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!