How to Use This Section
This chapter is examined in a very particular way. Almost every question is "name the organelle", "state its function", "give the one difference" or "which of these is not". There is very little to derive and almost nothing to calculate - the marks sit in the exact names, the exact numbers and the one line that separates two structures that look alike.
That makes this chapter one of the easiest to score full marks in, and one of the easiest to lose marks in through a swapped word. Cristae belong to the mitochondrion and cisternae to the golgi. The centriole is 9+0 and the axoneme is 9+2. The mesosome is prokaryotic and the ER is eukaryotic. Each of those pairs has cost more marks than any reasoning question in the chapter.
Work through the three tiers in order.
- Tier 1 - Concept Checks. One fact per question, straight from the text. If you cannot answer these without stopping, go back to the section they came from before moving on.
- Tier 2 - Application and Identification. A structure is described, or a small situation is given, and you name the organelle and justify it. This is exactly how the "which organelle is this" question is set.
- Tier 3 - Comparisons and Long Answers. The two-column differences and the joined-up descriptions - prokaryote against eukaryote, plant cell against animal cell, mitochondrion against chloroplast, and the full descriptions of the nucleus and the chromosome.
The fourteen chapter-end exercises are all answered somewhere in this chapter. The last block lists where each one is answered and works out the one that is not covered anywhere else.
The Facts These Questions Draw On
The cell and the cell theory. The cell is the fundamental structural and functional unit of all living organisms, because anything less than a complete structure of a cell does not ensure independent living. Antonie Von Leeuwenhoek first saw and described a live cell; Robert Brown discovered the nucleus in 1831. Schleiden (1838), a German botanist - all plants are composed of different kinds of cells. Schwann (1839), a German zoologist - animal cells have a thin outer layer, today known as the plasma membrane, and the cell wall is a unique character of plant cells. Virchow (1855) - new cells are formed from pre-existing cells, Omnis cellula-e cellula. Cell theory today: all living organisms are composed of cells and products of cells, and all cells arise from pre-existing cells.
Prokaryote against eukaryote. Cells that have membrane bound nuclei are eukaryotic; cells that lack a membrane bound nucleus are prokaryotic. Cytoplasm, a semi-fluid matrix, is present in both and is the main arena of cellular activities. No organelles, like the ones in eukaryotes, are found in prokaryotic cells except for ribosomes. Ribosomes are non-membrane bound and are present in all cells. The centrosome is a non-membrane bound organelle of animal cells. Mycoplasmas, the smallest cells, are 0.3 micrometre; bacteria 3 to 5 micrometre; human red blood cells about 7.0 micrometre; the largest isolated single cell is the egg of an ostrich; nerve cells are among the longest.
The bacterial cell. Prokaryotes are bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). Shapes - bacillus (rod like), coccus (spherical), vibrio (comma shaped), spirillum (spiral). The cell envelope is a tightly bound three layered structure - the outermost glycocalyx, then the cell wall, then the plasma membrane - and each layer performs a distinct function but they act together as a single protective unit. Glycocalyx - a loose slime layer or a thick tough capsule. The cell wall determines the shape of the cell and prevents the bacterium from bursting or collapsing; it is absent only in mycoplasma. Gram positive bacteria take up the gram stain, Gram negative do not. The genetic material is naked, not enveloped by a nuclear membrane; genomic DNA is a single circular chromosome, and plasmids are small circular DNA outside it, conferring characters such as resistance to antibiotics.
Mesosome, flagellum, pili, ribosomes, inclusions. The mesosome is a specialised differentiated form of cell membrane, the characteristic of prokaryotes, formed by extensions of the plasma membrane into the cell in the form of vesicles, tubules and lamellae. It helps in cell wall formation, in DNA replication and distribution to daughter cells, in respiration, in secretion, and increases the surface area of the plasma membrane and its enzymatic content. Chromatophores, in cyanobacteria, are membranous extensions containing pigments. The bacterial flagellum has three parts - filament, hook and basal body - and the filament is the longest portion. Pili are elongated tubular structures of a special protein; fimbriae are small bristle like fibres that help attach the bacteria to rocks in streams and to host tissues; neither plays a role in motility. Prokaryotic ribosomes are about 15 nanometre by 20 nanometre, made of 50S and 30S subunits forming 70S, and are the site of protein synthesis; several ribosomes on one mRNA form a polysome. Inclusion bodies store reserve material, are not bound by any membrane system and lie free in the cytoplasm - phosphate granules, cyanophycean granules, glycogen granules; gas vacuoles occur in blue green and purple and green photosynthetic bacteria.
The membrane. The eukaryotes include all the protists, plants, animals and fungi. The cell membrane is mainly composed of lipids and proteins; the major lipids are phospholipids arranged in a bilayer with the polar head towards the outer sides and the hydrophobic tails towards the inner part, which protects the nonpolar tail of saturated hydrocarbons from the aqueous environment. Cholesterol is also present. The human erythrocyte membrane has approximately 52 per cent protein and 40 per cent lipids. Depending on the ease of extraction, proteins are integral (partially or totally buried) or peripheral (on the surface). Singer and Nicolson (1972) proposed the fluid mosaic model - the quasi-fluid nature of lipid enables lateral movement of proteins within the bilayer, measured as fluidity, which matters for cell growth, formation of intercellular junctions, secretion, endocytosis and cell division.
