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EXCRETION AND HOMEOSTASIS

Q1. Explain the role of human skin in:

Thermo-regulation.(16mks)

when the temperature is low, the erector pili muscles contract; marking the hair follicles to stand erect; the hair follicles trap a layer of hair; between them which reduces heat loss due to poor heat conduction

the skin has a sweat gland which secretes sweat; when the temperature is high water from the sweat evaporates

blood vessels vasoconstrict; less blood flows close to the skin surface; hence less heat is lost to the environment

When the temperature is low no sweat is produced.

When the temperature is high the blood vessels vasodilate and this encourages loss and more heat; as more flow close to the skin surface

Protection        (4mks)

the skin has a cornified layer made of dead cells which protect the entry of bacteria and inner tissues from mechanical damage; the sebaceous gland, secrete sebum which has antiseptic properties hence protects the body from bacteria. The skin has melanin pigments which protect the body from harmful UV rays

Q2. EXCRETION AND HOMEOSTASIS

1. Explain the role of insulin in blood glucose regulation.      (2 marks)

  • When the blood sugar level is above normal, it stimulates the liver cells; to convert excess glucose to glycogen/increase the rate of respiration/ convert glucose to fats. (Thus lowering the blood glucose level back to normal);

2. Name three methods by which plants eliminate their waste.         (3 marks)

  •  Diffusion;
  • Exudation;
  • Deposition;
  • Leaf fall;
  • Transpiration                           Mark first three only (3 marks)

3. In an investigation two people M and N drunk some amount of strong glucose solution. Their blood sugar levels were immediately determined and thereafter at one hour intervals for the next six hours. The results were shown in the table below.

 

 

a).In the grid provided, plot a graph for the blood glucose level against time for person M and N.      (8 marks)

b). In man the normal blood sugar level is about 90ml/100ml of blood. Explain the change in the sugar level in person M during.

(i) The first 4 hours.    (2marks)

  • Blood sugar level rises rapidly in the first hour; due to absorption of glucose from the ileum into the bloodstream;        

ii) The 6th hour.     (2marks)

  • Blood sugar level then starts declining as the person secrets insulin hormone (from the pancreas); which stimulates the liver cells to convert excess glucose into glycogen; (to lower blood glucose level) 

c) i)  Suggest a possible reason for the high blood sugar in person N.                                                            (1mark)

  • failure of the pancreas to secrete sufficient hormone insulin; which acts on the liver cells to lower the blood sugar level when it rises above normal;

ii) How can the high blood sugar in person N be controlled.     (1 mark)

  • Regular intravenous injection of insulin; (so as to lower the blood sugar level);

d)The pancreas and the liver work together in the regulation of glucose in the blood.

  • The pancreas secretes glucagon hormone; which stimulates the liver cells to raise blood sugar levels by reducing oxidation of glucose/stimulates the conversion of stored glycogen to glucose.

4. State the role of these organs when the concentration of glucose in the blood is below normal.    (2 marks)

i) What would be the effect of removing the pancreas from the body?    (1 mark)

  • There would be no secretion of the hormone insulin; hence the blood sugar level would rise far above normal resulting in diabetes mellitus;

ii) Distinguish between diabetes mellitus and diabetes insipidus.       (2marks)

  • Diabetes mellitus – is a kidney disorder due to insufficient or no secretion of insulin by the pancreas, leading to high blood sugar level. (Hence presence of glucose in urine);
  •  Diabetes insipidus – kidney disorder due to failure of the pituitary gland to secrete enough antidiuretic hormone; hence little or no water re – absorption in the kidney tubules;     

iii). Name two substances that are excreted through diffusion process in plants.   (2mks)

  • .         Carbon (IV) oxide;
  •            Oxygen;        
  •            Water vapour;                                                                          Any 2 x 1

5. (a) Arrange the following nitrogenous waste products in order of decreasing toxicity:  urea, uric acid and ammonia.(1mk)

  • Ammonia, urea, uric acid;

(b) Explain why desert animals excrete their nitrogenous wastes in form of uric acid.    (1mk)

  • It requires little amount of water to eliminate/excrete;

6. Name one waste product that is transported in the blood but not removed by the kidneys.  (1mk)

  • Carbon (IV) oxide;

7.The table below compares the approximate concentration of certain substances in plasma glomeruli filtrate and urine.

