PHOTOSYNTHESIS and CELLULAR RESPIRATION

SECTION 1 Photosynthesis

Energy and Living Things Photosynthesis is the process in which light energy is converted into chemical energy. Autotrophs (plants and some bacteria) use the sun’s energy to carry out photosynthesis, and are therefore the foundation of all living systems.

Breaking Down Food For Energy Autotrophs are organisms that use energy from sunlight or from chemical bonds in inorganic substances to make organic compounds. Heterotrophs are organisms that must consume other organisms as food to get their energy.

Photosynthesis Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, carbon dioxide, and water to produce carbohydrates and oxygen. Photosynthesis has 3 stages: Stage 1: absorption of light energy Stage 2: conversion of light energy into chemical energy, temporarily stored in ATP and NADPH Stage 3: storage of chemical energy in ATP and NADPH powers the formation of organic molecules

Photosynthesis Pigments are light-absorbing substances that absorb only certain wavelengths of light and reflect all others. Chlorophyll is the primary pigment involved in photosynthesis. Chlorophyll absorbs mostly blue and red light and reflects green and yellow light. This reflection of green and yellow light makes many plants, especially their leaves, look green.

Photosynthesis occurs in the chloroplasts and uses the pigment chlorophyll.

Photosynthesis REACTANTS: water, carbon dioxide, light energy The following chemical equation summarizes photosynthesis: 6H2O + 6CO2 + light  C6H12O6 + 6O2 REACTANTS: water, carbon dioxide, light energy PRODUCTS: glucose, oxygen

Stages of Photosynthesis: STAGE 1 - The Light-Dependent Reactions STAGE 1: These reactions are called the “light reactions,” or “light-dependent reactions” because the reactions absorb light energy to make the organic compounds glucose and oxygen. STAGE 1 occurs in the chloroplasts on the thylakoid membrane where clusters of the pigment chlorophyll are embedded. Other pigments used are carotenoids that produce yellow and orange and red fall leaf colors, as well as the colors of many fruits, vegetables, and flowers. 2 accessory pigments of carotenoids are carotenes and xanthophylls

Photosynthesis: Where Does it Occur? Thylakoid membrane

Photosynthesis occurs in: Thylakoids Thylakoids are disk-shaped structures found in the chloroplasts of leaf cells that contain clusters of embedded pigments. These pigment molecules in the thylakoids of chloroplasts absorb light energy. Electrons in the pigments are “excited” by light, and jump from the chlorophyll molecules to other nearby molecules in the thylakoid membrane. The series of molecules along the thylakoid membrane that excited electrons pass through as they jump along the chlorophyll molecules is called the electron transport chain.

Photosynthesis: Stage 1 Light - Dependent Absorption of Light Energy Light and H20 are required The excited electrons that leave chlorophyll molecules must be replaced by other electrons. Plants get these replacement electrons from water molecules, H20. The water molecules are split by an enzyme inside the thylakoid. When water molecules are split, chlorophyll molecules take the electrons from the hydrogen atoms, H, leaving hydrogen ions, H+. The remaining oxygen atoms, O, from the disassembled water molecules combine to form oxygen gas, O2.

Photosynthesis: Stage 2 Conversion of Light Energy by Electron Transport Chains (ETC) Excited electrons lose some of their energy as they pass through these proteins. The energy lost is used to pump hydrogen ions into the thylakoid. As the process continues, hydrogen ions become more concentrated inside the thylakoid than outside, producing a concentration gradient across the thylakoid membrane. The hydrogen ions will diffuse back out of the thylakoid down their concentration gradient through specialized carrier proteins, or proton pumps.

Photosynthesis: Stage 2 These proteins act as both ion channels as well as enzymes. As H+ pass through the channel portion of the protein, the protein catalyzes a reaction in which a phosphate group is added to ADP molecules to form ATP (ADP + P = ATP). Thus, the movement of hydrogen ions across the thylakoid membranes through proton pumps provide the energy to produce ATP molecules. inner thylakoid membrane outer thylakoid membrane

Two Electron Transport Chains The first electron transport chain lies between two large clusters of pigment molecules and is used to form ATP. A second electron transport chain lies next to the sight of the first electron transport chain. In this second chain, excited electrons combine with hydrogen ions (H+) and an electron acceptor called NADP+ to form NADPH. NADPH is an electron carrier and is important in photosynthesis because it carries high energy electrons needed to produce organic molecules.

