Calvin Cycle

• Also know as Calvin – Benson Cycle• Light Independent Reactions• (Dark Reactions – not a good term as it implies

this only occurs at night)

• The production of ATP AND NADPH the light reaction of photosynthesis

Figure 7.9

Thylakoidcompartment(high H+)

Thylakoidmembrane

Stroma(low H+)

Light

Antennamolecules

Light

ELECTRON TRANSPORT CHAIN

PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

ATP and NADPH power sugar synthesis in the Calvin cycle

• The Calvin cycle occurs in the chloroplast’s stroma – This is where carbon

fixation takes place and sugar is manufactured

INPUT

Figure 7.10A OUTPUT:

CALVINCYCLE

Figure 7.10B

• Details of the Calvin cycle INPUT:

Step Carbon fixation.

In a reaction catalyzed by rubisco, 3 molecules of CO2 are fixed.

11

Step Energy consumption2

3 P P P6

6

2

ATP

6 ADP + P

6 NADPH

6 NADP+

6 PG3P

Step Release of one molecule of G3P.

3

CALVINCYCLE

3

OUTPUT: 1 PGlucoseand other compounds

G3P

Step Regeneration of RuBP.

4

G3P

4

3 ADP

3 ATP

3CO2

5 P

RuBP 3-PGA

Step 1Carbon Fixation

• CO2 is incorporated (fixed) into a five-carbon sugar named ribulose bisphosphate (RuBP).

• The enzyme that does this is RuBP carboxylase or rubisco. – The most abundant protein on Earth.

Step 2Energy consumption

• ATP and NADPH2 (from the light reaction) are used to make the three-carbon precursor that will be used to make glucose and other sugars.

Step 3Glucose

• For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P)

• Used to make Glucose

Step 4Regeneration of RuBP

• ATP is used to regenerate RuBP from G3P

Energy cost of Calvin Cycle

• For each G3P synthesized, the cycle spends:• 9 ATP • 6 NADPH2. • Both are made in the light reaction

• http://highered.mcgraw-hill.com/sites/0070960526/student_view0/chapter5/animation_quiz_1.html

Review: Photosynthesis uses light energy to make food molecules

• A summary of the chemical processes of photo-synthesis

Figure 7.11

Light

Chloroplast

Photosystem IIElectron transport

chains Photosystem I

CALVIN CYCLE Stroma

Electrons

LIGHT REACTIONS CALVIN CYCLE

Cellular respiration

CelluloseStarch

Other organic compounds

• Many plants make more sugar than they need

– The excess is stored in roots, tubers, and fruits

– These are a major source of food for heterotrophs

C4 and CAM plants have special adaptations that save water

• Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle

– In these types of plants, stomata on the leaf surface close when the weather is hot

– This causes a drop in CO2 and an increase in O2 in the leaf

– Photorespiration may then occur

• No sugar or ATP

• Photorespiration in a C3 plant

CALVIN CYCLE

2-C compound

Figure 7.12A

• Some plants have special adaptations that enable them to save water

CALVIN CYCLE

4-C compound

Figure 7.12B

– Special cells in C4 plants—corn and sugarcane—incorporate CO2 into a four-carbon molecule

– This molecule can then donate CO2 to the Calvin cycle

3-C sugar

• The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism

CALVIN CYCLE

4-C compound

Figure 7.12C

– They open their stomata at night and make a four-carbon compound

– It is used as a CO2 source by the same cell during the day

3-C sugar

Night

Day

• Due to the increased burning of fossil fuels, atmospheric CO2 is increasing– CO2 warms Earth’s surface by trapping heat in the

atmosphere– This is called the greenhouse effect

PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE

Figure 7.13A & B

Sunlight

ATMOSPHERE

Radiant heat trapped by CO2 and other gases

• Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect

– Unfortunately, deforestation may cause a decline in global photosynthesis

• Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer

• His research focuses on how certain pollutants (greenhouse gases) damage that layer

Figure 7.14A

Figure 7.13A & B

Sunlight

ATMOSPHERE

Radiant heat trapped by CO2 and other gases

• Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect

– Unfortunately, deforestation may cause a decline in global photosynthesis

7.14 Talking About Science: Mario Molina talks about Earth’s

protective ozone layer • Mario Molino received a

Nobel Prize in 1995 for his work on the ozone layer

• His research focuses on how certain pollutants (greenhouse gases) damage that layer

Figure 7.14A

• The O2 in the atmosphere results from photosynthesis

– Solar radiation converts O2 high in the atmosphere to ozone (O3)

– Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

• Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer

Figure 7.14B

Sunlight

Southern tip of South America

• International restrictions on these chemicals are allowing recovery

Antarctica