2. Life Processes in Living Organisms Part 1

Explanation: In aerobic respiration one glucose molecule is completely oxidized. Each NADH₂ gives 3 ATP and each FADH₂ gives 2 ATP in the electron transfer chain.

• Glycolysis: 2 ATP + 2 NADH₂ (2 × 3 = 6 ATP) = 8 ATP
• Pyruvic acid to acetyl-CoA: 2 NADH₂ (2 × 3) = 6 ATP
• Krebs cycle (2 turns): 6 NADH₂ (18 ATP) + 2 FADH₂ (4 ATP) + 2 ATP = 24 ATP

Total = 8 + 6 + 24 = 38 ATP.

Explanation: Glycolysis occurs in the cytoplasm. One glucose molecule (6 carbon) is oxidized step by step. It gives two molecules each of pyruvic acid, ATP, NADH₂ and water.

Explanation: Prophase-I has five stages. In the third stage, pachytene, crossing over takes place between the chromatids of homologous chromosomes. Parts of chromosomes are exchanged, so genetic recombination occurs.

Explanation: In metaphase the nuclear membrane disappears completely. The chromosomes become fully condensed and arrange themselves on the equatorial (central) plane. Spindle fibres join their centromeres to the centrioles.

Explanation: Cells make phospholipids from fatty acids. Phospholipids are the main building material of the plasma membrane.

Explanation: During heavy exercise, muscles need energy quickly but do not get enough oxygen. So they perform anaerobic respiration. Lactic acid collects in the muscles and we feel tired.

The process of taking in food and using its nutrients for energy, growth and repair of the body is called nutrition.

The components of food that the body needs for energy, growth and proper working are called nutrients. Examples: carbohydrates, proteins, fats, vitamins, minerals, water and fibres.

Proteins are macromolecules formed by bonding together many amino acids.

The step-by-step oxidation of glucose in the cells to release energy in the form of ATP is called cellular respiration.

Cellular respiration in which glucose is completely oxidized with the help of oxygen, producing carbon dioxide, water and energy, is called aerobic respiration.

The process in the cytoplasm in which one molecule of glucose is oxidized step by step into two molecules of pyruvic acid is called glycolysis.

• Complete oxidation of glucose takes place in aerobic respiration, through glycolysis, the Krebs cycle and the electron transfer chain.
• In the electron transfer chain, NADH₂ and FADH₂ are oxidized to form ATP. Oxygen accepts the hydrogen at the end and forms water.
• Without oxygen this chain stops, so the Krebs cycle also stops. Glucose is then oxidized only partly, as in fermentation.

Hence oxygen is necessary for complete oxidation of glucose.

• We cannot digest fibres, but they help in the digestion of other substances.
• They add bulk to food and help to push out undigested substances (egestion). This prevents constipation.
• We get fibres from leafy vegetables, fruits and cereals.

Hence fibres are one of the important nutrients.

• A new organism is formed from an existing one only by cell division.
• A multicellular organism grows because its cells divide by mitosis.
• Wounds heal, a weak (emaciated) body is restored and blood cells are formed by mitosis.
• Gametes and spores are formed by meiosis, which is needed for reproduction.

Hence cell division is an important property of cells and organisms.

• Higher plants and animals normally perform aerobic respiration.
• When oxygen in the surroundings becomes less, they cannot get enough energy by aerobic respiration.
• Then they perform anaerobic respiration to get energy. For example, seeds germinating in soil submerged under water, and our muscle cells during heavy exercise.

• In the Krebs cycle, acetyl-CoA first combines with oxaloacetic acid.
• The first product formed is citric acid.
• Citric acid has three carboxylic (–COOH) groups, so the cycle is also called the tricarboxylic acid (TCA) cycle.

Hence the Krebs cycle is also known as the citric acid cycle.

