Chapter 6:Photosynthesis

How Plants Make Food and the Importance of Captured Energy

Historical Perspective• Life started on this planet about 3 ½ billion years ago, but

without photosynthesis it would not have thrived.

• Cyanobacteria (formerly called blue-green algae) were the first simple organisms to master photosynthesis. Later, these evolved into the chloroplasts of plants.

• Photosynthesis profoundly changed the planet. The carbon-dioxide and ammonia rich atmosphere slowly changed to an oxygen /nitrogen atmosphere, poisoning many of the original organisms, but providing a new, more efficient way of releasing energy for the survivors.

• The oxygen also produced a thin layer of ozone (O3) above the atmosphere, that protected us from solar radiation, and paved the way for organisms to live on land.

• Plants use these things to make food:– Water (H2O)

– Carbon Dioxide (CO2)

– Energy (from sunlight)– Chlorophyll (a green pigment that acts as an

organic catalyst: C55H72O5N4Mg)

• Overall reaction:

6 CO2 + 6 H2O + light → C6H12O6 + 6 02Glucose(a simple sugar)

Chloroplast

• Part of a plant cell where photosynthesis takes place.

• Contains:– Outer Membrane– Inner Membrane– Stroma (fluid)– Thylakoids – Chlorophyll

Plant cells (stained)

See Diagram on Page 112

Thylakoid

• A Thylakoid is a small, flat disk or sac inside a chloroplast. Several thylakoids stacked together make a granum (pl. grana)

• The surface of thylakoids is where most of the LIGHT reactions in photosynthesis take place.

Electron Micrograph of Thylakoid

Stroma

• The fluid inside the chloroplast, but outside of the thylakoids.

• The reactions of the Calvin Cycle occur in the stroma.

Pigment

• A pigment is a coloured compound

• Pigments capture certain colours of light

• The most important pigments in plant cells are:– Chlorophyll-a (green primary pigment)– Chlorophyll-b (blue-green accessory pigment)– Carotenoids (yellow, orange and brown

accessory pigments)

ATP

• Adenosine triphospate

• Important energy-releasing compound

• Breaks down into ADP and phosphate

NADP, NADPH

• Nicotinamide Adenine Dinucleotide Phosphate

• This chemical compound transfers hydrogen ions (protons) from place to place

• When its not carrying the hydrogen its called NADP, When it is carrying hydrogen it is NADPH

RuBP

• Ribulose biphosphate• A simple sugar

(monosaccharide)• Has only 5 carbon

atoms (glucose has 6)• Used in

photosynthesis

PGA, PGAL

• Phospho-glyceric acid, Phosphoglyceraldehyde

• Simple organic compounds

• They have only 3 carbon atoms

• They can be used to make sugars (like glucose)

Photosynthesis• Photosynthesis is the

process by which plants convert the energy of sunlight into chemical energy in organic compounds

• Carbon dioxide and water are “fixed”, or joined together to make simple sugars (eg. Glucose)

This is a fancy

way of saying that

plants make their

own food

Diagram Overview of Photosynthesis

Photosynthesis

Light Reactions

Calvin Cycle

Sunlight is absorbedElectrons are transportedWater molecules are splitOxygen is releasedHydrogen is retained as NADP•H

Carbon dioxide taken inCO2 is joined to other molecules.PGAL is made.

Consists of

1

2

The process by which plants make their own food…

WaterSunlightCO2

inpu

t

oxygenglucose(food)

output

is called

where

Goes to next step

where

makes

Glucose (food)

Main Steps in Main Steps in PhotosynthesisPhotosynthesis

The Light ReactionsThe Light Reactions The parts of photosynthesis that require The parts of photosynthesis that require

sunlight and water. Oxygen is producedsunlight and water. Oxygen is produced The Calvin CycleThe Calvin Cycle (A.K.A. the Dark Reactions)(A.K.A. the Dark Reactions)

The parts of photosynthesis that can take The parts of photosynthesis that can take place with or without sunlight. Carbon place with or without sunlight. Carbon dioxide is dioxide is fixedfixed or converted into organic or converted into organic compounds. compounds.

The Light ReactionsThe Light Reactions Absorption of Light by ChlorophyllAbsorption of Light by Chlorophyll

Light energy excites a chlorophyll-a electronLight energy excites a chlorophyll-a electron Electron TransportElectron Transport

Excited electrons pass from molecule to molecule Excited electrons pass from molecule to molecule through the thylakoid membrane. through the thylakoid membrane.

Water molecules are split:Water molecules are split: 2 H2 H220 0 → 4 H→ 4 H++ + 4 e + 4 e-- + O + O2 2 oxygen is releasedoxygen is released

Chemiosmosis -- HChemiosmosis -- H++ is used to is used to make: make: NADPH NADPH (NADP + H(NADP + H++ → NADPH)→ NADPH) ATPATP (ADP + P → ATP)(ADP + P → ATP)

The Calvin CycleThe Calvin Cycle(formerly called The Dark Reactions)(formerly called The Dark Reactions)

COCO22 is bonded to RuBP is bonded to RuBP (a 5-Carbon sugar) (a 5-Carbon sugar)

This splits into 2 PGAThis splits into 2 PGA (a 3-Carbon acid) (a 3-Carbon acid) The 2 PGAs are converted to PGALsThe 2 PGAs are converted to PGALs

ATP is used up (it gives up its phosphate)ATP is used up (it gives up its phosphate) NADPH is used (it gives its hydrogens)NADPH is used (it gives its hydrogens)

Most of the PGAL is converted into RuBPMost of the PGAL is converted into RuBP to to restart the cycle, but…restart the cycle, but…

Some PGAL is used to make organicSome PGAL is used to make organic compoundscompounds (like glucose) (like glucose)

