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Everything you need to know about AP Bio Unit 3! Enzymes, Photosynthesis, Respiration

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Main Ideas and Lessons (AP Bio Unit 3: Cellular Energetics)

1) Enzymes: what they are and how they work

  • Definition / role

    • Enzymes are biological catalysts.
    • Usually proteins (some RNAs can also act as enzymes).
    • They lower activation energy, increasing reaction rate.

  • Specificity

    • Enzymes are highly specific: the active site matches the shape/charge of the substrate.

  • Structure and why conditions matter

    • Enzymes have complex shapes due to secondary, tertiary, and quaternary structures held by:
      • Hydrogen bonds
      • Ionic bonds
      • Hydrophobic clustering
    • Changing pH, temperature, or ion concentration disrupts these interactions → active site shape changes → substrate can’t bind properly.
    • This creates an optimum (pH/temperature/ions) where enzyme function is highest.

  • Denaturation

    • Denaturation = enzyme shape changes in a way that lowers or eliminates function.

  • Effects of environment

    • pH
      • Activity peaks at a pH optimum.
      • Above/below optimum reduces activity due to bond disruption → denaturation.
    • Temperature
      • Up to a point, activity increases because molecules move faster and collide more often.
      • Past a threshold, increased temperature causes denaturation → reduced activity.

  • Reversible vs. irreversible denaturation

    • Reversible
      • Returning to optimal conditions can restore original shape and function.
    • Irreversible
      • Shape permanently altered (analogy: cooking an egg).

  • Substrate concentration

    • Low substrate → fewer enzyme–substrate encounters → slow reaction rate.
    • Increasing substrate → faster reaction rate.
    • Saturation: with enough substrate, enzymes are “fully engaged” → rate plateaus.

  • Inhibition types

    • Competitive inhibition
      • Inhibitor competes for the active site.
      • Decreases reaction rate by preventing substrate binding.
    • Non-competitive inhibition
      • Inhibitor binds at an allosteric site (not the active site).
      • Alters active site shape (“ripple effect”) → substrate can’t bind effectively.

2) Cell energy foundations: pathways, energy types, ATP, and coupling

  • Metabolic pathway

    • A linked series of enzyme-catalyzed reactions inside a cell.
    • Reactions share intermediates:
      • Initial reactant → intermediates → final product
    • Pathways can be:
      • Linear (e.g., glycolysis)
      • Cyclical (e.g., Krebs cycle, Calvin cycle) where starting molecules are regenerated

  • Autotrophs vs. heterotrophs

    • Autotrophs: produce their own food
      • Photoautotrophs (plants, cyanobacteria): use light to make organic compounds via photosynthesis
      • Chemoautotrophs (some bacteria/archaea): use chemosynthesis, oxidizing inorganic substances (e.g., iron, sulfur, hydrogen sulfide)
    • Heterotrophs
      • Get energy/matter from organic compounds made by other organisms
      • Can be consumers, decomposers, or parasites

  • Exergonic vs. endergonic reactions

    • Exergonic
      • Releases energy and increases entropy
      • Example: burning paper/wood; generally respiration and many hydrolysis reactions
    • Endergonic
      • Requires energy and decreases entropy
      • Example: photosynthesis; many dehydration synthesis reactions

  • ATP structure and function

    • ATP consists of:
      • Ribose (5-carbon sugar)
      • Nitrogenous base adenine
      • Three phosphate groups
    • ATP stores/releases energy to power cellular work:
      • Building ATP: ADP + Pi → ATP
      • Using ATP: removing terminal phosphate converts ATP → ADP + Pi

  • Energy coupling

    • Links an exergonic reaction to an endergonic reaction so the endergonic step proceeds.
    • Examples:
      • Cellular respiration: exergonic reactions drive ATP formation (endergonic)
      • Muscle contraction: ATP breakdown (ATP → ADP + Pi) powers endergonic work

3) Photosynthesis: big picture, phases, and key mechanisms

A. Photosynthesis big picture

  • Overall purpose

    • Uses light energy to convert CO₂ + H₂O into carbohydrates, releasing O₂.

  • Equation

    • 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂

  • Thermodynamics

    • Endergonic because it:
      • Builds high-energy glucose from low-energy inputs (CO₂, H₂O)
      • Decreases entropy (more organized products vs diffuse gases)

  • Evolution and consequences (high level)

    • Appeared ~3.5 billion years ago (after life ~3.8 bya).
    • Produced an oxygen-rich atmosphere (enabled aerobic metabolism).
    • Produced an ozone layer, enabling life on land.