Photosynthesis: Stage 3 The Light-Independent Reactions The Storage of Chemical Energy Stage 3 of photosynthesis is known as the Calvin cycle and occurs in the stroma. The Calvin cycle creates complex carbohydrates that store energy. Stage 3 of photosynthesis is also known as the “light-independent reactions” or “dark reactions” because these series of reactions do not need light to occur.

Photosynthesis: The Light-Independent Reactions Stage 3 of photosynthesis is sometimes called carbon dioxide fixation because in a series of enzyme-assisted chemical reactions within the chloroplasts, CO2 molecules adhere to existing carbon compounds to form sugars for long-term energy storage. The energy used in the Calvin cycle is supplied by ATP and NADPH that was made during Stage 2. In a series of enzyme-assisted chemical reactions within the chloroplast called carbon dioxide fixation, CO2 molecules adhere to existing carbon compounds to form sugars for long-term energy storage. This process called the Calvin Cycle uses the energy made in the 2nd stage of photosynthesis, and is often referred to as dark reactions, or light independent reactions.

Three Factors That Affect Photosynthesis 1.) amount of light – The rate of photosynthesis increases as light intensity increases until all the pigments are being used. At this saturation point, the reactions of the Calvin cycle cannot proceed any faster. 2.) concentration of carbon dioxide – Once a certain concentration of carbon dioxide is present, photosynthesis cannot proceed any faster. 3.) range of temperature – Like all metabolic processes, photosynthesis involves many enzyme-assisted chemical reactions. Unfavorable temperatures may inactivate certain enzymes.

SECTION 2 Cellular Respiration

Cellular Respiration

Cellular Respiration Before energy from food can be utilized, it must be transferred to ATP in a process called cellular respiration. Cellular respiration is the set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. To put it simply, cellular respiration is the process where cells produce energy from carbohydrates.

Cellular Respiration Cellular respiration is the opposite of photosynthesis. The reactants of photosynthesis – carbon dioxide and water – are the products of cellular respiration. The products of photosynthesis – glucose and oxygen – are the reactants of cellular respiration. Cellular respiration releases much of the energy in food to make ATP. ATP provides cells with energy they need to carry out the activities of life.

Cells Transfer Energy From Food To ATP When cells break down food molecules, some of the energy is released into the atmosphere as heat, while the rest is stored temporarily in molecules of ATP. Adenosine triphosphate (ATP) is a nucleotide with two extra energy-storing phosphate groups. ATP molecules are often called the “energy currency” of a cell.

NADPH ATP Glucose Sucrose

Adenosine Triphosphate RED = ribose (a 5-carbon sugar) BLUE = adenine (a nitrogenous base) GREEN = phosphate groups

ATP Stores and Releases Energy The energy from ATP is released when the bonds that hold the phosphate groups together are broken. The removal of a phosphate group from ATP (3 phosphates) produces ADP (adenosine diphosphate -- 2 phosphates), which releases energy in a way that enables cells to use the energy. Cells use energy released by this reaction to power metabolism.

ATP FYI: The human body uses about 1 million molecules of ATP per second per cell. There are more than 100 trillion cells in the human body. That is about 1 X 1020, or 100,000,000,000,000,000,000 ATP molecules used in the body each second!

Cellular respiration can be aerobic respiration (with oxygen) or anaerobic respiration (without oxygen). Cellular respiration begins in the cytoplasm, and ends in the mitochondria. Cellular respiration takes place in the two stages of glycolysis, then aerobic respiration.

Cellular Respiration REACTANTS: glucose, oxygen, ADP, extra phosphate The chemical formula for cellular respiration is: C6H12O6 + 6O2 + ADP + P  6CO2 + 6H2O + ATP REACTANTS: glucose, oxygen, ADP, extra phosphate PRODUCTS: carbon dioxide, water, ATP The process summarized by the equation begins in the cytoplasm of a cell and ends in the mitochondria.

