The Chemistry of Respiration
Animals obtain carbohydrates from food. The carbohydrate molecules are broken down by enzymes in the digestive system into simple sugars such as glucose, which are carried to cells by the circulatory system.
The reactions of respiration can be summarized in a word equation:
glucose + oxygen (long right arrow) carbon dioxide + water + energy
The corresponding chemical equation is:
C6H12O6 + 6O2 (long right arrow) 6CO2 + 6H2O + energy
This equation is somewhat misleading because it implies that respiration involves a single stage. In fact, it involves three main stages, each of which includes many complex reactions.
Glycolysis
Glycolysis is the first stage of respiration. It takes place in the cell cytoplasm and does not require oxygen. During glycolysis each molecule of glucose, which contains six carbon atoms, is split into two halves, each containing three carbon atoms. Each of these is converted into a compound called pyruvic acid, which is used in the second stage. Glycolysis produces a net gain of two molecules of ATP for each molecule of glucose.
The Krebs Cycle
The second stage of respiration, the Krebs cycle, takes place inside cell organelles called mitochondria. The molecules of pyruvic acid enter the matrix (internal space) inside a mitochondrion, where they are converted into a two-carbon compound called acetyl coenzyme A by removal of a molecule of carbon dioxide. The acetyl coenzyme A is then joined to a four-carbon compound, oxaloacetate, to produce a six-carbon molecule. This then passes through a cyclic series of reactions that remove hydrogen atoms and carbon dioxide molecules until the six-carbon molecule has been converted back into oxaloacetate. The cycle then starts again.
The reactions of the Krebs cycle contribute little energy directly, but they supply high-energy hydrogen atoms to molecules embedded in the mitochondrion’s inner membrane, where the third stage of respiration takes place.
The Electron Transport Chain
The electron transport chain is a “chain” of enzymes (not structurally linked together) embedded in the mitochondrion’s inner membrane. Here, high-energy hydrogen atoms from the Krebs cycle and oxygen molecules are used to generate most of the chemical energy obtained during respiration. The hydrogen atoms are split into electrons, which are passed along the chain of enzymes, and protons. The protons are shunted across the inner mitochondrial membrane to a space under the outer membrane. This results in a steep electrochemical gradient, with a large surplus of positively charged protons (H+) just outside the membrane, and a surplus of negatively charged electrons (e-) just under it. According to the chemiosmotic theory (first put forward by Peter Mitchell), this gradient pushes protons back across the membrane through specialized channels in the stalked particles that line the inner mitochondrial membrane. As each proton passes through a channel, its energy is used to convert a molecule of ADP (adenosine diphosphate) into ATP. The Krebs cycle and electron transport chain together give a net yield of 36 ATP molecules for each molecule of glucose.
After returning across the inner mitochondrial membrane, the protons are recombined with electrons to make hydrogen atoms. Finally, these are combined with oxygen to make molecules of water, which is produced as a waste product of respiration. Some desert animals obtain almost all their water this way, and are able to survive for months without drinking.
Anaerobic Respiration
A modified form of respiration, anaerobic respiration, does not use oxygen. Anaerobic respiration gives less energy than aerobic respiration, producing a net gain of only two molecules of ATP for each molecule of glucose. It occurs in muscles when prolonged or hard exercise has used up their oxygen supply. It also occurs in many anaerobic microorganisms, such as the tetanus bacterium, and in yeast, which is a type of fungus. Yeast produces ethanol (alcohol) as a by-product of anaerobic respiration, a property exploited in the brewing industry to make wine and beer.
Cell Processes
THOUSANDS of complex chemical reactions take place inside living cells every second. For example, molecules such as proteins, lipids, and carbohydrates are made by joining smaller molecules together. In plant cells, food is made through a process called photosynthesis. In multicellular organisms, cell division produces cells that become specialized to carry out particular functions. The energy required to fuel all these processes is obtained from glucose molecules, through a process called respiration.
Photosynthesis
Photosynthesis is a chemical process used by plants and many microorganisms to convert the inorganic compounds carbon dioxide and water into organic compounds (carbohydrates), using the Sun’s energy to drive the reaction. It is the most important synthetic process in the living world. About 50 billion tonnes of carbon are fixed into organic compounds by photosynthetic organisms each year, much of this by phytoplankton living in the surface of the oceans. The organic compounds produced are the ultimate source of food for all animals. The continual production of oxygen as a waste product of photosynthesis maintains the high level of atmospheric oxygen (about 21 per cent) needed for respiration.
Photosynthesis takes place in the chloroplasts in the cells of green plants. These are tiny, membrane-bound bodies containing millions of molecules of the green, light-trapping pigment chlorophyll. Chloroplasts are found in all photosynthetic organisms, except for photosynthetic bacteria. They are thought to have originated from photosynthetic bacteria that invaded other cells and later lost the ability to survive independently (the endosymbiotic theory).
The Chemistry of Photosynthesis
The process of photosynthesis can be written as a word equation:
Carbon dioxide + water + sunlight → glucose + oxygen.
This is a useful summary, but it makes photosynthesis look like a one-step process. In fact, there are two stages.
