Mrs Hills revision summary of Year 2 A level Biology
Chapter 5.1 Temperature control
The body needs communication systems to allow it to control the internal conditions around
an optimum value. This includes, temperature, pH, toxin levels and water/salt
concentrations. The external environment is changing constantly (this could be outside the
body or the tissue fluid outside the cells) and so a negative feedback mechanism is used to
balance out the effects. The communication system works over the whole body enabling
specific, rapid, long and short-term responses to be achieved. The two communication
systems are hormonal (slow response and long acting) and nervous (fast response, short
acting).
Negative feedback works in that when the condition is removed from the optimum (too high
or too low) a receptor detects this and stimulates a response. This will generally over
compensate (especially if it is a hormonal response) and therefore is taken the other side of
the optimum value. By constantly checking and rebalancing we create the optimum
conditions.
Positive feedback builds on the value that is already moved away from optimum, this is
important in the release of oxytocin to stimulate contractions when giving birth, but also
plays a role in the damage to the body from heat stroke or hypothermia.
Ectotherms (cold blooded animals) need behavioural responses as well to maintain their
body temperature. They do this by moving into/out of sunny areas depending on the time of
day and the conditions. The advantage is that they don’t waste energy keeping warm so all
their food energy goes into metabolic processes, but it does make them less active in cooler
temperatures making them more vulnerable to predation.
Endotherms (warm blooded animals) also have behavioural responses of moving into/out of
sunny areas but they can also exhibit physiological responses such as sweating and shivering.
The advantage of being an endotherm is that your body temperature is regulated and they
can inhabit colder areas of the planet, but they are at risk of over heating in hot climates and
they need more food to maintain their body temperatures.
The thermoregulatory centre in the hypothalamus, which monitors blood temperature,
controls temperature regulation. Peripheral temperature receptors in the skin will also send
signals to the brain to initiate behavioural responses before the core temperature is
affected.
,Chapter 5.2 Excretion
Liver:
All blood vessels/cells relating to the liver are Hepatic.
The liver has 3 major blood vessels, (hepatic artery coming direct from the heart, Hepatic
portal vein which brings blood from the intestines to the liver for processing after digestions
and Hepatic vein which takes blood from the liver back to the heart). The liver itself is
arranged in lobes which are divided into lobules to maximise contact between capillaries
and hepatocytes.
The hepatic artery and hepatic portal vein enter the liver in parallel and branch off into inter-
lobular vessels. Within the lobule the blood mixes from these two vessels in a sinusoid. A
sinusoid contains Kupffer cells which are specific macrophages to break down old red blood
cells into bilirubin (contained in bile). Liver cells also produce bile and release it into
canaliculi (which join to form the bile duct). At the end of the sinusoid the blood has been
regulated and modified (sugars, toxins removed etc) and it is emptied into the hepatic vein.
In order to do carry out all the cellular functions of the liver a hepatocyte has a simple
cuboidal shape with many microvilli to increase surface area. Within the cytoplasm there is a
dense collection of organelles to carry out protein synthesis, transformation and storage of
carbohydrates, synthesis of cholesterol and bile salts, detoxification and many other
processes.
The liver is involved in the detoxification of alcohol, which is a multistep enzyme controlled
reaction. The process involves converting ethanol to ethanal using ethanol dehydrogenase
that will remove H2 and use it to reduce NAD. Then the ethanal is reacted with ethanal
dehydrogenase (again removing H2 to reduce NAD) and forming ethanoic acid. The acid is
then formed into acetyl coenzyme A (See respiration chapter). NAD is also needed to
breakdown fatty acids so if too much is used up in breaking down alcohol the fatty acids
build up leading to cirrhosis of the liver as fats are stored in the hepatocytes.
The other main function of the liver is to break down excess proteins to form urea. This is
known as the ornithine cycle.
The first stage is to deaminate the amino acid (ie remove the amine group) to form
ammonia and a keto acid. The ammonia then moves into the ornithine cycle to form the less
toxic and more soluble urea.
