Photosynthesis

Watch this: http://youtu.be/g78utcLQrJ4

D. Photosynthetic Reaction1.In 1930 C. B. van Niel showed that O2 given off by photosynthesis comes from water and not from CO2. Redox reactions ultimately transfer electrons from water to carbon dioxide

2. The net equation reads:

What is a plant anyway?

7.1 Photosynthetic OrganismsA. Photosynthesis transforms solar energy

B. Organic molecules built by photosynthesis provide both the building blocks and energy for cells.

Figure 7.1b

C. Plants use the raw materials: carbon dioxide (from stoma) and water (from roots)D. Chloroplasts carry out photosynthesisPhotosynthesis takes place in the chloroplasts!

Figure 7.1c

E. Chlorophylls and other pigments involved in absorption of solar energy reside within thylakoid membranes of chloroplasts

Figure 7.2

Stoma (stomata pl.) allow for gas exchange

Quick Check -1.  Plant

2.  Thylakoid

3.  Photosynthesis

4.  Organic Molecules

5. Grana

6. Chloroplast

7.2 Plants as Solar Energy Converters

A. Solar Radiation - Only 42% of solar radiation that hits the earth’s atmosphere reaches surface; most is visible light. Light is composed of packets of energy called photons.

B. Photosynthetic Pigments - Pigments found in chlorophyll absorb various portions of visible light; absorption spectrum.

1. Two major photosynthetic pigments are chlorophyll a and chlorophyll b.2. Both chlorophylls absorb violet, blue, and red wavelengths best.3.  Most green is reflected back; this is why leaves appear green.

4. Carotenoids are yellow-orange pigments which absorb light in violet, blue, and green regions.5. When chlorophyll breaks down in fall, the yellow-orange pigments in leaves show through.

Color

• The color that we see when looking at a pigmented object is the wavelengths that are reflected or transmitted by the pigmented object. In other words, we see the reflected wavelengths. A green leaf reflects green. Green is not absorbed by the plant. Chlorophyll absorbs in the red, blue and violet wavelengths.

Fall Foliage Slideshow

Fall Leaves• Why do leaves turn orange/red/brown in the fall?

• What are the major pigments which absorb light?

• Why do leaves appear green?

• The reason why there are accessory pigments in a leaf is to absorb energy in parts of the spectrum that chlorophyll can’t

E. Two Sets of Reactions in Photosynthesis

1. Light reactions cannot take place unless light is present. They are the energy-capturing reactions. They convert light energy to chemical energy. When chlorophyll absorbs a photon, one of its electrons is raised to the excited state.

Excited Electrons

• When a photon hits a molecule, it releases energy as light, loss of an electron, fluorescence and heat

• Light reactions, and photosystems are located in the thylakoid membrane as well as the H+ ion gradient

b. Chlorophyl within thylakoid membranes absorbs solar energy and energizes electrons.c. Energized electrons move down the electron transport system; energy is captures and used for ATP production.d. Energized electrons are also taken up by NADP+, becoming NADPH.

2. Calvin Cycle Reactionsa. These reactions take place in the stroma; can occur in either the light or the dark.b. These are synthesis reactions that use NADPH and ATP to reduce CO2.

The products of the light reactions are ATP and NADPH

What you should know by now..

1.  The equation for photosynthesis.  Write it!

2.  The structure of a chloroplast.  Sketch it!

3.  Compare the two stages of photosynthesis and their products.  Chart it!

**Things are about to get much more difficult**

7.3 The Light Reactions

1.  Two paths operate within the thylakoid membrane

                         noncyclic              and                cyclic

                          *straight line                       *in a circle

2.  Both paths use ATP, but the noncyclic also produces NADPH(this is where we pick up electrons!)

3.  PHOTOPHOSPHORYLATION = ATP production(phosphorylation means adding a P to ADP ATP)

4. Photosystems are located in the thylakoid membrane which is where the light reactions occur.

1. Light hits photosystem II (yes, II comes before I)and exites an electron in H202.  The primary electron acceptor passes the electron down the ETC and generates ATP3. Light is required for PSI, but not water, it generates NADPH

Something trivial....

Photosystem I and Photosystem II are named based on when they were discovered, PSII was established first.

