a2 biology ocr unit f214: communication, homeostasis and energy module 2.1 excretion ... · ·...

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Andy Todd 2014 1

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A2 Biology OCR

Unit F214: Communication, Homeostasis and Energy

Module 2.1 Excretion

Notes & Questions


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Define the term excretion.

Excretion – The removal of metabolic waste from the body

Metabolic waste – any substance made by metabolism that is toxic or made in excess

Metabolism – The sum total of all cell reactions

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

There are two main types of metabolic waste from the human body

Carbon Dioxide

o Produced from every living cell in the body as a result of respiration

o What happens if carbon dioxide levels rise too much

1 Hydrogen ions competes with oxygen for space on the haemoglobin molecule.

If there is too much carbon dioxide then the amount of oxygen transported will be greatly reduced inside the red blood cells.

2 Carbon dioxide binds directly with haemoglobin to create carbaminohaemoglobin.

Carbaminohaemoglobin has a lower affinity for oxygen than haemoglobin and so less oxygen is transported.

3 Excess carbon dioxide dissolves in the blood plasma. Once dissolved it combines with water to produce carbonic acid.

The carbonic acid disassociates to release Hydrogen ions (H +).

The Hydrogen ions lower the pH of the blood plasma and makes it more acidic.

Proteins in the blood acts as buffers to resist the change in pH.

If the change is small the increased H + are detected by the respiratory centre in the medulla oblongata of the brain.

This causes an increase in breathing rate to remove the excess carbon dioxide.

o Respiratory acidosis can be caused by;

Emphysema

Chronic bronchitis

Asthma

Severe pneumonia

Blockage of an airway


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o If pH drops beneath 7.35 it results in...

Slowed or difficult breathing

Headache

Drowsiness

Restlessness

Tremors

Confusion

Rapid heart rate

Increased blood pressure

Toxic effect of Carbon dioxideTissue Fluid Capillary with blood plasma

Body Cell

Red Blood Cell

CO2

CO2

100% 95%

5%

CO2 + H2O = H2 CO3- (Carbonic acid)

HCO3-

(Hydrogencarbonate)

H+

85%

HCO3-

Hb

HHb

Haemoglobinic

acid

85%

10%Carboaminohaemoglobin

1

Carbonic

anhydrase

2

3

Urea

o Produced in the liver to remove excess amino acids.

o Too much protein in the diet leads to too many amino acids

o Too many amino acids will have a toxic effect on the body

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


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The Liver

Hepatic Vein (100%)

Heart

Heart

Hepatic Portal Vein (80%)

Gall

Bladder

Hepatic Artery

(20%)

Intestine

Bile

Ox

yg

en

ate

d B

loo

d

De

ox

yg

en

ate

d

Blo

od


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Hepatocytes

o Cuboidal in shape

o Many Microvilli

o Dense cytoplasm with many organelles

o Metabolic functions include;

o Protein synthesis

o Transformation and storage of carbohydrates

o Synthesis of Cholesterol.

o Synthesis of Bile salts.

o Detoxification

Kupffer Cells

o Specialised macrophages

o Move within sinusoids

o Involved in the breakdown and recycling of red blood cells

o Haemoglobin is brokendown into Bilirubin

o Bilirubin is excreted as part of the bile and in faeces. It gives faeces its brown pigment.


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Describe the roles of the liver in detoxification.

The liver carries out a wide range

o Control of blood glucose levels, amino acids and lipid levels

o Synthesis of red blood cells in the fetus, bile, plasma proteins, cholesterol

o Storage of vitamins A, D and B12, iron, glycogen

o Detoxification of alcohol, drugs

o Destruction of red blood cells

Detoxification

o Liver detoxifies many compounds

o Some, e.g. hydrogen peroxide produced by body

o Others, e.g. alcohol consumed

o Toxins rendered harmless by oxidation, reduction, methylation or combination with another molecule.

Detoxification of alcohol

Alcohol depresses nerve activity

Also a rich store of energy, which could be used for respiration (or for getting fat)

Alcohol broken down by hepatocytes

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

Nitrogenous wastes must be removed because the amino group is highly toxic, but proteins and amino acids are very high in energy, so it would be wasteful to excrete them.

