Photosynthesis - Energy for Life

Photosynthesis

• Transforms solar energy into the chemical energy of carbohydrates.
• Occurs in plants, algae, and cyanobacteria.
• Producers feed themselves and all consumers.
• It occurs usually within chloroplasts.
• Chlorophyll-containing organelles trap solar energy to reduce carbon dioxide to carbohydrate.
• Converts light energy into chemical energy, stored in sugars or other organic compounds.
• Takes place in green portions of plants; carbon dioxide enters through stomata, water is absorbed by roots.
• $CO<em>2$ and $H</em>2O$ diffuse into mesophyll cells and then into chloroplasts.

Chloroplast

• Double membrane surrounds stroma.
• The third membrane forms thylakoids (Grana—stacks).
• Chlorophyll and other pigments reside within the thylakoid membrane and absorb solar energy.
• Carbon dioxide will be reduced in the stroma into carbohydrates.
• Glucose is the chief organic energy source for most organisms.

Light Energy

• Pigment molecules absorb solar energy.
• Solar energy is described by wavelength and energy content.
• Shorter wavelengths contain more energy; longer wavelengths contain less.
• Vision and photosynthesis use visible light.

Photosynthetic Pigment Molecules

• Most photosynthesizing cells have chlorophylls and carotenoids.
• Chlorophyll a and b absorb violet, blue, and red wavelengths better.
• Leaves appear green because green is reflected.
• Accessory pigments (carotenoids) appear yellow or orange because they reflect those colors.

Photosynthetic Process

• Begins with $CO<em>2$ and $H</em>2O$.
• Hydrogen atoms from water are added to carbon dioxide using solar energy.
• Oxygen is a by-product of the oxidation of water.
• The end product is glucose $(C<em>6H</em>{12}O_6)$.
• $(CH<em>2O) + O</em>2 + \text{Solar energy}$
• Oxidation: $H_2O$
• Reduction: $CO<em>2$ to $(CH</em>2O)$

Reactions

• Light reactions:
  • Occur in the thylakoid membrane.
  • Chlorophyll absorbs solar energy.
  • Water is oxidized, releasing electrons, hydrogen ions, and oxygen.
  • ATP is produced in the electron transport chain.
  • $NADP^+$ becomes $NADPH$.
• Calvin cycle reactions:
  • Occur in the stroma.
  • $CO_2$ is taken up.
  • ATP and NADPH are used to reduce $CO_2$ to a carbohydrate.

Light Reaction

• Two photosystems (PSII and PSI) are used.
• Consist of a pigment complex and an electron acceptor.
• Electrons capture the sun’s energy and store it in the form of a hydrogen ion $(H^+)$.
• Gradient used to produce ATP; NADPH is also produced.
• Photosystem II:
  • Absorption of solar energy energizes electrons.
  • Electrons escape to an electron acceptor molecule and are sent through the electron transport chain.
  • Replacement electrons are obtained by splitting water, releasing oxygen gas.
• Photosystem I:
  • Absorption of solar energy energizes electrons.
  • Electrons are captured by another electron acceptor molecule.
  • Electrons and a hydrogen are passed to $NADP^+$ to become NADPH.
  • Replacement electrons come from the electron transport chain.

Thylakoid Membrane

• PSII, PSI, and the electron transport system are located within the thylakoid membrane.
• ATP synthase complex is also located here.

ATP Production

• The thylakoid space is a reservoir for $H^+$.
• Each time water is split, 2 $H^+$ remain in thylakoid space.
• Energy from electrons is used to pump more $H^+$ from the stroma into the thylakoid space.
• This establishes an $H^+$ gradient, creating potential energy.
• $H^+$ flows down the concentration gradient through the ATP synthase complex, producing ATP.
• $NADP^+$ accepts electrons and $H^+$ to become NADPH.