GCSE Biology Unit 1a&b

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  • Created by: R.Perkins
  • Created on: 06-06-12 17:56

What's a stimulus?

A stimulus, or stimuli (plural form) is a change in your environment, which you may need to react to. These stimuli can affect the body in a positive or negative way. To ensure reactions occur in the appropriate stimulus, the body continously monitors what's going on in the body.

A stimulus can be in various forms; they include:

  • Light
  • Sound
  • Touch
  • Pressure
  • Chemical
  • Change in position
  • Change in temperature
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What are receptors and sense organs? What are the


These consist of groups of cells, sensitive to stimuli. They have the ability to change energy from the stimulus (e.g. light energy) into electrical impulses, which can travel through the nervous system

Sense Organs

There are 5 sense organs, which contain receptors.

  • Eyes - these contain light receptors, allowing us to see objects and others
  • Ears - these contain sound and 'balance' receptors, allowing us to pick up the sound of the modern world.
  • Nose- consisting of smell receptors, the nose is extremely sensitve to chemical stimuli
  • Tongue- consisting of taste receptors, the tongue is sensitive to bitter, salt, sweet, sour and savoury (containing MSG) foods, which is a chemical stimuli
  • Skin - The skin contains receptors sensitive to touch, pressure and temperature change.
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How does the Central Nervous System (CNS) coordina

The Central Nervous System is where all the information, picked up from the several sense organs located around the body, is sent. This is also where reflexes and action are coordinated.

The Central nervous system consists of the organs, the brain and spinal cord only. Along the CNS are a series of neurones, which transport electrical impulses (information picked up from the receptors) to and from the CNS.

Instructions to solve the situation, are sent from the CNS ,to the effectors, which respond accordingly.

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Name the neurones found in the body

Sensory Neurones - these carry the electrical impulses, from the receptors around the body, to the CNS.


Order is..

Relay Neurones - the electrical impulses travel along these, which are found along the spinal cord.

Motor Neurones - these neurones carry the electrical impulses to the effectors. (Muscles and/or glands)

Remember STORM - Sensory first, then relay and then motor.

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What's an effector?

Muscles and glands are known as effectors, as they respond to the electrical impulses, picked up by the receptors, and cause an effect. Muscles and galnds respond in various different ways.

Muscles - Contract to a response

Glands - Secrete hormones

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What is a reflex? Can you name an example?

Reflexes help prevent further pain and injury through an automatic response to a stimulus.


If someone was to shine a bright light into your eye, the pupil dcreases in size, restricting the full amount from entering the eye. Doing so, prevents later damage to the eye.

Another, would be when the body experiences shock. The hormone, adrenaline, is released automatically, it doesn't just wait for when you decide you're shocked.

The Reflex Arc

The Reflex Arc travels through the CNS and does not involve alert from the brain, as it takes a while to make a decision, related to the situation picked up. (See later card for more information)

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What are synapses and what are their purpose?

Synapses connect neurones together, through a small gap. This is so electrical impulses can travel along them.

The signal from the nerves is transferred by chemicals, which diffuse across this gap.

These chemicals, later, set off a new electrical impulse, which is transported to the next neurone.

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Name the steps in the reflex arc

1. The neurones in the reflex arcs go through the spinal cord, when a stimulus is picked up and doesn't always require the brain's input. (Sometimes, it can be an unconscious part of the brain, used in this process)

2. When the stimulus is detected by the receptors, the impulse is sent along a sensory neurone to the spinal cord.

3. In the spinal cord, the impulse is transferred from sensory neurones, to relay neurones.

4. The relay neurones pass on the 'message' to the motor neurones.

5. The motor neurones pass on the impulse to the effectors. (Muscles and glands)

6. The muscle contracts or the gland secretes a hormone to deal with the change.

As this process does not involve the brain, the response to the stimulus is quicker, as we do not have to think about it.

