How do auxins make a plant bend towards light?

Auxins are produced at the shoot tip and move down the plant. When light shines from one side, auxins accumulate on the shaded side. This causes cells on the shaded side to elongate more than those on the lit side, making the shoot bend towards the light. This is called positive phototropism.

What is the difference between auxins and gibberellins?

Auxins mainly control tropisms (growth towards light/gravity) and root development, while gibberellins promote seed germination, stem elongation, and flowering. For example, auxins are used in rooting powders, whereas gibberellins are used to break seed dormancy and produce larger fruits.

Why do roots grow downwards and shoots grow upwards?

This is due to gravitropism. In roots, auxin accumulates on the lower side, but here it inhibits cell elongation, so the upper side grows faster, bending the root downwards. In shoots, auxin on the lower side stimulates cell elongation, causing the shoot to bend upwards. This ensures roots grow into the soil and shoots grow towards light.

How is ethene used in the food industry?

Ethene gas is used to ripen fruits artificially. Fruits like bananas, tomatoes, and apples are often picked unripe and then exposed to ethene in storage rooms to trigger ripening just before they reach shops. This allows for longer transport times and controlled ripening.

What are selective weedkillers and how do they work?

Selective weedkillers are synthetic auxins that mimic natural auxins. They are sprayed on lawns or crops and only affect broad-leaved weeds, not grasses. The synthetic auxins cause uncontrolled growth in the weeds, leading to their death, while grasses are unaffected because they have different hormone responses.

Do plants have hormones like animals do?

Yes, plants produce hormones, but they are different from animal hormones. Plant hormones are simpler chemicals that act locally or are transported through the plant. They control growth, development, and responses to the environment, such as phototropism and fruit ripening. Unlike animal hormones, they are not produced in glands.

Plant hormones

WJEC

GCSE

This topic explores the role of plant hormones in controlling and coordinating growth and development. It specifically covers the mechanisms of phototropism and gravitropism, as well as the functions and applications of auxins, gibberellins, and ethene in plant growth.

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Mark Points

Topic Overview

Plant hormones, also known as phytohormones, are chemical messengers that regulate growth, development, and responses to environmental stimuli in plants. In the WJEC GCSE Biology course, you will focus on auxins, gibberellins, and ethene (ethylene). Auxins control phototropism (growth towards light) and gravitropism (growth in response to gravity), while gibberellins promote seed germination and stem elongation. Ethene is a gas that speeds up fruit ripening and leaf fall. Understanding these hormones is crucial for explaining how plants adapt to their surroundings and for applications in agriculture and horticulture, such as using rooting powders or controlling fruit ripening.

This topic builds on your knowledge of plant cells and tissues, and it connects to broader concepts like homeostasis and response mechanisms in organisms. By studying plant hormones, you will see how even simple organisms use chemical signals to coordinate complex behaviours. In exams, you are expected to describe experiments that demonstrate tropisms, explain the role of auxins in bending responses, and evaluate the use of plant hormones in commercial settings. Mastering this topic will help you appreciate the sophistication of plant life and its practical importance in food production and gardening.

Plant hormones are a key part of the 'Response and Regulation' unit in WJEC GCSE Biology. They illustrate the principle that all living things must respond to changes in their environment to survive. For plants, which cannot move, hormonal control is essential for optimising growth towards light and water. This topic also introduces the idea of using scientific knowledge to solve real-world problems, such as using synthetic auxins as weedkillers or gibberellins to produce seedless grapes. By the end of this topic, you should be able to explain how these hormones work at a cellular level and apply your understanding to unfamiliar scenarios.

Key Concepts

Core ideas you must understand for this topic

→Auxins are produced in the shoot tip and move down the plant; they cause cell elongation on the shaded side of a shoot, making it bend towards light (phototropism). In roots, high auxin concentrations inhibit cell elongation, causing roots to bend downwards (positive gravitropism).
→Gibberellins promote seed germination by stimulating the production of enzymes that break down starch in the seed, and they also cause stem elongation and flowering in some plants.
→Ethene (ethylene) is a gaseous hormone that triggers fruit ripening and leaf abscission (fall). It is used commercially to ripen fruits like bananas and tomatoes after transport.
→Commercial uses of plant hormones include rooting powders (auxins) for cuttings, selective weedkillers (synthetic auxins) that kill broad-leaved weeds, and gibberellins to produce larger fruits or delay senescence.
→Tropisms are directional growth responses: phototropism (light), gravitropism (gravity), and thigmotropism (touch). Auxin distribution is key to these responses.

What You Need to Demonstrate

Key skills and knowledge for this topic

Explanation of auxins in phototropism (positive response of shoots to light)
Explanation of auxins in gravitropism (positive response of roots to gravity)
Identification of the roles of auxins, gibberellins, and ethene
Description of practical applications of plant hormones in controlling growth

Marking Points

Key points examiners look for in your answers

Explanation of auxins in phototropism (positive response of shoots to light)
Explanation of auxins in gravitropism (positive response of roots to gravity)
Identification of the roles of auxins, gibberellins, and ethene
Description of practical applications of plant hormones in controlling growth

Examiner Tips

Expert advice for maximising your marks

💡Ensure you can distinguish between the effects of auxins, gibberellins, and ethene.
💡Be prepared to explain how hormones coordinate growth in response to environmental stimuli like light and gravity.
💡When describing tropism experiments, always state the control (e.g., a plant grown in uniform light) and explain how you would measure the response (e.g., angle of bend or length of shoot). Use terms like 'positive' and 'negative' correctly.
💡For questions on commercial uses, link the hormone to its specific effect: e.g., 'Auxins in rooting powder stimulate root growth from cuttings' or 'Ethene is used to ripen fruits in storage.' Avoid vague answers like 'hormones help plants grow.'
💡Remember that auxin concentration gradients cause unequal growth. In shoots, more auxin on the shaded side leads to faster cell elongation, bending towards light. In roots, more auxin on the lower side inhibits elongation, bending downwards. This difference is a common exam point.

Common Mistakes

Pitfalls to avoid in your exam answers

Misconception: Auxins are only involved in phototropism. Correction: Auxins also control gravitropism, apical dominance, and root development. Their effect depends on concentration and the organ (shoot vs root).
Misconception: Plants only have one type of hormone. Correction: Plants produce multiple hormones (auxins, gibberellins, ethene, cytokinins, abscisic acid) that interact to regulate growth and development.
Misconception: Gibberellins cause fruit ripening. Correction: Gibberellins promote seed germination and stem growth; ethene is the hormone responsible for fruit ripening.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Plant cell structure and function (cell wall, vacuole, chloroplasts).
•Basic understanding of diffusion and active transport (for hormone movement).
•Photosynthesis and factors affecting growth (light, water, minerals).

Study Guide Available

Comprehensive revision notes & examples

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Practice questions tailored to this topic

Plant hormones

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Study Guide Available

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Ready to test yourself?

Related Topics in WJEC GCSE Biology

Biodiversity

Cell biology

Cell metabolism

Communicable disease

Topic Synopsis

Key Concepts & Core Principles

Exam Tips & Revision Strategies

Examiner Marking Points

Plant hormones

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

How do auxins make a plant bend towards light?

What is the difference between auxins and gibberellins?

Why do roots grow downwards and shoots grow upwards?

How is ethene used in the food industry?

What are selective weedkillers and how do they work?

Do plants have hormones like animals do?

Before You Start

Study Guide Available

Likely Command Words

Ready to test yourself?

Related Topics in WJEC GCSE Biology

Biodiversity

Cell biology

Cell metabolism

Communicable disease