Download The Working Cell: Energy from Food

The Working Cell:
Energy from Food
Chapter 7
Sunlight Powers Life
• Autotrophs: self-feeders
– Photosynthesis
– Producers
• Heterotrophs: other eaters
– Consumers
• Cellular Respiration: chemical process that
uses oxygen to convert chemical energy stored
in organic molecules into another form of energy
– Where does this occur in animal cells?
Food Stores Chemical Energy
• Kinetic energy: energy of motion
• Potential energy: energy that is stored
due to an object’s position or arrangement
• Thermal energy: (heat) total amount of
energy associated with the random
movement of atoms and molecules in a
sample of matter
• Chemical energy: potential to do work
due to the arrangement of atoms with the
molecules
• Working cells are similar to a car
engine…they produce carbon dioxide and
water as their “exhaust”
• Cells are much more efficient than
automobile engines- they convert about
40% of the energy from food into useful
work…the other 60% is converted to
thermal energy, which is lost from our
bodies in the form of heat
• calorie: amount of energy required to
raise the temperature of 1 gram of water
by 1o Celsius
• A calorie is such a tiny unit of energy so
people usually express the energy in food
in kilocalories
• 1 kilocalorie= 1000 calories
• The calories shown on a food label are
actually kilocalories
You can calculate the number of calories in a
peanut
• First you dry the peanut, then burn it under an
insulated container of water
• Burning the peanut converts its stored energy to
thermal energy, releasing heat
• Then you measure the increase in water
temperature
• A peanut has about 5,000 calories…what is that
in kilocalories?
5 kcal
• Cells use enzymes to break down organic
molecules through the more controlled
process of cellular respiration…the
released energy is easier to manage for
work
• Just a handful of peanuts provides enough
fuel to power an hour-long walk
ATP provides energy for cellular
work
• ATP: adenosine triphosphate
• The triphosphate tail is the business end of
ATP…it is the source of energy used for most
cellular work
More pictures of ATP
Examples of Cellular Work
• Chemical work: building large molecules
such as proteins
• Mechanical work: contraction of a muscle
cell
• Transport work: pumping solutes across a
cell membrane
• ATP is continuously converted to ADP as your
cells do work
• ATP is “recyclable”!!
• ADP can be converted back to ATP by adding a
third phosphate group…this requires
energy…the source of energy is the organic
molecules in food
• This cycle is fast repeating…a working muscle
cell recycles all of its ATP molecules about once
each minute!! (that’s 10,000,000 ATP molecules
spent and regenerated per second
We know that energy stored in
food is converted to energy
stored in ATP…but, how is
Oxygen involved?
Electrons “fall’ from food to
oxygen during cellular
respiration
• Cellular respiration is an aerobic
process…it requires oxygen!
• Respiration is used to describe breathing
• Breathing for a whole organism is not the
same as cellular respiration, but the two
processes are related
Overall equation for cellular respiration
Why does the process of cellular
respiration release energy?
• An atom’s positively charged nucleus
exerts an electrical “pull” on negatively
charged electrons
• When an electron “falls” toward the
nucleus, potential energy is released
• Oxygen attracts electrons very strongly…it
is sometimes called an “electron grabber”
• Carbon and hydrogen atoms exert much
less pull on electrons
• Cellular respiration is
a controlled fall of
electrons…like a
step-by-step “walk” of
electrons down an
energy staircase
• Cellular respiration
unlocks the energy in
glucose in small,
manageable
amounts…the
formation of ATP
molecules
• Oxygen only comes in
as an electron
acceptor at the end
Cellular respiration converts
energy in food to energy in ATP
• Structure of mitochondria
• All the chemical processes that take place
in cells make up the cell’s
metabolism…cellular respiration is one
type of chemical process
• Cellular respiration consists of a series of
reactions and is referred to as a metabolic
pathway
“Road Map” of cellular respiration
Stage I: Glycolysis
• Glycolysis is the chemical break down of a glucose molecule
– Splitting of sugar
– Takes place outside the mitochondria in the cytoplasm of the cell
– Two ATP molecules are used as an “investment”
– Glucose is split into two three-carbon sugars, each with a
phosphate group
– Each of these pyruvate molecules then transfers electrons and
hydrogen ions to a carrier molecule called NAD+
– NAD+ is converted to NADH
– Pyruvates lose the phosphate groups to form two pyruvic acids
– Four new ATP molecules are produced…a net gain of two ATP
molecules (“payment”)
Fermentation
• Fermentation= cellular process of making
ATP without oxygen
• Makes ATP entirely from glycolysis…
remember that glycolysis does not use
oxygen
• Doesn’t seem very efficient, but if enough
sugar is burned, fermentation can
regenerate enough ATP molecules for
short bursts of activity
• Lactic acid is a waste product of
fermentation
• Temporary build up of lactic acid in
muscles contributes to fatigue after
exercising
• Our bodies consume oxygen to convert
lactic acid back to pyruvic acid
• We gain the oxygen supply by breathing
heavy or stop exercising
Fermentation can be yummy!!
