Amino acid degradation and the urea cycle

As well as being oxidized occasionally to pro­duce energy, amino acids are also broken down during the normal turnover of protein in the tissues. If the amino acid unit is not needed in forming new protein, it is oxidized. Excess amounts of amino acids absorbed in food are also oxidized. The amino group re­moved at the start of amino acid oxidation has to be expelled from the body. This is because it contains ammonia, a very poisonous sub­stance. An intermediate in the citric acid cycle accepts the amino group and then releases it as an ammonium ion. Another intermediate

also readily accepts amino groups, forming as­partate. Aspartate plays an important part in ni­trogen elimination. Nitrogen is the main con­stituent of ammonia. So it has to be converted to a form in which it can be passed out of the body in small volumes of water. This com­pound is urea. The urea cycle pathway in which it is made occurs in most land animals. Fishes and some other aquatic animals excrete ammonia directly. This is because they have plenty of water available and are not likely to dehydrate. Before entering the cycle, the am­monia reacts with bicarbonate and ATP to form carbamoyl phosphate. This then reacts with ornithine to produce citrulline. This com­pound combines with aspartic acid to form ar-ginosuccinic acid. This acid, in the next stage, breaks down into the by-product fumaric acid and arginine. Finally, arginine breaks down to ornithine (for reuse in the cycle) and urea.

Photosynthesis

Photosynthesis is the means by which plants, and ultimately animals, derive the energy they need for their metabolic processes. Light en­ergy is used to synthesize (produce) organic molecules from water and carbon dioxide. In some ways, it is therefore a reverse of the process of respiration. It takes place in organ­elles (minute specialized parts of a cell) of the plant called chloroplasts. Chloroplasts have structural and functional analogies to mi­tochondria. Chloroplasts are thought to have originated from blue-green algae in the sea, mitochondria from aerobic bacteria on land. Mitochondria, however, are found in all or­ganisms. Chloroplasts occur only in green algae and higher plants. Certain bacteria and the blue-green algae also photosynthesize. But the process takes place in structures that are simpler than chloroplasts. Enormous amounts of light energy are absorbed by plants each year during photosynthesis. Much of this oc­curs in marine algae.

Pi

(b). ph

Bi

te

Light of wavelength 680nm

Light of wavelength 700nm

Electrons

T

Photosynthetic pigment II

T

Hydrogen

Hydrogen carrier system

Electrons

I

Photosynthetic pigment

Photocenter II

Water

c

I

Electrons

Oxygen

Hydrogen ions

~7\

ADP ^_p

T

NADPH„

NADP + electrons

Biochemistry: Biochemical energy 121

The light reaction

Photosynthesis occurs in two stages. The first stage needs the presence of light, whereas the second can take place in darkness. The light reaction is dependent on the green plant pig­ment chlorophyll. This is a porphyrin com­pound containing magnesium. It is related to hemoglobin. There are several similar pig­ments in chlorophyll. Only the blue-green chlorophyll (a) is common to all plants. Most plants also contain yellow-green chlorophyll (b), xanthophyll (yellow), carotene (orange), and pheophytin. Pheophytin is a gray pigment that is probably a breakdown product of chloro­phyll. Each pigment absorbs light of a slightly different wavelength. The purpose of the light reaction is to release hydrogen from water. This reduces carbon dioxide to carbohydrates. It takes place in structures called thylakoids, which resemble stacked plates. All the later re­actions occur in the liquid, or stroma, of the chloroplast.

There are two types of light centers in thylakoid membranes. These centers are called photosystems I and II. Absorption of light of a particular wavelength by pigment at center II excites the molecule in center I. Elec­trons are released. This process reduces (com­bines with hydrogen) a benzene compound called quinone on the other side of the mem­brane. The electrons are replaced from water molecules. This process releases oxygen gas and hydrogen ions. Absorption of light at an­other wavelength by chlorophyll at center I also causes electron release. This time, a phos­phate (NADP) is reduced. Both quinone and the phosphate are hydrogen carriers. Their re­duction by electrons produced in the light re­actions also requires the uptake of hydrogen ions. A compound of quinone and hydrogen is then reoxidized by a sequence of carriers (in­cluding cytochromes). Electrons are eventually transferred to replace those ejected from cen­ter I. Hydrogen ions are released inside the

thylakoid. The net effect of the two pho­tosystems is that the phosphate is reduced; water is split; oxygen is released; and hydro­gen ions are separated across the membrane. These ions then return. A special enzyme is used to conserve the energy released by this process as ATP. This is the same principle that underlies ATP production by mitochondria.

The dark reaction

The dark reaction begins with the combination of a carbon dioxide molecule with a five-car­bon sugar. This splits to form two further mol­ecules of a glyceric acid. The acid is then re­duced by the hydrogen carried by a phosphate compound. This forms a three-car­bon triose phosphate. This can then be used to build up the hexose sugars that are later stored as starch.

But not all the triose phosphate goes to make sugars. Some has to be used in reform­ing a type of phosphate. By doing so, carbon dioxide continues to be taken up by a pathway known as the Calvin cycle. Some of the steps of this cycle—including the reduction of phos-phoglyceric acid—require ATP. This is avail­able as a result of the light reactions. Studies with radioactive carbon have shown that phos-phoglyceric acid is also vital in building up amino acids and fats.

The dark reaction/below) is the second part of photo­synthesis. The light reac­tions have generated en­ergy (in the form of ATP) and NADPH₂ (a reduced phosphate). These are now used to build up complex organic molecules such as starch, fats, and proteins. The various stages of the dark reaction are explained in detail in the main text.

Date: 2015-12-11; view: 1394