Photosynthesis

Capturing Solar Energy

Overview of Photosynthesis

(glucose)• Occurs in some bacteria or chloroplast of plants and

protists • Carbon dioxide (CO2) is reduced to produce glucose

• Energy required is captured from the sun by chlorophyll

• Photosynthesis has a Light reaction and a Dark reaction

Fig. 7.2

A more colorful photosynthesis Fig 6-7

What is chlorophyll?Chloroplasts contain several pigments:

– Chlorophyll a– Chlorophyll b– Carotenoids

Chlorophyll absorbs Red and Blue light from the visible spectrum but reflects Green light.

Fig. 6-3

Light absorption in photosynthetic bacteria (7.5)

Light-Dependant Reaction• Occurs on thylakoid membranes of the

chloroplast• Only occurs in the presence of light• Two parts: Photosystems and Electron

Transport Chains (E.T.C.)• Transfers light energy to energy molecules –

ATP and NADPH

Light Reaction (7.10)

Photosystem II• Water loses 2 electrons and releases O2 .

• Light energizes the 2 electrons • e- passes through E.T.C. which produces ATP• ATP is carried over to the Calvin cycle

Photosystem I• Electrons are energized again • Energized electrons bond with a proton and

NADP+ to form NADPH• NADPH is carried over to the Calvin Cycle.

Light Reaction (7.14)

Light Independent (Dark) Reaction• Doesn't require light.• Occurs in stroma of the chloroplast

Calvin Cycle1. 6 CO2(1C) bonds with 6 RuBP(5C) and splits

to form 12 PGA (3C)2. ATP and NADPH (From Light Reaction)

reduces 12 PGA to 12 PGAL(3C)3. 2 PGAL forms Glucose (6C) (or other sugars)

remaining 10 form 6 RuP(5C)4. More ATP turns 6 RuP into 6 RuBP

Dark Reaction

Regulation of Photosynthesis

• Require CO2, H2O and Light

• Plants need to regulate the absorption of CO2 and H2O

• Stomata – openings allow CO2 in, O2 & H2O out.

Stomata – open and close with the help of guard cells (NIB)

Gas exchange (7.2)

Some plants are specialized to survive in dry climates.

Types of Plants

• C3 plants:– Use CO2 directly from the air

– Are very common and widely distributed

• C4 plants:– Close their stomata to save water during hot and

dry weather – Can still carry out photosynthesis

• CAM plants:– Are adapted to very dry climates – Open their stomata only at night to conserve water

C4 and CAM (7.14)

How much photosynthesis? (Do not copy)

• Photosynthetic organisms absorb more energy than humans consumes in a year

• In all, photosynthetic organisms convert around 104,900,000,000,000 Kg of carbon into organic compounds each year

• Bacteria have higher SA/V and consequently more efficient at absorbing light and producing glucose

Photosynthesis Again