Chapter 8 Cellular Energy Biology

Section 8.1 How Organisms Obtain Energy Main idea – All living organisms use energy to carry out all biological processes Objectives Summarize the two laws of thermodynamics Compare and contrast autotrophs and heterotrophs Describe how ATP works in a cell

Transformation of Energy All cellular activities require energy; the ability to do work Thermodynamics is the study of the flow and transformation of energy in the universe

Laws of Thermodynamics The 1st Law – “Law of conservation of energy” – Energy can be converted from one form to another, but it cannot be created nor destroyed The 2nd Law – Energy that is “lost” is generally converted to thermal energy – “entropy increases” Entropy – measure of disorder or unusable energy, in a system.

Autotrophs & Heterotrophs Autotrophs – organisms that make their own food Chemoautotrophs – uses chemicals as a source of energy Photoautotrophs – convert light energy from the Sun into chemical energy Heterotrophs – organisms that need to ingest food to obtain energy

Metabolism All of the chemical reactions in a cell are referred to as the cell’s metabolism Metabolic pathway is a series of chemical reactions in which the product of one reaction is the substrate for the next reaction Catabolic pathways – releases energy by breaking down larger molecules into smaller ones Anabolic pathways –uses the energy released by catabolic pathways to build larger molecules from smaller molecules The continual flow of energy within an organism is the result of the relationship of anabolic and catabolic pathways

Photosynthesis Photosynthesis is the anabolic pathway in which light energy from the Sun is converted to chemical energy for use by the cell 6CO2 + 6H2O  C6H12O6 + 6O2

Cellular Respiration Cellular Respiration is the catabolic pathway in which organic molecules are broken down to release energy for use by the cells C6H12O6 + 6O2  6CO2 + 6H2O + ATP

Photosynthesis & Cellular Respiration Form a Cycle

ATP: The Unit of Cellular Energy Adenosine Triphosphate (ATP) is the most important biological molecule that provides chemical energy ATP is made of an adenine base, a ribose sugar, and three phosphate groups ATP releases energy when the bond between the second and third phosphate groups are broken, forming adenosine diphosphate (ADP) and a free phosphate group Energy is stored in the phosphate bond formed when ADP receives a phosphate group and becomes ATP

Section 8.2 Photosynthesis Main idea – Light energy is trapped and converted into chemical energy during photosynthesis Objectives Summarize the two phases of photosynthesis Explain the function of a chloroplast during the light reactions Describe and diagram electron transport

Overview of Photosynthesis Photosynthesis is a process in which light energy is converted into chemical energy 6CO2 + 6H2O  C6H12O6 + 6O2 Photosynthesis occurs in two phases Phase 1-Light-dependent reactions-light energy is absorbed and then converted into chemical energy in the form of ATP and NADPH Phase 2-Light-independent reactions-the ATP and NADPH formed in phase 1 is used to make glucose

Chloroplasts

Phase 1: Light Reactions Chloroplasts – capture light energy in photosynthetic organisms; disc-shaped organelles that contain two main compartments Thylakoids are flattened saclike membranes that are arranged in stacks (grana) and are the location of light-dependent reactions Stroma are the fluid spaces outside the grana and are the location of light-independent reactions

Pigments Pigments are light-absorbing molecules found in the thylakoid membranes of chloroplasts Chlorophylls are the major light-absorbing pigments in plants; reflecting green light Accessory pigments allow plants to trap additional light energy Carotenoids – reflect yellow, orange and red light

Electron Transport See Figure 8.8 on page 225 Activated electrons are passed from one molecule to another along the thylakoid membrane in a chloroplast. The energy from electrons is used to for a proton gradient. As protons move down the gradient, a phosphate is added to ADP, forming ATP

Electron Transport (cont.) Light energy absorbed by photosystem II is used to split a molecule of water. When water splits, oxygen is released from the cell, protons (H+; hydrogen ions) stay in the thylakoid space and an activated electron enters the electron transport chain As electrons move through the membrane, protons are pumped into the thylakoid space At photosystem I, electrons are re-energized and NADPH is formed

Electron Transport (cont.) Chemiosmosis: Protons accumulate in the thylakoid space, creating a concentration gradient When protons move across the thylakoid membrane through ATP synthase, ADP is converted to ATP (photophosphorylation) http://highered.mheducation.com/olc/dl/120072/bio13.swf

Phase 2: Calvin Cycle Light-independent Reactions Calvin Cycle – the second phase of photosynthesis in which energy is stored in organic molecules such as glucose See Figure 8.9 on p. 226 First step – carbon fixation - 6CO2 + 6 RuBP (ribulose 1,5-biphosphate a 5-carbon compound) to form 12 3-PGA (3-phosphoglycerate, a 3-carbon molecule) Second step – the chemical energy stored in 12 ATP and 12 NADPH is transferred to the 12 3-PGA to form 12 G3P (glyceraldehyde 3-phosphate, high energy molecules)

