Cell Respiration
and Metabolism
Chapter 5
Topics
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Energy Production
Glycolysis
Krebs Cycle
Oxidative Phosphorylation
Glycogenesis & Glycogenolysis
Lipid Metabolism
Protein Metabolism
Energy Production
• Refers to ATP (adenosine triphosphate) synthesis
• Three metabolic pathways for this:
Glycolysis (splitting sugar)
 In the cytosol
Tricarboxylic Acid Cycle (TCA) (Citric Acid
Cycle, Krebs Cycle)
 In the mitochondrial matrix
Oxidative Phosphorylation
 Responsible proteins located in the inner
mitochondrial membrane
• Metabolism: reaction in body that involves
energy and energy transformation; categories =
anabolism, catabolism
Energy Production
• Glucose, fatty acids, and amino acids are
the primary sources of energy for ATP
synthesis.
• Broken down into smaller molecules via
catabolism to transfer chemical-bond
energy to ATP.
Glycolysis
• Breakdown of glucose to release energy
• Enzymes catalyze reactions
• Last step requires oxygen (from blood) = aerobic
cell respiration
• Oxidation-reduction reactions
 Oxidation = loss of electrons
 Reduction = another molecule accepts the
electrons
• Yields 2 ATP for each glucose molecule
• Anaerobic respiration can occur in red blood cells,
heart, and skeletal muscle, yielding lactic acid and
2 ATP
Glycolysis
Glucose “powers” ATP synthesis. Glucose enters the
cell, where it is metabolized for energy.
Fig 5.1
Glycolysis
• Glucose → 2 molecules of pyruvic acid
• Also 2 NADH (reduced form of NAD), 2
ATP, and 2 H+
• The co-enzymes NADH and FADH2 are
energy-rich molecules that provide much of
the electrons for the electron transport
system critical for ATP synthesis.
Glycolysis
Glycolysis: A net gain of 2 molecules of ATP.
Fig 5.2
Glycolysis
In glycolysis, 1 glucose
is converted into 2
pyruvic acids. Note the
production of 2 NADH
and 2 net ATP (4 are
produced but 2 are used
in the pathway).
Fig 5.3
Lactic Acid Pathway
Anaerobic conditions can occur in periods of high
energy demand; NADH is oxidized and lactate 
lactic acid is formed, increasing acidity in the
tissue. LDH = lactic acid dehydrogenase.
Fig 5.4
Citric Acid Cycle / Krebs Cycle
• Enzymes catalyze reactions
• Produces a large amount of NADH and FADH2,
which provide electrons for ATP formation
• Pyruvic acid leaves cytoplasm to enter the interior
of mitochondria (matrix)
• Each pyruvic acid molecule → 1 acetyl coenzyme
(acetyl CoA) + 1 CO2
• Two turns of the citric acid cycle
• 1 ATP produced in each turn, resulting in 2 ATP
total
Fig 5.6
Citric Acid Cycle
The pyruvic acid
resulting from
glycolysis enters the
mitochondrial
matrix, where the
citric acid cycle
occurs. The end
products include 2
ATP molecules.
Fig 5.7
In aerobic
conditions,
two spins of
the Krebs
cycle
occur for
each glucose
that enters
glycolysis.
Citric Acid Cycle
Oxidative Phosphorylation
• Proteins in inner mitochondrial membrane serve as
electron transport system during cellular respiration
 They pick up electrons from NADH and FADH2 and
transport them along the pathway.
 Oxidizes NADH and FADH2 (they transfer electrons to
electron transport system; their H+ is used to
phosphorylate ADP to ATP)
 Reduces other molecules
• This generates energy, which is used to phosphorylate
ADP into ATP
• Oxidative phosphorylation: production of ATP via pairing
of electron transport system with phosphorylation of ADP
• Produces 36-38 ATP molecules, of which 30-32 enter
cytoplasm
Oxidative Phosphorylation
Fig 5.9
(1) H+ is pumped from the matrix into the membrane.
(2) A large H+ gradient (difference) exists between the
membrane and cytoplasm.
(3) H+ diffuses, resulting in ATP production.
Glycogenesis & Glycogenolysis
• Glucose is stored as glycogen in liver and skeletal
muscles.
 Glycogenesis: formation of glycogen from glucose
• Glycogen can be converted to glucose when needed
for energy.
 Glycogenolysis: conversion of glycogen to glucose
 Skeletal muscles use the glucose they generate.
 Liver can also synthesize new glucose =
gluconeogenesis
 Lactic acid in skeletal muscles can be transformed
into glucose via gluconeogenesis (in liver) = Cori
cycle
Glycogenesis & Glycogenolysis
Fig 5.10
The interconversion of glucose and glycogen occurs
through the processes of glycogenesis and
glycogenolysis.
Lipid Metabolism
• If food/energy intake exceeds immediate needs,
glucose is converted into glycogen and stored as fat
instead of metabolized.
• Lipogenesis: synthesis of fat, in white adipose tissue
(white fat) and liver when blood glucose is elevated
after a meal
• Lipolysis: break down of stored fat by lipase enzymes,
into glyerol and free fatty acids to be used for energy
 Used for cellular respiration after gluconeogenesis
by liver
Protein Metabolism
• Amino acids are used for protein synthesis.
• Ingested amino acids replace the protein that is
metabolized daily.
 8 essential amino acids are obtained from food
• Beyond this, excess amino acids are used for energy,
converted to glucose, or converted to fat, after amine
groups are removed.
Interconversion of Glycogen, Fat, & Protein
Interconversions of the molecules that serve as building blocks
and as fuels = metabolism of carbohydrate, fat, and protein.
Fig 5.18