Chapter 7 // Pt1: Intro to Cellular Respiration

Chapter 7

Intro to Cellular Respiration

Metabolism is a characteristic of life

Early Cells

• First cells around 3.5 BYA

There was no oxygen in the atmosphere. Life had to be anaerobic.

Cellular Respiration

• Any pathway that uses an electron transfer chain to harvest energy from organic molecules to make ATP
• Breaking down glucose to produce ATP
    * Ex: electron transfer phosphorylation in light-dependent reactions of photosynthesis
• Photosynthesis v. cellular respiration
    * Electron phosphorylation in photosynthesis uses inorganic compounds
      * Ex: CO₂
    * Cellular respiration uses organic compounds
      * Ex: glucose
• Opposite to photosynthesis (CO₂ to O₂), cellular respiration is O₂ to CO₂

✨Oxygen✨

Early photosynthetic organisms evolved from cyclic light reactions (no oxygen) to noncyclic light reactions (produces oxygen) ← how oxygen came to be on Earth

• Oxygen was toxic
• It polluted the air
• O₂ easily steals electrons = dangerous free radicals = dying cells
• Great Oxidation Event
    * Killed off most anaerobic life
    * Survivors = life that was away from O₂
      * Deep water, muddy sediments, etc.
    * Caused by cyanobacteria 2.5 BYA
    * Triggered an ice age
    * Helped give rise to multicellularity
    * Explosion of minerals

Aerobic organisms started evolving

• Antioxidants - minimizes damages caused by O₂
• First aerobic organisms - could live in presence of oxygen
• Oxygen using metabolism evolved
    * Aerobic organism’s metabolism evolved to include oxygen
    * Aerobic respiration
    * Used oxidative properties of oxygen as part of their respiration

Aerobic Cell Respiration Happens in the Mitochondria

• Folded membrane makes ATP very efficiently
• Electron transfer chain in membrane
    * Similar to chloroplast in photosynthesis
• Hydrogen ions creates gradients to power ATP synthesis
• Oxygen molecules uses electrons at ends of chains

Free Radicals

 ****

• O₂ usually finds hydrogens to bind with to make H₂O
• Sometimes oxygen atoms escape, missing an electron = free radical
• Enzymes clean up the mess, most of the time
    * The mitochondria can’t detoxify free radicals on it’s own
    * Must use antioxidant enzymes in cytoplasm
    * Neutralizes the free radical
    * Ex: one of these enzymes is catalase

Lots of foods have antioxidants too: Vitamins C and E, beta carotene

Oxygen causes oxidative stress

• If free radicals accumulate they stop the mitochondria from functioning 💀
• Then the cell stops functioning
• Causes tissue damage called oxidative stress
• Oxidative stress causes lots of problems
    * Aging, cancer, Parkinson’s, autism
• Oxidative stress damage can be inherited
    * It damages the mitochondria which can cause mitochondrial disorders
    * Congenital blindness, deafness, diabetes, seizures, strokes

Why is ATP so important?

• Used by nearly all cellular reactions as energy
• It stores energy from bonds
• Cells harvest energy from organic molecules by breaking carbon backbone
• This releases energy
• Energy capture by ATP molecules

Aerobic Respiration

Cellular respiration using oxygen

• Different than respiration (breathing) but breathing is a part of cell respiration
• Breathing bring O₂ to cells
• Photosynthesis and cellular respiration work together
    * Photosynthesis produces oxygen which is inhaled
    * Exhaled in CO₂ and water which are used for photosynthesis
• 4 stages of aerobic respiration:
    * Glycolysis
    * Acetyl CoA formation
    * Citric acid cycle
    * Electron transfer phosphorylation
• Aerobic cell respiration often paired with fermentation
    * Breaking down glucose to make ATP but no oxygen is used
    * Anaerobic respiration
    * Pro: no harmful oxygen
    * Con: doesn’t make a lot of ATP
    * Only some tiny unicellular organisms can sustain themselves with fermentation

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