Chapter 8: Cell The Unit of Life

Ans: The incorrect statement is (a) Robert Brown discovered the cell. As he discovered a nucleus in the cell.

Ans: New cells generate from (c) pre-existing cells

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Ans: The correct statement is (c) In prokaryotes, there are no membrane-bound organelles.

Ans: Mesosomes are formed by the infoldings of plasma membranes. They are mainly found in bacteria. Functions of Mesosomes are listed below:

i) These extensions help in cell wall formation and DNA replication .

ii) They help in distribution of equal chromosomes in daughter cells.

iii) They contain enzymes for aerobic respiration and also helps in secretion processes and to increase the surface area of the plasma membrane and enzymatic content

Ans: Neutral solutes do not carry any charge because they move across the plasma membrane through osmosis. while polar molecules are charged molecules so they cannot pass through the non-polar membrane. To cross the non-polar membrane they require a carrier protein that facilitates its transport inside the cell. If this transport takes place against the concentration gradient, it will require energy in the form of ATP.

Ans: Mitochondria and chloroplasts are the two double membranous organelles. They both are also called semi-autonomous organelles because they contain their own DNA molecules. They also have 70S types of ribosomes that are found in the cytoplasm.

Characteristics of Mitochondria are:

1) Mitochondria are rod-shaped structures and generally found in the cytoplasm of cells with other organelles. 

2) They are principally concerned with energy generation in the form of ATP by converting chemical energy. 

3) Mitochondria are surrounded by two membranes – outer and inner where the outer membrane covers the organelle. However, the inner membrane is folded and forms a layered structure. This layered structure contains several finger-like projections called cristae.

4) The inner membrane is folded and forms a layered structure. This layered structure contains several finger-like projections called cristae. 

5) The inner mitochondrial membrane also possesses F0F1 particles called oxysomes. These oxysomes are responsible for ATP generation by using electron transport systems. 

6) The Citric Acid cycle takes place in the inner mitochondrial membrane that encloses a mitochondrial matrix.

Functions of Mitochondria are:

1) Mitochondria are called the powerhouse of a cell because it generates ATP by cellular respiration.

2) They provide energy in the form of ATP to perform all the important activities of living cells.

3) They are regarded as semi-autonomous organelle because they have their own DNA and ribosomes. 

4) Citric acid cycle taking place in the matrix of mitochondria where it generates several metabolic intermediates. These intermediates are required for the biosynthesis of various amino acids and proteins.

Characteristics of Chloroplast are:

1) The Chloroplasts are also double membrane-bound organelles and are found all over the cytoplasm of plant cells.

2) They contain chlorophyll that makes them appear green in colour.

3) The chloroplast is made up of two membranes i.e., inner and outer. The space limited by the inner membrane of the chloroplast is known as the stroma that contains metabolic enzymes and multiple copies of the chloroplast genome.

4) There are a number of organized flattened membranous sacs called the thylakoids present in the stroma.

5) These thylakoids are stacked one over the other to form a granum that looks like a stack of coins. 

6) Apart from this, there are flat membranous tubules called the stroma lamellae. They connect the thylakoids of the different grana.

7) A lumen is a space that is formed by the membrane of the thylakoids. 

Functions of Chloroplast are:

Chloroplasts are also known as the kitchen of the cell.

1) They trap solar energy and utilize it for manufacturing food for plants. They are involved in the process of photosynthesis.

2) Chloroplast contains the enzymes required for the synthesis of carbohydrates and proteins

Ans: In prokaryotic terms pro means ‘primitive’ and karyon means ‘nucleus’, which means prokaryotic cells have a very primitive and less defined nucleus. They do not possess any membrane-bound organelles like mitochondria, chloroplast, Golgi body, Endoplasmic reticulum, etc. Examples include archaea and bacteria. Other characteristics of prokaryotic cells are given below:

1)  Most of the prokaryotic cells are unicellular.

2) The size of a prokaryotic cell varies from 0.5 – 5 µm and is generally small in size.

3) The nuclear region of a prokaryotic cell is poorly defined and the genetic material is present in the region of the cytoplasm.

4) In prokaryotic cells, the DNA is found naked which means DNA is not associated with histone proteins. 

5) They have single, circular chromosomes as genetic material. except for the genomic DNA, they also contain circular plasmid DNA.

6) Mesosomes are the specialized membranous structures that are found in prokaryotic cells. It is formed by the invagination of the cell membrane and these extensions help in the synthesis of the cell wall and replication of DNA. 

7) Prokaryotic cells contain cell walls as an outer covering that gives shape to the cells. 

Ans: A multicellular organism has cells as a basic structural unit in its body. The cells are arranged in a manner to form tissues like blood, bone, etc. and these tissues are arranged in a manner to form organs like the heart, kidney, or other body organs. These organs form an organ system such as the digestive system, reproductive system, and respiratory system, etc. and various organ systems of the organism perform together to form a complete individual.

Ans: There are several organ systems that function together to form an organism. Each organ system like the nervous system, digestive system, circulatory system, etc., includes several organs. And these organs are formed by several types of tissues. A tissue is formed combinedly by the cells that interconnect with each other and perform a shared function. A cell can do all an organism can do this is the reason the cells are called the basic building blocks of all organisms.