Cell: The Unit of Life Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1Exactly one of the four is false, so identify it and give the reason, and then confirm the other three.
2. 2Option (a) is the incorrect one. Robert Brown did not discover the cell. It was Robert Hooke who described the first cell, in 1665, in the cork of the Calotropis, naming the dead rectangular boxes he saw the cells. Robert Brown's contribution was a different one, for in 1831 he discovered the nucleus in the orchid cells of Orchis, and he also described the cell in an unstained material and the Brownian movement. So the discovery of the cell belongs to Hooke and the discovery of the nucleus to Brown.
3. 3Option (b) is correct. Matthias Schleiden, a botanist, and Theodor Schwann, a zoologist, together formulated the cell theory in 1838-1839, having proposed independently that all plants and all animals are made up of cells and that the cell is the basic structural and functional unit of the body. So the two names are to be kept and the statement stands.
4. 4Option (c) is correct. Rudolf Virchow, in 1855, gave the third proposition of the cell theory, that cells are formed from pre-existing cells, omnis cellula-e cellula, and he based it on his own and on others' observations and rejected the earlier view of spontaneous generation. This is the correction of the last of Schleiden and Schwann's original statements.
5. 5Option (d) is correct. In a unicellular organism a single cell is the whole body, and it carries out all the life activities, the metabolic and the reproductive, by itself. This is exactly why a single cell is the basic unit of life, and it is the reason the cell can be called both structural and functional.

Step-by-step solution

1. 1The answer is (c) pre-existing cells, and the whole question turns on the last of Virchow's propositions.
2. 2Rudolf Virchow in 1855 stated that every cell arises from a pre-existing cell, omnis cellula-e cellula, and this is the third and final proposition of the cell theory, added to the two of Schleiden and Schwann. The first was that all living organisms are composed of cells and their products, and the second that the cell is the basic structural and functional unit of all living organisms, and Virchow added the third, that all cells arise from pre-existing cells, having tested and rejected the earlier idea of spontaneous generation.
3. 3The two wrong options are worth explaining. The idea of spontaneous generation, which is the nearest of them to option (d) abiotic materials, held that life arose from non-living matter, and Virchow's work, along with that of Pasteur, showed that it does not happen; life is continuous and a cell never arises de novo. Option (a), bacterial fermentation, is a metabolic process of some bacteria and has nothing to do with the origin of cells. Option (b), regeneration of old cells, is wrong because a cell divides to produce two daughter cells rather than being built up again out of old cell substance; the old cell is replaced by the new ones, and this is a division, not a regeneration of the old material.
4. 4The supporting evidence is worth one line. Cells divide, and each division gives two daughter cells, so the number of cells increases only by the division of cells already present, and this was demonstrated for every kind of cell and for unicellular organisms, where the whole organism divides and becomes two organisms.

Step-by-step solution

1. 1The three matches are (a) to (ii), (b) to (iii) and (c) to (i), and each is decided by the structure of the organelle and the position of the infolding or sac.
2. 2(a) Cristae — (ii) Infoldings in mitochondria. The cristae are the folds of the inner membrane of the mitochondrion that project inwards into the matrix, and they are the sites of the electron transport system and of the ATP synthesis, so infoldings of the inner membrane increase the surface available for these reactions.
3. 3(b) Cisternae — (iii) Disc-shaped sacs in Golgi apparatus. The cisternae are the stacks of flattened, disc-like or tubular sacs of the Golgi apparatus that make up the Golgi body, and they are bounded by a single membrane and involved in the modification, packaging and transport of the proteins and lipids received from the endoplasmic reticulum.
4. 4(c) Thylakoids — (i) Flat membranous sacs in stroma. The thylakoids are the flat membranous sacs or discs lying within the stroma of the chloroplast, and they are arranged in stacks called grana, singular granum, joined by stroma lamellae, and they contain the chlorophyll, so they are the site of the light reaction of photosynthesis.
5. 5It is worth adding the memory aid, since the question turns on confusing these three: the crista belongs to the mitochondrion, the cisterna to the Golgi and the thylakoid to the chloroplast, and the mitochondrion is the only one of the three where the named structure is a fold or infolding rather than a sac.

