Week 4 Monday: Plant Mineral Nutrition 2 (9/21)

Apoplastic movement

Through cell walls and intercellular spaces.

Symplastic movement

Through the cytoplasm and connected via plasmodesmata.

Casparian strip

Hydrophobic band made of lignin that surrounds the vasculature of roots. Forces the flow of water and anything else through the plant symplast to be subjected to transport proteins

Suberin lamellae

Layers of waxy hydrophobic suberin on the inner surface of cell walls just outside the plasma membrane

Passage cells

Thin-walled, unsuberized cells in the root endodermis and exodermis that allow water and mineral ions to pass into the vascular cylinder (so they’re inside the root)

Root cap

Protects the meristematic cells at the growing tip, guides root growth, and plays a signalling role

What must water do to be transported to the top of the plant?

In order to be transported to the top of the plant, minerals must pass through the Casparian strip and the suberin lamellae

Is apoplastic movement into the vasculature possible?

The lignified Casparian strip and the suberinized suberin lamellae prohibit apoplastic movement into the vasculature. ▪ Therefore, mineral elements must pass symplastically through passage cells, which are highly selective for what can and cannot enter.

Active transport

Against/up the concentration gradient (low to high concentration), energy required

Passive transport

Down/with the concentration gradient (high to low concentration), no energy required

Primary vs. secondary active transport

Primary active transport directly uses chemical energy from ATP to move molecules across a membrane, while secondary active transport uses an electrochemical gradient created by primary active transport to move molecules without directly using ATP

(for example, using a symport to move H+ along its concentration gradient and another molecule against its concentration gradient)

Symport

Pumps two molecules in the same direction

Antiport

Pumps two molecules in the opposite direction

Uniport

Pumps one molecule in one direction

Steenberg effect

Adding a deficient nutrient to a plant temporarily causes its internal concentration of that nutrient to decrease

This happens because the initial application of the missing nutrient triggers a massive, rapid burst of plant growth and biomass production that outpaces the rate of nutrient uptake, effectively diluting the nutrient within the plant's tissues

Michaelis-Menten

A mathematical model of enzyme action with two parameters: V_max and K_m

• V_max: maximum velocity of the enzymatic reaction

• K_m: substrate (the molecule the enzyme reacts with) concentration at which half of the enzyme’s active sites are occupied by substrate

K_m and affinity

Low K_m is high affinity

High K_m is low affinity

Competitive inhibition effect on Km and Vmax

Same Vmax

Increases the apparent Km value

Noncompetitive inhibition effect on Km and Vmax

Different Vmax

No change to Km

Noncompetitive inhibitors bind to an allosteric site, making the enzyme unable to bind to the substrate. This means a portion of the enzymes no longer work and Vmax is decreased. Km stays the same because the same substrate concentration is required to bind to 50% of the enzymes

Mechanism I vs II nutrient uptake

Mechanism I (High-Affinity Transport System/HATS): Operates at low nutrient concentrations. High affinity (Low Km), low Vmax

Mechanism II (Low-Affinity Transport System/LATS): Operates at high nutrient concentrations. Low affinity (high Km), high Vmax

High affinity = low Km because it requires a low concentration of the nutrient to start working

Is nitrate uptake active or passive?

Active

Is ammonium uptake active or passive?

Mostly passive through ion channels, but can also use K+, Mg2+, and Ca2+ uptake routes

Calcium

Immobile, and leads to disorders even when it is provided in adequate amounts

Strategy I iron uptake: who uses it and what steps does it have?

Who uses it? Most non-graminaceous plants, including dicots and non-grass monocots

Acidification, reduction, and transport

Strategy II iron uptake: who uses it and what steps does it have?

Who uses it? Graminaceous plants (grasses, such as maize, rice, barley, and wheat

Chelation and direct uptake of complexes