During the first stage (the light reaction) chlorophyll traps energy from sunlight and transforms it into chemical energy. At the same time, water molecules are split into hydrogen and oxygen atoms. The oxygen is released into the air as a waste product. The second stage (the dark reaction) does not need light. The energy trapped in the first stage is used to join hydrogen (from water) with carbon `ioxide to make glucose. Glucose can be converted into starch for storage, cellulose for cell walls, fats for storage and cell structure, or proteins for chemical processes in the cells as well as cell structure.
A plant needs six molecules of carbon dioxide (CO2) and six molecules of water (H2O) to make just one molecule of glucose (C6H12O6); six molecules of oxygen (O2) are also produced. A simple chemical equation showing this process looks like this:
6CO2 + 6H2O + sunlight → C6H12O6 + 6O2.
The Light Reaction
The first step in the light reaction is the absorption of light by pigments. Most chlorophyll traps light energy in the violet and red parts of the spectrum, while different forms of chlorophyll and other pigments (carotenoids and phycobilins) absorb other wavelengths of light and transfer the energy to chlorophyll. Energy is passed between many chlorophyll molecules until it finally becomes concentrated in one of two reaction centres: photosystem I and photosystem II. These convert the light energy into chemical energy by producing two products: ATP and NADPH.
Formation of ATP
When a chlorophyll molecule in photosystem II traps light energy, it becomes chemically excited and donates an electron to a nearby electron carrier. Photosystem II then becomes an electron acceptor, extracting electrons from water molecules, and splitting water into hydrogen and oxygen. Having accepted electrons from water, the photosystem II chlorophyll molecules return to their normal, nonexcited state, and the oxygen atoms are released as oxygen gas. This is called the photolysis of water, or the Hill reaction.
The donated electron passes along a chain of molecules (the electron transport chain) embedded in membranes called thylakoids inside the chloroplast, eventually reaching a molecule of photosystem I chlorophyll. As each electron passes along this chain, energy is captured and used to convert the molecule ADP (adenosine diphosphate) into an energy-storing form called ATP (adenosine triphosphate). This is known as photophosphorylation.
Formation of NADPH
Light absorbed by photosystem I also excites electrons. These pass through another electron transport chain, which contains different carrier molecules (including: plastocyanin, a blue copper-containing protein; plastoquinone; cytochrome b6; and ferredoxin, an iron-containing protein). At the end of this chain, the electrons are transferred to an electron-acceptor enzyme called NADP (nicotinamide adenine dinucleotide phosphate), which is then reduced by reaction with the hydrogen ions (protons) released from water earlier in the process. As a result, NADP is converted into NADPH. The electrons lost by photosystem I are replaced by those passed along the electron transport chain from photosystem II.
So, from the light reactions, ATP, NADPH, and oxygen are produced.
The Dark Reaction
ATP and NADPH are subsequently used to drive the reactions of the Calvin cycle (named after the US biochemist Melvin Calvin, who first deduced the cyclical series of reactions that fix carbon into carbohydrates). In the Calvin cycle, atmospheric carbon dioxide is built up into carbohydrate skeletons, and these are used as frameworks for the synthesis of starch and cellulose, and for various other cellular processes. Unlike the light reaction, which takes place in the thylakoids, the dark reaction takes place in the space between the chloroplast’s thylakoids. This space is called the stroma.
The Calvin Cycle
The Calvin cycle involves a series of small steps, each controlled by a specific enzyme. At the end of the series of steps, the final product is converted back into the original starting molecule, and the cycle repeats itself.
The carbon dioxide absorbed by the chloroplast first binds to an acceptor molecule called ribulose biphosphate (a five-carbon sugar), which is regenerated on each turn of the cycle. Binding with carbon dioxide produces an unstable 6-carbon compound that immediately breaks down into two molecules of a 3-carbon compound, glycerate 3-phosphate (GP). The NADPH and ATP produced during the light reaction are then used to reduce these GP molecules to two molecules of a 3-carbon sugar, glyceraldehyde 3-phosphate (GALP). Some of the sugar molecules are removed to be converted into glucose, and the remainder are converted into new molecules of ribulose biphosphate, allowing the Calvin cycle to be repeated.
The glucose molecules are then converted into starch for short-term storage. The products of carbon fixation are not only used for energy – they can also be converted into structural and genetic compounds, such as carbohydrates, proteins, lipids, and nucleic acids.
Carbon Fixation in Tropical Plants
Certain tropical plants, such as sugar cane and maize, can capture carbon dioxide with a substance called phosphoenol pyruvate (PEP), before releasing the carbon dioxide into the Calvin cycle. These plants are called C4 plants because the immediate product of carbon fixation is a 4-carbon compound. (Other plants are called C3 plants because the first stable product of fixation is the 3-carbon compound GP.)
C4 plants have an exceptionally high affinity for carbon dioxide, which enables them to photosynthesize more effectively than C3 plants in very bright sunlight and at high temperatures – conditions often found in tropical environments.
Certain desert plants use the C4 system to store carbon dioxide during the day and convert it into carbohydrates at night. This allows them to keep their stomata closed during the heat of the day, preventing water loss, and to open them at night, allowing gas exchange and the Calvin cycle to take place.