B. Two phases of photosynthesis

  • Phase 1: Light reactions

    • Convert light energy → chemical energy stored in:
      • ATP
      • NADPH
    • Occur in thylakoid membranes.
    • Inputs: light + water
    • Outputs:
      • ATP, NADPH
      • O₂ as waste

  • Phase 2: Calvin cycle

    • Uses ATP + NADPH (from light reactions) and CO₂ to build carbohydrates (sugars).
    • Occurs in the stroma.

C. Chlorophyll and light absorption/action spectra

  • Chlorophyll role

    • Main pigment that absorbs light to support electron energy changes.
    • Structure supports placement in thylakoid membranes; key active portion is a ring with Mg in the center.

  • Absorption spectrum

    • Strong absorption in blue and red.
    • Poor absorption in green, so leaves appear green (reflecting green light).
    • Two main forms: chlorophyll a and chlorophyll b (different functional groups, similar overall absorption pattern).

  • Action spectrum

    • Shows which wavelengths drive photosynthesis most.
    • Blue and red drive most photosynthesis; green drives very little.
    • Engelmann experiment
      • Algae filament exposed to wavelength-separated light.
      • Aerobic bacteria grew best where oxygen production was highest (blue/red).

D. Chloroplast structure connected to functions

  • Location / organization

    • In plant cells (especially leaf top cells); multiple per cell.

  • Membranes and compartments

    • Outer membrane and inner membrane
    • Inner features:
      • Thylakoids (membrane-bound sacs), stacked into grana
      • Stroma surrounds thylakoids; Calvin cycle occurs here

  • Organelle origin note (vestiges)

    • DNA and ribosomes suggest evolutionary ancestry (independent-cell origin).

E. Light reactions: products, sites, and electron/proton machinery

  • Key components

    • Photosystems in thylakoid membranes:
      • Protein complexes with embedded chlorophyll
    • Photosystems convert light energy into electron energy and drive electron flow.
    • Water splitting occurs at photosystem II, producing:
      • O₂
      • protons

  • Important ordering note

    • Light reactions require photosystem II before photosystem I.

  • Electron flow & Z-scheme

    • Light boosts electrons (starting at photosystem II after excitation).
    • Electrons move through an electron transport chain.
    • Electron transport powers:
      • Proton pumps moving H⁺ from stroma → thylakoid space
      • ATP synthesis via ATP synthase as protons diffuse back (chemiosmosis)
    • Photosystem I boosts electrons again and helps produce NADPH via:
      • NADP⁺ reductase converting NADP⁺ → NADPH

  • Chemiosmosis concept (light reactions → ATP)

    • Proton pumps create an electrochemical (proton) gradient (both concentration and electrical gradients).
    • Protons return only through ATP synthase, converting:
      • ADP + Pi → ATP
    • Extra protons from water splitting strengthen the gradient, boosting ATP production.

4) Calvin cycle: phases and carbon accounting

  • Purpose

    • Convert CO₂ into sugars using energy carriers (ATP, NADPH).

  • Three phases

    1. Carbon fixation
      • CO₂ + RUBP combine via enzyme RuBisCO
      • Produces a temporary 6-carbon intermediate that splits into two 3-carbon molecules
    2. Energy investment and harvest
      • The 3-carbon molecule is:
        • reduced by NADPH
        • phosphorylated using ATP
      • Produces G3P (also called PG3P; names treated as interchangeable)
    3. Regeneration of starting compound
      • Regenerates RUBP to continue capturing CO₂

  • Carbon accounting method (exam-focused)

    • 3 RUBP (5 C each) + 3 CO₂ (1 C each):
      • 3×5 + 3×1 = 18 carbons
    • The 6-carbon intermediates split into six 3-carbon molecules:
      • 6×3 = 18
    • During harvest:
      • Some G3P is used; one G3P removed for sugar building
      • Remaining carbons are rearranged to regenerate 5 RUBP (returning to the starting carbon budget)

5) Cellular respiration: big picture and major stages

A. Big picture

  • Chemical equation

    • C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP (energy)

  • Thermodynamics

    • Exergonic: releases energy and increases disorder (entropy)

  • Where it occurs in eukaryotes (high-level map)

    • Glycolysis: cytoplasm
    • Link reaction: mitochondrial matrix entry/process
    • Krebs cycle: mitochondrial matrix
    • ETC + oxidative phosphorylation: mitochondrial inner membrane (uses intermembrane space)

  • Big stages described

    • ATP is produced using steps that generate NADH/FADH₂, then the ETC uses them for ATP synthesis.

