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
(a) Calculate the magnification of the image, showing your working. [2]
- Step 1: Measure the scale bar in the image.
From the image, the scale bar is labeled as 15 µm (micrometres).
Let’s say the bar measures 1.5 cm (15 mm) on your screen. - Step 2: Convert screen measurement to µm:
1.5 cm = 15 mm = 15,000 µm - Step 3: Apply the magnification formula:
\[
\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}} = \frac{15000\ \mu\text{m}}{15\ \mu\text{m}} = 1000\times
\]
(b) Predict how the stomatal density for the upper epidermis would compare. [1]
The stomatal density of the upper epidermis would be lower than that of the lower epidermis.
Why?
In most dicot plants (like Protium decandrum), the lower surface has more stomata to reduce water loss because it’s less exposed to direct sunlight. The upper surface has fewer stomata to limit evaporation.
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
a. i. Identify the dark structure indicated by I. → Nucleus
It’s the dense, dark oval on the edge of the muscle fibre a muscle fibre has many nuclei.
a. ii. Identify the protein in the thick filament (II). → Myosin
Found in the dark A-band; responsible for pulling actin during contraction.
a. iii. Identify the structure marked III. → Muscle fibre / Muscle cell
A long, multinucleated cell with myofibrils inside.
b. Does the tissue consist of cells? → Yes.
- Muscle fibres have one membrane (sarcolemma) → matches cell structure.
- They’re multinucleated → an exception to usual single-nucleus cell rule.
- Formed by fusion of cells → called syncytia.
c. Role of calcium in muscle contraction:
- Nerve impulse triggers calcium release from sarcoplasmic reticulum.
- Calcium binds to troponin, shifting tropomyosin.
- This exposes actin binding sites, allowing myosin to bind and contract the muscle.