General Science Quiz: Plant Physiology: Transpiration, Transport and Respiration

Dixon and Joly explained that water molecules cling to one another by cohesion and to the xylem walls by adhesion, forming an unbroken column; evaporation from the leaf surface creates a suction at the top that pulls this column upward.

It occurs through pores called hydathodes at the leaf margin or tip, usually in the early morning when the soil is moist, the air is humid and transpiration is low, so root pressure forces out drops of water with dissolved salts; Colocasia, grasses and tomato show it well.

Its sieve tubes, helped by companion cells, carry sucrose and other soluble food from a source such as a mature leaf to a sink such as a root, fruit or bud, and the movement can be upward or downward.

In light, potassium ions are actively pumped into the guard cells, their solute concentration rises, water enters by osmosis, the cells swell and because their inner walls are thicker they bend apart, opening the pore; when potassium leaves, the cells lose water and the pore closes.

Ganong's potometer and the simpler bubble potometer measure the water absorbed by a cut shoot in a given time, and since almost all of that water is lost as vapour, the reading is taken as the rate of transpiration.

Glycolysis, also called the Embden Meyerhof Parnas pathway, splits one molecule of glucose into two molecules of pyruvic acid in the cytoplasm, needs no oxygen and gives a net gain of two ATP along with two molecules of reduced NAD.

Pyruvic acid formed in glycolysis enters the mitochondrion, is converted to acetyl coenzyme A and is oxidised completely in the matrix through the cycle named after Hans Krebs, who received the Nobel Prize for the discovery; the cycle releases carbon dioxide and reduced coenzymes.

The respiratory quotient is the volume of carbon dioxide released divided by the volume of oxygen used. When glucose is oxidised completely, six molecules of oxygen are used and six of carbon dioxide are released, so the ratio is one.

In yeast the pyruvic acid formed by glycolysis is decarboxylated and then reduced to ethyl alcohol, a process called alcoholic fermentation, which is the basis of brewing and of the rising of dough.

Sugar is actively loaded into the sieve tubes at the source, water follows from the xylem by osmosis and raises the pressure there, and the resulting gradient pushes the whole solution to the sink, where sugar is unloaded and water returns to the xylem.

Glycolysis spends two molecules of ATP in the preparatory steps that phosphorylate glucose and fructose, and produces four by substrate level phosphorylation in the later steps, so the net gain is two ATP along with two molecules of reduced NAD.