Revision: Life Processes in Living Organisms Part -1 Science and Technology 2 SSC (English Medium) 10th Standard Maharashtra State Board

The process occurring in the cytoplasm where one glucose molecule is stepwise oxidized to form two molecules each of pyruvic acid, ATP, NADH₂, and water is called glycolysis.

The cyclic series of reactions occurring in the mitochondria, where acetyl-CoA is completely oxidized to produce CO₂, H₂O, NADH₂, and FADH₂, is called the tricarboxylic acid cycle or Krebs cycle.

The process occurring in the mitochondria, where NADH₂ and FADH₂ release electrons to form ATP and water, producing 3 ATP from each NADH₂ and 2 ATP from each FADH₂, is called the electron transfer chain reaction.

The breakdown of glucose in the presence of oxygen to produce carbon dioxide, water, and energy is called aerobic respiration.

It is a process of release of energy from food substances such as glucose and fats under the control of enzymes, to carry out life processes, by the living organisms.

The breakdown of glucose in the absence of oxygen to produce alcohol or lactic acid and a small amount of energy is called anaerobic respiration.

The centrosome (in animal cell) splits into two along with the simultaneous duplication of the centrioles contained in it. The daughter centrioles move apart and occupy opposite "poles" of the cell. Each centriole is surrounded by radiating rays and is termed an aster (aster : star).

Cell division is one of the most fundamental characteristics of life. This is the method which enables life to perpetuate generation after generation.

All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).

A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.

The two sister chromatids remain attached to each other at a small region called centromere.

• Glycolysis breaks one glucose into two pyruvic acid molecules in the cytoplasm.
• It has 10 steps: 5 preparatory (use ATP) and 5 pay‑off (produce ATP).
• There is a net gain of 8 ATP per glucose in the given scheme.
• With oxygen, pyruvic acid is fully oxidised to CO₂ and water.
• Without oxygen, pyruvic acid forms lactic acid or alcohol.
• Insulin promotes glycolysis by increasing glucose uptake.
• Glucagon and epinephrine help release glucose from glycogen, indirectly supporting glycolysis.

• It is a common oxidative pathway where acetyl Co‑A (from pyruvic acid via link reaction) is completely oxidised to CO₂.
• The cycle also supplies intermediates (e.g., α‑ketoglutarate, oxaloacetate) for synthesis of amino acids such as glutamate and aspartate.
• Per pyruvic acid, the cycle produces 3 CO₂, 4 NADH + 4H⁺, 1 FADH₂ and 1 ATP (or GTP) in the mitochondrial matrix.
• For each glucose (2 pyruvates), Krebs cycle output is 6 CO₂, 8 NADH + 8H⁺, 2 FADH₂ and 2 ATP molecules.
• Considering the whole respiratory pathway, glucose breakdown yields CO₂, 8 NADH + H⁺, 2 FADH₂ and 2 ATP at the Krebs‑cycle level.
• Because its intermediates are used both for breakdown (catabolism) and for biosynthesis (anabolism), the respiratory pathway is termed an amphibolic pathway.

• The electron transport chain is located on the inner mitochondrial membrane and contains a series of electron and proton carrier complexes (I–V).
• Complex I (NADH dehydrogenase) accepts electrons from NADH via FMN and Fe‑S centres, passes them to ubiquinone (Co‑Q), and pumps 4 H⁺ into the intermembrane space.
• Complex II (succinate dehydrogenase) receives electrons from succinate through FADH₂ and Fe‑S centres, passes them to Co‑Q, but does not pump protons.
• Complex III (cytochrome‑c reductase) transfers electrons from reduced Co‑Q (UQH₂) to cytochrome‑c and pumps 4 H⁺ into the intermembrane space.
• Complex IV (cytochrome‑c oxidase) passes electrons from cytochrome‑c to oxygen, reducing it to water and pumping additional H⁺ across the membrane.
• Complex V (ATP synthase) uses the proton gradient generated by complexes I, III and IV to synthesise ATP from ADP and Pi; this proton‑driven ATP formation is called chemiosmosis.
• Oxidation of each NADH yields about 3 ATP, while every FADH₂ yields about 2 ATP, though exact yields can vary with conditions and substrate.

• ATP formation is called phosphorylation and occurs in three ways: photophosphorylation, substrate-level phosphorylation, and oxidative phosphorylation.
• Photophosphorylation occurs during photosynthesis, while the other two occur during respiration.
• Substrate-level phosphorylation involves direct transfer of a phosphate group to ADP and occurs in the cytoplasm and mitochondrial matrix.
• Oxidative phosphorylation uses energy from oxidation of NADH and FADH₂ and occurs in the inner mitochondrial membrane.
• ATP is hydrolysed to release energy whenever the cell needs it for metabolic activities.

• Cellular respiration is the process where food (glucose) is oxidised inside the cell to release energy, stored as ATP via phosphorylation.
• Oxidation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 686 Kcal | Phosphorylation: ADP + iP + 7.3 Kcal → ATP
• First step occurs in cytoplasm — glucose breaks down into pyruvate (3-carbon molecule), releasing some energy.
• Without O₂ (Anaerobic) — Pyruvate → Ethanol + CO₂ (yeast) or Lactic acid (muscles). Less energy released.
• With O₂ (Aerobic) — Pyruvate breaks down in mitochondria into CO₂ + H₂O, releasing a large amount of energy as ATP.

• Cell division is a vital process for growth, repair, and the formation of new organisms, helping maintain life in all living beings.
• It occurs in two forms: mitosis (in somatic and stem cells) for producing diploid identical cells, and meiosis (in germ cells) for forming haploid gametes.
• Mitosis supports body growth and tissue repair, while meiosis ensures genetic variation and maintains chromosome number in reproduction.
• Before division, the cell’s chromosome number doubles (e.g., from 2n to 4n) to ensure accurate distribution during mitosis or meiosis.

Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:

1. Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
2. Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
3. Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
4. Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.

• Cytokinesis is the division of the cytoplasm that follows nuclear division (karyokinesis), resulting in the formation of two separate daughter cells.
• In animal cells, it occurs by the formation of a cleavage furrow, while in plant cells, a cell plate forms at the centre to divide the cytoplasm.

• Meiosis is a reduction division that results in haploid gametes (sex cells) with half the chromosome number.
• In humans, it occurs in the testes (to form sperm) and ovaries (to form ova).
• In flowering plants, it occurs in the anthers (to form pollen grains) and ovaries (to form ovules).
• Fertilisation restores the diploid (2n) number, maintaining chromosome count across generations.

• Meiosis is a two-stage process: Meiosis I and Meiosis II, responsible for forming haploid gametes from diploid germ cells.
• Prophase I is a complex phase subdivided into five stages where homologous chromosomes pair up and undergo crossing over, enabling genetic recombination.
• In Metaphase I, homologous pairs (not sister chromatids) align along the equatorial plate, attached to spindle fibers from opposite poles.
• During Anaphase I, homologous chromosomes are separated and pulled to opposite poles, reducing the chromosome number by half.
• Telophase I results in the formation of two haploid daughter cells, each with half the chromosome number, setting the stage for Meiosis II.

• Meiosis II is similar to mitosis and occurs in both haploid cells formed after Meiosis I, dividing the recombined sister chromatids.
• It includes four stages: Prophase II, Metaphase II, Anaphase II, and Telophase II, resulting in four haploid daughter cells.
• The process ensures genetic variation, as each resulting cell is genetically different from the parent and from each other.
• Meiosis II plays a vital role in sexual reproduction, producing gametes (in animals) or spores (in plants) with half the chromosome number.