cellular resperation

Cellular Respiration

Overview of Cellular Respiration

  • ATP (adenosine triphosphate) is utilized within cells and must be replenished through the process of cellular respiration.

  • Food energy contributes phosphates to ADP (adenosine diphosphate) molecules to reform ATP.

  • Cellular respiration requires oxygen, categorizing it as an aerobic process.

  • Many chemical reactions that comprise cellular respiration primarily occur within the mitochondria.

Electron Transfer Process

  • Oxidizing Agent (Electron Acceptor): Accepts an electron.

  • Reducing Agent (Electron Donor): Donates an electron.

  • Oxidation and reduction processes occur simultaneously and are interconnected.

Mitochondrial Structure

  • Mitochondria possess both an inner and outer membrane, with an intermembrane space situated between them.

  • Matrix: A semifluid medium that resides inside the mitochondrion.

Steps of Cellular Respiration

Step 1: Glycolysis

  • Location: Occurs in the cytoplasm.

  • A 6-carbon glucose molecule undergoes degradation into two 3-carbon pyruvic acid molecules.

  • Glycolysis is anaerobic; it does not require oxygen.

  • The process results in the production of 2 ATP molecules.

Step 2: Acetyl-CoA Formation

  • Location: Occurs in the mitochondria.

  • Oxygen is necessary for this reaction, indicating it is aerobic.

  • Pyruvate (the product of glycolysis) loses one carbon and two oxygens which are released as carbon dioxide.

  • Enzymes facilitate the linkage of coenzyme A to the acetate.

Step 3: Citric Acid Cycle

  • Location: Takes place in the mitochondrial matrix.

  • A series of chemical reactions, catalyzed by 8 different enzymes, generates ATP and releases carbon dioxide.

  • High-energy electrons are captured in the forms of NADH and FAD.

  • For each turn of the cycle, citrate loses a total of 8 electrons to electron acceptors such as NAD+.

Step 4: Electron Transport & Oxidative Phosphorylation

  • Location: Occurs in the inner membrane of the mitochondria.

  • The pathway of electrons from one carrier to another forms the electron transport chain, with each electron carrier transferring its electrons to the subsequent carrier (metaphorically described as a 'bucket brigade').

  • When a reduced carrier donates its electrons, it becomes oxidized.

  • Oxygen serves as the final electron acceptor.

Energy Harvesting Mechanisms

  • Proton Gradient: The flow of electrons through the electron transport chain creates a proton gradient.

  • Chemiosmosis: This process involves harnessing the energy stored in the chemical gradient. Certain machinery in the membrane is responsible for facilitating this process.

Proton Pumping Mechanism

  • The electron transport chain (ETC) actively pumps protons out of the mitochondrial matrix, contributing to the generation of an electrochemical gradient.

Substrates for Cellular Respiration

  • Fats, carbohydrates, and proteins can enter the cellular respiration pathway at various points.

Anaerobic Conditions & Fermentation

  • In the absence of oxygen following glycolysis, fermentation occurs as a means to regenerate more NAD+ from NADH.

  • In yeast, this results in the production of ethanol, while in animals, lactic acid is formed (often noted as painful).

Photosynthesis

General Overview of Photosynthesis

  • Sunlight encompasses a mixture of wavelengths, with reflected colors being those that are not absorbed by objects.

  • Photosynthesis serves as the reverse process of aerobic cellular respiration.

  • It entails combining carbon dioxide with water, utilizing light energy to produce glucose and oxygen.

  • Photosynthesis is performed by plants and certain bacteria but not by animals or fungi.

  • Carbon dioxide enters plants through openings known as stomata.

Pathways in Photosynthesis

1. Light Reactions

  • The light reactions generate ATP and NADPH.

  • Photosynthesis takes place in two phases, occurring in different regions of the chloroplast:

    • Light Reactions (occur in thylakoids): Light energy is absorbed by chlorophyll, exciting the electrons within it.

    • Excited electrons travel down an electron transport chain, yielding ATP.

2. Light-Independent Reactions (Calvin Cycle)

  • These reactions utilize ATP and NADPH produced from the light reactions to create glucose.

Chloroplast Structure

  • Thylakoids: Membranous structures within chloroplasts where light reactions occur.

Reaction Centers

  • Pigment molecules in an antenna complex capture light energy, transferring it to the photochemical reaction center.

Photosystem Characteristics

  • Protein-chlorophyll complexes in the thylakoid membrane trap light energy.

  • Photosystem I utilizes light at 700 nm (deep red).

  • Photosystem II utilizes light at 680 nm (red).

  • Light wavelengths correlate inversely with energy: shorter wavelengths equate to higher energy, while longer wavelengths yield lower energy.

Electron Pathways in Photosystems

  • Electrons can take two pathways:

    • Cyclic Pathway: Produces ATP.

    • Noncyclic Pathway: Produces NADPH.

  • Photosystem I (P700):

    • Absorbs a photon, becoming an electron donor; the excited electron is released to one of two transport chains (P700 becomes oxidized).

    • Cyclic flow returns the electron to P700, allowing for continuous repetition.

    • Non-cyclic electron flow generates NADPH, which serves as a reducing agent for glucose synthesis.

Photosystem II (P680)

  • Provides electrons to Photosystem I, where ATP is generated.

  • Electrons derive from the splitting of water molecules; P680 facilitates this process, resulting in oxygen as a waste byproduct.

Calvin Cycle Steps

  • Discovered by Calvin utilizing radioactive carbon, the Calvin Cycle entails three major steps:

    • Carbon dioxide fixation onto ribulose bisphosphate (RuBP).

    • Conversion of energy from ATP and NADPH into sugar.

    • Regeneration of ribulose bisphosphate.

  • The Calvin Cycle occurs in the stroma of the chloroplast.

Rubisco Enzyme

  • The rubisco enzyme catalyzes the addition of carbon dioxide to 5-carbon ribulose bisphosphate (RuBP), creating an unstable 6-carbon compound that quickly breaks down into two 3-carbon phosphoglycerate molecules.

  • It is noted that rubisco operates slowly at approximately 3 molecules per second; plants may contain up to 50% rubisco in terms of protein content.

Different Types of Plants

C3, C4, and CAM Plants

  • Guard Cells regulate stomatal openings which facilitate gas exchange in plants.

  • Transpiration: The movement of water from a plant through stomata.

  • Stomata conditions:

    • Open Stomata: Abundant carbon dioxide but with the risk of water loss.

    • Closed Stomata: Conserves water but limits photosynthesis.

  • Higher temperatures can negatively influence the rate of photosynthesis.

C3 Plants

  • The majority of plants are classified as C3 plants.

  • They often close their stomata to conserve water during hot conditions, potentially leading to photorespiration.

  • Photorespiration: A process that consumes ATP and releases carbon dioxide previously incorporated during the Calvin cycle.

C4 Plants

  • C4 plants have adapted mechanisms to avoid photorespiration.

  • An additional enzyme allows these plants to synthesize sugars even when stomata are nearly closed.

  • Examples include corn and sugar cane.

CAM Plants

  • CAM (Crassulacean Acid Metabolism) plants open their stomata exclusively at night.

  • Carbon captured at night is stored as an acid, which is then broken down during the day into carbon dioxide for photosynthesis.