Exam Review Notes on Cellular Respiration and Photosynthesis

Respiration Overview

• Key process: Breaking down sugar using oxygen.
• Products: Carbon dioxide (CO2) and water (H2O).
• Energy yield: 30-32 ATP molecules.
• Equation: C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + ATP

Anaerobic Respiration

• Involves the complete breakdown of sugar without oxygen.
• Less efficient than aerobic respiration.
• ATP yield: 2-30 ATP (variable depending on the process).
• Many different versions exist.

Fermentation

• Incomplete breakdown of sugar.
• Products still contain a lot of energy (e.g., ethanol).
• ATP yield: Only 2 ATP per glucose molecule.
• Example: Ethanol fermentation, where ethanol is energy-rich and burns.

ATP: The Energy Currency of the Cell

• ATP powers cellular activities.
• Used for muscle movement, active transport, and building molecules.
• De-energized form: ADP (adenosine diphosphate).

Phosphorylation

• Definition: Addition of a phosphate group to a molecule.
• In respiration: Phosphorylation is used to make ATP by adding a phosphate to ADP.
• Two Types:
  • Substrate-level phosphorylation: ATP made in a chemical reaction.
  • Oxidative phosphorylation: Associated with the electron transport chain (where most ATP is made).

Key Molecules Involved in Energy Storage

• ATP (Adenosine Triphosphate).
• NADH (Nicotinamide Adenine Dinucleotide).
• FADH2 (Flavin Adenine Dinucleotide).
• NADH and FADH2 store energy temporarily and are used to make ATP in the electron transport chain.

Breakdown of Glucose

• Energy from glucose is transferred to ATP, NADH, and FADH2.
• A significant portion of energy goes into NADH and FADH2 first.
• The energy from NADH and FADH2 is then extracted in the electron transport chain to produce ATP.

Glycolysis

• Location: Cytosol (cytoplasm) of the cell.
• Process: Glucose (6-carbon molecule) is broken down into two molecules of pyruvate (3-carbon molecule).
• Products: 2 ATP and 2 NADH.
• Does not occur in the mitochondria.

Krebs Cycle (Citric Acid Cycle)

• Location: Mitochondrial matrix.
• Pyruvate molecules from glycolysis enter the mitochondria to undergo the Krebs cycle.

Electron Transport Chain

• Location: Inner mitochondrial membrane.

Carbon Dioxide Production

• All carbons from the original glucose molecule end up as carbon dioxide (CO_2), which is exhaled.

Summary of Cellular Respiration Steps

• Glycolysis:
  • Location: Cytoplasm.
  • Glucose (6 carbon) is converted to 2 Pyruvate (3 carbons).
  • Net Production: 2 ATP, 2 NADH.
• Transition Step:
  • Pyruvate crosses into the mitochondria and loses a carbon.
  • Each pyruvate molecule becomes Acetyl CoA, producing NADH and releasing CO_2.
• Krebs Cycle:
  • Location: Mitochondrial matrix.
  • Acetyl CoA is completely broken down.
  • Net Production (per glucose molecule, which means two cycles): 2 ATP, 6 NADH, 2 FADH2, and releases CO_2.
• Electron Transport Chain and Oxidative Phosphorylation:
  • NADH and FADH2 deliver electrons, leading to ATP production.
  • Oxygen is the final electron acceptor, forming water.
  • ATP yield: 26-28 ATP.
  • Occurs in the inner mitochondrial membrane.

Acetyl CoA

• Delivers carbons to the Krebs cycle.
• Formed from pyruvate after it crosses into the mitochondria.

Citric Acid/Citrate

• The first molecule formed when acetyl CoA enters the Krebs cycle.

ATP Production in Cellular Respiration

• Most ATP is produced during electron transport and oxidative phosphorylation; approximately 26-28 ATP molecules.
• Glycolysis and Krebs cycle only produce 2 ATP molecules each.

Electron Transport Chain Details

• Electrons from NADH and FADH2 flow through the chain.
• The final electron acceptor is oxygen, which combines with electrons and hydrogen ions to form water.
  • Enzyme involved in ATP production: ATP synthase.

Importance of Oxygen

• If oxygen is absent, the electron transport chain stops.
  • Without electron transport, the Krebs cycle also stops because FAD and NAD are needed (which are produced by the electron transport chain).
  • In the absence of oxygen, only glycolysis continues (followed by fermentation).

