Chapter 7 - Nutrition in Plants

A simplified word equation for photosynthesis is: carbon dioxide + water --> glucose + oxygen + water.

What are the other conditions necessary for photosynthesis?

• humidity and light

• chlorophyll and light

• light and nutrients

• chlorophyll and nutrients

Answer explanation

Recall the definition of photosynthesis - that chlorophyll converts light energy to chemical energy in the form of glucose.

The graph shows the how the oxygen released by a plant changes over 10 hours.

At one point during the experiment, the plant was moved from a dark room to a sunny spot. When was this likely to be?

• beginning of the experiment

• 2nd hour

• 3rd hour

• sometime within the first 3 hours

Answer explanation

Oxygen release indicates the rate of photosynthesis since oxygen is produced during photosynthesis.

At the 2nd hour, the rate of O2 release begins to increase, indicating that the plant has begun to photosynthesise. Hence, this was the point when it was moved to the sunny spot.

The graph shows the how the oxygen released by a plant changes over 10 hours.

During which period did respiration occur?

• 0-3 hours

• 3-10 hours

• 0-10 hours

• 4-10 hours

Answer explanation

Recall that respiration occurs ALL the time in living things, including plants!

1 hour after the light source was switched on, the meniscus of the water in the syringe moved downwards. Which statement explains this observation?

• The water was pulled downwards by gravity

• Oxygen produced by the plant displaced the water

• Capillary action occurs, which causes water to move down the tube

• Plant respired and produced carbon dioxide, which displaced the water

Answer explanation

In narrow tubes, water will not move downwards because of surface tension, even in the presence of gravity.

Capillary action explains why water moves UP in a narrow tube against gravity, but not downwards.

The plant undergoes photosynthesis, producing O2 which is collected as a gas in space X. Since X increases in volume and water cannot be compressed, water is displaced downwards.

CO2 produced by respiration is used up during photosynthesis and not released as a gas.

What is the likely observation if the water in the syringe was replaced with dilute sodium bicarbonate solution?

• The meniscus moves downwards at a slower rate

• The meniscus moves downwards at the same rate

• The meniscus moves downwards at a faster rate

• The meniscus moves upwards instead

Answer explanation

When added to water, sodium bicarbonate increases the amount of CO2 present in the water.

As CO2 is no longer a limiting factor, the rate of photosynthesis is likely to increase hence the amount of O2 produced would increase at a higher rate.

Thus the meniscus moves downwards at a faster rate.

Which of the following statements about the fate of glucose formed during photosynthesis is NOT true?

• Glucose combines with mineral salts to form amino acids

• Glucose is used immediately in the leaf to form cellulose cell walls

• Excess glucose can be converted to lipids and used to make cell membranes

• Excess glucose is converted to starch for transport to storage organs

Answer explanation

Glucose is converted to sucrose for transport, and then converted to starch once it has been transported to the storage organ.

Starch cannot be transported as it is a polysaccharide; starch molecules are too large to be transported.

Why is the lamina of a leaf usually thin and flat?

• Prevents damage of chlorophyll in cells by excess light

• Enables sunlight to reach more guard cells

• Prevents leaf from being too heavy

Answer explanation

When plants are exposed to too much light, their chlorophyll may be damaged. Having thin leaves is a disadvantage as light is able to more easily penetrate the leaf.

Thin leaves ensure that the efficient gas exchange as CO2 has a shorter diffusion distance.

While thin leaves allows greater penetration of light to the underside where the guard cells are found, this is not the most important advantage of thinner leaves.

Why does the leaf contain many intercellular spaces?

• Allows the leaf to accumulate starch reserves during the day

• Leaf can store water for periods of dry weather

• Mesophyll cells have maximum surface area to volume ratio for photosynthesis

• Ensures that gaseous exchange with the environment is more efficient

Answer explanation

Starch reserves are not stored in the intercellular spaces in the leaf; rather, they are stored within the cells.

The water in intercellular spaces evaporates the most quickly since it is exposed to the surroundings via the stomata. This is hence not where the plant stores water.

Why do mesophyll cells need to maximise surface area to volume ratio? This option is incomplete.

What is/are the function(s) of layer A?

• protects inner layers of leaf

• minimises evaporation of water from leaf

• blocks excessive sunlight to prevent damage to chlorophyll

• allows gaseous exchange to occur through leaf surface

Answer explanation

Layer A is the epidermis, which is a waterproof, impermeable layer that does not allow gases to pass through.

Which of the following statements correctly describes layers B and E?

• Layer E tends to have a thicker cuticle than layer B

• Both layers B and E often comprise more than one layer of cells

• Layer E tends to contain more guard cells than layer B

• Cells in layer B contain more chloroplasts since they receive more sunlight

Answer explanation

The thickness of the upper and lower cuticle is approximately the same.

Layers B and E are te epidermal layers, which are both only one-cell-thick.

Layer E (lower epidermis) contains many stomata, which are regulated by guard cells. Layer B (upper epidermis) contains much fewer stomata and hence fewer guard cells.

Neither layer B nor E contain chloroplasts except in the guard cells.