The following diagram shows the position of endocrine glands in the human body. The hormone secreted by structure 3 is called:

13. The diagram shows the position of endocrine glands. Structure 3 is located in the adrenal gland, which sits atop the kidneys. The adrenal gland secretes adrenaline (also known as epinephrine).

• A. Thyroxine is secreted by the thyroid gland.
• B. Adrenaline is secreted by the adrenal gland.
• C. Progesterone is secreted by the ovaries.
• D. Testosterone is secreted by the testes.

The final answer is $\boxed{B}$

14. Sister chromatids are two identical copies of a chromosome joined together. The point where they are attached is called the centromere.

• A. Centrioles are involved in cell division in animal cells but are not the attachment point for chromatids.
• B. The centromere is the constricted region of a chromosome that links sister chromatids.
• C. A chromosome is the entire structure containing the chromatids.
• D. The spinal cord is part of the nervous system.

The final answer is $\boxed{B}$

15. To reduce water loss in extremely dry and hot environments, plants need adaptations that minimize transpiration.

• A. Having stomata sunken into pits helps reduce water loss by creating a humid microenvironment.
• B. A thin transparent upper epidermis allows light penetration but doesn't directly reduce water loss.
• C. A high density of stomata on the upper surface would increase water loss, as the upper surface is more exposed to direct sunlight and wind.
• D. Developing a very thin cuticle on leaf surfaces would increase water loss, as the cuticle is a waxy layer that prevents water evaporation. A thick cuticle would reduce water loss.

The final answer is $\boxed{A}$

16. A reflex action is an involuntary, rapid response to a stimulus. The pathway of a nerve impulse typically involves:

1. Receptor detects stimulus.
2. Sensory neuron transmits impulse from receptor to central nervous system.
3. Relay neuron (interneuron) in the central nervous system processes the impulse.
4. Motor neuron transmits impulse from central nervous system to effector.
5. Effector (muscle or gland) carries out the response.

Therefore, the correct sequence is Receptor $\to$ Sensory neuron $\to$ Relay neuron $\to$ Motor neuron $\to$ Effector.

• A. Effector to sensory neuron is incorrect.
• B. Receptor to sensory neuron to relay neuron to motor neuron is the correct sequence.
• C. Motor neuron to relay neuron is incorrect.
• D. Receptor to motor neuron to effector is incorrect as it skips the sensory and relay neurons.

The final answer is $\boxed{B}$

17. The structure that prevents backflow of blood from the aorta to the left ventricle is a valve. Blood flows from the left ventricle into the aorta. If blood were to flow back from the aorta into the left ventricle, it would pass through the aortic semilunar valve.

• A. The semilunar valve (specifically the aortic semilunar valve) prevents backflow from the aorta into the left ventricle.
• B. The bicuspid valve (mitral valve) prevents backflow from the left ventricle to the left atrium.
• C. The tricuspid valve prevents backflow from the right ventricle to the right atrium.
• D. Tendon supporting valve is not a specific valve type.

The final answer is $\boxed{A}$

18. The large intestine's primary function is to absorb water and electrolytes from indigestible food matter and to pass waste material from the body.

• A. Digestion of all types of food primarily occurs in the small intestine.
• B. The large intestine is connected to the small intestine, not directly to the stomach.
• C. The large intestine absorbs water.
• D. The small intestine is the longest part of the intestinal track.

The final answer is $\boxed{C}$

19. In the cell of an amoeba, the contractile vacuole functions in osmoregulation. Amoebas live in freshwater, which is hypotonic to their cytoplasm. Water constantly enters the cell by osmosis. The contractile vacuole collects excess water and expels it, preventing the cell from bursting.

• A. Reproduction is a separate process.
• B. Food storage is handled by food vacuoles.
• C. Osmoregulation is the regulation of water balance.
• D. Synthesizing proteins occurs on ribosomes.

The final answer is $\boxed{C}$

20. Spirogyra is a type of green algae. It is autotrophic because it contains chloroplasts, which are organelles responsible for photosynthesis.

• A. A cell wall provides structural support but doesn't make it autotrophic.
• B. Pyrenoid is a proteinaceous structure within the chloroplast that stores starch, but the chloroplast itself is responsible for photosynthesis.
• C. Mucilage is a slimy substance, not related to autotrophy.
• D. Chloroplasts contain chlorophyll and are the site of photosynthesis, making Spirogyra autotrophic.

The final answer is $\boxed{D}$

22. The alveolus is well-adapted for efficient gas exchange due to several features:

• 1. It is highly permeable: The alveolar and capillary walls are very thin, allowing gases to diffuse easily. (Correct)
• 2. It is highly vascularized: A rich network of capillaries surrounds each alveolus, ensuring a large blood supply for gas exchange. (Correct)
• 3. It has a thick epithelium: This is incorrect. The alveolar epithelium is extremely thin (squamous epithelial cells) to facilitate rapid gas diffusion. (Incorrect)

Therefore, statements 1 and 2 are correct. The final answer is $\boxed{A}$ (assuming A corresponds to "1 and 2 are correct" based on the provided directions table in the previous image).

23. Arteries and veins can be distinguished by several characteristics:

• 1. Arteries have many valves than veins: This is incorrect. Veins have valves to prevent backflow of blood, especially in the limbs, while arteries generally do not. (Incorrect)
• 2. Arteries carry blood at higher pressure than veins: This is correct. Arteries carry blood directly from the heart, which generates high pressure. (Correct)
• 3. Arteries have thicker walls than veins: This is correct. Arteries have thicker, more muscular and elastic walls to withstand the high pressure of blood pumped from the heart. (Correct)

Therefore, statements 2 and 3 are correct. The final answer is $\boxed{B}$ (assuming B corresponds to "2 and 3 are correct" based on the provided directions table in the previous image).

24. Phototropism, the bending of a plant shoot towards light, is primarily caused by the uneven distribution of auxins.

• 1. Uneven distribution of auxins: Auxins migrate to the shaded side of the shoot, promoting cell elongation there. (Correct)
• 2. Faster cell elongation on the shaded side: The higher concentration of auxins on the shaded side causes cells there to elongate more rapidly than cells on the illuminated side, leading to bending towards the light. (Correct)
• 3. Increase in cell division on the lighted side: Auxins primarily affect cell elongation, not cell division, in this context. Also, the effect is more pronounced on the shaded side. (Incorrect)

Therefore, statements 1 and 2 are correct. The final answer is $\boxed{A}$ (assuming A corresponds to "1 and 2 are correct" based on the provided directions table in the previous image).