Spring 2001

PHOTOSYNTHESIS

I. Virtually all energy on earth comes from sunlight

A. Plants use energy from the sun to make the bonds which hold organic molecules together

B. When these bonds are broken the energy is ultimately transferred to ATP, which is then moved about cells and organisms to power their needs

C. Since these molecules are synthesized from the energy in sunlight the process is called photosynthesis

1. Autotrophs = organisms that can make their own food

a. Plants are photoautotrophs because they use light to make their food

2. Heterotrophs = organisms that can not make their own food and therefore must get it from their environment

II. Photosynthesis

A. Electricity, one of our most common energy sources, consists of a flow of electrons

B. During photosynthesis the sun's energy is used to split water molecules, starting a flow of electrons

C. The energy from this flow of electrons is harnessed and used to make the bonds in organic molecules

D. 3CO2 + 6H20----- light -----> C3H6O3 + 3O2+ 3H20

1. Since organic molecules contain carbon, a supply of carbon is needed for this process, it comes from carbon dioxide in the atmosphere

2. Note the starting products are water, which supplies the electrons, and carbon dioxide, which provides the carbon

3. The end products are oxygen, a triose (two combined make glucose) and water

III. Importance of photosynthesis

A. Note that photosynthesis produces:

1. Glucose - which provides us with both food (alpha glucose polymers) and structural materials (beta glucose polymers)

2. Oxygen

a. The early atmosphere of the earth lacked oxygen and it took about 3 billion years of photosynthesis to produce the current 21% oxygen atmosphere we now enjoy

b. Some of this oxygen reacted with sunlight in the upper atmosphere to produce ozone (O3), which protects us from harmful solar radiation

IV. Energy transformations

A. First law of thermodynamics - energy can not be created nor destroyed, only changed from one form to another

B. Plants transform the energy from the sun into a more useable form, carbohydrates

C. Energy transformations in living systems involve the transfer of electrons and protons [H+] from one energy level to another, or from one atom or molecule to another

1. Reduction/ oxidation (REDOX) reactions:

a. Oxidation = loss of electrons, e.g. Na to Na+

b. Reduction = gain of electrons, e.g. Cl to Cl-

V. Overview of photosynthesis

A. Photosynthesis is actually a two step process:

1. Energy transduction reactions - energy from the sun is captured in energy carrying molecules. Since it must take place in sunlight it is often called the light-dependent reaction(s)

2. Carbon-fixation reactions - The energy from the transduction reactions is used to fix the carbon from atmospheric carbon dioxide into organic compounds. It does not have to take place in the light so it is often called light-independent reaction(s)

VI. Energy transduction reactions

A. The sun's energy is used to split water molecules which starts a flow of electrons

B. The energy from this flow of electrons is used to convert ADP to ATP and to form an additional electron carrier molecule called NADPH

C. The first step in the conversion of energy is the absorption of light by pigments

1. Pigment = a substance that absorbs light

2. Major photosynthetic pigments:

a. Chlorophylls - absorb violets, blues and reds, reflect green

b. Carotenoids - red, yellow and orange pigments. E.g. betacarotene is principle source of vitamin A required by humans for proper vision

c. Phycobilins - pigments found in cyanobacteria and some red algae

3. When chlorophyll pigments absorb light, electrons are boosted to a higher energy level and the energy is captured in a chemical bond

4. Photosynthetic pigments are embedded in the thylakoids, membrane bound sacs within the chloroplasts

a. 250 - 400 pigment molecules are organized into a photosystem

5. Photosystems have two components:

a. Antenna complex - pigment molecules which gather light energy and funnel it to the reaction center

b. Reaction center - a complex of proteins and pigment molecules that enable light energy to be converted to chemical energy

6. All the pigments within a photosystem can absorb photons (particles of light energy)

7. However, only one pair of chlorophyll molecules per photosystem can actually use the energy. These special chlorophyll molecules are at the core of the reaction center

8. Light energy gathered by the antenna pigments is transferred from one molecule to the next until it reaches the reaction center

9. When the reaction center absorbs the energy, electrons from one of the core molecules is boosted to a higher energy level and transferred to an acceptor molecule which initiates electron flow along an electron transport chain

a. There may be one or two electron transport systems and one or two photosystems working together

