Topic 6

• What is a stimulus?

  A stimulus is a detectable change in the environment
• Shoot tip cells in plant produce IAA, causing cell elongation
• What is a reflex?

  A reflex is a rapid, automatic response to a stimulus
• What is a taxes?

  When an organism moves its entire body to a favourable stimulus or away from an unfavourable stimulus
• What is a kinesis?

  When an organism changes the speed of movement and the rate it changes direction
• Pacinian Corpuscle is a pressure detector
• When pressure is applied to the Pacinian Corpuscle it deforms the neurone plasma membrane and stretches the stretch-mediated sodium channels so sodium ions diffuse in. This leads to the establishment of a generator potential. If the generator potential reaches threshold then an action potential in the sensory neurone is induced and an impulse is sent to the CNS and interpreted as pressure
• What are the two main photoreceptors found in the eye?

  Rods and Cones
• Rods process images in black and white. They can detect light in very low intensity, as many rod cells connect to one sensory neurone (retinal convergence)
• Cones produce images in colour. Action potentials can only be generated with high light intensity, hence why we cannot see colours in the dark
• Cardiac muscle is myogenic (will contract and relax without any nervous system input)
• An unstimulated neurone is at 70mV, this is called resting potential
• Heart beat stages

  SAN releases a wave of depolarisation across the atria, causing it to contract. AVN releases another wave of depolarisation, the bundle of His conducts the wave down the septum and the Purkyne fibres. This results in the walls of the ventricles contracting, although there is a short delay before this happens, whilst the AVN transmits the second wave. This allows enough time for the atria to pump all the blood into the ventricles. Finally the cells repolarise and the cardiac muscle relaxes
• Hormones are proteins produced in the glands and secreted into the blood
• Exocrine function: secretes digestive enzymes into the pancreatic duct
• Endocrine function: secretes hormones directly into the blood
• Endocrine function: Hormones are secreted from the cells in the islets of Langerhans. Alpha cells produce and secrete the hormone glucagon. Beta cells produce and secrete the hormone insulin. These are released directly into the blood.
• Control of blood glucose is a negative feedback process. If blood glucose levels get too high or low then the changes are detected by the beta and alpha cells in the islets of Langerhan.
• Islets of Langerhans are found in the pancreas
• What is glycogenesis?


  The codensation of glucose into glycogen. This occurs in the liver and is catalysed by enzymes there.
• What is glycogenolysis?


  The hydrolysis of glycogen to glucose. This occurs in the liver due to the second messenger model
• What is gluconeogenesis?


  Creation of glucose from other molecules, such as amino acids and glycerol in the liver.
• Second messenger model for for glucagon.


  Glucagon binds to glucagon receptors. Once bound, it causes a change in shape of the enzyme adenyl cyclase, which activates it. Activated adenyl cyclase enzymes convert ATP into cyclic AMP (cAMP). cAMP can trigger the enzyme which hydrolyses glycogen into glucose
• Blood with too low of a water potential is hypertonic. Too much water will leave the cells and move into the blood by osmosis. Cells will shrivel (crenation).
• Blood with too high of a water potential is hypotonic. Too much water will move from the blood into the cells by osmosis. Cells will burst (lysis).
• Osmoregulation occurs within the nephrons in the kidneys.
• Nephrons are long tubules surrounded by capillaries, the blood is filtered here to remove waste and selectively reabsorb useful substances back into the blood.