6.1: Lab 6 Background

Learning Objectives

• Explain ways of measuring and looking for evidence of photosynthesis.
• Describe how variation in leaf color is due to differences in photosynthetic pigments.

Introduction: Photosynthesis

Photosynthesis is the process by which some organisms take sunlight energy and convert it into stored chemical energy. We typically think of plants performing photosynthesis, but other microorganisms like bacteria and protists do photosynthesis as well. The overall chemical reaction for photosynthesis takes carbon dioxide (\(\ce{CO2}\)) and water (H2O), along with light energy, and turns them into a reduced carbon source like glucose (\(\ce{C6H12O6}\)), releasing oxygen gas (\(\ce{O2}\)) in the process (following the chemical equation below). The light energy is converted into chemical energy that gets stored in the covalent bonds of glucose or a similar molecule.

\[\ce{6 CO2 + 6H2O  ->[\text{light}] C6H12O6 + 6 O2} \nonumber\]

There are two parts of photosynthesis: the Light Dependent Reactions and the Calvin Cycle. The Light Dependent Reactions use water (H₂O) and light to store the energy in intermediate molecules such as ATP and NADPH. The Light Dependent Reactions produce oxygen gas (O₂) as well and do not occur when light is absent (such as during night time). The Calvin Cycle, sometimes called the Dark Reactions, uses the other reactant carbon dioxide (CO₂) along with the ATP and NADPH produced by the Light Dependent Reactions. This step takes the energy out of the ATP and NADPH molecules and stores it in new chemical bonds between carbon atoms forming a sugar molecule like glucose (C₆H₁₂O₆). These reactions can happen with or without light present.

For today’s lab we will be measuring photosynthesis. First, let’s think about some possible ways to see photosynthesis happening. We could look for an increase in products like glucose molecules! Maybe we could use a chemical reagent that changes color if glucose is present (does this sound familiar?). Another option is to look for the presence of oxygen gas. These two measurements could work but end up being pretty challenging. Most plants use the glucose they make to do other things rather than store it, making glucose hard to measure. Oxygen gas molecules are miniscule and immediately mix into the air. Sometimes we can see small gas bubbles forming on leaves under water but that is hard to compare and quantify. Other options include looking at the removal of reactants. Water molecules are hard to track since cells are made mostly of water. But we can track carbon dioxide pretty easily! If the plant is performing photosynthesis, then it would be using carbon dioxide and the levels of CO2 would decrease.

Today, we will be designing controlled experiments to answer the question: will photosynthesis happen more under a lamp (direct) or sunlight (indirect)? To do this experiment, we will be using the aquatic plant Elodea which must remain in water at all times to survive. We will be using a chemical indicator called phenol red that changes colors in response to pH changes. pH is a measure of hydrogen ions in a solution and is reported on a scale of 0-14 (Figure 1). Smaller numbers (less than 7) on the scale are considered acids; while larger numbers (greater than 7) are considered basic/alkaline. Right in the middle is a pH of 7 which is considered a neutral pH. Pure water has a neutral pH of 7. When CO₂ is added to water, the water becomes acidic. Phenol red is red/pink when CO₂ is absent (neutral or basic pH) and orange/yellow when CO₂ is present (acidic pH). We will first add CO₂ to the water by exhaling into the liquid through a straw until we get an orangey color. We will use this liquid for all tests. The process of photosynthesis takes time so let’s use multiple pieces of Elodea in each test tube and allow at least 30 minutes before collecting results.

Figure 1. The pH scale. (Image source)

Introduction: Chloroplasts & Pigments

Taking an even closer look, there are small organelles within leaf cells that perform photosynthesis called chloroplasts. Some plant cells have higher numbers of these organelles than others depending on their role for the plant. The palisade mesophyll cells in a leaf are filled with lots of chloroplasts since they perform the most photosynthesis for a plant. Plant cells have the ability to move their chloroplasts around within the cell to capture the most light possible. Within the chloroplasts are various pigments that have different light capturing abilities. A pigment is a chemical molecule that has the ability to capture light and emit a color that our eye interprets. Plants have evolved the ability to use pigments to capture the light’s energy and convert it into stored chemical energy during photosynthesis. The most abundant pigment in plants is called chlorophyll a, which reflects a bluish green color. A secondary pigment is chlorophyll b, which is a more yellowish green color. Other pigments include xanthophyll, which is a dull yellow color, and carotene, which is a bright yellow/orange color. Every plant will have evolved various quantities of these pigments and have the ability to change the amount of each pigment made. This helps explain the variation in leaf color across plant species. Today, we will investigate the movement of chloroplasts within cells and the various pigments in leaves.