PLANT GROWTH FACTORS: IAA and tropisms for A-level Biology. Phototropism and gravitropism.

Summary

In this educational video, Miss Estrich explores plant growth factors, focusing on indole acetic acid (IAA) and its role in plant tropisms. She explains that IAA, produced in shoot and root tips, influences cell elongation differently in shoots and roots. Phototropism is highlighted as a positive response in shoots, causing plants to bend towards light, while roots exhibit negative phototropism, bending away from light. Gravitropism is also discussed, with shoots showing negative gravitropism when placed horizontally, bending upwards, and roots demonstrating positive gravitropism, bending towards gravity. The video aims to clarify misconceptions about plant hormones and provides insights into how IAA directs plant growth responses.

Takeaways

• 🌱 Plants respond to stimuli through tropisms, which are growth and cell elongation responses.
• 🔍 Troisms can be positive (towards the stimulus) or negative (away from the stimulus), with key stimuli being light, gravity, and water.
• 🚫 Plants do not have hormones; instead, they have plant growth factors, with indole acetic acid (IAA) being a key auxin.
• 🌳 IAA is produced in the tips of roots and shoots and can diffuse to other parts of the plant to influence growth.
• 📈 High IAA concentrations in shoots cause cell elongation, while in roots, it inhibits cell elongation.
• 💡 Phototropism is the plant's response to light, with shoots bending towards the light due to IAA diffusion (positive phototropism).
• 🌑 Roots exhibit negative phototropism, growing away from light because they do not photosynthesize and seek deeper soil for anchoring.
• 🌐 Gravitropism is the response to gravity, with shoots bending upwards due to IAA moving to the lower side (negative gravitropism).
• 🌱 In roots, IAA moves towards gravity, causing roots to bend towards the ground for better anchoring (positive gravitropism).
• 📚 Understanding IAA and plant growth factors is crucial for students studying biology at the A-Level.

Q & A

What is the main difference between plant hormones and plant growth factors?

Plant hormones are chemicals transported in blood to cause a change, whereas plant growth factors are chemicals that influence plant growth but are not transported through blood, as plants do not have a blood system.

What is the role of auxins in plant growth?

Auxins are a group of plant growth factors that influence growth and cell elongation. They are particularly important in processes like tropisms, where they can cause cells to elongate or inhibit growth depending on the concentration and location in the plant.

What is the full name of the auxin commonly referred to as IAA?

The auxin commonly referred to as IAA is Indole Acetic Acid.

How does IAA distribution affect plant growth direction in relation to light?

In shoots, high concentrations of IAA cause cells to elongate, leading to growth towards the light (positive phototropism). In roots, high IAA concentrations inhibit cell elongation, causing roots to grow away from light (negative phototropism).

What is phototropism and how does IAA play a role in it?

Phototropism is the growth of a plant in response to light. IAA, produced in the shoot tip, diffuses towards the shaded side of the shoot under unilateral light, causing cells on that side to elongate and the plant to bend towards the light.

Can you explain the concept of gravitropism in plants as discussed in the script?

Gravitropism is the growth response of a plant to gravity. In shoots, IAA diffuses to the lower side causing cells there to elongate, leading to upward growth (positive gravitropism). In roots, IAA moves towards gravity and inhibits cell elongation, causing roots to bend towards gravity (positive gravitropism).

What would happen to a plant if it were placed in complete darkness in terms of gravitropism?

In complete darkness, a plant would still exhibit gravitropism. IAA would diffuse to the side of the shoot nearest the bottom due to gravity, causing those cells to elongate and the shoot to bend upwards.

How does the distribution of IAA in roots contribute to the anchoring of a plant?

In roots, IAA moves towards the lower side and inhibits cell elongation, causing the roots to bend towards gravity. This helps anchor the plant by growing deeper into the soil.

What are the three key stimuli that plants respond to in terms of tropisms?

The three key stimuli that plants respond to in terms of tropisms are light, gravity, and water.

Why is it advantageous for roots to grow away from the light source?

It is advantageous for roots to grow away from the light source because it allows them to penetrate deeper into the soil, which helps anchor the plant more effectively.