Plant Structures and Their Functions

What are photosynthetic organisms?

Organisms that use photosynthesis to produce organic substances such as glucose.

Why are photosynthetic organisms the main producers of food and biomass?

They convert light energy into chemical energy stored in glucose, forming the starting point of most food chains and producing biomass.

What is photosynthesis?

An endothermic reaction in which plants and algae use light energy to react carbon dioxide and water to produce glucose and oxygen.

What is the word equation for photosynthesis?

Carbon dioxide + water → glucose + oxygen.

Why is photosynthesis described as an endothermic reaction?

It requires an input of energy from light.

Where does the energy for photosynthesis come from?

Light energy from the Sun.

What are the raw materials for photosynthesis?

Carbon dioxide and water.

What are the products of photosynthesis?

Glucose and oxygen.

What is the role of chlorophyll in photosynthesis?

It absorbs light energy needed for photosynthesis.

What is a limiting factor?

A factor that limits the rate of a process when it is in short supply.

What are the three main limiting factors of photosynthesis?

Temperature, light intensity and carbon dioxide concentration.

How does light intensity affect the rate of photosynthesis?

As light intensity increases, the rate of photosynthesis increases until another factor becomes limiting.

How does carbon dioxide concentration affect photosynthesis?

Increasing carbon dioxide concentration increases the rate of photosynthesis until another factor becomes limiting.

How does temperature affect the rate of photosynthesis?

As temperature increases, the rate increases up to an optimum, then decreases because enzymes become denatured at high temperatures.

Why does increasing light intensity eventually stop increasing the rate of photosynthesis?

Another factor, such as carbon dioxide concentration or temperature, becomes limiting.

Why does increasing carbon dioxide concentration eventually stop increasing the rate of photosynthesis?

Another factor, such as light intensity or temperature, becomes limiting.

Why can low temperatures limit photosynthesis?

Photosynthesis involves enzyme-controlled reactions, and low temperatures reduce enzyme activity.

Why can very high temperatures reduce photosynthesis?

High temperatures can denature enzymes involved in photosynthesis.

What happens when several limiting factors are changed at the same time?

The factor in shortest supply relative to the plant's needs limits the rate of photosynthesis.

What is the principle of limiting factors?

The rate of photosynthesis is controlled by whichever required factor is most limiting at that particular time.

What happens if light intensity is increased when carbon dioxide concentration is the limiting factor?

There may be little or no increase in photosynthesis because carbon dioxide is still limiting.

What happens if carbon dioxide concentration is increased when light intensity is limiting?

There may be little or no increase in photosynthesis because insufficient light is available.

What is the core practical for Topic 6?

Investigate the effect of light intensity on the rate of photosynthesis.

How can the rate of photosynthesis be measured in an aquatic plant?

By measuring the volume of oxygen produced or counting oxygen bubbles produced in a set time.

How can light intensity be changed in the photosynthesis practical?

Change the distance between the light source and the plant.

What variables should be controlled in the light intensity practical?

Temperature, carbon dioxide concentration, plant species/size and volume of water should be kept constant.

Why should temperature be controlled in the light intensity practical?

Temperature is another limiting factor and could affect the rate of photosynthesis.

What is the independent variable in the light intensity practical?

Distance from the light source, which changes light intensity.

What is the dependent variable in the light intensity practical?

The rate of photosynthesis.

What is the inverse square law for light intensity?

Light intensity is inversely proportional to the square of the distance from the light source.

What is the equation for the inverse square law?

Light intensity ∝ 1/d².

What happens to light intensity if the distance from the light source doubles?

Light intensity decreases to one quarter.

What happens to light intensity if the distance from the light source triples?

Light intensity decreases to one ninth.

How is light intensity related to distance from a light source?

Light intensity is inversely proportional to distance squared.

What does directly proportional mean?

When one quantity increases by a certain factor, the other increases by the same factor.

What does inversely proportional mean?

When one quantity increases, the other decreases by a corresponding factor.

What is a root hair cell?

A specialised plant cell found on the surface of roots that absorbs water and mineral ions.

How is a root hair cell adapted to absorb water?

It has a long hair-like extension that provides a large surface area for water absorption.

Why does a root hair cell have a large surface area?

It increases the area available for water and mineral ion absorption.

How are root hair cells adapted to absorb mineral ions?

They contain many mitochondria to provide energy for active transport.

Why are mitochondria important in root hair cells?

They release energy needed for active transport of mineral ions.

How does water enter root hair cells?

Water enters by osmosis from the soil into the cell.

How do mineral ions enter root hair cells when their concentration is lower in the soil?

They enter by active transport using energy from respiration.

What is xylem?

A plant transport tissue that transports water and mineral ions from the roots to the rest of the plant.

How is xylem adapted for transporting water?

It consists of dead, hollow cells joined end-to-end, forming continuous tubes.

What is lignin?

A strong substance deposited in xylem cell walls that strengthens and waterproofs the tissue.

Why is xylem lignified?

Lignin strengthens the vessels and prevents them collapsing as water is transported through them.

Why are xylem cells dead?

Their lack of cell contents creates a hollow tube with minimal resistance to water flow.

