cells and control

• Mitosis


  Prophase - the nucleus starts to break down and spindle fibres appear.

  Metaphase - the chromosomes are lined up on the spindle fibres across the middle of the cell.

  Anaphase - chromosome copies are separated and moved to either end of the cell on the the spindle fibres.

  Telophase - membrane forms around each set of chromosome to form a nuclei.

  Cytokinesis - a cell surface membrane forms to separate the two cells. Cell walls form in plant cells.
• Stem cells 


  Stem cells are cells that have differentiated over time.

  In plants they can be found in meristems.

  The cells of an early stage embryo are called embryonic stem cells that can produce any type of specialised cell.

  By the time young animals have fully developed, stem cells can usually only make one type of specialised cell that is in the tissue around them, these are called are called adult stem cells.

  The adult stem cells in human tissues allow tissues to grow and to replace old or damaged cells.
• The Brain


  Cerebral cortex - makes up 80% of brain, used most for our senses, language memory, behaviour and consciousness. It is divided into two cerebral hemispheres each having slightly different functions. The right hemisphere communicates with the left side of the body and vice versa.

  Cerebellum - the base of the brain, split into two halves, controls balance, posture, the timing and fine control of muscle activity.

  Medulla oblongata - controls heart rate, breathing rate and reflexes. Connects the brain to the spinal cord.
• The nervous system


  The brain and the spinal cord make up the central nervous system and nerves make up the rest of the nervous system. This system allows all parts of your body to communicate using electrical signals called impulses.

  Anything your body is sensitive to is called stimulus. Sense organs, eyes, skin etc, contain receptor cells that detect stimuli.

  Receptor cells create impulses, which usually travel to the brain. The brain then processes this information and can send impulses to other parts of the body to cause something to happen, a response.
• The nervous system (neurones)


  The travelling, or transmission, of impulses is called neurotransmission and happens in neurones (nerve cells). Neurones have a cell body and long extensions to carry impulses.

  There are many types of neurones, eg. sensory neurones. Its function is to carry impulses from receptor cells to the CNS. A receptor cells impulse passes through a tiny branch called a dendrite. It is then transmitted along the dendron and the axon. A series of axon terminals allow impulses to be transmitted to other neurones.
• PET scans


  • the patient is given a small amount of a radioactive form of glucose
  • this travels to parts of the body where respiration is occurring rapidly
  • this shows changes in parts of the body, such as the brain, and may indicate damage or disease
  • the scanner detects the radioactivity and builds up images showing where the radioactive tracer is most concentrated
• CT scans


  • the patient is given a radioactive tracer which allows different parts of the brain to show up
  • a CT scan is carried out, taking many different x-rays of the skull and brain from many different angles.
  • a computer puts all these images together to give a 3D image.
• The reflex arc


  Reflex arcs involve on three neurones and impulses which pass to and from the spinal cord. This provides a fast response that does not involve the brain. Reflex arcs are:

  • immediate
  • involuntary
  • innate
  • invariable

  These reflexes help protect us from immediate harm, such as the eye blink reflex which protects the eye if something comes close to it.
• Synapses


  The point where two neurones meet is called a synapse. There is a small gap between neurones.

  The electrical nerve impulse cannot cross this gap, and the impulse is carried by an neurotransmitter

  1. electrical nerve impulse reaches end of axon
  2. electrical impulse causes neurotransmitter to be released from vehicles in the neurine into gaps between neurones
  3. neurotransmitter diffuses across the ap and fits into receptors, causing a new electrical impulse in next neurone
• The eye (receptor cells)


  Cones are receptor cells that are sensitive to the colour of light. Some cones detect red light while other detect green/blue. Cones generate impulses which reach the brain through the optic nerve.

  Rods are receptor cells that detect differences in light intensity. Rods work very well in dim light where as cones only work in bright light.

  The pupil is the dark centre of your eye where light enters. The amount of light entering is controlled by muscles in the iris, which can constrict and dilate the pupil. Bright light can damage the receptor cells in the retina.
• The eye (light rays)


  Light rays entering the eye need to b focused onto a point in the retina to produce a clear image. Most focusing is done by the cornea, which bends light rays to bring them together. The lens then fine tunes the focusing. Ciliary muscles make the lens fatter to focus light from near objects and thinner to focus light from distant objects.
• The eye (problems)


  short-sightedness - distant objects are blurred and is caused by the eyeballs being too long or the cornea being too curved. Corrected with a diverging lens.

  long-sightedness - close objects are blurred, caused by the the opposite problems. Corrected with a converging lens.
• Tissue culture


  Tissue culture is the growing of cells or tissues in a liquid containing nutrients or a solid medium. This is a useful way to grow many identical cells. These may form a callus (a clump of undifferentiated cells). Sometimes the cells are then treated to make them differentiate.
• Tissue culture (plants)


  Tissue culture is used to produce new plants of a very rare species which are at risk of extinction. It is also used to produce new individuals of plant species that may be difficult to grow from seed. The technique is also used to produce clones of GM plants.
• Tissue culture (medicine)


  Tissue cultures have many uses in medicine. Culturing a thin layer of cells on a solid medium makes it easier to study how cells communicate with each other. Cell cultures are also needed to study viruses, which cannot replicate outside of cells. Cultures of cancer cells have been developed to study how cancers develop and spread. Using cells cultures, scientists can investigate how infected cells respond to new medicines without risking harm to animals or humans.
• Spinal cord and brain problems

  Quadriplegia - damage to the neck can cause the loss of use of arms and legs.

  Nerve damage in lower spinal cord can cause loss of feeling or use in the legs.

  Brain tumours - can squash parts if the brain and stop them from working.
• Growth in plants

  Plant cells divide by mitosis, just behind the tips of shoots and roots. After this the cells grow by enlarging. Young cells have small vacuoles which take in water by osmosis and enlarge, causing the cells to elongate. These cells can differentiate into specialised cell types. Most plants can continue to grow and differentiate throughout life.

  Examples of specialised cells:

  • xylem
  • phloem
  • mesophyll cells
  • root hair cells
  • stoma cells
• Growth in animals

  In animals, a fertilised egg, or zygote, divides by mitosis to produce genetically identical daughter cells. These cells grow cand divide by mitosis and eventually differentiate into different types of cells to make up a whole organism. Differentiation creates specialised cells adapted to carry out a particular function.

  Examples are

  • nerve cells
  • smooth muscle cells
  • egg and sperm cells
  • bone cells