IM - BIOLOGY PAPER 2

• receptors -

  the cells that detect stimuli

  such as:

  taste receptors on the tongue

  sound receptors in the ear

  smell receptors in the nose

  light receptors in the eyes
• central nervous system (CNS)

  where all the information from the receptors is sent and where reflexes and actions are coordinated

  in vertebrates (animals with backbones) this consists of the brain and spinal cord only. in mammals the CNS is connected to the body by sensory neurons and motor neurons. neurons transmit information as electrical impulses to and from the CNS
• effectors -

  instructions from the CNS are sent along neurons to effectors. effectors are muscles and glands which respond to neuron impulses and bring about a response to the stimulus

  muscles contract

  glands secrete chemical substances called hormones
• different types of neurons -

  1. sensory neurons : carry information as electrical impulses from the receptors in the sense organs to the central nervous system
  2. relay neurons : carry electrical impulses from sensory neurons to motor neurons
  3. motor neurons : carry electrical impulses from the central nervous system to the effectors
• transmission of info to and from the CNS -

  • stimulus
  • receptors
  • sensory neuron
  • relay neuron in the CNS
  • motor neuron
  • effector
  • response
• synapse -

  the connection between two neurons, the nerve signal is transferred by chemicals which diffuse across the gap. these chemicals then set off a new electrical signal in the next neurone

  neurons deliver information really quickly because the signal is transmitted by electrical impulses. synapses slow down the transmission of nervous impulse because the diffusion of chemicals across the gap takes time
• reflexes -

  fast, automatic responses to certain stimuli. they bypass your conscious brain completely when a quick response is essential
• reflex arcs :

  1. when a stimulus is detected by receptors, impulses are sent along a sensory neurone to a relay neurons in the CNS
  2. when the impulses reach a synapse between the sensory neurone and the relay neurone, they trigger chemicals to be released. these chemicals cause impulses to be sent along the relay neurone
• reflex arc part two:

  3. when impulses reach a synapse between the relay neurone and a motor neurone, the same things happens. chemicals are released and cause impulses to be sent along the motor neuron

  4. the impulses then travel along the motor neurons to the effector

  5. if the effector is a muscle it will respond to the impulse by contracting, if its a gland it will secrete a hormone
• reflex arc -

  1. stimulus
  2. receptor
  3. sensory neurone
  4. relay neurone
  5. motor neurone
  6. effector
  7. response
• what are hormones?

  chemical messengers which travel in the blood to activate target cells.

  carried in the blood plasma and only affect particular organs.
• endocrine glands:

  • the pituitary glands - produces many hormones that regulate body conditions. these hormones act on other glands directing them to release hormones that bring about change.
  • the pancreas - produces insulin, regulates blood glucose levels
  • they thyroid - produces thyroxine, regulates rate of metabolism, heart rate and temperature
• endocrine glands -

  • the adrenal glands - produce adrenaline which prepares the body for a fight or flight response
  • the ovaries - produce oestrogen which is involved in the growth and maintenance of the uterus lining
  • the testes - produce testosterone which controls puberty and sperm production
• nerves -

  fast action

  act for a short time

  acts on a very precise area 


  hormones -

  slower action

  acts for a long time

  acts in a more general way
• thyroxine -

  • released by the thyroid gland in the neck
  • regulates basal metabolic rate
  • stimulates protein synthesis for growth and development
  • thyroxine is released in response to thyroid stimulating hormone (TSH) which is released by the pituitary gland
• when the level of thyroxine in the blood is higher than normal the secretion of TSH from the pituitary gland is inhibited reducing the amount of thyroxine released and levels fall back to normal

  when the level of thyroxine in the blood is low the secretion of TSH is stimulated again increasing the amount of thyroxine so levels in the blood rise back up to normal
• adrenaline -

  released by the adrenal glands

  released in the response to stressful or scary situations

  your brain detects the fear and sends a nervous impulse to the adrenal gland

  gets your body ready for flight or fight by triggering mechanisms that increase the supply of oxygen and glucose to cells in the brain and muscles
• what are mutations?

