P S Verma V K Aggarwal Biology%e2%80%932019 2020 for Class 9 Science Chapter 2 - The Fundamental Unit Of Life Cell

Answer:

Answer:

Answer:

Answer:

Answer:

Discovery of cell:
Cell was first discovered and named in 1665 by an English scientist Robert Hooke. He was examining a thin slice of cork and observed that the cork resembled the structure of honeycomb consisting of many little compartments. He called these compartments as cells (meaning 'little room' in Latin).
In 1838, Jakob Matthias Schleiden first proposed the idea that all plants consist of cells.
In 1839, Theodor Schwaan proposed that all animals and plants are made up of cells.

Answer:

Plasma membrane consists of two types of proteins that floats in the fluid phospholipid layer:
(i) Intrinsic proteins: They completely cover the lipid bilayer.
(ii) Extrinsic proteins: They occur either on the outer surface or on the inner surface of the lipid membrane.

Answer:

Plasma membrane serves as a barrier for the entry and exit of materials in the cells. It allows only certain selective materials to pass through it. Apart from this, it also performs certain physical and biological activities. These are as follows:
(i) Diffusion.
(ii) Osmosis.
(iii) Active transport.
(iv) Endocytosis.

Answer:

Diffusion is the process of movement of molecules from a region of high concentration to a region of lower concentration until an equilibrium is reached.
When concentration of gases such as CO₂ gets high in the cell as compared to the external environment, then CO₂ moves out of the cell by the process of diffusion via plasma membrane. Similarly, O₂ enters the cell by the process of diffusion when its concentration is reduced in the cell.

Answer:

Osmosis is the movement of water from a region of higher water concentration to the region of lower water concentration through a semi-permeable membrane.
Following experimental set up explains the process of osmosis.

Answer:

Answer:

Endocytosis:
The ingestion of materials by the cells through the plasma membrane is called endocytosis. It includes three processes—that is,
(a) phagocytosis (cell eating),
(b) photocytosis (cell drinking), and
(c) receptor-mediated endocytosis.
 All of these processes require energy, so these may be considered as different forms of active transport.

Answer:

When we place an eukaryotic cell in hypotonic solution, it swells up and ultimately bursts because of the large quantity of water entering the cell.
When we place an eukaryotic cell in isotonic solution, there is no change in its size, as the water concentrations in the external and internal media are the same.
When we place an eukaryotic cell in hypertonic solution, it gets plasmolysed because of the excess loss of water.

Answer:

(a) Ribosome is the smallest cell organelle.
(b) Nucleus is the largest cell organelle.
(c) ER studded with ribosomes is referred to as rough endoplasmic reticulum (RER).
(d) Genes are the functional segments of the DNA molecule.

Answer:

(a)

Answer:

Answer:

If the nucleus of a cell is removed, then the cell will die. This is because the nucleus of a cell controls all the metabolic activities in a cell, and if it is not present, then the metabolic functioning of the cell will stop and its protoplasm will dry up ultimately, causing cell death.

Answer:

Spinach looks green due to the presence of a green pigment called chlorophyll in them. Similarly, papaya looks yellow and edible part of watermelon looks red due to the presence of coloured plastids called chromoplasts in these fruits. Chromoplasts lack chlorophyll but contains carotenoid pigments that give different colours to these fruits.

Answer:

(a) Functions of endoplasmic reticulum are as follows:
(i) It forms a supporting skeletal framework of the cell.
(ii) It provides a pathway for the distribution of nuclear material from one cell to the other.

(b) Functions of lysosomes are as follows:
(i) They remove the worn out or poorly working cellular organelles by digesting them to make way for new cells.
(ii) They destroy foreign materials such as bacteria and viruses that enter the cell.

Answer:

(a) Ribosome is the cell organelle that synthesises protein.
(b) Leucoplast is the type of plastid that stores food.

Answer:

'Lysosomes are known as suicide bags' because the enzymes of lysosomes eat up their own cells if the cells get damaged. This is referred to as autolysis or self-digestion in which the cell is destroyed by the action of its own enzymes.

Answer:

(a) Cell inclusions: They are considered to be the non-living components of the cell that do not possess metabolic activity and are not bounded by membranes.
(b) Cytosol: It refers to the soluble part of cytoplasm that contains protein fibres and is located between the cell organelles.
(c) Protoplasm: It is the living content of the cell that is surrounded by the plasma membrane.
(d) Nucleoplasm: It is the ground substance of the nucleus enclosed by a nuclear envelope.

Answer:

Ribosomes are composed of RNA (Ribonucleic acid) and proteins. Ribosomes are synthesised in the nucleolus part of the nucleus as nucleolus is rich in RNA and protein molecules. Nucleolus is referred to as factory of ribosomes.

Answer:

(a) Mitochondria:
These are the tiny bodies of varying shape located in the cytoplasm. They are self-replicating organelles as they have their own DNA. They synthesise energy-rich compounds (ATP); hence, they are referred to as powerhouse of the cell.
(b) Plastids:
They are present in only plant cells. They have their own DNA and ribosomes and are self-replicating. Their function is to provide colour to plants (chromoplasts), to perform photosynthesis (chloroplasts), and to store food in the form of starch, fats, and proteins (leucoplasts).

Answer:

Name of cell organelles are as follows:
Nucleus, endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus, lysosomes, plastids, chloroplasts, vacuoles, peroxisomes, and centrosomes.

Answer:

The main functional regions of the cell are as follows:
(i) Nucleus: It is responsible for all the metabolic activities of the cell.
(ii) Mitochondria: It synthesises energy-rich compounds (ATP) in the cell that is then used in various processes.
(iii) Ribosomes: They are responsible for the synthesis of proteins in the cell.

