BIS 1A Handout 10 - Photosynthesis

The Process That Feeds the Biosphere

The chloroplasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules.

Plants and other autotrophs are the producers of the biosphere.

Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment.

Autotrophs are the ultimate sources of organic compounds for all heterotrophic organisms.

Photoautotrophs use light as a source of energy to synthesize organic compounds.

Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes.

Chemoautotrophs harvest energy from oxidizing inorganic substances, such as sulfur and ammonia.

Chemoautotrophy is unique to prokaryotes.

Heterotrophs live on organic compounds produced by other organisms.

These organisms are the consumers of the biosphere.

The most obvious type of heterotrophs feeds on other organisms. Animals feed this way.

Other heterotrophs decompose and feed on dead organisms or on organic litter, like feces and fallen leaves. Most fungi and many prokaryotes get their nourishment this way.

Photosynthesis converts light energy to the chemical energy of food

The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts.

Chlorophyll plays an important role in the absorption of light energy during photosynthesis.

Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf.

O2 exits and CO2 enters the leaf through microscopic pores called stomata in the leaf.

Veins deliver water from the roots and carry off sugar from mesophyll cells to nonphotosynthetic areas of the plant.

A typical mesophyll cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long.

Each chloroplast has two membranes around a central aqueous space, the stroma.

In the stroma is an elaborate system of interconnected membranous sacs, the thylakoids.

Chlorophyll is located in the thylakoids.

Photosynthetic prokaryotes lack chloroplasts, but have infolded regions of the plasma membranes, functioning similar to the thylakoid membranes of chloroplasts.

Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis.

The equation describing the process of photosynthesis is:

6CO2 + 12H2O + light energy ( C6H12O6 + 6O2+ 6H2O

C6H12O6 is glucose.

The overall chemical change during photosynthesis is the reverse of cellular respiration.

In its simplest possible form: CO2 + H2O + light energy ( [CH2O] + O2

[CH2O] represents the general formula for a sugar.

Hydrogen extracted from water is incorporated into sugar, and oxygen is released to the atmosphere (where it can be used in respiration).

Photosynthesis is a redox reaction.

It reverses the direction of electron flow in respiration.

Water is split and electrons transferred with H+ from water to CO2, reducing it to sugar.

Because the electrons increase in potential energy as they move from water to sugar, the process requires energy.

Here is a preview of the two stages of photosynthesis.

Photosynthesis is two processes, each with multiple stages.

The light reactions (photo) convert solar energy to chemical energy.

The Calvin cycle (synthesis) uses energy from the light reactions to incorporate CO2 from the atmosphere into sugar.

In the light reactions, light energy absorbed by chlorophyll in the thylakoids drives the transfer of electrons and hydrogen from water to NADP+ (nicotinamide adenine dinucleotide phosphate), forming NADPH.

Water is split in the process, and O2 is released as a by-product.

The light reaction also generates ATP using chemiosmosis, in a process called photophosphorylation.

The Calvin cycle is named for Melvin Calvin who, with his colleagues, worked out many of its steps in the 1940s.

The cycle begins with the incorporation of CO2 into organic molecules, a process known as carbon fixation.

The fixed carbon is reduced with electrons provided by NADPH.

ATP from the light reactions also powers parts of the Calvin cycle.

Thus, it is the Calvin cycle that makes sugar, but only with the help of ATP and NADPH from the light reactions.

While the light reactions occur at the thylakoids, the Calvin cycle occurs in the stroma.

The light reactions convert solar energy to the chemical energy of ATP and NADPH

Light is a form of electromagnetic radiation. The most important segment for life is a narrow band between 380 to 750 nm, the band of visible light.

While light travels as a wave, many of its properties are those of a discrete particle, the photon.

Photons are not tangible objects, but they do have fixed quantities of energy.

When light meets matter, it may be reflected, transmitted, or absorbed.

A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.

The light reaction can perform work with those wavelengths of light that are absorbed.

There are several pigments in the thylakoid that differ in their absorption spectra.

Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis.

The action spectrum of photosynthesis does not match exactly the absorption spectrum of any one photosynthetic pigment, including chlorophyll a.

Only chlorophyll a participates directly in the light reaction, but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.

Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.

Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light.

These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll.

They also interact with oxygen to form reactive oxidative molecules that could damage the cell.

When a molecule absorbs a photon, one of that molecule's electrons is elevated to an orbital with more potential energy.

Chlorophyll excited by absorption of light energy produces very different results in an intact chloroplast than it does in isolation.

In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems.

A photosystem is composed of a reaction center surrounded by a light-harvesting complex.

Each light-harvesting complex consists of pigment molecules (which may include chlorophyll a, chlorophyll b, and carotenoid molecules) bound to particular proteins.

When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the reaction center.

At the reaction center is a primary electron acceptor, which accepts an excited electron from the reaction center chlorophyll a.

There are two types of photosystems in the thylakoid membrane.

Photosystem I (PS I) has a reaction center chlorophyll a that has an absorption peak at 700 nm.

Photosystem II (PS II) has a reaction center chlorophyll a that has an absorption peak at 680 nm.

The differences between these reaction centers (and their absorption spectra) lie not in the chlorophyll molecules, but in the proteins associated with each reaction center.

