Introduction and Goals
The majority of life on Earth could not exist without photosynthesis. Recall from the tutorial on Thermodynamics (Tutorial 23) 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., an electron transport chain).
Photosynthesis can be divided into two parts: the light-dependent reactions and the light-independent reactions (also referred to as the "dark" reactions). This tutorial will cover the light-dependenet reactions. These reactions transform light energy into chemical energy that can be used by cells. The next tutorial will cover the light-independent reactions which synthesize sugar.
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.
- Illustrate the importance of redox reactions in photosynthesis
- Discuss the role of pigment molecules in photosynthesis
- Identify the role of Photosystems I and II in the light dependent reactions
- Illustrate the flow of electrons through the ETCs and Photosystems
- Demonstrate how the hydrogen ion gradient across the thylakoid membrane is produced
- Explain how ATP is produced through photophosphorylation
- Review the relationship between the light-dependent and light-independent reactions
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.
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.
The Wave-Like Character of Light
Photosynthesis requires sunlight. But what is sunlight? To answer this question, we will answer three questions:
- Where does sunlight come from?
- How does sunlight travel from place to place?
- What happens when sunlight reaches an object?
The answer to the first question is that sunlight comes from thermonuclear reactions in the sun, which release energy in the form of electromagnetic radiation.
The answer to the second question is that sunlight (and other forms of light) has both wave and particle characteristics and it travels from place to place as both a wave and as discrete particles of energy.
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. Visible 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 (Click image to enlarge)
The Particle-Like Character of Light
If light acts as a wave that travels through space, how can it also act as a particle? Light travels as "packets," or quanta of energy known as photons. Each photon has a specific amount of energy. Photons can "bump" into things in their path. These disturbances cause some of the effects of light in the world around us.
This brings us to the third question posed about light. What happens when light reaches another object? We have all seen light reflect off of a mirror, which allows us to see ourselves. We have also avoided wearing a black shirt (instead of 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.
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 by green plants, it is reflected.
Figure 2. Absorption spectra for various photosynthetic pigments. (Click image to enlarge)
Light Energy is Channeled by Photosystems
The leaves of deciduous trees are usually green in the summer and then turn colors and fall off in autumn. These bright fall colors come from red and orange pigments such as 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.
In eukaryotes the light-dependent reactions of photosynthesis occur in the thylakoids of chloroplasts (Fig. 3). Chloroplasts are flattened disc shaped organelles that contain stacks of thylakoids called grana. Thylakoids contain the essential components (e.g., pigments and electron transport chains) of the light reactions of photosynthesis. The thylakoid is covered with several types of pigments arranged into photosystems.
Figure 3. The structure of a chloroplast and a chlorophyll molecule. (Click image to enlarge)
Figure 4 shows the structure of a photosystem. In most land plants, there are two types of photosystems - photosystem I and photosystem II. Structurally, these photosystems are similar and the following description of the photosynthetic activity in a photosystem can be applied to either photosystem.
The various pigments in a photosystem (e.g., different forms of chlorophyll and carotenoids) each absorb sunlight of a different wavelength (shown in fig. 4). The absorbed energy raises electrons associated with the pigment molecule to a higher energy state. This causes the electrons to become unstable and the unstable electrons are passed to a specialized chlorophyll molecule at the center of the photosystem (the reaction center). From this reaction center, the electrons will be passed to an electron transport chain.
How does this movement of electrons occur? 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 of light can bump the electron into a higher energy state. When this happens, light energy has been absorbed and energy has been transformed.
Figure 4. The structure of a photosystem. (Click image to enlarge)
The Electron Transport Chains: The Z-Scheme
As you learned in Tutorial 25 (Energy III - The Krebs Cycle and Electron Transport Chain), an electron transport chain is a series of redox reactions. There are two electron transport chains used in photosynthesis and they are energetically linked to one another as electrons travel through both.
Figure 5 illustrates the movement of electrons through the two electron transport chains. Both photosystems are absorbing sunlight at the same time but let's start our analysis by looking at the activity in photosystem II. Light is absorbed by photosynthetic pigments and the absorbed energy excites an electron to a higher energy state. The excited electron moves around the photosystem until it is transferred to the primary electron acceptor molecule of the associated electron transport chain. The electron travels through the electron transport chain (via a series of redox reactions) until it is passed to a chlorophyll molecule in photosystem I. Sunlight is absorbed by the pigment molecules in photosystem I and the electron is once again excited and transferred along the electron transport chain associated with photosystem I. When this process is diagrammed, it looks like the letter Z on its side. An electron is excited to a higher energy state, falls back down in energy, is excited again, and falls back down again. Thus, the two electron transport chains are called the Z-scheme.
