The common statement "Animals respire, plants photo-synthesize' is only partially correct. Although it is true that plants are capable of producing their own organic compounds through the process of photosynthesis, plants are so versatile that they can also respire just like animals.
Photosynthesis is a complex scries of reactions involving the capture of light energy, conversion to chemical energy, and finally the synthesis of carbohydrates. It is one of the main biosvnthetic processes by which energy and carbon enter die network of living organisms. A summary of the ultimate reactants and products of photosynthesis can be stated as:
This equation is deceptive in its simplicity because photosynthesis involves numerous intermediate steps. Photosyndiesis can be divided into two series of reactions. The first series, light harvesting, results in the capturing of light energy and the temporary storage of this energy in ATP and NADPH. The second series of reactions, carbon fixation, uses the energy in the ATP and NADPH to synthesize sugars from C02.
Light harvesting starts with the absorption of light energy by the pigment chlorophyll embedded in the membranes of the chloroplasts within plant cells. The energy excites electrons within the chlorophyll molecule to a higher energy state. At diis energy state, the electrons are easily transferred to other chemicals that accept the electrons. The electrons arc transferred from one chemical to the next much like the way buckets of water arc passed from one person to die next in a bucket brigade. Eventually, the electrons are transferred to an electron acceptor called NADP* (nicotinamide adenine dinucleotide phosphate). To replace the lost electrons of the chlorophyll molecules, water (H:0) is split to producc electrons, protons (H4), and oxygen gas (02). In addi tion, a proton gradient is generated that is used to pro duce ATP (adenosine triphosphate from ADP
(adenosine diphosphate) and P: (inorganic phosphate). ATP is a temporary energy carrier. The NADP* is converted to NADPH by the adding of two electrons and one proton (H*). At the end of the light harvesting phase, NADPH, ATP, and O^ arc produced. A summary of die light harvesting phase can be stated as:
In the carbon fixation phase of photosynthesis, the energy and electrons of ATP and NADPH arc used to form carbon-carbon bonds. Carbon dioxide (CO*) enters a cyclic series of reactions (the Calvin cycle), and eventually sugars (CH:0). arc produced. The name of the enzyme that catalyzes die fusion of C()2 to the first chemical in the cycle (ribulose bisphosphate is rubisco. This is worth mentioning since rubisco is the most abundant protein on our globe. The first sugar to exit the cycle is a three-carbon sugar, G3P (glyceraldehyde 3-phosphatc). G3P can be used to generate other sugars, such as glucose (C6Hl2O0). The summary of the carbon-fixation phase of photosynthesis can be stated as:
Take some time to examine the three equations. Equation 1 is the summary of equations 2 and 3. What information is lost by only looking at equation 1? Which phase requires light? Which phase is not directly dependent on light, yet needs the products of the light-dependent phase?
The two phases of photosynthesis actually occur in two different locations within the chloroplasts. The light-harvesting phase occurs on the membranes of the thvlakoids, whereas the carbon-fixation phase occurs in the stroma, the space between the thvlakoids. Given this, what compounds cycle back and forth between the two phases? Does carbon enter the system in both phases?
All animals, including humans, ultimately depend on plants to produce the oxygen gas (O,) they need. 0: is actually a by-product of photosynthesis. We are lucky that plants produce oxygen. For example, an average hectare of corn produces enough oxygen per day to support 325 people. Where do the atoms that make up the 02 come from? If you remove a plant from the light, what happens to its oxygen production?
What is the fate of the products of photosynthesis? G3P can easily be converted into various six-carbon sugars. such as glucose or fructose, or stored as starch, a polysaccharide formed as a chain of glucose molecules. When these basic sugars arc combined with odier elements, such as nitrogen or phosphorus, all the other organic compounds in a plant, such as proteins, nucleic acids, lipids, or alkaloids, can be formed. In this way, plants make all their basic building blocks. And since animals are incapable of carbon fixation themselves, plants make the basic building blocks for animals as well. Animal food has to come from other organisms, and ultimately most of it comes from plants. In Laboratory Topic 10, you will relate your own needs lor energy to production of chemical forms of energy by plants. Of course, the plants could probably care less about the welfare of animals.
Respiration is another fundamental process of living organisms. Before we proceed, it is important to com pare two definitions of respiration. Many of you may consider respiration the process of breathing air in and out of your body. Mammals like yourself pull air into their lungs by contraction of the diaphragm and exhale the air as the diaphragm relaxes- The inhalation and exhalation of air is one valid definition of respiration. Humans even use breathing as a sign of life itself.
Biologists often use another definition of respiration to describe what happens to some of the components of the air. particularly CO* and 02, at the ccllular level. Cellular respiration is defined as the process by which cells release energy from organic compounds to generate ATP through a series of chemical reactions involving the transfer of electrons. In aerobic respiration, oxygen (02) is the final electron acceptor. In anaerobic respiration, or fcrmcnta tion, some other chemical is the final electron acceptor. The main results of cellular respiration arc organic compounds broken down to simpler compounds, with some energy becoming available tor use in other metabolic steps.
The overall proccss of aerobic cellular respiration can be stated as:
Equation 4: <CH2() . • o: — CO: f H>O -energy (ATP and hear)
Compare equation 4 with equation 1. On die surface, respiration appears to be merely the reverse of photosynthesis. But in reality, aerobic respiration is another complex series of reactions that can be divided into three phases: glycolysis, the Krebs cycle, and the electron transport chain.
In glycolysis, a moleculc of the six-carbon sugar glucosc is oxidized to two moleculcs of the three-carbon pyruvate, and some of the energy is rccapturcd in die production of ATP. The Krebs cycle completes the oxidation of pyruvate to produce carbon dioxide (C02) and reduced electron carriers. In the electron transport chain, a proton (H4) gradient drives the production of even more ATP and is coupled with the transfer of electrons to oxygen (On), producing water (H20). After the entire proccss of respiration is complete, much of the energy released from the glucose is rccapturcd in the production of ATP. Sincc no conversion of energy is 100% efficient, some of the energy is lost as entropy and is no longer available to the organism. The ATP, however, can be used for all die other normal processes of life, such as synthesis of new tissue, response to external stimuli, or movement of materials throughout die body.
Just like animals, plants use aerobic respiration to recapture the energy held in the sugar moleculcs produced during photosynthesis. Just like animals, plants use glycolysis, the Krebs cycle, and the electron transport chain to produce ATP. Plants, however, do nor need to consume preformed organic compounds as animals must. They produce their own organic compounds via photosynthesis. For this reason, some people say, "Plants make their own food." Again we emphasize, plants photosyn-thesize and respire. Animals only respire.
As long as the net production of new material via photosynthesis exceeds the breakdown of molecules to produce ATP via respiration (P > R), a plant will grow and increase in biomass. But sometimes, plants need to rely heavily on die energy stored in sugars, and respiration can exceed net gain from photosynthesis (R > P). This is common when photosyntheric tissues are not yet available, such as in a germinating seed or rcgrowth of buds in the tips of branches each spring. What do you suppose happens when a plant is kept in the dark for a long time or at night?
Take a look at equations 1 and 4 again. Notice how on die surface, carbon dioxide is consumed by photosynthesis, yet produced by respiration (fig. 5.1). If you had an experimental system with only plants, the balance between photosynthesis and respiration could be determined by monitoring the levels of carbon dioxide around a plant. In this activity, we will use a C02 sensor to measure the concentration of C02 in the air surrounding plants in various conditions and see what happens. Relate what you observe to what could be happening biologically.
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