12.6 Transport across Membranes

Question 1

Why is the fluid mosaic model referred to as ‘fluid’?

Answer 1

The phospholipid molecules form a double layer (bilayer) that is constantly moving around relative to one another, giving the membrane a fluid structure

Question 2

Why is the fluid mosaic model referred to as ‘mosaic’?

Answer 2

The proteins are unevenly distributed throughout the membrane so it is a mosaic

Question 3

Components within cell membrane

Answer 3

Aquaporins
Cholesterol
Channel proteins
Carrier proteins
Enzymes
Glycoproteins
Phospholipids
Receptor proteins

Question 4

Phospholipid function

Answer 4

• Hydrophobic tails attracted towards eachother
• Hydrophillic heads point either inwards/outwards
  = FORM PHOSPHOLIPID BILAYER
• Allows lipid soluble, non-polar molecules to pass through by SIMPLE DIFFUSION
• Prevents passage of small, polar molecules, e.g. ions

Question 5

Cholesterol function

Answer 5

• Decreases permeability and increases stability of the membrane
• More cholesterol = less fluidity of membrane

Question 6

Channel proteins function

Answer 6

• Allow specific charged ions/ small molecules to move across membrane by FACILITATED DIFFUSION
  e.g Na+ ions pass through Na+ channel proteins only

Question 7

Carrier proteins function

Answer 7

• Allow transport of ions, polar and large molecules, e.g. glucose, amino acids by FACILITATED DIFFUSION and ACTIVE TRANSPORT

Question 8

Receptor proteins function

Answer 8

• Other proteins act as specific receptors for complementary molecules
  e.g. insulin may bind to insulin receptor proteins
• Specific cells have specific receptors

Question 9

Enzymes function

Answer 9

• Enzyme active site is specific and complementary to substrate, allowing them to bind and form ESCs

Question 10

Glycoproteins function

Answer 10

• Composed of carbohydrates and protein
• On outer surface of membrane
• Important in cell recognition acting as antigens and immune cells detect shapes to identify if they are self or non-self cells
• Produced in golgi body

Question 11

Aquaporins function

Answer 11

• Special channel protein specific to water
• Lots of aquaporins in cell= very permeable to water= carries out osmosis easily

Question 12

Movement of substances in or out cells occurs by?

Answer 12

Simple diffusion
Facilitated diffusion
Osmosis
Active transport
Co-transport
Bulk transport (endo/exocytosis)

Question 13

Diffusion definition

Answer 13

Net movement of molecules from an area of higher concentration to a lower concentration across a partially permeable membrane.

Question 14

Ficks law

Answer 14

Rate of Diffusion= (Surface area x Concentration Gradient)/ Diffusion Distance

Question 15

Factors affecting rate of diffusion ROD

Answer 15

Surface area
Temperature
Concentration gradient
Diffusion distance

Question 16

How does temperature affect ROD

Answer 16

Increased KE
Faster ROD of molecules

Question 17

How does surface area affect ROD

Answer 17

Larger SA, more space for molecules to pass through
Faster ROD
More proteins can be present

Question 18

How does concentration gradient affect ROD

Answer 18

As concentration difference increase, ROD increase

Question 19

How does diffusion distance affect ROD

Answer 19

Shorter the diffusion distance (fewer membranes to cross), faster the molecules will travel

Question 20

Facilitated diffusion definition

Answer 20

Specific proteins help specific molecules to pass through phospholipid bilayer
Water soluble, charged, large molecules cant pass directly through bilayer, so need to pass through channel/carrier proteins.

Question 21

Is FD a passive or active process?

Answer 21

Passive, NO ATP REQUIRED

Question 22

when does facilitated diffusion plateau on a graph

Answer 22

When all the carrier proteins are saturated/ binding sites are full
Number of proteins is a limiting factor

Question 23

Osmosis definition

Answer 23

Net movement of water molecules from a high water potential to a lower water potential through a partially permeable membrane

Question 24

What happens when there is a higher WP outside and lower WP inside cell?

Answer 24

Water moves IN by osmosis
Animal cells- swelling and lysis occurs
Plant cells- swelling increasing mass (cellulose cell wall prevents lysis)

Question 25

What happens when there is a lower WP outside and higher WP inside cell?

Answer 25

Water moves OUT by osmosis, mass is lost.
Animal cells- shrivel
Plant cell membrane pulls away from cell wall- plasmolysed

Question 26

Isotonic

Answer 26

Equalised water potential inside and outside cell

Question 27

Active transport definition

Answer 27

• Transport of molecules against their concentration gradient
• Low to high
• Only uses carrier proteins
• ATP required

Question 28

Is active transport an active or passive process?

Answer 28

It is active as it requires ATP for a source of energy

Question 29

Process of exocytosis

Answer 29

• Uses golgi vesicles to transport large quantities of molecules from inside the cell to outside the cell
• Used to move enzymes and glycoproteins from golgi apparatus to cell surface membrane to SECRETE PROTEINS

Question 30

Is exocytosis an active or passive process?

Answer 30

Active
It requires ATP to move the vesicles to the cell surface membrane and the vesicle fuse with the membrane

Question 31

Process of endocytosis

Answer 31

• Cell surface membrane is pulled inwards to create a vesicle
• Any molecules nearby are enclosed into the vesicle

Question 32

Is endocytosis an active or passive process?

Answer 32

Active
Requires ATP to create the vesicle and move the vesicle

Question 33

Explain the arrangement of phospholipids in a cell-surface membrane.

Answer 33

• 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 34

Many different substances enter and leave a cell by crossing its cell surface membrane.
Describe how substances can cross a cell surface membrane.

Answer 34

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 35

The movement of substances across cell membranes is affected by membrane structure.

Describe how.

Answer 35

• 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;