PHOTOSYNTHESISSeptember 3, 2009

Biology I

Autotrophs and Heterotrophs

Plants and some other types of organisms are able to use light energy from the sun to produce food.

Organisms, such as plants, which make their own food, are called autotrophs.

Other organisms, such as animals, cannot use the sun’s energy to make their own food—these are known as heterotrophs. They get energy from the foods they consume.

Adenosine Triphosphate Cell activity is powered

by chemical fuels. One of the principle compounds living things use is adenosine triphosphate (also known as ATP).

Composed of adenine(which contains nitrogen), ribose(5-ring sugar), and three phosphate groups.

Adenine

Ribose

Phosphate Groups (3)

Adenosine DiphosphateAdenosine Diphosphate (ADP) is similar to ATP but it only has two phosphate groups instead of three. When cells have energy available, they can store small amounts of energy by adding a phosphate group to ADP molecules, as shown in the picture.

What is Photosynthesis? Definition: The process by which plants

and some other organisms use light energy to power chemical reactions that convert water and carbon dioxide (CO2) into oxygen(O2) and high-energy carbohydrates such as sugars and starches.

The following experiments have contributed to the modern understanding of the process of photosynthesis.

Van Helmont Jan van Helmont’s Experiment: He

devised an experiment to find out if plants grew by taking material out of the soil. He planted a seedling and watered it regularly.

After 5 years he concluded that most of the mass of the now 75kg tree, had come from water, because that was the only thing he added to the pot.

He gave us the “hydrate” part of carbohydrate, but where did the carbo- come in?

Priestley Joseph Priestley’s Experiment: After placing a

candle in a glass jar, he watched the flame gradually die out! He reasoned that something in the air was necessary to keep the flame burning. When the substance was used up, the candle went out.

That substance was oxygen. He also found that if he placed a live sprig of

mint under the jar and allowed a few days to pass, the candle could be relight and would remain lighted for a while. The mint leaf released oxygen.

Ingenhousz Jan Ingenhousz’s Experiment: He

expanded on Priestly’s experiment by showing that the candle could remain burning only if the plant was exposed to light.

The experiments performed by van Helmont, Priestley, Ingenhousz, and other scientists reveal that in the presence of light, plants transform carbon dioxide and water into carbohydrates and release.

Photosynthesis Equation LEARN BOTH OF THESE:

6CO2 + 6H2O -------→ C6H12O6 + 6O2

Carbon dioxide + water -------→ sugar + oxygen

Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into oxygen and high-energy sugar.

light

light

Light and Pigments In addition to water and carbon dioxide,

photosynthesis requires light and chlorophyll, a molecule in chloroplasts.

Plants gather the sun’s energy with light-absorbing molecules called pigments. The plants’ principal pigment is chlorophyll

There are two main types of chlorophyll: chlorophyll a and chlorophyll b.

Light and Pigment

Notice how chlorophyll a absorbs light in the violet and red regions of the visible spectrum, and chlorophyll b absorbs light in the blue and red regions of the visible spectrum

Chloroplasts In plants and other photosynthetic eukaryotes,

photosynthesis takes place inside chloroplasts. Chloroplasts contain saclike photosynthetic

membranes called thylakoids. Thylakoids are arranged in stacks known as

grana(plural) or granum(singular). Thylakoids contain clusters of chlorophyll and

other pigments and proteins known as photosystems that capture the energy of sunlight.

The Calvin cycle takes place in the stroma, the region outside the thylakoid membranes.

Chloroplast

Overview of Photosynthesis

Summary of Light-Dependent Reactions

A. Light is absorbed by chlorophyll or other pigments in photosystem II. (inner thylakoid space)

B. High-energy electrons from photosystem II move through the electron transport chain in photosystem I. The molecules in the electron transport chain use energy from the electrons to transport H+ ions from the stroma into the inner thylakoid.

C. As in photosystem II, pigments add energy from light to the electrons. The high energy electrons are then picked up by NADP+ to form NADPH.

Summary of Light-Dependent Reactions

D. The inside of the thylakoid membrane fills up with H+ ions. This action makes the outside of the thylakoid membrane negatively charged and the inside positively charged.

E. As H+ ions pass through ATP synthase, it converts ADP to ATP.

Please note: NADP+ is nicotinamide adenine dinucleotide phosphate. It is a carrier molecule that picks up 2 high energy electrons along with H+.

Light-Dependent Reaction Please note that this picture is upside down! The thylakoid space is at the bottom!

The Calvin CycleA. Six carbon dioxide molecules are combined

with six 5-carbon molecules to produce twelve 3-carbon molecules.

B. Energy from ATP and high-energy electrons from NADPH are used to convert the twelve 3-carbon molecules into higher-energy forms.

C. Two 3-carbon molecules are used to make a 6-carbon sugar.

D. The ten remaining 3-carbon molecules are converted back into six 5-carbon molecules, which are used in the next cycle.

Calvin Cycle

Twelve 3-carbon

molecules

Twelve 3-carbon

molecules

Ten 3-carbon

molecules

Six 5-carbon

molecules

12 ATP

12 ADP

12 NADPH

12 NADP+

Two 3-carbon

molecules

One 6-carbon sugar

6 ATP

6 ADP

6 carbon molecules and

CO2

A B

CD

Any Questions?