6.16.3 The Mechanism of Photosynthesis (Structured Question 1 & 2)

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Question 1:
Green plants synthesize their food through the process of photosynthesis. The chemical process of photosynthesis can be summarized as in the schematic diagram below.

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State the meaning of photosynthesis based on the schematic diagram. [2 marks]

Starting with water and carbon dioxide as the raw materials, describe how a green plant produces starch molecules. [8 marks]

It is found that the process of photosynthesis contributes to the balance of nature.
Explain how air pollution may have an effect on photosynthesis and the balance of nature. [10 marks]

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A process whereby a green plant produces glucose from carbon dioxide and water in the presence of chlorophyll and sunlight.

  • Chlorophyll absorbs light energy to produce ATP/ electrons.
  • Photolysis of water produces hydrogen ions and hydroxyl ions.
  • The H+ ions combine with electrons to form hydrogen
  • The hydrogen/ ATP will be used in the dark reaction
  • It occurs in the grana
  • Takes place in the absence of light
  • Carbon dioxide combines with hydrogen to form glucose and water
  • Glucose undergoes condensation and is converted to starch for storage
  • It occurs in a series of chemical reactions which require ATP
  • The reaction occurs in the stroma
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  1. The importance of photosynthesis for the balance of nature:
  • Absorbs carbon dioxide from the atmosphere
  • Replaces oxygen in the atmosphere
  • Maintains the percentage of oxygen in the atmosphere
  1. The sources of pollution are motor vehicles and urbanization which will liberate heavy smoke and increase the amount of particles in the air.
  2. The particles accumulate on the leaf surfaces. They cover the stomata and reduce the intensity of the light that reaches the leaves.
  3. As a result, the rate of photosynthesis decreases. Less carbon dioxide is reabsorbed from the atmosphere and less oxygen is released. It also promote the greenhouse effect and global warming.

    6.16.2 Food Digestion (Structured Question 1 & 2)

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    Question 1:
    (a) Diagram 1.1 shows human digestive system.
    Structure S in Diagram I is involved in the digestion of fat.

    Name structure S.
    (ii) Name the enzyme which digests fat in S.
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    Diagram 1.2 shows a mechanism of an enzyme reaction.
    Draw the diagram in the spaces III and IV provided in Diagram 1.2 to complete the mechanism of the enzyme reaction.

    Explain how starch is digested in the duodenum.

    A student eats too many oranges.
    Explain the effect of eating too many oranges on the digestion of starch in the duodenum.
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    In a food preparation process, a housewife carries out the following steps:
    Step 1: Marinate pieces of meat with strips of unripe papaya in a basin.
    Step 2: Immerse the basin in a container filled with water at 40oC for 30 minutes as in Diagram 1.3.

    Explain why the housewife carries out Step 1 and Step 2.

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    Structure S: Ileum

    Enzyme which digests fat in S is Lipase


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    The pancreas secretes an amylase into the duodenum which hydrolyses starch to maltose.

    - The duodenum medium becomes acidic
    - Which is not suitable for the action of the amylase on starch
    - Slows down the digestion of starch

    - Papaya contains papain/ protease
    - The pieces of meat and papaya strips increase the total surface area
    - To increase enzyme action
    - Protease will tenderize the meat
    - Takes place in water at 40oC, which is the optimum temperature for enzyme reaction

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    6.16.1 Malnutrition (Structured Question 1 & 2)

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    Question 1:
    (a)(i) After absorption of nutrients in the small intestine, the undigested substances in the colon result in the formation of faeces.
    Explain the formation of faeces in a human. [4 marks]

    Malnutrition is a condition due to taking an unbalanced diet in which certain nutrients are lacking, in excess or in the wrong proportions.
    Explain the effect on a child who is given insufficient amounts of any two nutrients of food for a long period of time. [6 marks]
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    Table below shows the food intake by a boy aged 15 years in his daily menu. The daily energy requirement for him is 12500 kJ.

