AQA A level Biology Topic 6 – Organisms respond to changes in their internal and external environments notes (A* achieved)

AQA AL Biology notes Topic 6 – Organisms respond to changes in their internal and external environments
Topic 6 – Organisms respond to changes in their internal and
external environments
Table of Content
6.1 Stimuli, both internal and external, are detected and lead to a response ... 2
6.1.1 Survival and response ................................................................................................ 2
6.1.2 Receptors................................................................................................................... 4
6.1.3 Control of heart rate .................................................................................................. 8

6.2 Nervous coordination ............................................................................... 9
6.2.1 Nerve impulses .......................................................................................................... 9
6.2.2 Synaptic transmission .............................................................................................. 13

6.3 Skeletal muscles are stimulated to contract by nerves and act as
effectors ................................................................................................ 17
6.4 Homeostasis is the maintenance of a stable internal environment ... 19
6.4.1 Principles of homeostasis and negative feedback .................................................... 19
6.4.2 Control of blood glucose concentration ................................................................... 21
6.4.3 Control of blood water potential ............................................................................. 26




1

,AQA AL Biology notes Topic 6 – Organisms respond to changes in their internal and external environments
6.1 Stimuli, both internal and external, are detected and lead
to a response
6.1.1 Survival and response
1. Stimulus - change in internal or external environment
a. Push stimulus - drives pest away from crop
b. Pull stimulus - attracts pest towards crop
2. Receptor - detects stimulus, specific to 1 type of stimulus
3. Coordinator - formulates a suitable response to a stimulus e.g. nervous system / hormonal system
4. Effector - produces response to a stimulus e.g. muscles / glands
5. Response

Tropisms and growth factors in (flowering) plants
• Tropism - growth of a part of a plant in response to a directional stimulus
o +ve tropism – growth towards stimulus
o -ve response – growth away from stimulus


Gravitropism and phototropism explained by indoleacetic acid (IAA)
How IAA results in phototropism in shoots How IAA results in gravitropism in roots
• Tip produces IAA • Tip produces IAA
• IAA diffuses down shoot • IAA diffuses down shoot
• ↑ IAA on shaded side • ↑ IAA on lower side of root
• IAA stimulates cell elongation / growth • IAA inhibits cell elongation / growth
• Shoot bends towards light (+ve tropism) • Root curves downwards towards gravity
(+ve tropism)




Explain how phototropism helps plant to survive. (3)
• Plant can get as much light as possible
• More photosynthesis takes place
• Allow more energy to be generated

What happen if shoot is removed?
• No auxin available
• Shoot stop growing

Explain why the student removed the shoot tip from each seedling at step 1 (2)
• Tips produces IAA
• Affect concentration of IAA thus shoot growth / elongation

2

, AQA AL Biology notes Topic 6 – Organisms respond to changes in their internal and external environments

Types of behaviour
Taxis (plural taxes)
• Directional responses to a stimulus by mobile organisms
o Towards a favourable stimulus (+ve taxis)
o Away from an unfavourable one (-ve taxis)
• E.g. Woodlice show a tactic response to light (phototaxis)
o Move away from light - keeps concealed under stones during day away from predators
o Move away from damp conditions - ↓ water loss - ↑ chances of survival


Kineses
• Movement is random
• Non-directional response to stimulus
• Results in changing movement
• Keeps organisms in favourable environment
• E.g. Woodlice show a kinetic response to humidity
o Move faster/change direction more often when air drier - ↑ chances of moving to an area
of higher humidity where lose ↓ water - ↑chances of survival

Reflex arc
Overview of nervous system




Protective effect of simple reflex
Stimulus → receptor → sensory neurone (in peripheral) → synapse → relay neurone (in CNS) → synapse →
motor neurone (in peripheral) → effector → response




3