Class 9 - Biology - Fundamental Unit of Life

Intext Questions-

Question 1:

Who discovered cells and how?

 

Answer:

Cells were discovered in 1665 by an English Botanist, Robert Hooke. He used a primitive microscope to observe cells in a cork slice.

Living cell is discovered by Leuwenhoek.

 

 

Question 2:

Why is cell called the structural and functional unit of life?

 

Answer:

Cells constitute various components of plants and animals. A cell is the smallest unit of life and is capable of all living functions.

Cells are the building blocks of life, in the same way as bricks are the building blocks of a building.

A group of cells performing a specific task form tissue; tissues group together to form organ; organs group together to form organ system and

organ systems group together to form an organism.

 

 

Question 1:

How do substances like CO2 and water move in and out of the cell? Discuss.

 

Answer:

The cell membrane is selectively permeable and regulates the movement of substances in and out of the cell.

Movement of CO2:

CO2 is produced during cellular respiration. Therefore, it is present in high concentrations inside the cell.

This CO2 must be excreted out of the cell. In the cell's external environment, the concentration of CO2 is low as compared to that inside the cell.

Therefore, according to the principle of diffusion, CO2 moves from a region of higher concentration (inside the cell)

towards a region of lower concentration (outside the cell). Similarly, O2 enters the cell by the process of diffusion

when the concentration of O2 inside the cell is low as compared to its

surroundings.

Movement of water:

Water moves from a region of high concentration to a region of low concentration

through the plasma membrane. The plasma membrane acts as a semi-permeable

membrane, and this movement of water is known as osmosis. However, the

movement of water across the plasma membrane of the cell is affected by the

amount of substance dissolved in water.

 

 

Question 2:

Why is the plasma membrane called a selectively permeable membrane?

 

Answer:

The cell membrane or plasma membrane is known as a selectively permeable

membrane, because it regulates the movement of substances in and out of the cell.

This means that the plasma membrane selectively allows the entry of only some substances

and prevents the movement of unwanted materials.

 

 

Question 1:

Can you name the two organelles we have studied that contain their own genetic

material?

 

Answer:

Mitochondria and chloroplasts are the two organelles that contain their own genetic material. Both these organelles have their own DNA and ribosomes.

These two organelles are also known as semi- autonomous organelle.

 

Question 2:

If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

 

Answer:

Cell is the smallest unit of life, which is capable of all living functions. If the organisation of a cell is destroyed due to some physical or chemical influence,

then the ability of the cell to perform all living functions such as respiration, nutrition, digestion, excretion, etc. would be affected.

 

Question 3:

Why are lysosomes known as suicide bags?

 

Answer:

Lysosomes are membrane-bound vesicular structures that contain powerful digestive enzymes(hydrolytic enzymes).

These enzymes are capable of breaking down any foreign food particle or microbes entering the cell. Sometimes,

lysosomes can cause self-destruction of a cell by releasing these digestive enzymes, hydrolytic enzymes, within the cells. Hence, they are also known as 'suicidal bags'.

 

Question 4:

Where are proteins synthesized inside the cell?

 

Answer:

Ribosomes are the site for protein synthesis. Ribosomes are very small structures found either in a free state,

suspended in the cytoplasm, or attached to the surface of the endoplasmic reticulum.

They are composed of ribonucleic acids and proteins. It has two subuints- one is large and other is small.

 

Exercise:

 

Question 1:

Make a comparison to write down ways in which plant cells are different from

animal cells.

 

Answer:

The cells of animals and plants have the following differences:

Plant cell

 Animal cell

1. It is larger than animal cell.

Generally smaller in size.

2. Cell wall is present.

Cell wall is absent.

3. Plastids are present.

No plastids except euglena.

4. Large vacuole.

Vacuoles are small comparitively.

5. Have simpler unit of Golgi apparatus, called dictyosomes.

Have single complex golgi body.

6. Lack centrosomes and centrioles.

 centrosomes and centrioles are present.

 

 

Question 2:

How is a prokaryotic cell different from a eukaryotic cell?

 

Answer:

Difference between prokaryotic cell and eukaryotic cell:

Prokaryotes

Eukaryotes

Cell size- small

large

Nuclear region is nucleiod and no presence of nuclear membrane.

 Double layered nuclear membrane is present.

Nucleolus is absent.

Nucleolus is present.

Membrane bound oragnelle are absent.

Membrane bound oragnelle are present.

Single chromosome present.

More than one chromosome is present.

Cell division- fission or budding.

Cell division by mitosis or meiosis.

 

 

 

 

Question 3:

What would happen if the plasma membrane ruptures or breaks down?

 

Answer:

Plasma membrane or Cell membrane is a selectively permeable membrane of the cell that maintains its homeostasis, i.e., constant internal composition of the cell.

It selects the entry and exit of components.

If it ruptures or breaks down the constant internal chemical composition of the cell will be lost and it will not be able to perform its basic functions.

Such a cell with ruptured plasma membrane is killed.

 

 

Question 4:

What would happen to the life of a cell if there is no Golgi apparatus?

 

Answer:

The materials synthesized in the ER are stored, sorted, modified, packaged and dispatched to various targets inside and outside the cell

through the Golgi apparatus packs products in vesicles, the secretary vesicles.

In some cases, complex sugars e.g. cellulose, may be made from simple sugars in Golgi apparatus.

The Golgi apparatus is also involved in the formation of the cells which will not be possible if Golgi apparatus is not there.

 

 

Question 5:

Which organelle is known as the powerhouse of the cell? Why?

 

Answer:

Mitochondria are known as the powerhouse of the cell because they contain enzymes that are needed for stepwise oxidation of food stuffs (carbohydrate, fats and lipids)

present in the cells to CO2 and water. Oxidation of food releases energy which is used to form high-energy ATP (adenosine triphosphate) molecules.

ATP is known as Energy Currency of the cell and it is used as cellular fuel.

Energy stored in ATP is used to bring about energy requiring activities of the cell such as photosynthesis, protein synthesis and muscle contraction.

 

 

Question 6:

Where do the lipids and proteins constituting the cell membrane get synthesised?

 

Answer:

Lipids and proteins are synthesised by Endoplasmic reticulum.

Rough Endoplasmic Reticulum (RER) - synthesizes proteins constituting cell

membrane.

Smooth Endoplasmic Reticulum (SER) - synthesizes lipids constituting cell

membrane.

 

 

Question 7:

How does Amoeba obtain its food?

 

Answer:

Amoeba has flexible cell membrane. It enables amoeba to engulf in food by the

process called endocytosis.

 

 

Question 8:

What is osmosis?

 

Answer:

The diffusion of water or solvent through a semi-permeable membrane from a

solution of lower concentration of solutes to a solution of higher concentration of

solutes, to which the membrane is relatively impermeable, is called osmosis.

“Osmosis” is a word used for diffusion in case of water.

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