cells and control

• mitosis


  cell division to grow or replace damaged cells in cell cycle

  makes 2 genetically identical diploid daughter cells

  same number of chromosomes

  can be used for asexual reproduction (strawberries)
• chromosomes

  contains genetic material

  coiled up lengths of dna molecules

  come in pairs (diploid)

  46 chromosomes (23 pairs) in human body
• interphase


  subcellular structures grow (mitochondria, ribosomes)

  dna duplicates so theres 2 sets

  happens before mitosis takes place
• 4 stages of mitosis

  1. prophase - when chromosomes coil up and condense, nuclear membrane breaks down (chromosomes lie free in cytoplasm)
  2. metaphase - when chromosomes line up at centre of cell
  3. anaphase - when spindle fibres pull chromosomes apart, each copy pulled to opposite ends of cell
  4. telophase - 2 nuclear membranes form around chromosomes
• cytokinesis

  happens before telophase ends, end of mitosis

  cell membrane divides to form 2 separate cells
• cell growth

  increase in size or mass

  due to cell differentiation, cell division

  plants grow by cell elongation
• cell growth in animals

  all happens by cell division

  grow when young, stop growing once reached full growth

  when young, cells divide at fast rate

  when adult, cell division for repair (replace old/damaged cells)

  cell differentiation lost at an early stage as cells are replaced
• cell growth in plants

  mostly by cell elongation

  cell division in tips of roots and shoots (meristem)

  grow continuously, differentiate continuously to develop new parts
• cancer

  uncontrolled cell division

  could be due to change in gene which controls cell division (the rate at which cells divide is controlled by genes)

  leads to mass of abnormal cells (tumour)

  if a tumour invades and destroys surrounding tissue, it is cancer
• growth percentile charts


  shows overall pattern in development, easy to highlight problems (obesity, malnutrition, dwarfism)

  measures length, mass and head circumference

  can help see inconsistent patterns (small baby with big head)
• stem cells

  undifferentiated cells

  can divide by mitosis to become specialised

  found in early human embryos (potential to produce any cell needed)

  adult stem cells found in bone marrow but they arent as versatile as embryonic stem cells (can only produce certain cells, not any cell)

  in adults, stem cells used to replace damaged cells (skin, blood)
• meristem


  plant tissue where cells which divide by mitosis are only found

  found in areas of plant when growing (root, shoot)

  produce unspecialised cells which divide to form any cell needed

  can divide and differentiate any cell for as long as plant lives (embryonic stem cells only as embryo)

  forms specialised cells like xylem and phloem
• stem cells and medicine

  • sickle cell anaemia can be cured with bone marrow transplant sometimes (adult stem cells produce new red blood cells). allograft (donor stem cells) or autograft (own stem cells transported)
  • extracting embryonic stem cells can be stimulated to differentiate into specialised cells
  • replace damaged cells due to disease or injury (cardiac stem cells transported into someone with heart disease)
• risks of using stem cells in medicine

  • tumour development as stem cells divide very easily
  • disease transmission as donor stem cells may be infected with a virus (live inside cells) so recipient gets sicker
  • rejection as the stem cells may be recognised as foreign and so body attacks new cells. immunosuppressant drugs can be taken but this makes them susceptible to more disease
• ethical issues of using stem cells in medicine


  argument of using human embryonic cells as each is a potential human life

  argument as curing suffering patients is more important than potential life
• central nervous system


  made up by brain and spinal cord
• spinal cord


  long column of neurones from base of brain down to spine

  neurones may branch off and connect with other parts of body

  relays information between brain and rest of body
• brain


  made up of billions of interconnected neurones

  • cerebrum - largest part of brain, divided into 2 cerebral hemispheres (2 halves). right hemisphere controls muscles on left side of body, left hemisphere controls muscles on right side of body. responsible for movement, intelligence, memory, language, vision
  • cerebellum - responsible for muscle coordination, balance
  • medulla obliganta - controls unconscious activities (breathing, blinking, heart rate)

