump by protein-carrier molecules
– this maintains a higher conc of sodium ions in the lumen of the intestine
– sodium ions diffuse into the epithelial cells down this conc gradient
– glucose / amino acids pass into the blood plasma by facilitated diffusion