Questions on plan diagrams and tissue sections

Question 1

Describe the layers of the wall of the trachea in order from the lumen outward, naming the tissue found in each layer.

Accepted Answer

B1 ciliated epithelium, with goblet cells among it, lining the lumen B1 loose tissue beneath it, containing mucous glands and blood vessels B1 smooth muscle, not as a complete layer but as a band joining the open ends of the cartilage at the back B1 cartilage, as an incomplete C-shaped ring B1 an outer layer of connective tissue, attaching the trachea to its neighbours

Question 2

Describe the distribution of xylem and phloem in a transverse section of the root and of the stem of a herbaceous dicotyledonous plant.

Accepted Answer

B1 in the root the vascular tissue forms a single central stele B1 the root's xylem forms a central star, with the phloem in separate patches between the arms B1 in the stem the vascular tissue is split into discrete bundles arranged in a ring near the outside B1 within each stem bundle the xylem lies on the inner side and the phloem on the outer side

Question 3

A transverse section of a herbaceous dicot stem is 4.0 mm across, and its ring of vascular bundles lies 0.50 mm in from the epidermis. A student draws a low-power plan diagram of the section as a circle 120 mm across. Calculate the magnification to be written on the student's plan, calculate how far in from its outer line the ring of vascular bundles must be drawn, and state which rule of the plan diagram convention the second calculation serves.

Accepted Answer

M1 magnification = size of drawing ÷ actual size, both in millimetres: 120 ÷ 4.0 A1 ×30, written on the plan with no unit, because a plan diagram must state its magnification or carry a scale bar A1 0.50 mm × 30 = 15 mm in from the outer line of the drawing B1 the tissues of a plan diagram must be drawn in their correct proportions, so the position of each boundary is measured, with an eyepiece graticule where needed, and scaled rather than estimated

Question 4

State the four named blood cells of a stained smear in order of increasing diameter, giving an approximate size in micrometres for each.

Accepted Answer

B1 erythrocyte, about 7 µm B1 lymphocyte, about 9 µm B1 neutrophil, about 12 µm B1 monocyte, up to about 18 µm

Question 5

A root carries its xylem as a central column, while a stem carries its lignified vascular bundles in a ring near its surface. Suggest why each arrangement suits the forces that organ must withstand.

Accepted Answer

B1 a root is loaded in tension, pulled by the shoot it anchors and by wind acting on the plant above B1 a central column resists pulling in the way a rope does, so strength down the middle suits the root B1 a stem is loaded in bending, and a hollow cylinder of strong material near the outside resists bending far better than the same material in the centre

Question 6

Identify each of the following cells seen on a stained blood smear, giving a reason for each: cell A is 12 µm across with a nucleus of three connected lobes; cell B is 18 µm across with a kidney-shaped nucleus; cell C is 7 µm across and has no nucleus.

Accepted Answer

B1 A is a neutrophil, because a nucleus of three or more connected lobes identifies it B1 B is a monocyte, the largest cell on a smear, with the kidney-shaped nucleus that separates it from a lymphocyte B1 C is an erythrocyte, because mature erythrocytes are the only blood cells with no nucleus

Question 7

On a micrograph of a blood smear taken at a magnification of 2000 times, a monocyte measures 36 mm across. Calculate the actual diameter of the monocyte in micrometres, and use your answer and the nucleus to explain how the monocyte is told apart from the lymphocyte beside it.

Accepted Answer

M1 actual size = image size ÷ magnification, with 36 mm as 36 000 µm: 36 000 ÷ 2000 = 18 µm B1 at about 18 µm the monocyte is the largest cell on the smear, about twice the diameter of a lymphocyte at about 9 µm B1 the monocyte's nucleus is kidney or horseshoe shaped and pushed to one side, whereas a lymphocyte's round nucleus almost fills the cell, leaving only a thin rim of cytoplasm

Question 8

Sections are taken through three different airways. Describe how the cartilage seen in each section allows the trachea, a bronchus and a bronchiole to be told apart.

Accepted Answer

B1 the trachea shows a single C-shaped ring of cartilage, incomplete at the back where smooth muscle spans the gap B1 a bronchus shows cartilage broken into separate curved plates with gaps between them B1 a bronchiole shows no cartilage, only epithelium, smooth muscle and elastic fibres

Question 9

Describe the distribution of xylem and phloem in a transverse section through the midrib of a leaf, and describe how this arrangement relates to the arrangement in the stem the leaf grew from.

Accepted Answer

B1 the xylem lies on the upper side of the vein and the phloem on the lower side B1 this is the same vascular bundle that ran through the stem, carried out sideways into the leaf B1 what was the inner face of the bundle in the stem, the xylem, becomes the upper face in the leaf, so the orientation is continuous

Question 10

A student cuts a sunflower stem twice, once transversely and once longitudinally through its centre, and the longitudinal section shows no vascular tissue at all. Explain why this section might show no vascular bundles, and explain why this does not mean the stem lacks vascular tissue.

Accepted Answer

B1 the vascular bundles are discrete and lie in a ring near the edge of the stem, rather than filling the whole cross-section B1 a longitudinal cut straight through the centre of the stem can pass entirely between two bundles, showing only pith and cortex B1 the transverse section already showed the ring of bundles, so the correct conclusion is that one section plane is insufficient, not that the tissue is absent

Question 11

Compare a low-power plan diagram with a high-power biological drawing, referring to what each records and how many cells each shows.

Accepted Answer

B1 a plan diagram records the positions, boundaries and proportions of whole tissues without drawing any individual cell, whereas a high-power drawing records the structure of a small number of representative cells in detail B1 a plan diagram is drawn at low magnification so a whole organ or section fits in view, whereas a high-power drawing is made at a magnification high enough to show detail within one or a few cells B1 combining the two, drawing cellular detail inside a plan diagram, breaks the convention, because a plan diagram's purpose is the tissue map rather than cell structure

Question 12

A cross-section is cut through a leaf blade away from the midrib, between two of the smaller veins. Suggest why this section is much less useful than a section through the midrib for showing the position of xylem relative to phloem.

Accepted Answer

B1 away from the midrib a vein is much smaller, with the xylem and phloem forming only a tiny bundle among the mesophyll, difficult to make out clearly B1 a section between two small veins might miss the vascular tissue almost entirely, showing mostly mesophyll and epidermis B1 the midrib is a fixed, predictable location carrying the largest vein, where the tissues are clearly separated and the upper-xylem, lower-phloem arrangement is easy to see

Question 13

State two rules of the low-power plan diagram convention, and state what a plan diagram records that a high-power drawing does not.

Accepted Answer

B1 any two of: tissue boundaries only with no individual cells, no shading, sharp unbroken lines, regions in correct proportion, ruled label lines touching the structure, magnification or scale bar stated B1 it records the positions and proportions of whole tissues within the organ, rather than the detail of any cell

Question 14

Explain why the cartilage in the tracheal wall forms an incomplete C-shape rather than a complete ring.

Accepted Answer

B1 the gap faces the oesophagus, which lies immediately behind the trachea B1 leaving the ring incomplete, with smooth muscle spanning the gap, allows the oesophagus room to bulge as swallowed food passes down it

Question 15

State the typical ratio of erythrocytes to white blood cells seen in a field of a blood smear, and state one feature of an erythrocyte's appearance that follows from its biconcave shape.

Accepted Answer

B1 roughly 700 erythrocytes for every 1 white blood cell B1 the biconcave disc is thinner in the middle, so that part of the cell stains more palely and the cell appears as a ring with a paler centre