SB2 Cells and Control

• The human body contains two copies of 23 chromosomes, 46 in total. 
• Cell cycle - process of growth and repair by new cells:

1. Interphase - cells make extra sub-cellular cell parts. DNA is replicated and copies of the chromosome stay attached (X shape).                                                                                                2. Cell division/mitosis - The cell splits to form two identical daughter cells. 

• Mitosis occurs in a series of stages: prophase (nucleus breaks down, spindle fibers appear), metaphase (chromosomes line up across middle of cell), anaphase (chromosome copies seperate to either end of cell on spindle fibres), telophase (membrane forms around each set of chromosomes to form nuclei) and cytokinesis (cell surface membrane forms and separates the two cells). 
• Asexual reproduction - reproduction of clones using one parent. Relies on mitosis. Much faster than sexual because organisms don't need others for reproduction, however, doesn't produce variation. 
• Cancer cells - changes in cells can turn them into cancer cells, they undergo uncontrollable cell division, producing lumps called tumors that can damage the body. 

• Growth - increase in size as a result of an increase in number or size of cells. Number of cell increases due to mitosis, and growth recorded over a period of time.
  
• Growh of babies - measured in mass (kg), length (cm) and head circumference (cm).
• Percentile growth charts - a chart used to compare the growth of a baby to other babies.
• For example, a baby born on the 10th percentile for weight, 90% will be heavier, 10% lighter.
• Cell differentation - process that changes less specialised cells into more specialised ones.

• Meristems -  an area where cells divide rapidly by mitosis, to increase in length (elongation), and differentiate into specialised cells that have different functions.
• Specialised cells, root hair cells and xylem cells, allow the plant to carry out many different processes effectively. 
• Measuring growth in plants - ((final value - starting value) ÷ starting value) x 100

• Stem cells - cells that divide rapidly by mitosis over a long period of time to produce cells that then differentiate. 
• Plant stem cells can produce any kind of specialsied cells but not animals.
• Embryonic stem cells - cells on an early-stage embryo that can produce any type of specialised cells.
• By the time the animal is fully developed, the stem cells can only produce the type of cells in tissue around them (adult stem cells).
• Stem cells is a way of treating diseases, such as replacing damaged cells. This is done by injectecting them into places needed. 
• Problems - if the cells continue to divide it could turn into cancer. Stem cells from one person could be killed by the immune system of the other person that are put into, called rejection. 

• Cerebral cortex - makes up 80% of brain. Used for most of our senses, language, memory, behavior and consciousness. Divided into 2 cerebral hemispheres, right communicates with left side of body and vice versa. 
• Cerebellum - at base of brain, divided into two halves, controls balance and posture. coordinates timming and fine control, making movements smooth. 
• Medulla oblongata - controls heart rate and breathing rate. responsible for reflexes, zomiting, sneezing. 
• The mass of neutrons that make up the medulla oblongata connect the brain to spinal cord. It carries info from brain to rest of body. 

• Scanning - allows people to looking into brain without surgery and without risk of damaging brain.
• CT scan - shows shapes and structures of brain. X-ray beam moves around head and detectors measure absorbtion. Computer uses info to produce a view in 'slices'. Differences in shapes of the brain can be linked to functions. 
• PET scan - shows brain activity. Patient is injected with radioactive glucose. Active cells take in more glucose than less active ones. Radioactive atoms cause gamma rays which scanner detects. More gamma rays come from parts containg active cells.
• Spinal cord damage - damage to spinal cord can reduce the flow of information between brain and parts of body. Nerve damage can cause loss of feeling in legs, damage in neck can cause loss of both legs and arms (quadriplegia).
• No adult stem cells can differentiate into neurons, so none can be made to repair damaged ones. Wires can be used to stimilate nerves and muscles, but patients don't regain movement of feeling. 
• Brain tumours - this may squash parts of brain and stop them from working. Tumours can be cut out using radiotherapy (high-energy x-ray beams) and chemotherapy (injecting drugs that kill dividing cells). The methods could damge body and chemotherapy may not work due to the blood barrier - filter that only allows certain substances into brain.

• Central nervous system (CNS) - control body, made up of brain and spinal cord. Nerves make up rest of nervous system, allowing parts to comunicate using impules (electrical signals). 
• Sense organs contain receptor cells that detect stimuli (anything your body is sensitive to ). They create impulses, travel to the brain which send impulses to other parts of the brain causing a response. 
• Neurotransmition - transmission of impulses and happens in neurones (nerve cells). 
• Sensory neurone - carry impulses from receptor cells to the CNS. A receptor cell impulse passes into a tiny branch (dendrite), transmitted along a dendron and the axon. Series of axon terminals allow impulses to be transmitted to other neurons. 
• Dendron and axons are long to allow fast neurotransmittion over long distances. There's a fatty layer surrounding them - myelin sheath. This insulates a neurone from neighbouring neurons, stopping signal from losing energy. Makes impulse jump along cell between gaps in myelin, speeds up neurotransmition.

• Cones - receptor cells in eyes that are sensitive to colour of light. Generate impulses which leads to brain through optic nerve. Cones works bad in the dark.
• Rods - receptor cells that detect light intensity, and works well in dark.
• Pupil - dark area in middle of eye where light enters. 
• There are muscles in the iris that controls the amount of light entering eye, which can constrict pupil (decrease diameter) or dilate it (make it bigger).
• Most light is focused by cornea, which bends (refracts) light rays, bringing them together. Lens then fine-tunes the focusing. 
• Ciliary muscles makes lens fatter to focus light from near objects and thinner for far objects. 
• For short-sighted people - eyeball is too long, or cornea is too curved and bends the rays too much. Opposite occurs with long-sighted people. Can be corrected using lens (short - diverging , long - converging).
• Cataract - protein builds up inside lens, making it cloudy. Can be fixed by replacing the clouded lens.
• Colour-blindness - cones don't work properly, cannot be corrected.

• When the brain coordinates a response to a stimulus, impulses are sent to effectors, muscles and glands, and these carry out the action. 
• Motor neurones - carry impulses to effectors.
• Relay neurones - found in spinal cord, linking motor and sensory neurones.
• Synapses - gap when one neurone meets another. When a impulse meets an axon terminal, a neurotransmitter substance is released into gap. This is deteted by next neurone and generates new impulse. This slows down neurotransmission, however, it creates new impulses so impulses don't need to split or lose strength and ensures that impulses flow in one direction.
• Reflex arc - pain receptors creates impulse to be transmitted in a sensory neurone, goes through spinal cord to relay neurone, to motor neurone, Impulses from motor neurone pass into the effectors (muscle cells) via synapses. The muscles then respond.