Edexcel Biology GCSE Topic 6 - Plant Structures and Their Functions

Flashcards reviewing photosynthesis, plant cell adaptations, transport mechanisms (transpiration and translocation), plant hormones, and commercial plant uses based on Edexcel Biology GCSE Topic 6.

What type of reaction is photosynthesis and where does it occur in plant cells?

Photosynthesis is an endothermic reaction that occurs in chloroplasts, transferring light energy from the environment to synthesise food.

What is the word equation for photosynthesis?

carbon dioxide + water → glucose + oxygen (in the presence of light)

What are the chemical formulas for carbon dioxide, water, oxygen, and glucose?

Carbon dioxide is $\text{CO}_2$, water is $\text{H}_2\text{O}$, oxygen is $\text{O}_2$, and glucose is $\text{C}_6\text{H}_{12}\text{O}_6$.

How do temperature, light intensity, and carbon dioxide concentration affect the rate of photosynthesis?

Increasing light intensity or $\text{CO}_2$ concentration increases the rate until another factor becomes limiting; increasing temperature increases the rate up to an optimum before enzymes denature and the rate decreases.

What is a limiting factor in photosynthesis?

An environmental condition in low levels (such as light intensity, temperature, or carbon dioxide concentration) that restricts any further increase in the rate of photosynthesis.

In this graph of photosynthesis rate versus light intensity, what demonstrates that light intensity is the limiting factor in the lower line?

The $50\,\text{lux}$ level restricts the rate of photosynthesis compared to the $100\,\text{lux}$ experiment, showing that increasing light intensity allows a higher rate.

What experimental setup is shown in this core practical diagram for measuring the rate of photosynthesis?

A sealed $100\,\text{ml}$ flask filled with water and pondweed at a measured distance along a $1\,\text{m}$ ruler from a lamp, connected to a gas syringe to measure oxygen volume change.

According to the Inverse Square Law, what is the mathematical relationship between light intensity and distance?

Light intensity is inversely proportional to the square of the distance: $\text{Light intensity} \propto \frac{1}{\text{distance}^2}$.

If a light source is moved from its original position to a distance $2\,\text{metres}$ away, what fraction of the original light intensity does the plant receive?

The plant receives $\frac{1}{4}$ of the original light intensity ($\frac{1}{2^2} = \frac{1}{4}$).

How are root hair cells adapted for absorbing water and mineral ions from soil?

They have a large surface area from root hairs, a large permanent vacuole affecting water movement speed, and mitochondria to supply energy for active transport of mineral ions.

How are xylem cells structurally adapted for transporting water and mineral ions?

They are hollow, lignified dead cells joined end-to-end into a continuous tube, with spiral lignin deposits that withstand internal water pressure.

How are phloem cells adapted to transport food substances throughout the plant?

They are living cells with end walls modified into sieve plates, supported by energy from companion cell mitochondria to transport sucrose.

What is transpiration and how is the transpiration stream maintained?

Transpiration is the loss of water vapour from leaves and stems through stomata; as water evaporates, water molecules pull each other up through the xylem, causing continuous uptake from soil.

How do guard cells control stomatal opening and closing?

Guard cells are kidney-shaped with thin outer and thick inner walls; when water is plentiful, they fill and open the stomata, and when water is scarce, they close them to prevent evaporation.

What is translocation?

The movement of food substances (such as sucrose) in the phloem from sources (where made) to sinks (where used or stored).

What adaptations do leaves have to maximise photosynthesis and gas exchange?

Stomata for controlled gas exchange, green chlorophyll for absorbing light, thin structure for short diffusion pathways, and a large surface area to absorb light.

How do humidity and air movement affect the rate of transpiration?

High relative humidity reduces the concentration gradient and decreases transpiration; increased air movement removes water vapour, increasing the concentration gradient and transpiration rate.

How is a potometer used to estimate the rate of transpiration?

A leaf shoot is placed in a capillary tube of water in a potometer, and the distance a bubble moves over a set time is measured to calculate water uptake rate.

What extreme adaptations do desert plants possess to survive scarce water conditions?

Small or absent leaves to reduce transpiration area, a thick waxy cuticle to block evaporation, and stomata that close when water is scarce.

How does auxin cause a plant shoot to bend towards light (positive phototropism)?

Auxin accumulates on the shaded side of the shoot and stimulates cell elongation there, causing the shoot to bend towards the light source.

How do horizontal plant roots exhibit positive gravitropism?

Auxin moves to the lower side of the root, where it inhibits cell growth relative to the upper side, causing the root to curve downwards into the soil.

What are three commercial applications of auxins?

Used as broad-leaved weedkillers, as rooting powders for plant cuttings, and as additives in growth media for tissue culture cloning.

What commercial uses do gibberellins have in plant crop management?

Breaking seed dormancy to trigger germination, increasing fruit size and weight, and encouraging faster flowering in plants.

How is ethene used commercially in the fruit industry?

Unripe fruit is transported firm to prevent damage, then exposed to ethene and warm temperatures to stimulate enzymes that control cell division and fruit ripening before sale.