1. The fundamental life processes of plants and animals depend on a variety of chemical reactions that occur in specialized areas of the organism’s cells. As a basis for understanding this concept: 1. f. Students know usable energy is captured from sunlight by chloroplasts and is stored through the synthesis of sugar from carbon dioxide. Photosynthesis is a complex process in which visible sunlight is converted into chemical energy in carbohydrate molecules. This process occurs within chloroplasts and specifically within the thylakoid membrane (light-dependent reaction) and the stroma (light-independent reaction). During the light-dependent reaction, water is oxidized and light energy is converted into chemical bond energy generating ATP, NADPH + H+, and oxygen gas.† During the light-independent reaction (Calvin cycle), carbon dioxide, ATP, and NADPH + H+ react, forming phosphoglyceraldehyde, which is then converted into sugars. Notes:

Photosynthesis

Photosynthesis relies on sunlight. In fact all life on earth relies on sunlight directly (producers) or indirectly (consumers). Photosynthesis is the trapping of light energy and converting it into chemical energy. Plants, algae, and cyanobacteria have independently evolved the ability to capture light energy and convert it into chemical energy.

This process requires special molecules, membranes and organelles. In plants the:

• molecule is chlorophyll • membranes are the thylakoids and • organelles are the chloroplasts.

Chlorophyll is a green pigment which releases an electron or two when it absorbs red or blue wavelengths of light (it reflects, thus its color, green light). Other pigments increase the range of light absorbed by plants. For example carotinoids absorb red and orange light and transfer the energy to a special chlorophyll molecule.

Thylakoid membranes contain all the molecular structures necessary to convert light energy into chemical energy. This process requires the following components:

• Photosystems -- are a large group of antenna pigments which capture and transfer light energy to a central chlorophyll molecule.

• Electron transport chains, similar to those found in mitochondria, which accept electrons given off by chlorophyll and use the electrons' energy to make NADPH (similar to NADH) and ATP.

• Other molecules associated with making ATP and splitting water molecules (photolysis).

Chloroplasts are the organelles responsible for both phases of photosynthesis -- the light dependent reactions as well as the light independent or dark reactions.

The structure of chloroplasts are essential to their function. Each chloroplast has two membranes which separate the organelle into two compartments. The outer compartment is called the stroma. It is here in the stroma where the dark reactions take place, converting CO2 and H2O into glucose. This process is called the Calvin Cycle.

The inner membrane is the thylakoid which forms stacks of membranes called grana -- a structure which maximizes the surface area necessary to capture and convert light energy efficiently.

Photosynthesis can be summarized by the equation below. Note that it is simply the opposite of the equation for respiration.

6CO2 + 6H2O C6H12O6 + 6O2

Photosynthesis generated O2

About 1 to 2 billion years after the origin of the earth photosynthetic bacteria evolved, and began to change the nature of the Earth's atmosphere irrevocably. Over the next 1 to 2 billion years they gradually increased the concentration of oxygen in the atmosphere to levels that were probably higher than we now enjoy. As a result, organisms evolved the ability to use the oxygen in metabolism. Because of the great oxidizing ability of O2 this allowed cells using aerobic metabolism to extract much more energy from food materials. Cells capable of aerobic metabolism then rapidly took over the biosphere which they still dominate.

The importance of photosynthesis is two fold.

1. It allows cells to directly trap the energy of the sun and convert it into hydrocarbon fuel, like glucose. 2. Photosynthesis splits molecules of water (H2O) releasing molecular oxygen (O2)

Photosynthesis can be summarized as a chemical reaction:

12 H2O + 6 CO2 ------> 6 O2 + C6H12O6 + 6H2O

12 molecules of water plus 6 molecules of carbon dioxide react together to yield 6 molecules of oxygen, one molecule of glucose, and six molecules of water.

This equation may seem nonsensical since you could easily subtract out six water molecules and get a balanced equation:

6 H2O + 6 CO2 ------> 6 O2 + C6H12O6

But as we will see, the 12 oxygen atoms in the 12 water molecules all go into forming the 6 O2 molecules, while half of the 12 oxygens in the 6 CO2 molecules are reduced to form H2O.

So, we see that photosynthesis both creates O2 for aerobic respiration, and glucose to be used as fuel in that respiration.