B. Overview of what happens in each phase

  1. Glycolysis

    • Glucose (6C) → 2 pyruvate (3C each)
    • Produces ATP and NADH
    • Runs without oxygen (anaerobic portion)

  2. Link reaction

    • Pyruvate (3C) → acetyl-CoA (2C)
    • Releases CO₂ and reduces NAD⁺ → NADH

  3. Krebs cycle

    • Acetyl-CoA is oxidized, generating:
      • NADH
      • FADH₂
      • ATP via substrate-level phosphorylation (and releases CO₂)
    • Runs twice per glucose

  4. Electron transport chain (ETC) + oxidative phosphorylation

    • NADH and FADH₂ donate electrons to ETC
    • ETC pumps H⁺ to create a proton gradient
    • Oxygen is the final electron acceptor
    • Proton gradient drives ATP synthase (most ATP) via chemiosmosis

C. Glycolysis: phases, inputs, outputs

  • Location: cytoplasm

  • O₂ requirement: none (anaerobic)

  • Three phases

    • Investment (ATP use)
      • Enzymes transfer phosphate from ATP → glucose
      • Produces fructose-1,6-bisphosphate
    • Cleavage
      • Fructose-1,6-bisphosphate splits into 2 G3P
    • Energy harvest
      • Each G3P is oxidized:
        • electrons transfer to NAD⁺ → NADH
      • ADP → ATP using energy from G3P

  • Outputs summary

    • Gross yield: 4 ATP produced
    • Net yield: 2 ATP (because 2 ATP are used in investment)
    • 2 NADH
    • 2 pyruvate (3C each)

D. Link reaction: explicit list

  • Pyruvate enters mitochondrial matrix
  • Pyruvate → acetyl-CoA (2C)
  • CO₂ released
  • NAD⁺ → NADH

E. Krebs cycle (K): explicit output facts

  • Location: mitochondrial matrix
  • Type: cyclical sequence
  • Per acetyl-CoA generates:
    • 1 ATP
    • 3 NADH
    • 1 FADH₂
  • CO₂ released
  • Oxaloacetate is both the starting and ending compound

6) ETC and oxidative phosphorylation + UCP/brown fat

  • How ATP is made

    • NADH and FADH₂ deliver electrons to ETC.
    • ETC protein complexes pump protons:
      • matrix → intermembrane space
    • Oxygen is the final electron acceptor.

  • Proton gradient drives ATP synthesis

    • Protons return through ATP synthase
    • ADP + Pi → ATP

  • Heat production instead of ATP (brown fat cells / uncoupling)

    • UCP/thermogenin provides an alternative proton channel in the inner mitochondrial membrane.
    • Protons leak back without passing through ATP synthase → less ATP, more heat.
    • ETC still runs, and electron movement still produces heat.

7) Similarities between mitochondria and chloroplasts (evolutionary takeaway)

  • Both systems:
    • Use electron transport chains to pump protons into a compartment → build a proton gradient
    • Use ATP synthase to make ATP via chemiosmosis
  • Evolution claim:
    • Similarities suggest a shared ancient ancestor.
    • ATP synthase likely evolved once and was inherited/shared between ancestors of mitochondria and chloroplasts.

8) Anaerobic respiration and fermentation

A. Aerobic vs. anaerobic respiration

  • Aerobic respiration

    • Requires oxygen
    • Includes glycolysis + link reaction + Krebs cycle + ETC
    • Approx ATP: ~32 ATP per glucose

  • Anaerobic respiration / fermentation (video context)

    • Happens when oxygen is lacking/insufficient or aerobic machinery is unavailable.
    • Depends on glycolysis, followed by fermentation
    • Produces 2 ATP per glucose total
    • Occurs in the cytoplasm (no mitochondria required for the described process)

B. What fermentation is and why it happens

  • Fermentation definition

    • Glycolysis + reactions that regenerate NAD⁺
    • NAD⁺ regeneration is essential because glycolysis requires NAD⁺.

  • Why fermentation occurs

    • Keeps glycolysis producing ATP when oxygen isn’t available.
    • Much less ATP than aerobic respiration, but prevents ATP shortage.

C. Alcohol vs. lactic acid fermentation

  • Alcohol fermentation (ethanol fermentation; yeast, bread, beer)

    • Pyruvate → ethanol + CO₂
    • NADH is oxidized → NAD⁺ regenerated (so glycolysis can continue)
    • CO₂ creates bubbles in beer and helps dough rise.

  • Lactic acid fermentation (muscle tissue under anaerobic conditions)

    • Pyruvate → lactic acid
    • NADH oxidized to NAD⁺, allowing glycolysis to continue
    • Associated with intense exercise when oxygen delivery can’t keep up → “lactic acid build-up” and fatigue.

Speakers / Sources Featured

  • Mr. W — “learn-biology.com” (presenter/voice in multiple promotional segments and instruction sections)
  • Thomas Engelman — referenced for the algae/prism oxygen production experiment (Engelmann experiment)
  • learn-biology.com — course/tutorial service mentioned (including reviews and free trial/promotions)
  • APBioSuccess.com — mentioned as a place to download a study checklist (apbiosuccess.com/checklist)

Original video