Introduction to Metabolism
Artika Nath
Biochemistry
School of Health Sciences
Objectives
We are now going to look at two major questions
in Biochemistry
 How do cells extract energy from their
environment
 How do cells synthesize the building blocks of
their macromolecules
 This leads to the study of …………………………
Metabolism
Is the sum total of all chemical reactions involved
in maintaining the living state of the cells, and
thus the organism.
Divided into two categories
1. Catabolism – break down of
molecules to obtain energy
2.
Anabolism- the synthesis of
all molecules needed by cells
e.g. DNA, protein etc.
Thermodynamics of Metabolism
• Metabolic reactions are catalyzed by enzymes and obey the
laws of thermodynamics
• (1) First Law of Thermodynamics (conservation of energy):
energy can be transformed from one form to another but
cannot be created nor destroyed. Energy in the universe is
constant.
• 2) Second Law of Thermodynamics: energy transformations
are never 100% efficient (some energy is lost as heat)
Major purpose living things require energy for:
 1. mechanical work in muscle
contraction and other
cellular movement
2. Active transport of
molecules and ions
3. Synthesis of bio-molecules
and simple precursors : proteins,
glycogen, fat
What carries energy????
The energy currency in combustion engines is heat: heat
is produced and used to expand the volume of a gas
• The energy currency or
coin of the cell
• Energy rich molecule
• Triphosphate contains 2
phosphoanhydride bonds
The 2 phosphate bonds (phosphoanhydride) bond)
is here the energy is stored in ATP
A large amount of energy is liberated when
1.
ATP → ADP + Pi (orthophosphate)
2.
ATP → AMP + PPi (pyrophospahte)
The free energy liberated with the hydrolysis of ATP is used to
drive reactions that require input f free energy
• ATP is continuously formed and consumed
• It’s the principal immediate donor and not long term
storage of energy
• Rate of turnover of ATP is high. A molecule of ATP is
consumed within a minute it is formed
• Resting person consumes ~ 40kg of ATP/ 24 hr.
Motion, Active transport,
Biosynthesis, Signal amplification
ADP
ATP
Photosynthesis or oxidation of fuel
molecules
Basic model of energy exchange in biological systems
How do cells make ATP
• By PHOSPHORYLATION... adding a phosphate to ADP
ADP + P
------>
ATP
3 mechanisms of phosphorylation:
1. substrate level phosphorylationwhere a substrate molecule ( X-p )
donates its high energy P to ADP
making ATP
2. Oxidative phosphorylation
e- transferred from organic molecules
and passed through a series of
acceptors to O2
3. Photophosphorylation
Occurs during photosynthesis –light
energy used to make ATP
Electron carriers
• When food molecule oxidized – electrons are removes
• These electrons are carried to oxygen
•
•
e-
How does the e- get to oxygen???????
ANS:
Electron carriers
O2
NAD+ (nicotinamide adenine dinucleotide) and
FAD (flavin adenine dinucleotide)
• Reduced form (with electrons bound) is
- NADH and FADH2
- NADH and FADH2 transfer e- to O2 in the mitochondria by
means of ETC > ATP generated in this process
Stage 1
Large molecules broken int o
smaller units
Stage 2
The numerous small units
are degraded to a few
simple units that play a
central role in metabolism.
Most re converted to the
acetyl unit of acetyl CoA.
Some ATP generated
Stage 3
Kreb cycle and oxidative
phosphorylation
Most energy generated
here
Carbohydrates
•2-3% of your total body weight
•Monosaccharides – the building block
e.g. glucose (the main energy supplying molecule of the body
Fructose- found in fruits; Galactose – present in milk;
Deoxyribose – in DNA; Ribose- RNA
•Glycogen – main storage (liver and skeletal muscle) form of
carbohydrate.
•Starch – main carbohydrate in food
•Cellulose- no digested by humans
•18-25% of body weight in lean adults
•Hydrophobic
•Usually combine with proteins (lipoproteins) and move in the
blood
•Hydrophobic , so cannot exert osmotic pressure on cell walls –
therefore stored easily.
•Triglycerides most plentiful lipid in your body. Each gram can
produce twice as much as energy compared to carbohydrates and
proteins
•Triglycerides stored unlimited in adipose tissues
Proteins
• 12-18% body weight
• Many functions:
- catalyst, transport and storage, movement, structural,
immune system and regulatory role
 Amino acids building blocks
 Cannot store proteins
1. What is a polysaccharide? A disaccharide? A monosaccharide? Give some
examples of monosaccharides. Which monosaccharide does the body use
for energy production?
2. How is carbohydrate stored in the body?
3. What is a polypeptide? A dipeptide? An amino acid? What distinguishes an
amino acid from a carbohydrate?
4. What does the body use proteins for?
5. What is a triglyceride? A fatty acid? Glycerol? Name some uses of lipids in
the body. How are excess fatty acids stored in the body?
6. Define metabolism.
7. Distinguish between anabolism and catabolism.
8. ATP is the cell's storehouse of energy. Where, within the ATP molecule, is
this energy stored? Where does that energy come from?
9. Which of the three nutrients produces more energy, gram for gram, when
oxidized?
10. What is the difference between glycogenesis and glycogenolysis?