Class 9 Biology Notes Chapter 9

Chapter 9: Plant Physiology

Exercise Solution

A. Select the Correct Answers for the Following Questions

1. Which of the following plant nutrients is required in large amount?

a) Iron
b) Zinc
c) Potassium
d) Boron

Correct Answer: (c) Potassium

2. Which element is required by plants for the formation of chlorophyll?

a) Phosphorus
b) Calcium
c) Magnesium
d) Sulphur

Correct Answer: (c) Magnesium

3. The primary function of root hairs is:

a) Transport of nutrients
b) Storage of food
c) Increase surface area for absorption
d) Synthesis of proteins

Correct Answer: (c) Increase surface area for absorption

4. Root hairs absorb salts from soil by:

a) Diffusion
b) Osmosis
c) Active transport
d) Bulk flow

Correct Answer: (c) Active transport

5. Water moves from the soil into root cells by:

a) Osmosis
b) Active transport
c) Diffusion
d) Filtration

Correct Answer: (a) Osmosis

6. Transpiration is regulated by:

a) Mesophyll
b) Guard cells
c) Xylem
d) Phloem

Correct Answer: (b) Guard cells

7. Under which condition will there be a high rate of transpiration?

a) High humidity
b) Low light intensity
c) Wind
d) Waterlogged soil

Correct Answer: (c) Wind

8. Which ion plays a role in the opening of stomata?

a) Sodium (Na⁺)
b) Potassium (K⁺)
c) Calcium (Ca²⁺)
d) Magnesium (Mg²⁺)

Correct Answer: (b) Potassium (K⁺)

9. In most plants, food is transported in the form of:

a) Glucose
b) Sucrose
c) Starch
d) Maltose

Correct Answer: (b) Sucrose

10. What is TRUE according to the pressure flow mechanism of food transport?

a) Water enters the source, creating pressure
b) Water is pulled from the sink
c) Movement of food in phloem is due to gravity
d) Solutes move from low to high concentration

Correct Answer: (a) Water enters the source, creating pressure

11. Succulent organs are present in:

a) Xerophytes
b) Hydrophytes
c) Mesophytes
d) Halophytes

Correct Answer: (a) Xerophytes

B. Write Short Answers

1. Define mineral nutrition in plants.

Answer: Mineral nutrition refers to the study of how plants acquire, absorb, transport, and utilize inorganic mineral ions from the soil for their growth, development, and metabolic processes.

2. Define macronutrients and micronutrients and give examples.

Answer:

Macronutrients: Essential plant nutrients required in relatively large amounts, such as nitrogen, phosphorus, potassium, and calcium.

Micronutrients: Essential plant nutrients required in trace or very small amounts, such as iron, zinc, boron, and manganese.

3. State the roles of nitrogen and magnesium in plants.

Answer:

Nitrogen: It is an essential component of amino acids, proteins, nucleic acids, and chlorophyll. It promotes vigorous vegetative growth.

Magnesium: It acts as the central structural component of the chlorophyll molecule and activates important enzymes involved in carbohydrate and ATP metabolism.

4. Define transpiration and its types.

Answer: Transpiration is the loss of water vapour from the aerial parts of plants, primarily leaves, into the atmosphere.

Its types include:

1. Stomatal Transpiration: Loss of water vapour through stomata. It accounts for approximately 90% or more of transpiration.

2. Cuticular Transpiration: Loss of water vapour directly through the leaf cuticle.

3. Lenticular Transpiration: Loss of water vapour through lenticels in woody stems and bark.

5. How is transpirational pull important in plants?

Answer: Transpirational pull creates a continuous negative pressure (tension) in the xylem vessels, drawing water and dissolved mineral ions upward from the roots to the uppermost leaves against gravity. It also helps cool the plant.

6. Transpiration is the loss of water from plants. Is it a harmful phenomenon? If not, what is its importance?

Answer: No, transpiration is not entirely harmful. It is often termed a “necessary evil.” Its importance includes driving the transport of water and minerals, cooling the plant’s leaf surface, and maintaining cell turgidity.

7. Differentiate between the following:

i. Xylem and Phloem

Xylem: Transports water and dissolved mineral ions upward from the roots to the shoots.

Phloem: Transports synthesized organic food, mainly sugars, throughout the plant from sources to sinks.

ii. Transpiration and Guttation

Transpiration: The loss of water as vapour through stomata and the cuticle, driven by evaporation.

Guttation: The expulsion of liquid water droplets from leaf margins through hydathodes, driven mainly by root pressure.

iii. Hydrophytes and Halophytes

Hydrophytes: Plants adapted to live completely or partially submerged in aquatic environments.

Halophytes: Plants adapted to grow in soils with high salinity, such as salty marshes.

iv. Hydrophytes and Xerophytes

Hydrophytes: Plants that live in water-abundant environments and generally have thin cuticles and reduced root systems.

Xerophytes: Plants that live in arid, water-scarce environments and may have thick cuticles, sunken stomata, and water-storing tissues.

v. Lenticular Transpiration and Stomatal Transpiration

Lenticular Transpiration: Occurs through lenticels, which are small pores in woody bark.

Stomatal Transpiration: Occurs through adjustable stomatal pores on leaf surfaces.

8. How do rubber and keekar plants excrete their wastes?

Answer: Rubber plants excrete wastes in the form of sticky latex stored in specialized latex vessels, while keekar (Acacia) trees excrete excess metabolic waste by producing gum that oozes out of their stems and bark.

