Photosynthesis: Capturing the Energy in Sunlight

Energy Needs of Living Things Organisms can be classified

according to how they get energy:

Heterotrophs must consume other other heterotrophs or autotrophs to get energy.

Autotrophs manufacture their own food from inorganic molecules and energy. Most autotrophs are photosynthetic.

Plants, cyanobacteria, and algae are photosynthetic autotrophs.

Photosynthesis is not one chemical reaction, but a series of linked chemical reactions called a biochemical pathway.

During photosynthesis, energy from the sun is used to link together carbon dioxide molecules with H2O molecules to produce glucose, a simple sugar, and a powerful energy source for living things.

Oxygen is a by-product of photosynthesis, and is released to the atmosphere.

Photosynthesis and Respiration

Photosynthesis and respiration are related. The reactants of one process are the products of the other.

Photosynthesis is a way to trap and store energy in the chemical bonds of glucose.

Respiration is a way to release the energy stored in the chemical bonds of glucose so that it can power the reactions of the cell.

Photosynthetic autotrophs carry out photosynthesis, but all living things carry out cellular respiration, at least in part, to obtain energy for cellular processes.

Photo-synthesis

Cellular Respiration

6CO2 + 6H2O + Energy

(Photosynthetic autotrophs only)

C6H12O6 + 6O2

(Glucose)

(All Cells)

Light Absorption in Plants In plants, photosynthesis is divided

into two groups of reactions: the light reactions and the dark reactions.

In plants and algae, photosynthesis takes place in specialized organelles called chloroplasts.

The structure of chloroplasts is the same, regardless of which organism contains them.

Chloroplasts are surrounded by not one, but two membranes. Inside the double membrane is another system of membranes arranges as many flattened sacs called thylakoids.

Thylakoids are connected to each other and are piled together into stacks called grana. The fluid-filled spaces between grana make up the stroma of the chloroplast.

Structure of a Chloroplast

Double Membrane

Thylakoid

Granum

Stroma

Light and Pigments Light is a form of energy that travels

through space in the form of electromagnetic waves.

Sunlight appears white, but it is a mixture of different colors, each of which has a different wavelength.

When sunlight is separated into its different colors, this is called the visible spectrum.

When sunlight hits an object, it can be absorbed, transmitted, or reflected.

Pigments are chemicals that absorb some wavelengths of light and transmit or reflect other wavelengths.

Pigments are used to absorb certain wavelengths of light to power the chemical reactions of photosynthesis.

The main photosynthetic pigments are several kinds of chlorophylls. The most common types are chlorophyll a and chlorophyll b.

Chlorophyll a is directly involved in the light reactions of photosynthesis. Chlorophyll b and other pigments capture light wavelengths that chlorophyll a cannot absorb.

Photosystems In the thylakoid membranes,

groups of pigment molecules cluster together in arrangements called photosystems.

These photosystems include chlorophyll a, chlorophyll b, and also yellow-orange pigments called carotenoids. In some plants, other accessory pigments are found.

There are two types of known photosystems: Photosystem I and Photosystem II. They work in a similar way, but they do different things in the system of Light Reactions.

In the thylakoid membrane, the two photosystems are embedded along with two electron transport chains.

These molecules perform the biochemical pathways of the Light Reactions of Photosynthesis.

Stroma of Chloroplast

Inside of Thylakoid

Thylakoid Membrane

Photosystem II Photosystem I

Electron Transport Chain Electron Transport Chain

e- e-

NADP + H+ NADPH

e-

H+

H+

ADP + PATP

The Light Reactions – Part 1 The light reactions can be grouped into five steps:1) Light energy strikes photosystem II and causes an electron in chlorophyll a to jump to a

higher energy level. The electron is said to be “excited.”2) The excited electron is passed to a primary acceptor in the electron transport chain. 3) The electron is now passed through a series of oxidation-reduction reactions down the

chain until all of its excess energy is bled off. This energy is used to move H+ ions into the thylakoid. The protons diffuse back through the thylakoid membrane and power the synthesis of ATP by the embedded enzyme ATP synthase. This process of ATP formation is called chemiosmosis.

4) Light energy also excites a chlorophyll a electron in photosystem I. The e- moves to another primary acceptor, and is replaced in the chlorophyll molecule by the e - from photosystem II.

5) The primary acceptor passes the e- down the second electron transport chain, and its energy is used to synthesize NADPH from NADP+.

H+

ADP + PATP

H+

e-

e-

e-

NADP + H+ NADPH

The Light Reactions – Part 2 In order for the light reactions of

photosynthesis to continue, the electrons lost from the chlorophyll a molecules in each of the photosystems must be replaced.

In the previous slide, it was stated that the electron lost from photosystem I was replaced by an electron from photosystem II. This is true, but how is the electron lost from photosystem II replaced?

