Question
The stomatal density is the number of stomata per area of leaf epidermis. A study was done on the stomatal density of the lower epidermis of leaves from Protium decandrum, a tree found in the Amazon forest.
The image shows the epidermis stripped from the lower surface of a leaf.
a. Calculate the magnification of the image, showing your working. . . . . . . . . . . . . . . . . . . . . x [2]
b. The mean stomatal density for the lower epidermis of P. decandrum was around 600 per mm2. Predict how the stomatal density for the upper epidermis would compare. [1]
▶️Answer/Explanation
Markscheme
Calculation: size of bar ÷ 15 μm (1.5 cm ÷ 15 μm or 15 000 μm ÷ 15 μm);
Answer: 1000 x;
(upper surface/epidermis usually has) fewer stomata/lower stomatal density/no stomata/OWTTE
Question
This light micrograph shows skeletal muscle.
a.i. Identify the dark structure indicated by I. [1]
a.ii. Identify the protein producing the thick filament in the dark band indicated by II. [1]
a.iii. Identify the structure indicated by III. [1]
b. Discuss whether the tissue shown in the micrograph consists of cells or not. [2]
c. Explain how calcium is involved in muscle contraction. [3]
▶️Answer/Explanation
Markscheme
nucleus ✔
myosin ✔
muscle fibre/muscle cell ✔
Reject myofibril because it would be much narrower – diameter
a. «muscle fibres are» multinucleate/contain many nuclei «whereas cells are expected to have only one/so muscle fibers are an exception to the cell theory» ✔
b. one cell membrane/sarcolemma enclosing a whole muscle fibre «as expected for cells» ✔
c. very large/much larger/longer/than most cells ✔
d. muscle fibres formed by fusion of cells/are syncytia ✔
a. action potential/nerve impulse causes release of calcium ✔
b. from sarcoplasmic reticulum/specialized endoplasmic reticulum ✔
c. binds to troponin ✔
d. causes tropomyosin to move/be removed «from binding sites» ✔
e. exposes myosin-binding sites on actin/allows myosin «heads» to bind to actin ✔