What is phloem?

A plant transport tissue that transports dissolved sugars such as sucrose around the plant.

What is translocation?

The transport of dissolved sugars, especially sucrose, through the phloem from sources to sinks.

Are phloem cells living or dead?

Phloem consists of living cells.

Why does phloem require energy?

Energy is needed for the active transport processes involved in moving sucrose around the plant.

What is transpiration?

The loss of water vapour from plant leaves, mainly through the stomata.

What is the transpiration stream?

The movement of water and mineral ions from the roots through the xylem to the leaves, driven by transpiration.

What are stomata?

Small pores in the epidermis of leaves that allow gas exchange and water vapour loss.

What are guard cells?

Specialised cells surrounding each stoma that control whether the stoma is open or closed.

How do stomata allow water to leave a plant?

Water evaporates from leaf cells and diffuses out as water vapour through open stomata.

Why are stomata important for photosynthesis?

They allow carbon dioxide to enter the leaf.

What happens when stomata are open?

Carbon dioxide can enter, but water vapour is also lost through transpiration.

What happens when stomata close?

Water loss decreases, but carbon dioxide entry and therefore photosynthesis may also decrease.

How does transpiration help transport water through a plant?

Water loss from the leaves creates a pull that draws water upward through the xylem.

What is translocation?

The movement of sucrose through the phloem from sources to sinks.

What is a source in translocation?

A region where sucrose is produced or released, such as photosynthesising leaves.

What is a sink in translocation?

A region where sucrose is used or stored, such as roots, fruits or growing tissues.

How is a leaf adapted for photosynthesis?

It has a large surface area, thin structure, many chloroplasts, air spaces and stomata.

Why does a leaf have a large surface area?

To absorb as much light as possible.

Why are many chloroplasts found in palisade cells?

They contain chlorophyll and absorb light for photosynthesis.

Why are leaves thin?

A short diffusion distance allows gases to move quickly into and out of the leaf.

Why do leaves have air spaces?

They allow carbon dioxide and oxygen to diffuse through the leaf.

Why are stomata important in leaves?

They allow carbon dioxide to enter and oxygen and water vapour to leave.

What is the role of the waxy cuticle on a leaf?

It reduces water loss by evaporation.

How does light intensity affect water uptake by a plant?

Higher light intensity usually increases transpiration, increasing water uptake by the roots.

How does air movement affect water uptake?

Increased air movement removes water vapour around the leaf, increasing the concentration gradient and increasing transpiration and water uptake.

How does temperature affect water uptake?

Higher temperature generally increases evaporation and transpiration, increasing water uptake, up to the point where excessive water loss causes stomata to close.

Why does increased air movement increase transpiration?

It removes humid air surrounding the leaf, maintaining a steep diffusion gradient for water vapour.

Why does increased light intensity usually increase transpiration?

Light causes stomata to open for photosynthesis, increasing water vapour loss.

What is the rate of transpiration?

The amount of water lost by a plant per unit of time.

How do you calculate rate of transpiration?

Rate = amount of water lost ÷ time taken.

What unit could be used for a transpiration rate?

For example, cm³ per minute or g per hour, depending on the measurements.

How can water uptake be measured experimentally?

Using a potometer to measure the movement of an air bubble in a capillary tube.

What does a potometer measure directly?

The rate of water uptake, which can be used as an estimate of transpiration rate.

What is an extreme environment for a plant?

An environment with conditions that make survival difficult, such as very dry, hot or cold conditions.

How can plants in dry environments be adapted by having smaller leaves?

Smaller leaves have a reduced surface area, reducing water loss through transpiration.

How can leaf shape reduce water loss?

Small, narrow or rolled leaves reduce the surface area exposed to the air and therefore reduce transpiration.

How does a thick waxy cuticle help plants in dry environments?

It reduces evaporation of water from the leaf.

How can stomata help plants survive dry conditions?

Stomata can close to reduce water loss when conditions are dry.

What are plant hormones?

Chemical substances that coordinate plant growth and development.

What is auxin?

A plant hormone involved in controlling growth responses such as phototropism and gravitropism.

What is phototropism?

The growth response of a plant to light.

What is positive phototropism?

Growth towards a light source.

How does auxin cause positive phototropism in shoots?

Auxin moves to the shaded side of the shoot, where it promotes cell elongation, causing the shoot to bend towards the light.

Why does a shoot bend towards light?

Cells on the shaded side elongate more than cells on the light side.

What is gravitropism?

The growth response of a plant to gravity.

What is positive gravitropism?

Growth towards gravity, as shown by roots.

What is negative gravitropism?

Growth away from gravity, as shown by shoots.

How does auxin affect roots during gravitropism?

High auxin concentrations inhibit root cell elongation, causing roots to bend and grow downwards.

How does auxin affect shoots during gravitropism?

Auxin promotes cell elongation in shoots, causing them to bend and grow upwards.

What are auxins used for commercially?

They are used in weedkillers and rooting powders.

How are auxins used in weedkillers?

High concentrations of auxins cause uncontrolled growth in weeds, eventually killing them.

How are auxins used in rooting powders?

They stimulate root growth in plant cuttings.