 

 

 

a) Account for the absence of

(i) Glucose in the urine.    (1mk)

  •   It is completely reabsorbed selectively in the proximal convoluted tubule

 (ii) Protein in glomerular filtrate.   (1mk)

  • Proteins have big molecules that cannot be ultra-filtered in the glomerulus.

b)  (i) Why is the percentage of urea highest in urine? Give two reasons.   (2mks)

  • All the urea is excreted since it has no use in the body i.e. it is toxic.
  • A lot of water is reabsorbed in the distal convoluted tubule leaving much urea.  Urea is actively secreted into the tubules;

c) How would the composition of urine differ from the one given above in case of:

     (i) High protein diet.      (2mks)

  • Urea concentration will increase due to an increase in the deamination process; to eliminate excess amino acids.

     (ii) Streneous exercise.      (2mks)

  • The concentration of urea will go down; because some urea will be excreted through sweating;

8.  Describe the regulation of blood sugar level in man.    (10mks)

  • When the blood sugar rises, it is detected by the hypothalamus; which stimulates the pancreas; to release insulin; through the blood, the insulin gets to the liver; where it stimulates the conversion of excess glucose to glycogen and fats which are stored; the insulin also enhances rapid breakdown of glucose in the muscle cell; these events lead to a fall in blood sugar back to normal;  When the blood sugar is low, the hypothalamus; stimulate the pancreas to release glucagon; which via the bloodstream; get to the liver, where it stimulates the liver to convert glycogen to glucose; and fats to glucose; there is also less breakdown of glucose in the cells; leading to a rise in the blood sugar level back to the norm; total 14mks, maximum 10mks.

9. (a) Given a sample of urine, name one test you would carry out to determine if it was obtained from a person suffering from diabetes mellitus.     (1 mark)

  • Reducing sugar test. (1mk);

(b) What results are expected if one is diabetic?  (2 marks)

  • When the mixture of urine and Benedict solution is heated the colour of the mixture turns from blue to green - to yellow – to orange/brown/red. (1mk); indicating the presence of reducing sugar      (1mk);

(c) Explain why sugar appears in the urine of a diabetic.     (2 marks)

10. (a) State three processes by which flowering plants excrete waste products and for each process name two waste products that are eliminated.     (6 marks)

  1. (a) Process                                    Products
  •    Exudation/guttation;    Resins, gums, latex, rubber, calcium oxalate, salts, water.(any two)  (2mks)
  •    Transpiration/Diffusion; Water, carbon (IV) oxide, oxygen. (any two)                               (2mks)
  •    Deposition/leaf fall;  Tannins, caffeine, morphine, nicotine, cocaine  (any two)           (2mks)

11. a) What is diabetes mellitus?          (1mk) 

  • A condition in which blood sugar level is high and uncontrolled. Caused by lack of insulin in the blood due to the malfunctions of the pancreas;  

b) How can it be controlled?       (1mk)

  • Administration of insulin injections/ tablets 
  • Avoid foods rich in sugar

12. Explain what happens to excess amino acid in the liver of humans   (3marks)

  • Amino acids are broken down into amino groups and carboxyl group deamination;
  • The amino group combines with hydrogen forming highly toxic ammonia which combines carbon (IV) oxide to form urea;    (3mks)

13. Explain why plants do not require specialized excretion organs (2marks)

  • Plants reuse some of their waste product; Animals do not
  •  Produce lower metabolic waste than animals; 

14. Explain the role of human skin in;-

(i) Thermoregulation       (14marks)

  • when the temperature is low (cold), the erector Pilli muscles contract; making hair follicles to stand erect; the hair follicles trap a layer of hair; between them which reduces heat loss due to poor heat conduction through them; when the temperature is high the erector Pilli muscle relax; thus making the hair lie flat; thus reducing the air trapped and more heat will be lost to the environment
  • The skin has sweat glands that secrete sweat; when temperatures are high water from the sweat evaporates taking away latent heat of vapourization; when the temperature is low, no sweat is produced.
  • When the temperature is high, the blood vessels vasodilate; and this encourages loss of heat; as more blood flow close to the skin surface; when temperatures are low, blood vessels vasoconstrict; less blood flows close to the skin surface; hence less heat is lost to the environment.
  • The skin has adipose tissue for insulation against heat loss.

