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Photosynthesis
The Big Picture
Photosynthesis and Respiration work
together in plants to make energy for
the plant – they are autotrophs
Animals only use cellular respiration –
why?
They are heterotrophs!
What is energy?
In cells, energy is stored as ATP (adenosine triphosphate)
ATP delivers energy to other cells
The energy in ATP is released and used for many chemical processes
When the energy is released, ATP becomes ADP – it loses a phosphate atom.
Therefore, the energy is stored in the phosphate bond!
ATP ADP + P + energy
ATP
3 phosphate
groups
Pentose
Sugar (ribose)
adenine
Does this structure look familiar?
Photosynthesis
Definition: process by which light energy is
converted to chemical energy
In English: Plants use light, carbon dioxide
and water to make sugar (food) and oxygen.
ATP is used as a fuel source to power
photosynthesis (like gas powers your car)
Where does
photosynthesis occur?
Photosynthesis occurs in the leaves of the
plants; special pores of the plants called
stomata open to take in carbon dioxide and
release oxygen
3 Stages of
Photosynthesis
Stage 1 – Energy is captured from sunlight
Stage 2 – Light energy is converted to
chemical energy, which is temporarily stored
in ATP and the energy carrier molecule
NADPH
Stage 3 – Chemical energy in ATP and
NADPH powers the formation of organic
compounds (sugars) using carbon dioxide
Photosynthesis:
Stage One
The leaves of a plant contain light-absorbing
substances called pigments
The main pigment of a plant is chlorophyll,
which is what makes plants look green.
The other pigments are called carotenoids
These pigments are located in the
chloroplasts of a plant cell
Photosynthesis:
Stage One
Chloroplasts contain stacks of disk-like
structures called thylakoids
Light energy hits the thylakoid and the energy is
transferred to electrons in chlorophyll; the
electrons get “excited”
Excited electrons leave chlorophyll and are
replaced by electrons in water
The water is split by an enzyme and electrons
from the hydrogen atom are taken by the
chlorophyll. The oxygen atoms from different
water molecules combine to make oxygen (O2)
Electrons Jumping
Photosynthesis:
Stage Two
The electrons (energy) from stage one is used to make ATP
The ATP is made by a process called the Electron Transport Chain
The electrons from stage one are passed through several molecules in the membrane of the thylakoid.
Another energy molecule called NADPH is also made in the electron transport chain.
Electron Transport Chain Animation
Photosynthesis:
Stage Two
Stage two makes ATP and NADPH; these are both important because they are needed for stage three.
ATP is used to power Stage Three
NADPH is used to make bonds in Stage Three
Stage Two is also known as the light dependent reactions – they need light to work
Photosynthesis:
Stage Three
In this stage, carbon dioxide molecules are used to make organic compounds (sugars) where chemical energy is stored.
This process is called carbon dioxide fixation
The most common method of carbon dioxide fixation is the Calvin cycle
The Calvin Cycle
Step one: each carbon dioxide molecule is
added to a 5-carbon compound by an
enzyme – this makes a 6-carbon compound
Step two: the 6-carbon compound splits into
two 3-carbon compounds. Then, phosphate
groups from ATP and electrons from
NADPH are added to turn the 3-carbon
compounds into 3-carbon sugars
The Calvin Cycle
Step three: one of the 3-carbon sugars is
used to make organic compounds (starch
and sucrose) – energy is stored in those
compounds for use later
Step four: the other 3-carbon sugars are
used to regenerate the 5-carbon compound,
completing the cycle and allowing it to start
over again
Factors that affect
Photosynthesis
Light
Carbon dioxide concentration
Water
See the whole cycle!
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