Download Enzymes & Energy

Cell Respiration & Metabolism
Physiology
Ch. 5
Carbohydrate Metabolism
Most dietary carbohydrate is burned as
fuel within a few hours of absorption
 Three monosaccharides are absorbed
from digested food - glucose, galactose,
and fructose, but the last two are quickly
converted to glucose
 All oxidative carbohydrate consumption
is essentially a matter of glucose
catabolism

C6H12O6 + 6 O2
6 CO2 + 6 H2O
Combustion -vs- Glucose Catabolism

Combustion carries out
the preceding reaction
in a single, uncontrolled
step, releasing energy
as heat

Cellular respiration occurs
in many small,
enzymatically-catalyzed
steps, storing much of the
energy as ATP
Three Major Pathways of Glucose
Catabolism
Glycolysis - splits a glucose molecule
into two molecules of pyruvic acid
 Anaerobic respiration - occurs in the
absence of oxygen; reduces pyruvic
acid to lactic acid
 Aerobic respiration - occurs in the
presence of oxygen and oxidizes
pyruvic acid to carbon dioxide and water

Fig 5.1
P. 105
Glucose + 2 NAD + 2 ADP + 2 Pi
2 pyruvic acid + 2 NADH + 2 ATP
Fig 5.2
P. 106
Phase 1.
Sugar activation
Step 1. - Phosphorylation
The enzyme hexokinase
transfers a Pi from ATP to
glucose, producing glucose
6-phosphate (G6P).
Keeps intracellular glucose
concentration low, favoring
continued diffusion of glucose.
Traps the glucose within the
cell, as phosphorylated molecules
cannot pass through the plasma
membrane.
Phase 1.
Sugar activation
Steps 2 & 3. - Priming
G6P is isomerized to form
fructose 6-phosphate (F6P).
It is phosphorylated again to
form fructose 1,6diphosphate by the action of
phosphofructokinase.
Primes the process by
providing activation energy
At this point, two molecules of
ATP have been consumed
Phase 2.
Sugar cleavage
Step 4. - Cleavage
F6P is split into
glyceraldehyde 3-phosphate
(GAP) and dihydroxyacetone
phosphate (DHAP).
GAP is on the direct pathway
of glycolysis; DHAP is not. The
two are isomers and readily
interconverted.
Phase 3.
Sugar oxidation and
ATP formation
Step 5. - Oxidation
Each GAP molecule is
oxidized by removing a pair of
hydrogen atoms.
A Pi is added (from the cell’s
pool of free phosphate ions) to
form 1,3-bisphosphoglycerate
(1,3-BPG)
NAD is reduced by the
hydrogens to NADH+ + H+.
Phase 3.
Sugar oxidation and
ATP formation
Step 6. - ATP formation from 1,3-BPG
A phosphate group is removed
from 1,3-BPG and transferred
to ADP, phosphorylating it to
ATP. 2 ATPs are made.
3-Phosphoglycerate is formed.
Phase 3.
Sugar oxidation and
ATP formation
Steps 7 & 8. - Isomerizations
The position of the
phosphate group is shifted
in the conversion of 3phosphoglycerate to 2phosphoglycerate.
Phosphoenolpyruvic acid is
created by the dehydration
of 2-phosphoglycerate. This
enol phosphate has a high
phosphate transfer
potential.
Phase 3.
Sugar oxidation and
ATP formation
Step 9. - ATP formation from
phosphoenolpyruvate
A phosphate group is
removed from
phosphoenolpyruvate and
transferred to ADP,
phosphorylating it to ATP. 2
more ATPs are made.
Two pyruvic acid molecules
are formed from the single
original glucose.
The tally from glycolysis…
Lactic Acid Pathway

Metabolic pathway by which glucose is converted
to lactic acid (anaerobic respiration):

Oxygen is not used in the process.

NADH + H+ + pyruvic acid

Produce 2 ATP/glucose molecule.
Fig 5.3
P. 106
lactic acid + NAD.
Lactic Acid Pathway

Some tissues better adapt to anaerobic
conditions:


RBCs do not contain mitochondria and only use
the lactic acid pathway.
Occurs in skeletal muscles and heart when ratio of
oxygen supply to oxygen need falls below critical
level.
• Skeletal muscle:
• Normal daily occurrence.
• Does not harm muscle tissue.
• Cardiac muscle normally respires aerobically:
• Myocardial ischemia occurs under anaerobic conditions.
Also study
Kreb’s, ETC,
lipid and amino
acid
metabolism in
Chapter 5