Calvin Cycle• 6 carbon dioxide

molecules join 6 RuBP molecules

• 6 unstable six-carbon sugars form

• They immediately split into 12 PGAs

• The PGAs transform into 12 PGALs (using up ATP + NADPH from light reactions)

• 10 PGALs are used to make 6 RuBPs

• 2 PGALs are used to make glucose

RuBP

CO2

Unstable 6-C

PGAPGA

PGAL

PGAL

START CYCLE AGAIN END

RuBP

GLUCOSE

Calvin Cycle• 6 carbon dioxide

molecules join 6 RuBP molecules

• 6 unstable six-carbon sugars form

• They immediately split into 12 PGAs

• The PGAs transform into 12 PGALs (using up ATP + NADPH from light reactions)

• 10 PGALs are used to make 6 RuBPs

• 2 PGALs are used to make glucose

RuBP

CO2

Unstable 6-C

PGAPGA

PGAL

PGAL

START CYCLE AGAIN END

Overall Equation for Overall Equation for Photosynthesis:Photosynthesis:

6 CO6 CO22 + 6 H + 6 H22O + light O + light → C→ C66HH1212OO66 + 6 0 + 6 022

Which means that:Which means that: 6 carbon dioxide molecules, plus6 carbon dioxide molecules, plus 6 water molecules, with6 water molecules, with enough sunlight, producesenough sunlight, produces One molecule of glucose.One molecule of glucose. 6 molecules of oxygen.6 molecules of oxygen.

Important!

Other PathwaysOther Pathways

The notes on photosynthesis describe the most The notes on photosynthesis describe the most common form… common form… C3 photosynthesis.C3 photosynthesis. Plants Plants that grow in hot, dry places use variations of that grow in hot, dry places use variations of photosynthesis.photosynthesis.

C4 photosynthesisC4 photosynthesis occurs in corn, sugar cane occurs in corn, sugar cane and crabgrass.and crabgrass.

CAM photosynthesisCAM photosynthesis occurs in cactus and occurs in cactus and pineapple.pineapple.

Follow up ActivitiesFollow up Activities

Draw the Calvin Cycle diagram (p. 117) in Draw the Calvin Cycle diagram (p. 117) in your notebook your notebook

Skim pages 111-120 in the textbook, looking Skim pages 111-120 in the textbook, looking particularly at the diagramsparticularly at the diagrams

Carefully read the Summaries (page 121)Carefully read the Summaries (page 121)

Leaf structureLeaf structure

Plural: stomata

Palisade Layer (lots of photosynthesis)

Palisade Layer (lots of photosynthesis)

Spongy Layer(some photosynthesis)

Spongy Layer(some photosynthesis)

Epidermis Layer(no photosynthesis)Epidermis Layer

(no photosynthesis)

AKA: vascular bundle

AssignmentAssignment

Page 122 #1 - 22Page 122 #1 - 22

PhotosynthesisPhotosynthesis

The Light Reactions , showing photosystemsThe Light Reactions , showing photosystems The Calvin Cycle.The Calvin Cycle.

AnswersPage 122 #1 to 22

1 Chlorophyll is well named because chloro- means green and chlorophyll is a green pigment.

2 Stroma is the liquid inside a chloroplast, stomata (s. stoma) are the small holes or pores on the underside of leaves.

3 Carbon fixation means taking carbon dioxide from the air and incorporating it into more complex organic compounds, like sugar

4 A biochemical pathway is a series of linked reactions, where the product of one reaction is used by the next reaction.

5 Calvin cycle does not belong, because the others are all part of the light reactions.

6 (a) O2 is a product, the others are used up.

7 (c) CO2 is a reactant in the Calvin cycle.

8 (b) accessory pigments absorb colours of light that chlorophyll a doesn’t.

9 (d) C4 plants fix CO2 into 4-carbon compounds

10 (c) Oxygen is produced when water is split.

11 (d) Light reactions take place on the thylakoid membrane

12 (a) During chemiosmosis ATP is formed

13 (c) Carbon fixation occurs during the Calvin cycle, not during the light reactions.

14 (b) most of the PGAL is recycled to make more RUBP and keep the cycle going.

15 (b) most of the Calvin cycle occurs in the stroma

16. Photosystem II is the starting point where sunlight causes chlorophyll-a to lose electrons. These electrons move through a transport chain to Photosystem I, where more light is absorbed and more electrons are released. The electrons are eventually replaced by electrons from water molecules that have been split.

17. The Calvin cycle is a biochemical pathway, because each time a substance is formed as a product of one reaction, it is immediately used as a reactant in another reaction.

18.Most CAM plants are found in hot, dry climates, where plants need to keep their stomata closed during the day, and only open them at night.

19.ATP is synthesized during the step called chemiosmosis, when H+ ions are diffusing through the membrane. They provide energy to put together the ATP

20 During the summer the leaves are rich in chlorophyll, which is bright green and masks most other pigments. In the fall the chlorophyll production stops and the chlorophyll begins to disappear, leaving behind other pigments (like yellow and orange carotenoids).

Note: In some plants the decaying pigments may also produce bright red colours

21.The structure of a chloroplast is well suited to its function. The chloroplast contains many thylakoids that provide lots of membrane surface for reactions to occur on. These membranes also contain chlorophyll and other pigments to absorb different colours of light. The chloroplasts contain fluids (stroma) where other reactions can occur.

22. In the Calvin cycle (1) CO2 combines with RuBP to form an unstable 6C sugar. This immediately breaks into two PGA molecules. The PGAL molecules are immediately changed into PGAL molecules (using up some NADH an ATP). Most of the PGAL is used to replace the RuBP, but some of it is changed into glucose.