It is replaced by electrons obtained by splitting water molecules.

Embedded next to photosystem II in the thylakoid membrane is a water-splitting enzyme.

For each 2 water molecules split, 4 electrons are available to replace lost electrons from chlorophyll a. Four protons are also released inside the thylakoid space, which help to power ATP synthesis.

Two oxygen atoms are also released. This is the molecular oxygen released to the atmosphere as a byproduct of photosynthesis.

Water-splitting enzyme

Photosystem II

2H2O

H+

ADP + PATP

H+

e- e-

4H+

O2

4e-

The Dark Reactions – Part 1 At the end of the Light Reactions of

photosynthesis, light energy has been used to make ATP and NADPH, both energy-storing molecules.

Living things are made of large molecules like carbohydrates, proteins, nucleic acids and lipids. Energy has been captured, but none of these molecules have been made during the Light Reactions.

Organic molecules are made during the Dark Reactions of photosynthesis. The energy harnessed in ATP and NADPH is used to build the carbohydrate molecule glucose, which can then be used to make other macromolecules.

The Dark Reactions of photosynthesis form a circular biochemical pathway called the Calvin Cycle.

In the Calvin Cycle, CO2 molecules are joined together to form glucose. This process is called carbon fixation. The Calvin Cycle takes place in the stroma of the chloroplast.

Light Reactions Summary Equation: (not balanced)

Light Energy + ADP + NADP+ + H2O ATP + NADPH + O2

Dark Reactions Summary Equation: (not balanced)

CO2 + ATP + NADPH C6H12O6 + ADP + NADP+

The Dark Reactions – Part 2 The Dark Reactions can be summarized

in three major steps: Step 1: CO2 diffuses into the stroma of

the chloroplast from the cell cytoplasm. There, an enzyme combines the CO2 with a 5-C compound called ribulose bisphosphate (RuBP) to make an unstable 6-C compound. This 6-C splits into two 3-C molecules of phosphoglycerate (PGA).

Step 2: PGA changes into another 3-C molecule, glyceraldehyde 3-phosphate (PGAL also called G3P) in a two-step process. During this process, the PGA gets a high energy phosphate group from ATP and then gets a proton from NADPH, and releases the phosphate group.

The products of the two reactions of Step 2 are PGAL, ADP, NADP+

and phosphate.The ADP, NADP+ and phosphate can be reused in the Light Reactions.

Step 3: Most of the PGAL (G3P) that has been made is converted back into RuBP to start the Calvin Cycle again. These reactions use an ATP. Some of the PGAL (G3P) leaves the Calvin Cycle pathway and is used to make glucose and other organic molecules.

The Calvin Cycle

C C C C C

RuBP

CCO2

C C C C C C

Two molecules of PGA

C C C C C C

Two molecules of PGAL (G3P)

Step 1

Organic Molecules

Step 3

ADP

ATP

Step 22 NADPH

2 NADP+

2 Phosphates

2 ATP

2 ADP

Photosynthesis – Other Information How much ATP and NADPH are

needed to make one molecule of glucose from CO2? Each turn of the Calvin Cycle fixes one molecule of CO2, so six turns of the Calvin Cycle will make one glucose molecule.

Since each turn of the Calvin Cycle uses 3 ATP and 2 NADPH, one molecule of glucose would “cost” 18 ATP and 12 NADPH molecules.

Glucose is not really made during the reactions of photosynthesis. The final product of the Calvin cycle is PGAL, which can then be made into glucose or a large variety of other organic molecules.

The light reactions of photosynthesis MUST take place during the day because they are “light dependent,” but the dark reactions of photosynthesis can take place during the day or the night. They are often called “light independent.”

Dark Reactions - Other Pathways The Calvin Cycle is the most

common pathway used by plants to fix carbon. There are other pathways that are also used.

The plants that use the Calvin Cycle to fix carbon are called C3 plants because the carbon compound PGA that is first formed is a 3-C compound. Most plants are C3 plants.

In climates that are very hot and dry, plants have problems with water loss. C3 plants do not flourish under these conditions. If they close their stomata to conserve water, they run out of CO2 for carbon fixation. If they open their stomata, they can dehydrate to death.

One alternative pathway for carbon fixation is called the C4 pathway. These plants (C4 plants) initially fix CO2 into a 4-C compound even when the CO2 levels inside the leaf are very low. These plants can then partially close their stomata and conserve water. Some C4 plants are corn, sugar cane, and crabgrass.

Another pathway is called CAM (crassulacean acid metabolism). CAM plants open their stomata at night and close them during the day. At night, they take in CO2 and fix it temporarily into certain compounds. During the day, CO2 is released from those compounds and then fed into the Calvin Cycle. Carbon can now be fixed with minimal water loss. Cacti and pineapples are CAM plants.