Step-by-step solution

1. 1Exactly one option is correct, and it is (c). Give the reason for it and the reason for rejecting each of the other three.
2. 2Option (c) is correct. Prokaryotic cells have no membrane-bound organelles; the prokaryotic cell is bounded by a plasma membrane of a single unit membrane with the transport and the secretory functions and it lacks the usual compartmentalisation. The cell envelope, the plasma membrane and the cell wall where present, is the only membrane system, and there is no endoplasmic reticulum, no Golgi, no mitochondria, no lysosomes and no nucleus, so the cell lacks the compartmentalisation that a eukaryotic cell has.
3. 3Option (a) is wrong because the prokaryotic cell has no true nucleus, since the genetic material is a naked, circular double-stranded DNA lying in the cytoplasm in a region called the nucleoid, without being surrounded by a nuclear membrane. So the nucleus is absent in the prokaryotes, and it is only the eukaryotic cell that has a membrane-bound nucleus and the rest of the membrane-bound organelles.
4. 4Option (b) is wrong because only the plant cells of the two have a cell wall, the plant cell wall being made of cellulose, hemicellulose, pectin and proteins, whereas the animal cells have no cell wall and possess only a plasma membrane. Fungi and bacteria also have cell walls, but the animal cell does not, so the two do not both possess one.
5. 5Option (d) is wrong on Virchow's principle, since cells are never formed from abiotic material, and every cell arises from a pre-existing cell, omnis cellula-e cellula, and this is the third proposition of the cell theory.

Step-by-step solution

1. 1Define the mesosome, then give the functions in a list. This is one of the NCERT points that is asked repeatedly, and the examiner wants the four functions.
2. 2A mesosome is a membranous infolding or invagination of the plasma membrane of a prokaryotic cell that projects inwards into the cytoplasm, and it appears as a vesicle, tubule or lamellar structure, and a group of these mesosomes is called a mesosome, or the plural mesosomata. It is a characteristic feature of the prokaryotic cell and is visible only in certain prokaryotes and in a few of the eukaryotes, having arisen by the infolding of the plasma membrane.
3. 3First function, respiration. The mesosome is a membranous structure that can act as an analogue of the mitochondrion, for it carries out the electron transport and the oxidative phosphorylation and so functions as a respiratory apparatus, since the prokaryotic cell has no mitochondria.
4. 4Second function, the formation of the cell wall. The mesosome helps in the formation of the new cell wall, since the infolding of the membrane and the new wall material secreted by it appear in the septa that form during cell division, so the mesosome is concerned in cell wall formation, in the septum and in the cell division itself.
5. 5Third function, DNA replication and the attachment of the DNA. The mesosome helps in the replication of the DNA and the formation of the daughter cells, and it is the site at which the circular chromosomal DNA is attached, so it assists in the distribution of the DNA to the daughter cells during division.
6. 6Fourth function, secretions. It also helps in the secretion processes and in the increase in the surface area of the plasma membrane, so a larger surface is available for the transport and the secretory activities that the membrane carries out. So the four functions to list are respiration, cell wall formation, DNA replication and helping in cell division and in secretions.

Step-by-step solution

1. 1The question is in three parts and must be answered in that order: the movement of neutral solutes, then the answer that polar molecules cannot move in the same way, then how they are transported. Keep the two processes strictly apart, since mixing simple diffusion with facilitated diffusion is the usual error.
2. 2Neutral solutes, that is small, non-polar and lipid-soluble molecules such as oxygen, carbon dioxide, nitrogen, water and glycerol, move across the plasma membrane by simple diffusion. The mechanism is that the membrane has a lipid bilayer, the lipids being arranged with their hydrophilic polar heads outward and their hydrophobic non-polar tails inward, so the interior of the bilayer is hydrophobic. A small non-polar molecule is soluble in this hydrophobic interior, so it passes from a region of higher concentration to a region of lower concentration, purely down the concentration gradient, at a rate that depends on the size of the molecule and on the concentration gradient, and no protein carrier and no energy is required, so it is passive.
3. 3Polar molecules cannot move across in the same way. The reason is structural: the interior of the lipid bilayer is hydrophobic and non-polar, so a polar or charged molecule is insoluble in it and cannot pass through the lipid phase, and it cannot cross without a protein. So the rate of passage of a polar molecule through the lipid phase is negligible, whatever the concentration gradient.
4. 4Polar molecules are therefore transported by facilitated diffusion, in which a protein helps them across, and there are two kinds of protein and the exam wants both named. Small and medium-sized polar molecules, such as glucose, sucrose and amino acids, are carried by carrier proteins, so the transport is carrier-mediated facilitated diffusion, and the solute binds to a specific site on the carrier, the carrier changes shape and releases the solute on the other side, and the process is still passive and still down the concentration gradient, with no energy being used. Large and macromolecules such as proteins and polysaccharides, and polar molecules generally, pass by the second method, in which the plasma membrane invaginates around them and the membrane bulges and pinches off to form a small membrane-bound vesicle, and this is called endocytosis, with phagocytosis for the solids and pinocytosis for the liquids; the reverse process by which the vesicle fuses and releases its contents is exocytosis.
5. 5So the contrast to close with is that the neutral solutes use the lipid phase directly by simple diffusion, and the polar molecules must use a protein, either a carrier for the small ones by facilitated diffusion, or the vesicle mechanism of endocytosis for the large ones. Neither method uses energy, so both are passive, and the difference is structural rather than energetic.