Transport. Passive transport needs no energy. Neutral solutes move by simple diffusion along the concentration gradient, from higher to lower. Movement of water by diffusion is called osmosis. Polar molecules cannot pass through the nonpolar lipid bilayer and need a carrier protein of the membrane to facilitate their transport. Active transport moves a few ions or molecules against the concentration gradient, from lower to higher, is energy dependent and uses ATP - the Na+/K+ pump is the example.
The cell wall. A non-living rigid structure forming an outer covering for the plasma membrane of fungi and plants. It gives shape to the cell, protects it from mechanical damage and infection, helps in cell-to-cell interaction and provides a barrier to undesirable macromolecules. Algal wall - cellulose, galactans, mannans and minerals like calcium carbonate; other plants - cellulose, hemicellulose, pectins and proteins. The primary wall of a young cell is capable of growth, which diminishes as the cell matures; the secondary wall forms on the inner side, towards the membrane. The middle lamella is a layer mainly of calcium pectate which glues neighbouring cells together. Plasmodesmata traverse the cell wall and middle lamella and connect the cytoplasm of neighbouring cells.
The endomembrane system. It includes the endoplasmic reticulum, golgi complex, lysosomes and vacuoles, because their functions are coordinated. Mitochondria, chloroplast and peroxisomes are not part of it, because their functions are not coordinated with these components - being membrane bound is not the test.
ER and golgi. The ER is a network or reticulum of tiny tubular structures scattered in the cytoplasm and divides the intracellular space into luminal (inside ER) and extra luminal (cytoplasm) compartments. RER bears ribosomes on its outer surface, is frequent in cells actively involved in protein synthesis and secretion, and is extensive and continuous with the outer membrane of the nucleus. SER has no ribosomes and is the major site for synthesis of lipid; in animal cells lipid-like steroidal hormones are synthesised in SER. Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus. The golgi has many flat, disc-shaped sacs or cisternae of 0.5 to 1.0 micrometre diameter, stacked parallel to each other, with a convex cis or forming face and a concave trans or maturing face, which are entirely different but interconnected. It principally performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell, and is the important site of formation of glycoproteins and glycolipids.
Lysosomes and vacuoles. Lysosomes are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus, very rich in almost all types of hydrolytic enzymes - lipases, proteases, carbohydrases - optimally active at the acidic pH, and able to digest carbohydrates, proteins, lipids and nucleic acids. The vacuole is the membrane-bound space found in the cytoplasm containing water, sap, excretory product and other materials not useful for the cell, bound by a single membrane called the tonoplast, and in plant cells it can occupy up to 90 per cent of the volume of the cell. The tonoplast transports ions and other materials against concentration gradients into the vacuole, so their concentration is significantly higher in the vacuole than in the cytoplasm. In Amoeba the contractile vacuole is important for osmoregulation and excretion; food vacuoles are formed by engulfing the food particles.
Mitochondria. Unless specifically stained they are not easily visible under the microscope. The number per cell is variable, depending on the physiological activity of the cells. Sausage-shaped or cylindrical, diameter 0.2 to 1.0 micrometre (average 0.5 micrometre), length 1.0 to 4.1 micrometre. Each is a double membrane-bound structure; the two membranes divide the lumen into an outer and an inner compartment, the inner compartment filled with a dense homogeneous substance called the matrix. The outer membrane forms the continuous limiting boundary of the organelle; the inner membrane forms infoldings called the cristae towards the matrix, and the cristae increase the surface area. The two membranes have their own specific enzymes. Mitochondria are the sites of aerobic respiration and produce cellular energy in the form of ATP, hence the 'power houses' of the cell. The matrix has a single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins, and the mitochondria divide by fission.
Plastids and the chloroplast. Plastids are found in all plant cells and in euglenoides, and are easily observed under the microscope as they are large. Based on the type of pigments they are chloroplasts, chromoplasts and leucoplasts. Chloroplasts contain chlorophyll and carotenoid pigments responsible for trapping light energy essential for photosynthesis. Chromoplasts contain fat soluble carotenoid pigments like carotene and xanthophylls, giving a yellow, orange or red colour. Leucoplasts are colourless plastids of varied shapes and sizes with stored nutrients - amyloplasts store carbohydrates (starch), as in potato; elaioplasts store oils and fats; aleuroplasts store proteins. Majority of the chloroplasts of green plants are found in the mesophyll cells of the leaves, are lens-shaped, oval, spherical, discoid or even ribbon-like, 5 to 10 micrometre long and 2 to 4 micrometre wide, 1 per cell in Chlamydomonas to 20 to 40 per cell in the mesophyll. They are double membrane bound and the inner chloroplast membrane is relatively less permeable. The space limited by the inner membrane is the stroma, holding organised flattened membranous sacs called thylakoids, stacked like piles of coins to form grana, joined by flat membranous tubules called stroma lamellae; the membrane of the thylakoids encloses a space called a lumen. The stroma contains enzymes required for the synthesis of carbohydrates and proteins, small double-stranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the thylakoids. Chloroplast ribosomes are smaller (70S) than the cytoplasmic ribosomes (80S). The grana is the site of the light reactions and the stroma of the dark reactions.