In deamination, the amine group is removed from the amino acid to form ammonia and a keto acid when added to oxygen. The keto acid enters the krebs cycle and is respired.

In the ornithine cycle ammonia is converted to urea and water. The urea is then transported to the kidneys for excretion from the body.

Ethanol Ethanal Ethanoic acid

Ethanoate

(Acetate) Acetyl Coenzyme A

Acetyl Coenzyme A – Enters mitochondria

and is respired to generate ATP

2H

NAD

NADH

Coenzyme A

Ethanol Dehydrogenase

Ethanal Dehydrogenase

2H

NAD

NADH


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Too Much

Body requires 40 – 60g of Protein a day

In developed countries people eat well in excess of this due

to heavy meat based diet as opposed to a pulses and grains

rich diet.

Catch 22

Excess amino acids cannot be stored as the amine group

makes them TOXIC.

The amino acid contains

lots of energy, so it would

be wasteful to excrete

the entire molecule

The amino acids are therefore processed in the Liver and only the toxic

amine group is excreted, while the rest is respired to create ATP.

The process consists of two steps;

1 Deamination

2 Ornithine Cycle

Deamination

Produces ammonia, which is highly toxic and very soluble

The ammonia must not accumulate

Also produces an organic compound, a keto acid, which can

enter respiration directly

C COOH

H

R

NH3C COOH

O

R

O2

Amino Acid Keto AcidOxygen Ammonia

Keto acid – Enters mitochondria

and is respired to generate ATP

NH2

Highly Toxic

& Soluble

2 2 21


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Ornithine Cycle

Ammonia is rapidly converted to something less toxic

In the ornithine cycle

Ammonia combines with carbon dioxide to produce urea and

water

Urea put back into blood, then filtered out by the kidneys

2 x Ammonia

2NH3 CO2

Urea

CO(NH2)2

Carbon

DioxideWater

H2O

2 x Amino Acid

2 x Keto Acid Ammonia

Oxygen

Citrulline

Ammonia

Arginine

Ornithine

NH3

NH3

CO2H2O

H2O

H2O

UreaCO(NH2)2

Ornithine Cycle

Deamination

Ornithine

Cycle


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

The role of the Kidney is to

o Filter Urea from the blood

o Osmoregulation - Control the amount of water that leaves the body


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


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

The blood of the renal artery consists of

• RBCs

• WBCs

• Platelets

• Plasma proteins

• Water

• Ions

•

• Glucose

• Amino acids

• Urea

Ultrafiltration – Bowmans Capsule

• The filtering of molecules from the blood by size (69,000RMM)

• Blood flows into the glomerulus via the afferent arteriole and leaves by the efferent arteriole.

• The afferent arteriole has a wider lumen that the efferent arteriole.

• This creates a bottleneck effect in the glomerulus (network of capillaries).

• This generates a high hydrostatic pressure inside the glomerulus compared to the lumen of the bowman’s capsule

Large molecules – RMM >69,000

Small molecules – RMM <69,000 Useful but not all needed

Small molecules – RMM <69,000 Useful all needed

Small molecule – RMM <69,000 Not useful – all must leave the n=body


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• As the hydrostatic is high small molecules are squeezed out of the blood and into the bowman’s capsule.

• This is known as ultrafiltration and the glomerulus and bowman’s capsule have a specialised exchange surface that allows this to occur.

Endothelium of capillaries- contains gaps from which blood passes through as well as the substances dissolved in it

Basement membrane- fine mesh of collagen fibres and glycoproteins

Filter by not allowing molecules with an RMM larger the 69000 to pass through (usually proteins)

Epithelium of Bowman’s capsule- made of specialised cells called podocytes.

Podocytes have finger like projections (foot processes) that allow fluid from the glomerulus to pass through into the Bowman’s capsule lumen

• The fluid that passes from the glomerulus into the bowman’s capsule is referred to as the Glomerular filtrate.

