S3 Forensics and the human body summary
Body organs and their functions
Cells and their specialised functions
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S3 Forensics and the human body summary
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Nervous system
The nervous system is made up of the brain, spinal cord and associated nerves. The brain
and the spinal cord together are known as the central nervous system (CNS).
Nerve cells pass electrochemical messages from one cell to another.
The gap between nerve cells is called a synapse and a chemical called a neurotransmitter
crosses this and passes the message to the next nerve cell.
Reflex actions
Some actions take place automatically to protect our body. These are involuntary actions
and include blinking and the knee jerk reaction. The information does not travel to the brain.
Brain
The brain is the control centre of the body and co-ordinates all of our organs and systems.
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DNA
Chromosomes are found in the nucleus and of a cell and are made from DNA. DNA is a
molecule that is responsible for passing on of genetic information. It is able to do this
because of its structure and its ability to form base pairs. DNA carries the code to make
proteins and proteins are responsible for the formation of characteristics. For example if
you can roll your tongue it is because you have been able to make the protein to do so.
Guanine always pairs with cytosine and adenine always pairs with thymine. The number of
DNA bases in a chromosome encodes information for the structure of a protein.
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Sex chromosomes
Chromosomes are also responsible for determining the sex of an individual. When sex cells
(gametes) are formed they only contain one set of chromosomes in comparison to the two
sets of chromosomes that all other body cells have. Females have the sex chromosomes XX
so all eggs contain an X chromosome. Males have the sex chromosomes XY so half their
sperm contain an X chromosome whilst the other half contains a Y chromosome.
Monohybrid crosses
These are genetic crosses where the parents are different from one another in one
characteristic. Letters are assigned to represent dominant and recessive genes and it is
possible to predict the genotype (genes – represented by letters) and the phenotype
(physical appearance) of offspring. Some individuals are homozygous (both genes are the
same e.g. HH), these individuals are also said to be true-breeding whilst some individuals are
heterozygous (genes are different e.g. Hh).
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If two heterozygous plants were crossed in the F2 generation (second generation) we would
get a ratio of 3:1 in terms of 3 yellow seeds for every one green seed.
We can work out the offspring of any monohybrid cross using the following model. Lets look
at a cross between two homozygous or true-breeding plants. One purple (PP) and one white
(pp). Already we know that purple flower colour is dominant to white flower colour.
Skeleton
The skeleton provides support, allows movement and is a framework for muscle attachment.
Cell division
Cell division increases the number of cells in organism which allows for growth and repair and
allows single-celled organisms to reproduce. The process is called mitosis and happens in
stages to ensure that no genetic information is lost as this would have an impact on the
daughter cell produced.
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S3 Forensics and the human body summary
Stage A
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The chromosomes inside the nucleus are copied (thus, copying the genetic
material, the DNA)
Stage B
The envelope around the nucleus disappears and the chromosomes become
visible.
Stage C
Stage D
Stage E
The chromosomes arrange themselves across the centre of the cell.
The two halves of the chromosome are then pulled to opposite ends of the cell
The nuclear envelope reforms around the chromosomes and the cytoplasm
divides to form two new daughter cells
Stem cells
Some unspecialised cells in the body are capable of repeated cell division and can produce
specialised cells. We have blood stem cells which can produce all of the different kinds of
blood cells, we have skin stem cells which can produce all of the different kinds of skin cells
as well as some others. At the moment the only type of stem cells that can produce all of
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the different kinds of cells in the human body are embryonic stem cells (ESC’s). These are
taken from leftover embryos used in IVF.
Stem cells can be used to treat diseases and replace damaged cells they are used in the
treatment of leukaemia, can be used to cure diabetes and are being looked at in the
treatment of Alzheimer’s.
There are ethical implications in the use of stem cells with the main issue being that
embryos have the potential to form a life.
Blood sugar and diabetes
Our body requires a constant internal environment to allow our cells to function properly.
Blood sugar levels rise when we eat and drink glucose and glucose is constantly required by
our cells for respiration. Our body has a system to ensure that our blood sugar levels remain
constant.
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Diabetics are unable to manage their blood sugar levels independently. Those with type 1
diabetes are unable to produce insulin and take daily insulin injections as a result. Type 2
diabetes occurs as a result of poor diet and exercise choices and can be reversed by
reducing dietary sugar intake and exercising. Both high and low blood sugar levels can lead
to diabetic comas.
Enzymes
Catalysts are substances that speed up chemical reactions whilst remaining unchanged. Our
cells produce biological catalysts called enzymes. We require enzymes as without them the
chemical reactions in our body would take place too slowly to keep us alive. Enzymes are
made of protein and speed up both degradation (breakdown) and synthesis reactions.
Enzymes are specific, this means that only one enzyme works with one substrate. This is
because of their unique shape. Enzymes and substrates fit together like locks and keys.
Enzymes have conditions at which they work best and these are their optimum conditions.
Enzyme activity will increase with an increase in temperature up until an optimum of around
37oC. Above this temperature the enzyme starts to become denatured and changes shape
meaning it no longer fits together with its substrate.
pH also affects enzyme activity, most enzymes operate best at around pH 7 with the
exception of pepsin which works best at acidic pH values of around 2-4.
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