Ribosomes. First observed under the electron microscope as dense particles by George Palade (1953). Composed of ribonucleic acid (RNA) and proteins and not surrounded by any membrane. Eukaryotic ribosomes are 80S, prokaryotic ribosomes are 70S. Each has a larger and a smaller subunit - 80S is 60S and 40S, 70S is 50S and 30S. "S" (Svedberg's Unit) stands for the sedimentation coefficient and is indirectly a measure of density and size, which is why the subunit values do not add up arithmetically.
Cytoskeleton and microbodies. An elaborate network of filamentous proteinaceous structures consisting of microtubules, microfilaments and intermediate filaments present in the cytoplasm is collectively referred to as the cytoskeleton, and it is involved in mechanical support, motility and maintenance of the shape of the cell. Microbodies are many membrane bound minute vesicles containing various enzymes, present in both plant and animal cells, and they are not part of the endomembrane system.
Cilia, flagella and the centrosome. Cilia and flagella are hair-like outgrowths of the cell membrane, covered with plasma membrane. Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid; flagella are comparatively longer and responsible for cell movement. Prokaryotic bacteria also possess flagella, but these are structurally different from eukaryotic flagella. The core, called the axoneme, possesses a number of microtubules running parallel to the long axis and usually has nine doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules - the 9+2 array. The central tubules are connected by bridges and enclosed by a central sheath, which is connected to one tubule of each peripheral doublet by a radial spoke, so there are nine radial spokes; the peripheral doublets are interconnected by linkers. Both cilium and flagellum emerge from a centriole-like structure called the basal body. The centrosome is an organelle usually containing two cylindrical structures called centrioles, surrounded by amorphous pericentriolar materials, lying perpendicular to each other, each with an organisation like the cartwheel - nine evenly spaced peripheral fibrils of tubulin protein, each fibril a triplet, adjacent triplets linked, with a proteinaceous hub in the central part of the proximal region connected to the peripheral triplets by radial spokes made of protein. Centrioles form the basal body of cilia or flagella, and the spindle fibres that give rise to the spindle apparatus during cell division in animal cells. The centriole is the 9+0 arrangement - triplets and no central tubule.
The nucleus. First described as a cell organelle by Robert Brown as early as 1831; the material of the nucleus stained by basic dyes was named chromatin by Flemming. The interphase nucleus is the nucleus of a cell when it is not dividing, and has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix, and one or more spherical bodies called nucleoli. The nuclear envelope consists of two parallel membranes with a space between them, the perinuclear space, 10 to 50 nanometre wide, and forms a barrier between the materials present inside the nucleus and those of the cytoplasm. The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it. The envelope is interrupted by minute pores formed by the fusion of its two membranes, and these nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions. Normally there is only one nucleus per cell, though some mature cells lack a nucleus - erythrocytes of many mammals and sieve tube cells of vascular plants. The nuclear matrix, or nucleoplasm, contains the nucleolus and chromatin. The nucleolus is not a membrane bound structure, its content is continuous with the rest of the nucleoplasm, it is a site for active ribosomal RNA synthesis, and larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.
Chromosomes. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA. A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes. Every chromosome, visible only in dividing cells, essentially has a primary constriction or the centromere, with disc shaped structures called kinetochores on its sides, and the centromere holds the two chromatids of a chromosome. A few chromosomes have non-staining secondary constrictions at a constant location, giving the appearance of a small fragment called the satellite. Based on the position of the centromere - metacentric (middle centromere, two equal arms), sub-metacentric (slightly away from the middle, one shorter and one longer arm), acrocentric (close to its end, one extremely short and one very long arm), telocentric (terminal centromere).
Tier 1 - Concept Checks
Question 1
Q. Who first saw and described a live cell, and who discovered the nucleus?
Answer. Antonie Von Leeuwenhoek first saw and described a live cell. Robert Brown discovered the nucleus, as early as 1831.
Question 2
Q. State the cell theory as it is understood today, and name the scientist who supplied its second half.
Answer. (i) All living organisms are composed of cells and products of cells. (ii) All cells arise from pre-existing cells. The second statement came from Rudolf Virchow in 1855, who first explained that cells divided and new cells are formed from pre-existing cells - Omnis cellula-e cellula.
Question 3
Q. What exactly separates a eukaryotic cell from a prokaryotic one?
Answer. Cells that have membrane bound nuclei are eukaryotic; cells that lack a membrane bound nucleus are prokaryotic. A prokaryote does have genetic material - what it lacks is the membrane around it.
Question 4
Q. Give the smallest cell, the largest isolated single cell and the longest cells.
Answer. Mycoplasmas, the smallest cells, are only 0.3 micrometre in length. The largest isolated single cell is the egg of an ostrich. Nerve cells are some of the longest cells.
