Photosynthesis

Obtaining Energy Photosynthesis converts light energy from the sun into chemical energy in the form of organic compounds through a series of reactions known as biochemical pathways. Autotrophs use energy from sunlight or from chemical bonds in inorganic substances to make organic compounds. Animals and other organisms that must get energy from food instead of directly from sunlight or inorganic substances are called heterotrophs.

Linking Photosynthesis and Cell Respiration The oxygen (O2) and some of the organic compounds produced by photosynthesis are used by cells in a process called cellular respiration.

Overview of Photosynthesis Photosynthesis can be divided into two stages: Light Reactions and Calvin Cycle In the light reactions, light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH. In the Calvin Cycle, organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH. Equation of Photosynthesis: 6CO2 + 6H2O light energy C6H12O6 + 6O

Light Reactions The light reactions begin with the absorption of light in chloroplasts, organelles found in the cells of plants, some bacteria, and algae. Light and Pigments White light from the sun is composed of an array of colors called the visible spectrum. Pigments absorb certain colors of light and reflect or transmit the other colors. Located in the membrane of the thylakoids of chloroplasts are several pigments, including chlorophylls (such as chlorophyll a and chlorophyll b) and carotenoids.

Converting light to energy The pigment molecules acquire some of the energy carried by the light. The acquired energy forces electrons to enter a higher energy level These energized electrons are said to be “excited.” The excited electrons have enough energy to leave the chlorophyll molecules. As the electrons move from molecule to molecule in this chain, they lose most of the acquired energy. The energy they lose is used to move protons into the thylakoid.

Converting light to energy An important part of the light reactions is the synthesis of ATP. The protons move through ATP synthase into the stroma, which produces ATP. In the stroma, the electrons combine with a proton and NADP+. This causes NADP+ to be reduced to NADPH.

Calvin Cycle The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle. In the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation.

Calvin Cycle The Calvin cycle, which occurs in the stroma of the chloroplast, is a series of chemical reactions that produces a three-carbon sugar from three carbon dioxides. Most of these three-carbon sugars are converted to a five-carbon sugar to keep the Calvin cycle operating. But some of the three- carbon sugars leave the Calvin cycle and are used to make organic compounds, in which energy is stored for later use.

Calvin Cycle

Photosynthesis

The C4 Pathway Some plants that evolved in hot, dry climates fix carbon through the C4 pathway. These plants have their stomata partially closed during the hottest part of the day. Certain cells in these plants have an enzyme that can store CO2 as a four-carbon compounds so they can keep doing the Calvin cycle.

CAM Pathway Some other plants that evolved in hot, dry climates store carbon through the CAM pathway. These plants bring in carbon dioxide at night and store so they can use it to do the Calvin cycle during the day. This lets them minimize water loss.

Factors that influence photosynthesis Light Intensity The rate of photosynthesis increases as light intensity increases, because more electrons are excited in both photosystems. Carbon Dioxide Levels Increasing levels of carbon dioxide also stimulate photosynthesis.

Factors that influence photosynthesis Temperature As temperature increases, the rate of photosynthesis increases to a maximum and then decreases with further rises in temperature. The stomata begin to close at higher temperatures to limit water loss and therefore impede the entry of carbon dioxide.