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.

• ATP (Adenosine Triphosphate) is the energy currency of the cell, storing energy in its phosphate bonds and releasing it when needed.
• ATP consists of adenine, ribose, and three phosphate groups, and is formed in cells to meet energy demands.
• When carbohydrates are lacking, the body uses lipids and proteins, which are converted into acetyl-CoA and oxidized via the Krebs cycle for energy.

1. Glucose is first broken down in the cytoplasm into a 3-carbon molecule called pyruvate, releasing some energy.
2. In the absence of oxygen, pyruvate is converted into ethanol and carbon dioxide (in yeast) or lactic acid (in muscles), releasing less energy.
3. In the presence of oxygen, pyruvate is broken down in the mitochondria into carbon dioxide and water, releasing a large amount of energy.
4. The energy released during respiration is used to make ATP, which is the energy currency of the cell and used for all life processes.

• 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.