Step-by-step solution

1. 1The two are the mitochondrion and the plastid, and the question is a drawing question, so describe the figure and the labels for each, and give the characteristics and the functions in a list.
2. 2First, why double membrane: a double membrane is a characteristic of the mitochondria and the plastids, since these two arose by endosymbiosis, and a cell that has double membrane organelles is regarded as having a double membrane, that is the whole cell is bounded by a double membrane. This is the premise of the question, so state it first.
3. 3The mitochondrion. Structure: a small, rod-shaped or oval organelle, and it is a double membrane organelle, the outer membrane and the inner membrane being two separate membranes, and the inner membrane is thrown into folds called cristae, which project inwards into the matrix, and the cristae are the site of the electron transport system and of the synthesis of ATP. Inside is the matrix, which contains circular double-stranded DNA, 70S ribosomes, and the components of protein synthesis, so the mitochondrion is partly self-contained and behaves as a semi-autonomous organelle, and many enzymes for the Krebs cycle are also present in the matrix. The two membranes together are called the envelope.
4. 4Mitochondrion, functions: it is called the powerhouse of the cell because it is the site of aerobic respiration, so it is the principal site of the synthesis of ATP by oxidative phosphorylation, it is the principal site of the Krebs cycle and of the oxidation of pyruvate, and therefore of carbohydrate and of fat metabolism. It also stores calcium ions, so it is concerned in the regulation of the calcium level of the cell, it is called the suicide bag of the cell because it is the site of the enzymes of apoptosis, it is a semi-autonomous organelle since it contains its own DNA, RNA, ribosomes and can make some of its own proteins, and it helps in the synthesis of certain amino acids. The diagram should be a cut-away showing the outer membrane, the inner membrane with its cristae, the matrix, the circular DNA and the 70S ribosomes.
5. 5The plastid, and the types. There are three kinds of plastid, and this is worth stating since it explains the double membrane: the chloroplast, the chromoplast and the leucoplast. The chloroplast contains chlorophyll and is the site of photosynthesis; the chromoplast contains coloured pigments, the carotenes and the xanthophylls, and gives the red, orange and yellow colour to flowers, to fruits such as the tomato and the carrot and to autumn leaves; and the leucoplast is the colourless plastid that stores food, the amyloplast storing starch as in the potato and the elaioplast storing oils and fats as in the groundnut, with the allanioplast storing protein.
6. 6The chloroplast, structure: it is a double membrane organelle, and the outer membrane is smooth while the inner membrane is thrown into folds, though the folds here are not called cristae. Inside the stroma there are flat membranous sacs called thylakoids, and the thylakoids are arranged into stacks called grana, singular granum, and the grana are connected by the stroma lamellae or the intergranal lamellae. The lamellae of two different grana are connected by the stroma lamellae, and the chloroplast also contains chlorophyll, the 70S ribosomes, circular DNA and the enzymes of the dark reaction, so it too is semi-autonomous. The diagram should be labelled for the outer and the inner membrane, the stroma, a granum, a thylakoid, the stroma lamella and the grana.
7. 7The plastid, functions: the chloroplast is the site of photosynthesis, and it converts light energy to chemical energy, storing the energy in the form of sugars, so it is the site of both the light and the dark reactions. The chromoplast gives the colour to the parts of the plant and so attracts the pollinators and the seed dispersing animals, and it is the source of the pigments such as the carotene in the carrot, the capsanthin in the red pepper and the lycopene in the tomato. The leucoplasts store reserve food, starch, oil and protein, which is the food the plant keeps for later use. And all three types, being derived from the same proplastid of the meristematic cells, are interconvertible, so a green chloroplast in a leaf can become a chromoplast in a flower and a fruit, which is worth adding.

