The Reactions of Photosynthesis

8-3

Inside a Chloroplast

• Where photosynthesis takes place

• Contains thylakoids, which are sac-like photosynthetic membranes

• Thyalkoids are arranged in stacks called grana

• Thylakoids contain proteins which organize chlorophyll and other pigments into clusters called photosystems– Photosystems are light-collecting units of

chloroplasts

• The reactions of the photosystems are divided into 2 types:– Light-dependent reactions– Light-independent reactions, or Calvin

Cycle

• Light-dependent reactions take place in the thylakoid membranes

• Light-independent reactions take place in the stroma, or region outside the thylakoids

Electron Carriers

• When sunlight strikes chlorophyll, it transforms the electrons into a high-energy state

• These high-energy electrons are like hot coals and need a special electron carrier

• Carrier molecules are a compound capable of transporting these high-energy electrons, along with most of their energy to other molecules

• This process is known as electron transport

• Electron carriers are known as electron transport chain

• The molecule NADP+ serves as an electron carrier– Its job is to hold 2 high-energy electrons along with

a hydrogen ion (H+)

• This converts NADP+ to NADPH– This conversion is one way in which some energy

from sunlight is trapped in chemical form

• NADPH then transfers the high-energy electrons to chemical reactions elsewhere in the cell

Light-Dependent Reactions

• Require light to happen

• Converts ADP to ATP and NADP+ to NADPH

• Produces oxygen gas

Step A

• Photosystem II absorbs light

• Light energy is absorbed by electrons, increasing energy levels

• Electrons are passed to electron transport chain (ETC)

• Enzymes on inner surface of thylakoid membrane break up each water molecule into 2 electrons, 2 H+ ions, and 1 oxygen atom– 2 e- replace 2 high-energy e- lost to ETC– Oxygen released to air– H+ ions are released inside the thylakoid

membrane

• This reaction is source of nearly all oxygen in the atmosphere

Step B

• High-energy electrons move through ETC from photosystem II to photosystem I

• Energy from the e- is used by ETC to transport H+ ions from stroma to inner thylakoid space

Step C

• Pigments in photosystem I use energy from light to reenergize e-

• NADP+ picks up high-energy electrons and H+ions to become NADPH

Step D

• More H+ ions are pumped across the membrane

• Inside of the membrane becomes positively charged and the outside of the membrane becomes negatively charged

• The differences in charges provide energy to make ATP

Step E

• In order for H+ ions to move across the membrane, the protein ATP synthase is needed

• As the H+ ions pass through ATP synthase, the protein rotates

• As it rotates, ATP synthase binds ADP and a phosphate together to make ATP

• Because of this system, light-dependent electron transport produces not only high-energy electrons, but ATP as well

• In summary, in light-dependent reactions:– Plants use water, ADP, and NADP+– They produce oxygen, and 2 high-energy

compounds: ATP and NADPH