Question 5
Q. Which organisms are represented by prokaryotic cells?
Answer. Bacteria, blue-green algae, mycoplasma and PPLO, where PPLO stands for Pleuro Pneumonia Like Organisms.
Question 6
Q. Name the four basic shapes of bacteria and describe each in one word.
Answer. Bacillus - rod like. Coccus - spherical. Vibrio - comma shaped. Spirillum - spiral.
Question 7
Q. Name the three layers of the bacterial cell envelope from outside inwards.
Answer. The outermost glycocalyx, then the cell wall, then the plasma membrane. Although each layer performs a distinct function, they act together as a single protective unit.
Question 8
Q. What is the mesosome, and name three of its functions.
Answer. It is a specialised differentiated form of cell membrane, the characteristic of prokaryotes, formed by the extensions of plasma membrane into the cell in the form of vesicles, tubules and lamellae. It helps in cell wall formation, in DNA replication and distribution to daughter cells, and in respiration. It also helps in secretion processes and increases the surface area of the plasma membrane and its enzymatic content.
Question 9
Q. Do pili and fimbriae help a bacterium to move?
Answer. No. Pili and fimbriae are surface structures of the bacteria but do not play a role in motility. Movement is the job of the flagellum, which is made of filament, hook and basal body, the filament being the longest portion.
Question 10
Q. Which subunits make up the prokaryotic ribosome, and what do ribosomes do?
Answer. 50S and 30S units, which when present together form 70S prokaryotic ribosomes. Ribosomes are the site of protein synthesis.
Question 11
Q. What are inclusion bodies, and are they membrane bound?
Answer. Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. They are not bound by any membrane system and lie free in the cytoplasm - phosphate granules, cyanophycean granules and glycogen granules.
Question 12
Q. Who proposed the fluid mosaic model, in which year, and what does it state?
Answer. Singer and Nicolson, in 1972. According to the model, the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer, and this ability to move within the membrane is measured as its fluidity.
Question 13
Q. On what basis are membrane proteins classed as integral or peripheral?
Answer. Depending on the ease of extraction. Peripheral proteins lie on the surface of the membrane, so they come off easily; integral proteins are partially or totally buried in the membrane, so they are hard to extract.
Question 14
Q. How do neutral solutes cross the plasma membrane, and what is osmosis?
Answer. Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, that is, from higher concentration to the lower, with no requirement of energy. Water may also move across the membrane from higher to lower concentration, and this movement of water by diffusion is called osmosis.
Question 15
Q. What is the middle lamella made of and what does it do?
Answer. The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together.
Question 16
Q. What are plasmodesmata?
Answer. The cell wall and middle lamellae may be traversed by plasmodesmata, which connect the cytoplasm of neighbouring cells.
Question 17
Q. Name the four organelles of the endomembrane system and the three that are left out of it.
Answer. In the system - the endoplasmic reticulum (ER), the golgi complex, lysosomes and vacuoles, because their functions are coordinated. Left out - mitochondria, chloroplast and peroxisomes, because their functions are not coordinated with the above components.
Question 18
Q. Name the two faces of the golgi apparatus and give its principal function.
Answer. A distinct convex cis or forming face and a concave trans or maturing face, which are entirely different, but interconnected. The golgi apparatus principally performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell, and it is the important site of formation of glycoproteins and glycolipids.
Question 19
Q. What are the lysosomal enzymes called, and at what pH do they work best?
Answer. They are the hydrolytic enzymes or hydrolases - lipases, proteases and carbohydrases - and all of them are optimally active at the acidic pH. They can digest carbohydrates, proteins, lipids and nucleic acids.
Question 20
Q. Which part of the chloroplast is the site of the light reactions and which of the dark reactions?
Answer. The grana, in the plastid, is the site of the light reactions and the stroma of the dark reactions. That follows the structure - chlorophyll pigments are present in the thylakoids of the grana, while the stroma contains enzymes required for the synthesis of carbohydrates and proteins.
Tier 2 - Application and Identification
Question 21
Q. A cell is only 0.3 micrometre long, has no cell wall and no membrane bound nucleus. Name it and say which group it belongs to.
Answer. It is Mycoplasma. It is a prokaryote - the prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO - and it is the smallest cell, only 0.3 micrometre in length. The missing wall fits too, because all prokaryotes have a cell wall surrounding the cell membrane except in mycoplasma.
Question 22
Q. One bacterium in a culture goes on growing on a plate of antibiotic that kills all the rest. Which piece of its DNA is responsible, and why?
Answer. The plasmid. Many bacteria have small circular DNA outside the genomic DNA, and these smaller DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria, and one such character is resistance to antibiotics. The genomic DNA, the single circular chromosome, is not the answer here.
Question 23
Q. An electron micrograph of a bacterium shows the plasma membrane folded deep into the cell as vesicles, tubules and lamellae. Name the structure and give one function.
Answer. It is the mesosome, a specialised differentiated form of cell membrane which is the characteristic of prokaryotes. Among its functions, it increases the surface area of the plasma membrane and its enzymatic content, and it helps in cell wall formation, in DNA replication and distribution to daughter cells, in respiration and in secretion processes.
