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Cells and Their
Housekeeping
Functions –
Metabolic Process
Shu-Ping Lin, Ph.D.
Institute of Biomedical Engineering
E-mail: [email protected]
Website: http://web.nchu.edu.tw/pweb/users/splin/
Date: 11.08.2010
Metabolism
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Cell metabolism: sum of all chemical reactions in living cell used for
production of useful energy and subsequent synthesis of cell constituents
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Intake of food by cells from bloodstream: messenger substances
(hormones) released from endocrine glands into blood stream to affect
metabolism of cells that have receptors for that hormone
Hormone:
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Anabolism: cells steadily remodel and replace their structures
Catabolism: structures worn out and no longer needed are broken down into small molecules
and either reused or excreted
Alter permeability of cell membrane to extracellular substances, Ex: glucose
Alter activity of key intracellular enzymes (pacemaker enzymes) controlling major chemical
pathways, Ex: insulin increases glucose uptake by muscle cells and increases storage of
glycogen. Type 1 diabetes (insulin deficiency) – depress glucose uptake and increase glycogen
breakdown, causing abnormally high levels of glucose in blood ↑osmotic pressure Remove
tissue water, cellular dehydration, and electrolyte loss Cells break down structural lipids and
proteins when glucose starves. Protein deficiency and weight loss in type 1 diabetes
Animals in low blood glucose levels Secreting epinephrine (adrenaline) from
adrenal gland and glucagon from pancreas Lead to an increase in conversion
of glycogen to glucose in liver. Opposite directions: glucagon and insulin
establish levels in circulation  Excess nutrients not immediately used are
stored as glycogen (in liver and skeletal muscle; sufficient for a few hours) and
fat reserve-triglyceride (sufficient fat stored for several weeks of starvation)
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Generation of Useful Energy from Food
First stage of metabolism: large
molecules split into smaller units in digestive
tract, no useful energy is produced; Ex:
proteins  20 amino acids, carbohydrates
 glucose, fats glycerol and fatty acids
Second stage: occurs in cytoplasm small
organic units convert into simple units, Ex:
sugars, fatty acids, glycerol, and amino
acids are converted into acetyl unit of acetyl
CoA; process does not require oxygen,
yields small amount of ATP
Third stage: useful food energy, citric acid
cycle and oxidative phosphorylation carried
out under aerobic conditions in
mitochondria
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Oxygen for contracting muscle cells is insufficient,
pyruvate convert to lactate releasing useful
energy But accumulation of lactate in muscle
tissues is responsible for muscle cramps. Yeast
(anaerobic organisms), pyruvate transform into
ethanol
Oxidation-Reduction Reactions
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Oxidation-reduction or redox reaction: food degradation, chemical reactions
in which one or more electrons are transferred from one reactant to
another, each reaction requires an electron donor and an electron acceptor
Oxidation
Give up e- by removing H, loss of electrons
Adds O
Called an electron donor or a reducing agent
(reduces the accepting molecule, makes it more -)
Releases energy - exergonic
Reduction
Gain e-(more -), addition of electrons
Removes O
Called an electron acceptor or oxidizing
agent (oxidizes donor molecule)
Stores energy - endergonic
Oxidation
Xe- + Y  X + YeReduction
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Xe- is being oxidized (losing e-), acts as a reducing agent because it reduces Y
Y is being reduced (gaining e-), acts as an oxidizing agent because it oxidizes Xe-
Electron-transfer potential of NADH convert into phosphate-transfer
potential of ATP  NAD+(oxidized form) +RH2 NADH (reduced form) +H++R
Redox potential: Oxidized form X  reduced form X-  G 0'  nFE 0'
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Chemical reactions: degradation of food are exergonic redox (△G<0)
NAD+
Oxidized form
NADH Reduced
form
Degradation of Glucose
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Glucose metabolism:
C6H12O6 + 6O2  6CO2 + 6H2O + energy
Blocks indicate 4 separate pathways in cellular energy
process, each pathway is composed of multiple
consecutive reactions catalyzed by enzyme. 
Anaerobic Respiration
Glycolysis in cytoplasm, others in mitochondria (called (Respiration without O2)
cellular respiration)
Glycolysis: consists of 10 reactions to convert glucose
into 2 molecules of 3-carbon compounds (i.e. pyruvic
acid and pyruvate)
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First 5 reactions consume energy: 2 ATP molecules are
used to phosphorylate and activate glucose to 3-carbon
sugar phosphate
Aerobic Respiration
nd
2 set of reactions: hydrogen atoms are removed (Respiration using O2)
(oxidation) by NAD+ forming NADH (reduction):
2NAD+ + 4H (oxidation)  2 NADH (that’s a total of 4 e) -- Four ATP were produced from energy released by
substrate-level phosphorylation
Exergonic process with ∆G = -140kcal/mol
Glycolytic pathway do not involve oxygen
Glucose+ 2Pi+ 2ADP+ 2NAD+  2 pyruvate+ 2ATP+ 2NADH+ 2H++ 2H2O
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Glycolysis is highly regulated.
blood to meet the need for ATP.
Cells acquire enough glucose from
Vesicular Transport mechanism
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The transport mechanism which
proteins use to progress
through the Golgi apparatus
Cisternal maturation model:
the cisternae of the Golgi
apparatus move by being built
at the cis face and destroyed at
the trans face.
Vesicular transport model:
Vesicular transport views the
Golgi as a very stable organelle,
divided into compartments in
the cis to trans direction.
Vesicular Transport
Type
Description
Exocytotic
vesicles
Example
Vesicle contains proteins destined for extracellular release.
After packaging the vesicles bud off and immediately move
(continuous) towards the plasma membrane, where they fuse and
release the contents into the extracellular space in a
process known as constitutive secretion.
Antibody
release by
activated
plasma B
cells
Secretory
vesicles
Vesicle contains proteins destined for extracellular release.
After packaging the vesicles bud off and are stored in the
cell until a signal is given for their release. When the
appropriate signal is received they move towards the
membrane and fuse to release their contents. This process
is known as regulated secretion.
Neurotrans
mitter
release
from
neurons
Lysosomal
vesicles
Vesicle contains proteins destined for the lysosome, an
organelle of degradation containing many acid hydrolases,
or to lysosome-like storage organelles. These proteins
include both digestive enzymes and membrane proteins.
The vesicle first fuses with the late endosome, and the
contents are then transferred to the lysosome via unknown
mechanisms
Digestive
proteases
destined for
the
lysosome
(regulated)