(ii) Protection                    (6marks)

  • Skin has a cornified layer made of dead cells; which protects the entry of bacteria and inner tissues from mechanical damage; the sebaceous gland, secret sebum; which has antiseptic properties; hence protects the body from bacteria. The skin has melanin pigment which protect the body from harmful U.V rays.

15. The diagram below illustrates part of a nephron from a mammalian kidney.

(a) Name the fluid found in the part labelled Q.     (1mk)

  • Glomerular filtrate;

(b) Identify the process responsible for the formation of the fluid named in (a) above.   (1mk)

  • Ultra-filtration / pressure filtration;

(c) Which two hormones exert their effect in the nephron?        (2mk)

  • Antidiuretic hormone / vasopressin; Aldosterone;

16. a)  Identify the source of urea that is removed via the kidneys in a healthy human being.   (1mk)

  • Deamination of excess proteins / amino acids in the liver;

b)  Explain why a pregnant woman excretes less urea compared to a woman who is non- pregnant.   (2mk)

  • Amino acids are used in the formation of foetal tissues; thus has less excess   to be eliminated;

17 a) Explain what happens in humans when the concentration of glucose in the blood rises above the normal level. (3)

  • Insulin is produced which increases oxidation of glucose; facilitate conversion of glucose into glycogen / fats for storage; inhibits conversion of glycogen into glucose; 

b) Describe the process of urea formation.       (7 marks)

18. Name two processes by which flowering plants excrete waste products.   (2mks)

  • Exuddation / Guttation;
  • Transpiration / diffusion;
  • Deposition / leaf fall / abscission.                                                 (any two)          (2mks)

19. Describe how excretion takes place in the 

      i) Mammalian kidney     (10 marks)

  • Blood reaches the kidney from the renal artery, which branches into the renal arterioles. Arterioles further branches into afferent arterioles, which drain into the glomerulus. Enclosed in the Bowman‟s capsule.
  • The afferent arteriole are wider than the efferent arterioles causing higher pressure to develop in the glomerulus, this causes ultrafiltration of the plasma into the Bowmans capsule, the liquid part of the blood is filtered out, forming glomerular filtrate. This filtrate contains both waste and useful products.
  • The filtrate moves to the proximal convolute tubules, where selective re-absorption of glucose, amino acids, and some water and vitamins takes place, re-absorption occurs by diffusion and active transport. 

 

  • The filtrate passes in to the loop of henle where the concentration of sodium and chloride ions are raised to favour the reabsorption of water by osmosis. The filtrate that remains mainly consist of excretory products namely : urea, uric acid, excess water and mineral salts, these are passed to the distal convoluted tubules, where the remaining useful substances mainly water and mineral salts are re-absorbed.
  • The urea, uric acid, ammonia ions, excess water and excess mineral salts form urine which is removed through the ureter to the urinary bladder and later excreted out of the body.

      ii) Green plants        (10 marks)

Diffusion :

  • waste product may be eliminated when concentration gradient exists, such products include carbon (iv) oxide through the lenticels and stomata. iii) Exudation refers to the process by which plants release wastes as components of their secretion such as latex, gums, salts resins, calcium pectate

Transpiration:

  • this is the process by which plants lose excess water in the form of water vapor through the stomata, lenticels and cuticle into the atmosphere

Guttation 

  • Leaves of certain plants such as potatoes and tomatoes have specialized gland called hydathodes at tips and margins which secrete droplets of water, this aids in elimination of excess water and some dissolved salts.

 Deposition 

  •  Plants store little waste products in their harmless and insoluble form in tissues and organs such as leaves, fruits and flowers. The wastes are eliminates when these parts die or are shed off from the plant. Such waste include alkaloids,(nicotine, caffeine, oxalates) glycosides (sugar compounds such as plants oils and tannins.

20. (a)  Explain why glucose does not appear in the urine of a healthy person even though it is filtered in the  Bowman‟s capsule of a  mammal.  (2mks)      

  • All is actively reabsorbed at the proximal convoluted tubule; 

(b) In a certain person, glucose appeared in the urine. State the disease the person was suffering from.        (1mk)

  • Diabetes mellitus;

 

 

 

 

Q3. Excretion and  Homeostasis Answers