IB Biology B2.3: Cell specialization worked solutions

Both cells keep the whole genome. Differentiation changes which genes are expressed, so the beta cell makes insulin and the muscle cell does not.

A stem cell renews itself by repeated division and also produces daughters that differentiate. A cell lacking either property is not a stem cell.

Every body cell type but not the placenta is the range of a pluripotent cell. A totipotent cell would also form placental cells; a multipotent cell is limited to one lineage.

A concentration gradient gives each cell positional information. Different concentrations switch on different genes, so the cells differentiate differently without any change in DNA.

3 cm is 30 000 \mum, far longer than the other values. Compare the same dimension: the egg and the red cell are given as diameters.

(a) Area scales with the square of the linear factor, 2^2=4 (1). (b) Volume rises by 2^3=8, so the ratio changes by \frac48=\frac12 and halves (1).

Microvilli are finger-like projections that multiply the membrane area on the lumen side. Mitochondria supply ATP; they do not add surface.

Type I cells are very thin for gas exchange. Type II cells hold lamellar bodies and secrete surfactant. The other rows swap the roles.

Both are striated, with myofibrils of sarcomeres. Branching is cardiac, many nuclei is skeletal, and voluntary control applies only to skeletal muscle.

(a) A niche is the small region of tissue, with its supporting cells and signals, in which stem cells are maintained and regulated (1). (b) Any two named niches with their output: bone marrow giving red cells, white cells and platelets (1); the base of a gut crypt giving lining cells, or a hair follicle giving follicle and skin cells (1). (c) A stem cell divides and one daughter stays in the niche as a stem cell, which keeps the reserve (1); the other daughter leaves and differentiates to replace lost cells, and signals from the niche regulate which happens (1).

(a) Area =6\times6^2=216\ \mum^2 (1); volume =6^3=216\ \mum^3 (1). (b) The ratio is 6/L, which for L=12 gives 0.5 per \mum (1). (c) Real cells are not cubes: they have folds, flattened regions and organelles, so the model gives a trend, not exact values (1).

(a) A: 3/5=0.60\ \mum^{-1} (1); B: 3/15=0.20\ \mum^{-1} (1). (b) Cell B has 3^2=9 times the membrane area but 3^3=27 times the volume (1), so each unit of cytoplasm has less membrane to supply oxygen and the supply falls behind the demand (1).

(a) All three descend from the zygote by mitosis, which passes on the same genome (1). (b) Actin filaments form part of the cytoskeleton (1), needed for the shape and movement of every cell type (1). (c) Different sets of genes are expressed in each cell type (1), so different proteins and mRNA are made (1), and these proteins give each cell its own structure and function (1).

(a) P forms body cells and placental cells: totipotent (1); Q forms every body cell type but no placenta: pluripotent (1); R forms only the cells of one lineage, blood: multipotent (1). (b) Cells of R are restricted to blood cell types, so they cannot form liver cells (1).

(a) Microvilli (1). (b) Microvilli greatly increase the membrane area for the cell volume (1), so more transport proteins can reabsorb glucose, ions and water per second (1). The basal infoldings add membrane that carries pumps and channels (1); the mitochondria beside them supply the ATP for active transport (1).

(a) Type II pneumocytes (1); their lamellar bodies (1). (b) Without surfactant the surface tension of the water film lining the alveolus is high (1), so the walls stick together and the alveoli collapse as air leaves (1); the area for gas exchange falls and the lungs are hard to inflate (1).

(a) One mark each for a drawn structure that is correctly placed and labelled with a line ending on it: nucleus in the head (1); acrosome as a cap at the front of the head (1); midpiece behind the head with mitochondria (1); tail extending from the midpiece, with a continuous cell outline (1). (b) Any two: the acrosome releases enzymes that digest a path through the outer layers of the egg (1); the mitochondria in the midpiece supply ATP (1); the tail beats to propel the sperm (1); the nucleus carries the haploid set of chromosomes (1).

