Cellular Respiration

Introduction To perform their many tasks, cells require energy from outside sources. In most ecosystems, energy enters as sunlight Light energy trapped in organic molecules (such as glucose) is available to all organisms.

Chloroplast

Chloroplasts, found in plants and eukaryotic algae, are the site of photosynthesis. – They convert solar energy to chemical energy and synthesize new organic compounds from CO 2 and H 2 O. Contains chlorophyll, which causes the green color of many plants. Has small quantities of DNA that help make own proteins.

Mitochondria

Mitochondria is the organelle that converts energy to forms that cells can use for work- “powerhouse”. Mitochondria are the sites of cellular respiration, generating ATP from the catabolism of sugars, fats, and other fuels in the presence of oxygen. Has small quantities of DNA that help make own proteins.

ATP (adenosine triphosphate) Adenosine: – Adenine – Ribose sugar Triphosphate tail: – Three phosphate groups Each phosphate group is negatively charged. The crowding of negative charge in the phosphate tail contributes to potential energy stored in the ATP.

The ATP Cycle ATP is “recyclable” ATP is continuously converted to ADP as cells do work. Analogy: like compressing a spring requires energy, adding the phosphate group requires energy. Where does the energy come from? The ATP cycle churns at an astonishing pace. Any guesses for how many ATP molecules are spent and regenerated per second?

Cell Respiration vs. Breathing Cellular respiration is an aerobic process, meaning that it requires oxygen. Although breathing for the whole organism is not the same as cellular respiration, the two processes are related.

Equation for Cellular Respiration Cellular Respiration’s main function is to generate ATP for cellular work. The process produces molecules for each glucose molecule consumed.

Electrons as an energy source If you burn sugar directly, it happens very quickly, releasing energy in the form of heat and light. In your cells, the ‘burning’ process happens in controlled steps, with some of the energy released being used to generate ATP molecules instead of being converted to heat and light.

Respiration occurs in three metabolic stages: 1. glycolysis, 2. the Krebs cycle 3. the electron transport chain (oxidative phosphorylation). NET 36 ATP formed Total 38 ATP formed Respiration involves glycolysis, the Krebs cycle, and electron transport: an overview Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.6 Boehm 2010

Glycolysis Glycolysis means “splitting of sugar” Glycolysis takes place outside the mitochondria in the cytoplasm of the cell. Upon splitting, each 3-carbon molecule transfers electrons and H+ to a carrier molecule called NAD+. In summary, 2 ATP molecules have been spent, and 4 ATP molecules have been produced. The pyruvic acid molecule still holds most of the energy of the original glucose molecule.

Stage 2: The Krebs/Citric Acid Cycle

Krebs Cycle… Upon diffusing into the mitochondrion, each pyruvic acid molecule loses a carbon molecule, and is converted into acetyl CoA, which then enters the Krebs cycle. The reactions in the Krebs cycle produce two CO2 molecules and 1 ATP molecule per acetyl CoA. NADH and FADH2 trap most of the energy released as electrons and H+

Electron Transport Chain Electron carriers, NADH and FADH2, pass the electrons on to other carriers in a series of transfers called the electron transport chain. Each carrier holds the electron more strongly than the carrier before it. Finally, oxygen—the electron grabber—pulls electrons from the final carrier molecule and joins them with H+ ions, forming water.

ETC and ATP synthase Action Analogy: potential energy much like water stored in a dam Protein structures called ATP synthases allow H+ to rush back across membrane into the area of lower concentration, using energy from the flow of H+ to convert ADP to ATP (~34 ATP generated).

Fermentation

Fermentation enables some cells to produce ATP without the help of oxygen Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic). Glycolysis produces a net yield of 2 ATP, compared to the 38 ATP produced by cellular respiration. However, by burning enough glucose, fermentation can regenerate enough ATP molecules for short bursts of activity, such as a short sprint. Under anaerobic conditions, and depending on the type of organism, either lactic acid or alcoholic fermentation will occur to produce ATP.

Lactic acid fermentation continued.. Lactic acid is produced in the human muscle cells when they’re not getting enough oxygen to support their energy requirements. The build up of lactic acid in muscles causes the fatigue and soreness a person feels after an intense exercise The lactic acid is eventually converted back into pyruvic acid in the liver.

Alcoholic Fermentation Yeast ( a microscopic fungus) is also capable of both cellular respiration and fermentation. In an anaerobic environemnt, yeast ferment sugar and other foods. However, fermentation in yeast produces ethyl alcohol, instead of lactic acid, as a waste product. This process also releases CO2, which forms the bubbles seen in bread. The alcohol evaporates during baking,

Other uses of fermentation for humans

Comparing Photosynthesis & Respiration PhotosynthesisCellular Respiration FunctionEnergy StorageEnergy Release LocationChloroplastsMitochondria ReactantsCO 2 and H 2 OC 6 H 12 O 6 and O 2 ProductsC 6 H 12 O 6 and O 2 CO 2 and H 2 O Equation6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O