Paper 2

• the nervous system is made up of the central nervous system (CNS)
• the CNS comprises the brain and spinal cord.
• the nervous system enables humans to react to there surroundings and coordinate there behaviours.
• the nervous system includes both voluntary and involuntary actions.
• nervous system responses: stimulus > receptor > coordination > effector > response.
• stimulus is a change of environment detected by receptors.
• information from reception s passes along cells to the CNS as electrical impulses.
• the CNS coordinates the body’s response to a stimulus.
• effectors enable a response
• the body responds to the stimulus
• Homeostasis is the regulation of internal conditions in response to internal and external changes to maintain optimum functioning conditions.
• Homeostasis maintains optimum conditions for all cell functions and enzyme action.
• Homeostasis controls: glucose, temperature, water.
• the automatic control systems of homeostasis may involve nervous responses or chemical responses.
• all systems involve: receptor cells, coordination centres, effectors.
• factors that affect reaction time: tiredness, distractions, caffeine, alcohol.
• Neurones carry electrical impulses around the body.
• synapses are gaps between neurons which allow electrical impulses to pass through.
• reflex actions of the nervous system are automatic and rapid.
• Differences in the characteristics of individuals in a population are called variation
• Variation is due to differences in:

  • inherited genes
  • developing environment
  • genes and the environment
• A mutation is a change in a DNA sequence
• mutations:

  • occur continuously
  • may lead to a new phenotype
  • can lead to a rapid species change
• selective breeding is the process by which humans breed plants and animals for particular genetic characteristics
• process of selective breeding:

  • chose parents with the desired characteristics
  • breed them
  • chose offspring and breed
  • continue over generations
• disadvantages of selective breeding:

  • can lead to inbreeding
  • reduces variation
• sexual reproduction:

  • two parents
  • meiosis
  • male and female sex cells
  • produces non identical offspring
  • wide variation of speices
• asexual reproduction:

  • one parent
  • mitosis
  • no fusion of gametes
  • produces offspring clones
  • no mixing of genetic information
• Meiosis is a type of cell division that makes gametes in the reproductive organs
• Meiosis halves the number of chromosomes in gametes
• fertilisation (joining of two gametes) restores the full number of chromosomes
• The fertilised cell divides by mitosis, producing more cells. As the embryo develops, the cells differentiate.
• genetic material in the nucleus of a cell is composed of DNA
• DNA is made of two strands forming a double helix
• DNA is contained in structures called chromosomes
• A gene is a small section of DNA on chromosomes which codes for a specific sequence of amino acids
• the genome of an organism is the entire genetic material of that organism
• some inherited disorders are due to alleles:

  polydactyly - extra fingers or toes is caused by a dominant allele

  cystic fibrosis is caused by a recessive allele
• gametes are specialised sex cells formed by meiosis
• chromosomes are long molecules made from DNA in the nucleus