NCERT Class 9 Science Chapter 5 The Fundamental Unit of Life

Chapter 5 The Fundamental Unit of Life

Intext Questions
Question 1: Who discovered cell, and how?
Answer:
In 1665, Robert Hooke observed slices of cork from the bark of a tree under a simple magnifying glass. He noticed small boxes or compartments the cork slice. The boxes were separated from each other by a wall or partition. He named each box as a cell. The cells which Hooke observed were actually dead cells of plants.

Question 2: Why is the cell called the structural and functional unit of life?
Answer:
All the basic functions of an organism takes place inside the cell. As bricks assemble to make a building, cell makes a body of an organism. A group of cells form tissues which perform specific functions. A group of tissues form organs, a group of organs form an organism. This is the reason why the cells are considered as structural and functional unit of life.

Question 3: How do substances like CO₂ and water move in and out of the cell? Discuss.
Answer:
Some substances such as carbon dioxide (CO₂) can move across the cell membrane by a process called diffusion. CO₂ (which is a cellular waste and requires to be excreted out by the cell) accumulates in higher concentrations inside the cell. In a cell’s external environment, the concentration of CO₂ is low as compared to that inside the cell. As soon as there is a difference of concentration of CO₂ inside and outside a cell, CO₂ moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process of diffusion.
The movement of water molecules through such a selectively permeable membrane is called osmosis. The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water. Thus, osmosis is the passage of water from a region of high water concentration through a selectively permeable membrane to a region of low water concentration till equilibrium is reached.

Question 4: Why is the plasma membrane called a selectively permeable membrane?
Answer:
Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. Therefore, plasma membrane is called a selectively permeable membrane.

Question 5: Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.

 Question 5

Answer:

Prokaryotic Cell

Eukaryotic Cell

1. Size: generally small (1 - 10 μm) 1 μm = 10⁻⁶ m

2. Nuclear region: poorly defined due to the absence of nuclear membrane and known as nucleoid

3. Chromosome: single

4. Membrane bound cell organelles absent

1. Size: generally large (5 - 100 μm)

2. Nuclear region: well defined and surrounded by a nuclear membrane

3. More than one chromosome

4. Membrane-bound cell organelles present

Question 6: Can you name the two organelles we have studied that contain their own genetic material?
Answer:
The two organelles that contain their own genetic material are
• Mitochondria
• Plastids

Question 7: If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Answer:
If the organisation of a cell is destroyed due to some physical or chemical influence then the cell will not be able to perform its life processes such as digestion, excretion etc. Lysosomes may burst and the enzymes will digest their own cell.

Question 8: Why are lysosomes known as suicide bags?
Answer:
When the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.

Question 9: Where are proteins synthesized inside the cell?
Answer:
Proteins are synthesized inside the ribosomes in the cell. Ribosomes are also known as protein factories. The ribosomes are rough like particles present on the surface of Rough Endoplasmic Reticulum (RER).

Exercise Questions
Question 1: Make a comparison and write down ways in which plant cells are different from animal cells.
Answer:

Plant cell

Animal cell

• Cell wall is present. Cell wall is made up of cellulose.

• Cell wall is absent.

• Plastids like chromoplast and leucoplast are present.

• Plastids are also absent.

• Presence of larger vacuoles.

• Small or absence of vacuoles.

• The nucleus is located towards periphery.

• The nucleus is located in the centre of the cell.

Question 2: How is a prokaryotic cell different from a eukaryotic cell?
Answer:

Prokaryotic cell

Eukaryotic cell

• Pro = primitive, karyon = nucleus

• Eu = true, karyon = nucleus

• The cell having nuclear material without membrane are called prokaryotic cell.

• The cell having well organised nuclear membrane are called eukaryotic cell.

• Membrane bound cell organelles are absent.

• Membrane bound cell organelles are present.

• Its size is generally small (1 - 10 μm) 1 μm = 10⁻⁶ m.

• Its size is generally large (5 - 100 μm).

• Chromosome: single

• Chromosome: more than one

• Example: bacteria, blue-green algae

• Example: human cheek cell, onion peel cell

Question 3: What would happen if the plasma membrane ruptures or breaks down?
Answer:
If the plasma membrane ruptures or breaks down, then the molecules of some substances will freely move in and out of the cells. As plasma membrane acts as a mechanical barrier, exchange of material from its surrounding through osmosis or diffusion in the cell wound not take place. As a result, the cell would die due to the disappearance of the protoplasm.

Question 4: What would happen to the life of a cell if there was no Golgi apparatus?
Answer:
Golgi apparatus consists of a system of membrane-bound vesicles which has the following function.
• Processing of the synthesised proteins and lipids
• Packaging of processed proteins and lipids
• Dispatch
• Formation of Lysosomes
In the absence of Golgi apparatus, these functions would stop and the cell would eventually die.

Question 5: Which organelle is known as the powerhouse of the cell? Why?
Answer:
Mitochondria are known as the powerhouses of the cell. Mitochondria have two membrane coverings. The outer membrane is porous while the inner membrane is deeply folded. These folds increase surface area for ATP generating chemical reactions. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work.

Question 6: Where do the lipids and proteins constituting the cell membrane get synthesised?
Answer:
The Rough Endoplasmic Reticulum (RER) has the particles called Ribosomes attached to it which help in synthesising of proteins. The Smooth Endoplasmic Reticulum (SER) helps in manufacturing of fat molecules or lipids.

Question 7: How does an Amoeba obtain its food?
Answer:
By the process of endocytosis, an Amoeba obtains its food. Since, its cell membrane is flexible, food particles are engulfed forming a food vacuole girdling it which is assisted by the pseudopodia. Amoeba secretes digestive enzymes to digest the food particles.

Question 8: What is osmosis?
Answer:
Osmosis is the special case of diffusion. It is the movement/migration of water from higher concentration of water to lower concentration region of water across semi-permeable membrane.

Question 9: Carry out the following osmosis experiment:
Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,
a) Keep cup A empty
b) Put one teaspoon sugar in cup B
c) Put one teaspoon salt in cup C
d) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the following:
i) Explain why water gathers in the hollowed portion of B and C.
ii) Why is potato A necessary for this experiment?
iii) Explain why water does not gather in the hollowed out portions of A and D.
Answer:

i) Water gathers in the hollowed portions of B and C as there is a difference of concentration of water in and out of the cell. So endosmosis occurs as the cells act as semi-permeable membrane.

ii) Potato A is essential for this experiment as it is significant to compare different potato cups B, C and D. The potato A in this experiment shows that the potato cavity on its own cannot bring water movement.

iii) Cup in A does not show any change in the water flow concentration for osmosis to occur which requires the concentration to be higher than the others. Cells in cup D are dead, thus there is no existence of a semipermeable membrane for water flow. As a result osmosis does not occur.

Question 10: Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?
Answer:
Mitosis is the type of cell division that is involved in the growth and repair of body while meiosis is a type of cell division which results in the formation of gametes.

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