* Cells & Control

• Chromosomes are coiled up lengths of DNA molecules which contain genetic information.
• When a cell divides by mitosis it makes two cells identical to the original cell - the nucleus of each new cell contains the same number of chromosomes as the original cell.
• The main stages of the cell cycle are:

  Interphase

  Prophase

  Metaphase

  Anaphase

  Telophase

  Cytokinesis
• The phrase to help you remember the cell cycle is:

  I Pooed Myself At The Circus.
• Interphase -> The cell grows and increases the amount of subcellular structures. it then duplicates its DNA - so there is one copy for each new cell.
• Prophase -> the chromosomes condense, getting shorter and fatter. The membrane around the nucleus breaks down and the chromosomes lie free in the cytoplasm.
• Metaphase -> the chromosomes line up at the centre of the cell.
• Anaphase -> spindle fibres pull the chromosomes apart. Then the chromatids are pulled to opposite ends of the cells.
• Telophase -> membranes form around each of the sets of chromosomes. These become the nuclei of the two cells - the nucleus has divided.
• Cytokinesis -> the cytoplasm and the cell membrane divide to form two separate cells.
• CELL DIFFERENTIATION -> the process by which a cell changes to become specialised for its job. Having specialised cells allows multicellular organisms to work more efficiently.
• CELL DIVISION -> by mitosis - the cell cycle.
• CELL ELONGATION -> where a plant cell expands, making the cell bigger and so making the plant grow.
• The rate at which cells divide by mitosis is controlled by the genes in an organism’s DNA. If there is a change in one of the genes that controls cell division, the cell may start dividing uncontrollably. This can result in a mass of abnormal cells called a tumour. If the tumour invades and destroys surrounding tissue it is called cancer.
• Percentile charts are used to monitor growth.

  
• Undifferentiated cells are called stem cells. Stem cells are found in early human embryos. These embryonic stem cells have the potential to divide and produce any kind of cell at all. Stem cells are very important for growth and development
• Adults have stem cells, but they are only found in certain places, like bone marrow. These aren’t as versatile as embryonic stem cells so they can’t produce any cell type, only certain ones.
• In plants, the only cells that divide by mitosis are found in plant tissues, called meristems. Meristem tissue is found in the areas of a plant that are growing, e.g. the roots and shoots. They produce unspecialised cells that can divide into any type of cell. These unspecialised cells go on to form specialised tissues like xylem and phloem.
• Stem cells can be used in medicine, for tumour development, disease transmission, and rejection.
• The brain and spinal cord make up the Central Nervous System (CNS).
• The brain is made up of three main structures:

  Cerebrum -> Largest part of the brain, divided in 2 halves (called the cerebral hemispheres), right hemisphere controls muscles on left side and vice versa. They are responsible for things like: movement, intelligence, memory, language + vision.

  Cerebellum -> responsible for muscle coordination and balance.

  Medulla Oblongata -> controls unconscious activities like breathing and your heart rate.

  
• Scanners are used to investigate brain functions:

  CT Scanning -> uses x-rays to produce an image of the brain, shows the main structures but doesn’t show the functions.

  PET Scanning -> use radioactive chemicals to show which parts of the brain are active, very detailed, shows the structure and the functions, shows if areas are unusually inactive or active.
• The nervous system is made up of neurones which go to all parts of the body. The body has lots of sensory receptors - groups of cells that can detect a change in your environment a stimulus. When a stimulus is detected by receptors, the information is converted to a nervous impulse and sent along sensory neurones to the CNS.
• The CNS coordinates a response. Impulses travel through the CNS along relay neurones. The CNS sends information to an effector along a motor neurone. The effector then responds accordingly. The time it takes you to respond to a stimulus is called your reaction time.
• Sensory Neurone:

  • one long dendron carries nerve impulses from receptor cells to the cell body, which is located in the middle of the neurone.
  • one short axon carries nerve impulses from the cell body to the CNS.

  
• Motor Neurone:

  • many short dendrites carry nerve impulses from the CNS to the cell body.
  • One long axon carries nerve impulses from the cell body to effector cells.

  
• Relay Neurone:

  • many short dendrites carry nerve impulses from sensory neurones to the cell body.
  • an axon carries nerve impulses from the cell body to motor neurones.

  
• The connection between two neurones is called a synapse. The nerve signal is transferred by chemicals called neurotransmitters, which diffuse across the gap. The neurotransmitters then set off a new electrical signal in the next neurone. The transmission of a nervous impulse is very fast, but it is slowed down a bit at the synapse because the diffusion of neurotransmitters across the gap takes time.

  
• Reflexes are automatic, rapid responses to stimuli - they can reduce the chances of injury.

  The passage of information in a reflex is called a reflex arc.

  • when a stimulus is detected by receptors, impulses are sent along a sensory neurone to a relay neurone.
  • when the impulse reaches a synapse, they trigger the release of neurotransmitters.
  • they then are released and travel along the motor neurone to the effector.
  • the muscle then contracts and moves.

  
• Very bright light can damage the eye - so you have a reflex to protect it. Light receptors in the eye detect very bright light and send a message along a sensory neurone to the brain. The message then travels along the relay neurone to a motor neurone, which tells circular muscles in the iris to contract, making the pupil smaller.

  
• Cornea -> refracts light into the eye.
• Iris -> controls how much light enters the pupil.
• Lens -> refracts light, focusing it onto the retina.
• Retina -> the light sensitive part and it’s covered in receptor cells called rods and cones, which detect light.
• Rods -> more sensitive in dim light but can’t sense colour.
• Cones -> sensitive to different colours but are not so good in dim light.
• The information from light is converted into electrical impulses.

  The optic nerve carries these impulses fro the receptors to the brain.

  
• To look at distant objects:

  • the ciliary muscle relaxes, which allows the suspensory ligaments to pull tight.
  • this pulls the lens into a less rounded shape so light is refracted less.

  
• To look at close objects:

  • the ciliary muscle contracts, which slackens the suspensory ligaments.
  • the lens becomes a more rounded shape, so light is refracted more.

  
• Long-sighted people are unable to focus on near objects:

  • this occurs when the lens is the wrong shape and doesn’t bend the light enough or the eyeball is too short.
  • light from near objects is brought into focus behind the retina.
  • you can use glasses or contact lenses with a convex lens to correct it.