Blood of renal artery Glomerular filtrate

RBC WBC

Platelets Plasma proteins

water water ions ions

glucose glucose

amino acids amino acids urea urea

>69,000 RMM

<69,000 RMM


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Selective Reabsorbtion: - (Proximal Convoluted Tubule)

• The selective reabsorption of useful molecules back into the blood

• Structures of the proximal convoluted tubule that makes it adapted to carry out selective reabsorption

• A folded basal membrane – increases surface area so more space for sodium/potassium pumps

• Many mitochondrion = lots of ATP available

• A close proximity to the blood vessels

• Microvilli – increases the surface area so there is more room for the co-transporter proteins

• The tubule itself is highly folded (convoluted) increases the surface area

1

2

3 4 6

6 5


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1 Na ions are actively transported out of the wall of the convoluted tubule and enter the surrounding tissue fluid.

2 This lowers the concentration of Na inside the cell.

3 Na moves into the cell down its concentration gradient through co-transporter proteins taking glucose and amino acids with it by facilitated diffusion.

4 Sodium is transported into the cell with Amino Acids or Glucose by facilitated diffusion.

5 As the concentrations of Amino Acids or glucose rise inside the cell, they diffuse into the tissue fluid; they may also be actively removed.

6 They then diffuse into the blood and are carried away.

7 The reabsorption of salts, glucose or amino acids reduces the water potential of the cell and ultimately the blood and increases the water potential in the tubule fluid. This sets up a water potential gradient so water will move out of the tubule and enter the cells and then be reabsorbed into the blood by osmosis.

8 Larger molecules, such as proteins, will be reabsorbed by endocytosis.

Structure of the cells of the proximal convoluted tubule

Blood of renal artery

Glomerular filtrate

Glomerular filtrate after selective reabsorption

RBC

WBC Platelets

Plasma proteins water water some water

ions ions some ions

glucose glucose amino acids amino acids

urea urea urea

Water reabsorption – The Loop of Henle

• In the loop of Henle, salts are transferred from the ascending limb to the descending limb. This means that the tissue fluid in the medulla has a very negative water potential, therefore water is lost by osmosis, particularly from the collection duct.

• The longer the loop of henle, the more concentrated the interstitial fluid. Therefore the more water can be reabsorbed back into the blood.

• Kangeroo rats have very long loops of Henle whereas beavers have very short.


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Explain, using water potential terminology, the control of the water content of the blood, with reference to the roles of the kidney, osmoreceptors in the hypothalamus and the posterior pituitary gland.

Osmoregulation – Collecting duct

o The collecting duct walls are impermeable to water.

o If more water needs to be conserved then the collecting duct walls can become permeable to water allowing more water reabsorption and increasing the concentration of the urine further.

1. The water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain

2. When the water potential of the blood is very low, osmoreceptors shrink, and stimulate neurosecretory cells in the hypothalamus.

3. These produce and release anti diuretic hormone (ADH) which flows down the axon to the posterior pituitary gland where it is stored until needed.

4. When the neurosecretory glands are stimulated they send action potentials down their axons and cause the release of ADH into the blood

5. It enters the capillaries running through the posterior pituitary gland. It is transported around the body and acts on the cells of the collecting ducts.

6. When it binds to the receptors, it causes a chain of enzyme catalysed reactions, the end result of which is the insertion of vesicles containing water-


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permeable channels (aquaporins) in the membranes of the cells in the collecting duct walls, so they are more permeable to water.

7. More water is reabsorbed, by osmosis, into the blood

8. Less urine, with a lower water potential is release.

9. Less ADH is released when the water potential rises again.

10. The ADH is slowly broken down and the collecting ducts receive less stimulus.

• Low water potential of the blood

Blood is concentrated (low water potential)

Increase in ADH secretion from the pituitary gland into the blood

Increases the permeability of the collecting duct walls

More water is reabsorbed into the blood by osmosis

Increases the amount of water conserved by the body

Urine is concentrated (yellow) and in small quantities


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• High water potential of the blood

Blood is dilute

Decrease in ADH secretion from the pituitary gland into the blood

Decreases the permeability of the collecting duct walls

less water is reabsorbed into the blood by osmosis

Decreases the amount of water conserved by the body

Urine is dilute (colourless) and in large quantities.

