Respiration and Photosynthesis

Respiration and Photosynthesis

Cellular Energy

• Metabolism – chemical reactions in a cell o Catabolic pathways – release energy by breaking down larger molecules o Anabolic pathways – use energy released by catabolic pathways

• Photosynthesis – anabolic pathway in which light from the Sun is converted to chemical energy

• Cellular respiration – catabolic pathway in which organic molecules are broken down to release energy

Thermodynamics

• Thermodynamics – study of the flow and transformation of energy in the universe o Laws of Thermodynamics – 1. Law of conservation of energy – energy can be converted from one form to another, but it cannot be created or destroyed 2. Energy cannot be converted without loss of usable energy (usually thermal energy)

ATP AND BIOLOGICAL ENERGY

• Adenosine triphosphate (ATP), the energy currency or coin of the cell

• ATP consists of the adenine nucleotide (ribose sugar, adenine base, and phosphate group

• ADP + Pi + energy ----> ATP

ATP Analogy: Rechargeable Batteries

• ATP is the higher energy form (the recharged battery) while ADP is the lower energy form (the used battery). When the terminal (third) phosphate is cut loose, ATP becomes ADP (Adenosine diphosphate; di= two), and the stored energy is released for some biological process to utilize. The input of additional energy (plus a phosphate group) "recharges" ADP into ATP (as in the analogy the spent batteries are recharged by the input of additional energy).

Cellular Respiration Equation (Reverse for Photosynthesis)

C6H12O6 + 6O2 = 6 Carbon Sugar oxygen

6CO2 + 6H2O + ENERGY Carbon dioxide water

Electromagnetic Spectrum

Fig. 9-4c, p. 194

Outer membrane

Inner membrane

Stroma

1 μm

Intermembrane space

Thylakoid membrane

In the chloroplast, pigments necessary for the light-capturing reactions of photosynthesis are part of thylakoid membranes, whereas the enzymes for the synthesis of carbohydrate molecules are in the stroma.

Granum (stack of thylakoids)

Thylakoid lumen

Photosynthesis: an overview• 2 major steps:

– Light reactions (“photo”) In the Light Dependent Processes (Light Reactions) light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction.

– Carbon fixation (Calvin Cycle): The ATP and NADPH are used to ‘fix’ carbon from carbon dioxide to make organics (sugar)

• Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule.

Photosystems

• Light harvesting units of the thylakoid membrane, where light energy is converted to chemical energy

• Composed mainly of protein and pigment antenna complexes

• Antenna pigment molecules are struck by photons (light)

Noncyclic Electron Transport• Light-dependent reactions

– form ATP and NADPH

Light Dependent Reactions• Photosystem II - Light energy splits water and causes the removal

of an electron from Photosystem II. Two electrons are replaced from a water molecule, breaking the water into H+ ions and 2 O- ions. These electrons are "boosted" to a higher energy state and are passed through a series of reactions and electron carriers (called an electron transport chain), eventually attaching them to a molecule in Photosystem I. These reactions result in the production of ATP.

• Photosystem I - Light acts on a molecule in Photosystem I, causing 2 electrons to be "boosted" to a still higher potential. These electrons are passed again through a series of reactions (another electron transport chain making more ATP). Eventually the electrons are attached to NADP+ and H+ to form NADPH, an energy carrier needed in the Light Independent Reaction. The electrons from Photosystem II replace the excited electrons in Photosystem I. There is thus a continuous flow of electrons from water to NADPH. This energy (ATP) is used in Carbon Fixation.

Chemiosmosis

• Chemiosmosis is the diffusion of ions across a selectively-permeable membrane. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane

• In photosynthesis - As electrons are passed from chlorophyll to NADP+, more hydrogen ions are pumped across the membrane. Inside of membrane becomes positively charged and outside negatively charged. Difference in charge provides the energy to make ATP o ATP synthase in membrane allows hydrogen ions to pass through it. As the ions pass through, the protein spins like a turbine catalyzing the combination of ADP and a phosphate group.

Chemiosmosis

The Calvin Cycle

Calvin Cycle (Light Independent)

• 6 carbon dioxide molecules eneter cycle from atmosphere and combine with 6 5 carbon molecules to make 12 3 carbon molecules

• Energy from ATP and NADPH is used to convert the 12 3 carbon molecules into high energy forms

• 2 of the 12 3 carbon molecules are removed from the cycle to be used by the plant to produce sugars, lipids, amino acids, etc. A single 6 carbon sugar is produced.

• Remaining 10 3 carbon molecules are converted back into 6 5 carbon molecules

Overview of 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. ATP is used by cells to do work.

Cellular Respiration Process Overview

• STAGE 1: GLYCOLYSIS– IT DOES NOT REQUIRE OXYGEN (ANAEROBIC)

•STAGE 2: THE PRODUCTS OF GLYCOLYSIS CAN FOLLOW EITHER OF TWO MAIN PATHWAYS, DEPENDING ON WHETHER THERE IS OXYGEN IN THE CELL. – AEROBIC RESPIRATION PATHWAY INCLUDES THE KREBS

CYCLE AND ELECTRON TRANSPORT AND REQUIRES OXYGEN

– ANAEROBIC RESPIRATION PATHWAY IS CALLED FERMENTATION (ALCOHOL OR LACTIC ACID FERMENTATION) AND DOES NOT REQUIRE OXYGEN

Aerobic Respiration• Glycolysis occurs in the cytoplasm

• All other stages in the mitochondria

Cellular respiration

• Glycolysis: cytosol; degrades glucose into pyruvate

• Kreb’s Cycle (Citric Acid): mitochondrial matrix; pyruvate into carbon dioxide

• Electron Transport Chain: inner membrane of mitochondrion; electrons passed to oxygen

Glycolysis

• 1 Glucose ---> 2 pyruvate

molecules• Energy investment

phase: cell uses ATP• Energy payoff phase:

ATP and NADH are produced

• Net energy yield per glucose molecule: 2 ATP plus 2 NADH; no CO2 is released; occurs aerobically or anaerobically

Glycolysis

Glycolysis I

Glycolysis II

Citric Acid Cycle

Citric Acid Cycle

Citric Acid Cycle

Electron transport chain

• NADH & FADH2 are used as carriers in the electron transport chain; oxygen is the final acceptor

• ATP synthase: produces ATP by using the H+ gradient (proton-motive force) pumped into the inner membrane space from the electron transport chain; this enzyme harnesses the flow of H+ back into the matrix to phosphorylate ADP to ATP (oxidative phosphorylation)

Electron Transport Chain

Summary of Glycolysis and Cellular Respiration

ATP Total Yield

• Glycolysis produces 2 ATP; aerobic respiration produces 34 more ATP

Review: Cellular Respiration

• Glycolysis: ATP

• Kreb’s Cycle: 2 ATP (substrate-level phosphorylation)

• Electron transport & oxidative phosphorylation: 2 NADH (glycolysis) = 6ATP 2 NADH (acetyl CoA) = 6ATP 6 NADH (Kreb’s) = 18 ATP 2 FADH2 (Kreb’s) = 4 ATP

• 38 TOTAL ATP/glucose

Aerobic and Anaerobic Pathways

• In the absence of oxygen, fermentation reactions produce alcohol or lactic acid but no additional ATP.

Anaerobic Pathways