Unit 6: Photosynthesis
Have you ever wondered how a plant grows from a tiny acorn into a giant oak tree? Where does all that biomass come from? How does it get the energy to grow? Photosynthesis is the fascinating process plants use to convert light energy to chemical energy. Because plants are at the bottom of the food pyramid in almost all ecological systems, understanding how they grow and develop will give you a greater understanding of your environment.
Completing this unit should take you approximately 4 hours.
Upon successful completion of this unit, you will be able to:
- explain the role of photosynthesis;
- describe where matter originates and ends up during photosynthesis;
- describe how photosynthesis converts low-energy molecules into energy-rich carbohydrates;
- explain the role of the light-dependent phase of photosynthesis;
- explain the role of the light-independent phase of photosynthesis, and describe how it is related to the light-dependent reactions;
- explain how energy is transformed and transferred during photosynthesis;
- explain how plants have adapted to deal with the problem of photorespiration;
- identify the differences in photosynthesis in reference to CAM and C-4 plants; and
- explain what the "carbon cycle" is and how it relates to the conservation of matter.
6.1: Overview of Photosynthesis
Photosynthesis is how green plants and other photosynthetic organisms use the power of sunlight to synthesize their own food from carbon dioxide and water. Photosynthesis harnesses sunlight by using green pigment chlorophyll and generates oxygen as a byproduct. The food and oxygen created by these autotrophs (organisms that make their own food) indirectly nourish and energize the whole earth.
Read this overview of the process of photosynthesis. The text discusses basic photosynthetic structures and the two cycles in photosynthesis: the light and dark cycles, which we will explore later in the unit.
Read this review of the light-dependent reactions of photosynthesis. Light dependent reactions require light energy and water to create products ATM, NADPH, and oxygen.
Read this review of the Calvin cycle, which is the light-independent reactions in photosynthesis. Light independent reactions require ATP, NADPH from the light reactions. and carbon dioxide to create sugar for the plant.
Watch this overview of photosynthesis, which gives a visual demonstration of how and where this process works inside the plant.
6.2: Photosynthesis and Photorespiration
The products of photosynthesis are sugars and oxygen. This anabolic process requires the reactants carbon dioxide and water. When there are significant amounts of these reactants, photosynthesis can nourish the plant and indirectly support life on earth. However, in environments where water and carbon dioxide are limited, there is a risk of photorespiration.
Watch this video, which reviews the structures and cycles of photosynthesis. Pay attention to the chemical reactions that occur throughout the process, where they take place, and the products of these reactions. You should be familiar with the two stages of photosynthesis and the products of each stage of the process.
Watch this video about the details of the light reactions of photosynthesis. After you watch, you should be able to list the components of the chloroplasts, describe the role of chlorophyll in the light reactions, describe the photosystems and their role in the light reactions, and trace the movement of electrons in noncyclic electron flow.
Watch this video about photophosphorylation of light reactions. After you watch, you should be able to describe the role of the cytochrome complex in the light reactions, trace the movement of electrons in cyclic vs. non-cyclic electron flow, and describe the role of ATP synthase in light reactions.
Watch this video about the light-independent Calvin cycle. You should be able to describe the role of light-dependent byproducts ATP and NADPH in the Calvin cycle, illustrate and describe the function of each of the three phases of the Calvin cycle, and discuss the products of the Calvin Cycle.
Watch this lecture, which discusses an alternate mechanism of the Calvin cycle called photorespiration. You should be able to describe what happens when ribulose-bisphosphate carboxylase-oxygenase (RuBisCO) interacts with O2 instead of CO2 because of concentration differences.
6.3: C-4 and CAM Photosynthesis
A crucial step in the Calvin cycle is the fixation step, which takes in carbon dioxide and joins it with an intermediate compound (ribulose bisphosphate), thus incorporating inorganic carbon dioxide into an organic compound. The enzyme that catalyzes this step is called RuBisCO (ribulose bisphosphate carboxylase oxygenase). RuBisCO can operate to join either carbon dioxide or oxygen to ribulose bisphosphate. However, joining oxygen instead of carbon dioxide is counterproductive because no carbon fixation (the purpose of the Calvin cycle) takes place. This counterproductive process (incorporating oxygen instead of carbon dioxide) is called photorespiration.
Two major categories of plant species have evolved ways around this problem:
- C-4 plants separate the process of carbon dioxide intake (which occurs in superficial cells called mesophyll cells) from the process of carbon fixation in the Calvin cycle (which occurs in deeper cells called bundle sheath cells).
- CAM plants take in carbon dioxide and store it in the form of organic acids during the night when their stomata are open. During the day, the organic acids get broken down to release the carbon dioxide for the Calvin cycle, while the stomata are closed (preventing oxygen from interfering).
These two types of plants more efficiently operate the Calvin cycle because photorespiration is minimized. As you review C-4 plants and CAM plants, notice that they accomplish the same thing in two different ways. After you have read this section, you should be able to define the purpose of the stomata, describe the two photosynthetic adaptations that minimize photorespiration, and list the types of plants that have these adaptations.Watch these videos to learn about C-4 and CAM photosynthesis. Make sure you are able to explain the differences between these unique types of photosynthesis processes.
6.4: The Carbon Cycle
The phrase carbon cycle refers to the many chemical transformations that occur involving compounds containing carbon. The carbon cycle is cyclic because there is a continuous alternation between the carbon of organic compounds and the carbon of inorganic compounds. Inorganic carbon dioxide gets fixed (by autotrophs) into organic compounds. These organic compounds get converted into other organic compounds (including simple organic compounds like monosaccharides, nucleotides, and amino acids, as well as complex macromolecules like polysaccharides, nucleic acids, lipids, and polypeptides).
The carbon in these organic compounds gets passed from organism to organism as they feed on each other. Some of the organic molecules get used as fuel by the organisms, and the oxidation of these organic fuels (to provide energy for the organisms) returns the carbon to inorganic form (carbon dioxide) to complete the cycle. In this cycle of transformations, carbon (matter) remains in the ecosystem (it is conserved). It is not destroyed; it is only transferred and transformed.
Watch this video for an overview of the carbon cycle. Note that the carbon cycle has serious implications for the regulation of our atmosphere and climate, such as the production and accumulation of greenhouse gasses. We will not discuss this phenomenon in detail in this course, but it is an important aspect of the study of environmental science.
Unit 6 Assessment
- Receive a grade
Take this assessment to see how well you understood this unit.
- This assessment does not count towards your grade. It is just for practice!
- You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.