NCERT Solutions for Class 11 Biology Chapter 8 - Cell : The Unit of Life

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(a) Robert Brown discovered the cell

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(d) abiotic materials

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1. (b) 2. (c) 3. (a)

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(d) Cells are formed de novo from abiotic materials

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Mesosome. It is the name given to infoldings of the cell membrane in a prokaryotic cell. Mesosomes may be in the form of vesicles, tubules and lamellae.
Functions of mesosomes. The functions performed by the mesosomes are as follows :

1. Mesosomes help in cell wall formation.
2. They help in DNA replication and distribution to daughter cells.
3. They also help in respiration, in secretion processes, increase plasma membrane surface area and enzymatic content.

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Neutral solutes move across the membrane by the process of simple diffusion along the concentration gradient i.e. from a region of higher concentration to the lower. An example of such movement of water molecules by diffusion across a semi-permeable membrane is called osmosis.
No, the polar molecules cannot move across the membrane in the same way because of non-polar lipid bilayer, so they require a carrier protein of the membrane to facilitate their transport.

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Two double membrane bound organelles are :
— Mitochondria
—Chloroplast
Characteristics of Mitochondria
Refer to Basis and basics
Characteristics of Chloroplast

1. Chloroplasts are a type of plastids which are a characteristic of green protists and plant cells.
2. They are usually spherical, ovoid, discoidal-shaped, cup or girdle-shaped.
3. They range in size from 4-10 mm in long axis.
4. Higher plants often have 20-40 chloroplasts per cell but may be as high as 500.
5. Chloroplasts are double membranous structures; the space between the membranes is called intermembrane space. Of the two, the inner chloroplast membrane is relatively less permeable.
6. The space limited by the inner membrane of the chloroplast is called the stroma that has a number of organized flattened membranous sacs called the thylakoids.
7. Thylakoids are arranged in stacks like the piles of coins called grana. In addition, there are flat membranous tubules called stromal lamellae connecting the thylakoids of the different grana.

Functions of Chloroplasts

1. The most important function of chloroplast is to carry out the process of photosynthesis by trapping the radiant energy of sunlight and transform it into the chemical energy of glucose formed from water and carbon dioxide.
2. Chloroplasts also supply oxygen, a by-product of photosynthesis, to all aerobic organisms for respiration.
3. They also temporarily store starch in the form of pyrenoids.
4. Chloroplasts utilise CO2 keeping its concentration in the air normal.

Structure of chloroplast

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Prokaryotic cells are primitive cells.

1. They are bounded by single membrane.
2. These cells lack membrane bound cell organelles.
3. They have circular DNA without proteins, forming a single chromosome and lying in direct contact with cytoplasm.
4. They have 70S ribosomes.
5. Sap vacuoles, mitotic apparatus and meiosis are absent.
6. They do not show cyclosis, phagocytosis, pinocytosis and exocytosis.
7. Flagella, if present, are simple and formed of a protein flagellin.
8. They multiply rapidly.

Examples. Bacteria, blue green algae, mycoplasma or pplo, spirochaetes and rickettsiae.

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Cells in multicellular organisms are grouped to form tissues, organs and organ systems. This help in easy, quick and efficient completion of physiological functions. e.g. digestive system, pumping activity of heart, relay of impulse by nervous tissue. Division of labour provides better coordination and organisation to multicellular organisms as the cells are not completely independent but show interactions among themselves so that they may function in a coordinated manner. This is, thus, a division of labour and the different cells become specialized through differentiation to carry out distinct activities of the body like movement (muscle cells) and conduction (nerve cells). So, a multicellular organism is a sum total of this coordination carried out by different cells.

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A cell is the structural and functional unit of life. This fact implies that all living organisms are made up of cells. A multicellular organism contains numerous cells, each adapted to perform a particular function. The processes carried out by complete living bodies are also performed by the cells e.g. respiration, reproduction etc.

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Nuclear pores are minute, circular perforations present at a number of places in the nuclear envelope. These are formed by the fusion of two nuclear membranes. There may be 1,000 to 10,000 pores per nucleus.
The nuclear pores are the passages through which movement of macromolecules like trnas, mrnas, ribosomal proteins, nucleotides, enzymes takes place in both directions.

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Endomembrane structures are those structures whose functions are coordinated upto a large extent. Lysosomal vesicles are very rich in almost all types of hydrolytic enzymes called hydrolases-lipases,proteases, carbo-hydrases etc. which can digest lipids, proteins, carbohydrates.
On the other hand, the vacuoles perform functions like maintenance of turgor pressure, absorption of excess water, storage of water soluble compounds and deposits of unwanted materials like toxins.

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(i) Nucleus (Fig. 8.14). The nucleus was first described by Robert Brown in 1831. It is the most important organelle of the cell and is the main seat for coordination of activities of the cell. The nucleus has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called nucleolus. The nucleus is bounded by two parallel membranes with an enclosed space called the perinuclear space which acts as a barrier between the materials of the nucleus and the cytoplasm. The outer membrane continues with the endoplasmic reticulum and also bears ribosomes on it. At a number of places, the nuclear envelope is interrupted by minute pores formed by the fusion of the two membranes.
The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. Nucleolus is not a membrane-bound structure and, therefore, its contents are continuous with the rest of the nucleoplasm. It is a site for active ribosomal RNA synthesis. The chromatin contains DNA and basic proteins called histones, some non-histone proteins and also RNA.

Structure of centriole of centrosome

(ii) Centrosome. Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar material called centrosphere. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin. Each of the peripheral fibril is a triplet. The adjacent triplets are also linked. The central part of the centriole is also proteinaceous and called the hub which is connected with tubules of the peripheral triplets by radial spokes made of protein. The centrioles form the basal body of the cilia or flagella, and the spindle fibres that give rise to spindle apparatus during cell division in animal cells.

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A notch like primary constriction present in the chromosome is called a centromere on the sides of which disc-shaped structures called kinetochores are present. Based upon the relative position of the centromere over a chromosome, the chromosomes are classified into various types which are as follows :

1. Metacentric Chromosome. In this type, the position of the centromere is exactly in the middle of the chromosome such that the two arms on either side of the chromosome are exactly equal in length.
2. Submetacentric chromosome. It has centromere nearer to one end of the chromosome resulting into one shorter and one longer arm.
3. Acrocentric Chromosome. In case of acrocentric chromosome, the centromere is situated close to one end forming one extremely short and one very long arm.
4. Telocentric Chromosome. When the centromere is terminal in position, the chromosome is of telocentric type.