BIOLOGY 5 Marks Questions

Factor Governing Cell Size: Like all other living things, cells have a definite age and a maximum size. This process of cell division goes on in the body all the time. It is on this account, the cells do not grow beyond a certain size and limit.

Surface	Volume ratio
A	6:1
B	3:1
C	2:1

Volume ratio decreases with the increase in size. The size of the cell is co-related to the functions it performs.
Surface: Volume ratio decreases with the increase in size. The size of the cell is co-related to the functions it performs. As cell increases in volume, its surface area is increased (not to some extent). The volume determines the chemical activity of the cells per unit of time. The surface area determines the amount of absorption as well as the amount of release of waste products by cells. As the living cell grows, its rate of waste product formation and the need for intake of substances from the outside increases at a faster rate than that of the surface area. The small size of the cells of the large/bigger organisms compensates the disproportionate increase in volume and surface area of the cells. The cells are tiny structures. They are small in volume to maintain a large surface area to volume ratio.

i. Nucleus: Nucleus is a double membrane structure, with minute pores in the membrane. The pores work like channels for passage of substances. The fluid-filled in the nucleus is called nucleoplasm. There is usually one nucleolus inside the nucleus. Sometimes many nucleoli can be found. There is a fine network of a thread like chromatins inside the nucleus. During the resting stage of a cell, structures inside the nucleus cannot be seen. They become visible only during cell division.
ii. Centrosome: In chromosome, there is one primary constriction, which contains two centromeres. These centromeres comprise the centrosome. Centrosome plays an important role during cell division.

Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms. In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid. The following are the important step of glycolysis.
• Glucose undergoes phosphorylation to produce glucose-6-phosphate.
• Fructose-6-phosphate is then converted into PGAL. (Phosphoglyceraldehyde).
• Each molecule of PGAL then undergoes several steps to produce Pyruvic Acid.
• There is a net gain of two molecules of ATP during glycolysis of one molecule of glucose.
Fate of Pyruvate: Aerobic Respiration: Pyruvic acid is completely oxidized to produce carbon dioxide and energy.
Anaerobic Respiration: Depending upon the availability of oxygen in some organisms. pyruvic acid is converted into ethanol and carbon dioxide when there is a complete absence of oxygen. In some other organisms, pyruvic acid is converted into lactic acid when there is an incomplete oxygen supply.

Meiosis is called reduction division because the number of chromosomes in daughter cells becomes half of the number of chromosomes in mother cells. In spite of this, meiosis enables the conservation of specific chromosome number of each species. In fact, has there been no meiosis, organisms would not have been able to evolve to sexual mode of reproduction. We know that fertilization involves fusion of male and female gametes. Thus, zygote gets the chromosome pool from two cells and the number of chromosomes in a zygote becomes double that of the gametes. To ensure conservation of specific chromosome number after fertilization, it is necessary that the gametes should have half the number of chromosomes compared to what it is in somatic cells.