Biology Chapter 6 - Photosynthesis

Photosynthesis

Is the process by which living plant cells, containing chlorophyll, produce food substances (glucose and starch), from carbondioxide and water, by using light energy. Plants release oxygen as a by-product during photosynthesis

Importance of Photosynthesis

(1) Food for all:
- Ultimately source of food for all living beings - dircetly and indirectly
(2) Oxygen to breathe in:
- Only biological process which release oxygen into the atmosphere. Oxygen supports all life on earth

Chlorophyll

(chloro: green; phyll: leaf) is the green pigment found in plants. It is contained in microscopic cell organelles called choloroplasts

Chloroplasts

• Minute oval bodies bounded by a double membrane, and their interior contains closely packed flattened sacs (thylakoids) arranged in piles (grana) lying in a colourless ground substance called stroma

• Ordinarily there may be 40-50 chloroplasts in a cell

• The pigment chlorophyll is contained in the walls of the thylakoids

• It is a highly complex substance, composed to carbon, hydrogen, oxygen, nitrogen and magnesium

• Chloroplasts are mainly contained in the mesophyll cells located between the upper epidermis and the lower epidermis (i.e. in palisade cells and spongy cells) of leaves.

• They are also found in the guard cells of the stomata and in the outer layers of young green stems

• There may be over 50,000 chloroplasts per sq.mm. of leaf surface

Types of cholorphyll

9 types of chlorophyll, two of which are:

1. Chlorophyll-a

2. Chlorophyll-b

   Best known and most abundant

Why does chlorophyll appear green?

• Chlorophylls absorb light at both ends of the visible sepctrum - i.e. blue and red lights

• They reflect the green light away

• The absorbed blue and red lights are most effectlive for photosynthesis

Chlorophyll and light intensity…………………………… Explain

• Chlorophyll is highly sensitive to light, so too much light may destroy it

• But formation of chlorophyll is dependant on exposure of the plant to light

• The grass growing in the shade under a stone turns yellowish due to the non-formation of new chlorophyll and due to the disintegration of the older one in the absence of light

Regulation of stomatal opening. Explain

• Stomata are minute openings occuring in large numbers on the lower surface of a leaf

• The main function of this is to let in CO₂ from the atmosphere into the leaf for photosynthesis

• When stomata are not in use for photosynthesis, i.e. when it is dark, they tend to close their openings so that water loss is minimalised from the leaves through transpiration

• When the light reappears, they reopen to allow CO₂ to diffuse in

• Transpiration occurs along with photosynthesis

• Due to this process, one can say that transpiration is the price which the plant pays for photosynthesis

• The closing and opening of stomata are on account of the movement of water in and out of the guard cells

• They have a thicker inner wall facing the opening and a thin outer wall on the oposite side

• Their cytoplasm contains chloroplasts

Opening and Closing of the Stomata

2 theories:

(1) Potassium ion concentration theory (recent)

(2) Sugar concentration theory (old)

Explain the process of Photosynthesis

• The mesophyll cells (both palisade and spongy) in a leaf are the principle centres of this activity

• During daytime, when sunlight falls on the leaf, the light energy is trapped by the chlorophyll of the upper layers of mesophyll, especially the palisade cells

• This energy is utilized in chemical processes involved in the manufacture of food, where the raw materials are CO₂ and H₂O

• Carbondioxide from the atmosphere enters the leaf by diffusion down a concentration gradient (higher conc. outside the leaf, and less conc. inside) through the stomata

• Water from the soil is taken up by the roots, sent up through the stem and finally to the leaves, where it is dictributed in the mesophyll tissue

• 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

  On light energy and chlorophyll

• Glucose molecule - simple sugar and soluble in water

• The 6 molecules of H₂O liberated at the end of the process are those that are re-formed during a chain on reactions and not out of the original ones

What are the two main phases of photosynthesis?

(1) Light-Dependant Phase

(2) Light-Independant Phase

Explain the Light-Dependant Phase

In textbook page no. 72

Explain the Light-Independant Phase

In textbook page no. 72 and 73

Adaptations of leaves to perform Photosynthesis

1. Large Surface Area - for max. absorption (Narrowing of leaves in certain plants to minimalize transpiration in order to conserve water, as in oleander and pine)

2. Leaf Arrangement - right angle to the light source to obtain max. light

3. Cuticle and Upper Epidermis - are transparent and water proof to allow light to enter freely

4. Numerous Stomata - allow rapid exchange of gases (oxygen and carbondioxide)

5. The thinness of leaves - reduces dist. between cells facilitating rapid transport

6. Chloroplasts - are conc. in the upper layers if the leaf to obtain light energy quickly

7. Extensive Vein System - for rapid transport to and from the mesophyll cells

End products of photosynthesis with their fate

1. GLUCOSE: Simple sugar

   (i) immediately consumed by plant cells

   (ii) stored in the form of insoluble starch

   (iii) converted into sucrose

   (iv) used in synthesising fats, proteins, etc.

2. WATER: Re-utilized in the continuance of Photosynthesis

3. OXYGEN:

   (i) some may be used in respiration in the leaf cells (the phenomenon is called photorespiration)

   (ii) major part is not required and hence, diffuses into the atmosphere through the stomata

Utilisation of Synthesized Food and its translocation

In textbook page no. 74

What are the four external factors that affect Photosynthesis?

In textbook page no. 74

What are the four internal factors that affect Photosynthesis?

In textbook page no. 74 and 75

What are the Dawn-to-Dusk- natural changes in environmental conditions and photosynthesis?

In textbook page no. 75

Plants which perform Photosynthesis even at night. Explain

In textbook page no. 75