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Name 4 examples where hormones are produced in the

Pituitary Gland- This is found in the brain. The gland produces many important hormones including LH and FSH (in females only) and ADH - a hormone to control water content in our bodies.

Pancreas- The Pancreas produces insulin, to control blood sugar content

Testes - (Males only) - Testosterone is produced here, responsible for the changes in the body, during puberty. (Eg. Extra hair in places, change in body proportion, greasy skin)

Ovaries- (Females only)- Responsible for the production of oestrogen,

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Compare hormonal responses to nerve responses


  • Chemical signal
  • Slow to act upon it. Can take days or even weeks to resolve the situation
  • Can affect various parts of the body can one
  • Involuntary response - normally affecting endocrine glands and involuntary muscles
  • Long lasting - affects the body for longer periods of time.


  • Electrical/Chemical signal
  • Quick to act upon a response.
  • Acts upon specific target organs and cells
  • Voluntary response - normally affecting exocrine glands and voluntary muscles
  • Doesn't last long. Lasts for roughly a few seconds.
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Name the 4 stages of the menstrual cycle

Stage 1

  • The lining of the uterus breaks down for roughly four days, resulting in bleeding. (Known as a 'period')

Stage 2

  • Between day 4 and 14 of the cycle, the lining begins to form, once again. This consists of a spongy layer, full of blood vessels, suitable for receving of a fertilised egg.

Stage 3

  • An egg is developed released from the ovaries at around the 14th day

Stage 4

  • The wall is kept in place for another 14 days. If no fertilised egg is implanted, the lining breaks down again and the cycle continues.
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Which hormones control the different stages? Are t

There are 3 main hormones, which control the activity of the menstrual cycle (not including progesterone, which keeps the lining in place for the 14 days).

FSH Follicle-Stimulating Hormone

Produced by the pituitary gland, FSH causes an egg to mature in the ovaries. Doing so, causes the ovaries to produce oestrogen.


Produced in the ovaries, oestrogen causes the pituitary gland to produce LH (Luteinising Hormone). It also prevents the further release of FSH.

LH Luteinising Hormone

Likewise with FSH, LH is produced in the pituitary gland. This hormone causes the release of the egg from the ovary at around the 14th day of the menstrual cycle.

Without one of these hormones, the menstrual cycle would be unable to occur. Each hormone links into another, depending on one other to complete certain tasks.

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How can hormones be used to reduce fertility?

Reducing Fertility

The hormone oestrogen can be used to prevent the release of an egg, therefore meaning that this hormone can be used as a form of contraception. Despite being the hormone, responsible for the release of the eggs, but if the level of oestrogen is kept at a constant, high level, FSH is secreted, preventing further development and production of the egg.


  • Effectiveness - the pill's over 99% effective regarding the prevention of pregnancy
  • Reducing the risks of getting some types of cancer.


  • There is still a slight chance, likewise with the majority of healthcare products, that this method of contraception may not prevent pregnancy
  • Side effects-  headaches, nausea, irregular periods and fluid retention (keeping excess fluids in the body) can be side effects of this contraception method.
  • The contracpetion does not protect against Sexually Transmitted diseases (STI's)
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How can hormones be used to increase fertility?

A lack of the hormone, FSH, prevents the egg maturing, therefore no eggs are released, meaning an unsuccessful pregnancy. FSH can be taken by women, going through this, which will therefore stimulate egg production in the ovaries, causing the pituitary to secrete LH. Doing so, allows the woman to become pregnant.


  • It helps a lot of women, who couldn't get pregnant the natural way, to have children


  • Not always effective - the procedure may have to be completed many times, before its actually successful. Doing so, makes this more and more expensive
  • Too many eggs could be stimulated, resulting in multiple pregnancies
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What's IVF?

IVF (In vitro fertilisation) is a porsedure involving the collection of the eggs from the woman and fertilising them, using the man's sperm. This is completed in a labortory.This is a rather complex, yet costly prosedure

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Pleased :)

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