Plants are undoubtedly humankind's favorite living things outside the animal kingdom. Apart from plants' ability to feed the world's people – without fruits, vegetables, nuts and grains, it's unlikely that you or this article would exist – plants are revered for their beauty and their role in all manner of human ceremony. That they manage to do this with without the ability to move or eat is remarkable indeed.
Plants, in fact, make use of the same basic molecule that all life forms do in order to grow, survive and reproduce: the small, six-carbon, ring-shaped carbohydrate glucose. But instead of eating sources of this sugar, they instead make it. How is this possible, and given that it is, why don't humans and other animals simply do the same thing and save themselves the trouble of hunting for, gathering, storing and consuming food?
The answer is photosynthesis, the series of chemical reactions in which plant cells use energy from sunlight to make glucose. The plants then use some of the glucose for their own needs while the rest remains available for other organisms.
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Components of Photosynthesis
Astute students might be quick to ask, "During photosynthesis in plants, what is the source of the carbon in the sugar molecule the plant produces?" You don't need a science degree to suppose that "energy from the sun" consists of light, and that light contains none of the elements that make up the molecules most often found in living systems. (Light consists of photons, which are massless particles not found on the periodic table of the elements.)
The easiest way to introduce the various parts of photosynthesis is to begin with the chemical formula that summarizes the whole process.
6 H2O + 6 CO2 → C6H12O6+ 6 O2
Thus the raw materials of photosynthesis are water (H2O) and carbon dioxide (CO2), both of which are abundant on the ground and in the atmosphere, while the products are glucose (C6H12O6) and oxygen gas (O2).
Summary of Photosynthesis
A schematic recap of the photosynthesis process, the components of which are described in detail in subsequent sections, is as follows. (For now, don't worry about abbreviations with which you may not be familiar.)
- CO2 and H2O enter the leaf of a plant.
- Light strikes the pigment in the membrane of a thylakoid, splitting the H2O into O2 and liberating electrons in the form of hydrogen (H).
- These electrons move down along a "chain" to enzymes, which are special protein molecules that catalyze, or speed up, biological reactions.
- Sunlight strikes a second pigment molecule, allowing the enzymes to convert ADP to ATP and NADP+ to NADPH.
- The ATP and NADPH are used by the Calvin cycle as a source of energy to convert more CO2 from the atmosphere into glucose.
The first four of these steps are known as the light reactions or light-dependent reactions, as they rely absolutely on sunlight to operate. The Calvin cycle, in contrast, is called the dark reaction, also known as light-independent reactions. While, as the name implies, the dark reaction can operate without a source of light, it relies on products created in the light-dependent reactions to proceed.
How Leaves Support Photosynthesis
If you have ever looked at a diagram of a cross-section of human skin (that is, what it would look like from the side if you could look at it all the way from the surface to whatever tissue the skin meets beneath), you might have noted that the skin includes distinct layers. These layers contain different components in different concentrations, such as sweat glands and hair follicles.
The anatomy of a leaf is arranged in a similar way, except that leaves face the outside world on two sides. Moving from the top of the leaf (considered to be the one that faces the light most often) to the underside, the layers include the cuticle, a waxy, thin protective coat; the upper epidermis; the mesophyll; the lower epidermis; and a second cuticle layer.
The mesophyll itself includes an upper palisade layer, with cells arranged in neat columns, and a lower spongy layer, which has fewer cells and greater spacing between them. Photosynthesis takes place in the mesophyll, which makes sense because it is the most superficial layer of a leaf of any substance and is closest to any light striking the leaf's surface.
Chloroplasts: Factories of Photosynthesis
Organisms that must get their nourishment from organic molecules in their environment (that is, from substances humans call "food") are known as heterotrophs. Plants, on the other hand, are autotrophs in that they build these molecules inside their cells and then use what they need of it before the rest of the associated carbon is returned to the ecosystem when the plant dies or is eaten.
Photosynthesis occurs in organelles ("tiny organs") in plant cells called chloroplasts. Organelles, which are present only in eukaryotic cells, are surrounded by a double plasma membrane that is structurally similar to that surrounding the cell as a whole (usually just called the cell membrane).
- You may see chloroplasts referred to as "the mitochondria of plants" or the like. This is not a valid analogy as the two organelles have very different functions. Plants are eukaryotes and engage in cellular respiration, and so most of them have mitochondria and chloroplasts.
The functional units of photosynthesis are thylakoids. These structures appear in both photosynthetic prokaryotes, such as cyanobacteria (blue-green algae), and plants. But because only eukaryotes feature membrane-bound organelles, the thylakoids in prokaryotes sit free in the cell cytoplasm, just like the DNA in these organisms does owing to the lack of a nucleus in prokaryotes.