  • a random change in an organisms DNA and can sometime be inherited
  • they occur continuously and spontaneously - when a chromosome isn't replicated properly or exposure to substances or radiation.
  • mutations change the sequence of the DNA bases in a gene which produces a genetic variant. this can lead to changes in the protein and amino acid codes for
• types of mutation -

  • insertions - a new base is inserted into the DNA base sequence where it shouldn't be. an insertion can change the way the bases are read changing the amino acids that they code for, they have a knock on effect so may change more than one amino acid
  • deletions - a random base is deleted from the DNA base sequence
  • substitutions - a random base in the DNA base sequence is changed into a different base
• gametes -

  • sperm and egg
  • contains 23 chromosomes
• sexual reproduction - 


  involves the fusion of male and female gametes. because there are two parents the offspring contain a mixture of their parent's genes and are genetically different to their parents.
• asexual reproduction -


  there's only one parent. there's no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring. the offspring are genetically identical to the parent
• meiosis -

  1. genetic information is duplicated and the chromosomes arrange themselves into pairs
  2. chromosome pairs line up down the middle of the cell
  3. the pairs are pulled apart
  4. they line up down the middle and the arms of the chromosomes are pulled apart again
  5. four genetically different gametes are produced which contain 23 chromosomes
• malaria -

  • caused by a parasite that is spread by mosquitoes
  • the parasite reproduces sexually in the mosquitoes and asexually in the human host
• many fungus can reproduce sexually and asexually - these release spores which can reproduce sexually and asexually 


  plants can reproduce sexually and asexually

  by runners and by bulbs
• males had a X and a Y chromosome: XY

  the Y chromosome causes male characteristics

  females have two XX chromosomes: XX

  the XX combination allows female characteristics to develop
• there is a 50% chance each sperm cell will get a X chromosome and 50% it will get a Y chromosome


  all eggs end up with one X chromosome
• who was Gregor Mandel?

  from all his experiments on pea plants Mendel reached these three important conclusions about heredity in plants:

  • characteristics in plants are determined by "hereditary units"
  • hereditary units are passed on to offspring unchanged from both parents, one unit from each parent.
  • hereditary units can be dominant or recessive - if an individual had both the dominant and the recessive unit for a characteristic, the dominant characteristic will be expressed
• importance of Mendel's work

  people didn't no about genes, DNA and chromosomes and background knowledge of experiments weren't fully understood.

  after his death people realised how significant his work was
• types of variation

  genetic variation

  environmental variation
• height intelligence

  and health is a combination of environmental and genetic variation.
• mutation -

  • a random change in an organisms DNA and occur continuously
  • a gene is altered and produces a genetic variant
• extinction -

  • environment changes too quickly
  • new predator kills them all
  • new disease kills them all
  • can't compete with other species for food
  • catastrophic events happens that kills them all
• Lamarck -

  • changes that an organism acquires during its lifetime will be passed on to its offspring
  • if a rabbit used its legs to rung a lot then its legs would get longer and the offspring would be born with longer legs
• selective breeding -

  humans artificially select the plants or animals that are going to breed so that the genes for particular characteristics remain in the population
• process of selective breeding -

  1. from existing stock, select ones which had the desired characteristics
  2. breed them with each other
  3. selects the best of the offspring and breed them together
  4. continue this process over several generations and the desirable trait gets stronger and stronger eventually all offspring will have then characteristic
• reducing the gene pool -

  • inbreeding reduces the number of different alleles in a population
  • can cause health problems as there more likely to inherit genetic defects
  • less chance of any resistant alleles being present in the population
• what are fossils?

  the remains of organisms from many years ago which are found in rocks. they provide the evidence that organisms lived years ago. fossils can tell us a lot about how much or how little organisms have changed over time. by comparing fossils with species that live today we can see that many of today's species have developed into much simpler organisms over millions of years
• • scientists believe may early organisms were soft-bodied, and soft tissue tends to decay away without forming fossils
  • fossils that did form millions of years ago may have been destroyed by geological activity so the fossil record is incomplete
• • a species is a group of similar organisms that can reproduce to give fertile offspring
  • speciation is the development of a new species, it occurs when populations of the same species become so different that they can no longer successfully interbreed to produce fertile offspring.
  • isolation is where populations of a species are separated. this can happen due to a physical barrier and can lead to speciation