10. The molecules of the electron transport chain harness the energy from the flow of electrons and use it to make ATP and another energy carrier called NADPH

a. A molecule of NADPH carries seven times more energy than an ATP molecule

11. Since a phosphate is added to ADP (converting it to ATP) using sunlight the process is called photophosphorylation

12. The energy captured in ATP and NADPH will be used to power the second phase of photosynthesis, carbon fixation

VII. Carbon fixation (light-independent reactions)

A. During carbon fixation reaction the energy in the ATP and NADPH produced in the transduction reactions is used to extract carbon from atmospheric (or dissolved in water) carbon dioxide to make organic compounds, chiefly glucose

B. The reactions are sometimes called the Calvin cycle for its discoverer. Since the initial product is a three carbon compound it is also often called the C3 or three carbon pathway

C. Carbon fixation occurs in the stroma or ground substance of the chloroplast

D. The starting compound is a 5-carbon sugar with 2 phosphates attached called RuBP (Ribulose 1,5 bisphosphate)

E. Carbon fixation (see F. 7-20, p. 141):

1. Carbon dioxide enters through small pores in the leaves called stomates and enzymes extract and bond the carbon to RuBP forming a 6 carbon compound

2. The 6 carbon compound is quite unstable and immediately hydrolyzed to two 3 carbon compounds called PGA. This is why it is often called C3 photosynthesis

3. The PGA is reduced to PGAL

4. Once 6 molecules of PGAL accumulate 5 are used to regenerate RuBP

5. The sixth one is used to form organic compounds

6. Note that each process is powered by energy from the transduction reactions

F. Although glucose is often represented as the byproduct of photosynthesis, very little glucose is actually generated. Most of the fixed carbon is converted either to sucrose (a disaccharide), the major transport sugar in plants, or starch, the major storage carbohydrate in plants

TWO VARIATIONS ON PHOTOSYNTHESIS

I. C4 photosynthesis

A. Plants that use only the Calvin cycle are called C3 and the process takes place in the mesophyll of the leaves

B. However is some plants the carbon dioxide is initially fixed into a 4-carbon compound called oxaloacetate

1. Therefore these plants are called C4

2. The oxaloacetate is then converted to either malate or aspartate, which is then transported to the bundle sheath cells

3. The malate or aspartate is decarboxylated and the resulting carbon dioxide enters the Calvin (C3) cycle

C. Why such a seemingly complex system?

1. See figures 7-21 & 7-22 - the enzyme RUBP carboxylase/oxygenase works differently depending on the relative concentrations of carbon dioxide and oxygen:

a. At high carbon dioxide levels it fixes carbon (carboxylase)

b. At low concentrations of carbon dioxide it acts as an oxygenase, consuming oxygen and releasing carbon dioxide (rather than fixing it)

c. This is called photorespiration

2. Photorespiration is a common phenomenon in C3 plants

a. As much as 50% of the fixed carbon may be reoxidized to oxygen, making C3 photosynthesis inefficient

b. When it is hot and/or dry they must close their stomates

c. This cuts off the supply of carbon dioxide so photorespiration increases

3. C4 plants are well adapted to high light intensities, high temperatures and dry conditions - photorespiration is nearly absent in them

a. This is because the enzyme PEP carboxylase takes the place of the RUBP carboxylase/oxygenase found in C3 plants

b. C4 grasses have net photosynthetic rates 2-3 greater than C3 plants.

c. Note the distribution of C4 grasses in North America on p. 147. There are many more C4 species in the hotter and drier areas

d. Many of our most important crops are C4 plants, e.g. corn and sugarcane

e. Crabgrass is also a C4 plant and it tends to overtake traditional C3 lawn grasses as the summer gets hotter and drier

II. CAM photosynthesis

A. Many succulent plants, e.g. cacti, have a system very similar to C4 plants

B. It is called CAM (crassulacean acid metabolism) because it was discovered in members of the stone crop family, the Crassulaceae

C. To prevent water loss they open their stomates at night and accumulate carbon as malate like C4 plants

D. The malate is converted to malic acid and stored until the following day

E. It is then decarboxylated and the carbon enter the Calvin cycle.