Define excretion and homeostasis. Describe the role of the kidney in osmoregulation and explain how the human…

Question

Define excretion and homeostasis. Describe the role of the kidney in osmoregulation and explain how the human body regulates blood sugar levels.

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SECTION A

1. (a) Define the following terms:

i) Excretion: The process by which metabolic waste products and excess substances are removed from the body of an organism.
ii) Homeostasis: The maintenance of a stable internal environment in an organism despite changes in external conditions.

(b) Describe the role of the kidney in osmoregulation.
The kidneys play a crucial role in osmoregulation by regulating the water and salt balance in the body. They filter blood, reabsorbing necessary water and salts back into the bloodstream, and excreting excess water, salts, and waste products as urine. This process ensures that the concentration of solutes in the body fluids remains within a narrow, healthy range.

(c) Explain how the human body regulates the amount of sugar level in blood.
Blood sugar regulation is primarily controlled by the hormones insulin and glucagon, produced by the pancreas. When blood sugar levels rise (e.g., after a meal), the pancreas releases insulin, which promotes the uptake of glucose by cells for energy or storage as glycogen in the liver and muscles, thus lowering blood sugar. When blood sugar levels fall, the pancreas releases glucagon, which stimulates the liver to convert stored glycogen back into glucose and release it into the blood, raising blood sugar levels.

2. (a) Describe the structure of:

i) A mould fungus: A mould fungus typically consists of a network of thread-like structures called hyphae, which collectively form a mycelium. Some hyphae grow into the substrate to absorb nutrients, while others grow upwards to produce sporangiophores that bear sporangia containing asexual spores.
ii) An Amoeba: An Amoeba is a single-celled eukaryotic organism characterized by its flexible cell membrane and cytoplasm that allows it to change shape constantly. It moves and feeds using temporary cytoplasmic extensions called pseudopods. It contains a nucleus, contractile vacuole for osmoregulation, and food vacuoles for digestion.

(b) How does a mould fungus carry out asexual reproduction?
Mould fungi primarily reproduce asexually through the production of spores. These spores are typically formed within specialized structures like sporangia (e.g., in bread mould) or at the tips of hyphae (e.g., conidia). When mature, these spores are released and dispersed by wind or water. If they land in a suitable environment with sufficient moisture and nutrients, they germinate and grow into new hyphae, forming a new mycelium genetically identical to the parent.

(c) Account for the various ways by which Fungi are beneficial to man.
Fungi are beneficial to humans in several ways:

Decomposers: They break down dead organic matter, recycling nutrients back into the ecosystem.
Food source: Many fungi, like mushrooms, are consumed as food. Yeasts are used in baking (bread) and brewing (alcohol).
Medicine: Some fungi produce antibiotics, such as penicillin from Penicillium mould, which are vital for treating bacterial infections.
Biotechnology: Fungi are used in industrial processes for producing enzymes, organic acids, and vitamins.
Symbiotic relationships: Mycorrhizal fungi form symbiotic relationships with plant roots, enhancing nutrient absorption for plants.

3. (a) State the biological importance of:

i) Nitrogen to plants: Nitrogen is an essential component of proteins, nucleic acids (DNA and RNA), chlorophyll, and ATP. It is crucial for plant growth, photosynthesis, and overall metabolic functions.
ii) Nitrogen fixation in nature: Nitrogen fixation is the process by which atmospheric nitrogen gas ( $\text{N}_2$ ) is converted into ammonia ( $\text{NH}_3$ ), a form usable by plants. This process, carried out by certain bacteria (e.g., Rhizobium in legumes), makes nitrogen available to ecosystems, supporting plant growth and the entire food web.

