Photosynthesis & Leaf Structure (Cambridge (CIE) IGCSE Biology)

True.

The carbohydrate sucrose is used for transport in the phloem.

Glucose is converted into sucrose before being transported around the plant inside phloem.

Glucose is used in respiration to release energy for cells.

Two essential mineral ions required by plants are:

• nitrates

• magnesium ions

True.

Nitrates are needed for the production of amino acids, which are joined together to build proteins.

Plants need magnesium ions for the production of chlorophyll, which is essential for light absorption in photosynthesis.

The purpose of destarching plants by placing in the dark for 24 hours is to ensure that any pre-existing starch in the leaves is used up so it does not affect the results.

True.

Removing chlorophyll by boiling in ethanol is an important step in testing leaves for starch. Removing the green colour allows the results of the starch test to be seen more clearly.

Iodine solution is used to test for starch; it turns blue-black in the presence of starch.

False.

A leaf that has been destarched and then covered with foil will not turn blue-black when iodine is added but will remain yellow-brown. This is because it will not have had light for photosynthesis, so will not have produced any new starch.

A variable that should be kept the same (control variable) when testing the requirement for carbon dioxide in photosynthesis could be:

• light intensity / whether or not the leaf was exposed to light

• the presence of chlorophyll

• the temperature of the room

Flames should not be present when boiling leaves in ethanol because ethanol is highly flammable. A water bath should be used for this step.

Variegated leaves are used when investigating the role of chlorophyll in photosynthesis because they have green and white regions, where the green regions do contain chlorophyll but the white regions do not. Demonstrating that, after exposure to light, the green regions do contain starch while the white regions do not, shows that chlorophyll is needed for photosynthesis.

The independent variable when testing for the importance of chlorophyll in photosynthesis is whether the area of leaf tested is green or white (i.e. whether or not the leaf contains chlorophyll).

When investigating the need for carbon dioxide in photosynthesis, the purpose of placing plants in a bell jar which contains a beaker of sodium hydroxide is to absorb carbon dioxide from the surrounding atmosphere.

The parts of leaves that were covered with aluminium foil during an investigation into the need for light in photosynthesis will remain orange-brown when tested with iodine. This is because these areas did not receive any sunlight and could not photosynthesise to produce starch.

The equipment shown in the image can be used to investigate the effect of light intensity on the rate of photosynthesis.

The independent variable is light intensity, which is controlled by altering the distance between the lamp and the beaker.

The dependent variable in this investigation is the rate of photosynthesis, which is measured by recording the volume of oxygen produced within a set time period.

Control variables for this investigation would include:

• maintaining the same environmental temperature

• using the same species, length and age of plant in each repeat

• dissolving sodium hydrogen carbonate (NaHCO₃) in the water to maintain carbon dioxide levels

Light intensity should be controlled when investigating the effect of temperature on the rate of photosynthesis as light intensity will affect on the dependent variable.

The predicted outcome in an investigation into the effect of carbon dioxide on the rate of photosynthesis is that the rate of photosynthesis will increase with increasing carbon dioxide concentration.

This increase will continue until another factor becomes limiting.

The number of bubbles produced decreases as the light moves further away from the pondweed because light intensity decreases with distance; this means that rate of photosynthesis decreases and so less oxygen is produced.

To measure the rate of photosynthesis we need to know the volume of gas collected and time period during which it has been collected.

Plants absorb oxygen in order to carry out aerobic respiration.

False.

There will be a net intake of oxygen by plants in the dark, as they are respiring but not photosynthesising.

True.

Carbon dioxide is an acidic gas when dissolved in water. Knowing this can help you to understand the results of a hydrogencarbonate indicator test.

Hydrogencarbonate indicator is useful for investigating gas exchange in aquatic plants because it shows the presence of carbon dioxide in solution.

Hydrogencarbonate indicator will turn yellow/orange at high carbon dioxide concentrations.

False.

Hydrogencarbonate indicator will turn purple at low carbon dioxide concentrations.

A red colour indicators atmospheric carbon dioxide levels.

The balanced chemical equation for photosynthesis is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

A limiting factor is a factor that limits the rate of photosynthesis.

The three limiting factors for photosynthesis are:

• temperature

• light intensity

• carbon dioxide concentration

Photosynthesis occurs slowly at low temperatures because molecules (such as enzymes involved in photosynthesis) have little kinetic energy. This means that fewer successful collisions take place.

False.

The rate of photosynthesis only increases with temperature up to a certain point, after which higher temperatures lead to a decrease in the rate of photosynthesis. This is because high temperatures denature enzymes involved in the reactions of photosynthesis.

At very high temperatures the enzymes that control photosynthesis denature, meaning that the reaction can no longer take place.

At point B the graph has levelled off, indicating that a factor other than carbon dioxide concentration is limiting, e.g. temperature or light intensity.

The higher the light intensity, the faster the rate of photosynthesis. This continues until another factor becomes limiting.

True.

In a graph of light intensity against rate of photosynthesis, the part of the graph where the line is horizontal is a point at which a factor other than light intensityis limiting. This factor could be temperature or carbon dioxide concentration .

Gas exchange in plants occurs via the stomata. Carbon dioxide diffuses into leaves while oxygen diffuses out.

True.

The waxy cuticle is a waterproof layer that reduces water loss by evaporation from the surface of leaves.

The palisade mesophyll layer contains tall, thin cells that pack together closely. These cells contain many chloroplasts to maximise light absorption.

True.

The air spaces allow the outer surfaces of many cells in the spongy mesophyll to come into direct contact with the air. This maximises the available surface area for gas exchange.

Guard cells can change shape to open or close the stomata. This allows plants to control the rate of water loss from leaves.

Vascular bundles in plants contain xylem and phloem for transport of useful substances, such as water, around the plant.

False.

Phloem vessels carry sucrose and amino acids around inside plants. It is the xylem vessels that carry water and dissolved minerals.

It is beneficial for leaves to be broad and thin because:

• being broad maximises surface area for light absorption

• being thin reduces diffusion distance for gas exchange

True.

The palisade mesophyll layer is located just beneath the waxy cuticle and upper epidermis. This means that it is ideally located for light absorption.

False.

The cells of the spongy mesophyll are not especially tall and narrow; this description applies to the cells of the palisade mesophyll.

Structure A is a stoma (plural stomata), or stomatal pore.

Structure B is the waxy cuticle.

Structure C is a palisade cell, identifiable by its tall, column shape and many chloroplasts.

The xylem and phloem are found within the vascular bundles of leaves.

Structure E is the upper epidermis.