Question 24
Q. A polar molecule crosses the membrane down its concentration gradient and no ATP is spent. How does it get across, and why can it not simply diffuse?
Answer. It uses a carrier protein of the membrane, which facilitates its transport across the membrane. It cannot simply diffuse because the polar molecules cannot pass through the nonpolar lipid bilayer - the hydrophobic tails face the inner part of the membrane, so the interior is oily and a polar molecule is stopped there. The move is still passive transport, since it is along the gradient with no requirement of energy.
Question 25
Q. Ions are being carried from a region of lower concentration to one of higher concentration and ATP is being consumed. Name the process and give the standard example.
Answer. Active transport. A few ions or molecules are transported across the membrane against their concentration gradient, that is, from lower to the higher concentration, and such a transport is an energy dependent process in which ATP is utilised. The example is the Na+/K+ pump.
Question 26
Q. A cell of the testis is making large amounts of a steroid hormone. Which kind of endoplasmic reticulum would you expect to be abundant, and why?
Answer. Smooth endoplasmic reticulum (SER), the ER in the absence of ribosomes. SER is the major site for synthesis of lipid, and in animal cells, lipid-like steroidal hormones are synthesised in SER. A steroid hormone is a lipid, so the lipid-making membrane is the one that expands.
Question 27
Q. A gland cell is packed with membranes that bear ribosomes on their outer surface and run continuously into the outer membrane of the nucleus. Name the structure and say what the cell is doing.
Answer. It is rough endoplasmic reticulum (RER) - ER bearing ribosomes on its outer surface. RER is frequently observed in the cells actively involved in protein synthesis and secretion, and these membranes are extensive and continuous with the outer membrane of the nucleus. So the cell is making and secreting protein.
Question 28
Q. A vesicle isolated from a cell is very rich in hydrolytic enzymes that work best at acidic pH. Name it and say which organelle made it.
Answer. It is a lysosome. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes - lipases, proteases and carbohydrases - and all of them are optimally active at the acidic pH. Lysosomes are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus, so the golgi apparatus made it.
Question 29
Q. An organelle has two membranes, the inner one thrown into infoldings, a single circular DNA molecule and 70S ribosomes. Name it and justify.
Answer. It is the mitochondrion. Each mitochondrion is a double membrane-bound structure, and the inner membrane forms a number of infoldings called the cristae towards the matrix, which increase the surface area. The matrix possesses a single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins.
Question 30
Q. A potato cell is full of colourless plastids packed with starch. Name them and the class of plastid they belong to.
Answer. They are amyloplasts, a kind of leucoplast. Leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients, and amyloplasts store carbohydrates (starch), for example in the potato. The other two are elaioplasts, storing oils and fats, and aleuroplasts, storing proteins.
Question 31
Q. A transverse section of a hair-like outgrowth of a cell shows nine peripheral doublets of microtubules around a central pair, with nine radial spokes. Name the structure and the arrangement.
Answer. It is the axoneme, the core of a cilium or a flagellum. The arrangement is the 9+2 array - nine doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules. The central tubules are connected by bridges and enclosed by a central sheath, and the central sheath is connected to one tubule of each peripheral doublet by a radial spoke, so there are nine radial spokes.
Question 32
Q. A section through a cylindrical organelle of an animal cell shows nine triplet fibrils of tubulin protein around a proteinaceous hub, with no central tubule. Name it and the arrangement.
Answer. It is a centriole, one of the two cylindrical structures of the centrosome. The arrangement is the 9+0 one - nine evenly spaced peripheral fibrils of tubulin protein, each fibril a triplet, with no central microtubule. The central part of the proximal region is proteinaceous and is called the hub, which is connected with the tubules of the peripheral triplets by radial spokes made of protein, giving an organisation like the cartwheel.
Question 33
Q. Under the microscope a cell shows unusually large and numerous nucleoli. What does that tell you about the cell?
Answer. That the cell is actively carrying out protein synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis, because such a cell needs many ribosomes, ribosomes need ribosomal RNA, and the nucleolus is a site for active ribosomal RNA synthesis.
Question 34
Q. Which one does not belong with the rest, and why: endoplasmic reticulum, golgi complex, lysosome, microbody?
Answer. The microbody. The other three are part of the endomembrane system, which includes only the ER, golgi complex, lysosomes and vacuoles, because their functions are coordinated. Microbodies are many membrane bound minute vesicles that contain various enzymes, present in both plant and animal cells, but they work separately, so they stay out - just as mitochondria and chloroplasts do. Being membrane bound is not enough to belong to the endomembrane system.
