Growth and Repair
The Cell Cycle has two parts: interphase and mitosis. During interphase, the cell haves extra sub-cellular parts and DNA replication occurs. Mitosis is the production of two diploid daughter cells, each with identical sets of chromosomes in the nucleus to the parent cell and are therefore clones.
Mitosis is needed for growth and repair. It is also used in asexual reproduction of plants and also some animals.
Normal cells only divide when required, but cancer cells undergo uncontrollable cell division, leading to lumps of cells called tumours growing, damaging the body and a chance of leading to death.
Growth is an increase in size or mass. Plants usually grow due to cell elongation, while cell division only happens in the tips of the roots or shoots. The meristem is a small area of undifferentiated cells where cells are rapidly dividing and differentiating into specialised tissues like root, phloem and xylem. In animals, growth occurs through division and differentiation. Differentiation is when a group of similar cells become different in form, allowing for functions to be performed. Specialised cells include neurones and red blood cells.
Human growth can be showed on a percentile chart. At a certain percentile, the percentage below will have a value for the variable below that on the chart for that percentile e.g. at 7th percentile for height, 75% of people will have a height equal to or lower than the value shown on the y-axis.
Stem Cells are unspecialised cells that divide by mitosis into more stem cells or differentiate into specialised cells. Adult stem cells are found in specialised tissue like bone marrow and can produce more of the cells in that tissue. Embryonic stem cells can produce any type of specialised cell.
There are benefits of using stem cells in medicine. Diseases can be cured, such as Type 1 Diabetes and the potential exists to defeat cancer and Parkinson’s. However, if stem cells divide too rapidly they themselves can cause cancerous tumours. They can be rejected by the immune system if the recipient’s white blood cells identify them as foreign.
The Brain
Cerebrum: largest structure, split into two parts. The right hemisphere controls the left half of the body and vice versa. Responsible for movement, language, intelligence, memory, language and vision
Cerebellum: muscle coordination and balance
Medulla Oblongata: unconscious activities e.g. breathing and heart rate, reflexes like sneezing
The spinal cord is long column of neurones that run from the brain to the lower back, branching off to connect with effectors and sensory organs. It relays information between brain and body.
CT Scanners use X-rays to produce an image of the brain, showing health of structures and the presence of tumours but not functions. PET Scanners use radioactive chemicals to show active parts of the brain and can therefore indicate functions. They produce detailed images of structures and functions in real time and can highlight disorders in cognitive thinking.
Damage to the spinal cord reduces flow of information between the brain and parts of the body, resulting in a loss of feeling in these areas. No adult stem cells can differentiate into neurones in the cord to cannot repair damage. Wires can stimulate nerves and muscles below the damage, but patients do not regain full movement or feeling. If a brain tumour forms, it can be cut out or killed using chemotherapy or radiotherapy. However, this may cause damage to the brain and chemotherapy may be blocked by the blood-brain barrier
Co-ordination
The brain and spinal cord form the central nervous system, and together with nerves form the nervous system, allowing for body parts to communicate using impulses
Neurones meet as a synapse, a small gap. A neurotransmitter is released into the gap and generates a new impulse, ensuring that impulses only flow in one direction and allowing for many fresh impulses to be generated in many neurones connected to one.
Reflex actions are automatic, extremely quick responses that protect the body, with reaction pathways called reflex arcs which does not require the brain
The Eye
Retina: contains light-receptive cells
Cornea: transparent layer responsible for refraction of light rays entering the eye
Iris: coloured part which expands and contracts to control the amount of light entering the eye
Lens: with ciliary muscle and suspensory ligament, control light focusing on the retina
Cones are receptor cells that are sensitive to the colour of light, with different types recognising blue, green, blue and combinations. Cones generate impulses which are processed into full colour vision. Rods detect differences in light intensity; they work well in dim light.
The amount of light entering the eye is controlled by the iris, which can constrict the pupil or dilate it, as bright light can damage cells on the retina
Light rays need to be focused on the retina. This is done by the cornea, which refracts light rays to make them converge. The lens fine-tunes the focusing, as the ciliary muscles contract as the suspensory ligaments slacken, causing the lens to become rounded and allow for focusing on a distant object with parallel rays.
If the ciliary muscles relax, the suspensory ligaments tighten and the lens is flatted to focus on a near object as rays are divergent.
Issues with the eye can often be fixed. However, colour-blindness, caused by faulty cones, cannot be corrected. Short-sightedness, caused by an overly-curved cornea or a long eyeball, can be corrected with a diverging lens, while long-sightedness, caused by the inverse, can be corrected with a converging lens to bend rays before they reach the eye. Contact lenses or glasses can fix these problems, or a laser can cut and reshape the cornea. Cataracts can be fixed by replacing the clouded lens with a plastic one.