Calvin Cycle (cont.) Third step – 2 G3P leave the cycle to form glucose and other organic compounds Final step – An enzyme called rubisco converts the remaining 10 G3P into RuBP. Plants use the sugars formed during the Calvin Cycle both as source of energy and as building blocks for complex carbohydrates, including cellulose, which provides structural support for the plant

Alternative Pathways Many plants in extreme climates have alternative photosynthesis pathways to maximize energy conversion C4 plants minimize water loss by closing stoma in hot days as they fix carbon dioxide into 4-carbon compounds instead of the 3-carbon molecules during the Calvin Cycle CAM plants (crassulacean acid metabolism) occurs in water-conserving plants as CO2 enters leaves at night fixing it into organic molecules. During the day, CO2 is released and enters the Calvin Cycle

8.3 Cellular Respiration Main idea – Living organisms obtain energy by breaking down organic molecules during cellular respiration Objectives Summarize the stages of cellular respiration Identify the role of electron carriers in each stage of cellular respiration Compare alcoholic fermentation and lactic acid fermentation

Overview of Cellular Respiration Organisms obtain energy in a process called cellular respiration The function of cellular respiration is to harvest electrons from carbon compounds, such as glucose, and use that energy to make ATP C6H12O6 + 6O2  6CO2 + 6H2O + ATP

Cellular Respiration (cont.) Two main parts Glycolysis Anaerobic process – do not require oxygen Aerobic Respiration Krebs cycle & electron transport Aerobic process – requires oxygen

Glycolysis Glucose is broken down in the cytoplasm through the process of glycolysis, refer to Figure 8.12 on p. 229 First, 2 phosphate groups are joined to glucose Second, the 6-carbon molecule is broken down into 2 G3P Next, two phosphates are added and electrons and hydrogen ions combine to produce 4 ATP and 2 NADH, respectfully Last, the 2 G3P are converted into 2 pyruvate molecules Glycolysis has a net yield of 2 ATP molecules

Krebs Cycle The series of reactions in which pyruvate is broken down into carbon dioxide is called the Krebs cycle or tricarboxylic acid (TCA) cycle. This cycle is known as the citric acid cycle, too The Krebs cycle occurs inside the mitochondria of cells, refer to Figure 8.13 on page 230

Krebs Cycle (cont.) Prior to the Krebs cycle, pyruvate reacts with coenzyme A (CoA) to form acetyl CoA At the same time CO2 is released and NAD+ is converted into NADH The reaction results in the production of 2 CO2 molecules and two NADH The cycle begins with acetyl CoA combining with a 4-carbon compound to form citric acid, a 6 carbon compound

Krebs Cycle (cont.) Then, citric acid is broken down in the next series of steps, releasing 2 molecules of CO2 and generating one ATP, three NADH, and one FADH2. FAD is another electron carrier similar to NAD+ and NADP+ Finally, acetyl CoA and citric acid are generated and the cycle continues The net yield from the Krebs cycle is 6 CO2 molecules, 2 ATP, 8 NADH, and 2 FADH2. NADH and FADH move on to play a significant role in the next stage of aerobic respiration

Electron Transport Refer to Figure 8.14 on p. 231 Electrons move along the mitochondrial membrane from one protein to another Electrons are transported to oxygen to form water Electron transport produces 24 ATP Each NADH molecule produces 3 ATP Each group of 3 FADH2 produces 2 ATP In eukaryotes, one molecule of glucose yields 36 ATP In prokaryotes, one molecule of glucose produces 38 ATP

Anaerobic Respiration The anaerobic pathway that follows glycolysis is anaerobic respiration, or fermentation Fermentation occurs in the cytoplasm and regenerates the cell’s supply of NAD+ while producing a small amount of ATP Two types Lactic acid fermentation Alcohol fermentation

Lactic acid fermentation & Alcohol fermentation Enzymes convert the pyruvate made during glycolysis to lactic acid When oxygen is absent or in limited supply, fermentation can occur Skeletal muscles during strenuous exercise Microorganisms to produce cheese, yogurt & sour cream Alcohol fermentation Occurs in yeast and some bacteria when pyruvate is converted to ethyl alcohol and CO2

Photosynthesis & Cellular Respiration Processes cells use to obtain energy Metabolic pathways that produce and break down simple carbohydrates The products of Photosynthesis are oxygen and glucose – the reactants needed for cellular respiration The products of cellular respiration – carbon dioxide and water – are the reactants for photosynthesis