B9.5 Tropic responses

Question 1

What is gravitropism?

Answer 1

Gravitropism is a response in which parts of a plant grow towards or away from gravity.

Question 2

What is phototropism?

Answer 2

Phototropism is a response in which parts of a plant grow towards or away from the direction from which light is coming.

Question 3

What is a tropism?

Answer 3

A tropism is a plant growth response to a stimulus.
e.g. light, water, gravity.

Question 4

What is a positive tropism response?

Answer 4

A positive response is a growth response TOWARDS a stimulus.

Question 5

What is a negative tropism response?

Answer 5

A negative response is a growth response AWAY from a stimulus.

Question 6

What are the 2 types of tropisms?

Answer 6

-Phototropism - a response to light.
-Gravitropism - a response to gravity.

Question 7

How are the roots positively gravitropic?

Answer 7

The roots of plants are positively gravitropic because they grow towards the stimulus of gravity (down).

Question 8

How are the shoots negatively gravitropic?

Answer 8

The shoots of plants are negatively gravitropic because they grow away from the stimulus of gravity (up).

Question 9

How are shoots positively phototropic?

Answer 9

Shoots of plants are positively phototropic because they grow towards the stimulus of light.

Question 10

How are roots negatively phototropic?

Answer 10

Roots of plants are negatively phototropic because they grow away from the stimulus of light.

Question 11

Where do the chemicals in plants go during phototropism?

Answer 11

Certain chemicals in plants regulate plant growth. Phototropism and gravitropism are examples of how chemicals like plant hormones regulate plant growth.
In phototropism, these chemicals move to the part of the plant receiving less light. This causes that part of the plant to grow more than the side receiving light, making the plant bend towards the light.

Question 12

Where do the chemicals go during gravitropism?

Answer 12

During gravitropism, the plant hormone gathers on the lower side of the plant. This has different effects on different parts of the plant:
-In the shoot, the chemical stimulates growth, so the stem curves upwards
-In the root, the chemical slows growth, so the root curves down.

Question 13

How to investigate phototropism?

Answer 13

Start with 3 identical plants set up.
-Plant A is in a box with light from ONE direction.
-Plant B is in a box with light from ONE direction and rotating on a clinostat.
-Plant C is in a dark box - control.
In plant A, the shoots grow towards the light due to positive phototropism.
In plant B, it grows upward due to the light falling equally on all sides of the plant - clinostat.
In plant C, it grows upwards because plants in the dark still grow but there was no light stimulus to grow towards.

Question 14

How to investigate gravitropism?

Answer 14

3 seedlings are placed in a dish pointing in different directions.
2 identical dishes are produced. One of these is placed on its side in the dark while the other is placed on a clinostat (coating surface) also in the dark.
The stationary plate (on its side) was affected by the stimulus of gravity. The radicles (roots) grew downwards and the plumules (shoots) grew upwards due to gravitropism. The dish on the clinostat was NOT affected by gravity and therefore the radicles and plumules both grew straight outwards from the seed.

Question 15

What is auxin?

Answer 15

Auxin is a plant hormone. It affects the growth of plants by stimulating or inhibiting cell elongation.

Question 16

Where is auxin produced?

Answer 16

Auxin is produced in the tips of the shoots.

Question 17

What does auxin bring about?

Answer 17

Auxin brings about plant growth (uneven cell elongation) which causes responses called tropisms.

Question 18

How does auxin enable phototropism?

Answer 18

Auxin is produced at the tips of the shoot and if light shines ALL around the tip, auxin is distributed evenly throughout and the cells in the meristem grow at the same rate. -This is what usually happens with plants growing outside.

Question 19

What happens when light shines on the shoot predominantly from one side?

Answer 19

When light shines of the shoot but predominantly from one side, the auxin produced in the top concentrates on the shaded side and diffuses down there, making the cells on that side elongate and grow faster than the cells on the sunny side. The unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of light.

Question 20

What is cell elongation?

Answer 20

Cell elongation is not the same as growth, it just means that the cells already there change shape because of the auxin.

Question 21

How does auxin cause gravitropism?

Answer 21

It is produced in the tips of the roots and shoots and diffuses downwards, accumulating on the bottom of the shoots/roots due to the pull of gravity.

Question 22

Why do shoots grow away from gravity? - negative gravitropism

Answer 22

Shoots grow away from gravity because gravity modifies the distribution of auxin so that it accumulates on the lower side of the shoot. Auxin increases the rate of growth in shoots, causing the shoot to grow upwards

Question 23

Why do roots grow towards gravity? -positive gravitropism

Answer 23

Roots grow towards gravity because of the higher concentration of auxin in the roots that results in a lower rate of cell elongation. The auxin that accumulates at the lower side of the root prevents cell elongation. As a result, the lower side grows at a slower rate than the upper side of the root. This causes the root to bend downwards.

Question 24

Explain how auxin works.

Answer 24

Auxin is mostly made in the tips of the growing stems and roots and can diffuse to other parts of the stems or roots; spreading from a high concentration in the shoot tips down the shoot to an area of lower concentration. Auxin stimulates the cells behind the tip to elongate (get larger); the more auxin there is, the faster they will elongate and grow.