    Based on the Table, state the value of energy contained in this daily food intake.
    Does the food intake satisfy his daily energy requirement? [2 marks]

    This boy takes this menu continuously for a long time. [8 marks]

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    • The contents in the small intestine that are not absorbed enter the colon.
    • The intestinal content consists of a mixture of water, indigestible food, bacteria, dead cells and pigments.
    • The contents move slowly along the colon by peristalsis.
    • Water is reabsorbed and faeces are formed.

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    • A child who is deficient in protein may suffer from kwashiorkor.
    • Protein is needed for normal growth. Lack of protein causes growth of the child to be stunted.
    • A child who is deficient in vitamin C may suffer from scurvy.
    • Vitamin C is needed for healthy gums and skin. Lack of vitamin C causes swollen and bleeding gums.
    • A child who is deficient in roughage may suffer from constipation.
    • Roughage is needed to stimulate peristalsis.
    • Lack of roughage causes difficulty in defaecation.
    Total energy = 8230 kJ
    The energy produced from the food taken daily is insufficient.
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    6.5.1 Functions of the Liver and Assimilation

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    Transport of Nutrients by Circulatory System
    1.   Glucose, amino acids, minerals, vitamins B and C are absorbed into blood capillaries of the villus and carried out of the small intestine to the liver by the hepatic portal vein.
    2.   The hepatic portal vein transports the nutrients absorbed from the small intestine to the liver for processing and prepared for metabolic process or assimilation.
    3.   Assimilation takes place in the cells where the nutrients are used to form complex compounds or structural components, for example, the transformation of amino acids to proteins.
    4.   Glycerol and fatty acids as well as vitamins A, D, E and K are absorbed into the lacteal of the villus and transported out of the small intestine by the thoracic duct to the lymphatic system. The lymph is then transported by the thoracic duct to the subclavian vein. The lipid droplets and fat-soluble vitamins are then transported to all the cells via the blood circulatory system.

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    Functions of the Liver
    1.   Regulation of blood glucose levels
    Convert excess glucose to glycogen for storage, and glycogen to glucose when the glucose level is low.
    2. Storage of nutrients
       (a)  Excess glucose is converted into fats in the liver and later stored in other parts of the body.
       (b)  The liver also stores iron and fat soluble vitamins such as A, D, E and K.
    3.   Deamination
    Breaks down excess amino acid to ammonia in a process called deamination. Ammonia is then converted to urea and is excreted in the urine.
    4.  Detoxification
    The liver detoxifies blood by removing and metabolizing toxic substances.
    5.  Production of bile
       (a)  Bile contains bile salts and bile pigments which are delivered to the duodenum.
       (b)  Bile emulsifies lipid droplets into tiny droplets. This increases the surface area of the action for the lipase.
    6.   Synthesis of plasma proteins
      The liver is the site of plasma protein synthesis, for example, fibrinogen and prothrombin, which are vital blood clotting agents, are synthesized in the liver.

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    1.   Assimilation is the process by which food materials after being absorbed are built up into complex constituents of the organism.
    2.   Glucose
      ·  Glucose is oxidised in body cells during respiration to produce energy for various
       activities such as cell division.
      ·  Excess glucose is converted into glycogen and stored in the liver.
      ·  When the blood sugar level falls and the body needs energy, the stored glycogen is converted back into glucose.
    3.   Amino Acids
      · Amino acids are absorbed by body cells to synthesise various types of proteins such as enzymes, antibody, plasma membrane and protoplasm.
      · Excess amino acids are broken down in the liver and converted into urea which is excreted in the urine through the process of deamination.
    4.   Lipids
       · Lipids (fat droplets) are absorbed by body cells to build the plasma membrane and cholesterol.
       · Phospholipids are components of the plasma membrane.
       · Excess fat are stored in adipose tissue underneath the skin as reserve energy and around internal organ.
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    6.5 Absorption and Assimilation of Digested Food

