Biology Edexcel Topic 6 (Controlling plant growth)

6.15B to 6.16B

What are plant tropisms

The growth of a plant in response to a stimulus

What is positive tropism

The growth of a plant towards a stimulus

What is negative tropism

The growth of a plant away from a stimulus

What are the two main types of plant tropism

Phototropism and geotropism (also known as gravitropism)

Define phototropism

A plant's growth response to light

Define geotropism (gravitropism)

A plant's growth response to gravity

What are auxins

A group of plant hormones involved in plant tropisms which control growth in plant root and shoot tips

Compare the effects of auxins in plant shoots and roots

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

Where are auxins produced

Root and shoot tips

How do auxins move through a plant

They diffuse through the plant in solution

Are plant shoots positively or negatively phototropic

Positively

How does positive phototropism affect shoot growth

Plant shoots will grow towards light

Explain why plant shoots are positively phototropic

When the shoot tip is exposed to light, the auxins in the shoot accumulate on the shaded side of the shoot, which causes the elongation of cells of the shaded side where the auxins are, making the shoot tip bend towards the light

Are plant shoots positively or negatively geotropic (gravitropic)

Negatively

How does negative geotropism (gravitropism) affect shoot growth

Plant shoots will grow upwards, against gravity

Explain why plant shoots are negatively geotropic (gravitropic)

When a shoot is placed horizontally, auxins accumulate on the lower side of the shoot due to gravity, which causes the elongation of cells on the lower side, where the auxins are, making the shoot bend upwards, against gravity

Are plant roots positively or negatively phototropic

Negatively

How does negative phototropism affect root growth

Plant roots grow away from the light (not that there should be any light underground)

Explain why plant roots are negatively phototropic

When a root is exposed to light, auxins accumulate on the shaded side of the root, causing the Inhibition of cell growth on the shaded side, where the auxins are, making the root grow away from the light

Are plant roots positively or negatively geotropic (gravitropic)

Positively

How does positive geotropism (gravitropism) affect root growth

Plant roots will grow downwards, with gravity

Explain why plant roots are positively geotropic (gravitropic)

When a root is place horizontally, auxins accumulate on the lower side due to gravity, which causes the inhibition of cell growth on the lower side, where the auxins are, making the root bend downwards, with gravity

Explain how plant tropisms increase the chance of survival

They enable plants to respond to their environment, shoot growth towards light maximises light absorption for photosynthesis and root growth downwards increases the uptake of water and minerals from the soil and enables anchorage of the plant body to the ground, making sure the plant is stable

Describe the role of auxins in commercial plant cultivation

It can be used for rooting powders, as they contain auxins that stimulate the growth of roots in cuttings, which enables rapid plant cloning, and it can also be used in selective weedkillers that target broad leaved plants (most weeds are broad leaved), altering growth patterns and making them grow too quick, killing them

What are gibberellins

Plant hormones that control germination and flowering

Describe the role of gibberellins in commercial plant cultivation

It can be used to initiate germination in seeds at times of the year when they naturally wouldn't or to ensure that all seeds in a batch germinate, it can also be used to trigger flowering in plants under irregular conditions or increase the fruit size by reducing the number of flowers produced by plants, and it can be used to produce seedless fruits by stimulating the 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, which enables fruits to be picked while they are unripe and less easily damaged during transport, and they can be stimulated to ripen when the shops want to sell them