HOMEOSTATSIS

• Homeostasis


  The maintenance of a constant internal environment in order to maintain optimum conditions for enzyme and cellular function
• Conditions maintained by homeostasis in the body


  • Blood glucose concentration
  • Body temperature
  • Water levels
• Types of responses used in body control systems


  • Nervous
  • Chemical
• Components of all control systems

  • Receptors
  • Coordination centres
  • Effectors
• Function of a receptor


  Detect changes in the environment (stimuli)
• Function of a coordination centre


  Obtain and process information from receptors
• Function of an effector


  Bring about responses to stimuli
• Nervous system

  Allows the body to react to its surroundings and coordinate an appropriate response
• How a stimulus leads to a response being carried out by the body

  1. Stimulus is converted into an electrical impulse by the receptors

  2. The electrical impulse passes along sensory neurones to the central nervous system (CNS)

  3. The CNS coordinates an appropriate response and an electrical impulse is sent along motor neurones to the effector, which carries out the response
• Sequence of events describing how the nervous system works

  Stimulus → receptor → coordinator → effector → response
• Reflex action

  An automatic and rapid response which does not involve any conscious input from the brain
• Reflex actions

  • Aid survival by preventing harm to the body
• How a reflex action occurs via a reflex arc


  1. The stimulus is detected by a receptor

  2. An electrical impulse passes along a sensory neurone to the spinal cord (part of the CNS)

  3. At a synapse between a sensory neurone and a relay neurone, a chemical diffuses across the gap and stimulates a new impulse which passes along the relay neurone

  4. The same process occurs at a synapse between a relay neurone and a motor neurone

  5. At the effector, an appropriate response is carried out
• Reflex pathway

  The coordination centre is a relay neurone found in the spinal cord/unconscious parts of the brain
• Conscious pathway


  The coordination centre is in the conscious part of the brain
• Cerebral cortex


  • Controls consciousness, intelligence, memory and language
• Cerebellum


  • Controls muscular coordination
• Medulla


  • Controls unconscious activities eg. breathing, heart rate
• Investigating and treating the brain

  • The brain is a complex and delicate organ
  • The brain is easily damaged and destroyed
  • Certain membranes prevent drugs from reaching the brain
  • The exact function of each part of the brain is not known
• Methods used by scientists to determine brain function

  • Studying patients with brain damage
  • Electrical stimulation of the brain
  • MRI scans
• Receptors of the eye

  • Sensitive to light intensity and colour
• Structures within the eye

  • Focusing on near or distant objects - accommodation
  • Adaptation to dim light
• Retina

  A light-sensitive layer found at the back of the eye. Light stimulates the retinal cells, resulting in impulses being sent to the brain.
• Tropism


  The response of a plant to a specific stimulus
• Optic nerve

  Connects the eye and the brain. It carries impulses to the brain so that an image can be visualised.
• Phototropism


  The response of a plant's shoot or root to light
• Sclera


  The tough outer layer of the eye which protects its internal structures.
• Gravitropism


  The response of a plant's shoot or root to gravity
• Cornea

  The curved transparent layer at the front of the eye. It lets light into the eye and allows light to be focused onto the retina.
• Auxin

  A hormone which controls the growth of a plant's shoots and roots. When auxin is unevenly distributed in a growing plant, the rate of growth will also be unequal.
• Iris


  A muscle which controls the size of the pupil by contracting or relaxing. This allows the eye to adjust to bright and dim lighting.
• Ciliary muscles and suspensory ligaments

  Hold the lens in place and control its shape.
• Positive phototropism in plant shoots

  1. One side of the shoot is in the light, causing auxin to move to the shaded side

  2. At the shaded side, the cells are stimulated to grow - the shoot bends towards the light

  3. As a result, photosynthesis can occur at a faster rate
• Negative gravitropism in plant shoots

  1. In a horizontal shoot, auxin accumulates in the lower side due to gravity

  2. Cells on the lower side of shoot grow more, causing the shoot to bend away from the direction of gravity

  3. As a result, more light is usually available for photosynthesis
• Iris

  A muscle which controls the size of the pupil by contracting or relaxing. This allows the eye to adjust to bright and dim lighting
• Ciliary muscles

  Hold the lens in place and control its shape
• Positive gravitropism in plant roots

  1. In a horizontal root, auxin accumulates in the lower side, causing cells to grow less

  2. The root bends in the direction of gravity

  3. As a result, more water and nutrients are available to the plant roots
• Suspensory ligaments

  Hold the lens in place and control its shape
• Gibberellins


  A type of plant hormone involved in seed germination
• How the iris alters the size of the pupil

  1. Bright light: Circular muscles contract, radial muscles relax - makes pupil smaller

  2. Dim light: Circular muscles relax, radial muscles contract - makes pupil larger