• Cells require energy to survive; they can either obtain this by themselves (autotrophs) or by consuming other organisms (heterotrophs)

• Use light energy to synthesis organic molecules from inorganic substances.
• Typically occurs in a process known as photosynthesis.
  
  carbon dioxide + water -> glucose + oxygen

• Obtain organic molecules from consumption of other organisms.
• Begins with the consumption of a 'primary producer' by a 'primary consumer', and ends with the decomposition of the apex predator.
• Energy is obtained through aerobic respiration.
  
  glucose + oxygen -> carbon dioxide + water

Fermentation

• Fermentation occurs when there is insufficient oxygen to perform aerobic respiration, causing anaerobic respiration/fermentation.
• In human cells, this forms lactic acid - this acid is the cause of the burning sensation when strenuous exercise occurs.
• In plant cells, this forms ethanol and carbon dioxide.