biology: cell membrane

Question 1

What is the fluid-mosaic model?

Answer 1

• arrangement of phospholipids and proteins in the membrane

Question 2

What is a phospholipid?

Answer 2

• cell membrane is made up of tiny molecules called phospholipids
• each phospholipid has 2 sections:
  
  • the phosphate group (head)
  • the fatty acids chains (tails)

Question 3

What is the phosphate group?

Answer 3

• negatively charged polar head
• hydrophilic (love water)
• heads point out, towards the outside and inside of the cell, where there is liquid

Question 4

What are the fatty acid chains?

Answer 4

• uncharged, non-polar tails
• hydrophobic (dislike water)
• the tails point inwards, towards centre of the bilayer

Question 5

What is the phospholipid bilayer?

Answer 5

• the plasma membrane which is a double layer
• substances can travel through this to enter/exit the cell

Question 6

What is the structure of a phospholipid?

Answer 6

• non-polar tails are hydrophobic, so inside of the cell membrane
• charged heads are hydrophilic, so attracted to water

Question 7

What is the function of phospholipids?

Answer 7

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

Question 8

What is the structure of surface proteins?

Answer 8

• proteins embedded in membrane, which don’t cross it, are called extrinsic proteins
• proteins spanning the membrane, these are called intrinsic proteins

Question 9

What is the function of surface proteins?

Answer 9

• provide structural support
• form recognition sites for identifying cells
• help cells adhere together
• act as receptors e.g. for hormones

Question 10

What is the structure of cholesterol?

Answer 10

• found within layer of phospholipids give the structure stability

Question 11

What is the function of cholesterol?

Answer 11

• reduce lateral movement of other molecules including phospholipids
• make the membrane less fluid at high temperatures
• prevent leakage of water and dissolved ions from the cell

Question 12

What is the structure of glycolipids?

Answer 12

• components of cellular membranes comprised of a hydrophobic lipid tail and one or more hydrophilic sugar groups linked by a glycosidic bond

Question 13

What is the function of glycolipids?

Answer 13

• act as recognition sites
• help maintain the stability of the membrane
• helps cells to attach to one another, so form tissues

Question 14

What is the structure of glycoproteins?

Answer 14

• proteins with a carbohydrate attached

Question 15

What is the function of glycoproteins?

Answer 15

• acts as recognition sites
• helps cells to attach to one other, so form tissues
• on outer bilayer, they provide receptor binding sites, can act as antigens
• allow cells to recognise one another (antigens)

Question 16

What is the structure of carrier/channel proteins?

Answer 16

• carrier proteins allow transport of certain substances across the membrane
• protein channels allow the transport of certain substances across the membrane

Question 17

What is the function of carrier/channel proteins?

Answer 17

• carrier protein = involved in facilitated diffusion and active transport of substances out or into the cell
• responsible for diffusion of sugars, amino acids
• take up glucose molecules and transport them inside the cell
• channel protein = provides a passageway for water and small, polar ions
• involved in facilitated diffusion

Question 18

What is the overall function of cell surface membrane?

Answer 18

• controls movement of substances into and out of cell
• selectively permeable

Question 19

Why don’t most molecules freely diffuse across lipid bilayer?

Answer 19

• not soluble in lipids
• too larger to pass through channels in membrane
• polar, have difficulty passing through non polar hydrophobic tails
• same charge as protein channels, repelled