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    6.5 Absorption and Assimilation of Digested Food
    1.   Ileum is the major site of nutrient absorption.
    2.   Soluble nutrients are absorbed by simple diffusion, facilitated diffusion or active transport.
    Adaptive characteristics of the digestive system
    1.   The wall of the small intestine is covered with epithelial cells that are specialized to complete the digestive process.
    2.   The small intestine in adults is a long and narrow tube about 6 metre long, is the longest section of the alimentary canal.
    3.   The small intestine contains small finger-like projections of tissue called villi which increase the surface area of the intestine and contain specialized cells that transport substances into the bloodstream. 
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    4.   Specific feature of the villi:
    (a)  there are numerous in number. This feature increases the internal surface area of the intestinal walls making available a greater surface area for absorption.
    (b)  they have moist and thin wall, one cell thick, which enables a shorter diffusion path.
    (c)  the epithelial cells contain microvilli which provide a large surface area for more rapid absorption.
    (d)  contain network of blood capillaries for the absorption and efficient transport of digested food.
    (e)  contain lacteals for more efficient absorption of fatty acids and glycerol into the blood stream. 

    5.   The absorbed food substances are transported to the various body tissues or organs to be assimilated or to be stored for later use.
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    6.4.2 Digestion in the Small Intestine

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    6.4.2 Digestion in the Small Intestine

    Digestion in the Small Intestine
    1.   The small intestine consists of the duodenum, jejunum and the highly coiled ileum.
    2.   The duodenum is the first part of the small intestine, receives chyme from the stomach and secretions from the gall bladder and pancreas.

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    Digestion in the duodenum 
    1.   Bile is an alkaline, greenish-yellow liquid produced in the liver and stored in the gall bladder. When stimulated, bile is transported from the gall bladder to the duodenum.
    2.   Bile does not contain any digestive enzymes.
    3.   Bile helps to reduce the acidity of the chyme and optimises the pH for enzyme action.
    4.   Bile salts emulsify the lipids (fats) and break them up into tiny droplets, thus providing a larger surface area for digestion by enzymes (lipase).
    1.   Pancreas secretes pancreatic juice which contains the enzymes pancreatic amylase, trypsin and lipase, transported by the pancreatic duct, to the duodenum.
    2.   The optimum pH required for the action of enzymes in the pancreatic juice is between 7.2 and 8.2.
    1.   In the duodenum, pancreatic amylase hydrolyses the starch to maltose. 

    2.   Trypsin hydrolyses polypeptides into shorter peptide chains. 

    3.   Lipase hydrolyses the lipids to glycerol and fatty acids.

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    Digestion in the ileum
    1.   Glands in the wall of the ileum secrete intestinal juice which contains digestive enzymes needed to complete the digestion of food.
    2.   At the end of the digestive process, all carbohydrates are digested into monosaccharides such as glucose, fructose and galactose.
    · Maltase hydrolyses maltose to glucose

    · Sucrase hydrolyses sucrose to glucose and fructose

    · Lactase hydrolyses lactose to glucose and galactose
    3.   Proteins are digested into amino acids and lipids into fatty acids and glycerol.
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    6.4.1 Human Digestive System

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    6.4.1 Human Digestive System
    1.      The human digestive system comprises of the alimentary canal which starts at the mouth and ends at the anus.
    2.      The parts of the human digestive system are:
          (a) mouth
          (b) oesophagus
          (c) stomach
          (d) duodenum (first part of small intestine)
          (e) ileum (rest of small intestine)
          (f) large intestine (caecum, colon, rectum)

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    Digestion in the mouth
    1.      Digestion of food begins in the mouth.
    2.      The chewing action breaks the food into smaller pieces. This makes it easier to swallow the food and increases the surface area for chemical digestion by enzymes.
    3.      There are three pairs of salivary glands in the mouth to produce saliva.
    4.      Saliva contains the enzyme salivary amylase which begins the hydrolysis of starch to maltose.
    5.      The food particles are rolled into a small ball called bolus.
    6.      During swallowing, the bolus enters the oesophagus

    1.      The bolus moves along the oesophagus by a process called peristalsis, a series of wave-like muscular contractions along the oesophageal wall.
    2.      Mucus secreted by the oesophagus wall to lubricate the passage of the bolus.
    3.      When the cardiac sphincter relaxes, the bolus enters the stomach.

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    Digestion in the stomach
    1.      Gastric glands in the walls of the stomach secrete gastric juice.
    2.      Gastric juice contains hydrochloric acid and the enzymes pepsin and rennin.
    3.      Hydrochloric acid
    (a) Creates an optimum pH (1.5-2.0) for the action of the enzymes pepsin and rennin.
    (b) Stops the activity of salivary amylase.
    (c) Kills bacteria that are present in the food.
    4.      Pepsin hydrolyses the protein molecules into smaller polypeptides.

    5.      Rennin coagulates milks by converting the soluble milk protein, caseinogen, into insoluble casein.

    6.      The regular churning movements of the stomach mix up the gastric juice with food.
    7.      The food remains in the stomach for 2 to 4 hours.
    8.      The semi-liquid food is called chyme.
    9.      The pyloric sphincter then relaxes. It allows the chyme to enter the duodenum, a little at a time.
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    6.4 Food Digestion

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    6.4 Food Digestion
    1.   Digestion is the process that breaks down complex food substances into simpler, soluble molecules that are small enough for the body to absorb.

    2.   Digestion breaks down
     (a) carbohydrates into glucose molecules,
     (b) proteins into amino acids, and
     (c) lipids into glycerol and fatty acids.

    3.   Digestion involves both physical and chemical processes.

    4.   Physical digestion
     (a) involves the breaking of large pieces of food into smaller pieces by chewing action of the teeth.
     (b) this action increases the surface area of the food particles for enzyme reaction.

    5.   Chemical digestion
      Digestive enzymes break down of large complex food particles into simple soluble molecules by specific digestive enzymes in the presence of water. The process called hydrolysis.
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    6.1 Types of Nutrition

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    6.1 Types of Nutrition
    Autotrophic Nutrition
    1.  An autotroph is an organism that synthesise complex organic molecules from inorganic molecules such as air and water.
    2.   Autotrophs are able to synthesise their food by
                (a)  photosynthesis
                (b)  chemosynthesis
    3.  Photosynthesis is the process in which green plants and algae, called photoautotrophs, produce organic molecules from carbon dioxide and water using sunlight as a source of energy.
    4.  Chemosynthesisis the process in which chemoautotrophs synthesise organic compounds by oxidizing inorganic substances such as hydrogen sulphide and ammonia.

    Heterotrophic Nutrition
    1.  Heterotrophs are organisms that cannot synthesise their own nutrients but instead must obtain the nutrients from other organisms.
    2.  Heterotrophic nutrition is a type of nutrition in which an organism obtains energy through the intake and digestion of complex organic substances into simpler, soluble substances which are then absorbed into their bodies.
    3.   Heterotrophs include animals, fungi and some bacteria.
    4.   Heterotrophs may practice holozoic nutrition, saprophytism or parasitism

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    Holozoic Nutrition
    1.   Most animals like humans, herbivores and carnivores are holozoic heterotrophs.
    2.   In holozoic nutrition, the organisms feed by ingesting solid organic matter which is subsequently digested and absorbed into their body.

    Saprophytic Nutrition (Saprophytism)
    1.   In saprophytism, the organisms called saprophytes, feed on dead and decaying organic matter.
    2.   Bacteria and fungi are examples of saprophytes.
    3.   Saprophytes are sometimes called decomposer.

    Parasitic Nutrition (Parasitism)
    1.   Parasitism is a close association in which an organism, the parasite, obtains nutrients by living on or in the body of another living organism, the host.
    2.   Parasites which live on the body of the host called ectoparasitesFor examples, fleas, ticks and leeches.
    3.   Parasites which live in the body of the host called endoparasites. For example, the tapeworms which infest the human intestinal tract.
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