Question

11. The citric acid cycle begins with the formation of a 6-carbon (6C) molecule from joining the 4-carbon (4C) oxaloacetate and the 2-carbon (2C) acetyl group (see below). By about halfway through the cycle, two carbons have been lost as carbon dioxide and a 4C molecule labeled D is regenerated. List two reasons why the cycle does not stop after production of D and instead must continue through a series of different 4C intermediates

12. Which of the following statements is TRUE?

(a)        Plant cells store all of their food reserves as starch, whereas animals store all of their food reserves as glycogen.

(b)       Glycogen stores more energy than starch because glycogen molecules have many more branch points, which when hydrolyzed, provide energy.

(c)        Animal cells can convert fatty acids to sugars.

(d)       Plants synthesize starch for the same reason that animals synthesize glycogen.

(e)        Protein is an important form of energy storage in animal cells under normal conditions.

Answer 1

11. The TCA cycle involves the oxidation of acetyl CoA to CO2 and H2O.
It produces 3 NADH, 1 FADH2, and 1 GTP per cycle.

The first step of the cycle involves condensation of acetyl CoA with Oxaloacetate to produce citrate.
Citrate undergoes two successive oxidative decarboxylations to produce Succinyl CoA.
By this step, two carbon atoms in acetyl CoA are removed as 2 CO2 molecules.
However, the cycle further continues to regenerate Oxaloacetate which is utilized during the first step. These steps also produce 1 NADH, 1 FADH2, and 1 GTP.

12. Option D is correct
There are several forms of reserved food materials in plants = Fluridean starch, Laminarin, Mannitol, and starch
There are several forms of reserved food materials in animals = Fats, and glycogen
Both starch and glycogen produce the same amount of energy as both are homopolymers of glucose. Their structural differences do not account for any energetics.
Animal cells cannot perform glyoxylate cycle i.e. conversion of fats to carbohydrates.

Plants and animals store excess glucose in the polymeric forms.
Ex: Strach and Glycogen