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2.20: Chloroplasts - Biology

2.20: Chloroplasts - Biology


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What do pancakes and chloroplasts have in common?

The chloroplast is the site of photosynthesis. Part of the photosynthesis reactions occur in an internal membrane within the organelle. The chloroplast contains many of these internal membranes, making photosynthesis very efficient. These internal membranes stack on top of each other, just like a stack of pancakes.

Stages of Photosynthesis

Photosynthesis occurs in two stages, which are shown in Figure below.

  1. Stage I is called the light reactions. This stage uses water and changes light energy from the sun into chemical energy stored in ATP and NADPH (another energy-carrying molecule). This stage also releases oxygen as a waste product.
  2. Stage II is called the Calvin cycle. This stage combines carbon from carbon dioxide in the air and uses the chemical energy in ATP and NADPH to make glucose.

The two stages of photosynthesis are the light reactions and the Calvin cycle. Do you see how the two stages are related?

Before you read about these two stages of photosynthesis in greater detail, you need to know more about the chloroplast, where the two stages take place.

The Chloroplast

Chloroplasts: Theaters for Photosynthesis

Photosynthesis, the process of turning the energy of sunlight into ‘‘food,’’ is divided into two basic sets of reactions, known as the light reactions and the Calvin cycle, which uses carbon dioxide. As you study the details in other concepts, refer frequently to the chemical equation of photosynthesis: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2. Photosynthesis occurs in the chloroplast, an organelle specific to plant cells.

If you examine a single leaf of a Winter Jasmine leaf, shown in Figure below, under a microscope, you will see within each cell dozens of small green ovals. These are chloroplasts, the organelles which conduct photosynthesis in plants and algae. Chloroplasts closely resemble some types of bacteria and even contain their own circular DNA and ribosomes. In fact, the endosymbiotic theory holds that chloroplasts were once independently living bacteria (prokaryotes). So when we say that photosynthesis occurs within chloroplasts, we speak not only of the organelles within plants and algae, but also of some bacteria – in other words, virtually all photosynthetic autotrophs.

High power microscopic photo of the upper part of a Winter Jasmine leaf. Viewed under a microscope, many green chloroplasts are visible.

Each chloroplast contains neat stacks called grana (singular, granum). The grana consist of sac-like membranes, known as thylakoid membranes. These membranes contain photosystems, which are groups of molecules that include chlorophyll, a green pigment. The light reactions of photosynthesis occur in the thylakoid membranes. The stroma is the space outside the thylakoid membranes, as shown in Figure below. This is where the reactions of the Calvin cycle take place. In addition to enzymes, two basic types of molecules - pigments and electron carriers – are key players in this process and are also found in the thylakoid membranes.

You can take a video tour of a chloroplast at Encyclopedia Britannica: Chloroplast:www.britannica.com/EBchecked/...in-plant-cells.

A chloroplast consists of thylakoid membranes surrounded by stroma. The thylakoid membranes contain molecules of the green pigment chlorophyll.

Electron carrier molecules are usually arranged in electron transport chains (ETCs). These accept and pass along energy-carrying electrons in small steps (Figure below). In this way, they produce ATP and NADPH, which temporarily store chemical energy. Electrons in transport chains behave much like a ball bouncing down a set of stairs – a little energy is lost with each bounce. However, the energy “lost” at each step in an electron transport chain accomplishes a little bit of work, which eventually results in the synthesis of ATP.

This figure shows the light reactions of photosynthesis. This stage of photosynthesis begins with photosystem II (so named because it was discovered after photosystem I). Find the two electrons (2 e-) in photosystem II, and then follow them through the electron transport chain (also called the electron transfer chain) to the formation of NADPH. Where do the hydrogen ions (H+) come from that help make ATP?

Summary

  • Photosynthesis occurs in the chloroplast, an organelle specific to plant cells.
  • The light reactions of photosynthesis occur in the thylakoid membranes of the chloroplast.
  • Electron carrier molecules are arranged in electron transport chains that produce ATP and NADPH, which temporarily store chemical energy.

Explore More

Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Photosynthetic Structures
  1. What are the functions of a plant's leaves?
  2. Where do the photosynthetic reactions occur?
  3. What is a stomata? What is their role?
  4. Describe the internal structure of a chloroplast.
  5. What reactions occur in the thylakoid membranes?

Review

  1. Describe the chloroplast's role in photosynthesis.
  2. Explain how the structure of a chloroplast (its membranes and thylakoids) makes its function (the chemical reactions of photosynthesis) more efficient.
  3. Describe electron carriers and the electron transport chain.

Chloroplasts - Show Me the Green

Chloroplasts are the food producers of the cell. The organelles are only found in plant cells and some protists such as algae. Animal cells do not have chloroplasts. Chloroplasts work to convert light energy of the Sun into sugars that can be used by cells. The entire process is called photosynthesis and it all depends on the little green chlorophyll molecules in each chloroplast.

Plants are the basis of all life on Earth. They are classified as the producers of the world. In the process of photosynthesis, plants create sugars and release oxygen (O2). The oxygen released by the chloroplasts is the same oxygen you breathe every day. Mitochondria work in the opposite direction. They use oxygen in the process of releasing chemical energy from sugars.


2.20: Chloroplasts - Biology

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In plants, photosynthesis takes place within the leaf's dense mesophyll cell layers where the highest number of chloroplasts is contained.

Scattered throughout these specialized double membrane organelles are another set of compartments. Fluid filled membranous sacs called thylakoids, which are interconnected and form into multiple stacks called grana.

On the outer edges of each granum, embedded within the thylakoid membranes, are multiprotein complexes such as the photosystems. These structures contain the antenna proteins bound with numerous pigment molecules, like chlorophylls, to absorb light and begin the first stage of light dependent reactions.

