Week 8

• Good because reactions will work

  better and you don’t lose the products
• Good because you can keep out

  molecules that you don’t want
• Bad because there can be osmotic

  problems
• Bad because hazardous by products

  can stay in the cell
• Homeostasis is the physiological process by which the internal

  systems of the body (e.g. blood pressure, body temperature, acid-

  base balance, solute concentration, water potential) are

  maintained at equilibrium despite variations in the external

  conditions
• if an animal uses homeostatic mechanisms to regulate internal conditions it is a regulator

  • e.g. endothermic animals and birds
• other animals allow some conditions to change in tune with the external environment -conformers this is dependent on a relatively stable environment
• In most animals, the majority of cells are bathed

  by internal fluids rather than the environment
• This is advantageous since there can be control of

  substrates needed for metabolism
• Osmoregulation -controlling internal solutes

  • Water and salts/ions

  • Avoiding build-up of toxic metabolic by-products

  • Dealing with ammonia (NH3)

  • Maintaining acid-base balance
• Regulating the composition of body fluids involves balancing uptake and loss of solutes and water with the external environment: Osmoregulation
• Leads to regulation of the composition of cytosol in cells too because of osmosis
• Many animals expend considerable energy on this
• Regulating solute concentration also regulates water content by osmosis
• Most animals dispose of toxic by-products of protein (and other)

  metabolism as aqueous solutions
• All animals must regulate H2O uptake and loss
• Too much H2O and cells rupture, too little and they shrivel
• H2O enters or leaves cells by osmosis-movement of H2O across a semi-permeable membrane
• This is dependent on a difference in osmolarity of the solutions either side of the membrane

  • Isoosmotic

  • Hyperosmotic

  • Hypoosmotic
• This is dependent on a difference in osmolarity of the solutions either side of the membrane

  • Isoosmotic

  • Hyperosmotic

  • Hypoosmotic

  • Osmolarity is defined as the total solute

  concentration (expressed as moles/litre)