Unit two. The Living Cell
In this forest glade, you can literally see the pulse of life flowing through the organisms of an ecosystem. Sunlight beams down, a stream of energy in the form of packets of light called photons. Everywhere the light falls, there are plants—trees and shrubs and flowers and grasses, all with green leaves intercepting the energy as it rains down. In the cells of each leaf are organelles called chloroplasts that contain light-gathering pigments in their membranes. These pigments, notably the pigment chlorophyll, which makes leaves green, absorb photons of light and use the energy to strip electrons from water molecules. The chloroplasts use these electrons to reduce CO2—that is, to add hydrogens (a hydrogen atom, you will recall, is just a proton with an associated electron)—and so make organic molecules. This process of capturing the sun’s energy to build molecules is called photosynthesis—literally, using “light to build.” In this chapter, we will delve into photosynthesis, tracing how light energy is captured, converted to chemical energy, and put to work assembling organic molecules. In the roots and other tissues of the plants, the opposite process is taking place. Organic molecules are being broken down in the process of cellular respiration to provide energy to power growth and cellular activities. These reactions, which take place largely in another kind of organelle called a mitochondrion, are the subject of the following chapter. Together, chloroplasts and mitochondria carry out a flow of energy driven by the power of sunlight.
6.1. An Overview of Photosynthesis
Life is powered by sunshine. All of the energy used by almost all living cells comes ultimately from the sun, captured by plants, algae, and some bacteria through the process of photosynthesis. Every oxygen atom in the air we breathe was once part of a water molecule, liberated by photosynthesis as you will discover in this chapter. Life as we know it is only possible because our earth is awash in energy streaming inward from the sun. Each day, the radiant energy that reaches the earth is equal to that of about 1 million Hiroshimasized atomic bombs. About 1% of it is captured by photosynthesis and provides the energy that drives almost all life on earth. Use the arrows on this page and the next three pages to follow the path of energy from the sun through photosynthesis.
Trees. Many kinds of organisms carry out photosynthesis, not only the many kinds of plants that make our world green, but also bacteria and algae. Photosynthesis is somewhat different in bacteria, but we will focus our attention on photosynthesis in plants, starting with this maple tree crowned with green leaves. Later we will look at the grass growing beneath the maple tree—it turns out that grasses and other related plants sometimes take a different approach to photosynthesis depending on the conditions.
Leaves. To learn how this maple tree captures energy from sunlight, follow the light. It comes beaming in from the sun, down through earth’s atmosphere, bathing the top of the tree in light. What part of the maple tree is actually being struck by this light? The green leaves are. Each branch at the top of the tree ends in a spread of these leaves, each leaf flat and thin like the page of a book. Within these green leaves is where photosynthesis occurs. No photosynthesis occurs within this tree’s stem, covered with bark, and none in the roots, covered with soil—no light reaches these parts of the plant. The tree has a very efficient internal plumbing system that transports the products of photosynthesis to the stem, roots, and other parts of the plant so that they too may benefit from the capture of the sun’s energy.
The Leaf Surface. Now follow the light as it passes into a leaf. The beam of light first encounters a waxy protective layer called the cuticle. The cuticle acts a bit like a layer of clear fingernail polish, providing a thin, watertight and surprisingly strong layer of protection. Light passes right through this transparent wax, and then proceeds to pass right on through a layer of cells immediately beneath the cuticle called the epidermis. Only one cell thick, this epidermis acts as the “skin” of the leaf, providing more protection from damage and, very importantly, controlling how gases and water enter and leave the leaf. Very little of the light has been absorbed by the leaf at this point—neither the cuticle nor the epidermis absorb much.
Mesophyll Cells. Passing through the epidermis, the light immediately encounters layer after layer of mesophyll cells. These cells fill the interior of the leaf. Unlike the cells of the epidermis, mesophyll cells contain numerous chloroplasts, which as you recall from chapter 4 are organelles found in all plants and algae. They are visible as green specks in the mesophyll cells in the cross section of the leaf above. It is here, within the mesophyll cells penetrated by the light beam, that photosynthesis occurs.
Chloroplasts. Light penetrates into mesophyll cells. The cell walls of the mesophyll cells don’t absorb it, nor does the plasma membrane or nucleus or mitochondria. Why not? Because these elements of the mesophyll cell contain few if any molecules that absorb visible light. If chloroplasts were not also present in these cells, most of this light would pass right through, just as it passed through the epidermis. But chloroplasts are present, lots of them. One chloroplast is highlighted by a box in the mesophyll cell above. Light passes into the cell to the chloroplast, and when it reaches the chloroplast, it passes through the outer and inner membranes to reach the thylakoid structures within the chloroplast, clearly seen as the green disks in the cutaway chloroplast shown here.
