Photosynthesis Using light energy to build sugars

Leaf structures for photosynthesis

Chloroplast Thylakoid membrane contains photopigments. Thylakoid arranged in stacked sacs called grana. Surrounding semi-fluid region is called stroma. Light reactions take place in grana, Calvin cycle in stroma.

Photosynthesis includes of take place in takes place in uses to produce use Light- dependent reactions Calvin cycle Thylakoid membranes StromaNADPH ATP Energy from sunlight ATPNADPHO2O2 Chloroplasts High-energy sugars Section 8-3 Concept Map Go to Section:

Role of Photopigments Capture light energy and release electrons Each photopigment absorbs only part of the spectrum (range of wavelengths) Unabsorbed wavelengths are reflected Chlorophylls a and b (reflect green) are most important

Role of Photopigments Chlorophyll b and other photopigments capture light energy and transfer it to chlorophyll a using electrons. Chlorophyll a also captures more light energy and releases more electrons. Replace electrons lost from chlorophyll b by taking them from water.

Absorption Spectra of Chlorophylls Chlorophyll b Chlorophyll a

How does water fit in? Energy from light also used to split water: –O 2 is released as a byproduct. –Electrons replace those lost from chlorophyll b. –H + ions are used to power construction of ATP from ADP and Phosphate.

Role of energy carrier molecules ATP and NADPH receive energy in light dependent reactions; can only hold the energy briefly. Carry the energy to the Calvin cycle to power carbon fixation. Build these by using H+ ions released from water and electrons from chlorophyll a.

PHOTOSYNTHESIS 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2

Chloroplast Sugars + O 2 CO 2 + H 2 O

H2OH2O O2O2 Sugars CO 2 Light- Dependent Reactions Calvin Cycle NADPH ATP ADP + P NADP + Chloroplast

The light dependent reactions: Energy from sunlight is used to: –Excite chlorophyll to release electrons –Split H 2 O: Electrons go to chlorophyll O 2 is released as a byproduct. H + ions are used to build ATP using chemiosmosis. –Electrons and H + ions are used to build ATP and NADPH using the electron transport chain.

The light reactions at work!

The light-dependent reactions Hydrogen Ion Movement Photosystem II Inner Thylakoid Space Thylakoid Membrane Stroma ATP synthase Electron Transport Chain Photosystem IATP Formation Chloroplast

The light reactions and chemiosmosis

The Calvin Cycle Reactions that actually produce high energy molecules, like glucose. Use ATP and NADPH from the light reactions to power “fixation” of carbon from CO 2 into an organic molecule. Also returns low energy ADP plus inorganic phosphate ions and NADP+ to be used by the light reactions.

The Calvin Cycle ChloropIast CO 2 Enters the Cycle Energy Input 5-Carbon Molecules Regenerated 6-Carbon Sugar Produced

The Calvin cycle

Photosynthesis summary

So what do we need for photosynthesis? Think about the equation: what reactants are required? How about light? –Quantity (intensity)? –Quality (color)? What else??? Environmental conditions can affect the rate of photosynthesis.

Environmental conditions: temperature When it gets too hot, plants lose too much water from their leaves. –What does this cause? Solution: prevent water loss by closing the stomata. Resulting problem: no CO 2 enters the leaf, and O 2 can accumulate.

Stomates Spider plantTobacco leaf

Special cases: CAM metabolism In very high heat, stomates close to conserve water.  No CO 2 can enter. Take in CO 2 at night and store it as a stable molecule (crassulacean acid). Use that molecule to release CO 2 when light is available but gas CO 2 is not. All done in mesophyll cells.  Think: different time, same place

Special cases: C4 metabolism When there is constant bright sunlight, store CO 2 as a 4 carbon molecule to supplement available gas.  Uses ATP Increases carbon fixation above normal Avoids photorespiration Original carbon fixation in mesophyll cells. Calvin cycle in bundle sheath cells.  Think: same time, different place

Figure C 4 leaf anatomy and the C 4 pathway

Figure C 4 and CAM photosynthesis compared