Biology

• Homeostasis


  The regulation of the internal conditions of a cell or organism in response to internal and external changes
• Importance of homeostasis


  • Keeps conditions constant for enzyme action and cell functions
  • Includes control: blood glucose concentration, body temperature, water and ion levels
• Control system process


  1. Receptors detect stimuli

  2. Coordination centres receive and process information from receptors

  3. Effectors bring about responses that restore optimum levels
• Negative feedback


  • A receptor detects a change in a stimulus
  • The coordinating centre compares the stimulus to a set point
  • An effector then produces a response to correct any difference from the set point
• Nervous system


  • Enables humans to react to their surroundings and coordinate their behaviour
  • Information from receptors passes to the central nervous system (CNS) (the brain and spinal cord)
  • The CNS coordinates the response of effectors, ie muscles contracting or glands secreting hormones
• Reflex action


  1. Pain stimulus is detected by receptors

  2. Impulses from the receptor pass along a sensory neurone to the CNS

  3. An impulse then passes through a relay neurone

  4. A motor neurone carries an impulse to the effector

  5. The effector (usually a muscle) responds, eg. to withdraw a limb away from the source of pain
• Synapse


  The gap between two neurones where an electrical impulse causes the release of a chemical that diffuses across the gap and generates an electrical impulse in the second neurone
• Neurones are not directly connected to each other
• To obtain reliable results in the reaction time experiment, large numbers of subjects need to be tested and averages taken
• There are limited risks with the reaction time experiment
• Sensory neurone


  Responsible for sending impulses from the receptors to the CNS
• Synapse


  The gap between two neurones
• Structures that make up the CNS

  • Brain
  • Spinal cord
• Endocrine system


  Made up of glands that secrete hormones directly into the bloodstream
• Hormones


  Chemical messengers that are carried in the blood to a target organ where they produce an effect
• Effects of hormones vs nervous system

  Slower and act for longer
• Pituitary gland


  A 'master gland' in the brain that secretes several hormones in response to body conditions
• Pituitary gland hormone secretion

  1. Secretes hormones

  2. Some hormones act on other glands to stimulate other hormones to be released

  3. Brings about effects
• Main glands that produce hormones in the human body

  • Pituitary
  • Thyroid
  • Pancreas
  • Adrenal
  • Ovary
• If the blood glucose concentration is too high, the pancreas releases more insulin</b>
• Insulin causes glucose to move from the blood into the cells in liver and muscle, where excess glucose is converted to glycogen for storage
• If the blood glucose concentration is too low, the pancreas releases glucagon
• Glucagon stimulates glycogen to be converted into glucose and released into the blood
• This is an example of negative feedback
• Type 1 diabetes


  Caused by the pancreas failing to produce sufficient insulin, resulting in uncontrolled high blood glucose levels, normally treated with insulin injections
• Type 2 diabetes


  Caused by the body cells no longer responding to insulin, has obesity as a risk factor, treated with carbohydrate-controlled diet and regular exercise
• Injecting insulin in people with Type 1 diabetes

  Lowers the blood glucose level by stimulating the liver to convert it into glycogen
• Injecting insulin in people with Type 2 diabetes


  Little effect because the cells do not respond to insulin
• Oestrogen


  The main female sex hormone produced by the ovaries
• Testosterone


  The main male sex hormone produced by the testes
• Menstrual cycle


  1. Eggs begin to mature and be released (ovulation)

  2. Sperm production is stimulated

  3. Several other hormones are involved
• Hormones involved in control of the menstrual cycle

  • Follicle stimulating hormone (FSH)
  • Oestrogen
  • Luteinising hormone (LH)
  • Progesterone
• Follicle stimulating hormone (FSH)


  Causes eggs to mature in the ovaries in the first part of the cycle, stimulates the ovaries to produce oestrogen
• Luteinising hormone (LH)


  Stimulates the release of the egg from the ovary (ovulation)
• Progesterone


  Maintains the lining of the uterus during the second half of the cycle, inhibits both FSH and LH release
• Hormonal contraception methods


  • Oral contraceptives (combined pill) containing oestrogen and progesterone
  • Injection, implant or skin patch of slow release progesterone
• Non-hormonal contraception methods


  • Barrier methods (condoms, diaphragms)
  • Intrauterine devices
  • Spermicidal creams
  • Surgical sterilisation
• IVF treatment


  1. Giving a woman FSH and LH to stimulate growth of many eggs

  2. Collecting the eggs

  3. Fertilising the eggs with sperm in the laboratory

  4. Inserting one or two embryos into the woman's uterus
• Asexual reproduction involves only one parent, no fusion of gametes, and produces genetically identical offspring (clones)
• Sexual reproduction involves the fusion of male and female gametes, leading to a mix of genetic information and variation in the offspring