Photosynthesis

Life in the Sun

Light is central to the life of a plant. Photosynthesis is the most important

chemical process on Earth. Autotrophs are the producers of the biosphere.▪ Plants, some protists, and some bacteria are

photosynthetic autotrophs. ▪ They produce the O2 for the atmosphere.▪ They provide food consumed by virtually all

organisms. Autotrophs convert light energy into chemical

signals.

Life in the Sun

Light can influence the architecture of a plant. Plants that get adequate light are often

bushy with deep green leaves. Without enough light, plants become tall

and spindly with small pale leaves. Too much sun can damage a plant.

Photosynthesis Overview Photosynthesis is the process by

which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide.

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS

Photosynthesis- The Cell Structure In most plants,

photosynthesis occurs primarily in the chloroplasts of the leaf cells. A chloroplast contains:▪ Stroma (a fluid)▪ Grana (stacks of thylakoids)▪ Thylakoids (contain chlorophyll)▪ Chlorophyll (green pigment that

captures certain wavelengths of light for energy)

Photosynthesis- The Process sunlight CO2 + H2O C6H12O6 + O2 Two linked sets of reactions:

Light Reactions▪ Convert light energy into chemical energy using

water▪ Produce O2

Calvin Cycle▪ Use chemical energy from light reaction▪ Produce sugar molecules from CO2

Phase 1 – Light Reactions

Take place in the thylakoids which are stacked into grana

Light-absorbing pigments (chlorophyll) are found in the thylakoids.

Phase 1 – Light ReactionsThylakoids

GOAL: CHANGE LIGHT ENERGY INTO CHEMICAL ENERGY

Absorption of light is the first step in photosynthesis.

Pigments in the thylakoid capture light energy. The light energy causes: Electrons to become excited in Photosystem II. A water molecule to split.

An electron to be released for use later. H+ to be released for use later. O2 to be released as a waste product.

Phase 1 – Light ReactionsThylakoids

Following the electrons: Excited electrons move to an electron-

acceptor molecule in the thylakoid membrane. The acceptor molecule moves the electrons

along a series of carriers to Photosystem I. Photosystem I moves the electrons to a

protein, which transfers the electrons to a final carrier: NADP+

An energy-storing molecule NADPH is formed. Another energy-storing molecule ATP is also

formed.

Phase 2 – The Calvin CycleStroma

GOAL: USE CHEMICAL ENERGY TO PRODUCE SUGAR

Carbon fixation: 6 CO2 molecules combine with 6 5-carbon compounds to form 12 3-carbon molecules called 3-phosphoglycerate (3-PGA).

Chemical energy from ATP and NADPH is transferred to the 3-PGA molecules to form 12 high energy molecules called glyceraldehyde 3-phosphates (G3P).

Two G3P molecules leave the cycle to be used for sugar production.

Finally, an enzyme called rubisco converts the remaining 10 G3P into 5-carbon molecules to combine with future CO2 to continue the cycle.

Phase 2 – The Calvin CycleStroma

An overview of photosynthesis

Light

Chloroplast

LIGHTREACTIONS

(in grana)

CALVINCYCLE

(in stroma)

Electrons

H2O

O2

CO2

NADP+

ADP+ P

Sugar

ATP

NADPH