Download Chapter 6 How Cells Harvest Chemical Energy

Chapter 6
Overview:
•Cellular Respiration
•Reactions involved in cellular respiration
•Glycolysis
•Krebs Cycle
•Electron Transport
•Fermentation
•Food used to produce ATP
•ATP used to produce food
INTRODUCTION TO CELLULAR
RESPIRATION
Photosynthesis and cellular respiration
provide energy for life


Nearly all the cells in our body break down
sugars for ATP production
Cellular respiration occurs in mitochondria
◦ Cellular respiration is a chemical process that harvests
energy from organic molecules
◦ Cellular respiration yields CO2, H2O, and a large
amount of ATP

The ingredients for photosynthesis are carbon
dioxide and water
– CO2 is obtained from the air by a plant’s leaves
– H2O is obtained from the damp soil by a plant’s roots
• Chloroplasts rearrange the atoms of these
ingredients to produce sugars (glucose) and
other organic molecules
– Oxygen gas is a by-product of photosynthesis

Cellular respiration and breathing are closely
related
– Cellular respiration requires a cell to exchange gases
with its surroundings
– Breathing exchanges these gases between the blood
and outside air

Cellular respiration breaks down glucose
molecules and banks their energy in ATP
◦ The process uses O2 and releases CO2 and H2O
Glucose
Oxygen gas
Carbon
dioxide
Water
Energy

• A calorie is the
amount of energy that
raises the temperature
of 1 gram of water by
1 degree Celsius
ATP powers almost
all cell and body
activities

Glucose gives up energy as it is oxidized
Loss of hydrogen atoms
Energy
Glucose
Gain of hydrogen atoms

Chemical reactions that transfer electrons from
one substance to another are called oxidationreduction reactions
– Redox reactions for short
• The loss of electrons during a redox reaction is
called oxidation
• The acceptance of electrons during a redox
reaction is called reduction
LEO the Lion Goes GER
Oxidation
[Glucose loses electrons (and hydrogens)]
Glucose
Oxygen
Carbon
dioxide
Reduction
[Oxygen gains electrons (and hydrogens)]
Water

Why does electron
transfer to oxygen
release energy?
– When electrons move
from glucose to oxygen,
it is as though they were
falling
– This “fall” of electrons
releases energy during
cellular respiration

The path that electrons take on
their way down from glucose to
oxygen involves many stops
An enzyme called
dehydrogenase and a
coenzyme called NAD+
(nicotinamide adenine
dinucleotide) play important
role in oxidizing glucose.

The first stop is an electron acceptor called
NAD+
– The transfer of electrons from organic fuel to NAD+
reduces it to NADH
• The rest of the path consists of an electron
transport chain
– This chain involves a series of redox reactions
– These lead ultimately to the production of large
amounts of ATP
•All of the reactions involved in cellular
respiration can be grouped into three main
stages
1. Glycolysis
2. The Krebs cycle
3. Electron transport

A molecule of glucose is split into two
molecules of pyruvic acid
• Glycolysis breaks a six-carbon glucose into two
three-carbon molecules
– These molecules then donate high energy electrons to
NAD+, forming NADH
•Glycolysis occurs in the cytoplasm

Glycolysis makes some ATP directly when
enzymes transfer phosphate groups from fuel
molecules to ADP (This process is called
substrate-level phosphorylation)
Enzyme

The Krebs cycle completes the breakdown of
sugar
• In the Krebs cycle, pyruvic acid from glycolysis
is first “prepped” into a usable form, Acetyl-CoA

The Krebs cycle extracts the energy of sugar by
breaking the acetic acid molecules all the way
down to CO2
– The cycle uses some of this energy to make ATP
– The cycle also forms NADH and FADH2
•The Krebs cycle and the electron transport
chain occur in the mitochondria

Electron transport releases the energy your cells
need to make the most of their ATP
• The molecules of electron transport chains are
built into the inner membranes of mitochondria
– The chain functions as a chemical machine that uses
energy released by the “fall” of electrons to pump
hydrogen ions across the inner mitochondrial
membrane
– These ions store potential energy

When the hydrogen ions flow back through the
membrane, they release energy
– The ions flow through ATP synthase
– ATP synthase takes the energy from this flow and
synthesizes ATP
1.
2.
3.
The electrons from NADH and FADH2 travel down the electron
transport chain to oxygen
Energy released by the electrons is used to pump H+ into the space
between the mitochondrial membranes
In chemiosmosis, the H+ ions diffuse back through the inner
membrane through ATP synthase complexes, which capture the
energy to make ATP
Rotenone
Cyanide,
carbon monoxide
Oligomycin
• Some of your cells can actually work for short
periods without oxygen
– For example, muscle cells can produce ATP under
anaerobic conditions
• Fermentation
– The anaerobic harvest of food energy
 Aerobic
metabolism
– When enough oxygen reaches cells to support energy
needs
• Anaerobic metabolism
– When the demand for oxygen outstrips the body’s
ability to deliver it
• Anaerobic metabolism
– Without enough oxygen, muscle cells break down
glucose to produce lactic acid
– Lactic acid is associated with the “burn” associated
with heavy exercise
– If too much lactic acid builds up, your muscles give
out

Human muscle cells can make ATP with and
without oxygen
– They have enough ATP to support activities such as
quick sprinting for about 5 seconds
– A secondary supply of energy (creatine phosphate)
can keep muscle cells going for another 10 seconds
– To keep running, your muscles must generate ATP by
the anaerobic process of fermentation

Various types of microorganisms perform
fermentation
– Yeast cells carry out a slightly different type of
fermentation pathway
– This pathway produces CO2 and ethyl alcohol
• The food industry uses yeast to produce
various food products

In alcoholic fermentation, pyruvic acid is
converted to CO2 and ethanol
– This recycles NAD+ to keep glycolysis working

In lactic acid fermentation, pyruvic acid is
converted to lactic acid
– As in alcoholic fermentation, NAD+ is recycled
• Lactic acid fermentation is used to make cheese
and yogurt
INTERCONNECTIONS BETWEEN MOLECULAR
BREAKDOWN AND SYNTHESIS



Polysaccharides can be hydrolyzed to
monosaccharides and then converted to glucose
for glycolysis
Proteins can be digested to amino acids, which
are chemically altered and then used in the
Krebs cycle
Fats are broken up and fed into glycolysis and
the Krebs cycle

In addition to energy, cells need raw materials
for growth and repair
◦ Some are obtained directly from food
◦ Others are made from intermediates in glycolysis and
the Krebs cycle

Biosynthesis consumes ATP

Biosynthesis of macromolecules from
intermediates in cellular respiration

All organisms have the ability to harvest
energy from organic molecules
◦ Plants, but not animals, can also make these
molecules from inorganic sources by the process
of photosynthesis