Step-by-step solution

1. 1This is a long-answer question worth well over the usual marks, so organise it under clear headings. The safest order is: what a prokaryote is, the size and the shape, the cell envelope, the lack of a membrane-bound nucleus, the ribosomes, the lack of other organelles, the cell wall, the genetic material, the inclusions, the flagella and the pili, and the division. The first thing to state is that the prokaryotic cell is the primitive cell and represents the earliest form of life, since a prokaryote is a primitive, simple, single-celled organism with a very simple organisation and a cell that lacks a well-defined nucleus and the other membrane-bound organelles.
2. 2The size and the shape: the prokaryotic cell is smaller than the eukaryotic cell, usually about 0.5 to 5 micrometres, so that in general a prokaryote ranges from 0.5 to 5.0 micrometres, while the smallest bacterium, the Mycoplasma, is only about 0.1 micrometre, and the largest, the giant bacterium of the deep sea, is a few micrometres. The shapes are diverse, and the commonest are the bacillus or rod-shaped, the coccus or spherical, the vibrio or comma-shaped, the spirillum or the spirillum, and the cilia or the flagella, and the commonest shape overall is the coccus, the bacillus being the second commonest.
3. 3The cell envelope and the plasma membrane: the cell is surrounded by a cell envelope consisting of the glycocalyx or the slime layer, the cell wall and the plasma membrane in that order from outside in. The glycocalyx is a mucilaginous and viscous layer, and in bacteria it is a capsule or a slime layer, while in the cyanobacteria it is a mucilaginous sheath. The plasma membrane is a single unit membrane of phospholipid, protein and carbohydrate, of the fluid mosaic type, and it carries out the transport and the secretory functions, and it is the site of respiration, since the mesosomes act as the respiratory apparatus. The cell wall is present in most prokaryotes and is made of peptidoglycan, the murein, or of a heteropolymer, the archaeal pseudopeptidoglycan in the archaebacteria, and the diaminopimelic acid is absent in the Gram-positive bacteria, and the peptidoglycan is a polymer of the N-acetyl glucosamine and the N-acetyl muramic acid units joined by peptide bonds, so a peptide-rich wall is present in the Gram-positive form. The eubacteria are distinguished by the muramic acid and the diaminopimelic acid, and the archaebacteria differ in having a pseudopeptidoglycan wall, a branched-chain lipid membrane and the absence of peptidoglycan. The Gram-positive and the Gram-negative bacteria are distinguished by their wall and their staining reaction, the Gram-positive having a thick peptidoglycan wall, the teichoic acids and the polysaccharides, and staining purple, and the Gram-negative having a thin peptidoglycan layer with an outer membrane of lipopolysaccharide and staining pink.
4. 4The absence of a membrane-bound nucleus is the defining character: the genetic material is not enclosed within a nuclear membrane, and the single circular double-stranded DNA molecule is naked and lies loosely in the cytoplasm in a region called the nucleoid, and a membrane-less body in which the DNA is seen is called the nucleoid, while the cytoplasm also has a few inf granules. There is no nuclear membrane, no nucleolus and no spindle, and there is no mitosis and no meiosis, since the division is amitotic or binary fission. The nuclear membrane is absent, and the absence of a membrane-bound nucleus is the single most important feature of a prokaryote, since it is what the word prokaryotic means, a nucleus before the true nucleus.
5. 5The ribosomes are the one organelle-like body that a prokaryote does have, and they are essential to note as a counter-example to the usual claim that a prokaryote has no organelles: the ribosomes are made of two subunits, the 30S and the 50S, which together give an S value of 70S, in contrast to the 60S and the 40S of the eukaryotic cell which together give 80S. So the ribosome of a prokaryote is 70S and smaller than the 80S of the eukaryote, and this difference in the sedimentation coefficient is the basis of the studies with the streptomycin and the chloramphenicol sensitivity, since an antibiotic that acts on the 70S ribosome will act on the prokaryote and not on the eukaryote.
6. 6The other organelles are absent: there is no endoplasmic reticulum, no Golgi apparatus, no lysosome, no mitochondria, no chloroplast and no vacuole, and the photosynthetic bacteria carry out photosynthesis without chloroplasts, by a special photosynthetic lamellar membrane. The cell organelles are thus absent, and the reason the prokaryotic cell is simpler is that the compartmentalisation that a eukaryotic cell depends on is not present. One exception to the rule about ribosomes is worth stating so the answer is not read as saying the cell has no organelles at all.
7. 7The genetic material and the inclusions: the single circular DNA molecule is naked and has no histone, in the eukaryote the DNA is associated with the histones and the non-histone proteins, so the prok DNA has no histone-like protein in the same way. The cytoplasm has many granules, and the ribosomes are one of them, and it also has the glycogen and the polyphosphate granules, the polyphosphate granules being called the volutin or the metachromatic bodies, and these act as the reserve food and the reserve phosphate. The cyanobacteria also have gas vacuoles, and some bacteria have the carboxysomes for photosynthesis, and the magnetosomes, the internal membranes containing magnetite, in the magnetic bacteria. The cell is devoid of any membrane-bound organelles, apart from the plasma membrane and the mesosomes.
8. 8The motility and the attachment structures: some bacteria show motility, and this is by flagellation, or by gliding, twitching or jerking, or it may be non-motile. The flagella are of three types: the bacterial flagellum, which is a naked, proteinaceous structure made of flagellin, and which is several times longer than the body and lacks the 9 plus 2 arrangement; the eukaryotic flagellum, which is a 9 plus 2 arrangement of microtubules and is present in the eukaryotes; and the archaebacterial or the methanogens flagellum, which is a thick, hollow, cylindrical structure. This is the type of flagellum that distinguishes the eubacteria from the archaebacteria and from the eukaryotes, and it is examinable. The pili, fimbriae and sex pili are the appendages concerned in the attachment of the cell to a surface and in the conjugation with the other cell, and they are much shorter, thinner and simpler than the flagella.
9. 9The division: the prokaryotic cell divides amitotically, by binary fission, and not by mitosis or by meiosis, since it has no spindle and no true nucleus. So the cell enlarges, the DNA replicates, the cell membrane invaginates, and the cell divides into two daughter cells. It may be a bacterium or a cyanobacterium, so the division is a simple one and the whole cell is the reproductive unit.

