RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Everything alive is built from cells, and this chapter takes you inside one to see how the space is divided up and who does what. The prokaryote-eukaryote contrast comes first, and then each organelle is examined for the specific job its structure makes possible — from the flattened stacks of the Golgi to the double membrane and 70S ribosomes of the mitochondrion. Pay attention to sizes and numbers here; examiners love asking why a mitochondrion has its own DNA.
Which of the following is present in a prokaryotic cell?
Answer
Mesosome is correct — it is an infolding of the prokaryotic plasma membrane that assists in respiration, secretion and DNA replication; prokaryotes have 70S ribosomes and possess neither a nuclear envelope nor membrane-bound organelles.
The convex face of the Golgi apparatus, which receives vesicles from the endoplasmic reticulum, is called the:
Answer
Cis face is correct — it is the convex forming face lying next to the endoplasmic reticulum, while the concave trans face on the other side is the maturing face from which processed material is dispatched.
Ribosomes of the type 70S are found in eukaryotic cells:
Answer
Inside mitochondria and chloroplasts is correct — these two organelles carry their own circular DNA and prokaryote-like 70S ribosomes, which is why they are described as semi-autonomous; the ribosomes of the eukaryotic cytoplasm and the rough endoplasmic reticulum are 80S.
The hydrolytic enzymes packed inside lysosomes work most effectively at:
Answer
Acidic pH is correct — the lysosomal interior is kept acidic by proton pumps, and the lipases, proteases, carbohydrases and nucleases within it are optimally active under those conditions, which also limits damage if a few enzymes leak into the neutral cytosol.
Assertion (A): Mitochondria and chloroplasts are described as semi-autonomous organelles. Reason (R): Both contain their own circular DNA and 70S ribosomes and can divide by fission independently of nuclear division.
Answer
Both A and R are true and R is the correct explanation of A — the presence of their own genome and protein-synthesising machinery lets these organelles make some of their own proteins and multiply on their own, though most of their proteins are still coded by nuclear genes, which is why they are only semi-autonomous.
State two ways in which a plant cell differs structurally from an animal cell.
Answer
A plant cell has a rigid cell wall made mainly of cellulose lying outside the plasma membrane, which gives shape, provides mechanical strength and prevents the cell from bursting when it takes in water; an animal cell has no cell wall and is bounded only by the plasma membrane. Secondly, a mature plant cell contains a single very large central vacuole, bounded by a membrane called the tonoplast, which may occupy up to ninety percent of the cell volume and keeps the cell turgid, whereas animal cells have only small, temporary vacuoles. Plant cells also possess plastids, including chloroplasts, which animal cells lack, while centrioles are typically present in animal cells and absent from most higher plant cells.
Explain the fluid mosaic model of the plasma membrane and state why the fluidity of the membrane matters.
Answer
According to the fluid mosaic model proposed by Singer and Nicolson, the plasma membrane consists of a bilayer of lipid molecules whose polar heads face the watery medium on either side and whose non-polar tails point inward, protected from water. Protein molecules form a mosaic within this sheet: integral proteins are partially or wholly buried in the bilayer and some span it completely, while peripheral proteins lie on the surface. Carbohydrate chains attached to some lipids and proteins project outward. The lipid is in a quasi-fluid state, so lipid molecules and many proteins can move laterally within the plane of the membrane. This fluidity is important because it allows the membrane to grow, to fuse with vesicles during endocytosis and secretion, to be pinched into two during cell division, and it lets carrier and receptor proteins move and cluster to perform transport and signalling.
Describe the structure of the mitochondrion and the chloroplast, and explain how the structure of each suits its function.
Answer
A mitochondrion is a sausage-shaped or cylindrical organelle, typically about 0.2 to 1.0 μm in diameter and 1.0 to 4.1 μm long, and its number in a cell depends on how much energy that cell uses. It is bounded by two membranes that create two distinct compartments. The outer membrane is smooth and forms a continuous limiting boundary, whereas the inner membrane is thrown into numerous deep infoldings called cristae that project into the interior. The space enclosed by the inner membrane is filled with a dense fluid called the matrix. This arrangement suits the mitochondrion's role as the site of aerobic respiration: the cristae greatly increase the surface area available for the electron transport chain and the ATP-synthesising particles studded on the inner membrane, so a great deal of ATP can be produced within a small volume, while the matrix holds the enzymes of the Krebs cycle that oxidise pyruvate. The matrix also contains a single circular DNA molecule, 70S ribosomes and the enzymes needed for protein synthesis, which allows the organelle to make some of its own proteins and to divide by fission, making it semi-autonomous. A chloroplast is a lens-shaped or oval plastid, generally 5 to 10 μm long and 2 to 4 μm wide, and a mesophyll cell may hold twenty to forty of them, which orient themselves so as to catch the maximum light. It too is bounded by a double membrane, the inner one being relatively less permeable, and the space inside is filled with a colourless ground substance called the stroma. Running through the stroma is an elaborate system of flattened membranous sacs called thylakoids; in places thylakoids are stacked one above another like a pile of coins to form grana, and these stacks are joined by flat membranes called stroma lamellae. The space inside a thylakoid is the lumen. This organisation matches the two phases of photosynthesis exactly: the chlorophyll and the other pigments and electron carriers are located on the thylakoid membranes, whose stacking multiplies the light-absorbing area and provides the enclosed lumen needed to build the proton gradient during the light reaction, while the stroma contains the enzymes of the Calvin cycle that fix carbon dioxide into sugar during the dark reaction. Like the mitochondrion, the stroma contains circular DNA and 70S ribosomes, giving the chloroplast the same semi-autonomous character.