Water is the source of the electrons that are used in photosynthesis. 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.
Remember, the electron transport chain of respiration occurs in the mitochondria, while the electron transport chains of photosynthesis are located in the chloroplasts.
Figure 5. The Z-Scheme of electron transport through the light-dependent stages of photosynthesis. (Click image to enlarge)
The Electron Transport Chain: Redox
Free energy is released during the redox reactions of the electron transport chains. This energy is used in two ways.
In the electron transport chain that follows photosytem II, the movement of electrons through the chain releases energy that is used to move protons across the thylakoid membrane just as in the electron transport chain of cellular respiration in the mitochondria. The protons are moved into the thylakoid space where they accumulate and produces a voltage across the membrane. Also associated with this electron transport chain is the enzyme ATP synthase. The ATP synthase uses the voltage formed by the build-up of protons in the thylakoid space to produce ATP. Functionally, the electron transport chain that follows photosystem II is similar to the electron transport chain of respiration; the accumulation of protons on one side of a membrane provides the free energy that ATP synthase needs to drive the production of ATP.
In the electron transport chain that follows photosystem I, the electrons that move through the chain are used to reduce NADP+ *to *NADPH.
The two products of the light-dependent reactions of photosystem are ATP and NADPH. The movement of high energy electrons releases the free energy that is needed to produce these molecules. The ATP and NADPH are used in the light-independent reactions to make sugar.
Where do the electrons come from? Water molecules are split at the beginning of the first electron transport chain. The splitting of water provides electrons that become energized in the photosystems, and protons that accumulate across the membrane.
As water is split and electrons and protons are removed, oxygen is released. Thus, oxygen is a by-product of photosynthesis. Recall from Tutorial 6 (Prokaryotes III - Evolution and Early Metabolism) the oxygen that is released as by-product of photosynthesis had a major impact on the early earth's atmosphere and is the source of the oxygen that we breathe.
The Generation of ATP
Light energy raises electrons associated with pigment molecules to a higher energy state and these electrons are used to reduce NADP+ to NADPH, and to supply energy for the accumulation of protons into the thylakoid space.
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). Figure 6 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.
Both the ATP produced by the electron transport chain following photosystem II and the NADPH produced by the electron transport chain following photosystem I will be used by the light-independent reactions to build a sugar. How a plant uses this ATP and NADPH to build a sugar will be covered in the next tutorial.
Figure 6. A comparison of ATP production in mitochondria (left) and chloroplasts (right). (Click image to enlarge)
This tutorial has focused on the light-dependent reactions of photosynthesis:
- light + water --> NADPH + ATP (oxygen is a by-product)
Water provides the electrons and protons needed to produce ATP and NADPH and oxygen is released as a by-product of the splitting of a molecule of water. Sunlight is absorbed by chlorophyll and other pigments and excites electrons to a higher energy state. Sunlight is the original source of energy for every step in the process.
NADPH and ATP provide the electrons and energy that will be used to build a sugar in the light-independent reactions of photosynthesis.
After reading this tutorial, you should have a working knowledge of the following terms:
- electromagnetic radiation
- granum (pl. grana)
- NADP+ and NADPH
- photosystems I and II
Case Study for Energy IV (Photosynthesis - Light-dependent Reactions)
Epipremnum aureum (common name Pothos) is a common tropical houseplant that originates from the Solomon Islands. The leaves of this plant can be solid green (as in the golden pothos in the picture on the right) or variegated as in Pothos ‘Marble Queen’ shown on the left. Your roommate is a plant biology major and wishes to study the effect of variegation on growth rates and chilling sensitivity in pothos. She collects the data in Table 1.
Help her with the following items:
- What effect does variegation seem to have on growth rate (as indicated by the mean number of leaves and vines)?
- Present a plausible hypothesis for why plants with variegated leaves show this difference in growth rate.
Table 1. Effect of color on growth and sensitivity to chilling at 50°F for 1 week for Epipremnum aureum 'Marble Queen' (Exp. 510 - 30 January to 17 April 1990).
- aColor was graded on a scale from 1 (white) to 5 (green).
**Significant at the 1% level within columns.
Table from: Mid-Florida Research & Education Centerhttp://mrec.ifas.ufl.edu/Foliage/Resrpts/rh_90_17.htm
Now that you have read this tutorial and worked through the case study, go to ANGEL and complete the tutorial practice problems to test your understanding. Questions? Either send your instructor a message through ANGEL or attend an online office hour (the times are posted on ANGEL).