Chapter 2. Photosynthesis

Objectives

By the end of the period, students will be able to:

Introduction

One of the most important natural processes that occurs in the world is photosynthesis. All organisms including people depend on plants as well as certain protists and bacteria for oxygen. In addition, the process of photosynthesis “fixes” carbon. That means that plants and protists take carbon in the form of CO₂ from the atmosphere and convert it into organic compounds that are the building blocks of all organisms. The process of photosynthesis is the means by which plants get and store energy. All organisms that feed themselves in this way are called producers or autotrophs. The term autotroph literally means “self-feeding.”

Organisms that derive their energy from eating plants or other organisms are called consumers, or heterotrophs. Consumers include herbivores as well as decomposers, predators, and scavengers.

What is photosynthesis? We can examine this question by reviewing the general equations for photosynthesis and respiration.

In photosynthesis, plants and other producers “tie-up” solar energy (sunlight) by using it to make energy-rich molecules of the sugar glucose. A by-product of the reaction is the production of molecular oxygen (O₂). The general chemical reaction of photosynthesis is:

Notice that in this reaction, carbon dioxide (CO₂) is combined with water to make glucose. To do this requires energy and that energy comes from light.

What has happened to the solar energy that was put into the left side of the equation? The energy tied up in the bonds of glucose or other organic molecules cannot be used directly by cells of plants or animals to drive cellular processes. Instead, cells must make ATP that can be used as an immediate source of energy to help push energy-requiring chemical reactions along. In plants and animals, the main process of making ATP is called cellular respiration.

In respiration, organic molecules (including carbohydrates such as glucose) react with oxygen. Enzymes control the rate at which this reaction occurs. When carbohydrates react with oxygen, there is a release of energy. The energy that is released in this process can be used to make ATP. Then, ATP can be used in cells to drive all energy-requiring cellular processes. Notice that in respiration, oxygen is consumed (used up) and CO₂ is produced.

The equation for cellular respiration is:

A more detailed reaction equation for photosynthesis is:

The specific reactions of photosynthesis are divided into two parts: the light-dependent reactions and the light-independent reactions. In eukaryotes, each set of reactions occurs in a different part of the chloroplast (Figure 7.1). The light-dependent reactions obviously require light and occur in association with the thylakoids. The light-independent reactions are where the carbon fixation occurs; this happens in the stroma. Use your text to review the structure and organization of chloroplasts as well as the details of the light-dependent reactions and the light-independent reactions.

LIGHT-DEPENDENT REACTIONS

These reactions begin as chlorophyll pigment captures light energy (in the thylakoids, see Figure 7.1), causing one of the electrons in the chlorophyll to move to a higher energy state. The energized electron is transferred to an acceptor molecule and is replaced with an electron from a water molecule. This causes the water molecule to split, and molecular oxygen is produced. Some of the energy from the energized electron changes ADP to ATP. In addition, the coenzyme NADP⁺ is reduced and forms NADPH. Thus, the products of the light-dependentreactions are ATP and NADPH. The ATP and NADPH produced in the light-dependent reactions are well suited to transfer chemical energy but they are not suited for long-term storage of chemical energy.

LIGHT-INDEPENDENT REACTIONS OR THE CARBON FIXATION REACTIONS

Carbon fixation occurs in the stroma of the chloroplast (again, see Figure 7.1). Although these are sometimes called the “dark reactions,” they do not require darkness but can proceed whenever there are products from the light-dependent reactions. In carbon fixation, the energy of ATP and NADPH is used in the formation of organic molecules from CO₂. Many plants use a sequence of reactions known as the Calvin cycle in the fixing of CO₂.

Light and its Measurement

One of the key ingredients for photosynthesis to occur is light. Visible light is a portion of the electomagnetic spectrum (shown on the back cover of your lab manual) and includes the wavelengths of energy that are visible to us. Roughly speaking, we can see wavelengths from about 400–700 nm. The visible spectrum can be subdivided by wavelength into the visible colors—violet or blue have relatively short wavelength, while reds have longer wavelengths.

We will measure the intensity of visible light by using a light meter that measures in foot candles or fc. A foot candle is a measure of light that is equivalent to one lumen per square foot, or the amount of light put out by one candle measured at one foot. One foot candle is barely enough light to read by.

Each lab will have one light meter that will be shared between groups. The light meter has a flat sensor that must be pointing toward the light source to be measured.

Here are the steps in measuring light

You should use the lowest setting possible to get the most accurate numbers.

How does light intensity change as a function of distance from a light source? Using the light at your table, make at least 10 measures at different distances from the light. Tools: one fluorescent light, light meter, meter stick. Measure the distance carefully.

Please recopy this data onto your Hand-In for today’s lab.

Table 7.1. Measures of light intensity as a function of distance from a light source.

What can you say about how light intensity changes with distance? How careful do you need to be to get an accurate measurement?

What measure do you get if you measure sunlight? What about indirect light from the windows? Sometime during the lab period make additional measurements of light intensity from several different sources.

Table 7.2. Measures of light intensity from various sources.

WHAT IS THE EFFECT OF LIGHT INTENSITY ON THE RATE OF PHOTOSYNTHESIS?

The main part of lab today will be to do an experiment to investigate the effects of light intensity on the rate of photosynthesis in an aquatic plant, Elodea. We will measure the production of oxygen and thus are measuring the light-dependent reactions.

Will the rate of photosynthesis increase with increasing light? One may be tempted to say “of course.” But, is it possible that an aquatic plant may be adapted to photosynthesize maximally at lower light levels than full sunlight, since in its natural habitat the light will be filtered through water? We can’t know without running the experiment.

Since a plant may be photosynthesizing and respiring at the same time, we need to be aware that what we measure will be a product of both processes. For this lab, we will estimate respiration by measuring O₂ change while the plants are kept in complete dark.

Respiration (R) involves consumption of oxygen and organic matter and the production of carbon dioxide.

Gross photosynthesis (GP) is the amount of measured photosynthesis, not taking into account losses by cellular respiration.

Net photosynthesis (NP) is the actual amount of photosynthesis after taking into account losses by cellular respiration.

The Experiment

Each section will run at least one experiment on the effects of light intensity on the rate of photosynthesis.

MATERIALS

Each table will have a block of wood and 4–6 large test tubes with a stopper fitted with a syringe and a capillary tube for measuring changes in the gases in the tube. Each table will also have aluminum foil and two lights. Your TA will help you to know specifically what treatment your lab team will do. The tubes will have a solution of sodium bicarbonate, which will assure that the plants have plenty of carbon. Each team should cover at least one of the tubes with aluminum foil.

Set up the manometer and allow it to sit for at least 5 minutes. At 5 minutes, start your trial by setting your light at the specified distance, and adjust the bubble in the capillary tube. You will measure the distance the bubble moves as an indirect measure of oxygen production or use.

You should now measure how much the bubble moves in 15 minutes. You may need to reset it multiple times. Be as accurate as possible. Carefully record your data on the sheet provided. Measure light intensity carefully.

Table 7.3. Raw data from whole class. Each team is a separate lab table (8 lab tables).