Chapter 1. Photosynthesis Rate Activity Lab

Photosynthesis Rate Activity Lab

by Lawton L. Roberts and Kirk A. Stowe

Most cells convert energy-rich carbohydrates (reduced forms of carbon) into energypoor compounds such as carbon dioxide (an oxidized form of carbon) to produce ATP through respiration. This is called cellular respiration. ATP represents usable chemical energy for cells. The general chemical formula for this process is:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP + HEAT

Heterotrophic organisms must obtain these energy-rich forms of carbon from their environment. That is, they must ingest them in their food. Conversely, autotrophs (most plants and some bacteria and protists) make their own carbohydrates (food).

This production of carbohydrates from carbon dioxide and water using sunlight is called photosynthesis (photo = light and synthesis = to combine). That is, plants capture a small proportion of the sun’s energy and store it as chemical energy in the bonds of carbohydrates. Since this is the most common way of capturing other forms of energy and storing it as the chemical energy needed for living, plants form the basis for most ecosystem functioning. Photosynthesis is a complex series of reactions that can be summarized as the following equation:

6 CO₂ + 6 H₂O + LIGHT ENERGY → C₆H₁₂O₆ (Glucose) + 6 O₂ ↑

There are two stages of photosynthesis: light reactions and the Calvin cycle (light-independent reactions or dark reactions). The light reactions are the energy-capturing part of photosynthesis, where solar energy converts into chemical energy of ATP and NADPH. The Calvin cycle is the synthesis part of photosynthesis, using ATP and electrons from light reactions to make sugars from CO₂.

The light-dependent reactions of photosynthesis occur on or in the thylakoid membranes that are in stacks, called grana, within the chloroplasts. In the light-dependent reactions, when a light photon hits the chlorophyll molecule (a light-absorbing pigment), its energy is captured and transferred to an electron. In this process the energy is then used to “split” water molecules into hydrogen ions and molecular oxygen and electrons in a process called photolysis. This is where the oxygen you breathe comes from. The electrons are transferred along an electron transport chain and are ultimately used to create NADPH by reducing NADP⁺. As the electrons pass down the transport chain they lose energy that is used to pump hydrogen ions into the thylakoid creating a gradient across its membrane. As we saw in our previous laboratory, molecules tend to diffuse from regions of high concentration to those of low concentration. The energy generated as the hydrogen ions move back across the thylakoid membrane is then used to drive the synthesis of ATP. Thus, the end result of the light-dependent reactions of photosynthesis is the production of the two cellular energy sources, ATP and NADPH. The general chemical equation for this process is:

2 H₂O + 2 NADP+ + 2 ADP + 2 P_i + LIGHT → 2 NADPH + 2 H⁺ + 2 ATP + O₂

The Calvin cycle (light-independent reaction) occurs in the stroma of the chloroplasts. The stroma is the fluid-filled area between the chloroplast outer membrane and the thylakoid disks, analogous to the cytoplasm of a cell, so the stroma can be thought of as “cytoplasm of the chloroplast.” In the Calvin cycle, the high-energy ATP and NADPH yielded from the light reactions are used as the energy and electron sources to “fix,” or reduce, carbon dioxide into carbohydrates (chemical energy). The general chemical equation of this process is:

3 CO₂ + 9 ATP + 6 NADPH + 6 H⁺ → C₃H₆O₃ + ADP + 9 P_i + NADP+ + 3 H₂O

Be aware that the light-dependent reactions occur when light is present while the lightindependent reactions occur both in the presence of light and in its absence as long as the products of the light-dependent reaction are present.

What Environmental Factors Influence the Rate of Photosynthesis?

1. Light Quantity: The most obvious environmental factor that may affect photosynthetic rate is light. Plants typically exhibit the highest photosynthetic rate during the midpart of the day when the amount of light reaching them is at its highest point. Further, many of us have noticed that most plants in our house tend to do better in a room that receives large amounts of sunlight, like a kitchen window that has a southern exposure, than they do in a room that receives less sunlight, like a bathroom. Thus, in general, the higher quantity of light (light intensity) that a plant receives the higher its photosynthetic rate, up to a limit. This limit is the light saturation point of a plant. Past this point, photosynthetic rate will start to decrease and very high light intensities may actually cause damage to the plant. This is called photo inhibition.

2. Light Quality: Chlorophyll, a plant pigment that is the basis of most photosynthesis, absorbs the photons of light that hit the plant. The light that hits a plant is typically white light, which contains various wavelengths that correspond to different colors or different qualities of light. Does chlorophyll absorb all of the wavelengths of light that make up white light? Well, the color of a plant, because of its chlorophyll, is green. This means that the chlorophyll molecules are reflecting rather than absorbing the wavelengths of light that correspond to green. That is why leaves are green. In fact, chlorophyll tends to absorb those wavelengths of light that correspond to the red and blue part of the white light spectrum.

3. Carbon Dioxide Concentration: Since carbon dioxide is necessary for photosynthesis to proceed, its concentration in the environment may also affect photosynthetic rate. This is an especially interesting question considering the idea that humans have significantly altered, i.e., increased, the concentration of carbon dioxide in the atmosphere. Since carbon dioxide is a necessary reactant in the process of photosynthesis, we may think of it as a substrate for photosynthesis.

4. Temperature: The temperature range over which plants can photosynthesize is surprisingly large. In many plants exposed to direct sunlight on a summer day, leaf temperatures can reach 35°C and continue to photosynthesize. Just like most chemical reactions, increases in temperature are followed by increases in photosynthetic rate because the chemical participants in the reaction are moving more rapidly and bump into each other more often. However, there is some optimal temperature above which point photosynthetic rate starts to decline. This optimum is species specific and depends on the environment in which the plant normally grows.

Laboratory Exercise I

Laboratory Exercise I

Measuring the Rate of Photosynthesis and Its Dependence on Light

In this exercise we will indirectly gauge photosynthesis and explore how different environmental factors affect this rate. We will measure the rate of photosynthesis by looking at the behavior of spinach leaf disks in the dark (dark treatment) or exposed to light (light treatment) while immersed in a NaHCO₃ solution. Leaf disks that have the gas removed from the air sacs of their leaf tissue will sink in this solution.

During the process of photosynthesis gas is produced, and those disks actively photosynthesizing will become buoyant and begin to float to the surface of the solution. Thus, we can indirectly estimate the relative photosynthetic rate in various treatments by counting the number of leaf disks floating over time.

From the equations on the preceding pages, we can see that oxygen is only produced in one part of the photosynthetic process. Form a hypothesis concerning the production of oxygen in leaf material that is exposed to light and other material that is not, in terms of the number of floating disks.

Hypothesis:

Laboratory Exercise II

Effect of an Environmental Factor on Photosynthesis Rate

During the time that your lab group is conducting the preceding exercise you should be developing a hypothesis about a specific environmental factor (light quantity, etc.) and photosynthetic rate. Each lab group will work with a different environmental factor. Check with your instructor before you begin. Pick one of the four given environmental factors listed per group.

Environmental factor to test:

Hypothesis: