Topic 2: Cells and Control

• Mitosis is useful in asexual reproduction, growth and repair as it's simple to do over the entire body. For example, a fertilized egg cell (ovum) divides by mitosis to form an embryo
• Diploid - 2 sets of chromosomes (46)

  Haploid - one copy of each chromosome (23)

  For example, gametes (egg/sperm) are haploid and liver cells are diploid
• Cancer forms using mitosis at rapid and uncontrollable rates which creates a tumour (cancerous cells)
• Organisms may rely on asexual reproduction because it's more time and energy efficient. However, this means no variation as the chromosomes are all the same
• The Cell Cycle has two main processes for growth and repair:

  • Interphase - DNA replication occurs as cell makes extra parts
  • Mitosis - parent cell divides to form 2 daughter cells
• The 5 Stages of Mitosis:

  1. Prophase - nucleus starts to break down and spindle fibres appear
  2. Metaphase - chromosomes line up on the spindle fibres across the middle of the cell
  3. Anaphase - chromosome copies separate and move to the opposite end of the cell on the spindle
  4. Telophase - membrane forms around each set of chromosomes to form nuclei
  5. Cytokinesis - cells' surfaces form to separate the 2 daughter cells
• Growth happens when there is an increase in the number of cells due to mitosis
• Differentiation - cells that change and become specialised for their function
• Cell differentiation is important as this allows tissues/organs to form and create a functioning body rather than a mass of cells. Once a cell becomes differentiated, it only expresses the genes that produce the proteins characteristic of itself
• Examples of specialised cells and structures (1):

  • Platelets - clot to help seal the wound by forming a scab, prevents bleeding and infection
  • Red Blood Cells - thin without a nucleus, large surface area, haemoglobin molecules carry oxygen
• Examples of specialised cells and structures (2):

  • Fat Cells - areas to allow fat to be stored until needed
  • Nerve Cells - long fibres that carry electrical impulses around the body
  • Muscle Cells - contains contractile proteins that can shorten the cell
• Percentile growth curves (graphs) shows the growth expected at different birth weights to see if the baby is growing normally
• Plant eukaryotic cells elongate then differentiate into cells needed for the plant to function. For example, it can change size, shape, metabolic activates and signal responsiveness of cells
• Meristems are a group of cells near the end of each shoot and root that allows plants to continue growing (divide rapidly by mitosis)
• Specialised Plant Cells:

  • Root Hair Cells - long so there's a large surface area for rapid absorption of water/mineral ions and lots of mitochondria for active transport (energy from glucose during respiration)
  • Xylem Cells - dead cells that lose their end walls so the xylem form a continuous hollow tube to transport water/mineral ions one way (thick walls stiffened with lignin)
• Root hair cells and xylem cells transport water and mineral ions from the roots to the stem then leaves for photosynthesis (elongated for maximum effect)
• Growth occurs due to cell elongation (zone of elongation) due to the intake of water (osmosis) in vacuoles. This means all plant cells are able to become longer as they grow
• Adult stem cells can be found in the nose, brain, eyes, blood, liver, bone marrow, skin and muscle. They can repair damaged tissue, grow and replace worn out organs
• Embryotic stem cells can differentiate into any type of cell and is found in the inner cell mass of the human blastocyst (early stage embryo).

  However, adult stem cells are only found in specific areas and are limited in cell types when differentiating
• Stem cells can be used to cure life threatening diseases and create artificial organs (Type 1 diabetes and spine/brain injuries)
• Advantages of Stem Cells:

  • treats diseases
  • found all over the body (accessible)
  • easy to extract
  • can (mostly) turn into any cell type
  • safer than organ donations


  Disadvantage of Stem Cells:

  • cells may continue to divide uncontrollably (tumours)
  • body may reject it (harmful)
• The Central Nervous System (CNS) is made from the brain and spinal chord. This allows the body to communicate via electrical signals called impulses
• Changes are sensed by sensory organs (eyes, ears, nose, tongue, skin) which have receptors that detect stimuli. The brain coordinates a response to the stimulus.

