Cellular Respiration & 'Do Plants Breathe?'

Respiration is the enzyme-controlled breakdown of C-C bonds of complex food molecules by oxidation, releasing energy that is trapped as ATP. The molecule broken down is the respiratory substrate - usually glucose, but fats, proteins and organic acids can also be used. ATP is the energy currency of the cell; the energy is released in small steps, not all at once.

Do plants breathe? - gaseous exchange

Plant cells do need O2 and release CO2, but plants have NO specialised respiratory organs or transport system for gases. Reasons:

  • Each plant part takes care of its own gas-exchange needs; there is little transport of gases from one part to another.
  • Living cells are located near the surface; the bulky, non-photosynthetic, dead cells (wood) demand little O2.
  • Plants have loosely packed parenchyma with large intercellular spaces, giving a continuous air-filled network.
  • Gases diffuse in and out through stomata (leaves/young stems) and lenticels (woody stems), and directly across root/cell surfaces.

So gaseous exchange in plants is by simple diffusion, over short distances, driven by concentration gradients.

Types of respiration by the substrate's fate

  • Aerobic - complete oxidation of glucose to CO2 and H2O using O2 (in mitochondria); large ATP yield.
  • Anaerobic (fermentation) - incomplete oxidation without O2; small ATP yield.

Both begin with the SAME first step - glycolysis.

Glycolysis - the EMP Pathway

Glycolysis (Greek glycos = sugar, lysis = splitting) is the Embden-Meyerhof-Parnas (EMP) pathway. It is the first step of both aerobic and anaerobic respiration, occurs in the cytoplasm, and is present in all living organisms. It needs no oxygen.

The scheme

  • One glucose (6-C) is partially oxidised to two molecules of pyruvic acid (pyruvate, 3-C).
  • Glucose is first phosphorylated to glucose-6-phosphate (enzyme hexokinase, using 1 ATP) -> fructose-6-phosphate -> fructose-1,6-bisphosphate (a second ATP is used) -> split into two 3-carbon triose phosphates -> a series of steps yields 2 pyruvate.
  • In the pay-off, ATP is formed by substrate-level phosphorylation and NAD+ is reduced to NADH + H+.

The net yield per glucose (learn these)

  • 2 pyruvate (3-C each)
  • 2 ATP net (4 ATP made - 2 ATP used = net 2)
  • 2 NADH + H+

One-liners: glycolysis = cytoplasm, in all organisms, no O2; glucose -> 2 pyruvate; net 2 ATP (substrate-level) + 2 NADH; no CO2 is released in glycolysis.

Fate of pyruvate

The pyruvate formed is the key that decides the next step; its fate depends on the cell/organism and O2:

  • Lactic acid fermentation (some microbes; muscle) - anaerobic.
  • Alcoholic fermentation (yeast) - anaerobic.
  • Aerobic respiration (mitochondria) - complete oxidation with O2.

Visual - Glycolysis (the EMP pathway)

Glycolysis: glucose to two pyruvate in cytoplasm, net 2 ATP and 2 NADH

Glycolysis in the cytoplasm: glucose (6-C) is phosphorylated (using 2 ATP), split into two triose phosphates, and converted to two pyruvate (3-C), giving a net 2 ATP and 2 NADH.