ATP Production in Absence of Oxygen

• Glycolysis alone produces only 2 ATP molecules per glucose.

Bacterial vs. Eukaryotic Cells

• Bacterial Cells:
  • Electron transport occurs in the cell membrane.
  • Glycolysis and Krebs cycle occur in the cytoplasm.
• Eukaryotic Cells:
  • Electron transport occurs in the inner mitochondrial membrane.
  • Kreb's cycle occurs in the mitochondrial matrix.

Fermentation (Detailed)

• If oxygen is not available, pyruvate undergoes fermentation.
• In humans/animals, pyruvate is converted to lactic acid (lactic acid fermentation).
• Yeast converts pyruvate into alcohol (alcohol fermentation).
• Net ATP yield from fermentation: 2 ATP (same as glycolysis).
• Oxygen is not required for fermentation.

Photosynthesis Overview

• Occurs in chloroplasts in plants.
• Plants use light to create chemical energy.
• Chemosynthesis: some organisms use chemical energy rather than light energy to produce sugars (e.g., bacteria in thermal vents use hydrogen sulfide).

Light and Energy

• Purple end of the spectrum has more energy than the red end.
• White light is a combination of all the visible colors.
• gamma rays = shortest wavelengths
• radio waves = longest wavelengths
• Visible lights = narrowest band in the middle.

Absorption of light by plants

• Plants don't absorb green light well.

Chloroplasts and Photosynthesis

• Thylakoids: Hollow pancake-like structures where light is absorbed via Photosystems.
• Stroma: Fluid outside the thylakoids.
• Light reactions: Occur in the thylakoids.
• Calvin cycle: Occurs in the stroma.

Chemical Equation of Photosynthesis

• Reactants: Water (H2O) and carbon dioxide (CO2).
• Products: Oxygen (O_2) and sugar (glyceraldehyde-3-phosphate or G3P).
• Equation:
  • H2O + CO2 \rightarrow O_2 + G3P

Photosynthesis Pigments

• Chlorophylls.
• Xanthophylls.
• Carotenoids.

Light Reactions

• Capture light energy and use it to create ATP and NADPH.
  • Photosystem: cluster of pigments; crucial part of the light reactions.
  • light comes in, water is coming in, oxygen is going out; ATP and NADPH are the energy sources. Those molecules in turn are the ones that power the Calvin cycle to make sugar.

Steps in Light Reactions

• Light is absorbed by pigments in the photosystem and the energy is transferred from one pigment to the next until it reaches the reaction center chlorophyll.
• Electrons are excited to high energy levels.
• Electrons are stolen away from the reaction center of Chlorophyll by a primary electron acceptor.
• These high-energy electrons are passed down an electron transport chain to produce ATP (in Photosystem II) and NADPH (in Photosystem I).
• The last electron acceptor in Photosystem II's electron transport chain is Photosystem I.
• Oxygen is produced in Photosystem II.

Calvin Cycle

• Occurs in the stroma.
• Carbon dioxide (CO_2) is fixed (incorporated into an organic molecule) using the enzyme Rubisco.
• The product is glyceraldehyde-3-phosphate (G3P).
• For every three carbon dioxide molecules that enter, one G3P molecule exits.
• G3P: Glyceraldehyde-3-phosphate = glycerol + aldehyde + a phosphate.

Stomata

• Opening in a leaf formed by two guard cells that allows gases to enter and exit.

Photorespiration

• Occurs when oxygen enters the Calvin cycle instead of carbon dioxide due to closed stomata in dry conditions.
• Rubisco binds to oxygen instead of carbon dioxide, resulting in no sugar production.
• C4 and CAM plants have adaptations to avoid photorespiration.
• C4 Plants: such as grasses.
• CAM Plants: cacti, pineapples, succulents, yuccas, which store carbon dioxide at night to use during the day.

Pep Carboxylase

• Critical enzyme in both C4 and CAM pathways.
• Captures carbon dioxide and converts it into a four-carbon molecule (malic acid or malate).
• In CAM plants, malic acid is stored at night and released during the day.
• In C4 plants, malic acid is a transport molecule.

Endoplasmic Reticulum Differentiation

• Rough ER: Has ribosomes on its surface which synthesize proteins; bumpy.
• Smooth ER: Lacks ribosomes and is involved in lipid synthesis and other metabolic processes. Not bumpy. Does not partake in protein production.