Chapter 5: Capturing and Releasing EnergyPart 1

Green Energy Today, the expression “Food is fuel” doesn’t just mean we eat

food to get energy. Since fossil fuels (such as gas and oil) are nonrenewable and

their prices are soaring, there is an increasing demand for biofuels, which are oils, gases, and alcohols that can be produced from non-fossilized organic matter like food crops (corn, soybeans, and sugarcane).

Not only do these foods contain high energy bonds that can be used by our bodies for energy, these same high energy bonds can be broken and used by our vehicles for fuel.

These high energy bonds are broken when our cells or cars use oxygen to burn the fuel and carbon dioxide is released.

Green Energy Plants use energy from sunlight to produce carbohydrates from

carbon dioxide and water. This process is called photosynthesis. This process not only produces food for plants, but almost all

other life on Earth as well. Most all living organisms extract the energy that is stored in the

carbohydrates produced during photosynthesis. These organisms are called heterotrophs, because they do

not make their own food but, rather, they must obtain energy from other sources, in this case, the carbohydrates produced during photosynthesis.

Since plants harvest energy from the environment (the sun) and get carbon from inorganic molecules (like carbon dioxide) to make their own food, they are called autotrophs.

Green Energy The reason food crops like corn are being used to produce

biofuels is that they are rich in sugars and oils that are easily converted by bacteria to biofuels such as ethanol.

Other plant materials, such as weeds or wood chips (or other agricultural waste products) are not used because they contain a large amount of cellulose.

Cellulose is a tough, complex carbohydrate that cannot be broken down by bacteria to produce biofuels as easily, so this would add to the cost of the biofuels.

Current research is focusing on finding a cost-effective way to break down the cellulose in weeds and other agricultural waste products so that food crops can be used to feed people, thus cutting down on the rising cost of these foods in the grocery store.

Capturing Rainbows Capturing the energy in sunlight is very complicated, which is

the reason why humans have still not figured out how to do it in an economically sustainable way.

This light travels in waves. The wavelength is related to the energy contained in the light.

Light is organized in packets of energy called photons. Photons that carry less energy travel in longer wavelengths

and appear as colors like red or orange to our eyes, while photons that carry more energy travel in shorter wavelengths and appear as colors like blue or purple to our eyes.

Capturing Rainbows

Low Energy High Energy

Capturing Rainbows Organisms that do photosynthesis use pigments to capture

sunlight. A pigment is an organic molecule that absorbs light of

specific wavelengths only. Wavelengths that are not absorbed are reflected. It is this reflected light that gives the pigment its characteristic

color. Foe example, if a pigment absorbs violet, blue, and green

wavelengths of light and reflects yellow, orange, and red, it will appear orange to us.

Capturing Rainbows Chlorophyll a is the most common pigment in plants,

photosynthetic bacteria, and protists. Since chlorophyll a absorbs purple and red wavelengths of

light and reflects green, it appears green to us. There are other accessory pigments that absorb and reflect

other wavelengths of light as well. This allows the plant to absorb additional wavelengths of

light, thereby effectively utilizing most of the spectrum of visible light.

Why, then, do leaves appear green until the fall?

Capturing Rainbows The reason is that there are so many chlorophylls in plants

that their green color masks all the other colors. However, in the fall, as the amount of daylight decreases, the

pigments in the leaves begin to break down. Since chlorophylls break down faster than the other

pigments, their green colors go away first, allowing us to see the reds, yellows, oranges of the other accessory pigments that are present in the leaves.

Capturing Rainbows

Storing Energy in Carbohydrates All life must have an input of energy to sustain life. However, not all forms of energy are capable of sustaining life. For example, sunlight is plentiful on Earth, but its energy can

not be used directly to power energy requiring reactions in the cell such as protein synthesis or DNA replication.

The process of photosynthesis is able to convert the light energy from the sun into chemical energy in the bonds of carbohydrates.

This chemical energy can be used to power the reactions that sustain life and can also be stored for later use.

Storing Energy in Carbohydrates

Photosynthesis occurs in photosynthetic cells in the leaves of plants.

