Unit Plant Science

Problem Area Managing Plant Growth

Lesson Energy Transformation in Plants: Photosynthesis

Student Learning Objectives 1. Identify the importance of photosynthesis. 2. Discuss the structural unit where the photosynthetic reaction occurs. 3. Describe the processes of photosynthesis. 4. Identify the factors that affect photosynthesis.

Terms ADP ATP Bundle sheath cells C3 plants C4 plants Calvin cycle Carotenoids Cellulose Chlorophyll Chlorophyll a Chlorophyll b Chloroplasts Electron transport chain Grana Light-dependent reaction Light-independent reaction Mesophyll

Terms cont. NADPH NADP+ P680 P700 Photosynthesis RuBP Stroma Thylakoids

Why is photosynthesis important? Photosynthesis is the process in which carbon dioxide and water are transformed, in the presence of light, into carbon-containing, energy-rich, organic compounds. A. Photosynthesis provides the fundamental energy foundation for all organisms. Photosynthesis occurs in green plants, seaweed, and algae. Plants can produce millions of glucose molecules per second. Excess glucose produced is converted into starch and carbohydrates and stored in the plants roots, stems, and leaves. Stored glucose can be converted to energy when required by the plant.

B. Humans and animals are unable to manufacture glucose for energy. The animal kingdom relies on plant material for glucose. Glucose is a significant energy source for humans and animals. C. Oxygen breathed by humans and animals is the oxygen produced through the photosynthetic reaction. Without the photosynthetic reaction, oxygen would not be replenished in the atmosphere. D. Humanity is reliant on the earliest products of photosynthesis. Fossil fuels such as natural gas, coal, and petroleum are composed of complex hydrocarbons. These hydrocarbons are the remnants of organisms that relied on photosynthesis millions of years in the past.

What is the structural unit in which photosynthesis takes place? Photosynthesis takes place in chloroplasts. Chloroplasts are organelles that contain green pigments known as chlorophyll. Chloroplasts are located generally in the cells of the mesophyll tissue found in leaves. Typically there are 20–100 chloroplasts in each mesophyll cell. A. The stroma is a fluid-filled region inside the chloroplast. The stroma contains most of the crucial enzymes obligatory for photosynthesis. Thylakoids are also found in the chloroplast. Thylakoids are interconnected sets of flat, disk-like sacs. Arranged in stacks, the thylakoids sacs bear a resemblance to a stack of coins and are called grana.

B. Chlorophyll is found in two forms. Chlorophyll a is bright green and is responsible for absorbing energy from the violet-blue to red, red-orange light wavelengths. Chlorophyll b is yellow-green and absorbs different wavelengths of light. Carotenoids are supplementary yellow and orange pigments that absorb energy from green- yellow-orange wavelengths. The energy absorbed by carotenoids can be transferred to chlorophyll a.

What are the photosynthetic processes? Plants convert light energy into chemical energy of sugar molecules through the process of photosynthesis. A. Using light energy from sunlight, chlorophyll manufactures ATP and NADPH. Plants use the energy held in the ATP and NADPH molecules to make carbohydrates. Examples of carbohydrates are sugars, starches, and cellulose. While sugars and starches are used for energy, cellulose is a building block for the cell walls of plants. B. Photosynthesis is a very complex process and is divided into two stages.

C. The light-dependent reaction relies on the flow of energy and electrons instigated by light energy. Light energy causes the electrons in chlorophyll to boost up and out of their orbit. The electrons immediately resume their orbit. The orbit jumping electrons causes a release of energy. This energy is transferred rapidly from one chlorophyll or pigment to the next. Photosynthesis begins when light strikes Photosystem I pigments and excites their electrons. The energy passes rapidly from molecule to molecule.

When the energy reaches the chlorophyll molecule P700, the P700 molecules uses the energy to boost its electrons. The electrons are then passed from one carrier to another, called an electron transport chain. Nicotine adenine dinucleotide (NADP+) lies at the end of the electron transport chain. Using the energy released by the electrons, two electrons from the chain combine with a hydrogen ion and NADP+ to form NADPH. NADPH can then be used to make carbohydrates.

D. The light-dependent reaction continues with Photosystem II. Light energy activates electrons in the Photosystem II pigments. These pigments transfer their energy from excited electrons to the chlorophyll molecule, P680. Energy is used to transfer electrons through a different electron transport chain. As the electrons pass through the chain, some of the energy is used to fuel the production of ATP.

ATP is formed by the addition of one phosphorus atom to adenosine diphosphate (ADP). The electron transport chain will eventually deliver the Photosystem II electrons to Photosystem I and the P700 electrons. The P680 electron is replenished by water absorbed from the plants roots. The oxygen from the water will diffuse out of the chloroplast and is released into the air through the stoma on the leaf.

E. The light-independent reaction does not require light. In this reaction ATP and NADPH are used to make high-energy carbohydrates. Termed the Calvin cycle, the light-independent reactions use the electrons and hydrogen ions associate with NADPH and the phosphorus associated with ATP to produce glucose. These reactions occur in the fluid surrounding the thylakoids. The light-independent reactions begins when carbon dioxide links to sugar molecules called ribulose bisphosphate (RuBP) in the process known as carbon fixation.

Initially a six-carbon molecule is formed. Instantly, the molecule splits into 2 three-carbon molecules. The three-carbon molecules join to form simple sugars, glucose, and fructose. It takes six turns of the Calvin cycle, involving the input of six CO2 molecules and hydrogen from NADPH and ATP, to result in one molecule of glucose. Most plants are referred to as C3 plants because the first product of carbon fixation is a three-carbon compound.

F. Many plants have the ability to fix carbon dioxide into four-carbon compounds. C4 plants originated in areas of high temperatures, high light intensities, and limited amounts of water. Corn, sugar cane, and crabgrass are C4 plants. C4 plants have specialized cells known as bundle sheath cells. Reactions of the C4 pathway take place in the mesophyll cells. In the process, the plants fix CO2 into four-carbon compounds. A result of the C4 pathway is an increased concentration of carbon dioxide in bundle sheath cells.

What factors affect photosynthesis? A. The first requirement for photosynthesis is a healthy, living plant. Healthy plants contain chlorophyll and the pigments necessary for photosynthesis. B. Carbon dioxide must be in ample supply. The higher the concentration of CO2, the higher the rate of growth. C. Water must be available for the plant to use. Stomata may close to conserve water, decreasing the absorption of CO2, shutting down photosynthesis. D. Light or radiant energy drives photosynthesis. Light must be present for photosynthesis to occur.

Review/Summary Why is photosynthesis important? What is the structural unit in which photosynthesis takes place? What are the photosynthetic processes? What factors affect photosynthesis?