Topic 2 ~ Cells and Control

• Chromosomes

  The nucleus contains genetic information in the form of chromosomes, which contain coils of DNA. A gene is a short section of DNA that codes for a protein and controls a characteristic. There are 23 pairs of chromosomes in each body cell, resulting in 46 chromosomes in total. Sex cells have half the number of chromosomes, resulting in 23 chromosomes in total. Diploid number of chromosomes is found in body cells (46 in humans), while haploid number is half this amount (23 in humans) and is found in gametes.
• Mitosis and the Cell Cycle

  Interphase: Cell grows, organelles increase, DNA is replicated, and energy stores are increased. Mitosis: Chromosomes line up at the equator of the cell, cell fibres pull each chromosome to either side, Cytokinesis: Two identical daughter cells form when the cytoplasm and cell membranes divide. Mitosis is important in growth, development, replacing damaged cells, and asexual reproduction, producing genetically identical diploid daughter cells.
• In animals, growth occurs via cell division and differentiation. Almost all cells differentiate at an early stage and then lose this ability. Specialised cells can make more of the same cell by undergoing mitosis.
• Muscular system cells

  • Bring about movement
• Circulatory system cells

  • Specialised to transport substances
• In animals, almost all cells differentiate at an early stage and then lose this ability
• Cell division in animals

  1. Most specialised cells can make more of the same cell by undergoing mitosis

  2. Red blood cells cannot divide and are replaced by adult stem cells
• In mature animals, cell division mostly only happens to repair or replace damaged cells, as they undergo little growth
• Growth in plants

  Occurs by cell division, differentiation, and elongation controlled by auxins
• In plants, many types of cells retain the ability to differentiate throughout life
• Cancer occurs as a result of small changes in cells, leading to uncontrolled cell division. The group of cells resulting from this uncontrolled division is called a tumour
• Percentiles charts are used to measure the growth rate of an organism
• The growth of babies can be measured using mass, length, or head circumference
• Being in a high percentile (e.g. 90th percentile or higher) can indicate a health problem
• Stem cells are undifferentiated cells that can undergo division to produce many more similar cells, some of which will differentiate to have different functions
• Types of stem cells

  • Embryonic stem cells
  • Adult stem cells
  • Meristems in plants
• Therapeutic cloning involves producing an embryo with the same genes as the patient to obtain embryonic stem cells for growing into needed cells or tissues
• Therapeutic cloning


  1. An embryo is produced with the same genes as the patient

  2. The embryo produced could then be harvested to obtain the embryonic stem cells

  3. These stem cells could be grown into any cells the patient needed, such as new tissues or organs

  4. The advantage is that they would not be rejected as they would have the exact same genetic make-up as the individual
• The brain, along with the spinal cord, makes up our Central Nervous System, or CNS
• The CNS is responsible for controlling consciousness, movements, thoughts, and emotions, among other things
• The brain is made up of several important structures, each contributing their own function
• Cerebral hemispheres


  • The two large cerebral hemispheres take up most of the skull and sit on the left and right-hand sides. Together, these two parts are known as the cerebrum and perform functions such as consciousness, memory, intelligence, visual and sensory processing
• Cerebellum


  • A large ‘lump-like’ structure found at the bottom of the brain, responsible for controlling fine movements of muscles
• Medulla oblongata


  • A small ‘elongated’ structure found in the brainstem, at the base of the brain, responsible for maintaining basic autonomic bodily functions such as breathing, digestion, swallowing, and sneezing
• Doctors often need to look inside the brain to examine brain tissue for injury and disease
• CT (Computerised Tomography) and PET (Positron Emission Tomography) scans are used to look inside the brain more easily
• CT scans fire X-Ray radiation at the brain from several different angles to generate a 3D image of the brain, useful for examining bleeding within the skull and damage to brain structures. Not recommended for pregnant women and children due to higher radiation exposure
• PET scans involve injecting a radioactive ‘tracer’ into the blood before the scan. Areas where the tracer builds up will be highlighted more brightly on the resulting scan, useful for identifying cancerous tumours
• Scan

  The scan itself is sensitive to the tracer, so areas where the tracer builds up (which will also be areas with greater blood flow) will be highlighted more brightly on the resulting scan. This is useful for identifying cancerous tumours, as these use more blood than normal tissue
• Reasons why investigating brain function and treating brain damage and disease is difficult

  • It is complex and delicate
  • It is easily damaged
  • Drugs given to treat diseases cannot always reach the brain because of the membranes that surround it
  • It is not fully understood which part of the brain does what
• Brain tumours can push against other structures and blood vessels in the brain, restricting their function. Often, tumours can be buried deep in the brain or spinal cord, making them especially difficult to remove
• The nervous system allows us to react to our surroundings, and coordinate actions in response to stimuli
• Reflex arc

  Automatic responses which take place before you have time to think are called reflexes. They prevent the individual from getting hurt. The information travels down a pathway called a reflex arc, allowing vital responses to take place quickly. This pathway is different from the usual response to stimuli because the impulse does not pass through the conscious areas of your brain
• Synapses


  When the impulse reaches the end of the first neuron, a chemical called a neurotransmitter is released into the synapse. This neurotransmitter diffuses across the synapse. When the neurotransmitter reaches the second neuron, it triggers the impulse to begin again in the next neuron. Different neurotransmitters have different effects on the frequency and speed of the impulse in the second neuron
• Some nerves are myelinated, i.e they are surrounded by a myelin sheath. The advantage of having nerves surrounded by myelin is that it allows the nerve transmission (or action potential) to travel faster. Myelin is produced by cells called Schwann cells
• Accommodation


  Allows the eye to change its shape to focus on near or distant objects and to dim light
• Retina

  Layer of light-sensitive cells found at the back of the eye. When light hits this, the cells are stimulated, and impulses are sent to the brain to create an image. Contains rod cells and cone cells, with rod cells being more sensitive to light for low light vision and cone cells for color vision
• Cornea


  See-through layer at the front of the eye that allows light through and focuses it onto the retina
• Iris


  Muscles surrounding the pupil that contract or relax to alter the size of the pupil based on light conditions
• Ciliary muscles and suspensory ligaments

  Hold the lens in place, control its shape, and allow focusing on objects at different distances