If this seems a useless cycle, remember that a plant does this to provide raw materials for its own use, including generating energy when light is not available, and animals consume the materials created by plants to provide for their needs. In effect, animals are living indirectly on the energy of the sun's radiation

Photosynthesis in plants

Photosynthesis in plants occurs in specialized structures called chloroplasts. Chloroplasts include stacked membranes called the thylakoid membrane system. Surrounding the thylakoids is a space called the stroma. In the thylakoid membranes the energy of sunlight is absorbed, and where energy (in the form of ATP) and reducing potential (in the form of reduced NADP, or NADPH) are formed.

What is the role of NADP in this reaction? What do you think its function is? It is not to carry hydrogen (protons) but rather to carry electrons. It is reduced (gains an electron) and that negative charge is balanced by a proton, giving the reduced form, NADPH.

In the stroma the oxidized form of carbon, CO2, is reduced by NADPH, and through phosphorylation, fused to form hexoses (6 carbon sugars) including glucose.

To absorb light energy plants employ a variety of pigment molecules. Most important among these are the chlorophylls.

They absorb light in the violet-to-blue range, and in the red. Because they absorb those colors they give the plant its green color.

Carotenoids also absorb in the violet-to-yellow range. Other pigments (phycobilins) absorb in the rest of the spectrum.

Only the chlorophyll molecules are directly involved in photosynthesis-the other molecules transfer their captured energy to chlorophylls located in what is called reaction centers.

Light-dependent reactions

The first half of photosynthesis is the generation of ATP and NADPH. Packets of light energy (or "quantas") are called photons. A photon striking, and being absorbed by a chlorophyll molecule imparts a small amount energy, which raises an electron to a higher energy level.

Remember that atoms are organized with their electrons in "shells" surrounding the nucleus. Each successive shell has a higher energy level. Normally all of the electrons in an atom will fill the lowest possible energy levels. The absorbed energy will kick an electron up to a higher energy level.

This is an unstable situation, and the electron soon returns to its original shell, and energy. Since "energy is neither created nor destroyed" the excess energy is passed on to another chlorophyll molecule, with the loss of some energy as heat.

Eventually, the energy may be absorbed by the chlorophyll at the reaction center. At that point the reactions leading to ATP and NADPH production can occur.

The pigment molecules are organized in groups of around 200 to 300 called "photosystems".

There are two types of photosystems (photosystem I and II) which have different functions in photosynthesis

Photosystem I and cyclic photophosphorylation

The oldest form of photosynthesis involves a cyclic pathway using photosystem I.

A molecule of chlorophyll P700 is excited by a photon.

It donates an electron to an electron transport chain which uses it to generate ATP molecules (we'll see how later).

At the end of the chain the electron is donated back to chlorophyll P700.

This form of photosynthesis generates only ATP without splitting water, generating oxygen, or fixing carbon (making carbohydrates from CO2). To increase their yield of energy, cells evolved a second photosystem (photosystem II) which allows much more efficiently captures light energy.

Photosystem II and the noncyclic pathway

[Showed the movie of noncylic photophosphorylation]

A closer look at ATP formation

How is ATP created in thylakoid membranes?

The key is the fact that in passing electrons down the electron transport chain protons are pumped from outside to inside the membrane.

In addition, protons outside the thylakoid in the stroma are consumed in forming NADPH.

This creates a proton gradient between the inside and outside of the thylakoid.

A proton pump present in the membrane transfers the protons out into the stroma, along the concentration gradient, the energy of that transfer is used to generate ATP.

Transfer of protons through the transporter induces a change in conformation which is then used to transfer a phosphate onto ADP to create ATP.

The "dark reaction" & the formation of hexoses

The ATP and NADPH molecules generated then are used in the construction of molecules of six carbon sugars, called hexoses, including glucose. The reaction occurs as a cycle, with the constant regeneration of the starting material

The cycle is called the Calvin-Benson cycle.

In the cycle a carbon dioxide is added to ribulose bisphosphate (RuBP) forming a six carbon sugar which splits to give two molecules of phosphoglycerate (PGA).

PGA is reduced to phosphoglyceraldehyde (PGAL) with the consumption of one ATP and one NADPH

Of twelve PGAL, two are used to form a phosphohexose (e.g., glucose-phosphate), the other ten recycle to make more RuBP (with the consumption of more ATP molecules)

The balance sheet of photosynthesis comes out even since each molecule of H2O split to form O2 yields 4 electrons which is used to generate 3 ATP and 2 NADPH. Fixing each molecule of CO2 uses 3 ATP and 2

NADPH. To make one molecule of hexose requires six turns of the Calvin-Benson cycle (converting the carbons of six CO2

molecules into the hexose) using a total of 18 ATPs and 12 NADPH.