• Glycolysis is the first step of cellular respiration. It occurs in the cytoplasm.
• It occurs in both aerobic and anaerobic respiration. Oxygen is not needed.
• One molecule of glucose (6 carbon) is oxidized step by step with the help of enzymes.
• At the end, two molecules each of pyruvic acid (3 carbon), ATP, NADH₂ and water are formed.
• It was discovered by Embden, Meyerhof and Parnas, so it is also called the EMP pathway.

Fate of pyruvic acid: In aerobic respiration, pyruvic acid is converted into acetyl-CoA, releasing 2 NADH₂ and 2 CO₂. Acetyl-CoA then enters the Krebs cycle. In anaerobic respiration, pyruvic acid is converted into alcohol or lactic acid by fermentation.

Mitosis occurs in somatic cells and stem cells. Before division, the cell doubles its chromosomes. Mitosis has two main steps: karyokinesis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Karyokinesis has four phases:

Cytokinesis: The cytoplasm divides and two daughter cells are formed. In animal cells a notch forms at the equator and deepens. In plant cells a cell plate forms along the middle of the cell.

Mitosis is needed for growth, wound healing, restoring an emaciated body and forming blood cells.

1. Leptotene: Chromosomes begin to condense. They look like long, thin threads.
2. Zygotene: Homologous chromosomes come close and form pairs. This pairing is called synapsis.
3. Pachytene: Each chromosome shows two chromatids, so each pair has four chromatids. Crossing over occurs between chromatids of homologous chromosomes, causing genetic recombination.
4. Diplotene: Homologous chromosomes start to move apart. They remain joined at the points of crossing over, called chiasmata.
5. Diakinesis: Chromosomes become short and thick. The nuclear membrane and nucleolus disappear and spindle fibres begin to form.

• Nutrition: The digestive system gives nutrients. Proteins build new cells and tissues.
• Respiration: Mitochondria oxidize food and release energy (ATP) for all activities.
• Circulation: Blood carries nutrients and oxygen to every cell.
• Excretion: Harmful wastes are removed, so cells work properly.
• Control and co-ordination: The nervous system and hormones control all these processes.
• Cell division: Mitosis increases the number of cells, so the body grows and wounds heal.

All these life processes work together, so the body grows and develops.

The Krebs cycle (tricarboxylic acid cycle) occurs in the mitochondria. It is the second step of aerobic respiration.

1. Acetyl-CoA (2C) combines with oxaloacetic acid (4C) to form citric acid (6C).
2. Citric acid is converted into α-ketoglutaric acid (5C). One CO₂ and one NADH₂ are released.
3. α-Ketoglutaric acid is converted into succinic acid (4C). One CO₂, one NADH₂ and one ATP are formed.
4. Succinic acid is converted into fumaric acid. One FADH₂ is formed.
5. Fumaric acid takes up water and becomes malic acid.
6. Malic acid is converted into oxaloacetic acid. One NADH₂ is formed. Oxaloacetic acid again combines with a new acetyl-CoA, so the cycle continues.

Overall reaction (for one acetyl-CoA):

One glucose gives two acetyl-CoA, so the cycle runs twice for each glucose molecule. The NADH₂ and FADH₂ then go to the electron transfer chain to form more ATP.

Corrected diagram:

Mistakes corrected: In the given diagram, “Proteins” was placed below “Lipid”, “Fats” below “Amino acids”, and “Krebs cycle” below “Glycolysis”. The arrows also went up to pyruvic acid at the top. The correct order is shown above.

Explanation:

• Carbohydrates are broken into glucose. Glucose forms pyruvic acid by glycolysis. Pyruvic acid forms acetyl-CoA.
• Fats (lipids) are converted into fatty acids. Fatty acids form acetyl-CoA.
• Proteins are converted into amino acids. Amino acids form acetyl-CoA.
• Acetyl-CoA is completely oxidized in the Krebs cycle in the mitochondria. CO₂, water and energy (ATP) are formed.

Fats and proteins are used for energy mainly when carbohydrates are not enough, for example during fasting.

Material: Onion, conical flask or glass of water, watch glass, iodine solution (or acetocarmine stain), glass slides, cover slips, forceps, blade and a compound microscope.