  
• CT scanner

  uses x-rays

  shows main structures in the brain, not functions

  slice images to get 3D model of brain

  e.g if area of brain is damaged and patient cant see, it controls vision

  
• PET scanner


  uses radioactive chemicals

  shows which part of brain is active when inside scanner

  very detailed - shows structure and function

  2D images

  shows if areas in brain are active/inactive (help study disorders)

  e.g alzheimers has reduced activity in some parts of brain, PET scans show reduction compared to normal brain

  
• difficulties when treating problems in cns

  • hard to repair damage and repair nervous tissue
  • not easy to access - hard to get into some parts of brain, may be at risk of damaging other areas of brain
  • treatment can lead to permanent damage (injuring spinal cord needs surgery, but may damage more during surgery)

  
• the nervous system


  made up of neurones

  stimulus (change in environment) detected by receptors (cells that detect stimuli) which send nerve impulses to the coordinator (brain/spinal cord, chooses response) which ultimately sends an effector (muscle/gland which causes response) to cause a response

  *stimulus-receptor-coordinator-effector-response*

  **stimulus-receptor-sensory neurone-cns-motor neurone-effector-response**
• neurones

  can be long which speed up impulse

  have cell body with nucleus

  cell body has extensions to connect to other neurones (dendrite, dendron carry nerve impulses towards cell body, axon carries nerve impulses away from cell body)

  some axons surrounded by myelin sheath (electrical insulator) whcich speeds up impulse
• sensory neurone


  one long dendron carries nerve impulses from receptor cells to cell body (middle of neurone)

  short axon carries nerve impulses from cell body to cns
• motor neurone


  lots of short dendrites carry nerve impulses from cns to cell body

  long axon carries nerve impulses from cell body to cns
• relay neurone


  lots of short dendrites carry nerve impulses from sensory neurone to cell body

  axon carries nerve impulse from cell body to motor neurone
• synapse


  connection between 2 neurones

  electrical impulses cant travel through synapses (chemical -neurotransmitter- released which diffuse across gap)

  1. nerve impulse arrives at synapse
  2. chemical messenger released into synapse
  3. neurotransmitter diffuses across synapse
  4. neurotransmitter binds with receptor on next neurone
  5. neurotransmitter binds with receptor to stimulate impuse in next neurone

  transmitting nerve impulse is very fast but slowed down accross synapse as diffusing neurotransmitters takes time
• reflexes and reflex arcs

  automatic, rapid responses to stimuli

  passing information in a reflex (receptor to effector) = reflex arc

  neurones in reflex arc go through spinal cord or unconscious part of brain
• how reflexes prevent injury

  1. stimulus detected by sensory receptors, impulses sent along sensory neurone to relay neurone in cns
  2. impulse reaches a synapse between sensory and relay neurone so neurotransmitters are released, which causes impulses to be sent along relay neurone
  3. synapse between relay and motor neurone, neurotransmitters released, causes impulse to be sent along motor neurone
  4. impulse travels across motor neurone to effector
  5. muscle/gland contracts and causes a response (move)

  dont spend time thinking about response, quicker than normal
• reflexes in the eye


  very bright light damages the eye so reflexes help protect it

  • light receptors detect very bright light and send message along sensory neurone to brain
  • message travels along relay and motor neurone, which tells circular muscles in iris to contract, making the pupil smaller so less light enters the eye
• Cornea refracts (bends) light into the eye
• Iris controls how much light enters the pupil
• Lens refracts light and focuses it onto the retina
• Pupil lets light into the eye when open, contracts when there is a lot of light
• Ciliary muscles change the shape of the lens to adjust focus
• Retina is the light-sensitive part of the eye, covered in light-sensitive cells (rods and cones) which detect color
• Rod cells are sensitive in dim light and cannot sense color
• Cone cells are sensitive to color (red, blue, green) and are found in the fovea
• Fovea is the most sensitive part of the retina, containing cones for color vision
• Blind spot is the point on the retina where the optic nerve enters, and there are no rods/cones