Tier 3 - Comparisons and Long Answers
Question 35
Q. Give the differences between a prokaryotic and a eukaryotic cell.
Answer.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No well-defined nucleus; the genetic material is naked, not enveloped by a nuclear membrane | An organised nucleus with a nuclear envelope |
| Genetic material | A single chromosome of circular DNA, often with plasmids | Organised into chromosomes |
| Membrane bound organelles | Absent - no organelles like the ones in eukaryotes are found except for ribosomes | Present - ER, golgi complex, lysosomes, mitochondria, microbodies, vacuoles |
| Compartments | Cytoplasm is one space | Extensive compartmentalisation of cytoplasm |
| Ribosomes | 70S - 50S and 30S | 80S - 60S and 40S; 70S inside mitochondria and chloroplasts |
| Special membrane | A mesosome, a specialised differentiated form of cell membrane | No mesosome |
| Size and multiplication | Generally smaller and multiply more rapidly | Larger, divide more slowly |
| Locomotory structures | Flagella structurally different from eukaryotic flagella | A variety of complex locomotory and cytoskeletal structures |
| Examples | Bacteria, blue-green algae, mycoplasma, PPLO | All the protists, plants, animals and fungi |
The one line that decides the question: cells that have membrane bound nuclei are eukaryotic; cells that lack a membrane bound nucleus are prokaryotic.
Question 36
Q. Give the differences between a plant cell and an animal cell.
Answer. All eukaryotic cells are not identical.
| Structure | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present - a non-living rigid structure outside the plasma membrane | Absent; the outer membrane is the delimiting structure |
| Plastids | Present - chloroplasts, chromoplasts, leucoplasts | Absent |
| Vacuole | A large central vacuole, up to 90 per cent of the volume of the cell | Small vacuoles, if any |
| Centrioles | Absent in almost all plant cells | Present, in the centrosome |
Plant cells possess cell walls, plastids and a large central vacuole which are absent in animal cells. Animal cells have centrioles which are absent in almost all plant cells - keep the word almost, because the statement is deliberately hedged.
Question 37
Q. Compare passive transport with active transport.
Answer.
| Feature | Passive transport | Active transport |
|---|---|---|
| Energy | No requirement of energy | Energy dependent - ATP is utilised |
| Direction | Along the concentration gradient, from higher to lower | Against the concentration gradient, from lower to higher |
| What moves | Neutral solutes by simple diffusion, water by osmosis, polar molecules through a carrier protein | A few ions or molecules |
| Example | Simple diffusion and osmosis | The Na+/K+ pump |
The word that separates the two columns is gradient. Note that transport through a carrier protein is still passive - only the doorway is different, the direction is still down the gradient and no ATP is spent.
Question 38
Q. Distinguish between rough endoplasmic reticulum and smooth endoplasmic reticulum.
Answer.
| Feature | Rough ER (RER) | Smooth ER (SER) |
|---|---|---|
| Ribosomes | Bears ribosomes on its outer surface | Ribosomes absent, so the membranes appear smooth |
| Found in | Cells actively involved in protein synthesis and secretion | Cells making lipids |
| Continuity | Extensive and continuous with the outer membrane of the nucleus | Not described as continuous with the nuclear membrane |
| Function | Protein synthesis and secretion | The major site for synthesis of lipid; in animal cells lipid-like steroidal hormones are synthesised here |
Both are the same organelle - a network or reticulum of tiny tubular structures scattered in the cytoplasm - which divides the intracellular space into luminal (inside ER) and extra luminal (cytoplasm) compartments.
Question 39
Q. Lysosomes and vacuoles are both endomembrane structures. How do they differ?
Answer.
| Feature | Lysosome | Vacuole |
|---|---|---|
| What it is | A membrane bound vesicular structure | The membrane-bound space found in the cytoplasm |
| Origin | Formed by the process of packaging in the golgi apparatus | A space bound by its own membrane |
| Membrane | Membrane bound | A single membrane called the tonoplast |
| Contents | Hydrolytic enzymes - lipases, proteases, carbohydrases - optimally active at the acidic pH | Water, sap, excretory product and other materials not useful for the cell |
| Function | Digestion - it can digest carbohydrates, proteins, lipids and nucleic acids | Storage; the tonoplast transports ions and other materials against concentration gradients into the vacuole |
| Size | A small vesicle | In plant cells, up to 90 per cent of the volume of the cell |
Vacuoles also take on special jobs - in Amoeba the contractile vacuole is important for osmoregulation and excretion, and food vacuoles are formed by engulfing the food particles. The lysosome has one job only - digestion.
Question 40
Q. Name the two double membrane bound organelles and compare them.
Answer. The mitochondrion and the chloroplast.
| Feature | Mitochondrion | Chloroplast |
|---|---|---|
| Where | In all eukaryotic cells | All plant cells and euglenoides; majority in the mesophyll cells of the leaves |
| Visibility | Not easily visible unless specifically stained | Easily observed as it is large |
| Shape and size | Sausage-shaped or cylindrical, diameter 0.2 to 1.0 micrometre (average 0.5), length 1.0 to 4.1 micrometre | Lens-shaped, oval, spherical, discoid or ribbon-like, length 5 to 10 micrometre, width 2 to 4 micrometre |
| Inner membrane | Forms infoldings called cristae towards the matrix, which increase the surface area | Relatively less permeable; encloses the stroma |
| Inner contents | Matrix - a dense homogeneous substance with a single circular DNA molecule, a few RNA molecules and 70S ribosomes | Stroma with thylakoids stacked as grana, stroma lamellae, enzymes for the synthesis of carbohydrates and proteins, small double-stranded circular DNA and 70S ribosomes |
| Pigments | None | Chlorophyll pigments present in the thylakoids, with carotenoids |
| Function | Site of aerobic respiration; produces cellular energy in the form of ATP, hence the 'power house' of the cell | Traps light energy essential for photosynthesis; grana - light reactions, stroma - dark reactions |
Both are double membrane bound, both carry circular DNA and 70S ribosomes, and neither is a part of the endomembrane system, because their functions are not coordinated with its components.
