AP Biology Unit 8J: Comparative Physiology: Transport & Regulation — practice questions — page 2

Gibberellins are a class of plant hormones (phytohormones) that play a critical role in regulating various developmental processes. Their primary and most well-known function is promoting **stem elongation** by stimulating both cell division and cell elongation between the nodes (internodal growth). ### Breaking down the options: 1. **Stem elongation:** Gibberellins ($GA$) trigger the expression of genes that loosen cell walls, allowing the stem to stretch. This is famously seen in 'bolting' (rapid stem growth) in rosette plants. 2. **Leaf fall (Abscission):** This is primarily controlled by **Ethylene** and **Abscisic Acid ($ABA$)**. 3. **Cell death (Senescence):** This is associated with **Ethylene** and sometimes nutrient stress, whereas Gibberellins usually delay senescence in some tissues. 4. **Stomatal closure:** This is the hallmark function of **Abscisic Acid ($ABA$)**, which acts as a stress hormone during drought. Therefore, the primary function of gibberellins is promoting stem growth.

Insulin is a vital hormone responsible for regulating blood glucose levels. It is produced by the **pancreas**, specifically within specialized clusters of cells known as the **Islets of Langerhans**. Inside these islets, the **beta cells** synthesize and secrete insulin into the bloodstream when glucose levels rise (e.g., after a meal). Let's evaluate the other options: 1. **Thyroid**: Produces hormones like thyroxine ($T_4$) which regulate metabolism. 2. **Liver**: While it stores glucose as glycogen in response to insulin, it does not produce insulin itself. 3. **Pituitary**: Known as the 'master gland,' it produces hormones that regulate other glands (like Growth Hormone or TSH), but not insulin. Therefore, the correct answer is the Pancreas.

The human body requires specific minerals and vitamins to function correctly. Iodine is a crucial trace element required for the synthesis of thyroid hormones like thyroxine ($T_4$) and triiodothyronine ($T_3$) by the thyroid gland. 1. **Iodine's Role**: Iodine is the core component of thyroid hormones which regulate metabolism, growth, and development. 2. **Deficiency Mechanism**: When iodine intake is low, the thyroid gland cannot produce enough hormones. In an attempt to capture more iodine from the blood, the pituitary gland secretes more Thyroid Stimulating Hormone ($TSH$), which causes the thyroid gland to enlarge. 3. **Resulting Condition**: This abnormal enlargement of the thyroid gland, visible as a swelling in the neck, is called **Goitre**. * **Anaemia** is caused by iron deficiency. * **Diabetes** is related to insulin production or sensitivity. * **Rickets** is caused by Vitamin D or Calcium deficiency.