Photosynthesis Notes

Photosynthesis

• Photosynthesis is the process where plants use light energy to produce glucose (C6H{12}O6) and oxygen gas (O2).

  • The overall reaction involves converting a gas (CO2) and a liquid (H2O) into a solid (glucose) and a gas (O_2).
  • This process rearranges these substances using the energy from light.

Energy Storage and Endergonic Reactions

• The light energy is stored in the glucose molecule.
• Photosynthesis is an endergonic reaction because it requires energy, specifically solar energy from the sun.
• The process is also anabolic, building a larger energy source.
• Dozens of intermediate steps occur that are not covered in detail.

Light Spectrum and Color Perception

• White light emits a full spectrum of energy, including visible light.
• The full spectrum includes X-rays, gamma rays, radio waves, microwaves, infrared, ultraviolet, and visible light.
• Visible light is a narrow band within this spectrum that our eyes can detect as color.
  • Color itself isn't a physical entity but is interpreted by our brain based on the wavelengths of energy received by receptors in our eyes.
  • Individual differences can exist in color perception.

Chlorophyll and Pigments

• Chlorophyll absorbs specific wavelengths of light and reflects others.
  • The reflected wavelengths are what we perceive as color (e.g., green).
  • Molecules have specific shapes that determine which colors they absorb and reflect.
• Photosynthetic pigments:
  • Chlorophyll a: primary photosynthetic pigment
  • Chlorophyll b: a minor pigment, absorbs slightly different wavelengths than chlorophyll a.
  • Carotenoids and Xanthophylls: other accessory pigments.
• A pigment, such as chlorophyll, reflects color and uses light energy to drive photosynthesis reactions.

Hemoglobin

• Hemoglobin is a pigment found in blood that carries oxygen and has a greenish color.

Heterotrophs

• Heterotrophs are organisms that obtain energy by consuming other organisms.
  • Herbivores: eat only plants.
  • Omnivores: eat both plants and animals.
  • Carnivores: eat only meat.

Photosynthesis Equation and Balancing

• Photosynthesis converts carbon dioxide (CO2) and water (H2O) into glucose (C6H{12}O6) and oxygen (O2).
• The balanced equation is: 6CO2 + 6H2O \rightarrow C6H{12}O6 + 6O2

Chloroplast

• The inner membrane resembles a bacteria's membrane, supporting the theory that chloroplasts were once bacteria.
• The stroma is the space within the inner membrane, similar to a bacteria's cytoplasm, containing ribosomes.
• Thylakoids are individual sac-like structures within the stroma.
• Granum is a stack of thylakoids.

Photosynthesis Stages

• Photosynthesis is divided into two main stages:
  • Light Reactions (or Light-Dependent Reactions):
    • Require light.
    • Involve chlorophyll in the thylakoids.
    • Water is split into hydrogen atoms and oxygen atoms.
    • Light energy excites electrons, raising them to a higher energy state.
    • Oxygen is formed and released.
  • Carbon Reactions (or Light-Independent Reactions/Dark Reactions):
    • Involve carbon dioxide.
• Light splits water molecules (H_2O) into hydrogen atoms and oxygen atoms.
  • Two water molecules yield four hydrogen atoms.

Electron Carriers

• NADP+ (Nicotinamide Adenine Dinucleotide Phosphate) acts as an intermediate electron carrier.
  • It accepts electrons and hydrogen ions to become NADPH.

inputs and outputs of light and carbon reactions

• Inputs to Light Reactions: Light, Water
• Outputs to Light Reactions: Oxygen, ATP. NADPH
• Inputs to Carbon Reactions: Carbon Dioxide, ATP, NADPH
• The hydrogen from water becomes part of glucose during the carbon reactions.
• The oxygen in the atmosphere comes from the splitting of water during photosynthesis, mainly by algae in the oceans.

Origin and Impact of Oxygen

• Early Earth's atmosphere lacked oxygen.
• Oxygen was initially a waste product of photosynthesis and was poisonous to early organisms.
• The buildup of oxygen led to adaptations, evolution, and the formation of the ozone layer (O_3), which blocks ultraviolet light.

ATP (Adenosine Triphosphate)

• ATP is a molecule, essentially an RNA nucleotide with adenine as its base and three phosphate groups.
• It's recycled and repurposed by the cell for various functions.
• Adenosine is the base (adenine) plus the ribose sugar.
• AMP (Adenosine Monophosphate) is an RNA nucleotide with adenine as its base and one phosphate group.
• Energy is stored in the bonds between the phosphate groups.

Phosphate Groups and Energy Storage

• Phosphate groups are negatively charged.
• Electrostatic repulsion exists between these negatively charged phosphate groups but covalent bonds are stronger.
• Adding more phosphate groups increases repulsion.
• ATP acts like a loaded crossbow, storing potential energy in its bonds.
  • Breaking a phosphate bond releases energy that can be used to drive chemical reactions.

ATP Functions

• Phosphorylating means adding a phosphate group.
• Mechanical functions: ATP changes the shape of enzymes.
• ATP is essential for life.
  • Drives almost all embryonic reactions.
  • Cells don't stockpile ATP and have only a 2-minute supply.
  • ATP is not shared between cells.