Essay on Fats in Plants

In this essay we will learn about Fats in Plants. After reading this essay you will learn about: 1. Introduction to Fats in Plants 2. Classification of Fats in Plants 3. Breakdown and Synthesis 4. Qualitative Tests.

Contents:

1. Essay on the Introduction to Fats in Plants
2. Essay on the Classification of Fats in Plants
3. Essay on Breakdown and Synthesis of Fats
4. Essay on the Qualitative Tests for Fats.

Essay # 1. Introduction to Fats in Plants:

Fat and fat-like substances are invariably present in all living cells, plants or animals. They are produced in all actively metabolising plant cells and they serve a number of indispensable roles in plants, particularly as reserve food substance just as does starch.

The fats and certain other compounds, more or less closely related, are often called lipides (lipids) or lipoids.

Lipids have been synthesised in the laboratory by man, but only slowly. In contrast to proteins, lipids are generally molecules of-small weight, seldom over 1,000. In general, lipids are insoluble in water, but soluble in the so-called fat-solvents — ether, chloroform, benzene, etc. Chemically they are esters of fatty acids and glycerol.

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Essay # 2. Classification of Fats in Plants:

The following simple classification of fats, found in plants, is universally accepted:

I. Simple Fats—Esters of Fatty Acids with Alcohols:

(a) Fats—esters of fatty acids with glycerol (glycerine)—fatty acids esterified with glycerol to form triglycerides. These are true fats and are indispensable as reserve food material and as a source of energy in all living cells.

(b) Waxes—esters of higher aliphatic fatty acids, with long chain alcohols other than glycerol. Waxes are sometimes important components of the cuticle of the epidermal cell walls. Waxy layer sometimes seems to inhibit bacterial, fungal or insect attack. In insectivorous Nepenthes, waxy interior of the pitcher effectively helps it to trap insects.

II. Compound Fats—Esters of Fatty Acids Containing Groups in Addition to Alcohol and Fatty Acid Radicals:

Phospholipids—fats containing phosphoric acid. Lecithin, the best known among phospholipids in plant cells, is an essential structural material for all living cells. Sulpholipids contain sulphur.

Galactolipids—are the major lipid constituents of green leaf tissue. The most impor­tant types in the chloroplasts, mentioned before, are monogalactosyl diglyceride and digalactosyl diglyceride.

III. Derived Fats and Related Compounds—Certain Substances Related to Lipids or Derived From Compounds in the above Groups by Hydrolysis:

(a) Fatty acids of various series.

(b) Terpenes and terpenoids:

These are formed from C_s-isoprene units e.g., camphor (monoterpene), gibbereliic acid (diterpene), oleanolic acid (triterpene). Tetra- and pentacyclic terpenoids also occur in plants.

(c) Sterols:

These are mostly complex alcohols of very large molecular weights, soluble in fat-solvents like ether, chloroform, etc. Collectively the sterols found in plants are called phytosterols, the most important of which are cholesterol, ergosterol, stigmasterol and sitosterol. These are related to the triterpenes and some are polycyclic.

Fats, like proteins and carbohydrates serve as reserve food in seeds. In almost all seeds, fat is found as the principal reserve material. In many seeds, fats make up nearly 35-50% of the total dry weight material while vegetative tissues in general contain less than 5% of fat on a dry weight basis.

Fats occur in leaves, stems, fruits, flowers and even in pollen grains but the largest concentration are certainly in the seeds. As fats are in­soluble in water, they are found in plant cells in the form of very small droplets or globules, dispersed in the protoplasm. Sometimes they are large enough to be seen under microscope, particularly when strained red with Sudan III.

Fats contain the same elements C, H and O as the carbohydrates but the proportion of carbon to oxygen is much greater in fats than in carbohydrates or in other words carbon in fats is in a more highly reduced form than in carbohydrates.

The principal fatty acids of higher plant fats are those containing 12-26 carbon atoms. The fatty acid lauric (12 carbon atoms), palmitic (16 carbon atoms), stearic (18 carbon atoms) and oleic (18 carbon atoms) are the commonly occurring fatty acids in the plant kingdom and almost all fats, identified in plants, contain these fatty acids as their major components.

Oleic acid, the most abundantly found fatty acid in plant, constitutes almost 35-40% of the total fatty acids produced in all the world’s edible fats. Fatty acids may be saturated or unsaturated having one to few double bonds.

Plant fats may be either liquid or solid at ordinary temperatures. The liquid fats are commonly referred to as oils while solid fats are fats proper. The solid fats generally contain a much larger proportion of saturated fatty acid than oil whereas oil contains proportionately a much higher percentage of unsaturated fatty acids (60-75% of linseed oil is unsaturated fatty acids).

Conversion of liquid plant oils such as from soybean, peanut, nuts, etc., into solid fat by artificial hydrogenation (direct addition of hydro­gen to convert a proportion of the unsaturated fatty acids of oil to solid fat) has become a profitable industrial practice for the manufacturing of solid oils and margarine.

When alcoholic or aqueous solution of fats are boiled with caustic alkali, e.g., NaOH or KOH, they are readily hydrolysed into corresponding fatty acids and gly­cerol, the alkali immediately combining with fatty acids to form soap. This process is termed saponification.

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Essay # 3. Breakdown and Synthesis of Fats:

Fat is hydrolysed by the enzyme lipase to glycerol and fatty acids. The fatty acid is then oxidised by α- or β-oxidation process. In the former case the carboxyl carbon is oxidised to CO₂, and the a-carbon, i.e. the carbon adjacent to it is oxidised to —CHO by fatty acid peroxidase.

An NAD-linked aldehyde dehydrogenase then oxidises the — CHO to — COOH, so that the resulting fatty acid molecule is now one carbon less than the initial molecule.

In the next turn similar reactions will follow (See Fig. 717a).

In the β-oxidation process, which is the usual mode of oxidation, the β-carbon is oxidised to =CO, in several steps, and at the end of every turn of the “cyclic” opera­tion, the fatty acid molecule becomes 2-carbon atoms less, because an acetyl coenzyme A (CH₃ COS CoA) molecule is liberated each time.

The fatty acid molecule has to be activated first by hooking on a coenzyme A (HS CoA) residue to the carboxyl end, using ATP energy in the process.

In subsequent reactions two hydrogens are removed from the β and y carbons, a water molecule is added to the double bond formed, and two hydrogens removed from the — CHOH group involving the β-carbon, resulting in its oxidation to = CO. One molecule of acetyl CoA is split off by β-ketothiolase.

The reaction sequence is shown in Fig. 717a. These reactions take place on the glyoxysomes and peroxisomes, where the enzymes are present. The NADH₂ and the FADH₂ formed are oxidised by the electron transport system (See later).

The acetyl CoA molecules may enter Krebs cycle or other metabolic pathways. If it enters Krebs cycle, 3 molecules of NADH₂ and one molecule of FADH₂ will be produced per turn of Krebs cycle resulting in the production of 12 molecules of ATP per molecule of acetyl CoA oxidised through the cycle (one molecule coming from the substrate level phosphorylation during conversion of succinyl CoA to succinate).

Thus, during oxidation of stearic acid, which is a saturated fatty acid having 18 carbons, 9 acetyl CoA will be produced, which may contribute 108 molecules of ATP; to this has to be added 40 (8×3 + 8 x 2 = 40) molecules of ATP obtained by the oxidation of one NADH₂ and FADH₂ produced in each of the 8 times of the β-oxidation process producing 9 molecules of acetyl CoA.

Since 1 ATP is used at the commencement of the reaction sequence, the total ATP yield per molecule of stearic acid oxidised this way would be 147. In terms of 6C oxidised the value would be 49 against 38 in the case of glucose, α-oxidation of fatty acids thus, is a more efficient process, a-oxidation however, is less efficient as only one NADH₂ (≡3 ATP) is produced per turn.

Although α and β-oxidation processes are apparently cyclic, since the number of carbon atoms decrease at the end of each turn, the initial molecule is not returned. It is thus more appropriate to refer to it as fatty acid oxidation helix. Lynen was the first to work out the /S-oxidation process.

