SIENE & GLOBAL ISSUES/BIOLOGY ELL BIOLOGY Reading Energy for Life every cell needs a source of energy As you learned in the Ecology: Living on Earth unit of Science and Global Issues, all cells need energy if they are to function. ellular respiration is the process by which cells break down complex s, such as sugars, to release energy. Some of the energy is released as heat while the rest is stored temporarily in other s, such as adenosine triphosphate (ATP). ATP is used when the cell needs energy. For example, whenever your muscles contract, ATP supplies the energy. As you also learned in the Ecology unit, photosynthesis is the process by which the cells of producers capture the sun s energy and store it in sugars. All producers and consumers ultimately depend on these sugars for their energy needs. The following equations summarize the two processes. ellular respiration: sugars 1 oxygen & carbon dioxide 1 water 1 energy Photosynthesis: carbon dioxide 1 water 1 light energy & sugars 1 oxygen As you can see from the equations, the components of cellular respiration and photosynthesis are nearly identical. In fact, cellular respiration is often described as the opposite of photosynthesis. The two processes, however, are far more complex than the equations indicate. photosynthesis Photosynthesis only occurs in certain pigment-containing cells of producers. This differs from cellular respiration, which occurs in all types of cells in all organisms. A pigment is a that absorbs light energy. All producers contain pigments, which are essential for photosynthesis. The most common pigment in producers is chlorophyll. hlorophyll is what makes the stems and leaves of producers green. In most producers the chlorophyll is contained in s. PLANT PLANT ELL mitochondrion nucleus cell wall cell membrane vacuole Light micrograph of Elodea cells 220
POTOSYNTESIS AND ELLULAR RESPIRATION ATIVITY 12 hloroplasts are relatively large organelles surrounded by two membranes. hloroplasts absorb energy from sunlight, as shown in the diagram below. Inside the is a fluid-filled space called the and stacks of connected membrane sacs called. The thylakoid membranes contain the chlorophyll, and the stacked structure greatly increases the membrane surface area that can absorb light. PLANT ELL ANIMAL ELL MITOONDRION cristae matrix LOROPLAST mitochondria Mitochondria are present in both plants and animal cells, while s are present only in plant cells. 221
SIENE & GLOBAL ISSUES/BIOLOGY ELL BIOLOGY the reactions of photosynthesis The reactions occurring in photosynthesis are grouped into two stages, lightdependent and light-independent reactions. The light-dependent reactions rely on chlorophyll and other pigments in the thylakoid membranes to harness the energy of light. In a complex series of reactions, water breaks down into oxygen, hydrogen ions (protons), and electrons. Each reaction requires a unique enzyme. The oxygen is released to the atmosphere, some of the energy from the reactions goes into making ATP from ADP, and the protons and electrons combine with the, NADP +, to form NADP. LIGT-DEPENDENT REATIONS LOROPLAST solar energy (sunlight) 2 O O 2 lightdependent reactions ADP + P ATP alvin O 2 with hydrogen The NADP and ATP are important to the light-independent alvin, which is a series of enzyme-catalyzed reactions that take place in the of the. During these reactions carbon dioxide combines with the hydrogen ions and electrons produced from water during the light-dependent reactions. This results in the production of the high-energy sugar. The is used in cellular respiration or is converted to another form, such as starch, and stored for later use. Starch is a type of carbo hydrate. A starch macro is made of many s linked together. 2 O O O O O starch O 222
POTOSYNTESIS AND ELLULAR RESPIRATION ATIVITY 12 The reactions in the alvin occur in the presence or absence of light and are referred to as light-independent reactions. During the alvin ATP and NADP are converted back into ADP and NADP+. These will be used again in the light-dependent reactions in the thylakoid membranes. LIGT-INDEPENDENT REATIONS LOROPLAST solar energy (sunlight) 2 O O 2 lightdependent reactions ADP + P ATP alvin O 2 with hydrogen The reactions of the alvin continue as long as carbon dioxide, ATP, NADP, and enzymes are present. ertain plant diseases, such as the plum pox virus that attacks several kinds of fruit trees, inhibit enzymes involved in the alvin. This prevents the plant from making the sugar it needs to support cellular respiration. Trees infected with plum pox may eventually stop producing fruit. Organisms that feed off the fruit may need to find another food source, and communities that rely on fruit sales may suffer economically. cellular respiration Each reaction in cellular respiration also requires a unique enzyme, and, as in photosynthesis, diseases or poisons may reduce the activity of these enzymes. This can have serious consequences for an organism, since cellular respiration is the process by which energy is released for cells to use. It is important not to confuse cellular respiration with breathing, which is often called respiration. Breathing gets oxygen into your lungs, but the oxygen has no purpose until it enters your cells and plays its part in cellular respiration. 223
SIENE & GLOBAL ISSUES/BIOLOGY ELL BIOLOGY In prokaryotes, such as bacteria, the reactions of cellular respiration that require oxygen occur on the cell membrane. In eukaryotes, these reactions take place in the cells mitochondria. Mitochondria are organelles that have two membranes, an and an that is folded within the outer, which play central roles in the reactions of cellular respiration. The contains specialized proteins that make the membrane very permeable to ions and small s. The is less permeable, but it contains transport proteins to allow for the passage of some s. The also contains enzymes needed for cellular respiration. The is folded to form many cristae, finger-like projections that provide a large surface area for the reactions of cellular respiration. Inside a mitochondrion is a space called the matrix. the reactions of cellular respiration ellular respiration begins with the process of glycolysis. Glycolysis means the breaking of sugar. This first step takes place in the. In glycolysis, the energy from ATP breaks a, releasing hydrogen ions and creating two smaller s of pyruvic acid. The, NAD +, combines with these hydrogen ions and with electrons to form NAD. Glycolysis also produces new ATP s, resulting in a net gain of 2 ATP s for each of that is broken down. MITOONDRION GLYOLYSIS * pyruvate oxygen glycolysis Krebs * electron transport chain cristae matrix 2 ATP O 2 2 ATP 2 O 32 ATP * electrons transported on s If oxygen is present, the pyruvic acid s produced during glycolysis are then further broken down, releasing large amounts of energy in a process called aerobic respiration. In eukaryotic cells, the pyruvic acid s first enter the mitochondrion where they are stripped of a carbon atom and are also temporarily 224