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 intakes place in uses

to produce to produce

use

Light-dependentreactions

Calvin cycle

Thylakoidmembranes Stroma NADPHATPEnergy from

sunlight

ATP NADPH O2 Chloroplasts High-energysugars

Section 8-3

Concept Map

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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:

– O2 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

6CO2 + 6H2O C6H12O6 + 6O2

Chloroplast

Sugars + O2CO2 + H2O

H2O

O2

Sugars

CO2

Light-Dependent Reactions

CalvinCycle

NADPH

ATP

ADP + PNADP+Chloroplast

The light dependent reactions: Energy from sunlight is used to:

– Excite chlorophyll to release electrons

– Split H2O:• Electrons go to chlorophyll• O2 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

HydrogenIon Movement

Photosystem II

InnerThylakoidSpace

ThylakoidMembrane

Stroma

ATP synthase

Electron Transport Chain Photosystem I ATP 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 CO2 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

CO2 Enters the Cycle

Energy Input

5-CarbonMoleculesRegenerated

6-Carbon SugarProduced

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 CO2 enters the leaf, and O2 can accumulate.

Stomates

Spider plant Tobacco leaf

Special cases: CAM metabolism

In very high heat, stomates close to conserve water. No CO2 can enter.

Take in CO2 at night and store it as a stable molecule (crassulacean acid). Use that molecule to release CO2 when light is available but gas CO2 is not.

• All done in mesophyll cells. Think: different time, same place

Special cases: C4 metabolism

When there is constant bright sunlight, store CO2 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 10.18 C4 leaf anatomy and the C4 pathway

Figure 10.19 C4 and CAM photosynthesis compared