Solutions-NCERT-Class-9-Science (Exploration)-Chapter-2-Cell: The Building Block of Life-CBSE

According to the Cell Theory, all cells arise from pre-existing cells. This concept was notably expanded upon in 1855 by the scientist Rudolf Virchow, who stated that new cells are formed only from those that already exist.

Scientists developed the microscope to overcome the limit of the human eye, which can see objects only up to about 0.1 mm clearly.

• Magnification and Resolution: Microscopes use one or more convex lenses to make tiny objects appear larger and clearer.
• Light Microscope: It uses visible light and lenses to observe small structures such as cells. It is commonly used in school and college laboratories.
• Electron Microscope: It uses a beam of electrons instead of light to produce extremely highly magnified images at the nanometre (nm) scale.
• Electron microscopes have helped scientists study the fine details of cells and cell organelles, which cannot be seen with a light microscope.

The cell is called the fundamental unit of life because it is the smallest unit that can perform all the activities of a living organism.

Structural Unit:

• All living organisms, from single-celled bacteria to multicellular humans, are made up of cells.
• In multicellular organisms, cells combine to form tissues, tissues form organs, and organs work together as organ systems.

Functional Unit:

• The cell is the basic unit of function in every living organism. A cell works like a tiny living factory, carrying out all the essential life processes.
• Different cell organelles perform specific functions, such as producing energy, making materials, and removing waste.

Thus, the cell is the basic structural and functional unit of life.

Cell division is the process by which a cell divides to form new cells.

There are two main types of cell division:

(1) Mitosis :

• Mitosis is the type of cell division used for growth and repair.
• One parent cell divides to produce two genetically identical daughter cells.
• The daughter cells have the same DNA and the same number of chromosomes as the parent cell.

(2) Meiosis :

• Meiosis occurs only in the reproductive organs to produce gametes (sperm and eggs).
• It involves two successive divisions of the parent cell.
• It produces four daughter cells, each having half the number of chromosomes of the parent cell.
• Meiosis creates genetic diversity, making each offspring unique.

The necessity of a cell wall in plants compared to its absence in animals is directly related to their different lifestyles and environmental requirements:

• Plant cells have a cell wall that provides protection, support, and shape. The cell wall helps plants stand upright and withstand wind and rain.
• Animal cells do not have a cell wall, making them flexible. This flexibility allows animals to move easily and their cells to change shape.

Consequences if a Plant Cell Wall Became as Flexible as the Cell Membrane

• The plant cell would lose its fixed shape.
• It would not be able to support the plant, causing stems and leaves to droop.
• The cell would be more easily damaged by pressure and environmental conditions.
• The plant would become weak and unable to remain upright.

• Equal Size: Potato pieces are cut into equal sizes to ensure a fair comparison between the samples.
• Initial Weight: The initial weight is recorded to compare it with the final weight after the experiment.
• Evidence of Osmosis: Any gain or loss in weight shows the movement of water into or out of the potato cells by osmosis.
• These steps make the experiment accurate and reliable.

• White flowers generally do not contain coloured pigments.
• White flowers contain leucoplasts, which are a type of plastid that lacks pigments and is colourless.
• White flowers lack the pigments found in chloroplasts (green) or chromoplasts (various bright colours) and instead contain colourless leucoplasts.

If skin cells started dividing by meiosis instead of mitosis, a cut on the skin would not heal properly.

• Mitosis produces identical body cells needed for growth and repair.
• Meiosis produces gametes (sperm and eggs) with half the number of chromosomes.
• As a result, new skin cells would not replace the damaged cells correctly.
• Therefore, the cut would heal very slowly or may not heal at all.

A cell has many small mitochondria instead of one giant mitochondrion because many small organelles provide a larger total surface area.

• A larger surface area allows more chemical reactions to occur at the same time, producing more ATP (energy).
• Many mitochondria can also spread throughout the cell, supplying energy where it is needed.
• A single giant mitochondrion would have a lower surface area relative to its volume, making energy production less efficient.

Therefore, many small mitochondria are more efficient than one large mitochondrion because they provide a greater surface area for energy production.

The correct explanation for these observations is statement (iii): Water moved into Cell X and moved out of Cell Y through the cell membrane.

This phenomenon is explained by the process of osmosis, which is the movement of water across a selectively permeable membrane.

• For Cell X: Pure water is a hypotonic solution, meaning it has a higher water concentration and lower solute concentration than the inside of the cell. Consequently, water moves from the surrounding medium into the cell, causing it to swell.
• For Cell Y: A concentrated salt solution is a hypertonic solution, meaning it has a lower water concentration and higher solute concentration than the inside of the cell. This causes water to move out of the cell into the surroundings, leading the cell to shrink.

The correct option is (i) Leucoplast | Cell wall.

Why the other options are incorrect:

• Option (ii): While mitochondria are present in plant cells, ribosomes are NOT absent in animal cells; they are found in both types of cells.
• Option (iii): While a cell wall is present in plant cells, the Golgi apparatus is NOT absent in animal cells; it is a standard organelle in both plant and animal cells.
• Option (iv): While lysosomes are found in eukaryotic cells, the Endoplasmic reticulum (ER) is NOT absent in animal cells; it is a large organelle that spreads throughout the cytoplasm of both plant and animal cells.