(a) In group 2 the number of stem cells per follicle falls from 12 to 3 (1) and regrowth falls from 94% to 21%, so factor N maintains the stem cell reserve and hair regrowth (1). (b) Adding factor N to group 3 restores values near normal, 11 stem cells and 88% regrowth (1), so the fall in group 2 was caused by the missing factor and not by other damage to the niche cells (1). (c) Without the receptor the stem cells stay low at 2 and regrowth at 18% even though factor N is present (1), so the factor acts directly on the stem cells through that receptor (1). (d) Any one: mice may use different niche signals from humans; the groups are small; follicle counts alone do not show hair quality (1).

(a) Supply =4.0\times4\pi r^2=16\pi r^2 and demand =1.0\times\frac43\pi r^3 (1); dividing gives \frac{16\pi r^2}{\frac43\pi r^3}=\frac{12}{r} (1). (b) 12/6=2.0 (1); supply equals demand when 12/r=1, so r=12\ \mum (1). (c) At 18 \mum the ratio is 12/18=0.67, so supply meets only two thirds of demand and the cell starves unless it divides (1). (d) For the same volume a flat disc has a larger membrane area, so supply per unit of demand is higher (1) and the path from membrane to centre is shorter, so it can be larger before supply falls short (1).

(a) X is cardiac muscle: branched, with one central nucleus (1); Y is skeletal muscle: long, unbranched and with many nuclei (1). (b) The cells are joined end to end and branch (1), so the stimulus is propagated through the wall and many cells contract together (1). (c) Y is one very large cell, formed by the fusion of many precursor cells, so it keeps many nuclei (1); many nuclei supply the mRNA for the very large number of proteins and myofibrils along its length (1).

(a) 82 units is at least 80, so type R (1); 40 units meets the lower limit of type S (1); 37 units is below 40, so type T (1). (b) Each concentration range switches on a different set of genes in the cells (1), so different proteins are made even though every cell has the same genome (1). (c) It becomes type R, because the local concentration, not the origin of the cell, decides its fate (1).

(a) RNA polymerase transcribes the gene into mRNA (1); the mRNA is translated at ribosomes to make the insulin polypeptide (1). (b) The factor binds the promoter and allows RNA polymerase to start transcription (1); it is present only in beta cells, so the gene is expressed only there (1). (c) Methylation blocks transcription of the gene (1), and the methyl pattern is copied during DNA replication, so each daughter cell inherits the silent gene and the identity is kept through mitosis (1).

(a) 0.15+0.10+0.15=0.40\ \mum (1). (b) Oxygen is a small non-polar molecule and passes through the phospholipid bilayer by simple diffusion (1), moving from high concentration in the alveolus to low concentration in the blood (1). (c) Any three: the barrier is only 0.40 \mum thick, so the path is short (1); the flat type I cells and many alveoli give a very large area (1); blood flow removes oxygen, keeping a steep gradient (1); breathing renews the air, keeping alveolar oxygen high (1).

(a) The DNA is replicated before division, giving two identical chromatids per chromosome (1), and mitosis shares one copy of each chromosome to each daughter (1). (b) A stem cell persists and keeps dividing, so every descendant inherits the mutation (1); a lining cell is short-lived and is shed, so its mutation does not last (1). (c) Mitosis keeps the diploid number of chromosomes (1); sperm must be haploid, which needs meiosis, so that fertilisation restores the diploid number and not double it (1).

(a) Contraction needs ATP (1), which a cell that contracts continuously makes by aerobic respiration in many mitochondria (1). (b) Calcium ions bind to troponin (1), which moves tropomyosin so that the binding sites on actin are exposed and myosin heads can attach (1). (c) Myosin heads attach to actin and pivot, pulling the actin filaments towards the centre of the sarcomere, using ATP (1); the filaments slide past each other so the sarcomere shortens although neither filament shortens (1).