Biology GCSE - Plant structures and their functions

Describe the two main stages of photosynthesis in general terms.

1. Chlorophyll absorbs light energy which is used to split water into oxygen gas waste product and hydrogen ions
2. Carbon Dioxide combines with hydrogen ions to form glucose

What factors affect the rate of photosynthesis

-temperature
-light intensity
-carbon dioxide concentration

What is a limiting factor?

A variable that limits the rate of a particular reaction.

Explain how temperature affects the rate of photosynthesis

- Higher temperatures provides more KE for enzymes involved in photosynthesis so the rate increases as temperature rises
- The optimum temperature is usually 25 degrees
- If the temperature becomes too high around 45 degrees enzymes become denatured and the rate of photosynthesis decreases

Explain how light intensity affects the rate of photosynthesis

Rate of photosynthesis is directly proportional to light intensity therefore as light intensity increases, the rate of photosynthesis increases

Why does the rate of photosynthesis eventually plateau even if light intensity continues to increase?

Another factor temperature or carbon dioxide emissions becomes limiting

How can the effect of light intensity on the rate of photosynthesis be measured in a lab?

- Using a light meter
- Using the inverse square law where:
light intensity is directly proportional = 1/distance squared

Why does the rate of photosynthesis decrease as the distance from a light source increases?

Light intensity is inversely proportional to the square of the distance from the light source. Thus, as the distance increases, light intensity decreases and the rate of photosynthesis decreases

Explain how carbon dioxide concentration affects the rate of photosynthesis

As carbon dioxide concentration increases, the rate of photosynthesis increases

Why does the rate of photosynthesis eventually plateau even if carbon dioxide concentration continues to increase?

Another factor temperature or light intensity becomes a limiting factor

Why do plants require water?

- photosynthesis
- maintenance of structure turgidity
- cooling effect

Are plant roots positively or negatively phototropic? How does this affect root growth?

- Negatively phototropic
- Plant roots grow away from the light

What is photosynthesis?

A chemical reaction which takes place inside photosynthetic organisms e.g., plants, algae converting light energy into chemical energy

Write the word equation for photosynthesis

carbon dioxide+ water -> oxygen + glucose

Write the symbol equation for photosynthesis.

6 CO2 + 6 H2O --> C6H12O6 + 6 O2

Why is photosynthesis important?

It produces glucose which has a wide range of uses:
- Used in respiration to release energy
- Converted to starch and stored - broken down to glucose when the plant requires energy
- Used to make complex organic molecules which are used for growth, making up an organism's biomass (transferred through food chains)

Where does photosynthesis take place?

within chloroplasts

What type of reaction is photosynthesis?

An endothermic reaction that takes in energy in the form of light

What is chlorophyll?

A green pigment found in the chloroplasts that absorbs light

Why do plants require mineral ions?

For growth e.g. nitrates are required to produce proteins

Which structure in plants is adapted for the uptake of water and minerals?

Root hairs

How is water transported into root hairs?

- Lower concentration of water in root hair cells than in the soil
- Water diffuses down its concentration gradient into root hair cells by osmosis

How are minerals transported into root hairs?

● Lower concentration of mineral ions in the soil than in the root
● Root hair cells take up mineral ions by active transport

Outline how plants are adapted for the absorption of water and minerals

Plant roots are composed of millions of root hair cells which have:
-Long hairs that extend from the cell body, increasing the surface area for absorption
- Many mitochondria which produce ATP for active transport of mineral ions

Name the two plant transport tissues

xylem and phloem

What is the function of the xylem?

Transports water and minerals up the plant, from the roots to the leaves via the transpiration stream

Describe how xylem cells in plants are adapted to their function

- Decomposed of dead cells laid end-to-end to form a long, hollow continuous column
-No end walls which provides little resistance to the passage of water
- Thick cell wall strengthened with lignin to provide support

What is the function of the phloem?

Transports sugars up and down the stem from photosynthetic tissues e.g. mature green leaves to non-photosynthetic tissues e.g. developing seeds via translocation

What are the two cell types that make up the phloem?

Sieve tube elements
Companion cells

Describe how the phloem is adapted to its function

● Sieve tube elements are long, thin cells, laid end-to-end with perforated end plates to enable the flow of sugars. They contain no nucleus and little cytoplasm to allow sugars to flow easily.
● Companion cells adjacent to sieve tube elements contain a dense cytoplasm, nucleus and mitochondria. They provide energy for processes in both cell types.

What is transpiration?

The loss of water vapour from the parts of a plant exposed to the air due to evaporation and diffusion

Where does the majority of transpiration take place?

Leaves

Describe the process of transpiration

● Water evaporates from the mesophyll cell surfaces and diffuses out of the stomata
● Water molecules (which have cohesive properties) are drawn up the xylem vessels to replace the water that has been lost
● This causes more water molecules to be absorbed from the soil into root hair cells

How does the transpiration stream transport mineral ions

Mineral ions are dissolved in the water that is carried by the transpiration stream

What are stomata?

They are pores found in the lower epidermis of a leaf which allow gas exchange

What are guard cells?

Specialised cells surrounding the stoma that change shape to control the size of the pore

How do guard cells control the size of the stomata?

To open the stomata:
Water enters guard cells. They swell and become turgid. They bend and draw away from each other, opening the stomata.

To close the stomata:
Water leaves guard cells. They become flaccid, closing the stomata.

What factors affect the rate of transpiration?