Cellular Respiration: Stage 1 Glycolysis Energy Investment and Energy Yielding Stage Stage 1 of cellular respiration is called glycolysis. Glycolysis is the stage of cellular respiration where glucose is broken down in the cytoplasm, converted to pyruvate, and produces a small amount of ATP and NADPH. Pyruvate is oxidized to Acetyl CoA and CO2 is removed. Glycolysis – uses 2 ATP, but produces 4 ATP – net gain = 2 ATP Anaerobic

Cellular Respiration: Stage 2 The Krebs Cycle (Citric Acid Cycle) Stage 2 of cellular respiration is known as the Krebs cycle and is also called aerobic respiration Cellular respiration is called an aerobic process because it requires oxygen. C6H12O6 + 6O2 + ADP + P  6CO2 + 6H2O + ATP A two-carbon molecule combines with a four-carbon molecule during the Krebs cycle.

Krebs Cycle

Cellular Respiration: Stage 2 The Krebs Cycle Pyruvic acid produced during glycolysis enters the mitochondria and is converted into carbon dioxide and water. ATP and NADPH are produced. The Krebs cycle produces 2 ATP for each molecule of glucose broken down.

Cellular Respiration: Stage 3 The Electron Transport Chain (ETC) Oxidative Phosphorylation (Chemiosis) If enough O2 is present, up to 34 ATP molecules can be formed from a single glucose molecule! At the end of the electron transport chain, oxygen (O2) acts as the final electron acceptor and combines with H+ ions to form water molecules (H2O).

Eukaryotes (Have Membranes) Total ATP Yield 2 ATP – glycolysis (substrate-level phosphorylation) 4 ATP – converted from 2 NADH – glycolysis 6 ATP – converted from 2 NADH – grooming phase 2 ATP – Krebs cycle (substrate-level phosphorylation) 18 ATP – converted from 6 NADH – Krebs cycle 4 ATP – converted from 2 FADH2 – Krebs cycle 36 ATP - TOTAL

Prokaryotes (Lack Membranes) Total ATP Yield 2 ATP – glycolysis (substrate-level phosphorylation) 6 ATP – converted from 2 NADH – glycolysis 6 ATP – converted from 2 NADH – grooming phase 2 ATP – Krebs cycle (substrate-level phosphorylation) 18 ATP – converted from 6 NADH – Krebs cycle 4 ATP – converted from 2 FADH2 – Krebs cycle 38 ATP - TOTAL

Fermentation: Occurs in the Absence of Oxygen If oxygen (O2) is not present in sufficient amounts, the electron transport chain in the mitochondrial membrane cannot function. Energy molecules (ATP and NADH) cannot be created in abundance. So, what does the cell do to continue to break down organic compounds and release energy if not enough oxygen is present?

Fermentation: Occurs in the Absence of Oxygen Fermentation is the anaerobic process that continues the breakdown of carbohydrates when there is not enough oxygen for aerobic respiration. There are two types of fermentation: 1.) lactic acid fermentation and 2.) alcoholic fermentation. Lactic acid and/or ethanol (alcohol) are the by-products of fermentation when the breakdown of carbohydrates occurs without oxygen.

Lactic Acid Fermentation Glycolysis occurs without oxygen. However, the Krebs cycle requires oxygen. In lactic acid fermentation, NAD+, an electron acceptor, is recycled and glycolysis can continue to produce ATP. Fermentation enables glycolysis to continue producing ATP as long as the glucose supply lasts. Lactate, an ion of lactic acid, can build up in muscle cells if not removed quickly enough and can cause “muscle burn” or muscle fatigue.

Alcoholic Fermentation Carbon dioxide is released during alcoholic fermentation by yeast. Carbon dioxide gas released by the yeast is what causes the rising of bread dough and the carbonation of some alcoholic beverages.

Alcoholic Fermentation Alcoholic fermentation is a two-step process: First, pyruvate is converted, releasing carbon dioxide. Second, electrons are transferred from a molecule of NADH to the two-carbon compound, producing ethanol. Alcoholic fermentation by yeast can be used to produce food and beverages such as yogurt, cheese, beer, and wine.

Production of ATP The total amount of ATP a cell is able to harvest from each glucose molecule that enters glycolysis depends on the presence or absence of oxygen. When oxygen is present, aerobic respiration occurs. When oxygen is not present, anaerobic respiration, or fermentation, occurs instead.

Production of ATP Most ATP is made during aerobic respiration. Glycolysis (Stage 1 of cellular respiration) can occur with or without oxygen, and produces a net gain of 2 ATP molecules. The Krebs cycle (Stage 2 of cellular respiration) produces 2 ATP molecules for each glucose molecule broken down. The electron transport chain (Stage 3 of cellular respiration) can produce up to 34 ATP molecules from a single glucose molecule.