homeostasis - biology paper 2

• Homeostasis


  The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes
• Enzymes and cells require very stable conditions in order to work
• The body has systems in place to keep the internal conditions optimum for the cells to function
• Internal conditions maintained by homeostasis

  • Blood glucose concentration
  • Body temperature
  • Water levels
• Automatic control systems


  • Involve the nervous system or hormones
  • Detect changes in the environment (internal or external) through receptor cells
  • Pass information to a Coordination Center (brain, spinal cord, pancreas)
  • Coordination Center sends instructions to an effector (muscle or gland)
  • Effector carries out the response to maintain optimum conditions
• A change to the environment is called a stimulus
• Nervous system


  Consists of two parts: central nervous system (brain and spinal cord) and other nerves running to and from the central nervous system
• Homeostasis


  1. Stimulus

  2. Receptor

  3. Coordination Center

  4. Effector

  5. Response
• Neuron


  Nerve cell
• Reflex arc

  1. Stimulus detected by receptor

  2. Electrical impulse passed along sensory neuron to central nervous system

  3. Electrical impulse passed along relay neuron in central nervous system

  4. Electrical impulse passed along motor neuron to effector

  5. Effector (muscle) contracts, response occurs
• Reflex actions are automatic and rapid, without conscious decision-making by the brain
• Reflexes help protect us from danger
• Mutation


  A change to a base in the DNA sequence
• Mutations happen all the time
• A single base change in the DNA sequence

  Does not change the amino acid sequence of the protein
• A different mutation in the DNA sequence


  Changes the amino acid sequence and alters the shape of the protein
• Mutations that change the shape of a protein


  • Can have a dramatic effect on the function of the protein, e.g. an enzyme's active site may change shape so it can no longer bind to the substrate
  • A structural protein like collagen may lose its strength
• Non-coding regions of DNA


  Regions that switch genes on and off, telling genes when to produce proteins
• Mutations in non-coding regions


  Can affect how genes are switched on or off, e.g. a gene may be turned on when it should be turned off, leading to uncontrolled mitosis and cancer
• Dominant allele


  An allele that will show in the phenotype even if there is only one copy present
• Recessive allele


  An allele that will only show in the phenotype if two copies are present, with no dominant allele present
• Homozygous


  Having two copies of the same allele
• Heterozygous


  Having two different alleles
• Genotype


  The alleles present in an organism
• Phenotype


  The characteristics caused by an organism's alleles
• Normal human cells contain 23 pairs of chromosomes
• One chromosome in each pair comes from the father, the other from the mother
• Allele


  A version of a gene
• Earwax alleles


  • Allele for wet earwax (capital E)
  • Allele for dry earwax (lowercase e)
• The gene for earwax has two alleles
• Dominant allele


  Shows in the phenotype even if only one copy is present
• Recessive allele


  Only shows in the phenotype if two copies are present with no dominant allele
• Most characteristics are controlled by many genes acting together
• Height is an example of a characteristic controlled by many genes
• Cystic fibrosis


  An inherited disorder of cell membranes
• Cystic fibrosis


  • Controlled by a single gene
  • Has two alleles
• Normal cell membrane function allele

  Dominant, symbol capital C
• Defective cell membrane allele

  Recessive, symbol lowercase c
• To have cystic fibrosis, a person must inherit two copies of the defective allele
• Person with one defective allele and one normal allele

  Carrier of the cystic fibrosis allele