Unit 3: Organization for matter and energy flow in organisms

Background and instructional suggestions

Students at the middle school level have learned about photosynthesis and the production of sugars (glucose) using light energy and carbon dioxide (MS-LS1-6) and that oxygen and food molecules can be rearranged and during that process energy is released (MS-LS1-7).

All living organisms need energy. When this unit is completed, students will understand how an autotrophic organism (producer, mostly plants and algae) transforms sunlight (or light energy) into useable forms of chemical energy for living organisms. This process involves two interdependent cellular processes, the first is the capturing of sunlight/light energy by chloroplasts and the second is using that energy to fix atmospheric carbon dioxide into a glucose molecule. This can then be stored by the producer as many glucose molecules attached together in the form of starch (where it takes up less room and therefore is easier to store) or it can be used directly as glucose (see below).

Heterotrophs (consumers or animals) ingest producers as food that will then be used for energy and building blocks for growth. Consumers often store energy in stacked glucose molecules in the form of glycogen (in higher animals glycogen is stored in liver and muscle tissues). Both plants and animals use cellular respiration as the process by which organic molecules are broken down to release energy and form molecules of adenosine triphosphate (ATP). The process of cellular respiration uses up oxygen and releases carbon dioxide. The ATP formed in cellular respiration has high levels of potential energy that allow cells to do work; and therefore, if there is no ATP then there is no life. The energy from ATP is released when it is converted back into adenine diphosphate (ADP). The interrelatedness of photosynthesis and cellular respiration is important to keep ATP available and at a relatively constant level in all cells.

The products of photosynthesis are used as the reactants for cellular respiration and vice versa. However, as stated in the second Law of Thermodynamics, as energy is transferred there will be less energy available in a useable form for the organism. This means that the overall processes are always losing energy and there needs to be a constant influx of energy into the system which comes from the sun as light energy. Some organisms that do not live in oxygen-rich environments (like organisms that live near thermal vents deep in the ocean) and must use a different energy pathway.

Taking the physical components of glucose (carbon, oxygen and hydrogen, the matter) students can build a glucose model and show how to split it up (with an emphasis on the components needed to build the glucose and the components left after the breakdown of the glucose). They should start with the atoms of carbon, hydrogen and oxygen and make the simple molecules of CO₂, H₂0, and O₂ and then trace the movement of these molecules. For example, the carbon dioxide and water will be united through photosynthesis and then released again in cellular respiration showing the importance of carbon cycling in this process (which directly links to and should be connected to carbon-cycling learning in earth science).

Carbon is structurally important to building all biological molecules, including the glucose molecule. Students can be guided to understand the importance of why carbon is an atom found in all living molecules noting the size of the atom, the ability to covalently bond to four other atoms, placement of their electrons and the ability to form single and double bonds with other atoms. Energy comes into the system and energy comes out of the system when glucose is formed and when it is processed, providing evidence in support of the First Law of Thermodynamics. It should be noted that students do not need to know the individual biochemical steps of these two processes but rather need to understand the connections between them. Students will need to understand that these processes happen in order for organisms to make ATP, the molecular source or “currency” of energy for the cell. When the outermost or third phosphate on ATP (adenosine triphosphate) is cleaved from the molecule by the addition of water in a chemical reaction, energy is released and this energy is used to make things happen in a cell. It should be noted that the energy released in this reaction is not all released from the system as heat, which happens in some instances. Rather, in this reaction, some energy released is used to do work such as contracting a muscle while running, jumping or swimming.

On a larger scale, energy is brought into ecosystems by photosynthetic conversion of light energy into chemical energy in producers (Figure 2). That energy then passes through the ecosystem in consumers and decomposers as it is broken down and used as ATP (made during respiration) in individual organisms during metabolism (cell functioning). At each stage of respiration, energy is also returned to the environment in the form of heat and chemical potential energy within carbon dioxide and water. Chemical potential energy is also passed to each stage in the form of matter (biomass) and also passed between organisms as matter through the food web, but the whole amount in the ecosystem decreases at each trophic level.

Figure 2. A model of the relationship between photosynthesis and respiration.