Chapter 8: Cell The Unit of Life

Ans: Nuclear pores are small holes present in the nuclear envelope of the nucleus. This nucleus envelope is formed by the fusion of two nuclear membranes. These small holes only allow some specific substances to be transferred into the cell cytoplasm and back to the nucleus. However, they allow few molecules like RNA and proteins to move in both directions between the nucleus and the cytoplasm.

Functions include:

• Preserve the stability of the genetic material.

• Retains the shape of the nucleus.

• Responsible for the movement of protein molecules between the nucleus and the cytoplasm.

Ans: Both lysosomes and vacuoles are single membranous structures and both perform different types of functions.

• Lysosomes can hydrolyze all types of organic substances, except cellulose because it contains hydrolytic enzymes that work under acidic pH. They perform phagocytic functions and hence, are known as suicidal bags.

• However, the vacuoles are non-cytoplasmic sacs covered by a membrane. It is found in animal and plant cells both that contain sap, water, excretory substances, etc. 

• The membrane surrounding the vacuole is called tonoplast which is semi-permeable in nature. Vacuole separates harmful substances from cell cytoplasm and maintains osmotic pressure or turgidity. 

• This contractile vacuole performs several functions like osmoregulation and excretion. 

• The other type of vacuole is known as food vacuole. It stores the food while gas vacuoles store metabolic gases and take part in buoyancy regulation.

Ans: The nucleus is a membrane-bound organelle that controls all the cellular activities of the cell. It plays an important role in cell division. It is relatively large and spherical in shape and is composed of the following structures:

• Nucleoplasm: It is the fluid, filled in the nucleus containing enzymes, proteins, nucleosides and other factors responsible for the functioning of the genetic material.

• Nuclear Envelope: It is a double membrane envelope in which the nucleus is constrained by the nuclear pore. It has tiny pores which serves as a channel for substance to pass in and out.

• Nucleolus: These are round, irregularly shaped, naked structures attached to the chromatin. It produces ribosome units and proteins.

• Chromatin: They are a fine network of thread-like structures  containing DNA and few proteins such as histones, RNA and non-histone proteins. These are found in the nucleus and responsible for condensation during the process of cell division.

Ans: The centrosome is made up of two cylindrical structures that lie perpendicular to each other called centrioles.

• These centrioles are linked with each other by interconnected fibers. Each centriole has a cartwheel-like arrangement which is made up of microtubule triplets that are evenly placed in a ring.

• A proteinaceous hub is present in the central part of a centriole which is connected to the triplets via radial spokes.

• The centrioles play a vital role in forming the spindle fibers and astral rays during cell division and also form the basal body of cilia and flagella.

Ans: Chromosomes are found in the nucleus of each cell. It is a thread-like structure that is not visible in the cell’s nucleus. But at the time of cell division, it becomes more tightly packed, and then only it is visible under a microscope. Each chromosome joined at the centromere or the primary constriction and hence consists of two chromatids. These centromeres are the point of attachment of spindle fibres and play a vital role in cell division.

On the basis of the position of the centromere, chromosomes are classified into the following types: 

(1) Acrocentric Chromosome: In this type of chromosome, the centromere is present at the sub-terminal. In the Anaphase stage chromosomes are J-shaped.

(2) Sub-metacentric Chromosome: In this type of chromosome, the centromere is sub-median and the anaphasic chromosome appears L-shaped.

(3) Metacentric Chromosomes: In this type of chromosome, the centromere is present in the middle and divides the chromosome into two equal parts. The chromosome appears V-shaped.

(4) Telocentric Chromosome: In this type of chromosome, the centromere is present at the terminal. The anaphasic stage appears l-shaped.

Depending upon the number of centromeres, a chromosome is of different types.

(i) Monocentric: with a single centromere

(ii) Dicentric: with two centromeres

(iii) Polycentric: with many centromeres

(iv) Acentric chromosome: there is no centromere