The glycerol produced after hydrolysis of fat with lipase is broken down through glycolysis and Krebs cycle reaction sequences after conversion to dihydroxyacetone phosphate via oxidation of a-glycerol phosphate which in turn is produced by α-glycerokinase action on glycerol using ATP.

Synthesis of Fats:

Fats are not synthesized by a reversal of the breakdown process. For synthesis of fatty acids acetyl CoA in the cytoplasm is carboxylated to malonyl CoA in presence of ATP. The malonyl part is then transferred to the SH group of acetyl carrier protein (ACP) also present in the cytoplasm. The malonyl-S-ACP then condenses with acetyl-S-ACP forming acetoacetyl S-ACP and regenerating ACP-SH and CO₂.

The acetoacetyl-SACP is then reduced to butyryl-S-ACP through several steps with the help of NADPH+H⁺.

The butyryl-S-ACP then condenses with a malonyl-S-ACP, followed by decarboxylation as before, and the fatty acid chain is elongated by C₂ fragments coming from acetyl coenzyme A and the process is repeated until a long chain fatty acetyl CoA is produced which is then released from the enzyme—a synthetase.

In the mitochondria acetyl CoA condenses repeatedly with a short chain fatty acid molecule until the long chain results. The cytoplasmic pathway is the major pathway.

For glycerol synthesis dihydroxy acetone phosphate is reduced by a NADP- linked dehydrogenase to glycerol phosphate, from which the phosphate residue is removed by a phosphatase. Fats are produced from glycerol and fatty acids by condensation reactions.

Conversion of Fat to Carbohydrate:

In germinating fatty seeds, like castor bean, fat is converted to carbohydrate by glyoxylic acid cycle and reversal of glycolysis.

The reactions take place on glyoxysomes. Fatty acids are first oxidised to acetyl CoA. The acetyl CoA then passes through Krebs cycle until isocitrate is formed, the isocitrate is broken down by isocitratelyase to glyoxylate and succinate. The glyoxylate condenses with acetyl CoA with the help of the enzyme malate synthase to form malate, which is then oxidised to oxalacetate by malic dehydrogenase.

The succinate finds its way to the mitochondria where it is also oxidised to oxalacetate. The oxalacetate then is decarboxylated to phosphoenol pyruvate which by reversal of glycolysis produces carbohydrates (Fig. 717b).

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Essay # 4. Qualitative Tests for Fats and Oils:

i. Study sections of Gladiolus leaves, onion scales and fatty seeds. Place sections in a soln. of Sudan III (0-5% in 70% alcohol) and allow to stand for 20 min. Wash with 50% alcohol and then water and mount in glycerine. Observe the red-stained oil drops which can be seen floating around the section as well as in the cell-inside.

ii. (a) Test section with 1% osmic acid (OsO₄) and observe the black-stained drops.

(b) Add a drop of 1 % osmic acid soln. to a little olive soil. A black colour is produced.

iii. Try solubilities of olive and other oils in water, ether, alcohol and chloroform. Observe that they are insoluble in water and alcohol but soluble in ether and chloro­form.

iv. In a beaker dissolve about 0-5 gm of Sudan III in 50 ml of 95% alcohol. Add 10 ml of olive (or any other oil) oil and stir well. After a short time the oil collects at the bottom and becomes deep orange-red.

v. Hydrolysis of fats—Saponification:

About 100 ml of olive oil are added to about 200 ml of 10% NaOH in a water-bath. Boil for half an hour and after cooling, add Na₂SO₄ (or saturated NaCl soln). The soap formed separates out and rises to the surface. Filter. Neutralise the filtrate with dilute H₂SO₄.

The liquid is evaporated and the residue treated with alcohol. This dissolves the glycerol formed, but not the sul­phate. A syrup like liquid is formed which is glycerol. Test for glycerol.

(а) To a little of the liquid, add a few crystals of KHS0₄ in a test tube and heat strongly. Pungent odour of acrylic aldehyde is easily detected.

(b) To a little of the soln. add a few drops of CuSO₄ soln. and then some NaOH. A blue, clear soln. is obtained. Glycerol prevents the precipitation of Cu (OH)₂ formed.