Question 41
Q. Compare the eukaryotic ribosome with the prokaryotic one, and explain why the subunit values do not add up.
Answer. Ribosomes are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane. Each ribosome has two subunits, a larger and a smaller subunit.
| Ribosome | Where | Larger subunit | Smaller subunit |
|---|---|---|---|
| 80S | Eukaryotic cells | 60S | 40S |
| 70S | Prokaryotic cells, and inside mitochondria and chloroplasts | 50S | 30S |
60 and 40 do not make 80, and 50 and 30 do not make 70, and that is not a misprint. "S" (Svedberg's Unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size. It records how fast a particle settles when it is spun, which depends on density and shape as well as size, so it is not a mass and the values cannot be added.
Question 42
Q. Describe the cytoskeleton and the microbodies, and say why a microbody is not counted in the endomembrane system.
Answer. The cytoskeleton is an elaborate network of filamentous proteinaceous structures consisting of microtubules, microfilaments and intermediate filaments present in the cytoplasm. It is made of protein threads, not of membrane, and it is involved in many functions such as mechanical support, motility and maintenance of the shape of the cell.
Microbodies are many membrane bound minute vesicles that contain various enzymes, and they are present in both plant and animal cells.
Why a microbody is left out. The endomembrane system includes only the endoplasmic reticulum, golgi complex, lysosomes and vacuoles, because their functions are coordinated. A microbody is membrane bound but works on its own, so it is excluded - and for the same reason mitochondria, chloroplast and peroxisomes are not considered as part of the endomembrane system. Being membrane bound is not the test; coordinated function is.
Question 43
Q. Distinguish clearly between the 9+2 array and the 9+0 arrangement, and state how the two structures are related.
Answer.
| Feature | Axoneme of cilium or flagellum - 9+2 | Centriole - 9+0 |
|---|---|---|
| Peripheral units | Nine doublets of radially arranged microtubules | Nine evenly spaced peripheral fibrils of tubulin protein, each a triplet |
| Central microtubules | A pair of centrally located microtubules | Absent |
| Central structure | The central tubules are connected by bridges and enclosed by a central sheath | The proteinaceous hub in the central part of the proximal region |
| Radial spokes | Nine, from the central sheath to one tubule of each peripheral doublet | From the hub to the tubules of the peripheral triplets, made of protein |
| Interconnections | Peripheral doublets interconnected by linkers | Adjacent triplets are also linked |
| Overall look | Nine doublets around a central pair | An organisation like the cartwheel |
How they are related. The centrioles form the basal body of cilia or flagella, and both the cilium and the flagellum emerge from that centriole-like structure called the basal body. So the 9+0 structure sits at the base and the 9+2 structure is the shaft that projects out of the cell. The shortest way to hold it is doublets with 9+2, triplets with 9+0.
Question 44
Q. Describe the structure of the interphase nucleus, part by part.
Answer. The nucleus was first described as a cell organelle by Robert Brown as early as 1831. The interphase nucleus is the nucleus of a cell when it is not dividing. Take it from the outside inwards.
1. The nuclear envelope. It consists of two parallel membranes with a space between them and forms a barrier between the materials present inside the nucleus and those of the cytoplasm.
2. The perinuclear space. The space between the two membranes, 10 to 50 nanometre wide.
3. The outer membrane. It usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
4. The nuclear pores. At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. They are the passages through which movement of RNA and protein molecules takes place in both directions.
5. The nucleoplasm. The nuclear matrix, or the nucleoplasm, contains the nucleolus and chromatin.
6. The chromatin. Highly extended and elaborate nucleoprotein fibres, named chromatin by Flemming because it is the material of the nucleus stained by the basic dyes.
7. The nucleolus. One or more spherical bodies present in the nucleoplasm. Its content is continuous with the rest of the nucleoplasm as it is not a membrane bound structure, and it is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.
One more fact worth adding. Normally there is only one nucleus per cell, but variations in the number of nuclei are also frequently observed, and some mature cells even lack a nucleus - erythrocytes of many mammals and sieve tube cells of vascular plants.
Question 45
Q. How does chromatin differ from a chromosome?
Answer. They are the same material in two different states.
| Feature | Chromatin | Chromosome |
|---|---|---|
| When seen | In the interphase nucleus - the nucleus of a cell when it is not dividing | Visible only in dividing cells; during different stages of cell division, cells show structured chromosomes in place of the nucleus |
| Appearance | A loose and indistinct network of highly extended and elaborate nucleoprotein fibres | A structured body with a primary constriction or centromere |
| Composition | DNA, some basic proteins called histones, some non-histone proteins and also RNA | The same material, condensed |
A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes - which is the packing problem the condensed chromosome solves.
