3.2.3 Transport across cell membranes

Question 1

What is meant by the ‘fluid mosaic’ model of the cell membrane?

Answer 1

Fluid - molecules can move
Mosaic - lots of different molecules like proteins and phospholipids

Question 2

What type of molecules are transported by active transport?

Answer 2

• Large
• Polar
• Water soluble

Question 3

What are the functions of phospholipids in the cell membrane?

Answer 3

• Let lipid soluble molecules enter and leave the cell
• Stop water soluble molecules from entering or leaving the cell
• Make the membrane flexible (self healing)

Question 4

What type of molecules are transported by simple diffusion?

Answer 4

• Small
• Non-polar (not water soluble)
• Lipid soluble

Question 5

Why does active transport require energy?

Answer 5

• Movement of particles against the concentration gradient
• Energy from the hydrolysis of ATP to change the shape of the carrier protein

Question 6

What are 6 functions of proteins in the cell membrane?

Answer 6

• Provide structural support
• Act as protein carriers to transport water soluble molecules
• Act as protein ion channels to transport water soluble molecules
• Form recognition sites to identify cells
• Help cells adhere together (eg to form tissue)
• Act as receptors (eg for hormones)

Question 7

What are the functions of glycoproteins in the membrane?

Answer 7

• Act as recognition sites (eg hormones)
• Help cells recognise each other
• Help cells attach to each other to form tissues

Question 8

What is water potential? What is the water potential of pure water? How does it change as more solutes are dissolved in?

Answer 8

• Pressure created by water molecules
• 0 (highest possible value)
• Becomes more negative

Question 9

How are some transport processes passive?

Answer 9

They rely on natural kinetic energy

Question 10

What is the function of cholesterol in the membrane?

Answer 10

• Add strength to membrane (hydrophobic so bind to phospholipid tails)
• Makes membrane less fluid at higher temperatures
• Reduce lateral movement of other molecules eg phospholipids
• Stop leakage of water and dissolved ions from cell

Question 11

What is osmosis?

Answer 11

• Movement of water from an area of higher water potential
• To an area of lower water potential
• Through a partially permeable membrane
• DOWN the concentration gradient

Question 12

What happens when lots of water moves into an animal cell vs a plant cell?

Answer 12

Animal: cell bursts - osmoticlysis
Plant: cell becomes turgid (can’t burst - cell wall)

Question 13

What are the functions of membranes within cells?

Answer 13

• Control entry and exit of materials in organelles
• Separate organelles from cytoplasm so specific metabolic reactions can occur inside them
• Provide and internal transport system (eg endoplasmic reticulum and vesicles)
• Isolate enzymes that may damage the cell (eg lysosomes)
• Provide a surface area for reactions to occur on

Question 14

What is the function of glycolipds in the membrane?

Answer 14

• Help maintain stability of the membrane
• Act as recognition sites
• Help cells attach to each other to form tissues

Question 15

What type of molecules are transported by facilitated diffusion?

Answer 15

• Large
• Polar
• Water soluble

Question 16

What proteins in the membrane are required for active transport?

Answer 16

Carrier proteins

Question 17

What form of transport is not specific?

Answer 17

Simple diffusion

Question 18

What is diffusion?

Answer 18

• Net movement of particles
• From an area of higher concentration
• To an area of lower concentration
• Down a diffusion gradient

Occurs until concentration of particles on each side is equal and a dynamic equilibrium is reached

Question 19

What are intrinsic and extrinsic proteins?

Answer 19

Extrinsic proteins: on surface or party embedded in membrane.
* Give mechanical support
* Act as cell receptors for molecules like hormones

Intrinsic proteins: span across phospholipid bilayer.
* Enzymes
* Carriers to transport water soluble molecules across membrane (protein channel or protein carrier)

Question 20

What happens when lots of water moves out of an animal cell vs a plant cell?

Answer 20

Animal: cell shrivels
Plant: plasmolysis occurs (cell membrane still tethered at points to cell wall)

Question 21

What proteins in the membrane are required for facilitated diffusion?

Answer 21

• Protein ion channels (usually for ions- small)
• Carrier proteins (larger molecules like glucose or amino acids)

Question 22

What features would you expect to find in a cell specialised for absorption?

Answer 22

• Folded microvilli so large surface
  area
• Large number of carrier /channel proteins so fast rate of absorption/ active transport/facilitated diffusion
• Large number of mitochondria to make more ATP/release energy for active transport