Photosynthesis

Textbook pages 97 – 103

Photosynthesis • process used by plants

using light energy to create sugar (glucose, C6H12O6) from carbon dioxide (CO2) and water (H2O)

• oxygen gas (O2) is produced as a waste product and the light energy used is stored as chemical energy

Investigating Photosynthesis

• Jan Van Helmont (1643)– Carefully massed soil,

and tree seedling and watered over 5 years.

– Determined most of the mass of the tree came from water.

Investigating Photosynthesis• Joseph Priestley (1771)

– Lit a candle and placed a bell jar over it, watching the flame go out.

– Reasoned that there was something in the air necessary to keep the flame going.

– When a plant was placed under the bell jar, discovered that the candle could be relit after several days.

– Reasoned that the plant released something into the air necessary for the flame.

Investigating Photosynthesis• Jan Ingenhousz (1779)

– Showed the effect observed by Priestley only occurred when plants were exposed to light.

Investigating Photosynthesis• Julius Robert Mayer

(1845)– Proposed that plants

convert light energy into chemical energy

Photosynthesis Equation6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2

Stages of Photosynthesis1. Capture energy from

sunlight2. Convert light energy

into chemical energy3. Use stored chemical

energy to produce organic compounds (food) from carbon dioxide

1. Capture energy from sunlight

1. Capture energy from sunlight• Pigments (chlorophyll) in the membranes of

the thylakoids absorb light energy– Gets transferred to electrons

• Electrons are replaced from water– H+ ions are left– Remaining O from into O2 gas as waste product

2. Converting light energy to chemical energy

2. Converting light energy to chemical energy

• Light dependent reactions• Occurs in thylakoid membranes• Use high-energy electrons to produce two

high-energy products– Convert ADP into ATP– Convert NADP+ into NADPH

2. Converting light energy to chemical energy

2. Converting light energy to chemical energy

• Electron transport chains (ETC)– A series of molecules through which excited

electrons are passed– Energy from the electrons is used to pump H+

ions across the thylakoid membrane– A second ETC provides energy to make

NADPH, a high energy electron carrier

2. Converting light energy to chemical energy

2. Converting light energy to chemical energy

• Production of ATP– A concentration gradient is present now for H+

ions– Passing of ions back out of the thylakoid

provides energy to make ATP

2. Converting light energy to chemical energy

2. Converting light energy to chemical energy

• Both ATP and NADPH are used in the next step– ATP carries necessary energy– NADPH carries high-energy electrons

3. Use stored chemical energy to make food

3. Use stored chemical energy to make food

• Light independent reactions– Calvin Cycle is one example

• Occur in stroma inside chloroplasts, but outside of thylakoids

• Using energy from ATP and electrons from NADPH, CO2 is fixed into sugars

Investigating Photosynthesis• Melvin Calvin (1948)

– received the Nobel prize in chemistry (1961) for discovering pathways that carbon follows to make glucose in photosynthesis

Factors that affect Photosynthesis• light intensity• CO2 concentration• water levels• temperature