Step-by-step solution

1. 1Begin by defining division of labour, then show why it arose and how it is achieved, and end with its consequences. The mechanism is the important part: the reason a multicellular organism can specialise is that it arises from a single cell by repeated division, so that all its cells descend from one original zygote and share the same genetic material, and the specialisation of the groups of cells is therefore possible because the cells are not all the same and each group can take on one function while the others do the others.
2. 2Definition: division of labour is the phenomenon in which the different cells of a multicellular organism are grouped into specialised units or groups, and each such group performs one particular function, that is a specific chemical or structural role, so that the work of the organism as a whole is divided among the groups and the organism as a whole can perform its life activities more efficiently. In a unicellular organism, by contrast, all the life activities, the metabolism, the growth, the excretion and the reproduction, are carried out within a single cell.
3. 3How it is achieved: a multicellular organism is formed by the repeated mitotic division of a single cell, the zygote, and the cells so produced do not all remain alike. A part of the cells keeps dividing and retains the capacity to divide, and becomes the meristematic tissue, and a part of them stops dividing, changes its shape and begins to perform a particular function, and becomes the permanent or differentiated tissue. The differentiated cells of one kind form a tissue, the tissues of one type combine to form an organ, and the organs work together as an organ system, and these together constitute the whole organism. The division of the cells is therefore differential, so the cells end up specialised.
4. 4The consequences are the ones to list. First, each specialised group of cells can do its own work far more efficiently, so the organism as a whole becomes more efficient at the tasks it performs, and the example to give is the human body, in which the muscles contract, the neurons conduct, the lungs respire and the kidneys excrete, each tissue doing its own work and all of them together keeping the organism alive. Second, division of labour is responsible for the differentiation and the specialisation that produce the complex organisation of the body and the organ systems. Third, it raises the efficiency of the organism, since specialised cells are more effective at one function than a general cell is, so the organism can sustain a larger body and a higher rate of activity. Fourth, the whole organism becomes integrated, since the different specialised groups do not act independently but are co-ordinated, so the organism is a co-ordinated whole and not a collection of parts. Fifth, and this is the biological point, the division of labour makes the individual cell a functional unit of a larger whole, and it is the reason a multicellular organism has a well organised and integrated body plan.
5. 5So the explanation closes: division of labour means that the cells of a multicellular organism become specialised, each group performing one function, and it arises because the cells arise by repeated mitotic division of a single zygote and then differentiate into meristematic and permanent tissues which combine into organs and organ systems, and its effects are that the organism is more efficient, more organised and integrated, as in the human body where the muscle, nerve, respiratory and excretory tissues each do their own work for the good of the whole.

Step-by-step solution

1. 1This is a discussion question and the answer is a set of reasons why the cell is called the basic unit of life. The NCERT points should be taken one at a time, and each is worth a short paragraph with a reason and an example.
2. 2First, the cell is the basic structural unit, that is the basic unit of structure, because all living organisms are made up of cells, and the whole structure of the organism is built on the cell. This was the first proposition of the cell theory of Schleiden and Schwann, that all living organisms are composed of cells and their products, and a multicellular organism is not a mass of independent cells but an assembly of cells arranged in tissues, organs and organ systems, so the cell is the smallest unit that can show all the characteristics of life.
3. 3Second, the cell is the basic functional unit, because all the metabolic and physiological activities of the organism, the processes by which it lives, are performed in the cell, and the cell is the smallest unit that can carry out all the life activities. These include the metabolic activities of ingestion, digestion, respiration, excretion and synthesis, the reproductive activity by which the cell reproduces, and the genetic continuity by which the cell passes its information on to the daughter cells. So the cell is both the structural and the functional unit of life.
4. 4Third, the cell is the basic unit of reproduction. All cells reproduce, and the reproduction of a cell is the basis of all reproduction in organisms, so in a unicellular organism the cell divides to produce two new organisms, and in a multicellular organism the cell divides to produce the many cells of the body and to produce the gametes that give rise to a new organism. So no new organism arises except through the division and the growth of cells.
5. 5Fourth, the cell is the basic unit of metabolism and of heredity. The cell is the site of all the chemical reactions of the organism, the metabolism, since the enzymes, the mitochondria and the chloroplasts are all within it, and the genetic material, the DNA, is within the cell, so the cell carries both the information and the machinery by which that information is expressed. The hereditary characters pass from cell to cell and from parent to offspring through the cell, and it is the cell that carries the chromosomes.
6. 6Fifth, the cell is the basic unit of growth and of development, since an organism grows not by an increase in the size of its material but by an increase in the number of its cells, and every living organism begins as a single cell, the zygote, which divides to form the whole organism. And a cell is the smallest unit that can show all the characters of life, since the structures smaller than a cell, such as the organelles, cannot live independently, while the cell is capable of living independently in the case of a unicellular organism.
7. 7So the discussion closes on the two key sentences of the cell theory: the cell is the basic structural and functional unit of life, and all new cells arise from pre-existing cells. The two together mean that the cell is both the smallest unit of life and the unit by which life is continued, and this is what the question asks to be discussed.