Renu is correct.

• Plastids are present in all living plant cells, including root cells.
• Rohit is partially correct, roots do not contain chloroplasts because they are underground and do not perform photosynthesis.
• Instead, root cells contain leucoplasts, which are colourless plastids that store food such as starch, oils, and proteins.
• Therefore, plastids are not absent in roots; only chloroplasts are absent.

Hence, Renu's statement is correct because roots contain leucoplasts, not chloroplasts.

Similarities :

• Both are double-membrane organelles.
• Both contain their own DNA and ribosomes.
• Both are involved in energy conversion and can replicate independently.

(ii) Mitochondria, Nucleus

(i) Hypothesis:

• The researcher wants to test that water moves into or out of plant cells by osmosis depending on the concentration of the surrounding solution.

(ii) Improvement:

• Cut the carrots into equal-sized pieces.
• Measure and record their initial and final weight to compare the changes accurately.

(iii) Reason:

• In plain water, water enters the carrot cells by osmosis, making the cells turgid, so the carrot remains stiff and crunchy.
• In the concentrated salt solution, water moves out of the carrot cells by osmosis, making the cells flaccid, so the carrot becomes rubbery and limp.

(i) Why does water gather in Cup B and Cup C?

• Sugar in Cup B and salt in Cup C create a concentrated solution inside the potato cups.
• Water moves from the beaker into the potato cups through the living cell membranes by osmosis.
• Therefore, water collects in the hollowed portions of Cups B and C.

(ii) Why is Cup A necessary?

• Cup A acts as the control for the experiment.
• It shows that without sugar or salt, water does not collect in the potato cup.

(iii) Why does water not gather in Cups A and D?

• Cup A: There is no concentration difference, so osmosis does not occur.
• Cup D: The potato is boiled, so its cell membranes are damaged and no longer selectively permeable. Therefore, osmosis does not occur, and water does not collect in the hollowed portion.

(ii) SER — Lipid and cellulose synthesis : (Incorrect)

• While the Smooth Endoplasmic Reticulum (SER) is involved in the synthesis and storage of fats (lipids) and hormones, it is not responsible for the synthesis of cellulose

If all the mitochondria are removed from a eukaryotic cell:

• The cell would not be able to produce enough ATP (energy) because mitochondria are the powerhouses of the cell.
• Most cellular activities, such as growth, repair, active transport, and cell division, would stop due to lack of energy.
• As a result, the cell would stop functioning and eventually die.

Therefore, mitochondria are essential for the survival and proper functioning of eukaryotic cells.

The phenomenon that inhibits the formation of tumors in the human body is contact inhibition.

• Contact inhibition is the process in which normal animal cells stop dividing when they come into contact with neighbouring cells, preventing uncontrolled cell growth and tumor formation.
• If contact inhibition is lost, cells may divide continuously, leading to the formation of tumors or cancer.

Can plants also develop tumors?

• Yes, plants can develop tumors, but not due to the absence of contact inhibition. Plant tumors are usually caused by infections, such as the bacterium Agrobacterium tumefaciens, which induces uncontrolled growth (crown gall disease).
• These tumors generally do not spread (metastasize) like cancer in animals because plant cells are enclosed by rigid cell walls.

The cell membrane is made up of proteins and lipids.

• Proteins are synthesized by the Rough Endoplasmic Reticulum (RER).
• Lipids are synthesized by the Smooth Endoplasmic Reticulum (SER).
• Both proteins and lipids are transported to the Golgi apparatus, where they are modified, packaged into vesicles, and then carried to the cell membrane, where they become part of the membrane.

Path of Cell Membrane Components :

• Proteins: RER → Golgi apparatus → Vesicles → Cell membrane
• Lipids: SER → Golgi apparatus → Vesicles → Cell membrane

Labelled Diagram

If gametes were formed by mitosis instead of meiosis:

• Gametes would have the same number of chromosomes as body cells instead of half.
• After fertilization, the zygote would have double the normal number of chromosomes.
• The chromosome number would keep doubling in each generation.
• This would lead to abnormal development, and normal reproduction would not be possible.

Therefore, meiosis is essential to maintain the correct chromosome number from one generation to the next.

(i) Which scientific concept has the farmer applied?

• The farmer has applied the principle of osmosis in food preservation.

(ii) How do salt and sugar prevent spoilage?

• High concentrations of salt or sugar draw water out of bacteria and fungi by osmosis.
• Loss of water causes these microorganisms to dehydrate and stop growing, thereby preventing food spoilage.

(iii) Suggest a healthy recipe for food preservation.

• A healthy recipe is lemon pickle or amla murabba, prepared using salt or jaggery and stored in clean, airtight glass jars.

(iv) What scientific values are addressed?

• Scientific application of osmosis in food preservation.
• Prevention of food wastage.
• Sustainable use of farm produce.
• Value addition and entrepreneurship to increase farmers' income.