Inside the Chloroplast
All the important events of photosynthesis happen inside the chloroplast. The journey of light into the chloroplasts ends when the light beam encounters a series of internal membranes within the chloroplast organized into flattened sacs called thylakoids. Often, numerous thylakoids are stacked on top of one another in columns called grana. In the drawing below, the grana look not unlike piles of dishes. While each thy- lakoid is a separate compartment that functions more-or-less independently, the membranes of the individual thylakoids are all connected, part of a single continuous membrane system. Occupying much of the interior of the chloroplast, this thylakoid membrane system is submerged within a semiliquid substance called stroma, which fills the interior of the chloroplast in much the same way that cytoplasm fills the interior of a cell. Suspended within the stroma are many enzymes and other proteins, including the enzymes that act later in photosynthesis to assemble organic molecules from carbon dioxide (CO2) in reactions that do not require light and which are discussed later.
Penetrating the Thylakoid Surface. The first key event of photosynthesis occurs when a beam of sunlight strikes the surface membrane of a thylakoid. Embedded within this membrane, like icebergs on an ocean, are clusters of lightabsorbing pigments. A pigment molecule is a molecule that absorbs light energy. The primary pigment molecule in most photosystems is chlorophyll, an organic molecule that absorbs red and blue light, but does not absorb green wavelengths. The green light is instead reflected, giving the thylakoid and the chloroplast that contains it an intense green color. Plants are green because they are rich in green chloroplasts. Except for some alternative pigments also present in thylakoids, which we will discuss later, no other parts of the plant absorb visible light with such intensity.
Striking the Photosystem. Within each pigment cluster, the chlorophyll molecules are arranged in a network called a photosystem. The light-absorbing chlorophyll molecules of a photosystem act together as an antenna to capture photons (units of light energy). A lattice of structural proteins, indicated by the purple element inserted into the thylakoid membrane in the diagram on the facing page, anchors each of the chlorophyll molecules of a photosystem into a precise position, such that every chlorophyll molecule is touching several others. Wherever a photon of light strikes the photosystem, some chlorophyll molecule will be in position to receive it.
Energy Absorption. When a photon of sunlight strikes any chlorophyll molecule in the photosystem, the chlorophyll molecule it hits absorbs that photon’s energy. The energy becomes part of the chlorophyll molecule, boosting some of its electrons to higher energy levels. Possessing these more energetic electrons, the chlorophyll molecule is said to now be “excited.” With this key event, the biological world has captured energy from the sun.
Excitation of the Photosystem. The excitation that the absorption of light creates is then passed from the chlorophyll molecule that was hit to another, and then to another, like a hot potato being passed down a line of people. This shuttling of excitation is not a chemical reaction, in which an electron physically passes between atoms. Rather, it is energy that passes from one chlorophyll molecule to its neighbor. A crude analogy to this form of energy transfer is the initial “break” in a game of pool. If the cue ball squarely hits the point of the triangular array of 15 billiard balls, the two balls at the far corners of the triangle fly off, and none of the central balls move at all. The kinetic energy is transferred through the central balls to the most distant ones. In much the same way, the photon’s excitation energy moves through the photosystem from one chlorophyll to the next.
Energy Capture. As the energy shuttles from one chlorophyll molecule to another within the photosystem network, it eventually arrives at a key chlorophyll molecule, the only one that is touching a membrane-bound protein. Like shaking a marble in a box with a walnut-sized hole in it, the excitation energy will find its way to this special chlorophyll just as sure as the marble will eventually find its way to and through the hole in the box. The special chlorophyll then transfers an excited (high-energy) electron to the acceptor molecule it is touching.
The Light-Dependent Reactions. Like a baton being passed from one runner to another in a relay race, the electron is then passed from that acceptor protein to a series of other proteins in the membrane that put the energy of the electron to work making ATP and NADPH. In a way you will explore later in this chapter, the energy is used to power the movement of protons across the thylakoid membrane to make ATP and another key molecule, NADPH. So far, photosynthesis has consisted of two stages, indicated by numbers in the diagram to the lower left: Q capturing energy from sunlight—accomplished by the photosystem; and Q using the energy to make ATP and NADPH. These first two stages of photosynthesis take place only in the presence of light, and together are traditionally called the light-dependent reactions. ATP and NADPH are important energy-rich chemicals, and after this, the rest of photosynthesis becomes a chemical process.
The Light-Independent Reactions. The ATP and NADPH molecules generated by the light-dependent reactions described above are then used to power a series of chemical reactions in the stroma of the chloroplast, each catalyzed by an enzyme present there. Acting together like the many stages of a manufacturing assembly line, these reactions accomplish the synthesis of carbohydrates from CO2in the air Q. This third stage of photosynthesis, the formation of organic molecules like glucose from atmospheric CO2, is called the Calvin cycle, but is also referred to as the light-independent reactions because it doesn’t require light directly. We will examine the Calvin cycle in detail later in this chapter.
This completes our brief overview of photosynthesis. In the rest of the chapter we will revisit each stage and consider its elements in more detail. For now, the overall process may be summarized by the following simple equation:
Key Learning Outcome 6.1. Photosynthesis uses energy from sunlight to power the synthesis of organic molecules from CO2 in the air. In plants, photosynthesis takes place in specialized compartments within chloroplasts.