Outline the problems that arise from kidney failure and discuss the use of renal dialysis and transplants for the treatment of kidney failure.

Problems: • Unable to remove excess water & waste products from the body

• E.g. urea & excess salts • Inability to regulate urea and salt levels • Death Dialysis

• Waste, excess fluids and salts are removed from the body by passing the blood over a dialysis membrane.

• This allows the exchange of substances between the blood and the dialysis fluid, which has the same concentration of substances as blood plasma.

• Substances diffuse from both sides to create the correct concentration of substances.

Haemodialysis:

• Blood is drained form a vein and returned to a vein. • Blood is passed into a dialysis machine • It passes through pumps to keep blood hydrostatic pressure the same as inside the

body • Heparin is added to the blood so it does not clot whilst inside the dialysis machine. • Heparin is not added to the blood before it heads back into the body so it can regain

its normal clotting function when it is back in the body.

• This is carried out three times a week for several hours at a time. • It is very restrictive to normal lifestyles


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Peritoneal dialysis • The body’s own abdominal membrane is used as a filter.

• Patient is more mobile while carrying out dialysis • Better quality of life compared to haemodialysis


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Transplant

Advantages • No dialysis • Less limited diet • Better physical feeling • Better quality of life • No longer ‘chronically ill’

Disadvantages

• Need immunosuppressants for life of kidney • Major surgery • Risk of infection • Need frequent checks in case of rejection • Side effects of medication


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Describe how urine samples can be used to test for pregnancy and detect misuse of anabolic steroids.

Pregnancy

immobilised

antibody

free

antibody

attached to

coloured

marker

CG

position A

absorbent

membrane

urine sample

containing CG

• A human embryo secrets human chorionic gonadotrophin (hCG) as soon as it is implanted on the uterine lining.

• The hormone can be detected in the mother’s urine after as few as 6 days. • The hCG hormone is smaller than 69,000RMM • hCG acts as an antigen. • Pregnancy tests contain monoclonal antibodies which are complementary to hCG • The antibodies are tagged with a blue bead. • -antibody complex moves along the strip • The hCG-anitbody complex will bind to band of immobilised complementary

antibodies, • Forms a blue line •


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Anabolic Steroids • Urine samples are tested using gas chromatography

1. The sample is vaporised in the presence of a gaseous solvent 2. It is passed down a long tube lined with an absorbing agent. 3. Each substance dissolves differently in the gas and stays there for a unique, specific time- the retention time 4. Eventually, the substance leaves the gas and is absorbed by the lining 5. It is then analysed to make a chromatogram 6. Standard samples of drugs and urine samples are run so drugs can be identified and quantified in the chromatogram.


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1. The mammalian liver is made up of lobules that consist of liver cells (hepatocytes) arranged in plates.

The figure below shows a section of a liver lobule and its associated blood vessels.

A

BC

D

hepatocyte

branch of

hepatic vein

Name structures A to D.

A .......................................................................

B .......................................................................

C .......................................................................

D .......................................................................

[Total 4 marks]


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2. The table below shows the mass of different substances excreted by a volunteer during two 24 hour periods. During the first 24 hour period, the volunteer was fed a protein-deficient diet; during the second 24 hour period, the volunteer was fed a protein-rich diet. All other variables were kept constant.

mass of substance excreted / g

substance excreted protein-deficient diet protein-rich diet

urea 2.20 14.70

uric acid 0.09 0.18

ammonium ions 0.04 0.49

creatinine 0.60 0.58

(i) Calculate the percentage increase in urea excreted when the volunteer switched from a protein-deficient to a protein-rich diet. Show your working.

Answer = ...................................................%

[2]

(ii) Describe how excess protein is converted into urea.

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[3]

[Total 5 marks]


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3. Blood enters the kidneys through the renal arteries and the human kidneys process

1200 cm3 of blood every minute. This 1200 cm3 of blood contains 700 cm3 of plasma.

As this blood passes through a glomerulus, 125 cm3 of fluid passes into the renal tubule.

(i) Name the process by which the fluid passes from the glomerulus into the renal tubule.

.........................................................................................................................

