Spectrum Biology - September 2016

LEARN THRU PR BLEMSO

RAINBOW

This section is targetted to provide learning through problems along with the tips to memories them.

uWhich is a better suffix for Krebs’ work, a cycle

or a pathway?

•Since, the citric acid produced in the initial condensation

reaction of acetyl Co-A with oxaloacetic acid runs through

the ‘‘mill’’ and eventually yields another molecule of

oxaloacetic acid, the sequence may be seen as returning to

its starting point in typical cyclical fashion. A new acetyl

Co-A will then join the oxaloacetic acid to initiate another

turn of the merry-go-round. An apt characterisation of the

Krebs’ cycle would be a metabolic system in which the

products of glycolysis are first modified and then ground

down to low-energy products with the help of ancillary

pathways closely connected to the main cycle.

Memorising Tip

To remember the KTC = Krebs’ or TCA or Citric Acid Cycle remember,

‘Citric Acid Is Krebs’ Starting Substrate For Mitochondrial Oxidation’.

These stand for the substrate in order

Citric Acid

Iso-Aconitase

Iso-Citrate

Alpha-Ketoglutarate

Succinyl Co-A

Succinate

Fumarate

Malate

Oxaloacetate

vHow does the prophase of meiosis I differ from

the prophase of mitosis?

•The prophase of the first meiotic division is of much longer

duration than the prophase of mitosis. It is subdivided into

the following functional stages:

1.Leptotene Chromosomes have begun to coil and

condense. Replication has, of course, occurred during the

S-period of interphase, but individual chromatids cannot

yet be seen. The nuclear membrane begins to breakdown

and centrioles, when present move toward the poles.

Nucleoli disappear as discrete structures.

2.Zygotene Homologous chromosomes begin the process

of synapsis by making contact at several points along

their length. Spindle fibres begin to appear.

3.Pachytene Synapsis is complete with each gene locus

of one homologue closely apposed to the corresponding

gene locus of the other homologue. Since, two

chromosomes are joined, the resultant configuration is

called a bivalent (two parts).

4.Diplotene The two chromatids of each of the paired

chromosomes are now distinctly visible and the entire

structure is known as a tetrad because of its

four-stranded appearance.

5.Diakinesis At this stage the synapsed chromosomes

seem to be pulling apart and are held together only at

specialised places called chiasmata. The thickened

chromosomes of diakinesis migrate toward the

equatorial plane.

The major distinctions of the prophase of meiosis I are

its great length, synapsing (joining) of homologous

chromosomes, exchange of complementary regions

between homologous chromosomes (crossing over) and

attachment of spindle fibres along one side only of whole

chromosomes so that each chromosome moves to the

pole opposite that to which its homologue migrates.

Memorising Tip

For the different stages of prophase of the first meiotic division

remember ‘Lazy Zebras Ponder Dire Disasters’.

These stand for the stages in order, i.e. Leptotene, Zygotene,

Pachytene, Diplotene, Diakinesis.

wThe stomata are tiny lengthwise openings in

the leaf, about 0.0001 mm wide at their

maximum opening. They are concentrated on

the abaxial surface of most leaves and range in

frequency from 100-100,00 per cm^2. They are

enclosed by a pair of guard cells, which

resemble two bow-legged sausages. Just above

the stoma and within the leaf is a large air

space. Unlike the other cells of the abaxial leaf

epidermis, the guard cells contain chloroplast.

What mechanism does this suggest for opening

and closing the stomata?

•The guard cells function through osmotic changes in their

cytoplasm. During the daylight hours, when conditions are

optimal for active photosynthesis, the guard cells pile up

sugar molecules. These sugar molecules increase the

osmotic pressure, which leads to an uptake of water and an

increase in the turgor (internal pressure) of the cell. When

the cells start to swell, they pull apart so that the stomatal

opening (pore) increases in size. This permits CO 2 and water

vapour to pass into the leaf and oxygen, a byproduct of

photosynthesis, to pass out. Thus, the leaf opens its interior

to the atmosphere only when gas exchange is necessary for

photosynthesis.

When conditions for photosynthesis are poor, the guard

cells do not produce sugars and osmotic pressure is reduced.

With the loss of turgor pressure, the guard cells become

limp, the walls partially collapse and the opening is occluded

by the now flaccid cell walls. This prevents water loss at a

time when photosynthesis is not occurring at appreciable

levels.

To be Continued at Page 63

Sunflowers use their circadian clock, acting

on growth hormones, to follow the Sun

during the day as they grow. In simpler

words Growing sunflowers begin the day

with their heads facing east, swing west

through the day, and turn back to the east

at night.