Photosynthesis: Light and Chlorophyll

• Photosynthesis converts light energy into chemical energy.
• Plants synthesize organic compounds (e.g., glucose) from inorganic compounds (CO2 and H2O) using sunlight.
• This is a two-step process.
  • Light-dependent reactions: convert light energy into chemical energy (ATP).
  • Light-independent reactions: use chemical energy to synthesize organic compounds (e.g., glucose).
• Organic molecules from photosynthesis are used in cellular respiration to provide energy for the organism.

Sunlight

• Sunlight is white light, composed of all colors of the visible spectrum.
• Colors are different wavelengths of light, ranging from ~400 nm to 700 nm.
• The visible spectrum colors (longer to shorter wavelength): Red, Orange, Yellow, Green, Blue, Indigo, Violet (R.O.Y.G.B.I.V.).

Chlorophyll

• Chlorophyll is the main photosynthetic pigment, with different molecules having unique absorption spectra.
• Chlorophyll is the primary site of light absorption.
• When chlorophyll absorbs light energy, it releases electrons to make ATP (chemical energy).

Light Absorption

• Chlorophyll mainly absorbs red and blue light.
• Chlorophyll reflects green light, which is why leaves appear green.
• Deciduous trees reduce chlorophyll production in winter due to less sunlight, allowing other pigments (e.g., xanthophylls, carotenoids) to change the leaf color.

Light Dependent Reactions

• Light energy produces ATP.
• Light splits water molecules (photolysis) into oxygen and hydrogen.
• Light stimulates chlorophyll to release electrons, producing ATP.
• Photolysis produces oxygen and hydrogen:
  • Hydrogen is taken up by NADP+ to form NADPH.
  • Splitting water releases electrons, replacing those lost by chlorophyll.
• ATP and hydrogen (NADPH) are used in light-independent reactions.

Light Independent Reactions

• ATP and hydrogen (from NADPH) fix carbon molecules to create organic molecules.
• These reactions produce more complex organic molecules (e.g., sugars).
• Organic molecules can be stored for cellular respiration.

Measuring Photosynthesis Rate

• The rate can be measured directly by oxygen production or carbon dioxide uptake, or indirectly by measuring an increase in biomass.
• Water cannot be measured because it is involved in many things.

Measuring CO2 Uptake

• Place a plant in an enclosed space with water.
• CO2 interacts with water, producing bicarbonate and hydrogen ions, increasing acidity.
• The change in pH measures CO2 uptake (increased CO2 uptake = more alkaline pH).

Measuring O2 Production

• Submerge a plant in an enclosed space with water attached to a sealed gas syringe.
• Oxygen gas produced bubbles out and is measured by the change in water level.

Measuring Biomass (Indirect)

• Glucose production can be indirectly measured by changes in plant weight.
• The plant should be dehydrated before weighing.
• Glucose production can be determined by measuring the change in starch levels.
• Starch is identified via iodine staining (turns purple) and quantitated using a colorimeter.

Factors Affecting Rate

• Factors include temperature, light intensity, and carbon dioxide concentration.

Temperature

• Photosynthesis is controlled by temperature-sensitive enzymes.
• As temperature increases, the rate of photosynthesis increases until enzymes denature above a certain temperature.

Light Intensity

• As light intensity increases, the rate of photosynthesis increases to a maximum.
• Further increases in light intensity will not affect the rate, as chlorophyll becomes saturated.
• Different wavelengths of light have different effects (e.g., green light is not used).

CO2 Concentration

• As CO2 concentration increases, photosynthesis rate increases to a maximum.
• Further increases in CO2 concentration will not affect the rate, as carbon fixation enzymes become saturated.