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Energy IV - Photosynthesis (Light Reactions)

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Terms

You should have a working knowledge of the following terms:

  • algae
  • carotenoid
  • chlorophyll
  • chloroplast
  • electromagnetic radiation
  • granum (pl. grana)
  • NADP+ and NADPH
  • photochemistry
  • photon
  • photophosphorylation
  • photosynthesis
  • photosystems I and II
  • pigment
  • thylakoid
  • wavelength
  • Z-scheme

Introduction and Goals

The majority of life on Earth could not exist without photosynthesis. Recall from the tutorial on Thermodynamics that during photosynthesis light energy is converted to chemical energy. Specifically, it is the process whereby plants, protists, and some bacteria use light, water, and carbon dioxide to make sugars. Photosynthesis is not the exact opposite of cellular respiration, but rather a separate process that just so happens to contain many similar features (e.g., the electron transport chain).


Photosynthesis can be divided into two parts: the light reactions and the light-independent reactions (also referred to as the "dark" reactions). This tutorial will cover the light reactions. This is when light energy is transformed into energy that can be used by cells. The next tutorial will cover the light-independent reactions, which is when sugar is actually made.
By the end of this tutorial you should have an understanding of:

  • Plants' requirements for photosynthesis and respiration
  • The light reactions of photosynthesis
  • Characteristics of light
  • Channeling of light energy
  • Electron transport chains and the Z-scheme
  • Generation of ATP.


Photosynthesis' Light Reactions: An Overview


Scientists are always looking for alternative energy sources. Everyone has heard of solar-powered cars, and we tend to think of them as a new technology. Plants, however, have always been solar powered. In fact, humans (and other heterotrophs) are dependent on the ability of plants to transform light energy into chemical energy via the process of photosynthesis. Humans transform stored chemical energy into mechanical energy and heat. Solar-powered cars rely on the sun's energy being transformed into electrical energy (photochemistry). In each of these cases energy is being transformed but never created or destroyed. This should be familiar as the first law of thermodynamics.
Metabolism refers to all of the chemical processes in a cell. It is usually divided into two categories: catabolism and anabolism. Cellular respiration is an example of catabolism, whereas photosynthesis is an anabolic process.
While most people think of plants when photosynthesis is mentioned, algae actually undergo much more photosynthesis than plants. It has been estimated that about 90% of the oxygen produced from photosynthesis comes from algae and phytoplankton living in oceans. Plants, protists, and even some bacteria undergo photosynthesis.

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Plants Undergo Photosynthesis and Cellular Respiration


cellular respiration: sugar + oxygen --> carbon dioxide + water + energy to do cellular work
photosynthesis: light energy + carbon dioxide + water --> oxygen + sugar
Many plants have cells, tissues, and organs that do not photosynthesize. For example, most roots have no photosynthetic capacity. Typically sugars are produced in the photosynthetic tissues and transported to other areas of the plant for immediate nutrition and storage. As already discussed, cellular respiration is required for cells to convert the chemical energy in sugars into ATP. Therefore plants must also undergo cellular respiration in all cells in order to function properly.
Remember, plants have mitochondria and chloroplasts. Therefore they are able to carry out photosynthesis and cellular respiration.

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The Wave-Like Character of Light

Photosynthesis requires light. But what is light? To answer this question, we will answer three other questions in the next few pages:

  • Where does light come from?
  • How does light travel from place to place?
  • What happens when light reaches an object?
    Sunlight comes from thermonuclear reactions in the sun, which release energy in the form of electromagnetic radiation.
    Light has wave and particle characteristics.

First, let's look at the wave properties. Light is similar to x-rays, microwaves, and radio waves because they all have a periodic fluctuating character (a wave) that travels through space. They differ because of their wavelengths (the peak-to-peak distance between oscillations). Importantly, electromagnetic waves can transfer energy to objects in their paths. Light, as shown in Figure 1, is the portion of the electromagnetic spectrum that has wavelengths detectable by the human eye. These are wavelengths of about 400-700nm.

Figure 1-The Electromagnetic Spectrum






The Particle-Like Character of Light

If light acts as a wave that travels through space, how can it also act as a particle that bumps into things as it travels? Light travels as "packets," or quanta of energy known as photons. Each photon has a specific amount of energy. In fact, photons do "bump" into things in their path. These disturbances cause some of the effects of light in the world around us.

This brings us back to the third question posed about light. What happens when light reaches another object? We have all seen light reflect off of a mirror, hence we can see ourselves. We have also avoided wearing a black shirt (compared to a white shirt) in the summer because of the heat it absorbs. These two properties of reflection and absorption can help us to understand how plants use light.
Because light consists of photons of energy, reflection can be thought of as photons bouncing off of a leaf. However sometimes the photons of light are absorbed by the leaf to do work. Pigments are compounds that absorb light. Different pigments absorb different wavelengths of light. If a structure (e.g., a leaf) lacks pigments that absorb light in a given wavelength range, then that wavelength of light will be reflected. In other words, the color perceived is that which is not absorbed.


Figure 2

As shown in Figure 2, many colors are absorbed by chlorophyll and carotenoid pigments. Importantly, note that green light is absorbed poorly, which is why chlorophyll-containing leaves appear green to our eyes; green light is not absorbed, it is reflected.