Meanwhile the second stage, the Calvin cycle, takes place in the stroma, the aqueous cavity outside of the thylakoid's lipid bi-layer. With both processes working together, plants produce their own food thanks to the biochemical factories found in the chloroplast.

9.3: Anatomy of Chloroplasts

Green algae and plants, including green stems and unripe fruit, harbor chloroplasts&mdashthe vital organelles where photosynthesis takes place. In plants, the highest density of chloroplasts is found in the mesophyll cells of leaves.

A double membrane surrounds chloroplasts. The outer membrane faces the cytoplasm of the plant cell on one side and the intermembrane space of the chloroplast on the other. The inner membrane separates the narrow intermembrane space from the aqueous interior of the chloroplast, called the stroma.

Within the stroma, another set of membranes form disk-shaped compartments&mdashknown as thylakoids. The interior of a thylakoid is called the thylakoid lumen. In most plant species, the thylakoids are interconnected and form stacks called grana.

Embedded in the thylakoid membranes are multi-protein light-harvesting (or antenna) complexes. These complexes consist of proteins and pigments, such as chlorophyll, that capture light energy to perform the light-dependent reactions of photosynthesis. These processes release oxygen and produce chemical energy in the form of ATP and NADPH.

The second part of photosynthesis&mdashthe Calvin cycle&mdashis light-independent and takes place in the stroma of the chloroplast. The Calvin cycle captures CO2 and uses the ATP and NADPH to ultimately produce sugar.

Chloroplasts coordinate the two stages of photosynthesis. Photosynthesis releases oxygen and sugars&mdashthe basis of plant biomass which directly or indirectly feeds most life on Earth.

Jensen, Poul Erik, and Dario Leister. &ldquoChloroplast Evolution, Structure and Functions.&rdquo F1000Prime Reports 6 (June 2, 2014). [Source]

Bobik, Krzysztof, and Tessa M. Burch-Smith. &ldquoChloroplast Signaling within, between and beyond Cells.&rdquo Frontiers in Plant Science 6 (2015). [Source]


Artificial chloroplasts turn sunlight and carbon dioxide into organic compounds

Just like mechanics cobble together old engine parts to build a new roadster, synthetic biologists have remade chloroplasts, the engine at the heart of photosynthesis. By combining the light-harvesting machinery of spinach plants with enzymes from nine different organisms, scientists report making an artificial chloroplast that operates outside of cells to harvest sunlight and use the resulting energy to convert carbon dioxide (CO2) into energy-rich molecules. The researchers hope their souped-up photosynthesis system might eventually convert CO2 directly into useful chemicals—or help genetically engineered plants absorb up to 10 times the atmospheric CO2 of regular ones.

“[This] is very ambitious,” says Frances Arnold, a chemical engineer at the California Institute of Technology who wasn’t involved in the research. She says the work’s effort to reprogram biology could improve attempts to convert CO2 directly into useful chemicals.

Photosynthesis is a two-step process. In chloroplasts, chlorophyll molecules absorb sunlight and pass the extra energy to molecular partners that use it to generate the energy-storing chemicals adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). A suite of other enzymes working in a complex cycle then use ATP and NADPH to convert CO2 from the air into glucose and other energy-rich organic molecules that the plant uses to grow.

CO2 conversion starts with an enzyme called RuBisCO, which prompts CO2 to react with a key organic compound, starting a chain of reactions needed to make vital metabolites in plants. As effective as photosynthesis is, it also has a problem, says Tobias Erb, a synthetic biologist at the Max Planck Institute for Terrestrial Microbiology. “RuBisCO is superslow,” he says. Each copy of the enzyme can grab and use just five to 10 CO2 molecules per second. That puts a speed limit on how fast plants can grow.

In 2016, Erb and his colleagues sought to ramp things up by designing a new set of chemical reactions. Instead of RuBisCO, they substituted a bacterial enzyme that can catch CO2 molecules and force them to react 10 times faster. In combination with 16 other enzymes from nine different organisms, this created a new CO2-to-organic-chemical cycle they dubbed the CETCH cycle.

That took care of the second step. But to get the whole process to run on sunlight—the first step—Erb and his colleagues turned to chloroplast components called thylakoid membranes, pouchlike assemblies that hold chlorophyll and other photosynthesizing enzymes. Other researchers had previously shown that thylakoid membranes can operate outside plant cells. So Erb and his colleagues plucked thylakoid membranes from spinach leaf cells and showed that their assemblies, too, could absorb light and transfer its energy to ATP and NADPH molecules. Pairing the light-harvesting thylakoids with their CETCH cycle system allowed the team to use light to continually convert CO2 to an organic metabolite called glycolate, they reported yesterday in Science .

In order to integrate the light-harvesting apparatus with the CETCH cycle, the researchers had to make a few tweaks, Erb notes, swapping in and out a few of the CETCH pathway’s enzymes. To optimize the full ensemble, Erb and his colleagues teamed up with Jean-Christophe Baret, a microfluidics expert at the Paul Pascal Research Center. Baret’s team designed a device that generates thousands of tiny water droplets in oil and injects each one with different amounts of thylakoid membrane assemblies and CETCH cycle enzymes. That allowed the researchers to home in on the most efficient recipe for producing glycolate. Further comparisons of all the possible combinations and concentrations of different elements could make the process even more efficient, Arnold comments. “This is a nice way to do it.”

Erb says he and his colleagues hope to modify their setup further to produce other organic compounds that are even more valuable than glycolate, such as drug molecules. They also hope to more efficiently convert captured CO2 into organic compounds that plants need to grow. That would open the door to engineering the genes for this novel photosynthesis pathway into crops to create novel varieties that grow much faster than current varieties—a boon for agriculture in a world with a booming population.