CH 4 CONCEPTS

Chlorophyll

Chlorophyll molecules embedded in the thylakoid membrane absorb light energy. These molecules are the most important pigments for absorbing the light energy used in photosynthesis. A chlorophyll molecule has a hydrophobic "tail" that embeds the molecule into the thylakoid membrane. The "head" of a chlorophyll molecule is a ring called a porphyrin. The porphyrin ring of chlorophyll, which has a magnesium atom at its center, is the part of a chlorophyll molecule that absorbs light energy. Examine the illustration to view the chlorophyll molecule and to learn which wavelengths of light are absorbed by this pigment. Compare the absorption spectrum of chlorophyll with the action spectrum of photosynthesis that Engelmann found.

Chloroplast Structure

Chloroplasts are the sites of photosynthesis. These double-membrane bound organelles enclose additional membranes called thylakoids. The disc-shaped thylakoids possess an interior space. The thylakoids are stacked to form grana, which are suspended in the stroma of the chloroplasts. Demonstrate your knowledge of chloroplast structure by placing the labels in their correct locations.

Structure of a Leaf

Leaves are a plant's main photosynthetic organs. Leaf structure is closely associated with its photosynthetic function. Leaves must permit carbon dioxide access to the photosynthetic cells but impede water from diffusing out. The oxygen that is a waste product of photosynthesis must be allowed to escape from the leaf.

Structure of a Mesophyll Cell

Mesophyll cells are specialized for photosynthesis. These cells in the middle of the leaf contain many chloroplasts, the organelles that perform photosynthesis. As you demonstrate your knowledge of plant cell structure by placing the labels in the appropriate locations, remember that gas exchange (oxygen and carbon dioxide) takes place within the chloroplasts, and the food produced by the chloroplasts must move out of the cells to other parts of the plant.

An Overview of Photosynthesis

Photosynthesis converts light energy into the chemical energy of sugars and other organic compounds. This process consists of a series of chemical reactions that require carbon dioxide (CO2) and water (H2O) and store chemical energy in the form of sugar. Light energy from light drives the reactions. Oxygen (O2) is a byproduct of photosynthesis and is released into the atmosphere. The following equation summarizes photosynthesis: 6 CO2 + 6 H2O → 6(CH2O) + 6 O2 sugar Photosynthesis transfers electrons from water to energy-poor CO2 molecules, forming energy-rich sugar molecules. This electron transfer is an example of an oxidation-reduction process: the water is oxidized (loses electrons) and the CO2 is reduced (gains electrons). Photosynthesis uses light energy to drive the electrons from water to their more energetic states in the sugar products, thus converting solar energy into chemical energy.

Cooperation of the Light Reactions and the Calvin Cycle

Photosynthesis depends on an interaction between two sets of reactions: the light reactions and the Calvin cycle. Chlorophyll and the other molecules responsible for the light reactions are built into the thylakoid membranes. The enzymes that catalyze the Calvin cycle are located in the stroma. Beginning with the absorption of light by chlorophyll, the light reactions convert light energy into chemical energy in the form of ATP and NADPH. The ATP provides the energy, and the NADPH supplies the electrons for the Calvin cycle, which converts carbon dioxide to sugar. The ADP and NADP+ that result from the Calvin cycle shuttle back to the light reactions, which regenerate ATP and NADPH.

Electromagnetic Energy

The solar energy called visible light drives photosynthesis. Solar radiation is composed of electromagnetic energy that travels through space in a manner analogous to the motion of waves in water. The distance between the crests of waves is called the wavelength. The shorter the wavelength, the greater the energy for each unit (photon) of electromagnetic energy. Click and drag the "slider" button on the illustration below to investigate the type of electromagnetic energy that we call "light." Remember, energy cannot be created or destroyed. When light is absorbed by a green plant, a small portion of that energy is converted into chemical energy in the process of photosynthesis.