Photosynthesis and Cell
Respiration
How do organisms obtain energy?
• Autotrophs
– Able to produce their own food
• Ex. Plants use sunlight to produce energy
• Photosynthesis
• Heterotrophs
– Unable to produce their own food, rely on
other organisms as a source of energy
• Ex. Animals need to eat to survive
• Cellular Respiration
Organisms rely on PHOTOSYNTHESIS and
CELLULAR RESPIRATION for their energy needs
Heat
Sunlight
Photosynthesis
Cellular
respiration
Visual Summary 6.1
Photosynthesis!
• What happens during photosynthesis?
– Plants (autotrophs) use energy from sunlight to
convert water and carbon dioxide into
carbohydrates (energy) and oxygen
• 6CO2 + 6H2O (light) → C6H12O6 + 6O2
Reactants?
Carbon dioxide (CO2) and Water (H2O)
Products?
Glucose—Sugar (C6H12O6) and Oxygen (O2)
Photosynthesis: Light and Pigment
• In addition to water and carbon
dioxide, photosynthesis needs
light and chlorophyll (a molecule
in chloroplasts)
• Light is converted into chemical
energy
• Occurs in chloroplasts
• Chlorophyll absorbs the light (it
makes plants green!)
The Two Parts of Photosynthesis
• Light-dependent
reactions
– Need light to happen
– Occurs in Chloroplast
– Produces Oxygen
• Light-independent
reactions (Calvin Cycle)
– Does not need light
– Use energy molecules
(NADPH) to produce
sugars for the plant
Cellular Respiration
• Heterotrophs need to
consume other
organisms in order to
survive
– Glucose (sugar) is a
main source of this
energy
Cellular Respiration
Cellular respiration… is the process that releases
energy by breaking down food molecules
(glucose) in the presence of oxygen
6O2 + C6H12O6  6CO2 +
Oxygen + glucose 
6 H2O + Energy
carbon dioxide + water
+
energy
Compare this to the equation for photosynthesis….what do you notice?
Cellular Respiration
• Occurs in three different steps:
– Glycolysis: one molecule of glucose is split in half
• Occurs in the cytoplasm
– Krebs Cycle: the half-glucose molecules are broken
down into carbon dioxide
• Occurs in the Mitochondria
– Electron Transport: high energy electrons are used to
produce energy for the cell
• Occurs in the Mitochondria