These topic 6 notes cover everything in the AQA specification. They are detailed with diagrams and include phrases used in mark schemes, helping you to learn what points to write in exam questions as well as helping things stick in your mind. Made using CGP guide, AQA textbook and class notes. A* a...
Stimuli And Responses
Animals increase their chances of survival by responding to changes in their external environment. They also
respond to changes in their internal environment to make sure that the conditions are always optimal for their
metabolism.
- A stimulus is any change in the external or internal environment that leads to a response in the
organism
- Receptors detect stimuli- they can be cells or proteins on cell surface membranes. They are specific to
one type of stimulus
- Effectors are cells that bring about a response to a stimulus, to produce an effect. They include muscle
cells and cells found in glands
- Receptors communicate with effectors via the nervous system or the hormonal system
Tactic Responses- Taxes:
A taxis is a simple response whose direction is determined by the direction of the stimulus. As a result, a motile
organism responds directly to environmental changes by moving its whole body either towards a favourable
stimulus or away from an unfavourable one
Taxes are classified according to whether the movement is towards the stimulus (positive taxis) or away from
the stimulus (negative taxis) and also by the nature of the stimulus
e.g. single-celled algae will move towards light (positive phototaxis)
Kinetic Responses- Kineses:
A kinesis is a form of response in which 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
If an organism crosses a sharp dividing line between a favourable and an unfavourable environment, it’s rate of
turning increases. This raises its chances of a quick return to a favourable environment
e.g. Woodlice lose water from their bodies in dry conditions. When they move from a damp area into a dry one,
they move more rapidly and change direction more often. This increases their chance of moving back into the
damp area.
Tropisms:
A tropism is the growth of a plant in response to a directional stimulus.
A positive tropism is growth towards the stimulus
A negative tropism is growth away from the stimulus
Plant shoots grow towards light (positive phototropism) and away from gravity (negative gravitropism) so that
their leaves are in the most favourable position to capture light for photosynthesis
Plant roots grow away from light (negative phototropism) and towards gravity (positive gravitropism). They are
able to absorb water and mineral ions.
,Growth Factors:
Plants respond to directional stimuli using specific growth factors- these are hormone-like chemicals that speed
up or slow down plant growth
Growth factors are produced in the growing regions of the plant (e.g. shoot tips, leaves) and they move to where
they’re needed in the other parts of the plants
Growth factors called auxins stimulate the growth of shoots by cell elongation- this is where cell walls become
loose and stretchy so the cells get longer
High concentrations of auxins inhibit growth in roots though
Indoleacetic acid (IAA) is an important auxin that’s produced in the tips of shoots in flowering plants. IAA is
moved around the plant to control tropisms- it moves by diffusion and active transport over short distances, and
via the phloem over long distances. This results in different parts of the plant having different concentrations of
IAA leading to an uneven distribution.
Phototropism in flowering plants:
1. Cells in the tip of the shoot produce IAA, which is then transported down the shoot
2. The IAA is initially transported evenly throughout all regions as it begins to move down the shoot
3. Light causes the movement of IAA from the light side to the shaded side of the shoot. This causes the
elongation of shoot cells due to a greater concentration of IAA.
4. The shoot tip bends towards the light
Gravitropism in flowering plants:
1. Cells in the tip of the root produce IAA, which is then transported along the root
2. The IAA is initially transported to all sides of the root
3. Gravity influences the movement of IAA from the upper side to the lower side of the root
4. As IAA inhibits the elongation of root cells, the cells on the lower side elongate less
5. This causes the root to bend downwards towards the force of gravity
The Nervous System:
The central nervous system (CNS) is made up of the brain and the spinal cord
The peripheral nervous system (PNS) is made up of neurones that connect the CNS to the rest of the body
There are 3 main types of neurones:
Sensory neurones transmit electrical impulses from receptors to the CNS
Motor neurones transmit electrical impulses from the CNS to effectors
Relay neurones transmit electrical impulses between sensory neurones and motor neurones
The motor nervous system can be further subdivided:
- The voluntary nervous system, which carries nerve impulses to body muscles and is under voluntary
(conscious) control
- The autonomic nervous system, which carries nerve impulses to glands, smooth muscle and cardiac
muscle and is subconscious
, Detecting Stimuli:
1. A stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone
2. When an electrical impulse reaches the end of a neurone, chemicals called neurotransmitters take the
information across to the next neurone, which then sends an electrical impulse
3. The CNS processes the information and sends impulses along motor neurones to an effector
Reflexes:
A reflex is where the body responds to a stimulus without making a conscious decision to respond.
Reflex Arc e.g. response to heat:
1. Thermoreceptors in the skin detect the heat stimulus
2. The sensory neurone carries impulses to the relay neurone
3. The relay neurone connects to the motor neurone
4. The motor neurone sends impulses to the effector
5. Your muscle contracts to withdraw your hand
Importance of reflex arcs:
- They are involuntary so do not require the brain, thus leaving it free to carry out more complex
responses
- They protect the body from harm
- They are fast because the neurone pathway is short with very few synapses
- The absence of any decision-making process also means the action is rapid
Receptor cells in the nervous system:
- When a nervous system receptor is in its resting state, there’s a difference in charge between the inside
and outside of the cell- this is generated by ion pups and channels.
- This means there’s a voltage/ p.d across the membrane
- The potential difference when a cell is at rest is called its resting potential.
- When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing
more ions to move in and out of the cell- altering the p.d.
- The change in potential difference due to a stimulus is called the generator potential
- A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change
in p.d, so a bigger generator potential is produced
- If the generator potential is big enough, it’ll trigger an action potential- an electrical impulse along a
neurone.
- This is only triggered if the generator potential reaches a certain level called the threshold level.
- Action potentials are all one size, so the strength of the stimulus is measured by the frequency of action
potentials
- If the stimulus is too weak, the generator potential won’t reach the threshold, so there’s no action
potential
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller Astudent100. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £7.99. You're not tied to anything after your purchase.
