Chapter 6: Solute Transport

Question 1

what is ion diffusion driven by?

Answer 1

chemical potential x membrane permeability

Question 2

what is the chemical potential difference driven by?

Answer 2

concentration differences, pressure differences, and electrical potential differences (voltage differences)

Question 3

passive transport

Answer 3

diffusion down a chemical potential gradient

Question 4

active transport

Answer 4

against a chemical potential gradient - often uses energy derived from ATP hydrolysis. Often driven by a proton motive force provided by ATPase

Question 5

what do all living cells exhibit?

Answer 5

• a membrane potential due to the asymmetric ion distribution between the inside and outside of the cell
• can be determined by probing the cell and external solution with microelectrodes

Question 6

what can the nernst equation give?

Answer 6

the concentration gradient expected across a membrane at equilibrium of the voltage across the membrane is known

Question 7

what is the goldman equation?

Answer 7

• modified version of the nernst equation
• predicts the diffusion potential from Na, K, and CL
• for most ions, experimentally determining concentration different from that predicted by the Goldman equation due to active transport

Question 8

how is potassium uptaken?

Answer 8

• taken up passively by both unless concentration is very low - then it is taken up actively

Question 9

how is sodium and calcium uptaken?

Answer 9

• pumped actively out of the cytosol into the vacuole and extracellular spaces

Question 10

how are protons and anions uptaken?

Answer 10

• excess protons are pumped actively out of the cytosol into the vacuole and extracellular spaces
• anions are take up actively in the cytosol

Question 11

transporters

Answer 11

• facilitate the passage of select ions and other polar molecules into or out of the cell
• usually highly specified for a given ion but not always
  example: a K transporter might also transport Na but not as efficiently

Question 12

channels

Answer 12

• passive transporters; solutes diffuse through extremely rapidly due to electropotential gradients
• may have structures called gates that open and close the channel is response to external stimuli
• specialized to only inward and outward transport

Question 13

carrier

Answer 13

• passive transporter; do not have pores that completely extend across the membrane
• substance binds to carrier protein and a conformational change in the protein deposits the substance on the other side of the membrane
• about 106 times slower than a channel protein

Question 14

pumps

Answer 14

active transporter; move ions against the electrochemical gradient
- two kinds: primary and secondary

Question 15

primary active transport

Answer 15

moves ions across a membrane against the
gradient by coupling the pump directly to a source of energy such as
ATP hydrolysis, or electron transport chains

Question 16

Secondary active transport

Answer 16

couples uphill transport (pumping) of one
ion with downhill transport of another.
- ATPase is commonly involved, moving protons across the membrane and generating a proton-motive force

Question 17

symport

Answer 17

ions move in the same direction

Question 18

antiport

Answer 18

ions move in opposite direction

Question 19

Michaelis-Menten kinetics

Answer 19

• analyzes the function of transport proteins

- the rate of ion uptake ( = rate of protein activity) is plotted against the concentration of the ion

Question 20

saturation curve

Answer 20

• Michaelis-menten kinetics
• the parameters of the MM curve are the Vmax (max rate of uptake) and the Km (the ion concentration necessary to drive 50% Vmax)
• if MM kinetics observed, the role of a protein in transport is indicated

Question 21

how can protein transporters be discoverd?

Answer 21

• a gene sequence similarity with transporter genes known from other organisms (orthologs)
• purifying the transport protein
• finding the gene that restores transport function in mutants that are deficient in that transport capacity

Question 22

how are ions absorbed?

Answer 22

• mostly through root hairs
• may enter the root crossing into the epidermal cells through the plasma membrane
• or can be diffused into the root via the walls between epidermal cells

Question 23

what happens to ions after being uptaken by the root hairs?

Answer 23

• ions can diffuse to the stele via the symplast or move with water into the apoplast or move cell to cell across membranes
• at the endodermis, ions must enter the symplast because of the casparian strip, which allows the plant to control ions entering the xylem and prevent diffusion of ions back out of the xylem into the soil
• in the stele, xylem parenchyma cells may pump ions into the xylem