Light intensity, temperature, humidity and wind

Describe how high light intensity affects the rate of transpiration

- High light intensity greater number of stomata open to allow gas exchange for photosynthesis
- Rate of photosynthesis increases so more water is taken up from the soil, pushing water up the xylem
- More water vapour diffuses across the stomata therefore rate of transpiration increases

Describe how low light intensity affects the rate of transpiration

At a low light intensity, fewer stomata are open so the rate of transpiration decreases

Describe how temperature affects the rate of transpiration

- Temperature increases, water molecules have more KE so rate of diffusion increases
- Photosynthesis also increases so more water is taken up from the soil, pushing water up the xylem
- more water vapour diffuses out of the stomata therefore rate of transpiration increases

Describe how air movement affects the rate of transpiration

- Air movement increases, high water gradient maintained between the air spaces in the leaf and atmosphere
- Increased rate of diffusion of water molecules out of stomata
- Rate of transpiration increases

What apparatus is used to measure the rate of transpiration?

Potometer

What is assumed when measuring the rate of transpiration using a potometer

rate of water uptake = rate of transpiration

How can the rate of transpiration be calculated using a potometer?

rate of transpiration = distance moved by bubble/time taken

What is translocation?

The movement of sugars sucrose, amino acids etc. up and down a plant, from the source to the sink, via the phloem. Requires ATP

Describe how leaves are adapted for photosynthesis and gas exchange

- Broad large SA for light absorption
- Thin short diffusion distance for gases, allows light to reach all cells
- Vascular bundles xylem and phloem form a network to deliver water and remove glucose. And provides support
- Photosynthetic pigments e.g., chlorophyll absorb light

Describe how tissues of the leaves are adapted for photosynthesis and gas exchange

- Palisade mesophyll layer receives most light so contains greatest concentration of chloroplasts
-Upper epidermis transparent allows light to reach palisade layer
-Spongy mesophyll layer air spaces increases rate of diffusion
- Lower epidermis contains many stomata for gas exchange

Describe how plants are adapted to live in hot, dry conditions

- Small leaves/spines reduce SA for water loss
- Thick waxy cuticle reduces evaporation, conserving water
-Thick stem provides a storage of water
- Shallow but widespread roots large SA to absorb water
- Stomata sunken in pits and leaves curled reduces air flow, lowering diffusion gradient and reducing water loss by evaporation
-Stomata close to reduce water loss

What are plant tropisms?

The growth of a plant in response to a stimulus

What is positive tropism?

when the plant grows towards the stimulus

What is negative tropism?

When a plant grows away from a stimulus

What are the two main types of plant tropism?

Phototropism and gravitropism

Define phototropism

plant's growth in response to light

Define gravitropism

a plant's response to gravity

What are auxins?

● A group of plant hormones involved in plant tropisms
● Control growth in plant roots and shoot tips

Compare the effects of auxins in plant shoots and roots

Auxins stimulate growth in plant shoots
Auxins inhibit growth in plant roots

Where are auxins produced?

tips of shoots and roots

How do auxins move through the plant?

They diffuse through the plant in solution

Are plants positively or negatively affected by phototropic? How does this affect shoot growth?

- Positively phototropic
- Plant shoots grow towards the light

Explain why plants shoots are positively phototropic

- Shoot tip exposed to light
- On the shaded side of the shoot, auxin accumulates
- Elongation of cells on the shaded side
- Shoot tip bends towards the light

Are plant shoots positively or negatively phototropic? How does this affect shoot growth?

- Negatively gravitropic
- Plant shoots grow upwards, away from gravity

Explain why plant shoots are negatively gravitropic

- Shoot placed horizontally
- Due to gravity, auxin accumulates on the lower side of the shoot
- Elongation of the cells on the lower side
- Shoot bends upwards growing away from gravity

Explain why plant roots are negatively phototropic

● Root exposed to light
● On the shaded side of the root, auxin accumulates ● Inhibition of cell growth on the shaded side
● Root grows away from the light

Are plant roots positively or negatively phototropic? How does this affect root growth?

-Positively gravitropic
- Plant roots grow downwards, towards gravity

Explain why plant roots are positively gravitropic

● Root placed horizontally
● Due to gravity, auxin accumulates on the lower side of the root
● Inhibition of cell growth on the lower side
● Root bends downwards growing towards gravity

Explain how plant tropisms increase chance of survival

- They enable plants to respond their environment
- Shoot growth towards the light maximises light absorption
- Root growth downwards increase the uptake of water and minerals from the soil and enables anchorage of the plant body to the ground

Describe the role of auxins in commercial plant cultivation

- Rooting powders contain auxins that stimulate the growth of root in cuttings. Enables rapid plant cloning.
-Used in selective weedkillers that target and alter growth patterns in broad-leaved plants most weeds, killing them.

What are gibberellins?

Plant hormones that control germination and flowering

Describe the role of gibberellins in commercial plant cultivation

- Initiate germination in seeds at times of the year when they naturally wouldn't. Ensure all seeds in a batch germinate
- Trigger flowing in plants under irregular conditions. Also increase fruit size by reducing the number of flowers produced by plants.
- Produce seedless fruits by stimulating growth of fruit from unpollinated flowers

How do gibberellins trigger germination?

In the presence of water, gibberellins break seed dormancy, initiating germination.

Why is ethene important in commercial plant cultivation?

- Ethene stimulates enzymes that control fruit ripening
- Enables fruit to be picked while they are unripe and less easily damaged. They can be stimulated to ripen during transportation to shops.