Step-by-step solution

1. 1Define the pore in one sentence, then describe the structure of the pore complex briefly, then state the functions as a list, since the functions carry the marks.
2. 2Definition: a nuclear pore is a small, protein-lined aperture or opening present in the nuclear envelope, that is in the nuclear membrane, through which the movement of the molecules between the nucleus and the cytoplasm takes place. So the nuclear pore is a hole in the nuclear membrane, and it is not an empty hole, because it is occupied by a complex of proteins called the nuclear pore complex, which is a large structure, and a number of these pores occur all along the nuclear envelope.
3. 3The structure to mention is that the pore is plugged by a globular protein structure, so that in the electron micrograph the pore appears filled or plugged, and a nuclear pore of about 100 nanometres in diameter is commonly observed with a central plug, and the pore complex is made of about thirty different proteins, and the envelope is a two-membrane structure with the pores traversing both membranes so that the perinuclear space is continuous with the cytoplasm through the pore.
4. 4The functions, and these are the marks, are four. First, the pore is the main channel of communication between the nucleus and the cytoplasm, and it permits the passage of RNA, of the ribosomal subunits, and of the proteins, the enzymes and the other macromolecules that the nucleus makes and the cytoplasm needs, so the nucleus and the cytoplasm are in constant exchange. Second, it regulates the two-way traffic, so it allows the passage of substances outward from the nucleus and inward to it, thereby regulating the exchange and making the nucleus and the cytoplasm functionally integrated, and it controls what leaves the nucleus and what enters it. Third, the pores help in the exchange of RNA and proteins between the two, so RNA synthesised in the nucleus can pass out and the proteins made in the cytoplasm can pass in, and the ribosomal proteins are made in the cytoplasm but assembled in the nucleolus, so the pores are what allow this. Fourth, the nuclear pore is involved in the nuclear import of the proteins that carry the nuclear localisation signal, and it is the point at which the protein crosses into the nucleus, so the pore is the gate that all traffic between the two compartments must pass through.
5. 5So the answer to state is that the nuclear pore is the aperture in the nuclear envelope that provides the only route of communication between the nucleus and the cytoplasm, and its functions are to permit this two-way transport, to regulate it, to allow RNA and proteins to pass in either direction, and so to keep the nucleus and the cytoplasm functionally integrated.

