2.4 - Main Organ for Photosynthesis

Necessity of Photosynthesis in Plants

• Plants are autotrophs that produce glucose through photosynthesis.
• Glucose is oxidised to generate energy for life processes such as growth and reproduction.
• Leaves are the main photosynthetic organs; young stems and other green plant parts also carry out photosynthesis.

Adaptations of the Internal Leaf Structure for Photosynthesis

• Upper and lower epidermis: transparent, waxy cuticle allows sunlight to reach the palisade mesophyll; stomata in the lower epidermis allow gaseous exchange when open.
• Palisade mesophyll: closely packed with chloroplasts to absorb maximum sunlight; chlorophyll absorbs light energy.
• Spongy mesophyll: contains chloroplasts and many air spaces for efficient gaseous exchange.
• Vascular bundle: xylem transports water and mineral salts to the leaf; phloem transports sucrose from the leaf to the rest of the plant.

Structure of a Chloroplast

• Thylakoid: disc-shaped sac bearing photosynthetic pigments on its surface; site of the light-dependent reaction.
• Granum (plural: grana): stack of thylakoids that increases surface area for photosynthesis.
• Stroma: colourless fluid containing enzymes and starch granules; site of the light-independent reaction.
• Lamellae: structures connecting the grana.
• Leaf photosynthetic pigments include chlorophyll a, chlorophyll b, carotenoid, xanthophyll, and pheophytin.

Light-Dependent and Light-Independent Reactions

Light-Dependent Reaction

1. Occurs in the thylakoids.
2. Photosynthetic pigments on the thylakoid surface absorb light energy.
3. Light energy excites electrons in chlorophyll pigments to a higher energy level.
4. Excited electrons pass through a series of electron carriers; their energy generates ATP.
5. NADP${}^{+}$ accepts the electrons and combines with H${}^{+}$ from photolysis to form NADPH, a reducing agent.
6. Chlorophyll obtains replacement electrons from water through photolysis.
7. Photolysis breaks water into hydrogen ions ($H^{+}$) and hydroxide ions ($O{H}^{-}$) in the presence of light energy and chlorophyll.
8. Hydroxide ions lose electrons and form oxygen and water.

Light-Independent Reaction

1. Occurs in the stroma.
2. A 5-carbon organic compound fixes carbon dioxide to form a 6-carbon organic compound.
3. NADPH and ATP from the light-dependent reaction reduce the organic compound to glucose monomers.
4. Glucose monomers condense to form starch; starch granules are stored in the chloroplast stroma.

Overall Equation for Photosynthesis

$12{\text{H}}_2\text{O}+6{\text{CO}}_2\stackrel{\text{light energy, chlorophyll}}{\to }{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2+6{\text{H}}_2\text{O}$

Comparison of the Two Reactions

Similarities:

• Both occur in chloroplasts.
• Both are catalysed by enzymes.

Aspect	Light-dependent reaction	Light-independent reaction
Site	Thylakoids	Stroma
Reactant	Water	Carbon dioxide
Main process	Photolysis of water	Reduction of carbon dioxide
Product	Oxygen and water	Glucose
ATP	Produced	Used

Environmental Factors Affecting the Rate of Photosynthesis

Carbon Dioxide Concentration

• Increasing carbon dioxide concentration increases the rate until another factor, such as temperature or light intensity, becomes limiting.
• Beyond that point, a further increase in carbon dioxide concentration does not increase the rate.

Light Intensity

• Increasing light intensity increases the rate until the light saturation point.
• Beyond the saturation point, temperature or carbon dioxide concentration limits the rate.

Temperature

• Photosynthetic reactions are catalysed by enzymes, so temperature affects the rate.
• The optimum temperature differs among plant species but is generally $25^{\circ }\text{C}$ to $30^{\circ }\text{C}$.
• Excessively high temperature denatures enzymes and stops photosynthesis.

Investigating Environmental Factors

• Use a cut shoot of Hydrilla sp. in sodium hydrogen carbonate solution and count oxygen bubbles released in 5 minutes.
• Investigate one factor at a time: distance from a lamp for light intensity, water-bath temperature, or sodium hydrogen carbonate concentration for carbon dioxide concentration.
• Keep the Hydrilla species and size, lamp voltage, and all non-investigated factors constant.
• Take three readings and calculate the mean number of bubbles for each level of the manipulated variable.
• Plot mean bubble count against distance, temperature, or sodium hydrogen carbonate concentration to infer the effect on photosynthesis rate.

Effects of Light Intensity and Light Colour

• The photosynthesis rate changes with light intensity and light colour.
• The visible spectrum is violet, indigo, blue, green, yellow, orange, and red; each colour has a different wavelength.
• Photosynthesis rate is highest in red and blue light.
• Chlorophyll absorbs red light; carotenoid absorbs blue light and transfers its energy to chlorophyll.
• Red and blue light provide sufficient energy to excite electrons in the light-dependent reaction.
• Chlorophyll reflects green light, so chlorophyll-containing leaves appear green.