HAPPY NEW YEAR

Aims for today

• Review what we have covered so far

• complete a reflexes exam question

• Learn about plant tropisms (yes this is your last plant lesson of the course!!)

Quick biol5 refresher test1. Define a reflex

2. Movement directly towards a stimulus is called…

3. Woodlice exhibit which type of behavioural response to humidity?

4. Which part of our nervous system is responsible for unconscious control?

5. Which type of neurone is found only in the grey matter of the spinal cord?

Plant tropisms

Learning outcomes- you will be able to

• Describe positive and negative tropisms

• Explain their survival value to plants

• Explain the role of plant growth substances in tropisms

• Interpret the results of experiments carried out to investigate tropisms

What are tropisms?

• A tropism is a growth movement of part of a plant in response to a directional stimulus

Examples

• Plant shoots grow towards light – positive phototropism

• Plant roots grow away from light – negative phototropism

• Plant roots grow towards water - positive hydrotropism

Plant growth factors or regulators

• They are a bit like hormones but…

• They affect growth and are produced in small quantities

• They are made by cells located throughout the plant

• They affect the tissues that release them rather than acting on a distant target organ.

Examples of plant growth factors

• Indoleacetic acid• Stimulates cell elongation and shoot

growth

Giberellin

• Involve in stem elongation

Abscisic acid

• Inhibits plant growth, causing dormancy.

We will look at the effect of IAA

• This basically works by stimulating cell elongation on one side of a plant shoot, causing the shoot to bend.

• Watch the following animations then complete the diagram on your worksheet

• http://www.youtube.com/watch?v=Ze8NV7cvW8k

• http://bcs.whfreeman.com/thelifewire/content/chp38/3802001.html

• http://www.youtube.com/watch?v=HvZJqWSdMMk&feature=related

Effect of IAA on roots

• Slightly oddly, in roots IAA inhibits growth.

• When it accumulates on the bottom side of a root, because of gravity, it will inhibit growth and cause the root to grow downwards.

• Draw an annotated sketch on your worksheet to show this

Interpreting data

• The biologist Boysen- Jenson carried out a series of experiments to show the influence of plant growth regulators

• You need to be able to interpret these experiments and those of other workers

Interpreting data

• Study the experiments outlined on page 8 of your booklet

• For each one try to describe what each one shows and give an explanation – there is space for this on page 8 of your booklet

•Removing tip stops growth•Replacing tip enables growth to continue, but at a slower rate•Something must pass from the tip to the rest of the shoot that stimulates growth

•The signal from the tip to the rest of the shoot that stimulates growth must be able to pass through agar•This suggests it might be a chemical small enough to diffuse through the agar

•Something must have diffused from the tip into the agar. This then diffused into the rest of the shoot when the agar was placed on the cut shoot and stimulated growth.

What control should be used here?•Agar that has not had a tip on it should be placed on a cut shoot.

•Covering the shoot tip prevents the response to unidirectional light. This suggests that it is the tip that is sensitive to light.

•A chemical diffused into the agar block. When the block was placed on the side of the cut shoot more of the chemical diffused onto this side.•This caused more growth of the cells on this side and the tip bent over.

Uses of plant growth factors

• As selective weedkillers. Too much IAA can inhibit growth!

Stimulating the formation of fruits

• Or in this case growing giant cabbages!

As hormone rooting powders

Now complete…

• Application exercise p 158-160 (tropisms)

• Then do….Q1 p180/181

q 2 p153• 2 a) i) arc shows 3 neurones1 You must show 3 distinct neurones (the

middle one in the grey matter) with

correct route through dorsal and ventral

roots and indication of synapses.

• ii) neurones labelled sensory, relay /intermediate, motor; 1

• iii) muscle labelled as effector; 1

• b) i) rapid response to stimulus;

which is automatic / involuntary / not under conscious control; 2

It is important that you refer to stimulus

and response for the first marking point.

ii) avoids damage to tissues;

role in learning / homeostasis;

posture / balance;

escape from predators;

finding food / mate;2 max.

Avoid responses such as ‘it prevents you getting hurt’. Reflexes help an organism survive by responding to changes in their environment.

Q1 p180

• 1 a) grows towards light for photosynthesis 1

This is an example of phototropism.

Tropisms are responses to directional stimuli that can maintain flowering plants in favourable environments.

• b) size/species/age of shoot / temperature /time / size of agar block 1

Light intensity is an incorrect answer;

two of the experiments are carried out in

total darkness.

c) i) against hypothesis 1: similar amounts of IAA in light and dark;

• ii) in support of hypothesis 2:

• C has more IAA on shaded side; 2

For the first mark you could use the appropriate letters, for example similar amounts of IAA in A and D. The difference in total amounts in the light and dark are insignificant.

d) diagram showing growth of shoot to right 1

The concentration of IAA will be greater on the left-hand side of the shoot, which stimulates these cells to elongate more.