NCERT Solutions for Class 8 Science Chapter 8 Cell Structure And Function

Q.1 Indicate whether the following statements are True (T) or False (F).
(a) Unicellular organisms have one-celled body. (T/F)
(b) Muscle cells are branched. (T/F)
(c) The basic living unit of an organism is an organ. (T/F)
(d) Amoeba has irregular shape. (T/F)

Answer :

a) Unicellular organisms have one-celled body. ( T )
b) Muscle cells are branched. ( F )
c) The basic living unit of an organism is an organ. ( F )
d) Amoeba has irregular shape. ( T )

EXPLANATION

1) Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism. Unicellular organisms include bacteria, protists, and yeast etc.

2) Muscle cells are not branched as they form the framework of our body. Hence they are spindle shaped.

3) The basic living unit of an organism is a cell. Organ is formed when a group of tissues perform a specific function together and tissues are formed when a group a cells perform a specific function together. So even the basic unit of an organ is a cell.

4) Amoeba has irregular shape because it shows amoeboid movement. In amoeboid movement, amoeba moves through cytoplasm filled projections called as pseudopodia.

Q.2 Make a sketch of the human nerve cell. What function do nerve cells perform?

Answer :

Function of nerve cells:

Neurons, also known as nerve cells, send and receive signals from the brain.

Specialized projections in the nerve cell, called as axons, allow neurons to transmit electrical and chemical signals to other cells.

A neuron or nerve cell communicates with other cells via specialized connections called synapses. Thus it helps to control and coordinate the working of different parts of the body.

Q.3 Write short notes on the following.
(a) Cytoplasm
(b) Nucleus of a cell

Answer :

(a) Cytoplasm:

Cytoplasm is the gelatinous liquid that fills the inside of a cell. It is contained within cells in the space between the cell membrane and the nuclear membrane. It is composed of water, salts, and various organic molecules. The cytoplasm is responsible for holding the components of the cell and protects them from damage. It stores the molecules required for cellular processes and is also responsible for giving the cell its shape.

(b) Nucleus of a cell

Nucleus of a cell is an important component of a cell. It is located at the centre of the cell. It is separated from the cytoplasm by a membrane called nuclear membrane. The nucleus is a membrane-bound organelle that contains genetic material. It has a fluid like substance called as nucleoplasm which is protected inside the nuclear membrane. Nuclear membrane has pores present in it, called as nuclear pores, which is responsible for the exchange of food, gaseous particles and waste between the nucleus and the cytoplasm.

Q.4 Which part of the cell contains organelles?

Answer :

Cytoplasm contains organelles.

Cytoplasm is the gelatinous liquid that fills the inside of a cell.

It is composed of water, salts, and various organic molecules.

The cytoplasm is responsible for holding the components of the cell and protects them from damage.

It stores the molecules required for cellular processes and is also responsible for giving the cell its shape.

Q.5 Make sketches of animal and plant cells. State three differences between them.

Answer :

FIGURE OF ANIMAL CELL

FIGURE OF PLANT CELL

Animal cell

(i) Plasma membrane or cell membrane is the outermost covering of animal cell and is made up of proteins and lipids.

(ii) Plastids are not present in animal cells.

(iii) Very small vacuoles are present in animal cells.

Plant cell

(i) Cell wall is the outermost covering of the plant cell and is made up of cellulose.

(ii) Plastids are present in plant cells.

(iii) Large central vacuoles are present in plant cells.

Q.6 State the difference between eukaryotes and prokaryotes.

Answer :

Prokaryotes

(i) Most of them are unicellular organisms.

(ii) They don't have a well defined nucleus.

(iii) Nuclear membrane is absent or present in scattered form.

(iv) They don't have membrane bound organelles which means that the organelles present inside the cell doesn't have membrane.

(v) Example: bacteria and blue-green algae.

Eukaryotes

(i) Most of them are multicellular organisms.

(ii) They have a well defined nucleus.

(iii) Nuclear membrane is perfectly defined and structured.

(iv) They have membrane bound organelles which means that the organelles present inside the cell have perfectly defined membrane.

(v) Example : Plant and animals

Q.7 Where are chromosomes found in a cell? State their function.

Answer :

Chromosomes are thread-like structures located inside the nucleus of animal and plant cells.

Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA).

Chromosomes carry the basic genetic material i.e., DNA which is responsible to provide hereditary characteristics and genetic information to the various cells.

Chromosomes transfers the parental character to the offspring by transfer of genetic material.

Q.8 Cells are the basic structural units of living organisms’. Explain.

Answer :

A cell is defined as the smallest basic unit of life as it is responsible for all the life processes. Thus they are called the structural, functional and biological units of all living beings. A cell can replicate itself independently.

Different cells combine to form tissues. Tissues are groups of cells that have a similar structure and act together to perform a specific function.

Tissues combine to form organs. An organ is a self-contained group of tissues that performs a specific function in the body. The heart, liver, and stomach are examples of organs in humans.

Organs combine to form organ system such as respiratory system, excretory system, circulatory system and reproductive system.

All the systems and functions are regulated at the cellular level and are gradually segregated to different parts such as when some cells perform a specific function, they are called as tissues and when some tissues perform a specific function, they are called as organ and when organs perform a specific function, they are called as organ system. As a result of this, it is concluded that, cells are the basic unit of every system or process, thus becoming the structural unit of living organisms.

Q.9 Explain why chloroplast are found only in plant cells?

Answer :

Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight and converts it and stores it in the energy-storage molecules ATP and NADPH.

Chloroplasts are found only in plant cells because chloroplasts contain chlorophyll which traps sunlight and uses it to prepare food for plants by the process of photosynthesis, while animal cells doesn't require chloroplast because they are heterotrophic organisms and thus they depend on other organisms for their food. As they don't prepare their own food hence they don't need sunlight as a result, they don't need chlorophyll and chloroplast in their cell.

Q.10 Complete the crossword with the help of clues given below.

Across

1. This is necessary for photosynthesis.
3. Term for component present in the cytoplasm.
6. The living substance in the cell.
8. Units of inheritance present on the chromosomes.

Down

1. Green plastids.
2. Formed by collection of tissues.
4. It separates the contents of the cell from the surrounding medium.
5. Empty structure in the cytoplasm.
7. A group of cells.

Answer :

Across

1. Chlorophyll
3. Organelle
6. Protoplasm
8. Genes

Down

1. Chloroplasts
2. Organ
4. Membrane
5. Vacuole
7. Tissue


EXPLANATION

1) Chlorophyll is necessary for photosynthesis because it allows plants to absorb light energy from the sunlight.

2) Organelles that are present in the cytoplasm, is a subcellular structure that performs one or more specific functions in the cell.

3) Protoplasm is the living part of a cell that is surrounded by a plasma membrane. It is the colourless material comprising the living part of a cell, including the cytoplasm, nucleus, and other organelles.

4) A gene is the basic physical and functional unit of heredity. They are made up of DNA. They are present in the chromosome.

5) Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight and converts it and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water in plant and algal cells.

6) An organ is a self-contained group of tissues that performs a specific function in the body.

7) The cell membrane is a biological membrane that separates the interior of all cells from the outside environment and thus protects the cell from its environment.

8) A vacuole is a membrane-bound organelle. Vacuoles are closed sacs, made up of membranes with inorganic or organic molecules inside, such as enzymes.

9) Tissues are groups of cells that have a similar structure and act together to perform a specific function.