Question 46
Q. Describe the four types of chromosome based on the position of the centromere.
Answer. Every chromosome, visible only in dividing cells, essentially has a primary constriction or the centromere, which holds the two chromatids of a chromosome, and on the sides of the centromere, disc shaped structures called kinetochores are present. The centromere divides the chromosome into arms, and where it sits decides how long each arm is. Based on the position of the centromere, the chromosomes can be classified into four types.
| Type | Position of the centromere | The arms it gives |
|---|---|---|
| Metacentric | Middle | Two equal arms of the chromosome |
| Sub-metacentric | Slightly away from the middle | One shorter arm and one longer arm |
| Acrocentric | Close to its end | One extremely short arm and one very long arm |
| Telocentric | Terminal | A terminal centromere, so effectively a single arm |
Sub-metacentric and acrocentric are the pair that gets confused, since both give unequal arms. Hold on to the strength of the word - sub-metacentric is only slightly off the middle, while acrocentric is close to the end and its short arm is extremely short. Separately, a few chromosomes have non-staining secondary constrictions at a constant location, which give the appearance of a small fragment called the satellite.
The Chapter-End Exercises
All fourteen exercises at the end of this chapter are answered somewhere in this chapter. The table shows where each one is worked out, so you can check your own attempt against a full answer.
| Exercise | Answered as |
|---|---|
| 1. Which of the following is not correct - Robert Brown discovered the cell | Question 10 of the What Is a Cell, and the Cell Theory section |
| 2. New cells generate from | Question 9 of the What Is a Cell, and the Cell Theory section |
| 3. Match the following - Cristae, Cisternae, Thylakoids | Question 47 below, since it is not covered anywhere else |
| 4. Which of the following is correct - in prokaryotes there are no membrane bound organelles | Question 5 of the An Overview of the Cell section |
| 5. What is a mesosome in a prokaryotic cell, and its functions | Question 1 of the Prokaryotic Cells - Mesosomes, Flagella, Ribosomes and Inclusion Bodies section |
| 6. How do neutral solutes move across the plasma membrane | Question 3 of the Transport Across the Cell Membrane, and the Cell Wall section |
| 7. Name two cell-organelles that are double membrane bound | Question 11 of the Mitochondria section |
| 8. What are the characteristics of prokaryotic cells | Question 13 of the Prokaryotic Cells - Mesosomes, Flagella, Ribosomes and Inclusion Bodies section |
| 9. Multicellular organisms have division of labour | Question 11 of the An Overview of the Cell section |
| 10. Cell is the basic unit of life | Question 2 of the What Is a Cell, and the Cell Theory section |
| 11. What are nuclear pores, and their function | Question 7 of the The Nucleus section |
| 12. Lysosomes and vacuoles differ in terms of their functions | Question 11 of the Lysosomes and Vacuoles section |
| 13 (i). Structure of the nucleus with a labelled diagram | Question 14 of the The Nucleus section |
| 13 (ii). Structure of the centrosome with a labelled diagram | Question 15 of the Cilia, Flagella, Centrosome and Centrioles section |
| 14. What is a centromere, and classification by its position | Question 13 of the Chromosomes and the Position of the Centromere section |
Question 47
Q. Match the following. Column I - (a) Cristae, (b) Cisternae, (c) Thylakoids. Column II - (i) Flat membranous sacs in stroma, (ii) Infoldings in mitochondria, (iii) Disc-shaped sacs in Golgi apparatus. This is one of the chapter-end exercises.
Answer. (a) Cristae - (ii) Infoldings in mitochondria. (b) Cisternae - (iii) Disc-shaped sacs in Golgi apparatus. (c) Thylakoids - (i) Flat membranous sacs in stroma.
| Column I | Column II |
|---|---|
| (a) Cristae | (ii) Infoldings in mitochondria |
| (b) Cisternae | (iii) Disc-shaped sacs in Golgi apparatus |
| (c) Thylakoids | (i) Flat membranous sacs in stroma |
Why each pair goes together.
- Cristae. In the mitochondrion, the inner membrane forms a number of infoldings called the cristae (singular: crista) towards the matrix, and the cristae increase the surface area. Note that they are folds of the inner membrane - the outer membrane forms the continuous limiting boundary of the organelle and stays smooth.
- Cisternae. The golgi apparatus consists of many flat, disc-shaped sacs or cisternae of 0.5 to 1.0 micrometre diameter, stacked parallel to each other, with a convex cis or forming face and a concave trans or maturing face.
- Thylakoids. In the chloroplast, a number of organised flattened membranous sacs called the thylakoids are present in the stroma, arranged in stacks like the piles of coins called grana, and the membrane of the thylakoids encloses a space called a lumen. The phrase "in stroma" in the option is what pins this one to the chloroplast.
One warning about the word cisternae. It is also used for the flattened sacs of the endoplasmic reticulum, so the word on its own does not belong to the golgi alone. In this item the only Column II option describing them is "Disc-shaped sacs in Golgi apparatus", so that is the pairing the question wants - and it matches the description of the golgi exactly, flat, disc-shaped sacs stacked parallel to each other.