Identify and describe What Are Hormones?, How Do Plant Hormones Travel? and Tropisms: How Plants Respond to Stimuli in the context of control and coordination.
BEEZYREVISOR
GCSE Biology Revision
Learn it. Recall it. Revise it.
GCSE Biology revision
Control and coordination
Plant hormones (biology only)
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Explain the relationships between What Are Hormones?, How Do Plant Hormones Travel? and Tropisms: How Plants Respond to Stimuli and how they contribute to control and coordination.
Apply and analyse knowledge of What Are Hormones?, How Do Plant Hormones Travel? and Tropisms: How Plants Respond to Stimuli to interpret unfamiliar information about control and coordination.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
What Are Hormones?
- Hormones are chemical messengers that cause an organism to respond to its environment by triggering specific actions in target tissues.
- In humans, hormones are secreted by glands directly into the bloodstream as part of the endocrine system, where they bind to receptors on target tissues.
- The endocrine system produces slower but longer-lasting effects compared to the nervous system, which uses fast electrical impulses for short-lived responses.
- Every cell in a plant contains the genetic coding to produce hormones, meaning all plant cells can manufacture and respond to chemical messengers.
How Do Plant Hormones Travel?
- Unlike animals, plants do not have blood, so hormones travel through the vascular tissues — the xylem and phloem.
- The xylem transports water unidirectionally from roots to leaves, while the phloem transports dissolved sugars and hormones in both directions.
- Plasmodesmata are microscopic channels in plant cell walls that allow hormones and chemical signals to pass directly between neighbouring cells.
Tropisms: How Plants Respond to Stimuli
- A tropism is a growth response to a stimulus, where a positive tropism means growth towards the stimulus and a negative tropism means growth away from it.
- Phototropism is a plant's growth response to light, whilst geotropism (also called gravitropism) is a plant's growth response to gravity and the direction of the soil.
- Shoots display positive phototropism (growing towards light) and negative geotropism (growing away from the soil), whilst roots display the opposite responses.
- These tropisms ensure shoots grow upwards towards light for photosynthesis and roots grow downwards into the soil to absorb water and mineral ions.
Auxin and Cell Elongation
- Auxin is a plant hormone produced at the root and shoot tips whose primary role is to promote cell elongation and growth.
- When light hits one side of a shoot, auxin becomes more concentrated on the shaded side, causing those cells to elongate more and producing a curvature towards the light.
- Experiments removing shoot tips stop auxin production and halt growth entirely, whilst covering tips with foil caps distributes auxin evenly, preventing directional bending.
- It is important to describe the plant's response as cell elongation on the shaded side rather than simply 'bending' towards the light.
Meristems: Plant Stem Cells
- Meristems are undifferentiated plant cells (equivalent to stem cells in animals) that have not yet become specialised, found in the roots and shoots of growing plants.
- Auxin is concentrated in meristem regions, where it drives the cell division and elongation needed for plant growth.
Required Practical: Investigating the Effect of Light on Germinating Seeds
- The aim of this practical is to investigate the effect of light intensity on the growth of newly germinated seeds.
- Seeds are placed on damp cotton wool in petri dishes and kept in a warm space until they germinate (begin to sprout), with non-germinated seeds removed before testing.
- One dish is placed in full light, one in partial light, and one in a dark cupboard, with the height of each seedling measured daily for one week.
- The independent variable is light intensity, the dependent variable is the height (length) of the seedlings, and control variables include the number of seeds, species of plant, and volume of water provided.
- Mean average heights are calculated and compared across conditions, with plants in full light expected to show the greatest growth.
Ethene: The Ripening Hormone
- Ethene is a plant hormone (also a hydrocarbon alkene with a carbon–carbon double bond) that promotes the ripening of fruit and helps control cell growth.
- In agriculture, ethene is sprayed onto harvested fruits during storage so they ripen at the correct time before reaching supermarkets, preventing them from rotting during transport.
- Bananas naturally release ethene, which is why placing an unripe avocado next to a banana causes it to ripen more quickly.
Gibberellins: Ending Dormancy and Promoting Growth
- Gibberellins are plant hormones that end seed dormancy, triggering germination so that seeds begin to sprout and grow.
- Gibberellins also promote flowering in plants and increase fruit size, working alongside ethene to regulate plant development.
Commercial Uses of Plant Hormones
- Auxin is used in rooting powders, where plant cuttings are dipped into the powder before being placed in soil, stimulating the development of a new root system.
- Selective weedkillers used in agriculture contain auxin, which disrupts the growth of broad- leaved weeds without harming narrow-leaved crop plants.
- Tissue culture is a scientific method that uses plant hormones to clone plants repeatedly, producing many genetically identical plants from a small sample of cells.
Plants and Their Own Version of Puberty
- Just as hormone levels increase in humans during puberty, plants undergo their own developmental stage called the vegetative state, during which they begin to develop leaves.
- Leaves are a critical organ for plants as they capture sunlight energy for photosynthesis, requiring the plant to grow towards light sources efficiently.