OCR Biology A2 Unit 1 Notes-Excretion

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EXCRETION: the removal of metabolic waste from the body, accumulation would be toxic.

DEAMINATION: the removal of an amine group from an amino acid to produce ammonia.

Why must CO2 be removed? Reduces oxygen transport (combines with haemoglobin when it releases H+ ions), forms carbaminohaemoglobin (CO2 combines with haemoglobin)-has lower oxygen affinity, respiratory acidosis-combines with H2O to make carbonic acid (H2CO3) which dissociates to release H+, making the blood more acidic.

Why must nitrogenous compounds be removed? Amino acids transported to the liver, deamination-ammonia-converted to urea, transported to kidneys for excretion. Keto acid used in respiration or it is converted to carbohydrate or fat for storage.

Deamination: ammonia + CO2 --> urea + H2O (2NH3 + CO2 --> CO(NH2)2 + H2O.

The liver is supplied with blood from two sources: hepatic artery: from aorta, hepatic portal vein: deoxygenated blood from digestive system (rich in products of digestion.) Blood leaves via the hepatic vein which rejoins the vena cava and blood returns to normal circulation. Bile duct-carries bile from the liver to the gall bladder where it is stored until required to aid digestion of fats in the small intestines.

Division into lobes-->lobules-->interlobular vessels-->liver cell (microvilli.)

Sinusoid-blood from two vessels in mixed and passes along this chamber, lined by liver cells and emptied into the hepatic vein.

Bile made by liver cells and released into bile canaliculi.

Liver cells: many functions-protein synthesis, transformation and storage of carbohydrates, synthesis of cholesterol and bile salts, detoxification-dense, specialised cytoplasm.

Kupffer cells: macrophages, move in sinusoids, breakdown and recycling of old red blood cells, produces bilirubin-excreted as part of bile and in faeces.

Functions of the liver: control of blood glucose levels, amino acid and fat levels, synthesis of red blood cells, bile, plasma proteins, cholesterol, storage of vitamins (A,D,B12), iron, glycogen, detoxification of alcohol, drugs, breakdown and destruction of hormones/red blood cells.

Deamination: NH2CHRCOOH + O2 --> RCOCOOH + NH3.

Ornithine cycle: 2NH3 + CO2 --> CO(NH2)2 + H2O.


Detoxification: catalase-converts hydrogen peroxide to oxygen and water. Alcohol: ethanol is converted to ethanal by ethanol dehydrogenase, ethanal is converted to ethanoic acid by ethanal dehydrogenase, ethanoic acid (acetate) which then combines with CoA to form acetyl CoA which can then enter respiration.

The H atoms released in this process combine with NAD to form 2NADH + 2H+.

NAD is also required to break down fatty acids for use in respiration. If the liver has to detoxify too much alcohol it has insufficient NAD to deal with the fatty acids. These are stored as lipids in hepatocytes -fatty liver-leads to hepatitis or cirrhosis.

The kidney: supplied by renal artery and drained by renal vein, removes waste from blood, produces urine-passes out of kidney down ureter to bladder where it can be stored before being released. Structure: capsule, cortex, medulla, pelvis, ureter.

NEPHRON: functional unit of the kidney, microscopic tubule that receives fluid from the blood capillaries in the cortex and converts this to urine, which drains into the ureter.

GLOMERULUS: fine network of capillaries that


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