(b) Describe the role of the following processes in the circulation of carbon in nature:

i) Photosynthesis: Photosynthesis is the process by which green plants, algae, and some bacteria convert atmospheric carbon dioxide ( $\text{CO}_2$ ) and water into glucose (organic carbon) and oxygen, using sunlight as energy. This removes $\text{CO}_2$ from the atmosphere and incorporates carbon into organic compounds, forming the base of most food chains.
ii) Respiration: Respiration is the process by which living organisms break down organic compounds (like glucose) to release energy, producing carbon dioxide ( $\text{CO}_2$ ) and water as byproducts. This process releases carbon back into the atmosphere, completing the carbon cycle. Both plants and animals respire.

(c) How does deforestation affect the carbon cycle?
Deforestation significantly impacts the carbon cycle by reducing the number of trees, which are major carbon sinks. Trees absorb large amounts of carbon dioxide from the atmosphere during photosynthesis. When forests are cleared, this carbon absorption capacity is lost. Additionally, burning or decomposing felled trees releases the stored carbon back into the atmosphere as $\text{CO}_2$ , contributing to increased atmospheric carbon dioxide levels and enhancing the greenhouse effect.

SECTION B

4. (a) State the differences between aerobic and anaerobic respiration.

Aerobic respiration requires oxygen, completely breaks down glucose, produces a large amount of ATP (around 30-32 molecules per glucose), and yields carbon dioxide and water as byproducts.
Anaerobic respiration does not require oxygen, incompletely breaks down glucose, produces a small amount of ATP (2 molecules per glucose), and yields byproducts like lactic acid (in animals) or ethanol and carbon dioxide (in yeast).

(b) Describe an experiment to show that carbon dioxide is produced by a potted plant during respiration.

Place a healthy potted plant in a bell jar.
Place a small beaker containing limewater (calcium hydroxide solution) inside the bell jar next to the plant.
Limewater turns cloudy in the presence of carbon dioxide.
Seal the bell jar to make it airtight.
Cover the bell jar with a dark cloth or place it in a dark room to prevent photosynthesis, ensuring only respiration occurs.
After a few hours, observe the limewater.
Observation: The limewater will turn cloudy.
Conclusion: The cloudiness indicates the presence of carbon dioxide, which was produced by the potted plant during respiration in the absence of light.

(c) What is the disadvantage of putting a potted plant inside a living room at night?
At night, plants perform respiration but not photosynthesis (due to lack of light). During respiration, plants take in oxygen and release carbon dioxide. If a large number of potted plants are kept in a poorly ventilated living room at night, they can significantly reduce the oxygen concentration and increase the carbon dioxide concentration in the air, potentially causing discomfort or health issues for occupants due to reduced oxygen availability.

5. (a) Describe the structure of the mammalian skin.
The mammalian skin consists of two main layers:
- The epidermis is the outermost layer, composed of stratified squamous epithelium. It provides protection against pathogens, UV radiation, and water loss. Its outermost cells are dead and keratinized.
- The dermis is the layer beneath the epidermis, made of connective tissue. It contains blood vessels, nerve endings, hair follicles, sweat glands, and sebaceous glands. It provides strength, elasticity, and nourishment to the epidermis. Below the dermis is the subcutaneous layer (hypodermis), primarily composed of adipose tissue, which provides insulation and energy storage.
(b) What are the differences between the epidermis of the mammalian skin and that of a leaf?
- Mammalian skin epidermis: Composed of multiple layers of cells (stratified), with the outermost layer being dead, keratinized cells. It contains hair follicles, sweat glands, and sensory receptors. Its primary function is protection, thermoregulation, and sensation.
- Leaf epidermis: Typically a single layer of cells, covered by a waxy cuticle to prevent water loss. It contains stomata (pores) flanked by guard cells, which regulate gas exchange and transpiration. Its primary function is protection, gas exchange, and regulation of water loss.
(c) How is temperature regulation carried out in the mammalian skin during overheating?
During overheating, the mammalian skin helps regulate temperature through several mechanisms:
- Sweat production: Sweat glands in the skin produce sweat, which evaporates from the skin surface. This evaporation carries away heat, cooling the body.
- Vasodilation: Blood vessels in the dermis dilate (widen), increasing blood flow to the skin surface. This allows more heat to radiate away from the body into the environment.
- Hair relaxation: Hair erector muscles relax, causing hairs to lie flat against the skin, which reduces the insulating layer of trapped air and allows heat to escape more easily.
6. (a) Define the following terms as used in genetics:
- i) Homozygous: An individual having two identical alleles for a particular gene (e.g., AA or aa).
- ii) Heterozygous: An individual having two different alleles for a particular gene (e.g., Aa).
- iii) Recessive: An allele that is only expressed phenotypically when two copies are present (i.e., in a homozygous recessive individual, aa). Its effect is masked by a dominant allele in a heterozygous state.
- iv) Dominance: The phenomenon where one allele (the dominant allele) completely masks the expression of another allele (the recessive allele) in a heterozygous individual.
(b) A farmer grows plants with rose flowers that are either red in colour or white. He realized that when he crossed a pure breed red flowered plant with the pure breed white flowered plant, he crossed the pure breed red flowered plant with the pure breed white flowered plant. Using genetic symbols and diagrams explain the results of this cross.
Assumption: The repeated phrase "he crossed the pure breed red flowered plant with the pure breed white flowered plant" implies that the F1 generation was pink, indicating incomplete dominance.

