Homeostasis

Cells function  most efficiently if they  are kept  in near  constant conditions. Cells in multicellular animals are surrounded by tissue fluid. The composition, pH and temperature of tissue fluid are kept  constant by exchanges with the blood as discussed in the section on Transport in mammals. In mammals, core temperature, blood glucose concentration and blood water potential are maintained within narrow  limits to ensure the efficient  operation of cells. Prior knowledge for this section includes an understanding that  waste products are excreted from the body – a role that  is fulfilled by the kidneys  – and an outline of the structure and function  of the nervous and endocrine systems. In plants,  guard cells respond to fluctuations in environmental conditions and open  and close  stomata as appropriate for photosynthesis and conserving water.

Candidates will be expected to use  the knowledge gained  in this section to solve problems in familiar and unfamiliar contexts.

Learning outcomes

Candidates should  be able to:

14.1  Homeostasis in mammals

Homeostasis in mammals requires complex systems to maintain  internal conditions near  constant.

The kidneys  remove wastes from the blood and are the effectors for controlling the water potential of the blood.

a)   discuss the importance of homeostasis in mammals and explain the principles  of homeostasis in terms of internal and external stimuli, receptors, central  control,  co-ordination systems, effectors (muscles and glands)

b)   define  the term  negative feedback and explain how it is involved in homeostatic mechanisms

c)   outline  the roles of the nervous system and endocrine system in co-ordinating homeostatic mechanisms, including thermoregulation, osmoregulation and the control of blood glucose concentration

d)   describe the deamination of amino  acids  and outline  the formation of urea  in the urea  cycle (biochemical detail of the urea  cycle is not required)

e)   describe the gross structure of the kidney and the detailed structure of the nephron with its associated blood vessels using photomicrographs and electron micrographs

f) describe how the processes of ultrafiltration and selective reabsorption are involved with the formation of urine in the nephron

g)   describe the roles of the hypothalamus, posterior pituitary, ADH and collecting  ducts in osmoregulation

h)   explain how the blood glucose concentration is regulated by negative feedback control mechanisms, with reference to insulin and glucagon

i) outline  the role of cyclic AMP as a second messenger with reference to the stimulation of liver cells by adrenaline and glucagon

j) describe the three main stages of cell signalling in the control of blood glucose by adrenaline as follows:

•   hormone-receptor interaction at the cell surface
•   formation of cyclic AMP which binds to kinase  proteins

• an enzyme cascade involving activation  of enzymes by phosphorylation to amplify the signal

k)   explain the principles  of operation of dip sticks  containing glucose oxidase and peroxidase enzymes, and biosensors that can be used for quantitative measurements of glucose in blood and urine

l) explain how urine analysis  is used in diagnosis with reference to glucose, protein  and ketones

14.2  Homeostasis in plants

Stomatal aperture is regulated in response to the requirements for uptake of carbon  dioxide for photosynthesis and conserving water.

a)   explain that  stomata have daily rhythms of opening and closing and also respond to changes in environmental conditions to allow diffusion of carbon  dioxide and regulate water loss by transpiration

b)   describe the structure and function  of guard cells and explain the mechanism by which they  open  and close  stomata

c)   describe the role of abscisic acid in the closure of stomata during times of water stress (the role of calcium  ions as a second messenger should  be emphasised)