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OCR Biology A2

F214 Communication, Homeostasis and Energy 4.2 Excretion

JWh

8/8/2014

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4.2.1 Define the term excretion;

The process of excretion is the removal of waste from metabolic reactions from the body.

As this is the unwanted waste products of metabolic reactions they must have been formed within the cell.

Excretion must not be confused with egestion, which is the removal of undigested food or chemicals which have never been part of a cellular reaction (e.g. faeces).

The main two compounds which are produced are carbon dioxide and urea

Explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste, from the body;

The need to excrete carbon dioxide is shown by looking at a condition associated with high blood CO2 concentration.

CO2 is an acid gas and build- up of H+ decreases the pH of the body causing Respiratory Acidosis.

There are three methods of transporting carbon dioxide around the body for removal, outlined below:

1. As Hydrogen Carbonate ions – a process which also produces H+ within Red Blood Cells.

2. Carried around directly dissolved in the blood in solution.3. Combining directly with Haemoglobin to form

Carbaminohaemoglobin

Haemoglobin has a higher affinity to carbon dioxide than it does for oxygen, so formation of carbaminohaemoglobin reduces the efficiency of oxygen carriage around the blood.

When one haem group has taken up a molecule of carbon dioxide it cannot take on a molecule of oxygen.

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Hydrogen ions, which are produced alongside hydrogen carbonate ions alter the blood pH level too, which means that enzyme action is slowed.

Failure to excrete carbon dioxide from the blood appropriately will result in respiratory acidosis.

Symptoms of respiratory acidosis includes: an increased urge to breathe, or hyperventilation.

If this occurs for a prolonged period, there can be permanent damage to organs, in particular the brain

Urea is produced by oxidising excess amino acids in the liver in a process called deamination.

The ammonium ions produced are combined with CO2 in the Ornithene Cycle to produce urea.

The urea which is produced there is then transported, to the kidneys,

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where it will be excreted through urine. The urine is produced in the kidneys and temporarily stored in the

bladder.

Describe, with the aid of diagrams and photographs, the histology and gross structure of the liver;

Hepatocytes (liver cells) carry out many metabolic functions. It is essential that the organ has a rich blood supply to as many cells

as possible, and the liver has many secure connections to the blood system.

The liver receives oxygenated blood from the hepatic artery and deoxygenated blood from the digestive system through the hepatic portal vein.

This blood supplied is rich in the products of digestion, and may contain toxic substances that have been absorbed in the intestine.

Blood leaves the liver through the hepatic vein which rejoins the vena cava and returns to systemic circulation.

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There is also the bile duct leaving the liver, where bile is secreted. This has both digestive and excretory functions. The bile duct runs from the liver to the gall bladder, where bile is

stored until needed for digestion.

The arrangement of cells within the liver has been adapted so that each cell has a rich blood supply from adjacent blood vessels.

The liver is divided into lobes which are further divided into small lobules.

The vessels of the hepatic portal vein and hepatic artery, both of which supply the liver with blood, split into smaller vessels, and run alongside other vessels called inter-lobular vessels

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The Hepatic Artery, Hepatic Portal Vein and Bile duct form a TRIAD around each lobule.

Describe the formation of urea in the liver, including an outline of the ornithine cycle;

The body needs proteins to function Excess amino acids from proteins which we do not need cannot be

stored, because their amine groups make them toxic. The molecule is modified under two processes so that the amine

component can be excreted alone.

Deamination This is the removal of the amine group from an amino acid. Deamination produces the compound ammonia, which is very

toxic and so must not be allowed to accumulate. Another by-product of the process is keto acid, an organic

compound which directly enters the mitochondrion and is respired aerobically.

The ornithine cycle.

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Ammonia is very soluble and very toxic, so it needs to be converted into a less harmful substance.

The ammonia is combined with carbon dioxide to produce urea in the ornithine cycle.

Describe the roles of the liver in detoxification;

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Detoxification of alcohol The liver is able to detoxify many harmful substances Alcohol and drugs can be detoxified in the liver. Hepatocytes contain enzymes which have detoxification functions,

such as catalase, which converts hydrogen peroxide (very toxic) into water and oxygen.

Alcohol (ethanol) is broken down in the liver using the enzyme ethanol dehydrogenase to produce ethanal, which is broken down using ethanal dehydrogenase into ethanoic acid.

