1. Phospholipids: There are five types of phospholipids in the cell membrane. Lecithin, cephalin, glycolipid, glycophosphotide and phosphotidic acid. Phospholipids are arranged in two layers. Each layer is composed of numerous lipid molecules. Each lipid molecule has two parts. head and tail The large outer part is called the head. It is spherical or oval and composed of phosphates. It is polar and hydrophilic. The two filament-like parts attached to the head are called tails. It is composed of fatty acids. The tail is non-polar and hydrophobic. Between the head and the tail is the gyserol. Lipid molecules are always moving, shaking and bouncing around each other. This type of movement is called flip flop movement. As the layers move between the two layers due to frictional motion, the membrane feels like a fluid.
2. Proteins: There are three types of proteins in the plasma membrane. These are-
(i) Peripheral Protein: The protein which is located at the edge of the lipid layer is called peripheral protein. It looks round. It is enzyme in nature. It remains attached to the cytoskeleton. Cyclins are produced from abundant pollen grains by the action of membrane bound enzymes.
(ii) Intrinsic Protein: The protein that is in the intrinsic state inside the lipid layer is called intrinsic protein. It looks oval. Acts as a carrier and transports essential substances. It maintains cell-to-cell adhesion.
(iii) Intermembrane Protein: The protein that extends from one end of the lipid layer to the other end is called an intermembrane protein. It is the largest protein. It changes its structure by taking energy from the ancha. As a result, a hole is created through it. Essential substances move through these pores. It acts as a receptor for hormones, neurotransmitters, mediated endocytosis, insulin etc. Various molecules, ions and electrons move through channels, pumps and electron transport chains.
3. Glycocalyx (Carbohydrate): Carbohydrate chains are present on the outside of the cell membrane. Carbohydrate chains are attached to lipids to form glycolipids and proteins to form glycoproteins. Glycoproteins and glycolipids together are called glycocalyx or cell coat. It acts as a cell-marking factor. It helps in the movement of essential substances.
4. Cholesterol: Cholesterol is a type of steroid. It has a claw on its head and is water absorbent. The other part is water repellent. Cholesterol is sandwiched between phospholipid molecules. It is irregularly arranged. Cholesterol is higher in animal cells and lower in plant cells.
Protein molecules are scattered between phospholipids in the cell membrane. Because of this, the protein molecules look like a mosaic when viewed from the surface of the cell membrane. To explain this situation in one word, the cell membrane model has been named ‘Fluid Mosaic Model’.

1. NADH-Q reductase: a 26-subunit compound.
2. Cytochrome reductase: A 10-subunit compound.
3. Cytochrome oxidase: An 8 subunit compound.
4. Cytochrome-C: A small protein.
** Ubiquinone: A non-protein compound.

1. Step 1: NADH2 is oxidized to release high-energy electrons (e-) and energy. The released electron carrier is taken up by NADH-Q reductase. The energy generated here drives the proton (H+) out of the inner membrane. Here an ATP is produced.
2. Step 2: Electrons are then transferred from NADH-Q reductase to the carrier ubiquinone (Co-Q). Next the electron (e-) from ubiquinone reaches the carrier cytochrome-b.
3. Step 3: FADH2 is oxidized to release high energy electrons (e-) and energy. The released electron carrier is taken up by cytochrome-b. The energy generated here drives the proton (H+) into the intermembrane space. Electrons then pass from cytochrome-B to carrier cytochrome-C.
4. Step IV: Electrons (e-) are transferred from cytochrome-C to cytochrome oxidase. Here the existing proton (H+) is sent to the intermembrane space. Finally, electrons are released into the mitochondrial matrix and combine with O2 to form H2O water.
5. Fifth step: Protons from the intermembrane space are re-entered into the matrix by ATP synthase in the process of chemi-osmosis. Here the energy released combines ADP and Pi to form ATP.
6. Sixth step: Electrons (e-) and protons (H+) react with oxygen in the matrix to produce H2O. The process stops when an oxygen vacancy occurs in the matrix.

The system through which electrons are transferred from one place to another and ATP and H2O are produced during the flow of electrons is called the electron transport system. It is also called respiratory chain. Energy is released at specific locations as electrons flow in this process. This energy combines ADP and Pi to form ATP. Thus the production of ATP with the help of the energy released in the oxidation process is called oxidative phosphorylation.
ADP + Pi → ATP
Several electron transport proteins combine to form multi-proteins. The inner membrane of mitochondria contains four multi-proteins (in order of strength) and transfer electrons. In this process NADH2, NADPH2, FADH2 and FADPH2 are oxidized to NAD, NADP, FAD and FADP and release energy. Under the influence of this energy, ATP is produced by combining ADP and Pi. Normally 2 of each FADH2 or FADPH2 and 3 of each NADH2 or NADPH2 are formed. [Currently, NADH2 = 2.5 ATP, FADH2 = 1.5 ATP]

1. Krebs cycle is the main center of energy production. This energy is used in various metabolic functions – mineral salt absorption, water absorption, transport, movement, growth, flowering etc.
2. α-ketoglutaric acid and oxaloacetic acid produced in this cycle help in nitrogen metabolism.
3. This process involves the production of amino acids.
4. Various types of organic acids produced in this cycle are used in metabolism.
5. In this cycle succinyl CO is produced. Succinyl Co-A is one of the building blocks of chlorophyll, cytochrome, hemoglobin, phycobilin etc.
6. Organisms release CO2 through this cycle.
7. Each molecule of glucose oxidized produces 24 ATP.
8. Pyrimidines and alkaloids are produced from oxaloacetic acid.
9. Oxidation of carbohydrates, fatty acids and amino acids occurs through the Krebs cycle.
10. Oxidative co-enzymes NADH+H+ and FADH2 are synthesized through the Krebs cycle.
11. This cycle links sugar metabolism with nitrogen metabolism.
12. This cycle is called amphibolic pathway as both addition and dissociation reactions occur.
13. This cycle is linked to the glyoxylate cycle.