Figure 7.5

We’ve used our electrons here to form ATP

We use these electrons to go to the Calvin Cycle

Indicate which system  (PS1 or PS2 or BOTH)

____1.  Splits water____2.  Produces NADPH____3.  Has an electron transport chain____4.  Requires light____5.  Utilizes a primary electron acceptor____6.  Occurs in the thylakoid____7.  Requires the input of H20____8.  The cyclic path____9.  Uses chlorophyll____10.  Releases oxygen

Are you still confused?    This is pretty hard to visualize, but through the magic of technology, we can watch these processes as animations

McGraw Hill Animation

Forest Biology - The Light Reactions

7.3   Light Reactions      A.  Two Pathways   B.  Noncyclic   C.  Cyclic

   D.   ATP Production  -->  CHEMIOSMOSIS

           When H20 is split, two H+ remain, oxygen is released           These H+ are pumped from the stroma into the thylakoid            This creates a gradient used to produce ATP from ADP

ATP is the whole point of Photosystem II and will be used to power the Light Independent Reactions (Calvin Cycle)

Figure 7.7

Chemiosmosis is difficult to visualize.

So... you get to color it! Yay! coloring!

The Calvin Cycle  

Also called

*The Light Independent Reactions*The Dark Reactions

*Named after Melvin Calvin, who used a radioactive isotope of carbon to trace the reactions.

The Calvin Cycleis a series of reactions producing carbohydrates.

carbon dioxide fixation, carbon dioxide reduction, and regeneration of RuBP.

FIXATION

REDUCTION

REGENERATION

B. Fixation of Carbon Dioxide

1. CO2 fixation is the attachment of CO2 to an organic compound called RuBP.2. RuBP (ribulose bisphosphate) is a five-carbon molecule that combines with carbon dioxide.

3. The enzyme RuBP carboxylase (rubisco) speeds this reaction; this enzyme comprises 20–50% of the protein content of chloroplasts, probably since it is a slow enzyme.

Calvin Cycle Animation

C. Reduction of Carbon Dioxide

1. With reduction of carbon dioxide, a PGA 

(3-phosphoglycerate [C3])

molecule forms.

D. Regeneration of RuBP

1. Every three turns of Calvin cycle, five molecules of PGAL are used to re-form three molecules of RuBP.2. Every three turns of Calvin cycle, there is net gain of one PGAL molecule; five PGAL regenerate three molecules of RuBP.

Figure 7.8

E. The Importance of the Calvin Cycle1. PGAL, the product of the Calvin Cycle can be converted into all sorts of other molecules.2. Glucose phosphate is one result of PGAL metabolism; it is a common energy molecule.

Figure 7.9

Factors that Affect Photosynthesis 1. Light Quality (color)2. Light intensity3. Light Period4. Carbon Dioxide Availability5. Water Availability

In order for photosynthesis to occur, plants must open tiny pores on their leaves called STOMATA.

Opening these pores can lead to loss of water.

Alternative Pathways The Calvin Cycle is the MOST Common Pathway for Carbon Fixation. Plant Species that fix Carbon EXCLUSIVELY through the Calvin Cycle are known as C3 PLANTS.   Plants in hot dry environments (C4 plants) have a problem with water loss, so they keep their stomata partly closed... this results in CO2  deficit (Used in Calvin Cycle), and the level of O2

RISES (as Light reactions Split Water Molecules). CAM plants conserve water by opening their stomata only at night

Figure 7.10

C4 plants and CAM (Crassulacean acid metabolism) plants use an alternate pathway to FIX carbon dioxide from the air.

Figure 7.11THE CAM PATHWAY - Plants that use the CAM Pathway open their stomata at night and close during the day. At night, CAM Plants take in CO2 and fix into organic compounds. During the day, CO2 is released from these Compounds and enters the Calvin Cycle. Because they have their stomata open only at night, they grow slow.

Quick Practice

Quick Practice

thylakoid

O2

stroma

grana

Pg 129b

Light & H2O

O2

CO2

glucose

ATP

NADPH

ADP

NADP

A = photosystem IIB = photosystem IC = H20D = Electron Transport ChainE = ATP Synthase

AB = ATPAC = phospholipidsAD = light (energy)