Photosynthesis Notes

Photosynthesis

Photosynthesis is the fundamental process by which plants manufacture food molecules (carbohydrates) from raw materials (CO2 and H2O) using energy from light.

The equation for photosynthesis is:
$6CO<em>2 + 6H</em>2O + \text{energy from light} \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$

Carbon dioxide + water + energy from light produces glucose and oxygen.

Photosynthesis in Flowering Plants

Photosynthesis takes place in the green portions of plants.

• The leaf of the flowering plant contains mesophyll tissue.
• Cells containing chloroplasts are specialized to carry out photosynthesis.

The raw materials for photosynthesis are carbon dioxide and water.

• Roots absorb water that moves up vascular tissue.
• Carbon dioxide enters a leaf through small openings called stomata and diffuses into chloroplasts in mesophyll cells.
• The thylakoid membranes of chloroplasts contain chlorophyll and other pigments that can absorb the solar energy that drives photosynthesis.
• Electrons are energized in the process.
• Then, carbon dioxide is reduced to form a carbohydrate.
• In the stroma, $CO<em>2$ combines with $H</em>2O$ to form $C<em>6H</em>{12}O_6$ (sugar).

Leaves and Photosynthesis

Leaf cross-section includes:

• Cuticle
• Upper epidermis
• Mesophyll
• Lower epidermis
• Stoma
• Leaf vein

Chloroplast structure:

• Outer membrane
• Inner membrane
• Stroma
• Thylakoid membrane
• Thylakoid space
• Granum (stack of thylakoids)

Autotrophs and Heterotrophs

Autotrophs are organisms that can produce their own food using photosynthesis.

Heterotrophs are organisms that obtain food from other organisms.

The Process of Photosynthesis

Photosynthesis consists of two main stages:

• Light Reactions
• Calvin Cycle Reactions

Light Reactions

• Take place only in the presence of light.
• Also known as photochemical reactions.
• Are fast energy-capturing reactions.
  • Chlorophyll absorbs solar energy.
  • Split water to release oxygen, electrons, and protons.
  • Energizes electrons.
  • Electrons move down an electron transport chain.
    • Pumps $H^+$ into thylakoids.
    • Used to make ATP out of ADP and NADPH out of NADP.

Calvin Cycle Reactions

• Take place in the stroma.
• $CO_2$ is reduced to a carbohydrate.
• Use ATP and NADPH to produce carbohydrate.

Photosynthesis involves oxidation and reduction.

• Oxidation is the loss of electrons, and reduction is the gain of electrons.
• In photosynthesis, carbon dioxide is reduced, and water is oxidized.

Oxidation and Reduction in Photosynthesis

$CO<em>2 + H</em>2O \rightarrow (CH<em>2O) + O</em>2$

• $CO<em>2$ is reduced to $(CH</em>2O)$.
• $H<em>2O$ is oxidized to $O</em>2$.

Overview of Photosynthesis

Light reactions occur in the thylakoid membrane:

• $H<em>2O$ is converted to $O</em>2$.
• ADP + P are converted to ATP.
• NADP+ is converted to NADPH.

Calvin cycle reactions occur in the stroma:

• $CO<em>2$ is converted to $CH</em>2O$.
• ATP and NADPH are used in the process.

Cellular Respiration

Electrons are removed from substrates and received by oxygen, which combines with $H^+$ to become water.

Glucose is oxidized and $O_2$ is reduced.

$C<em>6H</em>{12}O<em>6 + 6O</em>2 \rightarrow 6CO<em>2 + 6H</em>2O + \text{energy}$

• Glucose ($C<em>6H</em>{12}O_6$) is oxidized.
• Oxygen ($6O_2$) is reduced.

Photosynthetic Pigments

• Pigments are chemicals that absorb certain wavelengths of light.
• Wavelengths that are not absorbed by pigments are reflected or transmitted.

Primary Photosynthetic Pigments

• Chlorophyll a & b are the primary pigments in green plants that absorb red and blue/violet light and reflect green light.

Accessory Pigments

• Xanthophyll and carotene.
• Carotenoids are accessory pigments that absorb light in the violet-blue-green range and reflect yellow and orange light.
• Each pigment absorbs a particular wavelength of light in the visible spectrum.

Chlorophyll Production

The decomposition rate of chlorophyll remains constant, so the green color starts to fade from leaves.

Light regulates chlorophyll production, so as autumn days grow shorter, less chlorophyll is produced.

Photosynthetic Pigments (Absorption Spectrum)

• Chlorophyll a
• Chlorophyll b
• Carotenoids

Light Reactions: Noncyclic Electron Pathway

Photosystem II

• $H<em>2O$ is split into 2H+ and $O</em>2$
• Solar energy is captured.

Electron transport chain

• Generates ATP.

Photosystem I

• Generates NADPH.

ATP and NADPH are used in the Calvin cycle to convert $CO<em>2$ to $CH</em>2O$.

Photosynthetic Reactions: Overview

Light Reaction

• Chlorophyll absorbs solar energy.
• This energizes electrons.
• Electrons move down the electron transport chain.
  • Pumps $H^+$ into thylakoids.
  • Used to make ATP out of ADP and NADPH out of NADP.

Calvin Cycle Reaction

• $CO_2$ is reduced to a carbohydrate.
• Reduction requires the ATP and NADPH produced in the light reaction.