• Define energy as it relates to biological systems
• Discuss energy transfer and use in biological systems
• Distinguish (or Compare and contrast) between photosynthesis and cellular respiration
• Discuss basics of reactions in photosynthesis and cellular respiration
• List the key molecules and cell organelles involved in photosynthesis and cellular respiration
• Explain the role of photosynthesis and cellular respiration in energy transfer
• Discuss how photosynthetic processes in plants adapt to illumination conditions/availability of sunlight/access to sunlight

Energy

What comes to mind when you read the word ‘energy’? Can you make concept connections between ‘photosynthesis’ and ‘energy’? In this lab series, you will explore how energy in the form of sunlight can be transformed to support the energy needs of all living beings.

Consider Figure on the right. What are the different forms of energy in this Figure?

Energy Transfer in Biological Systems

Energy can be defined as “the capacity to cause change, or to move matter in a direction it would not move if left alone.” ¹ The primary source of this energy on Earth is the Sun, the closest star to Earth. The energy of the Sun is absorbed by the ocean and land masses which allows the Earth to have an average surface temperature that is well above freezing (average 14°C). ² The energy of the Sun is also absorbed by chlorophyll in the chloroplasts of leaves where this energy, carbon dioxide, and water participate as reactants in a process called photosynthesis. In a two-step process, plants use the energy of the Sun to, first, produce carbohydrates (simple sugars), water, and oxygen (photosynthesis in chloroplasts) then, in the second step, use these products to produce complex carbohydrates (cellulose and starch), proteins (and enzymes), and fatty acids. Energy is also used and released in cellular (aerobic) respiration), a process that occurs in the mitochondria of plants and animals.

Photosynthesis and cellular respiration both use energy and release energy. Energy can be regarded as “trapped” in the molecular arrangements of chemical bonds. Breaking chemical bonds releases copious amounts of energy which can be harnessed to “cause change” (for example, breaking bonds in fossil fuels as part of gasoline engine combustion to cause automobile tires to rotate, permitting the vehicle to move forward) or to “move matter in a direction it would not move if left alone” (for example, permitting you to move your muscles against the force of gravity when you do a push up). The energy transfer through photosynthesis and cellular respiration represent two distinct forms of energy: Biomass (in the form of cellulose/starch) and stored chemical energy (in the form of ATP molecules), respectively. In addition to these energy products, photosynthesis also produces oxygen and cellular respiration also produces carbon dioxide and water. Measurement of the amount of oxygen and carbon dioxide produced can be used as an indirect measure of the amount of energy transfered, or, alternately, used to measure the rates of photosynthesis and cellular respiration.

Products of Photosynthesis and Cellular Respiration

Chemically speaking, the oxygen you breathe is a waste product (or by-product) of photosynthesis. Evolutionarily speaking, this waste product is your lifeline and the lifeline for all members of the Kingdom Animalia. Alternately, carbon dioxide, one of the waste products of cellular respiration, provides a lifeline for plants. As evident in the two reactions shown below, the products of photosynthesis are the reactants (or substrates) for cellular respiration. Therefore, these two processes in plants must be carefully balanced to assure that plants can retain sufficient amounts of carbon in order to grow—Remember, all living organisms, including plants, are carbon-based.

Photosynthesis (a two-step process in chloroplasts in leaves). Notice that both sunlight and enzymes are involved.

H2O+CO2−→−−−enzymessunlightO2+C6H12O6(sugar)H2O+CO2→enzymessunlightO2+C6H12O6(sugar)

Cellular respiration (in mitochondria in all parts of plants). Notice that enzymes are also are involved in this reaction.

C6H12O6+6O2−→−−−enzymes6CO2+6H2O+ATPC6H12O6+6O2→enzymes6CO2+6H2O+ATP

Ecologists and plant physiologists refer to a measurement of the carbon balance as net photosynthesis,³ or as net carbon gain.⁴

Net photosynthesis = Photosynthesis – RespirationNet photosynthesis = Photosynthesis – Respiration

Net carbon gain = Carbon gain in photosynthesis – Carbon loss in respirationNet carbon gain = Carbon gain in photosynthesis – Carbon loss in respiration

During full sunlight, net photosynthesis is positive, whereas, in the dark, there is a net loss of carbon because photosynthesis ceases while cellular respiration continues.⁵ Cellular respiration takes place in all parts of plants (leaves, stems, and roots), which means that plant growth is dependent on distribution of carbon (CO₂ and sugars) to all parts of the plant. Carbon distribution in a plant is critical to both cellular respiration and plant growth, but the primary means for obtaining carbon is through photosynthesis. In the absence of photosynthesis (that is, when there is no sunlight), measuring the change in CO₂ production provides an estimate of the rate of cellular respiration.⁶

If you measure CO₂ and O₂ concentration levels in a closed system (no gases allowed in or out of the system), then net photosynthesis (in full sunlight) and cellular respiration (in total darkness) would show the trends shown in the figures below.