Light Energy is Channeled by Photosystems

Figure 3

The leaves of deciduous trees turn colors and fall off in autumn. The multitude of colors come from pigments (e.g., carotenoids). These pigments exist in the leaf all summer, but it is not until the pigment chlorophyll is depleted that the other colors become obvious. Why might a plant have these other pigments?

In eukaryotes the light reactions of photosynthesis occur in the thylakoids of chloroplasts. Chloroplasts appear as stacked structures. These stacks are called grana, which consist of thylakoids that are actually flattened membranes. Thylakoids house the essential components of the light reactions of photosynthesis. The thylakoid is covered with several types of pigments arranged in photosystems. (This figure depicts chlorophyll molecules embedded in a photosystem.) There are two types of photosystems, and each consists of a few hundred pigment molecules.

Figure 4

When a photon strikes a pigment molecule, its energy is passed from pigment molecule to pigment molecule until it reaches the reaction center.
The various pigments each absorb light of a different wavelength (shown here), and they send this energy to a specialized chlorophyll molecule at the center of the photosystem (the reaction center). The chlorophyll molecules of photosystem I and photosystem II are different but both play an important role in the electron transport chains of photosynthesis.

We know that light is absorbed by pigments, but how does the plant make use of the photons of energy? A pigment is a molecule made up of atoms that contain electrons.

Recall from earlier tutorials, electrons are found in association with atoms, where they can participate in bond formation. Electrons have varying amounts of energy. In a non-illuminated pigment (one in the dark), critical electrons are usually found in a low energy state. However, the energy from an incoming photon can bump the electron into a higher energy state. If a photon of light resonates with a particular pigment, it has enough energy to raise a particular electron in the pigment to the next state. When this happens, light has been absorbed and energy has been transferred. Each photosystem has an electron acceptor that ultimately renews the energized electrons.

The Electron Transport Chain: The Z-Scheme

You were introduced to the electron transport chain in the tutorial on cellular respiration.

It is simply a series of redox reactions. There are two electron transport chains used in photosynthesis. They are energetically linked to one another, and electrons traverse through both.

Figure 5

First, light is absorbed by pigments and its energy is channeled to chlorophyll, where it excites an electron. (This is photosystem II.) The electron is then transferred to the primary electron acceptor. From there it rolls downhill, energetically speaking. It loses some energy as it is passed from molecule to molecule through the chain. The electron ends up at a second type of chlorophyll molecule in photosystem I. Light is absorbed and the electron is once again excited and transferred along the chain. When this process is diagrammed, it looks like the letter Z on its side. The electron gets excited, comes back down in energy, gets excited again, and comes back down again. Thus, the two electron transport chains are called the Z-scheme.

Keep in mind, water is the source of the electrons. As shown in this figure, electrons are removed from water in photosystem II, and consequently, these electrons become energized into a more energetic state. Protons and oxygen are released as by-products of this reaction.
The electron transport chain of respiration occurs in the mitochondria, whereas the electron transport chains of photosynthesis are located in the chloroplasts. The thylakoid membrane is home to the many proteins involved.

The Electron Transport Chain: Redox

Free energy is released during redox reactions. In photosynthesis this energy is used for two things. First, hydrogen ions are moved across the membrane. The hydrogen ions really only consist of a single proton, and will be referred to that way throughout this tutorial. Remember that the electron loses some energy as it is passed along the electron transport chain because energy is required to pump the protons. The protons are moved into the thylakoid space where they accumulate; energy is required because these protons are being moved against a concentration gradient. (In other words, this is not passive diffusion.) The second use of free energy is reduction of an ultimate electron acceptor. At the end of the first electron transport chain, the electron is transferred to another molecule of chlorophyll. At the end of the second electron transport chain, the final acceptor of the electron is NADP+, which is reduced to NADPH.

Remember that this process is happening inside the cell. Where do all of these electrons come from? You learned earlier that water is the source. Water molecules are split at the beginning of the first electron transport chain. This provides electrons that become energized in the photosystems, and protons that accumulate across the membrane.

As water is split and electrons are removed (along with protons), oxygen is released. Thus, oxygen is a by-product of photosynthesis.


The Generation of ATP

Light energy can be transduced to reduce NADP+ to NADPH, and to supply energy for the accumulation of protons across the thylakoid membrane and into the thylakoid space. This occurs in algal protists and plants. (Bacteria have no chloroplasts, but they do sequester protons in one area.) The thylakoid membrane prevents protons from moving back across to the other side.

Figure 6

The charge separation across the thylakoid membrane represents stored energy in the form of a voltage (similar to what happens within mitochondria during cellular respiration). This figure compares ATP synthesis in the two organelles. As shown, chloroplasts also make use of the voltage via ATP synthase proteins in the membrane. These are similar to the ATP synthase molecules associated with cellular respiration. In a process called photophosphorylation, they harness the energy from protons moving across the membrane to make ATP from ADP and phosphate.


Summary

This tutorial has focused on the light-dependent reactions of photosynthesis, or: light + water --> NADPH + ATP; water provided the electrons and protons, resulting in oxygen as a by-product. Light excites the electrons, and it is the original source of energy for every step in the process. The transfer of electrons in the electron transport chain moves protons. This creates a voltage that is used to make ATP. The electrons complete their journey by reducing NADP+ to NADPH.

NADPH and ATP hold energy that can be used to build molecules. Next we will explore how these molecules are used to synthesize sugars.