Step-by-step solution

1. 1The question first asks for the similarity, since both are endomembrane structures, and then the difference in function. The similarity is one of origin, since the endomembrane system is the network of membranes formed by the endoplasmic reticulum, the Golgi apparatus, the lysosomes and the vacuoles, and the lysosomes and the vacuoles are both endomembrane, and both arise in the endomembrane system, the lysosome being formed from the Golgi and the vacuole from the endoplasmic reticulum. Then the functional difference is the whole answer, and the way to give it is by contrast, since a lysosome is a small, spherical, single-membraned body containing a battery of digestive or hydrolytic enzymes, and a vacuole is a membrane-bound sac, large, and its function depends on the type of vacuole.
2. 2The difference in origin and in size first. A lysosome is small, spherical and usually 0.5 to 1 micrometre in diameter, and it is formed from the Golgi apparatus, and it is filled with acid hydrolases, the digestive enzymes. A vacuole is much larger, and in the plant cell it is very large, occupying most of the cell and displacing the cytoplasm to a thin peripheral layer, and it is formed by the endoplasmic reticulum, and it is bounded by a tonoplast.
3. 3The difference in function, which is the point of the question. A lysosome is the cell's suicide bag and its recycler: it contains about thirty to forty different types of hydrolytic enzymes, the acid hydrolases, and it performs intracellular digestion, breaking down the complex biomolecules, the carbohydrates, the lipids and the proteins, into their simple monomers, and it is the digestive organelle of the cell. It is also the scavenger of the cell, since it breaks down the worn-out and damaged organelles, the old proteins and the foreign matter that has entered the cell, and this is why the lysosome is called the garbage disposer, and it is the prot scavenger of the cell, so the old organelles and the foreign particles are degraded and the raw materials are returned to the cytoplasm for reuse. The number of lysosomes is greatest in the cells of the secretory tissues and in the phagocytic cells, and the lysosome is acidic, with a pH of about 5, and its membrane is a single unit membrane.
4. 4A vacuole is a contrasting organelle, and here the animal and the plant vacuoles differ, so state both. In the plant cell, the vacuole is large, membrane-bound and filled with cell sap rich in sugars, salts, pigments, proteins and organic acids, and it maintains osmotic pressure, turgidity and the turgor pressure that keeps the plant rigid, it stores the waste products and the metabolic by-products, it maintains the acidity and the pH of the cell, and it stores proteins in the seeds as in aleurone grains and carbohydrates as in the potato tuber, and it is the place of storage and of maintaining the internal conditions of the cell. So a plant vacuole is a contractile, or a permanent, vacuole, since it is permanent and non-contractile in the plant, unlike the animal vacuole. In the animal cell the vacuole is small and few, and it is a contractile vacuole, and it is contractile in some lower animals such as Amoeba and Paramecium, and its function is to pump out the excess water and the waste, and so to osmoregulate, that is to maintain the water balance and the osmotic equilibrium, and to keep the cell turgid. A non-contractile vacuole is present in Amoeba and serves for storage, while the contractile vacuole is present in Paramecium and serves for osmoregulation, and in Paramecium the two posterior contractile vacuoles collect the excess water by osmosis and expel it.
5. 5So the comment to make is that the two are alike in being derived from the same endomembrane system, since both arise within it, the lysosome from the Golgi and the vacuole from the endoplasmic reticulum, and both are bounded by a single membrane; but they are unlike in function, since the lysosome is an intracellular digestive and scavenging organelle that degrades the macromolecules and the worn-out organelles with hydrolytic enzymes and returns the raw materials to the cytoplasm, whereas the vacuole is a large storage and regulatory organelle that maintains the turgidity, the pH and the osmotic conditions of the cell and stores sugars, salts, pigments and waste in the plant, or that expels water in the contractile vacuole of the lower animals, so the lysosome digests and recycles and the vacuole stores and regulates.

Step-by-step solution

1. 1This is a drawing question with two parts, so describe the labelled diagram of the nucleus and of the centrosome separately, and give the structure and the function of each part as it is labelled.
2. 2(i) The nucleus. Structure: the nucleus is a rounded, usually spherical, membranous organelle delimited by a double membrane, since it is bounded by two membranes, an outer and an inner, with a space of 20 to 40 nanometres between them called the perinuclear space, and the two together form the nuclear envelope. The outer membrane is continuous with the endoplasmic reticulum and bears ribosomes on it, so the nuclear envelope is continuous with the ER membrane and is often studded with ribosomes. The nuclear envelope is interrupted at a number of places by pores, and these are the nuclear pores.
3. 3The nuclear content, and this is what the diagram must label. Within the envelope is the ground substance called the nucleoplasm or the karyolymph, and also the chromatin, which is the material of the nucleus, a mixture of DNA and the histone and non-histone proteins, and it is in one of two states. When the cell is not dividing, the chromatin is loosely organised and is called the euchromatin, which is less staining, and when the cell is dividing the chromatin condenses into the compact, deeply staining chromosomes, the heterochromatin. The nucleolus is the rounded, dense, non-membranous body lying within the nucleoplasm, and it is the site of the ribosomal RNA synthesis and the assembly of the ribosomal subunits, so the nucleolus is called the site of ribosome formation, and the nucleus as a whole controls the metabolism, the growth and the reproduction of the cell, since it contains the genetic material.
4. 4The diagram of the nucleus should therefore show: the nuclear envelope drawn as a double line, the perinuclear space between the two membranes, a nuclear pore in the envelope, the nucleoplasm filling the interior, the chromatin or the chromosome material, and the nucleolus as a dense rounded body inside. One or two chromosomes may be shown in the condensed form if the drawing is meant to represent the dividing cell.
5. 5(ii) The centrosome. Structure: the centrosome is a cell organelle that is present in the cytoplasm of all the animal cells and of the lower plants, the lower algae, the fungi, but is absent in the higher plants. It lies near the nucleus, usually in the cytoplasm close to the nucleus, and it is the only organelle that is without a membrane, that is non-membranous. Its structure is of two components: the centrioles and the perinuclear or centrosphere or matrix, and the centrosome is a microtubular organisation, since it is made of microtubules.
6. 6The centriole: each centrosome has a pair of centrioles, lying at right angles to each other, so the two are perpendicular, and they are called the daughter centrioles. Each centriole is a cylinder about 500 nanometres long and 170 nanometres in diameter, and its wall is made of nine sets or triplets of microtubules, and the triplets are arranged in a ring, so the centriole has a cartwheel or a whorl appearance in a cross section, and the microtubules are of the 9 plus 0 form, since the central part has no microtubules and only the peripheral ring of nine triplets is present, and this is in contrast to the 9 plus 2 arrangement of the eukaryotic flagellum. The centre of each centriole is called the centriole, or the hub, and the peripheral fibres or the cartwheel are the nine triplets, and the peripheral fibrous or granular material is the pericentriolar material, and it is this pericentriolar material that forms the spindle fibres, and that is the real centre of the cell.
7. 7The perinuclear material or the centrosphere: this is the matrix around the centrioles, in which many microtubules are arranged irregularly, and they are the ones that form the spindle at the time of division. The function of the centrosome follows from this: it is the microtubular-organising centre of the cell, and it is the structure from which the spindle fibres of the mitotic division arise, so it is the centre of the cell and it is the centre of the cell division, and it is also the part that forms the basal bodies of the cilia and the flagella, and it helps in the cell movement and in the astral body formation during division.
8. 8The diagram of the centrosome should be drawn as a pair of centrioles in transverse section, two rings lying at right angles to each other, and each ring should show the nine peripheral triplets arranged in a cartwheel, with the central hub and no central microtubules, so the 9 plus 0 pattern is visible, with the perinuclear material around them drawn as a field of dots or lines for the microtubules.

