Topic 4: Transport across Membranes

Question 1

The lipid bilayer is semipermeable, what does this mean?

Answer 1

Some molecules can pass in and out freely

-Small, hydrophobic molecules (oxygen, CO2, hydrocarbons)

Question 2

What is a transport protein?

Answer 2

Integral membrane Protein that aids in the movement of some molecules across the membrane
-large, hydrophilic molecules, charged molecules can’t pass through on their own

Question 3

What is Passive Transport?

Answer 3

Movement across a membrane that does not require an input of energy from the surroundings
-Favourable or Spontaneous process predicted by the 2nd law of thermodynamics (energy diffuses into available space; entropy increasing)

Question 4

What are some types of passive transport?

Answer 4

Diffusion
Facilitated Diffusion
Osmosis

Question 5

What is Active Transport?

Answer 5

Movement (Across a membrane) that Does require an input of energy from the surroundings

Question 6

What is Diffusion?

Answer 6

Tendency of molecules to spread out into available space.
Dictated by the 2nd law of thermodynamics
*Movement of molecules from an area of high concentration to low concentration releases energy

Question 7

What is Facilitated Diffusion?

Answer 7

Diffusion of substances aided by transport proteins.
*Passive
Carrier Proteins allow specific molecules to pass in/out of the cell based on concentration gradient.
-releases energy

Question 8

What type of molecules would use Protein channels?

Carrier proteins?

Answer 8

• Small, polar, charged molecules (IONS) often use ion channels
• Larger molecules often use carrier proteins

Question 9

How are ions transported across the membrane?

Answer 9

Ions can move passively via channels but may also be transported actively
*Ion Channels are specific for certain ions

Question 10

What is the overall result of transporting ions in cells?

Answer 10

Results in voltage or Membrane Potential

• separation of charges across the membrane
• specific accumulation of ions on different sides of the membrane

Question 11

What is the Electrochemical Gradient?

Answer 11

Combination of Chemical (concentration gradient) and electrical (membrane potential) forces that regulate the diffusion of ions across the membrane

Question 12

What is Osmosis?

Answer 12

Diffusion of water across a selectively permeable membrane directed by solute concentration

Question 13

What is Hypertonic solution relative to a hypotonic solution?

Answer 13

Hypertonic: contains higher concentration of solute
Hypotonic: Contains less solute concentration

Question 14

How does water move through lipids?

Answer 14

Through Aquaporins

Question 15

What happens to animal cells in a hypotonic solution?

Answer 15

Net movement of water is into the cell - cell swells and bursts (lysed)

Question 16

What happens to an animal cell in a hypertonic solution?

Answer 16

Net movement of water out of the cell - cell loses water and shrivels

Question 17

What happens to a cell that contains a cell wall in a hypotonic solution?

Answer 17

Net movement of water into the cell - cell wall provides back pressure that keeps cell from lysing
-Turgid (normal)

Question 18

What happens to a cell that contains a cell wall in a hypertonic solution?

Answer 18

Net movement of water out of the cell - cell membrane separates from cell wall
-plasmolysis

Question 19

What happens to a cell that contains a cell wall in an isotonic solution?

Answer 19

Net movement is equal - results in wilting

Question 20

What is Active Transport?

Answer 20

• Used to build up concentration gradients (potential energy) and to establish membrane potentials
• Movement Against a concentration gradient
• Requires Energy*

Question 21

What are the sources of energy for Active Transport?

Answer 21

ATP hydrolysis, energy stored in concentration gradients

Question 22

What is a proton pump? Where does it get its energy?

Answer 22

Protein that transports protons across a membrane (such as to create acidity in lysosomes) sets up a concentration gradient and establishes a membrane potential
-Energy from ATP hydrolysis - phosphate from ATP binds to the proton pump - changes shape of pump

Question 23

Mechanism of Sodium Potassium Pump (animal cells)

Answer 23

1. 3Na+ bind to the transport protein on the inside face of the cell - binding stimulates ATP hydrolysis - released phosphate attaches to transport protein
2. The transport protein changes shape, now facing the outside of the cell - the 3Na+ dissociate
3. 2K+ bind - causes the transport protein to change shape - results in release/dissociation of the phosphate from the transport protein
4. The transport protein undergoes another shape change, now facing the inside of the cell - the 2K+ dissociate
5. Repeat

***Main reason for electrochemical gradient

Question 24

Electrogenic Pump

Answer 24

-Contributes to the electrochemical gradient of the cell
(eg proton pumps and Sodium-Potassium pumps)
-Generates voltage across the membrane by separating charges

Question 25

What is an example of coupling passive and active transport? Explain the process.

Answer 25

Sucrose-H+Cotransporter (sucrose proton cotransporter)

1. Proton pump first establishes a membrane potetial and H+ gradient - uses the energy of ATP Hydrolysis
2. Proton gradient is the energy source to drive sucrose through the cotransporter - High G, unstable, low entropy
3. Protons diffuse back into the cell through a cotransporter protein - passive process: facilitated diffusion
4. Diffusion of protons releases energy - changes the shape of the cotransporter protein and makes a site available for sucrose to bind
5. Sucrose is actively transported into the cell - against concentration gradient - energy source directly from the proton concentration gradient

Question 26

How are large molecules transported?

Answer 26

Uses ATP

-Cell surrounds molecules in membrane package (vesicles)

Question 27

Exocytosis?

Answer 27

Molecules out - secretion via endomembrane system

Question 28

Endocytosis?

Answer 28

Molecules in

Question 29

Phagocytosis?

Answer 29

Cell engulfs material with pseudopodia - membrane remodeling

Question 30

Pinocytosis?

Answer 30

Non-specific engulfing of liquids and dissolved solutes