Calvin Cycle

The following formula summarizes the Calvin cycle.

C5 + CO2 + ATP + NADPH ---> C6H12O6

where C5 is a fivecarbon molecule, such as pyruvate, when is recycled as glucose is synthesized. The Calvin cycle is the last step in photosynthesis. The purpose of the Calvin Cycle is to take the energy from photosystem I and fix carbon. Carbon fixation means building organic molecules by adding carbon onto a chain. In order to do this, you have to start with an organic molecule, a starter molecule. In this case, the starter molecule is a 5-chain carbon compound (C5). I'll skip the names of most of the organic molecules in this process. We'll cover it in AP Biology.

The first step in the Calvin cycle is for the 3C5 to bind with 3CO2, producing a six 3-carbon organic molecules (6C3). Next, 6ATP and 6NADPH energizes the binding of a C3 to make a 6-carbon molecule (C6), glucose. The remaining 5C3 continues moving through the Calvin cycle, being turned back into the starter C5 organic molecule.

The following chemical equation summarizes the Calvin cycle.

Photosynthesis

Photosynthesis is the process where plants convert sunlight into energy, then store it as carbohydrates, sugars, such as glucose. Photosynthesis may be the most important process in ecosystems, for it both brings in energy needed within the ecosystem, and produce oxygen (O2) needed for cellular respiration, and the production of more ATP.

Photosynthesis has three basic steps:

1. Energy is captured from the sunlight. 2. Light energy is converted into chemical energy in the form of ATP and NADPH. 3. Chemical energy is used to power the synthesis or organic molecules (e.g. carbohydrates) from carbon

dioxide (CO2).

This process can be summed with the following chemical equation:

CO2 + H2O + light ---> C6H12O6 + O2

In terrestrial plants, this process takes place in leaves, specifically within the organelle chloroplast So what exactly happens?

First lets look at the structure of chloroplast. Chloroplast contains stacks of flattened organelles called a thylakoid. One stack of thylakoids is called a grana. Grana float within a cytoplasm-like fluid in the chloroplast called stroma.

How does this work? Think light as a packet of energy, like a battery, called photons. When sunlight shines

on a plant, the photons hits the plants, plugging into pigments called chlorophyll. Chlorophyl fills the thylakoids . But photons have different colors, similar to having AA and AAA batteries. AA batteries are longer than AAA batteries, and they also have slightly different charges. So each battery needs a different type of plug, and so does the different color photons. So there are two primary types of chlorophyll, chlorophyll a and chlorophyll b. Photons with a wavelength peaking at 680nm plugs into chlorophyll a, while photons with a wavelength peaking at 650nm plugs into chlorophyll b. By having both pigments, plants more than double the amount of photons that is can convert. Other pigments, such as carotenoids, also increases the range of photons which can be captured. But carotenoids are not a good at absorbing photons as chlorophyll.

Once chlorophylhas absorbed the photons, the energy is transferred down a chain until ATP and NADPH is charged. This process occurs in two separate, but connected systems, photosystem I and photosystem II. Photosystem I picks up photons at 700nm, while photosystem II photons at 680nm. The energy absorbed by photosystem II is passed on to photosystem I, which charges NADPH.

When light hits a plant, chlorophyll absorbs the photons. The energy that it absorbs is picked up by photosystem II. Photosystem II takes the energy, along with H2O, and passes that energy to an electron acceptor, Q. O2 is released at this point. The electron acceptor, Q, now has energy. Q takes that energy, and shuttles it off to photosystem I. As that energy is being shuttled from Q to photosystem I, ADP gets charged, becoming ATP. Photosystem I now takes that energy, and charges NADP+. When NADP+ is charged, it loses a hydrogen, and becomes NADPH. Notice that NADPH has now loosed its positive charge by picking up a negative electron. This entire process can be summarized with the following chemical equation.

H2O + light + ADP + P ---> O2 + ATP + e-

After the above steps occur in photosystem II, the electron is finally sent to photosystem I, where the following happens.

e- + NADP+ + H ---> NADPH

Now there are two high energy molecules, fully charged and ready to be used. Plants makes more energy that it needs immediately, so the NADPH and ATP is used to make glucose as follows:

CO2 + ATP + NADPH ---> C6H12O6

This happens through another process called the Calvin cycle.