3.6.1.1 Survival and Response

Question 1

What is a stimulus?

Answer 1

A detectable change in the internal or external environment of an organism that leads to a response in the organism

Question 2

What is a receptor?

Answer 2

Detects stimuli

Specific to one type of stimulus

Question 3

What is a coordinator?

Answer 3

It formulates a response to a stimulus

This may be at the molecular level or involve a large organ such as the brain

Question 4

What does an effector do?

Answer 4

It produces a response

Question 5

What is the general sequence of events for the reaction to a stimulus?

Answer 5

Stimulus
Receptor
Coordinator
Effector
Response

Question 6

What is a taxis?

Answer 6

A simple response whose direction is determined by the direction of the stimulus
Towards stimulus = positive taxis
Away from the stimulus = negative taxis

Question 7

What is a kinesis?

Answer 7

A form of response where the organism does not move towards or away from a stimulus
Instead it changes the speed at which it moves and the rate at which it changes direction
Important when the stimulus is less directional e.g. humidity and temperature

Question 8

What is a tropism?

Answer 8

The growth of a plant in response to a directional stimulus
Towards stimulus = positive
Away from stimulus = negative

Question 9

What is the name for the response of plants to a light stimulus?

Answer 9

Phototropic

Question 10

What is the name for the response of plants to a gravity stimulus?

Answer 10

Gravitropic

Question 11

What is the name for the response of plants to a water stimulus?

Answer 11

Hydrotropic

Question 12

What are plant growth factors?

Answer 12

Hormone-like substances which are involved in the response to external stimuli
They affect growth
They affect tissues which release them rather than a target organ

Question 13

What is the process of phototropism?

Answer 13

Cells in the tip of the shoot produce IAA which is transported down the shoot
IAA is initially transported evenly as it moves through the shoot
Light causes the movement of IAA from the light to the dark side of the shoot
This creates a greater concentration on the shaded site
IAA causes the cells on the shaded side to elongate more
The shaded side elongates which causes the root tip to bend towards the light

Question 14

What does IAA do?

Answer 14

It causes elongation of the shoot cells
It controls the bending of the shoots and roots in response to light
High concentrations inhibit cell growth in the roots and increase elongation in the shoots

Question 15

What is the process of gravitropism?

Answer 15

Cells in the root tip produce IAA which are transported along the root
IAA is initially transported to all sides
Gravity influences IAA to move from the upper to lower side of the roots
IAA inhibits elongation of root cells so it inhibits the cells on the lower side
This causes the root to bend downwards towards gravity

Question 16

Why does IAA only affect young cells?

Answer 16

IAA increases plasticity of the cell walls
The response only occurs in young cell walls where cells are able to elongate
Older cells develop greater rigidity

Question 17

What is the acid growth hypothesis?

Answer 17

Hydrogen ions are actively transported by active transport from the cytoplasm into spaces in the cell wall, causing the cell wall to become more plastic
This allows the cells to elongate by expansion

Question 18

What is a behaviour?

Answer 18

The response of an organism to the environment which increases the chance of survival

Question 19

What is a reflex?

Answer 19

An involuntary response which follows a specific pattern to a given stimulus