1. Condensation: Acetyl CoA reacts with oxaloacetic acid to produce citric acid under the action of acetyl CoA condensing enzyme with the help of citrate synthetase.
2. Isomerization: Under the action of aconitase enzyme, cis-aconitic acid is produced from citric acid and then isocitric acid from cis-aconitic acid.
3. Dehydrogenation: Oxalosuccinic acid is produced from isocitric acid by the action of isocitrate dehydrogenase enzyme. At this time, NADP participates in the reaction and produces NADPH+H+. The reaction is bidirectional.
4. Decarboxylation: Under the action of oxalo succinate carboxylase enzyme, oxalo succinic acid is converted into alpha keto glutaric acid.
5. Oxidative decarboxylation: Succinyl Co is produced from alpha ketoglutaric acid with the help of alpha ketoglutarate dehydrogenase enzyme. At this time NADP participates in the reaction NADPH+H+
produces
6. ATP Synthesis: Succinic acid is produced from succinyl CoA in the presence of succinyl CoA synthetase enzyme. During this reaction, ADP takes part and produces ATP. The reaction is bidirectional.
7. Dehydrogenation: Fumaric acid is produced from succinic acid by the action of succinyl dehydrogenase enzyme. FAD participates in the reaction and produces FADH2. The reaction is bidirectional.
8. Hydration: Fumaric acid is converted to malic acid in the presence of fumarase enzyme. The reaction is bidirectional.
9. Dehydrogenation: Oxaloacetic acid is produced from malic acid with the help of malate dehydrogenase enzyme. At this time, NADP participates in the reaction and produces NADPH+H+.
The oxaloacetic acid produced then re-enters the Krebs cycle and keeps the cycle going.

1. Condensation: Acetyl CoA reacts with oxaloacetic acid to produce citric acid under the action of acetyl CoA condensing enzyme with the help of citrate synthetase.
2. Isomerization: Under the action of aconitase enzyme, cis-aconitic acid is produced from citric acid and then isocitric acid from cis-aconitic acid.
3. Dehydrogenation: Oxalosuccinic acid is produced from isocitric acid by the action of isocitrate dehydrogenase enzyme. At this time, NADP participates in the reaction and produces NADPH+H+. The reaction is bidirectional.
4. Decarboxylation: Under the action of oxalo succinate carboxylase enzyme, oxalo succinic acid is converted into alpha keto glutaric acid.
5. Oxidative decarboxylation: Succinyl Co is produced from alpha ketoglutaric acid with the help of alpha ketoglutarate dehydrogenase enzyme. At this time NADP participates in the reaction NADPH+H+
produces
6. ATP Synthesis: Succinic acid is produced from succinyl CoA in the presence of succinyl CoA synthetase enzyme. During this reaction, ADP takes part and produces ATP. The reaction is bidirectional.
7. Dehydrogenation: Fumaric acid is produced from succinic acid by the action of succinyl dehydrogenase enzyme. FAD participates in the reaction and produces FADH2. The reaction is bidirectional.
8. Hydration: Fumaric acid is converted to malic acid in the presence of fumarase enzyme. The reaction is bidirectional.
9. Dehydrogenation: Oxaloacetic acid is produced from malic acid with the help of malate dehydrogenase enzyme. At this time, NADP participates in the reaction and produces NADPH+H+.
The oxaloacetic acid produced then re-enters the Krebs cycle and keeps the cycle going.

1. The Krebs cycle is the third stage of the cycle.
2. The cycle occurs in the mitochondria of cells.
3. This process requires oxygen.
4. The amount of energy generated in this process is high.
5. CO2 is produced in this phase.
6. In this process complete oxidation of respiratory material takes place.
7. The first compound produced in this cycle is citric acid.
8. This process is called TCA cycle or amphibolic pathway.

The process in which acetyl CoA reacts with oxaloacetic acid to produce citric acid and finally oxaloacetic acid is called the Krebs cycle. In 1937, the British biochemist Sir Hans Krebs first observed these reactions in the breast meat of pigeons, so it is called the Krebs cycle. Oxaloacetic acid is a stable compound called a resident molecule. Acetyl CoA is the chemical link between glycolysis and the Krebs cycle.
The first compound produced in this cycle is citric acid, also known as the citric acid cycle. Again, some compounds produced in this cycle have three carboxylic (-COOH) groups, so it is called Tricarboxylic Acid cycle. It is also called amphibolic pathway.

Pyruvic acid is oxidized to acetyl Co, producing NADPH+H+ and CO2. It is an oxidative decarboxylation process. Pyruvic acid produces acetyl Co-A stepwise in oxidative decarboxylation process. The enzyme pyruvic dehydrogenase plays a role in this reaction. The process is –
1. In the presence of decarboxylase enzyme, pyruvic acid and thiamine pyrophosphate (TPP) react to form thiamine pyrophosphate compound (TPP).
2. In the presence of trans acetylase enzyme, TPP compounds and lipoic acid react to produce acetyl lipoic acid. At this time TPP was released.
3. Acetyl lipoic acid reacts with Co-A to form Acetyl Co-A. At this time lipoic acid is oxidized.