Why does milk appear white?

Why does milk appear white?

The normal milk ranges in colour from yellowish creamy white (cow milk) to creamy white (buffalo milk). The colour of milk is due to the combined effect of colloidal casein particles and the dispersed fat globules, both of which scatter light and carotene and to some extent xanthophylls, which impart a yellowish tint to milk. The intensity of yellow colour of cow milk is dependent upon factors such as breed, feed, size of fat globule present in milk, fat percentage.

Photorespiration

Pentose phosphate pathway-2

Active Absorption

Active Absorption

Movement of ions from the outer space of the cell to the inner space is generally against the concentration gradient and hence requires energy. This energy is obtained through metabolism either directly or indirectly. Various evidences indicate the active uptake of ions by carrier mechanism.

Sub Topics

• Carrier mechanism

• Ion traffic into the root

• Translocation of solutes

• Goldacre's Theory

• Cytochrome Pump Salt Respiration or Electron Transport Theory

Carrier mechanism

In carrier mechanism, activated ions combine with carrier proteins and from ion carrier complex. This complex moves across the membrane and reaches the inner space by the expenditure of energy.



Within the cytoplasm, the complex breaks to release the ions. The carrier moves out of the cytoplasm and is again ready to attach another ion to from a complex.

Ion traffic into the root

Mineral nutrients absorbed from the root has to be carried to the xylem. This transport follows two pathways namely apoplastic pathway and symplastic pathway.

In apoplastic pathway, mineral nutrients along with water moves from cell to cell through spaces between cell wall by diffusion. The ions, which enter the cell wall of the epidermis move across cell wall of cortex, cytoplasm of endodermis, cell walls of pericycle and finally reach the xylem.

In symplastic pathway, mineral nutrients entering the cytoplasm of the epidermis move across the cytoplasm of the cortex, endodermis of pericycle through plasmodesmata and finally reach the xylem.



Translocation of solutes

P.R. Stout and Dr. Hoagland have proved that mineral nutrients absorbed by the roots are translocated through the xylem vessel. Mineral salts dissolved in water moves up along the xylem vessel to be transported to all the parts of the plant body. Translocation is aided, by transpiration. As water is continuously lost by transpiration on the upper surfaces of the plant, it creates a transpirational pull, by which water along with mineral salts is pulled up along the xylem vessel.

Active absorption of energy can be achieved only by an input of energy. Following evidences show the involvement of metabolic energy in the absorption of mineral salts.

• Higher rate of respiration increases the salt accumulation inside the cell.

• Respiratory inhibitors check the process of salt uptake.

• By decreasing oxygen content in the medium, the salt absorption is also decreased.

These evidences indicate that salt absorption is directly connected with respiratory rate and energy level in the plant body, as active absorption requires utilization of energy.

Goldacre's Theory

Contractile proteins of membrane show their existence in folded or unfolded condition. Proteins in unfolded conditions are able to bind ions by free valencies exposed at membrane surface. Proteins in folded (contracted) condition release ions as free valencies of proteins get satisfied in folded condition. In this theory role of carrier has been emphasised with utilisation of ATP energy. This theory however has not been proved.



Diagrammatic Representation of the Goldacre Concept

Cytochrome Pump Salt Respiration or Electron Transport Theory

This theory was proposed by H. Lundegardh, who suggested that anions could be transported across the membrane by cytochrome system. Energy is supplied by direct oxidation of respiratory intermediates.



Diagrammatic representation of cytochrome pump hypothesis On salt absorption, anions (A^-) are actively absorbed via a cytochrome pump and cations (M⁺) are passively absorbed.

The rate of respiration, which is solely due to anion absorption, is called as anion respiration or salt respiration. The original rate of respiration (without anion respiration) can be observed in distilled water and is called ground respiration.

Total respiration (R₁) = Ground respiration (R_g) + Salt or anion respiration (R_a).

Krebs Cycle

Krebs Cycle

 After the glycolysis takes place in the cell's cytoplasm, the pyruvic acid molecules travel into the interior of the mitochondrion. Once the pyruvic acid is inside, carbon dioxide is enzymatically removed from each three-carbon pyruvic acid molecule to form acetic acid. The enzyme then combines the acetic acid with an enzyme, coenzyme A, to produce acetyl coenzyme A, also known as acetyl CoA.

Once acetyl CoA is formed, the Krebs cycle begins. The cycle is split into eight steps, each of which will be explained below.

Diagram of Krebs Cycle

Step 1

The acetic acid subunit of acetyl CoA is combined with oxaloacetate to form a molecule of citrate. The acetyl coenzyme A acts only as a transporter of acetic acid from one enzyme to another. After Step 1, the coenzyme is released by hydrolysis so that it may combine with another acetic acid molecule to begin the Krebs cycle again.

Step 2

The citric acid molecule undergoes an isomerization. A hydroxyl group and a hydrogen molecule are removed from the citrate structure in the form of water. The two carbons form a double bond until the water molecule is added back. Only now, the hydroxyl group and hydrogen molecule are reversed with respect to the original structure of the citrate molecule. Thus, isocitrate is formed.

Step 3

In this step, the isocitrate molecule is oxidized by a NAD molecule. The NAD molecule is reduced by the hydrogen atom and the hydroxyl group. The NAD binds with a hydrogen atom and carries off the other hydrogen atom leaving a carbonyl group. This structure is very unstable, so a molecule of CO₂ is released creating alpha-ketoglutarate.

Step 4

In this step, our friend, coenzyme A, returns to oxidize the alpha-ketoglutarate molecule. A molecule of NAD is reduced again to form NADH and leaves with another hydrogen. This instability causes a carbonyl group to be released as carbon dioxide and a thioester bond is formed in its place between the former alpha-ketoglutarate and coenzyme A to create a molecule of succinyl-coenzyme A complex.

Step 5

A water molecule sheds its hydrogen atoms to coenzyme A. Then, a free-floating phosphate group displaces coenzyme A and forms a bond with the succinyl complex. The phosphate is then transferred to a molecule of GDP to produce an energy molecule of GTP. It leaves behind a molecule of succinate.

Step 6

In this step, succinate is oxidized by a molecule of FAD (Flavin adenine dinucleotide). The FAD removes two hydrogen atoms from the succinate and forces a double bond to form between the two carbon atoms, thus creating fumarate.

Step 7

An enzyme adds water to the fumarate molecule to form malate. The malate is created by adding one hydrogen atom to a carbon atom and then adding a hydroxyl group to a carbon next to a terminal carbonyl group.

Step 8

In this final step, the malate molecule is oxidized by a NAD molecule. The carbon that carried the hydroxyl group is now converted into a carbonyl group. The end product is oxaloacetate which can then combine with acetyl-coenzyme A and begin the Krebs cycle all over again.

Summary

In summary, three major events occur during the Krebs cycle. One GTP (guanosine triphosphate) is produced which eventually donates a phosphate group to ADP to form one ATP; three molecules of NAD are reduced; and one molecule of FAD is reduced. Although one molecule of GTP leads to the production of one ATP, the production of the reduced NAD and FAD are far more significant in the cell's energy-generating process. This is because NADH and FADH₂ donate their electrons to an electron transport system that generates large amounts of energy by forming many molecules of ATP.

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