During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic.

Noncyclic electron flow, the predominant route, produces both ATP and NADPH.

Photosystem II absorbs a photon of light. One of the electrons of P680 is excited to a higher energy state.

This electron is captured by the primary electron acceptor, leaving the reaction center oxidized.

An enzyme extracts electrons from water and supplies them to the oxidized reaction center. This reaction splits water into two hydrogen ions and an oxygen atom that combines with another oxygen atom to form O2.

Photoexcited electrons pass along an electron transport chain, producing ATP.

Meanwhile, light energy has excited an electron of PS I's P700 reaction center. The photoexcited electron was captured by PS I's primary electron acceptor, leaving it oxidized. An electron that from from PS II reduces the PS I reaction center.

Photoexcited electrons are passed from PS I's primary electron acceptor down a second electron transport chain through the protein ferredoxin (Fd), reducing NADP+ to NADPH.

The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH.

Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow.

Excited electrons cycle from their reaction center to a primary acceptor, along an electron transport chain, and return to the oxidized P700 chlorophyll.

As electrons flow along the electron transport chain, they generate ATP by cyclic photophosphorylation.

There is no production of NADPH and no release of oxygen.

Noncyclic electron flow produces ATP and NADPH in roughly equal quantities.

However, the Calvin cycle consumes more ATP than NADPH.

Cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.

Chloroplasts generate ATP by chemiosmosis.

The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space inside the thylakoid.

The thylakoid membrane makes ATP as the hydrogen ions diffuse down their concentration gradient from the thylakoid space back to the stroma through ATP synthase complexes.

When chloroplasts are illuminated, the pH in the thylakoid space drops to about 5 and the pH in the stroma increases to about 8, a thousandfold different in H+ concentration.

The light-reaction "machinery" produces ATP and NADPH on the stroma side of the thylakoid.

The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

The Calvin cycle regenerates its starting material after molecules enter and leave the cycle.

Carbon enters the cycle as CO2 and leaves as sugar.

The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

Each turn of the Calvin cycle fixes one carbon.

For the net synthesis of one G3P molecule, the cycle must take place three times, fixing three molecules of CO2.

To make one glucose molecule requires six cycles and the fixation of six CO2 molecules.

Phase 1: Carbon fixation

In the carbon fixation phase, each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).

This is catalyzed by RuBP carboxylase or rubisco.

Rubisco is the most abundant protein in chloroplasts and probably the most abundant protein on Earth.

The six-carbon intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate for each CO2.

Phase 2: Reduction

ATP and NADPH reduce each 3-phosphoglycerate to G3P.

After three rounds of the cycle, we would have six molecules of G3P (18C).

One of these can exit the cycle and be used by the plant cell.

Phase 3: Regeneration

The other five G3P (15C) remain in the cycle to regenerate three RuBP. In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle to regenerate three molecules of RuBP.

For the net synthesis of one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH.

The G3P from the Calvin cycle is the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates.

Alternative mechanisms of carbon fixation have evolved in hot, arid climates

One of the major problems facing terrestrial plants is dehydration.

The stomata are not only the major route for gas exchange (CO2 in and O2 out), but also for the evaporative loss of water.

When their stomata partially close on a hot, dry day, CO2 levels drop as CO2 is consumed in the Calvin cycle.

At the same time, O2 levels rise as the light reaction converts light to chemical energy.

When the O2:CO2 ratio increases, rubisco can add O2 to RuBP.

When rubisco adds O2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called photorespiration.

The two-carbon fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes.

Unlike normal respiration, this process consumes ATP.

Unlike photosynthesis, photorespiration consumes organic molecules.

Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration.

C4 plants first fix CO2 in a four-carbon compound.

Several thousand plants, including sugarcane and corn, use this pathway.

The key enzyme, phosphoenolpyruvate carboxylase, adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetate.

PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot (i.e., on hot, dry days when the stomata are closed).

The mesophyll cells pump these four-carbon compounds into bundle-sheath cells.

The bundle-sheath cells strip a carbon from the four-carbon compound as CO2, and return the three-carbon remainder to the mesophyll cells.

The bundle-sheath cells use rubisco and the Calvin cycle with an abundant supply of CO2.

C4 photosynthesis minimizes photorespiration and enhances sugar production.

A second strategy to minimize photorespiration is found in succulent plants, cacti, pineapples, and several other plant families.

These plants are known as CAM plants for crassulacean acid metabolism.

They open their stomata during the night and close them during the day.

During the night, these plants fix CO2 into a variety of organic acids in mesophyll cells.

During the day, the light reactions supply ATP and NADPH to the Calvin cycle, and CO2 is released from the organic acids.

Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle.

In C4 plants, carbon fixation and the Calvin cycle are spatially separated.

In CAM plants, carbon fixation and the Calvin cycle are temporally separated.

Both eventually use the Calvin cycle to make sugar from carbon dioxide.

Here is a review of the importance of photosynthesis.

Sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons to synthesize all the major organic molecules of cells.

Plants also store excess sugar by synthesis of starch.

Heterotrophs, including humans, may completely or partially consume plants for fuel and raw materials.

On a global scale, photosynthesis is the most important process on Earth.

It is responsible for the presence of oxygen in our atmosphere.