Biology Paper 2

• Thyroxine is produced from the thyroid gland, which stimulates the basal metabolic rate. It controls the speed at which oxygen and food products react to release energy for the body to use. Thyroxine plays an important role in growth and development. Thyroxine levels are controlled by negative feedback.
• Hypothalamus and pituitary gland


  Have important roles in detecting and controlling thyroxine levels
• Regulation of thyroxine levels

  1. Low thyroxine levels stimulate hypothalamus to release TRH

  2. TRH causes pituitary to release TSH

  3. Thyroid releases more thyroxine

  4. Blood levels return to normal
• Regulation of thyroxine levels

  1. Normal thyroxine levels inhibit TRH release from hypothalamus

  2. This inhibits release of TSH from pituitary

  3. Normal blood levels are maintained
• Adrenaline


  • Produced by adrenal glands in times of fear or stress
  • Targets vital organs
  • Increases heart rate
  • Boosts delivery of oxygen and glucose to brain and muscles
  • Prepares body for 'flight or fight'
  • Not controlled by negative feedback
• Effects of adrenaline release

  1. Increases breathing rate

  2. Increases heart rate

  3. Increases conversion of glycogen to glucose for more energy in muscles

  4. Diverts blood away from digestive system towards muscles
• Effects of adrenaline


  • Allow body to prepare for action in situations where a quick response may be essential
• Blood glucose concentration is monitored and controlled by the pancreas. If the blood glucose concentration is too high, the pancreas produces the hormone insulin that causes glucose to move from the blood into the cells. In liver and muscle cells excess glucose is converted to glycogen for storage
• Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin. It is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections. In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas. A carbohydrate controlled diet and an exercise regime are common treatments. Obesity is a risk factor for Type 2 diabetes.
• If the blood glucose concentration is too low, the pancreas produces the hormone glucagon that causes glycogen to be converted into glucose and released into the blood.
• In blood glucose regulation, the hormone insulin plays a key role. When blood sugar rises in the blood, insulin sends a signal to the liver, muscles and other cells to store the excess glucose. Some is stored as body fat and other is stored as glycogen in the liver and muscles. Whereas, if the blood glucose level is too low, the liver receives a message to release some of that stored glucose into the blood. This change is brought about by another hormone produced by the pancreas called glucagon.
• Water leaves the body via the lungs during exhalation. Water, ions and urea are lost from the skin in sweat. There is no control over water, ion or urea loss by the lungs or skin. Excess water, ions and urea are removed via the kidneys in the urine. If body cells lose or gain too much water by osmosis they do not function efficiently.
• The digestion of proteins from the diet results in excess amino acids which need to be excreted safely. In the liver these amino acids are deaminated to form ammonia. Ammonia is toxic and so it is immediately converted to urea for safe excretion.
• The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.
• People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis.
• Puberty


  Reproductive hormones cause secondary sex characteristics to develop
• Oestrogen


  Main female reproductive hormone produced in the ovary
• Ovulation


  1. Eggs begin to mature

  2. One egg is released approximately every 28 days
• Testosterone


  • Main male reproductive hormone produced by the testes
  • Stimulates sperm production
• Hormones involved in the menstrual cycle

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


  • Causes maturation of an egg in the ovary
• Luteinising hormone (LH)


  • Stimulates the release of the egg
• Oestrogen and progesterone


  • Involved in maintaining the uterus lining
• Plants produce hormones to coordinate and control growth and responses to light (phototropism) and gravity (gravitropism or geotropism). Unequal distributions of auxin cause unequal growth rates in plant roots and shoots
• Gibberellins are important in initiating seed germination. Ethene controls cell division and ripening of fruits.
• Plant growth hormones are used in agriculture and horticulture. Auxins are used:

  • as weed killers

  • as rooting powders

  • for promoting growth in tissue culture.

  Ethene is used in the food industry to control ripening of fruit during storage and transport.

  Gibberellins can be used to:

  • end seed dormancy

  • promote flowering

  • increase fruit size