C. Write Answers in Detail

1. Describe the events involved in the opening and closing of stomata.

Answer:

Opening of Stomata (Daylight): Guard cells actively accumulate potassium ions (K⁺) from neighbouring epidermal cells, decreasing the water potential inside the guard cells. Water enters by osmosis, causing the guard cells to swell and bow outward because of their thicker inner walls. This opens the stomatal pore.

Closing of Stomata (Night or Stress): Potassium ions (K⁺) flow out of the guard cells into surrounding cells, followed by water loss through osmosis. The guard cells lose turgor pressure, become less curved, and close the stomatal pore.

2. Explain the internal structure of a root and describe the uptake of salts and water by the root.

Answer:

Internal Structure of the Root: From outside to inside, a root consists of the epidermis bearing root hairs, cortex, endodermis with waterproof Casparian strips, pericycle, and central vascular cylinder containing xylem and phloem.

Uptake of Water: Water enters root hairs through osmosis, moving down a water potential gradient. It then moves across the cortex and reaches the xylem.

Uptake of Salts: Mineral ions are actively transported across root cell membranes against a concentration gradient, requiring metabolic energy in the form of ATP.

3. Describe temperature, wind, and humidity as factors affecting the rate of transpiration.

Answer:

Temperature: Higher temperatures increase kinetic energy and the rate of evaporation, thereby increasing the rate of transpiration.

Wind: Wind sweeps away humid air accumulating around the leaf surface. This maintains a steep water vapour concentration gradient and accelerates transpiration.

Humidity: High atmospheric humidity increases the water vapour content of the surrounding air, slowing evaporation and lowering the rate of transpiration.

4. Describe the mechanism of transport of water and salts in plants.

Answer: Water and dissolved salts move upward through the xylem via the Cohesion-Tension Theory. Transpiration at the leaves creates a negative pressure or tension that pulls water upward. Water molecules form an unbroken column in the xylem vessels because of hydrogen bonding (cohesion) and adhere to the vessel walls (adhesion). This pulls water continuously from the roots to the leaves. Root pressure also provides a minor pushing force.

5. Explain the mechanism of food translocation by the Pressure Flow Mechanism.

Answer: Proposed by Ernst Münch, the Pressure Flow (Mass Flow) Hypothesis explains sugar transport in the phloem.

At the Source (Leaves): Sugars are actively loaded into phloem sieve tubes, lowering the water potential. Water enters the sieve tubes from adjacent xylem vessels by osmosis, creating high hydrostatic pressure (turgor pressure).

At the Sink (Roots and Fruits): Sugars are actively unloaded and consumed or stored, increasing the water potential. Water leaves the phloem and returns to the xylem, lowering the pressure.

This pressure gradient forces sugar-rich sap to flow in bulk from the source to the sink.

6. How do plants excrete extra water and salts from their bodies?

Answer: Plants eliminate extra water primarily through transpiration (evaporation) and guttation (exudation of liquid droplets through hydathodes).

Excess salts are removed or managed in several ways:

1. Excretion through salt glands in halophytic plants.

2. Accumulation in older leaves that eventually fall off.

3. Storage in vacuoles as relatively harmless crystals, such as calcium oxalate.

7. Describe the process of gaseous exchange in plants.

Answer: Plants exchange gases (O₂ and CO₂) with the atmosphere through stomata in leaves and young stems, and through lenticels and loose bark tissues in woody stems.

During daylight, photosynthesis consumes CO₂ and releases O₂. During respiration, which occurs both day and night, plants consume O₂ and release CO₂ through diffusion.

8. Describe the mechanisms and adaptations in plants for the excretion of wastes.

Answer: Plants use several mechanisms to remove or manage waste products:

1. Storage in Shedding Parts: Waste products are stored in falling leaves, bark, or fruits, which are later shed.

2. Storage as Crystals: Metabolic wastes are stored as relatively inert crystals, such as calcium oxalate or calcium carbonate, in plant vacuoles.

3. Secretion of Substances: Gums, resins, latex, and mucilage are secreted through specialized ducts or surface glands.

4. Release of Gases: Gaseous substances, including CO₂ and O₂, pass directly through stomata and lenticels.

9. Explain osmotic adjustments in hydrophytes, xerophytes, and halophytes.

Answer:

Hydrophytes: These plants live in water-abundant environments. They generally have less need for specialized osmotic adjustments to overcome water shortage because water is readily available.

Xerophytes: These plants live in arid climates. They accumulate high concentrations of solutes in their cell vacuoles to lower their water potential, enabling them to absorb scarce moisture from dry soils.

Halophytes: These plants live in salty soils. They accumulate mineral salts and organic solutes in their cells to maintain a sufficiently low water potential, allowing them to absorb water from saline soil instead of losing it through osmosis.

D. Inquisitive Questions

1. Why do plants transpire more on a windy day compared to a humid one?

Answer: On a windy day, moving air rapidly disperses the layer of moisture that accumulates around the leaf stomata. This maintains a steep concentration gradient of water vapour between the moist intercellular spaces of the leaf and the surrounding air. The rapid removal of water vapour accelerates evaporation and increases the rate of transpiration.

In contrast, on a humid day, the surrounding air already contains a high amount of moisture. This reduces the concentration gradient and slows down evaporation and transpiration.

Notes by Sir Dilawar

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