Thermodynamics, Energy & Metabolism

Laws of Thermodynamics

• Thermodynamics studies energy flow and transformation
• The 1st Law, or Law of Conservation of Energy: energy can change forms but cannot be created or destroyed
• The 2nd Law: energy conversion is never perfectly efficient
• Lost energy typically becomes thermal energy
• Entropy measures unusable energy within a system, such as a food chain

How Organisms Obtain Energy

• Heterotrophs must ingest food for energy (e.g., humans)
• Autotrophs produce their own food, mainly via photosynthesis (e.g., plants)

Metabolism

• This encompasses all cell chemical reactions
• A metabolic pathway is a series of reactions where one reaction's product becomes the substrate for the next

Catabolic Reactions

• Energy is released by breaking down larger molecules into smaller ones

Anabolic Reactions

• Use energy from catabolic pathways to build larger molecules

• This creates a continuous energy flow in organisms

• Photosynthesis is an anabolic pathway, converting sunlight into glucose

• Cellular respiration is a catabolic pathway, breaking down organic molecules like ATP to release energy for cell use

ATP Structure and Function

• Adenosine Triphosphate (ATP) is the main energy carrier in cells
• All cell chemical reactions use ATP
• ATP is synthesized via cellular respiration and consists of:
  • Adenine base
  • Ribose sugar
  • Three phosphate groups
• When the bond between the second and third phosphate groups breaks, ATP releases energy and becomes ADP (adenosine diphosphate) and a free phosphate group

Photosynthesis

• Light energy is captured and converted into chemical energy
• The process converts light energy into a usable energy form for organisms
• Plants use it to create glucose, which is a primary energy source for life on Earth
• Photosynthesis is also used by protists like algae and some bacteria
• Photosynthetic organisms are crucial for life
• The reactants needed include:
  • Light, sourced from the sun
  • Carbon Dioxide (CO2), obtained from the air
  • Water (H2O), absorbed from the roots
• The products are:
  • Glucose (C6H12O6), a type of sugar
  • Oxygen (O2)
• Photosynthesis occurs in two phases:
  • Light-dependent reactions (or light reactions), which take place in the thylakoids
  • Chloroplasts, disk-shaped organelles with 2 compartments, capture light energy

Thylakoid

• Membranes that contain stacks called grana, and this is where light reactions occur

Photosystems I and II

• Crucial for light reactions
• Contain light-absorbing pigments and protein complexes

Stroma

• Its the fluid-filled space outside the grana
• It is where light-independent reactions take place

Light-Independent Reactions

• Sometimes called dark reactions or the Calvin cycle

• Enzyme-assisted reactions use chemical energy from ATP and NADPH to synthesize glucose

• Light-independent reactions do not need light

• They occur in the stroma of the chloroplast

• They require CO2

• ATP and NADPH provide the fuel

• Glucose is produced from CO2 and hydrogen

• Plants use the sugars from the Calvin cycle for energy, storing it as starch or using it for cell wall structure (cellulose)

• Photosynthesis happens in plant cell chloroplasts

• Factors affecting photosynthesis rate include:

  • Light intensity where more light increases photosynthesis up to a limit
  • Temperature
  • CO2 concentrations
  • Water

Pigments

• Light-absorbing molecules in thylakoid membranes

• Different pigments absorb and reflect different light wavelengths

• Chlorophylls are the main light-absorbing pigments in plants

• Chlorophylls absorb mostly blue-violet light and reflect green light

• Types of chlorophyll include:

  • Chlorophyll a
  • Chlorophyll b
  • Carotenoids

• Light absorption is the initial step

• Light energy excites electrons in chlorophyll within the thylakoid

• Energized electrons split water molecules

• Oxygen is released via stomata on the leaf underside

• Activated electrons moving along the thylakoid membrane (electron-transport chain) generate energy in the form of NADPH

• Energy from protons (H+) is used to synthesize ATP

• Temporarily stored energy in ATP and NADPH is used in the Calvin cycle

Alternative Pathways

• Many plants in harsh climates use alternative photosynthetic pathways

• C4 plants thrive where light energy is abundant but water is scarce

• Examples of C4 plants:

  • Sugarcane
  • Corn
  • Some grasses

• CAM plants conserve water

• They open stomata at night, have thick cuticles, and include species like:

  • Cacti
  • Orchids
  • Pineapple

Cell Respiration

• (Chemiosmosis/ATP Production)
• Living organisms break down organic molecules for energy via cellular respiration
• Its a catabolic pathway that releases energy for cell use
• The equation is opposite to photosynthesis:
  • Reactants: Glucose (C6H12O6), obtained by eating; Oxygen (O2), obtained by breathing
  • Products: Heat, Carbon Dioxide (CO2), and Water (H2O)

Mitochondria Parts and Functions

Glycolysis

• Takes place in the cytoplasm
• It involves the splitting of glucose
• It doesn't need oxygen
• Final glycolysis products per glucose molecule:
  • 2 ATP
  • 2 NADH molecules
  • 2 pyruvate molecules
• While 4 ATP are produced, 2 are used in the process

Aerobic Respiration

• This happens in the mitochondria
• It uses oxygen to break down glucose molecules
• 36 ATP results from one glucose molecule
• Aerobic respiration follows glycolysis in the mitochondria
• 2 pyruvate molecules are transported into the mitochondria and turned into acetyl CoA, which enters the Krebs cycle

Krebs Cycle

• Pyruvate is broken down inside a series of reactions into carbon dioxide
• Occurs twice for each glucose molecule, producing:
  • 8 NADH
  • 2 FADH2
  • 2 ATP
  • 6 CO2

Electron Transport and Chemiosmosis

• NADH & FADH2 enter the electron transport chain in the mitochondria cristae
• High-energy electrons and hydrogen ions from NADH and FADH2 convert ADP to ATP
• Oxygen is the final electron acceptor
• Protons and electrons form water
• 32 more ATP are produced

Anaerobic Respiration

• The initial step in anaerobic respiration is glycolysis
• Anaerobic means without oxygen
• Plants and animals cannot survive on anaerobic respiration alone
• It is not very efficient
• The two types are:
  • Lactic acid fermentation occurs in animal cells and causes muscle burn during intense activity
  • Alcohol fermentation occurs in plants and produces alcohol like wine and beer

Comparing Photosynthesis and Cellular Respiration

• Both photosynthesis and cellular respiration exchange oxygen and carbon dioxide
• Photosynthesis creates food, while cellular respiration breaks down food to produce energy