Light Reactions of Photosynthesis

How is most energy constantly transferred into the biosphere?

Through living things

What does many chemical reactions that cause a transfer of energy involve?

The transfer of electrons between atoms

What is it called when a substance loses electrons?

The substance is oxidized

Whenever one substance ___ electrons (is oxidized) another substance ___ them (is reduced)

loses, gains

What is it called when a substance gains electrons?

The substance is reduced

What is the oxidation-reductions reactions known as?

Redox reactions

What happens when a substance is oxidized?

It loses an electron and becomes positively charged. It loses energy

What happens when a substance is reduced?

It gains energy as energy-rich electrons are transferred

Organic molecules are:

Oxidized as they lose electrons and hydrogen ions and reduced as they gain electrons and hydrogen ions

What happens when light energy is absorbed from the sun by chloroplasts?

It is transformed into chemical energy (sugar)

Thylakoids

Flattened membrane-bound sacs

Granum (grana)

A stack of thylakoids

Stroma

A thick solution surrounding thylakoids

What is the structure of the cloroplast?

Double membrane; contain thylakoids, granum, and stroma

What are the two stages of photosynthesis?

Stage I: Light Reactions, Stage II: Calvin Cycle

What happens is Stage I: Light Reactions?

Light energy, absorbed by pigment molecules embedded in thylakoid membranes, is converted into two forms of unstable chemical energy. Water is split releasing oxygen gas

What are the two forms unstable chemical energy in Stage I?

ATP and NADPH

NADPH

Electron carrier; carries energized electrons and hydrogen ions to Stage 2. Provides high energy electrons and hydrogen ions needed to make sugar (C6H12O6) in stage 2

Where does stage I: Light Reactions take place?

Thylakoid disks

What happens in Stage 2: Calvin Cycle (Light Independent Reactions)?

Stable organic molecules (sugars) are formed using carbon dioxide and chemical energy stored in ATP and NADPH produced during the light reactions

Where does Stage 2 the Calvin Cycle take place?

The stroma

Photosystem

A cluster of pigment molecules in the thylakoid membrane. There are two of these; and they are connected by an electron transport chain

Electron Transport Train

A series of molecules in the thylakoid membrane that transport electrons

ATP Synthase

An enzyme that contains a Hydrogen ion channel and generate ATP

What are the reactions associated with the First Photosystem? (steps needed to "charge" the thylakoid, preparing to make ATP

1. Light is absorbed by pigment molecules (this energy is passed from accessory pigments, chl b and carotenoids, to a pair of chl a molecules)

2. Energy forces electrons of chl a to enter a higher energy level; energized electrons are "excited"

3. Excited electrons have enough energy to leave chl a. Electrons are ejected out of chl a molecule

4. Excited electrons are then passed down an electron transport chain

5. As the electrons are passed along the chain they release most of their acquired energy

6. The energy released from electrons is used to pump hydrogen ions into the thylakoid. Thylakoid fills up with hydrogen ions and a hydrogen ion gradient is formed. The hydrogen ion gradient is stored energy and will be used to make ATP

Restoring the first photosystem

Electrons lost form chl a in the first photosystem are replaced by splitting of water molecules

What is the splitting of water molecules chemical equation

2H20 --> 4H+ + 4e- + O2

Chemiosmosis

How ATP is made. 1. Hydrogen ions have accumulated inside the thylakoid through the splitting of water and the pumping of hydrogen ions. 2. Hydrogen ion gradient- there are more hydrogen ions inside the thylakoid than the stroma. 3. Enzyme ATP Synthase, is a "rotary motor" that makes ATP by adding a phosphate group to ADP as it spins. 4. Concentration gradient of Hydrogen ions aids in ATP synthesis- Hydrogen ions diffuse through ATP synthase causing it to spin

Reactions associated with the second photosystem (making NADPH)

1. light energy is simultaneously absorbed by the second photosystem

2. The excited electrons are ejected from chl a

3. NADP+ accepts the excited electrons (high in energy) and hydrogen ions to form NADPH

4. NADP+ is reduced to NADPH and obtains energy as it accepts high energy electrons and hydrogen ions

Restoring the second photosystem

electrons lost from the second photosystem are replaced with electrons from the end of the electron transport train (these electrons originated from the first photosystem)