Ammonia + CO2 + ornithine makes Citrulline and water. A second ammonia is added to the
Citrulline to form arginine and water. A single water molecule is then added back in to the
arginine to form urea [CO(NH2)2] and ornithine. The ornithine is then recycled again so the
only net products/reactants are 2 ammonia (NH3), 1 water, 1 Carbon dioxide and 1 urea
molecule per cycle.
Kidney:
The other organ that is vital in excretion is the kidney. This regulates the water/salt
concentrations of the body by responding to hormone levels released by the pituitary gland
and also filters all the toxins, urea out of the blood. The kidney is divided into 3 main
regions, the outside (cortex), the inner layer (Medulla) and the central region (pelvis). The
nephron which does the filtration and reabsorption of solutes runs across the cortex and
medulla and then eventually into the pelvis where it connects with collecting ducts that run
to the ureter and onto the bladder. All blood vessels associated with the kidney are called
Renal.
, The renal artery runs into the kidney and splits into many arterioles. The afferent arteriole
runs into a region of cells in the cortex called the glomerulus. Here the vessels are under
very high pressure and so ultrafiltration occurs forcing molecules with a molecular mass of
less than 69000 into the Bowman’s capsule. The Bowman’s capsule has 3 layers of cells, the
endothelium of the capillary which has small pores (fenestrations), the basement layer
which then prevents the larger proteins from being filtered and the epithelial cells on the
podocytes which allows the fluid into the nephron.
Once in the nephron the filtrate is reabsorbed into the capillaries, (Efferent arterioles) which
run in a counter current direction to the filtrate. The next region of the nephron is the
Proximal convoluted Tubule (PCT) which reabsorbs all the sugars, most of the minerals and
85% of the fluid that was filtered. This is selective reabsorption and works because the cells
have a large surface area (due to microvilli) on the surface connecting with the nephron
fluid. This membrane has cotransporter proteins to move glucose and amino acids in
association with sodium into the cell out of the nephron. The opposite membrane near the
capillary has sodium/potassium pumps to maintain the concentration gradient for sodium
ions (allowing the co-transporter proteins to work). The concentration gradient of glucose is
naturally maintained and glucose diffuses into the blood stream. The sodium/potassium
pump is an active process so there are many mitochondria in these cells of the PCT.
The filtrate then flows into the loop of Henle. In the descending limb Na and Cl ions diffuse
into the nephron due to the high concentration in the surrounding tissue fluid. Since the
concentration is so high outside water will also move out by osmosis and can be moved from
tissue fluid back into the capillaries. At the bottom of the loop (in the medulla region)
sodium and chlorine ions will diffuse out of the nephron into the surrounding tissues while
in the ascending limb they are actively transported out into the surrounding tissue. This
means the water potential in the medulla is always lower than the nephron so water is
removed by osmosis. The remaining urea and water left in the nephron moves into the distal
convoluted tubule (DCT) and then into the collecting duct.
The collecting duct has cells that are lined with special water channels called aquaporins
which are stimulated by the hormone ADH (anti diuretic hormone) which does the final
osmoregulation of the fluid to form the urine. If there is a high salt concentration in the
blood ADH is released so more water is reabsorbed in the collecting duct making small
volumes of concentrated urine. If there is a low salt concentration ADH is not released so
water is not taken up and we have larger volumes of dilute urine. The hormone ADH is
released by the posterior pituitary gland and is transported in the blood stream. It has a half-
life of about 20mins and so it is broken down continuously to prevent over stimulation of the
aquaporin channels.
Kidney failure can occur for 2 reasons. The ultrafiltration is not fine enough so large proteins
make their way into the nephron filtrate and cannot be reabsorbed, or because the selective
reabsorption mechanism is compromised. This can result in blood or sugars in the urine. The
main cause is diabetes. Kidney failure can be treated in two main ways, dialysis or
transplant.
Dialysis uses a partially permeable membrane to act as the filter and passes the blood over it
with a specifically balanced dialysis fluid on the other side. This can be done in 2 ways.
Haemodialysis – the method familiar from GCSEs, which involves blood from an artery or
vein being passed into a dialysis machine, and then pumped back into the body. This is time
consuming and requires you to be sitting attached to the machine for 8 hours at a time, 3
times a week.