This end product combines with coenzyme-A to produce acetyl coenzyme-A which can be directly respired

The hydrogen atoms released in this reaction can also combine with a molecule of NAD to produce reduced NAD.

Describe, with the aid of diagrams and photographs, the histology and gross structure of the kidney;

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The human body has two kidneys.The urinary system consists of four major parts:

1. the two kidneys2. the ureters which transport urine from the kidney to the bladder3. the bladder temporarily stores urine4. the urethra to take urine to the outside world

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There are approximately one million tubules within a kidney called nephrons.

A nephron exists through each section of the kidney: the outer cortex, the medulla in the middle, and the innermost pelvis

The nephron A single nephron consists of the:a. Bowman’s Capsule in which there is a network of capillaries called

the Glomerulus - ultrafiltrationb. Proximal (first) Convoluted Tubule which runs from the capsule

to the loop of Henle – absorption of useful material.c. Loop of Henle, creates the conditions for water reabsorption.d. Distal Convoluted Tubule – ion exchange.e. Collecting Duct which connects to many nephrons and ultimately

ending up in the pelvis region – water absorption.

Describe, with the aid of diagrams and photographs, the detailed structure of a nephron and its associated blood vessels;

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Bowman’s Capsule – as blood enters the Bowman’s Capsule, under high pressure, it passes through tree layers as it moves from the blood vessel in the Glomerulus to the Nephron proper.

Endothelial cells

The endothelial cells lining the glomerular capillaries are thin and flat with a large nucleus.

The cells contain pores, which allow plasma components to cross the vessel wall, but not blood cells, platelets or large proteins.

Basement membrane

The basement membrane is a continuous layer of connective tissue and glycoproteins.

It is a non-cellular structure that prevents any large molecules from being filtered.

It is the only real barrier to molecules being forced out of the blood

Epithelial lining

The epithelial lining of Bowman’s capsule consists of a single layer of

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cells; podocytes, which rest on the basement membrane. The podocytes have large extensions which extend out from the cell

body and are embedded in the basement membrane surrounding a capillary.

This leads to the formation of slit pores, which control the movement of substances through the final layer of the filter.

The podocytes have a well-developed Golgi apparatus, used to produce and maintain the glomerular basement membrane.

Describe and explain the production of urine, with reference to the processes of ultrafiltration and selective reabsorption;

Ultrafiltration The kidney is concerned with filtering out waste products from the

blood plasma, which are to be excreted in the urine. It is important that useful substances are kept. All molecules from the blood plasma below a certain size are passed

through the filtration membrane, and those materials which need to be kept are selectively reabsorbed later on.

The afferent arteriole which supplies the glomerulus has a much larger width than the efferent arteriole leading away from the glomerulus.

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This means that there is an increased pressure inside the glomerular capillaries than in most capillaries.

This forces fluid from the blood through from the blood capillaries into the renal capsule.

This process is called ultrafiltration. The pressure forces molecules within the blood with a relative

molecular mass (RMM) of below 69,000 through the glomerular capillaries and into the Bowman’s capsule.

This pressure is called filtration pressure. Molecules of water, glucose, urea, amino acids and ions are all

removed from the blood plasma and filtered into the nephron. Proteins are too big to fit through the gaps in the basement

membrane, and so remain in the blood. Similarly, all blood cells do not enter the glomerular filtrate

Selective reabsorption The filtrate which results in the Bowman’s capsule flows along the

nephron, and it is at the proximal convoluted tubule that the majority of substances are reabsorbed.

This process is known as selective reabsorption. All of the glucose and amino acids are reabsorbed back into the

blood. 80% of the Sodium ions (and Cl-), as well as 80% of the water are

reaborbed.Selective reabsorption occurs in a several staged process:

1. On the cell surface membrane closest to the blood capillary, there are sodium-potassium pumps which actively transport sodium ions out of the cell.

2. As sodium ions are pumped out of the cell, the concentration of sodium ions inside the cell drops

3. This allows the transport of sodium ions back into the cell from the surface membrane in contact with the proximal convoluted tubule – the ions flow in through co-transporter proteins which couple the

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movement of sodium ions into the cell with the movement of glucose and amino acids (this is facilitated diffusion)

4. As sodium moves into the cell, taking glucose and amino acids in with it, the concentration of glucose and amino acids within the cell lining the convoluted tubule increases, and so these substances are pumped out of the cell and into the blood capillary via active transport, using carrier proteins.

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5. The movement of ions, glucose and amino acids from the convoluted

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