HOW CELLS RELEASE
ENERGY
Chapter 8
All cells (prokaryotic & eukaryotic)
require energy to:
•
•
•
•
combat entropy
carry out day-to-day functions
repair/replace worn out organelles
reproduce
What form of energy do cells use?
ATP
How do cells obtain ATP?
All cells must make their own ATP
from nutrients they have either
synthesized (autotrophs) or consumed
(heterotrophs).
Most cells break down nutrients to
make ATP by:
• Cellular respiration (aerobic process)
• Fermentation (anaerobic process)
Aerobic respiration occurs in cells which
contain mitochondria ( plants, protista, fungi
and animals)
Bacteria and archaea – aerobic respiration
occurs without special organelle to extract
energy.
Anaerobes obtain energy in the absence of
oxygen by fermentation. They use nitric oxide
instead of oxygen as the terminal electron
acceptor.
A. Cellular Respiration (aka. Aerobic
Respiration)
Biochemical pathways that extract
energy from nutrients, in the
presence of oxygen.
Occurs in cells of most eukaryotes &
some prokaryotes.
General equation for cellular respiration
of glucose:
C6H12O6 + 6O2  6CO2 + 6H2O + 30 ATP
Cellular respiration occurs in 3
stages:
Eukaryotic cells
Cytoplasm
Prokaryotic cells
Glycolysis
Krebs Cycle
Mitochondria
Cytoplasm
Electron
Cell membrane
Transport Chain
Types of phosphorylation
Substrate level phosphorylation
•An enzyme oxidizing an energy-rich substrate,
liberating energy to directly attach inorganic phosphate
to a second substrate. Second reaction transfers this
phosphate to ADP.
•Simpler and more direct mechanism for producing ATP.
•Accounts for small % of ATP production.
Chemiosmotic phosphorylation (oxidative)
This mecahnism uses the energy in a proton gradient to
add phosphate to ADP.
Overview of cellular
respiration
1. Glycolysis (“glucose-splitting”)
Glucose (6C) is split into two pyruvate
(3C) molecules.
• does not require oxygen
• energy harvested/glucose:
2 ATP (via substrate-level
phosphorylation)
2 NADH (actively transported into
mitochondria of eukaryotic cells for
use by the electron transport
chain)
First half of
glycolysis
activates
glucose by
investing 2
ATP
molecules.
Second half of
glycolysis
extracts
energy by
releasing 4
ATP
molecules.
Pyruvic acid
must be
converted to
Acetyl CoA
before it can
enter Krebs
cycle.
•Enzymes allow a cell to rearrange molecules as
they extract energy in a controlled manner.
•The first half of glycolysis splits glucose into
two molecules of the three carbon PGAL. Energy
is used.
•In the second half of glycolysis, PGAL is
oxidized as NAD+ is reduced to NADH and two
ATP are formed.
•PGAL is rearranged to form pyruvic acid.
2. Krebs Cycle (aka. citric acid cycle)
Acetyl CoA is broken down completely
to CO2.
• cells use carbon skeletons of
intermediates to produce other
organic molecules (amino acids).
• energy harvested per acetyl CoA:
1 ATP (via substrate-level
phosphorylation)
3 NADH
1 FADH2
Thus far, how much useable energy
has been produced from the
breakdown of 1 glucose molecule?
4 ATPs
The electron transport chain is
needed to harvest the potential
energy in NADHs & FADH2s.
•Enzymes form cycles as the product of one
becomes the substrate of the next and the
starting molecule is ultimately re-formed.
•The krebs cycle takes the remaining carbons
from glucose and extracts energy as ATP and
NADH.
•The enzymes of the first energy-capturing
step in the mitochondria releases carbon
dioxide and reduces NAD+ to NADH.
•A carrier coenzyme transfers the remaining
carbons to the krebs cycle.
3. Electron Transport Chain (ETC)
Series of proteins & electron carriers
embedded in the inner mitochondrial
membrane (eukaryotes) or cell
membrane (prokaryotes).
• O2 is the final electron acceptor
• H2O is the final product
• energy harvested/NADH: 2.5 ATPs
(via chemiosmotic phosphorylation)
•
energy harvested/FADH2: 1.5 ATPs
(via chemiosmotic phosphorylation)
How many ATPs can 1 glucose yield?
Negative feedback regulates aerobic respiration
NADH and citric acid from the
Krebs cycle, in addition to ATP at
high concentartions, bind to the
regulatory enzyme
phosphofructokinase in a way that
temporarily halts its ability to
catalyze a reaction in
glycolysis(1,2).
3. High concentrations of ADP
restore the enzyme’s activity.
•The membrane structure within the
mitochondria allows the cell to form gradients
and use them as sources of potential energy.
•ETC uses the energy from NADH and FADH2
to pump hydrogen ions out of the
mitochondrial matrix.
•Protons reenter the matrix only through the
ATP synthase complex , which phosphorylates
ADP to ATP.
•Oxygen is required as final acceptor of
the energy-depleted electrons. Without
oxygen the system stops.
•Each NADH yields 2.5 ATP and each
FADH2 yields 1.5 ATP via this process.
The net energy extracted from glucose is
30 to 32 ATP.
•Feedback mechanisms regulate the activity
of enzymes to ensure the cell has the
energy it cells.
Can cells use proteins & lipids to
produce energy?
•Most biochemicals can be used as a source
of energy.
•Cells process the molecules to form
intermediates that can enter various points
in cellular respiration.
•Fat becomes acetyl CoA and enters the
Krebs cycle directly.
•Amino acid enter various points in the
Krebs cycle or glycolysis.
B. Fermentation
Biochemical pathways that extract
energy from nutrients, in the
absence of oxygen.
Glycolysis produces pyruvic acid which
is broken down in fermentation
1. Alcoholic fermentation
Pyruvic acid is broken down to ethanol
and carbon dioxide.
Ex. yeast (used in production of baked
goods & alcoholic beverages)
Fermentation
2. Lactic acid fermentation
Pyruvic acid is broken down to lactic
acid.
Examples:
• certain bacteria (used in production of
cheese & yogurt)
•
human muscle cells in oxygen debt
Photosynthesis, glycolysis & cellular
respiration are interrelated.
•In the absence of oxygen, cells find a different
final electron acceptor or another mechanism or
regenerating their limited supply of NAD+.
•Anaerobic pathways use different enzymes to
regenerate NAD+, producing a variety of byproducts in the process.
•The most common are lactic acid and ethanol.
•In human bodies, lactic acid is the by-product
of exercise without enough oxygen.
•Other by-products like ethanol come from
bacteria or yeast and have great commercial
value.
•The energy pathways are the core of all
metabolic processes and evolved slowly beginning
with glycolysis.
•The Krebs cycle added more levels of efficiency
•Endosymbiosis formed a variety of organisms
with differing capabilities.