Changes in CO₂ and O₂ concentration move in opposite directions and in step with one another during both processes.

Orientation to the Model Photosynthetic System

In this set of laboratory exercises, you will quantify the rate of photosynthesis as well as the rate of cellular respiration by measuring oxygen production and carbon dioxide production, respectively. You will use different levels of illumination, including 0% illumination (darkness/no sunlight), in two different model leaf systems.

As an orientation to the model leaf system, imagine a very large tree. Leaves at the top of the tree receive plenty of sunlight; these leaves would be considered ‘sun-adapted.’ Leaves on the lower branches of this large tree will, on average, receive much less sunlight; these leaves would be considered ‘shade-adapted.’ The different sunlight conditions for leaves on the same tree mean that the rate of photosynthesis will differ for leaves at different heights of the tree. The two model leaf systems in this laboratory will be one system that is sun-adapted and another that is shade-adapted.

Botanists and ecologists have studied the structural and functional differences between sun-adapted and shade-adapted plants. The results reveal two main differences between sun-adapted and shade-adapted plants:

• Shade-adapted plants produce more chlorophyll than sun-adapted plants produce
• Shade-adapted plants produce less of the key enzymes that catalyzes photosynthesis and cellular respiration than sun-adapted plants produce ⁷

Given these differences between sun-adapted and shade-adapted leaves, how do you think the net rate of photosynthesis and the rate of cellular respiration compare for sun-adapted and shade-adapted leaves? You will be able to answer this question after conducting the set of experiments in this laboratory.

What is a Rate?

In this set of laboratory exercises, you will record rates of photosynthesis and cellular respiration. To be more precise, you will measure changes in the concentration of reaction products of photosynthesis and cellular respiration over a specified period of time. In this set of experiments, you will measure production of one reaction product for photosynthesis—Oxygen (gas) and one for cellular respiration—Carbon dioxide (gas) over a specified period of time. As a reminder, plants are autotrophs/producers (that is, they make their own food) which means that cellular respiration is dependent on the products of photosynthesis. Think about that dependent relationship. What could that dependency mean for the rates of cellular respiration in shade-adapted leaves?

Measuring Gas Products

There are many ways to measure the presence of oxygen or carbon dioxide. For example, a micro-ecosystem could be constructed in a flask (as shown below) then exposed to sunlight. The distance moved on the gauge would mirror the net change in the amount of gas produced as a function of the duration of the illumination. A simple system like this one would measure net gas production for both gases; it would not distinguish between oxygen production and carbon dioxide production.

Fortunately, because these gases are very reactive with other elements and molecules, special biosensors have been developed that specifically measure one or the other of these gases. In this set of experiments, you will use gas sensors referred to in this lab as an O₂ probe and a CO₂ probe. There are many common uses of oxygen sensors. Can you think of a common application of oxygen sensors? They are helpful and often essential when it is important to monitor oxygen levels.

Procedure I Overview

Net Photosynthesis Rates ‐ 100% Illumination: You will determine net photosynthesis rates for sun-adapted and shade-adapted leaves under full illumination conditions.

Procedure II Overview

Respiration Rates ‐ 0% Illumination: You will determine respiration rates for sun-adapted and shade-adapted leaves under zero illumination conditions.

Procedure III Overview

Low-Light Net Photosynthesis Rates ‐ 15% Illumination: You will determine net photosynthesis rates for sun-adapted and shade-adapted leaves under low illumination conditions.

Summary of Formulas Needed for Calculations

Determine the rate of change (or simply the rate) using timed data.

rate=changetime intervalrate=changetime interval

Connection: When sunlight is present the rates of change in oxygen and carbon dioxide are direct measurements of the net photosynthesis rate. In the absence of sunlight (0% illumination) the rates of change in oxygen and carbon dioxide are direct measurements of the respiration rate.

Example: O₂ rate

O2rate=change in oxygen concentrationtime intervalO2rate=change in oxygen concentrationtime interval

Sample Calculation: Determine the oxygen rate if the oxygen concentration increases by 22.05 ppm during a 25.0 second time interval. Note: ppm stands for parts-per-million and is a unit for measuring concentration.

O2rate=22.05ppm25.0s=0.882ppm/sO2rate=22.05ppm25.0s=0.882ppm/s

Example: CO₂ rate

CO2rate=change in carbon dioxide concentrationtime intervalCO2rate=change in carbon dioxide concentrationtime interval

Sample Calculation: Determine the carbon dioxide rate if the carbon dioxide concentration decreases by 7.84 ppm during a 35.0 second time interval. Note: ppm stands for parts-per-million and is a unit for measuring concentration.

CO2rate=−7.84ppm35.0s=−0.224ppm/sCO2rate=−7.84ppm35.0s=−0.224ppm/s

Please note that either rate (O₂ rate or CO₂ rate) could be positive or negative depending on the level of illumination.