Compare prokaryotic and eukaryotic cells under the headings of genetic material, organelles, ribosomes, cell wall and cell division.
Answer
The genetic material of a prokaryotic cell is a single circular molecule of naked DNA that is not enclosed by any membrane; it lies in a region of the cytoplasm called the nucleoid and is not associated with histone proteins, though additional small circular DNA molecules called plasmids may be present and often carry genes for resistance to antibiotics. In a eukaryotic cell the DNA is linear, wound around histone proteins to form chromatin, organised into a definite number of chromosomes, and enclosed within a nucleus bounded by a double nuclear envelope perforated by pores, with one or more nucleoli inside where ribosomal RNA is made. As regards organelles, a prokaryotic cell has none of the membrane-bound kind: there are no mitochondria, no plastids, no endoplasmic reticulum, no Golgi apparatus and no lysosomes, and functions such as respiration are carried out on infoldings of the plasma membrane called mesosomes while photosynthetic bacteria use membranous chromatophores. A eukaryotic cell possesses the full range of membrane-bound organelles, so its activities are compartmentalised, with each organelle providing a distinct internal environment. Ribosomes in prokaryotes are of the 70S type, made of a 50S and a 30S subunit, and they lie free in the cytoplasm, often several joined along a messenger RNA as a polysome. Eukaryotic cytoplasmic ribosomes are of the larger 80S type, made of 60S and 40S subunits, and may be free or attached to the endoplasmic reticulum; the 70S type reappears only inside mitochondria and chloroplasts. The prokaryotic cell wall, when present, is a rigid layer containing peptidoglycan, and outside it there may be a loose slime layer or a firm capsule, together called the glycocalyx; the wall prevents bursting and determines whether the bacterium stains as Gram positive or Gram negative. Among eukaryotes, a cell wall is present in plants and fungi but is made of cellulose in plants and chitin in fungi, while animal cells have no wall at all. Finally, prokaryotes divide by simple binary fission or by budding, without any spindle or condensed chromosomes, and exchange genes only by processes such as conjugation, whereas eukaryotes divide by mitosis or meiosis, in which chromosomes condense and are separated by a spindle formed of microtubules.
A researcher studies two cell types from the same animal. Cell type A is a secretory cell of a digestive gland: it has an extensive rough endoplasmic reticulum, a very prominent Golgi apparatus and many vesicles near the surface. Cell type B is a muscle cell of the heart: it has few Golgi stacks but an exceptionally large number of mitochondria whose cristae are densely packed. (a) Explain why cell type A has so much rough endoplasmic reticulum and such a prominent Golgi apparatus. (b) Name the process by which the vesicles near the surface of cell A release their contents outside. (c) Account for the very large number of densely folded mitochondria in cell B. (d) If a drug destroyed the lysosomes of cell A, suggest one consequence for the cell.
Answer
(a) Cell type A manufactures and exports digestive enzymes, which are proteins. The ribosomes on the rough endoplasmic reticulum synthesise these proteins and pass them into its lumen for transport, and the Golgi apparatus then receives them at its cis face, modifies them, for example by adding sugar groups to form glycoproteins, concentrates them and packs them into secretory vesicles at its trans face. A cell with a heavy export load therefore needs both structures to be well developed. (b) The vesicles fuse with the plasma membrane and discharge their contents outside the cell by exocytosis, a process made possible by the fluid nature of the membrane. (c) The cardiac muscle cell contracts continuously throughout life and therefore has a very high and unremitting demand for ATP. Mitochondria are the sites of aerobic ATP production, so a large number are needed, and densely packed cristae provide a very large inner membrane surface on which the electron transport chain and ATP-synthesising particles are arranged, maximising ATP output per unit volume. (d) Without lysosomes the cell would lose its hydrolytic enzyme packages and so could not digest worn-out organelles or material taken in from outside. Damaged structures and ingested particles would accumulate in the cytoplasm, the recycling of their molecules would stop, and the cell would gradually become clogged and lose efficiency.
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