What Are Thylakoids For?
In plants, the thylakoid membrane is actually continuous with the membrane of the chloroplast itself. Thylakoids are therefore like organelles within organelles. They are arranged in round stacks, like dinner plates in a cabinet – hollow dinner plates, that is. These stacks are called grana, and the interiors of the thylakoids are connected in a mazelike network of tubes. The space between thylakoids and the inner chloroplast membrane is called the stroma.
Thylakoids contain a pigment called chlorophyll, which is responsible for the green color most plants exhibit in some form. More important than offering the human eye a lustrous appearance, however, chlorophyll is what "captures" sunlight (or for that matter, artificial light) in the chloroplast and, therefore, the substance that allows photosynthesis to proceed in the first place.
There are actually several different pigments contributing to photosynthesis, with chlorophyll A being the primary one. In addition to chlorophyll variants, numerous other pigments in thylakoids are responsive to light, including red, brown and blue types. These can relay incoming light to chlorophyll A, or they may help keep the cell from being damaged by light by serving as decoys of a sort.
The Light Reactions: Light Reaches the Thylakoid Membrane
When sunlight or light energy from another source reaches the thylakoid membrane after passing through the cuticle of the leaf, the plant cell wall, the layers of the cell membrane, the two layers of the chloroplast membrane and finally the stroma, it encounters a pair of closely related multi-protein complexes called photosystems.
The complex called Photosystem I differs from its comrade Photosystem II in that it responds differently to different wavelengths of light; in addition, the two photosystems contain slightly different versions of chlorophyll A. Photosystem I contains a form called P700, while Photosystem II uses a form called P680. These complexes contain a light-harvesting complex and a reaction center. When light reaches these, it dislodges electrons from molecules in the chlorophyll, and these proceed to the next step in the light reactions.
Recall that the net equation for photosynthesis includes both CO2 and H2O as inputs. These molecules pass freely into the cells of the plant owing to their small size and are available as reactants.
The Light Reactions: Electron Transport
When electrons are kicked free of chlorophyll molecules by incoming light, they need to be replaced somehow. This is done mainly by the splitting of H2O into oxygen gas (O2) and free electrons. The O2 in this setting is a waste product (it is perhaps difficult for most humans to envision newly created oxygen as a waste product, but such are the vagaries of biochemistry), whereas some of the electrons make their way into chlorophyll in the form of hydrogen (H).
Electrons make their way "down" the chain of molecules embedded into the thylakoid membrane toward the final electron acceptor, a molecule known as nicotinamide adenine dinucleotide phosphate (NADP+ ). Understand that "down" does not mean vertically downward, but downward in the sense of progressively lower energy. When the electrons reach NADP+, these molecules combine to create the reduced form of the electron carrier, NADPH. This molecule is necessary for the subsequent dark reaction.
The Light Reactions: Photophosphorylation
At the same time that NADPH is being generated in the system described previously, a process called photophosphorylation uses energy liberated from other electrons "tumbling" in the thylakoid membrane. The proton-motive force connects inorganic phosphate molecules, or Pi, to adenosine diphosphate (ADP) to form adenosine triphosphate (ATP).
This process is analogous to the process in cellular respiration known as oxidative phosphorylation. At the same time ATP is being generated in the thylakoids for the purpose of manufacturing glucose in the dark reaction, mitochondria elsewhere in plant cells are using the products of the breakdown of some of this glucose to make ATP in cellular respiration for the plant's ultimate metabolic needs.
The Dark Reaction: Carbon Fixation
When CO2 enters plant cells, it undergoes a series of reactions, first being added to a five-carbon molecule to create a six-carbon intermediate that quickly splits into two three-carbon molecules. Why isn't this six-carbon molecule simply made directly into glucose, also a six-carbon molecule? While some of these three-carbon molecules exit the process and are in fact used to synthesize glucose, other three-carbon molecules are needed to keep the cycle going, as they are joined to incoming CO2 to make the five-carbon compound noted above.
The fact that energy from light is harnessed in photosynthesis to drive processes independent of light makes sense given the fact that the sun rises and sets, which puts plants in the position of having to "hoard" molecules during the day so they can go about making their food while the sun is below the horizon.
For purposes of nomenclature, the Calvin cycle, the dark reaction and carbon fixation all refer to the same thing, which is making glucose. It is important to realize that without a steady supply of light, photosynthesis could not occur. Plants can thrive in environments where light is always present, as in a room where the lights are never dimmed. But the converse is not true: Without light, photosynthesis is impossible.