• Anaerobic= with out oxygen
• Yeast are forced to ferment sugar when they are placed
in an anaerobic environment
• Fermentation in yeasts produces alcohol, alcoholic
fermentation, and carbon dioxide
• Bread, beer, wine, etc.
• Some bacteria found in stagnant ponds or
deep in the soil are actually poisoned if
they come into contact with oxygen
• All of their ATP is generated by
fermentation
Stage II: The Krebs Cycle
• Hans Krebs 1930s
• Krebs Cycle finishes the breakdown of pyruvic
acid molecules to carbon dioxide
• More energy is released
• The fluid matrix in the inner membrane of the
mitochondrion contains the enzymes for the
Krebs cycle
• The pyruvic acid molecules diffuse into the
mitochondrion
• They then lose a CO2 molecule…the resulting
molecule is then converted to a two-carbon
compound called acetyl coenzyme A (also
known as acetyl CoA)
• The acetyl CoA then enters the Krebs Cycle
• Once in the Krebs cycle, each acetyl CoA
joins a four-carbon acceptor molecule
• The Krebs cycle produces two CO2
molecules and one ATP molecule per
acetyl CoA molecule
• But…electron carriers trap most of the
energy
• At the end of the cycle the four-carbon
acceptor molecule has been regenerated
and the cycle can continue
• Since each turn of the Krebs cycle breaks
down one acetyl CoA molecule, the cycle
actually turns twice for each glucose
molecule, producing a total of four CO2
molecules and two ATP molecules
Stage III: Electron Transport Chain
and ATP Synthase Action
• Final stage of cellular respiration
• Occurs in the inner membrane of mitochondria
• Electron transport chain= sequence of
electron carrier molecules that transfer electrons
and release energy during cellular respiration
• Carrier molecule (NADH) transfers electrons
from the original glucose to an electron transport
chain
• Electrons are transported through the chain
being pulled to oxygen at the end of the chain
• At the end of the chain the oxygen and hydrogen
ions combine to form water
• Each transfer in the chain releases a small
amount of energy
• This energy is used to pump hydrogen ions
across the membrane
• This pumping action stores potential energy
• Mitochondria have protein structures called ATP
synthases
• Hydrogen ions pumped by electron
transport rush back through the ATP
synthase
• The ATP synthase uses energy from the
flow of hydrogen ions to convert ADP back
to ATP
• Generates up to 34 ATP molecules for
every one glucose molecule
• This process is similar to how a dam
works!
Adding up the ATP molecules
• Glycolysis
= 2 ATP
• Krebs cycle = 2 ATP
• ATP synthase = 34 ATP
Total
38 ATP
Cellular
respiration
generates
has three stages
oxidizes
uses
ATP
produce
some
glucose and
organic fuels
(a)
C6H12O6
energy for
produces
many
(b)
(d)
to pull
to
electrons down
(c)
cellular work
by process called
chemiosmosis
uses
H+ diffuse
through
ATP synthase
uses
(e)
pumps H+ to create
H+ gradient
(f)