  Neurotransmission is when an impulse is sent out via the nerve cells. Effector cells (muscles/glands) carry out the action
• Motor Neuron - axon carries messages across the body over long distances

  Structure:

  • dendrites receive impulses from other neurons
  • myelin sheath on the axon increases the speed of the nerve impulses (fatty insulation on axon) then carry impulses to effector cells
  • axon terminals pass impulses to effector cells
  • dendrites ---> cell body ---> axon ---> axon terminal
• Relay Neuron - found in the spinal chord to link motor and sensory neurons meaning they have one axon as well as several dendrites

  Structure:

  • multiple dendrites to receive impulses
  • long axon connects cell body to axon terminal
• Synapses generate more impulses and allow them to flow in one direction. This is because they transmit impulses across the body through 2 neurons, a neuron and muscle cell, nerves to the brain and vice versa
• Reflex Arc - a neutral pathway that controls a reflex

  They're fast as the action can occur by activating spinal motor neurons without the delay of routing signals to the brain.

  Structure:

  1. Stimuli causes sensory neurons to send impulses
  2. Spinal chord receives impulses and immediately passes it to the relay neuron
  3. Relay neuron sends impulses to the motor neuron and muscles move
• Sensory Neuron - detect and transmit impulses through the CNS

  Structure:

  • dendrites receive impulses
  • travel through dendron to cell body
  • travel from cell body through the axon to the axon terminal
• The brain is mainly made of neurons and can be classified into 4 main parts:

  • Cerebral Cortex (cerebrum)
  • Cerebellum
  • Medulla Oblongata
  • Spinal Cord
• Cerebral Cortex (cerebrum):

  • makes up 80% of the brain
  • controls personality, senses, language, memory and consciousness
  • divided into 2 hemispheres (right hemisphere controls left side of the body and left hemisphere controls the right)
• Cerebellum:

  • coordinates timing
  • enables fine control over muscle activity which makes movements smooth
  • found at the base of the brain
  • has 2 hemispheres
• Medulla Oblongata:

  • controls unconscious movements like breathing
  • controls reflexes like sneezing
  • connects to the spinal chord
• Spinal Chord:

  • width of a finger
  • transfers important information as the bridge between the brain and body
  • made of nerves
• CT Scans:

  • uses x-rays (rotated in machine)
  • safer and more accurate than open brain surgery (advantage)
  • detects physical brain issues easily (advantage)
  • detectors in the machine measure the absorption of the x-rays to create the pictures
  • computer produces images in slices to show brain structure
• PET Scans:

  • uses radioactive glucose as a tracer
  • shows brain structure and activity in real time (advantage)
  • collision of negative and positive ion creates a found point that is detected by the machine to build live images
  • causes parts of the brain to become more active
  • active cells absorb more glucose for respiration
• Damage to the spine is difficult to treat as nervous tissue cannot be repaired and may cause permanent damage.

  Similar to the brain as it's not easily accessed and tumours cannot be surgically removed in certain areas.
• The brain area damaged causes different effects on a person. For example, a person with a damaged cerebellum would have poor muscle control, balance and reaction time
• The Eye Structure:

  • Pupil (hole through lens) - dark area in the middle of the eye where light enters
  • Iris (coloured ring) - muscles in the iris control light intensity by dilating or constricting
  • Cornea (clear covering) - outer surface focuses most of the light into the retina
  • Lens - focuses light on retina
  • Ciliary Muscles - changes lens shape for finer focusing
  • Optic Nerve - neurons carry nerve impulses from the cones and rods to the brain
• The retina is at the back of the eye near the optic nerve so information from receptor cells can quickly be processed by the brain:

  • Cone Cells - sensitive to the colour of light (green/blue) so the information is combined in the brain to see colours (bright light)
  • Rod Cells - detect differences in light intensity
• Common eye defects:

  • Colour blindness
  • Cataracts
  • Long-sightedness
  • Short-sightedness