BIOLOGY 5 Marks Questions

An animal cell has the following cell structures:
• Plasma membrane
• Endoplasmic reticulum
• Mitochondria
• Golgi body
• Lysosomes
• Ribosomes
• Vacuoles
• Nucleus
• Centriole



i. Plasma Membrane: This is also known as cell membrane. Plasma membrane is made up of lipid and protein. It is semi - permeable in nature. Certain substances are transported through plasma membrane by passive transport. Some substances get transported by osmosis and some by active transport. Active transport involves use of some carrier to facilitate transport. Apart from transport of materials, plasma membrane gives a shape and size to the animal cell.
ii. Endoplasmic Reticulum: These are networks of fine tubules extending from plasma membrane to nucleus. They work like pipelines and facilitate transport of substances from outside the cell to nucleus and cytoplasm. Depending on presence or absence of ribosomes ER can be either rough or smooth.
iii. Golgi Body: This is composed of many sack like structures stacked one over another. The function of golgi body is to package different materials, like carbohydrate, protein and lipid.
iv. Lysosome: Lysosome is a small spherical structure filled with digestive enzymes. The digestive enzyme helps in digesting foreign materials and waste products. Sometimes the lysosome digests the contents of cytoplasm which in turn kills the cell itself. That is why lysosome is also known as 'suicide bag of the cell'.
v. Ribosome: Ribosomes are small dot-like structures. They are made of two subunits. The function of the ribosome is to synthesize protein.
vi. Vacuoles: These are small fluid-filled structures. Vacuoles help in maintaining osmotic pressure inside the cell.
vii. Mitochondria: Mitochondria is a double membrane structure. The inner membrane is projected in finger-like structures, called cristae. The presence of cristae helps in increasing the inner surface are of mitochondria. Aerobic respiration takes place in the mitochondria and energy released is stored in the form of ATP (Adenosine triphosphate).
viii. Nucleus: Nucleus is covered by a nuclear membrane. The nucleus contains chromosomes which are genetic materials. Nucleus also controls various functions of the cell.
ix. Centriole: These are spindle-like structures. During cell division, they form spindle fibres.

• Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce
adenosine triphosphate (ATP).
• Although the many forms of life on earth use a range of different nutrients, almost all carry out oxidative phosphorylation to produce ATP, the molecule that supplies energy to metabolism.
• This pathway is an efficient way of releasing energy, compared to alternative fermentation processes such as anaerobic glycolysis.
• During oxidative phosphorylation, electrons are transferred from electron donors to electron acceptors such as oxygen, in redox reactions. These redox reactions release energy, which is used to form ATP.
• In eukaryotes, these redox reactions are carried out by a series of protein complexes within mitochondria, whereas, in prokaryotes, these proteins are located in the cells' inner membranes. These linked sets of enzymes are called electron transport chains.

All these happen in the two haploid nuclei simultaneously.
i. Prophase-II takes a short time. Spindle formation begins and the chromosomes become short. Two chromatids are joined to a single centromere. Nuclear membrane and nucleolus disintegrate.
ii. Metaphase-II At the equator, the chromosomes align at the equator and spindle is formed. The centromere of every chromosome is joined to the spindle fibre and centromere also divides.
iii. Anaphase-II The daughter chromosomes are formed. Chromatids move towards their poles with the spindle fibres.
iv. Telophase-II Reaching the poles, chromosomes from nuclei which are haploid (n) daughter nuclei. Again nuclear membrane is constructed. Nucleolus now becomes clearly visible.
v. Cytokinesis Occurs and four daughter cells are formed which are haploid (n). It may occur once or twice (i.e., in meiosis-I and II) or only after the meiosis-II cell division.

Meiosis II
i. Prophase II: Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles normal mitosis. The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.



ii. Metaphase II: At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
iii. Anaphase II: It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell.
iv. Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope.
Cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells.