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SS1: BIOLOGY - 1ST TERM

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  1. Introduction to Biology | Week 1
    6 Topics
    |
    1 Quiz
  2. Recognizing Living Things | Week 2
    3 Topics
    |
    1 Quiz
  3. Organisation of Life | Week 3
    3 Topics
  4. Classification of Living Organisms | Week 4
    7 Topics
    |
    1 Quiz
  5. Kingdom Prokaryotae / Monera & Kingdom Protista | Week 5
    3 Topics
    |
    3 Quizzes
  6. Kingdom Fungi & Kingdom Plantae | Week 6
    9 Topics
    |
    2 Quizzes
  7. Kingdom Animalia I | Week 7
    6 Topics
    |
    1 Quiz
  8. Kingdom Animalia II | Week 8
    5 Topics
    |
    1 Quiz
  9. The Cell | Week 9
    3 Topics
    |
    1 Quiz
  10. The Cell Structure and Functions | Week 10
    4 Topics
    |
    1 Quiz
  11. The Cell and its Environment | Week 11
    4 Topics
    |
    1 Quiz
  12. Nutrition in Plants | Week 12
    4 Topics
    |
    1 Quiz



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Photosynthesis is the process in which green plants synthesize organic substances from carbon dioxide and water, using energy absorbed from sunlight by chlorophyll.

It is often represented by this equation.

\( \scriptsize 6CO_2 \: + \: 6H_2O \: \overset{Light\:energy}{\normalsize \rightarrow} \: \scriptsize \underset{Glucose}{C_6H_{12}O_6 }\: + \: 6O_2 \)

This means that the reactants, six carbon dioxide molecules and six water molecules, are converted by light energy captured by chlorophyll (implied by the arrow) into a sugar molecule and six oxygen molecules, the products.

The equation helps to highlight the following:

  • The raw materials of photosynthesis are carbon (IV) oxide from the air and water from the soil. Aquatic plants are surrounded by water which has carbon (IV) oxide dissolved in it.
  • Synthesis is an anabolic process which needs energy. In photosynthesis, green plants use sunlight or radiant energy from the sun. They may also use strong artificial light for photosynthesis
  • The chlorophyll absorbs radiant energy from the sun. The chlorophyll which is green in colour is present in the chloroplasts.
  • Glucose (simple sugar) is the first product of photosynthesis.
  • Oxygen diffuses out as a byproduct, a little of this oxygen may be used for respiration but most of it diffuses from the plant into the surroundings.
  • During the day, sugars are usually produced more rapidly than they can be removed from the leaf. The surplus is converted into starch and stored temporally.
  • At night, the starch is gradually hydrolyzed to sugar and carried from the leaf to other parts of the plant. Starch is sometimes called the first visible product of photosynthesis because the simplest way to find out whether photosynthesis has taken place is to test for starch in the leaf.

Conditions Necessary for Photosynthesis:

Following are the conditions necessary for photosynthesis:

1. Sunlight
2. Chlorophyll
3. Carbon Dioxide
4. Water

Mechanism of Photosynthesis:

Photosynthesis consists of a series of complex reactions which are endothermic.

These reactions are grouped into two stages.

1. Light reaction.
2. Dark reaction.

Light Reaction /Stage:

It occurs during the day or in the presence of sunlight. The chlorophyll captures radiant energy from the sun and the electrons are excited. The radiant energy trapped is used to split water into hydrogen ion (H+) and hydroxyl ion (OH)¯. The splitting of the water molecule into hydrogen ion and hydroxyl ion is known as the photolysis of water.

\( \scriptsize 2H_2O \underset{chlorophyll}{\overset{radiant\:energy}{\normalsize \rightarrow}} \scriptsize \underset{2NADPH}{2H^+} \: + \: 2OH^{-}\)

The hydroxyl ion OH¯ undergoes further reaction to release oxygen. Oxygen given out during photosynthesis occurs in the light stage.   

\( \scriptsize 4OH^- \: \rightarrow \: 2H_2O \: + \: O_2 \)

The hydrogen atom released during the photosynthesis of water is picked up by a coenzyme (NADP) to form NADPH and ATP (Adenosine Triphosphate).

Note that the chloroplast contains ADP (Adenosine Diphosphate)

The extra energy not used in the splitting of water molecules is released to ADP to form ATP.

\( \scriptsize 2NADPH \: + \: 2ADP \: + \: 2P \: + \: 2H_20 \underset{chlorophyll}{\overset{sunlight}{\normalsize \rightarrow}} \scriptsize 2NADPH_2 \: + \: 2ADPH \)
 

Dark Reaction / Stage:

This stage does not require light energy. In the dark reaction, carbon (IV) oxide is reduced by combining with atoms of hydrogen provided by the enzyme NADPH2 to form sugar. This takes place through a series of small steps, each controlled by a specific enzyme. The energy needed for the formation of sugar is provided by ATP formed during the light stage.

 ATP + NADPH2 + CO2 → ADP + NADP + glucose..

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