Homeostasis

Question 1

Define, with examples, the term homeostasis.

4 marks

Answer 1

keeping conditions constant/ within narrow limits
within the body/ internal environment
e.g., temperature in humans kept at 37 degrees C/ other example
e.g., blood sugar/ glucose in humans kept within limits/ other example

Question 2

Describe homeostasis in relation to blood glucose concentration in humans.

6 marks

Answer 2

homeostasis is maintaining internal environment at constant levels/within narrow limits
homeostasis involves both nervous and endocrine systems
low blood glucose triggers glucagon release
glucagon is produced å-islet cells in pancreas
glycogen is converted to glucose
high blood glucose concentration triggers insulin release
insulin produced by ß-islet cells in pancreas
glucose taken up by (liver/muscle) cells
glucose converted to glyocgen
blood gluose levels controlled by negative feedback
correct reference to lowering or raising blood glucose levels

Question 3

Explain how blood glucose concentration is controlled in humans.

9 marks

Answer 3

homeostasis maintains the internal blood glucose levels between narrow limits
70-110 mg glucose per 100 ml blood
blood glucose level is maintained by negative feedback
islets in pancreas monitor blood glucose levels
after meal blood glucose increases
high blood glucose stimulates release of insulin
(release of insulin) by pancreatic islets/ by ß-cells
causes muscles/ adipose tissue and liver to store glycogen
glucose stored in the form of glycogen (in muscle/ liver)
storage lowers blood glucose levels
if blood glucose levels drops glucagon secreted
secrete glucagon by pancreatic islets/ by å-cells
this causes liver to break down glycogen (to glucose)
glycogen breakdown causes blood glucose level increase

Question 4

Describe the response of the human body to low external temperatures.

4 marks

Answer 4

thermoreceptors/ sensory input
hypothalamus acts as a thermostat
metabolic rate increases
shivering / goose bumps / hairs raising / swet glands inactive
vasoconstriction of skin arterioles
blood flow from extremities is reduced / blood flow to internal organs is increased
increased activity
heat is transferred in blood

Question 5

Explain how pre-synaptic neurons can either encourage or inhibit post-synaptic neurons.

6 marks

Answer 5

neurotransmitters released by pre-synaptic neurons
diffuse across synapse
bind to specific receptors on post-synaptic membranes
some neurotransmitters increase permeability of post-synaptic membrane to positive ions
causing localized depolarization
which helps an action potential to form / raises membrane above threshold
e.g. acetylcholine or other example

other cause negatively charged chloride ions to move across post-synaptic membrane into the cell / K ions move out of the post-synaptic nerve cell
e.g. GABA / other example
leading to hyper-polarization
which inhibits actions potentials