Let R be the allele for red flowers and W be the allele for white flowers. Since the cross of pure red and pure white results in pink, this is an example of incomplete dominance.

Parental Generation (P):
Pure Red (RR) $\times$ Pure White (WW)

Gametes:
R W

First Filial Generation (F1):
$\begin{array}{|c|c|} \hline & \text{W} \\ \hline \text{R} & \text{RW} \\ \hline \end{array}$
All F1 offspring will be heterozygous (RW) and have pink flowers.

Explanation: When a pure breed red flower (RR) is crossed with a pure breed white flower (WW), all the offspring in the F1 generation inherit one allele for red (R) and one allele for white (W). Due to incomplete dominance, neither allele is fully dominant, resulting in an intermediate phenotype, which is pink flowers.

(c) What would be the outcome of a cross between the pink flowers and the red parent?
Cross: Pink flowers (RW) $\times$ Red parent (RR)

Gametes:
From Pink (RW): R, W
From Red (RR): R, R

Punnett Square for F2 Generation:
$\begin{array}{|c|c|c|} \hline & \text{R} & \text{R} \\ \hline \text{R} & \text{RR} & \text{RR} \\ \hline \text{W} & \text{RW} & \text{RW} \\ \hline \end{array}$
Genotypic Ratio: 1 RR : 1 RW
Phenotypic Ratio: 1 Red : 1 Pink

Outcome: The offspring would be 50% red flowers (RR) and 50% pink flowers (RW).

7. (a) Define the following terms:
- i) Stimulus: Any detectable change in the internal or external environment that can cause a response in an organism.
- ii) Tropism: A growth movement of a plant in response to an external stimulus, where the direction of growth is determined by the direction of the stimulus (e.g., phototropism towards light).
- iii) Reflex action: A rapid, involuntary, and unconscious response of the body to a stimulus, mediated by a reflex arc (e.g., withdrawing hand from a hot object).
- iv) Effector: A muscle or gland that responds to a nerve impulse from the central nervous system, carrying out the final action or response to a stimulus.
**(b) Describe

Define excretion and homeostasis. Describe the role of the kidney in osmoregulation and explain how the human body regulates blood sugar levels.

Step-by-step solution

Place a healthy potted plant in a bell jar.

Place a small beaker containing limewater (calcium hydroxide solution) inside the bell jar next to the plant.

Seal the bell jar to make it airtight.

Cover the bell jar with a dark cloth or place it in a dark room to prevent photosynthesis, ensuring only respiration occurs.

After a few hours, observe the limewater.

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