[1]

(ii) Calculate the percentage of plasma that passes into the renal tubule.

Show your working and give your answer to one decimal place.

Answer = ................................................ %

[2]

[Total 3 marks]


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4. The first stage in the formation of urine is glomerular filtration. This results in the production of glomerular filtrate in the Bowman’s (renal) capsules. Below is a diagram that shows the structures and forces involved in the filtration process.

efferentarteriole

glomerularcapillaries

bloodhydrostaticpressure 8 kPa

water potentialof blood –4 kPa

proximal convoluted tubule

A

Bowman’s(renal)

capsule

capsular fluidhydrostatic

pressure2.7 kPa

afferent arteriole

(a) The normal blood hydrostatic pressure in other capillaries is 3.3 kPa.

(i) Using the diagram, explain why the blood pressure in the glomerular capillaries is considerably higher than in other capillaries.

................................................................................................................

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[2]


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(ii) Using the data given in the diagram, calculate the effective filtration pressure.

Answer = ........................................kPa

[2]

(b) The presence of protein molecules in the urine of an individual is a sign of kidney disease or kidney damage.

(i) Explain why it is unusual for protein molecules to appear in the urine.

................................................................................................................

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[2]

(ii) Explain why protein in the urine is often a symptom of chronic high blood pressure.

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[2]


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(c) A complex barrier exists between the blood plasma in the glomerular capillaries and the fluid in the renal capsule.

Describe in detail the structure of the region labelled A on the diagram above.

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[4]

(d) Coffee contains the drug caffeine, which inhibits the release of ADH.

Describe and explain the effect of drinking coffee on the volume and concentration of urine produced.

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[4]

[Total: 16 marks]


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5. The figure below is an electronmicrograph of a transverse section of part of a proximal convoluted tubule.

(i) Name the tissue that lines the proximal convoluted tubule.

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[1]

(ii) Name the structures indicated by X.

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[1]


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(iii) The table below shows the approximate concentration of some of the substances in the blood plasma, the glomerular filtrate and the urine leaving the collecting duct.

substance

concentration in blood plasma

(g dm–3)

concentration in glomerular

filtrate

(g dm–3)

concentration in urine leaving

collecting duct

(g dm–3)

amino acids 0.50 0.50 0.00

glucose 1.00 1.00 0.00

inorganic ions 7.30 7.30 15.60

nitrogenous waste (not including urea)

0.03 0.03 0.28

protein 80.00 0.00 0.00

urea 0.30 0.30 21.00


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Some of the changes observed between the glomerular filtrate and the urine are as a result of activity in the proximal convoluted tubule.

With reference to the table above, explain how these observed changes in concentration are brought about by the proximal convoluted tubule.

In your answer, you should use appropriate technical terms, spelt correctly.

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[4]

[Total 6 marks]


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6. The figure below shows diagrams of nephrons from the kidneys of three different mammals, X, Y and Z.

X Y Z

B

A

C

D E

A

B

C

D

E

A

B

C

D

not drawn to scale

X Y Z

name of mammal

beaver house mouse desert living gerbil

water potential of urine

high low very low

Explain the relationship between the length of the section D in the nephrons and the water potential of the urine each mammal produces.

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[Total 3 marks]


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7. The presence of CG in the urine can be used in pregnancy testing. Information about a

pregnancy testing kit is given below and in the figure.

• An absorbent membrane is dipped into urine.

• The membrane contains free antibodies that are specific to CG.

• The free antibodies are attached to coloured markers.

• There is a line of immobilised antibodies above position A.

• A positive result is shown by a coloured line at position A.

immobilised

antibody

free

antibody

attached to

coloured

marker

CG

position A

absorbent

membrane

urine sample

containing CG

Reproduced with the permission of Nelson Thornes Ltd from ‘New Understanding Biology for Advanced Level’ Fourth Edition

by Glen Toole and Susan Toole (978-0-7487-3957 -8), first published in 1999.


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Using information from the figure, explain how the presence of CG in the urine results in a coloured line at position A.

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[Total 4 marks]

a2 biology ocr unit f214: communication, homeostasis and energy module 2.1 excretion ... · ·...

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