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