Photosynthesis and Cellular Respiration CELL ENERGETICS Photosynthesis and Cellular Respiration

Photosynthesis All energy for the planet comes from the sun. Photoautotrophs are organisms that can convert light energy to chemical energy. This process is called photosynthesis and can be summarized in the following equation: light energy 6H2O + 6CO2 6O2+ C6H12O6

Photosynthesis Photosynthesis takes place in the chloroplasts of plant cells and consists of 2 sets of reactions. Light dependent reactions take place in the thylakoid and use water and light energy. Oxygen is a product of these reactions. Light independent reactions take place in the stroma and use carbon dioxide. Carbohydrates are a product of this reaction.

Light Dependent Reactions Sunlight strikes chlorophyll molecules. The energy of sunlight splits water molecules (photolysis). Oxygen atoms are released. Hydrogen atoms are stripped of their electrons, which are passed to the electron carrier NADP+ forming NADPH and H+. The concentration gradient of hydrogen is used to drive an ATP synthase, making ATP.

Light Independent Reactions Carbon dioxide, ATP, and the electrons carried by NADP+ are used to build sugar. The molecule RuBP rearranges carbon dioxide to form an intermediate molecule called PGA . PGA is converted to PGAL, using ATP and the NADP+. Some PGAL is used to make glucose and some is used to regenerate RuBP to continue the process.

Light Dependent Reactions Photosynthesis CO2 Water ADP + Pi + NADP+ RuBP PGAL Light Independent Reactions ATP + NADPH + H+ Light Dependent Reactions O2 Carbohydrates

Cellular Respiration Neither autotrophs nor heterotrophs can utilize the energy of the carbohydrate as it is. Both types of organisms must convert the carbohydrate to ATP, the energy currency of the cell, in order to carry out metabolic activity. This process is called cellular respiration and can be summarized in the following equation: C6H12O6 + 6O2 6H2O + 6CO2 + ATP

Cellular Respiration Cellular respiration takes place in the cytoplasm and the mitochondria of eukaryotic cells and consists of 3 sets of reactions. Glycolysis occurs in the cytoplasm and breaks down glucose to pyruvate, producing 2 molecules of ATP. The Kreb’s Cycle occurs in the mitochondrial matrix and breaks down pyruvate, producing CO2 and 2 molecules of ATP. Oxidative phosphorylation occurs in the mitochondrial matrix, uses oxygen, and produces 32 molecules of ATP.

Glycolysis Glycolysis breaks down glucose, a 6-carbon sugar, into 2 molecules of pyruvate, a 3-carbon intermediate. When glucose is broken down, some of the hydrogen atoms are stripped of their electrons, which are passed to the electron carrier NAD+, forming NADH and H+. An initial investment of 2 ATP are used to reconfigure the glucose atoms, but 4 ATP are produced in the process for a net total of 2 ATP.

Kreb’s Cycle Pyruvate is converted to Acetyl-Coenzyme A in the cytoplasm of the cell. Acetyl-CoA moves into the mitochondrial matrix, and through a series of reactions, carbon and oxygen molecules are stripped, releasing CO2, and electrons are passed to NAD+ and FAD, forming NADH and FADH2 and many H+. These reactions generate 2 molecules of ATP.

Oxidative Phosphorylation The only substances that move into the mitochondrial membrane are the electron carriers, NADH and FADH2, and the hydrogen protons. The hydrogen electrons are passed through a series of protein complexes and ultimately to oxygen, which accepts them. Just like in the light dependent reactions of photosynthesis, the hydrogen protons develop a steep concentration gradient, which drives the production of ATP by an ATP synthase. 32 molecules of ATP are produced.

Cellular Respiration Electrons carried via NADH GLYCOLYSIS Glucose  Pyruvate KREB’S CYCLE OXIDATIVE PHOSPHORYLATION ATP ATP, CO2 ATP, H2O Electrons carried via NADH Electrons carried via NADH and FADH2 O2

Fermentation If oxygen is present, aerobic cellular respiration is done. In the absence of oxygen, anaerobic respiration takes place. Glycolysis occurs. Following glycolysis, the pyruvate is converted to either lactic acid or ethanol. This is fermentation. Fermentation involves very few chemical reactions (compared to cellular respiration) but only produces 2 molecules of ATP.