Step-by-step solution

1. 1Define the centromere, give the four types of chromosomes on the basis of the centromere position, and then describe the diagram for each. This is a drawing question as well, and the four types must each be sketched with the centromere visibly in the right place.
2. 2Definition: a centromere is a specialized region of the chromosomes, a constricted, slightly narrow and non-staining part of the chromosome, and it is present on both the chromatids, and it is the region at which the two chromatids of a chromosome remain joined, so it holds the sister chromatids together. It is a non-staining or lightly staining constriction because the chromatin here is uncoiled, and it bears a disc-shaped or kinetochore, a disc-like structure on the surface of the centromere, and the kinetochore is the part to which the spindle fibres attach, and it is the kinetochore that moves towards the poles during division. The centromere is therefore the point of attachment of the spindle, and the position of the centromere is fixed for a particular chromosome and is constant.
3. 3Now the four types, and the classification rests on one question only: where on the length of the chromosome the centromere lies. The four positions give the four types, and the two ends of the chromosome are the two arms, the p or the short arm, called the p arm, and the q, the long arm, called the q arm. So the classification is made by the position of the centromere, which determines the relative length of the two arms, and the type is constant for a species.
4. 4(i) Metacentric, where the centromere lies in the middle of the chromosome, so that the two arms are of equal length, the p and the q being the same. The diagram is a straight rod with the centromere marked exactly at the centre, and the two arms labelled as equal, and the example to give is the chromosomes of man and of most higher organisms, and the spindle fibres attach on the centromere in such a way that the chromosome lies at the equator with the two arms extended.
5. 5(ii) Submetacentric, where the centromere lies off the middle, nearer to one end than to the other, so that the two arms are of unequal length, with the p arm shorter than the q arm. The diagram is a rod with the centromere marked a little to one side of the centre, and the shorter arm labelled p and the longer arm q, and the example is the V chromosome, the small one, of man, and the chromosome II of Drosophila.
6. 6(iii) Acrocentric, where the centromere lies very close to one end of the chromosome, so that one arm is very short and the other very long, the p being very short and the q very long. The diagram is a rod with the centromere marked almost at the tip, leaving only a very small p arm, and the example is the Y chromosome of man, and the satellite chromosomes, since in a satellite chromosome the short arm ends in a knob-like body called the satellite, as in the case of the Y chromosome.
7. 7(iv) Telocentric, where the centromere lies at or very near the terminal end of the chromosome, so that the chromosome has only one arm, since the p arm is absent and the whole chromosome is a single q arm. The diagram is a straight rod with the centromere marked at the very tip, so that no p arm can be drawn, and the example is the chromosome of the grasshopper and of some insects such as the Drosophila, though it is rare in the higher plants, and it is present in the acrocentric and telocentric forms that two arms are unequal or only one is present.
8. 8So the diagram should show all four side by side, with the same length of chromosome in each and only the centromere position moved: in the centre for metacentric, slightly off-centre for submetacentric, near the end for acrocentric and at the very end for telocentric, and the arms labelled p and q in each, which makes the point that the position of the centromere is what determines the two unequal arms and so the type.