These cells contain chloroplasts, which are organelles that are specialized to carry out photosynthesis.

Plant chloroplasts contain two outer membranes. Within these two membrane is a somewhat fluid substance called

the stroma. Floating in the stroma are the DNA of the chloroplast, ribosomes,

and a third, folded membrane called the thylakoid membrane. The folds of the thylakoid membrane form stacks of interconnected

disks called the thylakoids. The space enclosed by the thylakoid membrane is one continuous

compartment.

A Chloroplast

Storing Energy in Carbohydrates Clusters of light-capturing pigments are embedded in the

thylakoid membrane. These clusters absorb different wavelengths/energies of

light. These clusters of pigments are called photosystems. Photosystems are clusters of hundreds of hundreds of

pigments and other molecules that work as a unit to absorb light to be used for photosynthesis.

Storing Energy in Carbohydrates

Storing Energy in Carbohydrates The reactions of photosynthesis can be divided into two stages. The first stage occurs in the thylakoid membranes. Since these stages are driven by light, the reactions that occur

during this stage are called the light-dependent reactions. The light dependent reactions convert light energy to the

chemical energy in the bonds of ATP. The second stage of photosynthesis occurs in the stroma. These reactions are called the light-independent reactions

because light does not power them. The light independent reactions run on the energy that was

formed during light-dependent reactions. They use carbon dioxide and water to store the energy from the

light-dependent reactions in carbohydrates.

The Light-Dependent Reactions The light-dependent reactions convert energy from sunlight

to chemical energy in the form of ATP. They also result in the production of NADPH, which is

composed of the coenzyme NADP+ that is carrying hydrogen ions/electrons.

The Light-Dependent Reactions These reactions begin when a pigment in the thylakoid membrane

absorbs a photon lo light. This light energy excites one of the electrons in the pigment,

causing it to jump to a higher energy level. The excited electron then quickly releases its energy and drops

back down to its ground state. When the energy that was released as the excited electron

dropped back to its ground state the pair of chlorophyll a molecules at the center of a photosystem, it ejects electrons out of those chlorophyll a molecules.

These electrons then enter an electron transfer chain in the thylakoid membrane.

These electrons then pass from one molecule in the electron transfer chain to another.

The electrons release a little bit of their energy each time they are transferred.

The Light-Dependent Reactions The energy released by the electrons during their transfer in the electron

transfer chain is used to move hydrogen ions (H+) across the thylakoid membrane.

This sets up and maintains a hydrogen ion gradient across the thylakoid membrane.

These hydrogen ions are being pumped from the thylakoid compartment into the stroma, where they are unable move back through by crossing the membrane.

However, there are transport proteins in the thylakoid membrane called ATP synthases.

When hydrogen ions cross the membrane through this transport protein, it causes this protein to add a phosphate group to ADP, producing ATP, which is released into the stroma.

ADP + Pi ATP

The process by which the flow of electrons through an electron transport chain results in the formation of ATP is called electron transfer phosphorylation.

The Light-Dependent Reactions After the electrons have moved through the first electron

transport chain, they are then accepted by a second photosystem in order to replenish electrons it has lost.

But how does the first photosystem get its electrons replenished.

Electrons that have been lost by the first photosystem to the second photosystem are replaced when water molecules in the thylakoid compartment are split.

When electrons are taken from a water molecule, it splits apart into H+ ions and oxygen gas.

The oxygen gas then diffuses out of the cell, while the H+/electrons are used by the photosystem to replace the ones that were lost to the second photosystem.

The Light-Dependent Reaction What happens in the second photosystem? This photosystem absorbs light energy just as the first one

did. This energy causes electrons to be ejected from the pair of

chlorophyll molecules at its center. These electrons then enter a second, different electron

transport chain. At the end of this electron transport chain, NADP+ accepts

the electrons along with H+, forming NADPH.

Therefore, the products of the light-dependent reactions are ATP and oxygen gas (from the first photosystem) and NADPH (from the second photosystem).

The Light-Dependent Reactions

Photosystem II

Photosystem I