Answer:

(a) Chromatin is located in the nucleoplasm of the nucleus.
(b) Chromosomes are located in the nucleus.
(c) Tonoplast forms the outer covering of the vacuole.
(d) Nucleolus is located in the nucleoplasm of the nucleus.

Answer:

Genes are the functional segments of DNA. They contain the hereditary information of the cell. They are located in the nucleus of the cell.

Answer:

'Lysosomes are also called digestive bags' because they destroy the foreign materials such as bacteria and viruses that enter the cells by digesting them and protects the cell from viral and bacterial infections.

Answer:

Mitochondria is known as the 'power plant' of eukaryotic cell. Its function is to provide energy to the cell. It synthesises energy-rich compound (ATP) by the process of cellular respiration that stores energy. The energy stored in ATP is used by the cell for various processes.

Answer:

Centrioles are hollow and cylindrical structures made up of microtubules. Two centrioles combine to form a centrosome.
The function of a centriole is to form spindle fibres during cell division in animal cells.

Answer:

Lipids and proteins constituting the plasma membrane are synthesised by the endoplasmic reticulum.
Smooth endoplasmic reticulum (SER) is involved in the synthesis of lipids, whereas rough endoplasmic reticulum (RER) synthesises proteins.

Answer:

                                   Prokaryotic cell

Answer:

Plasmolysed cell refers to a cell that has shrunk due to loss of water. It occurs if the cell is kept in a hypertonic solution in which the concentration of water is less than that of the cell. Water crosses the plasma membrane in both directions, but more water leaves the cell than it enters, causing the cell to shrink.

Answer:

Fluid mosaic model of plasma membrane:



It was proposed by Singer and Nicolson to explain the ultrastructure of the plasma membrane of the cell. It can be described as 'a number of protein icebergs floating in a sea of lipids'. This is the most accepted model as it describes the properties as well as the organisation of plasma membrane.
According to this model, plasma membrane is made up of a bilayer (two molecules thick layer) of phospholipids. The following are the two types of proteins that float in the layer of phospholipid fluid:
(i) Intrinsic proteins: It completely covers the lipid bilayer.
(ii) Extrinsic proteins: It occurs either on the outer surface or on the inner surface of the lipid membrane.
Proteins and lipids provide flexibility to the plasma membrane that helps it in the movement of molecules in and out of the cell.

Answer:

Following are the main cell organelles:
(i) Nucleus: It controls all the metabolic activities of a cell and regulates the cell cycle.
(ii) Endoplasmic reticulum: They provide skeletal framework to the cell and are concerned with the production of lipids and proteins.
(iii) Ribosomes: They play an important role in protein synthesis.
(iv) Mitochondria: They are the powerhouse of the cell as they provide energy to the cell by performing cellular respiration.
(v) Golgi apparatus: It is involved in the synthesis of cell wall, plasma membrane, and lysosomes.
(vi) Lysosomes: They are the cellular housekeepers and form a kind of garbage disposal system of the cell by removing worn out or damaged cellular organelles.
(vii) Vacuoles: They help in maintaining the osmotic pressure in the cell. and also in storing excess nutrients and molecules.
(viii) Peroxisomes: They carry out certain oxidative reactions (detoxification) and remove the toxic substances from the cell.
(ix) Centrosome: It helps in cell division in animal cell.
 

Answer:

Nucleus:
Nucleus is a large, centrally located spherical component of cell. It is enclosed by two nuclear membranes, collectively called nuclear envelope that are connected to the endoplasmic reticulum. Nuclear envelope contains many nuclear pores and encloses the nucleoplasm (liquid ground substance). Nuclear pores help in the transfer of materials between the nucleoplasm and the cytoplasm. Nucleolus (one or more in number) and chromatin material are embedded in the nucleoplasm. Nucleolus is rich in proteins and RNA molecules and acts as the site for ribosome formation; hence, it is referred to as ribosome factory.
The chromatin material is a thin thread-like inter-coiled mass of chromosomes and contains DNA and proteins. It is formed by repeated units of nucleosomes. It is condensed into two or more thick ribbon-like chromosomes during cell division.


Functions of nucleus:
(i) It controls all the metabolic activities of the cell.
(ii) It regulates the cell cycle.
(iii) It is responsible for the transfer of heredity traits from parent to offspring.

Answer:

(a) Golgi apparatus:



Golgi apparatus consists of a set of membrane-bounded, fluid-filled vesicles, vacuoles, and flattened cisternae. Cisternae are stacked together in parallel rows. They are formed at one end of the stack, called cis face of Golgi, and are budded off as vesicles at the other face, called trans face of Golgi.
Golgi apparatus arises from the membrane of smooth endoplasmic reticulum. Golgi saccules (cisternae at cis and trans face) are constantly renewed. Distal saccules (trans face) disappear after vesicle formation.
The main function of Golgi apparatus is to secrete vacuoles or secretory vesicles that contain cellular secretions such as enzymes, proteins, and cellulose.

(b) Mitochondria:



Mitochondria are tiny bodies of varying shapes and sizes distributed in the cytoplasm. They are bounded by a double membrane envelope. The inner membrane is folded several times forming cristae. Each cristae are studded with small round bodies known as F₁ paricles or oxysomes. Matrix of mitochondria contains ribosomes, DNA molecule, and phosphate granules.
The main function of mitochondria is to synthesise ATP (energy-rich molecule) and provide energy to the cell via cellular respiration; hence, they are called as powerhouses of the cell.