Bond energy

  • The same quantity of energy is involved when a particular bond is being broken or formed.
  • The atoms in small inorganic molecules like water and carbon dioxide are joined by strong bonds, meaning that they require a lot of energy to break and release a lot when they form.
  • Organic molecules like glucose contain many more bonds; these are weaker by comparison.
  • They release less energy when they form and require less energy to be broken.



An represents light in this equation

6CO₂ + 6H₂O C₆H₁₂O₆+6O₂

Autotrophs and Heterotrophs

  • Autotrophs such as plants can synthesise complex organic molecules such as carohydrates, lipids, proteins, nucleic acids and vitamins from simple inorganic molecules like CO₂ and energy sources.
  • Organisms that can photosynthesise are described as photoautotrophs


  • Nitrifying bacteria get energy from reactions such as oxidising nitrite to nitrate
  • Bacteria living in darkness by thermal ocean vents


  • Cannot make their own complex organic molecules from inorganic small molecules like CO₂ and H₂O but feed on and digest complex organic molecules into simpler soluble ones.


  • 2-10 µm in length
  • Usually disk shaped (sometimes varying)
  • Surrounded by an envelope or double membrane
  • Outer membrane is permeable to many small ions
  • The inner membrane is less permeable and has embedded in it folded into llamellae (thin plates) which are stacked vertically. Each stack is called a granum.
  • Between grana are integranal llamellae

Photosynthetic Pigments

  • Absorb certain wavelengths of light

  • Funnel-shaped structures called photostems push light to a point

  • Proteins embedded in grana hold photostems in place

  • The fluid-filled stroma contains enzymes needed to catalyse reactions of the light-dependent stage of photosynthesis

  • Many grana, consisting of stacks of 100 thylakoid membranes provides a large surface area for the photosynthetic protein.

  • Grana surrounded by stroma so products of the light-dependent reaction (needed for light-independent reaction) can readily pass into stroma.

  • LIR - light independent reaction

  • LDR - light dependent reaction

  • Chloroplasts can make some of their proteins they need for photosynthesis using genetic instructions in chloroplast DNA, and the chloroplast ribosomes assemble the proteins.

  • Leaves contain a variety of pigments

  • Each absorbs a different range of light wavelengths

  • Maximises the amount of sunlight that can be absorbed