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Summary AS and A level Biology
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AS and A level Biology
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• Different parts of organisms perform different functions, and it is essential that informationcan pass between these parts, so their activities are coordinated.
• Sometimes, the purpose of this information transfer is to coordinate the regulation of
substances within the organism, such as the control of blood glucose concentrations in
mammals.
• Sometimes, the purpose may be to change the activity of some part of the organism in
response to an external stimulus.
• In animals there are two types of information transfer that are used to coordinate the body’s
activities:
→ Nerves that transmit information in the form of electrical impulses.
→ Chemical messengers called hormones that travel in the blood.
• Coordination in plants is mostly achieved with hormones that are also known as plant
growth regulators, but lants also use electrical impulses.
→ Low temperatures slow down metabolic reactions.
→ At high temperatures, proteins, including enzymes, are denatured and cannot function.
→ If the water potential decreases, water may move out of cells by osmosis, causing
metabolic reactions in cells to slow or stop.
→ If the water potential increases, water may enter the cell causing it to swell and maybe
burst.
→ Glucose is the fuel for respiration, so lack of it causes respiration to slow or stop, depriving
the cell of an energy source.
→ Too much glucose may cause water to move out of the cell by osmosis, disturbing the
metabolism of the cell.
,• A vital function of control systems in mammals is to maintain a stable internal environment
(homeostasis).
• Internal environment means the conditions inside the body in which cells function.
• For a cell, its immediate environment is the tissue fluid that surrounds it.
• Many features of its environment affect how the cell functions.
• Homeostatic mechanisms work by controlling the composition of blood, which therefore
controls the composition of tissue fluid.
• Most control mechanisms in living organisms use a negative feedback control loop to
maintain homeostatic equilibrium.
• This involves a receptor or sensor and an effector.
• The receptor detects changes in the parameter being regulated.
• This is known as the input.
• This sets off a series of events cumulating in some action, by the effector, which is called the
output.
• Continuous monitoring of the parameter by the receptor produces continuous adjustments
of the output, which keep the parameter fluctuating around a particular ideal value or set
point.
• In negative feedback systems, a rise in the parameter results in something happening that
makes the parameter decrease.
• If there is a fall in the parameter, then something happens to make it increase.
• There are a few instances of the opposite thing happening in living organism.
• If a person breathes air that has a very high carbon dioxide content, this produces a high
concentration of carbon dioxide in the blood.
• This is sensed by carbon dioxide receptors, which cause the breathing rate to increase.
• So, the person breathes faster, taking in even more carbon dioxide, which stimulates the
receptors even more, so they breathe faster and faster.
• This is an example of positive feedback.
• Positive feedback cannot play any role in keeping things constant.
• This method of control is involved in several biological processes including the transmission
of nerve impulses.
• Many of the metabolic reactions occurring within the body produce unwanted
substances.
• Some of these are toxic and the removal of these unwanted products of metabolism is
known as excretion.
• Many excretory products are formed in humans, but two are made in much greater
quantities than others.
• These are carbon dioxide and urea.
• Carbon dioxide is produced continuously by cells that are respiring aerobically.
• The waste carbon dioxide is transported from the respiring cells to the lungs, in the
bloodstream.
• Gas exchange occurs within the lungs, and carbon dioxide diffuses from the blood into
the alveoli; it is then excreted in the air we breathe out.
• Urea is produced in the liver.
, • It is produced from excess amino acids and is transported from the liver to the kidneys,
in solution in blood plasma.
• The kidneys remove urea from the blood and excrete it, dissolved in water, as urine.
• If more protein is eaten than is needed, the excess cannot be stored in the body.
• It would be wasteful, however, simply to get rid of all the excess, because the amino acids
provide useful energy.
• To make use of this energy, the liver removes the amino groups in a process known as
deamination.
• In the liver cells, the amino group (-NH2) of an amino acid is removed, together with an extra
hydrogen atom.
• These combine to produce ammonia (NH3).
• The keto acid may enter the Krebs cycle and be respired, or it may be converted to glucose,
or converted to glycogen or fat for storage.
• Ammonia is a very soluble and highly toxic compound.
• In many aquatic animals, ammonia diffuses from the blood and dissolves in the water
around the animal.
• However, in terrestrial animals, ammonia would rapidly build up in the blood and cause
immense damage.
• Damage is prevented by converting ammonia immediately to urea, which is less soluble
and less toxic.
• Several reactions are involved in combining ammonia and carbon dioxide to form urea.
• An adult human produce around 25-30g of urea per day.
• Urea is the main nitrogenous excretory product of humans.
• We also produce small quantities of other nitrogenous excretory products, mainly creatine
and uric acid.
• A substance called creatine is made in the liver from certain amino acids.
• Much of this is used n the muscles, in the form of creatine phosphate, where it acts as an
energy store.
• However, some is converted to creatinine and excreted.
• Uric acid is made from the breakdown of purines from nucleotides, not from amino acids.
• Urea diffuses from liver cells into the blood plasma.
• All of the urea made each day must be excreted, or its concentration in the blood would
build up and become dangerous.
• As the blood passes through the kidneys, the urea is filtered out and excreted.
• Each kidney receives blood from a renal artery and returns blood via a renal vein.
• A narrow tube, called the ureter, carries urine from the kidneys to the bladder.
• From the bladder a single tube, called the urethra, carries urine to the outside of the body.
• A longitudinal section through a kidney shows that it has three main areas.
• The whole kidney is covered by a fairly tough capsule, beneath which lies the cortex.
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