The Working Cell: Energy from Food

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The Working CellEnergy from Food

• Chapter 7

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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?

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Food Stores Chemical Energy

• Kinetic energy energy of motion
• Potential energy energy that is stored due to
  an objects 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

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• Working cells are similar to a car enginethey
  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 workthe other 60 is
  converted to thermal energy, which is lost from
  our bodies in the form of heat

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• 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

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• 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 calorieswhat is that in
  kilocalories?
• 5 kcal

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• Cells use enzymes to break down organic molecules
  through the more controlled process of cellular
  respirationthe released energy is easier to
  manage for work
• Just a handful of peanuts provides enough fuel to
  power an hour-long walk

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ATP provides energy for cellular work

• ATP adenosine triphosphate
• The triphosphate tail is the business end of
  ATPit is the source of energy used for most
  cellular work

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More pictures of ATP
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• 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

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• 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 groupthis requires energythe
  source of energy is the organic molecules in food
• This cycle is fast repeatinga working muscle
  cell recycles all of its ATP molecules about once
  each minute!! (thats 10,000,000 ATP molecules
  spent and regenerated per second

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• We know that energy stored in food is converted
  to energy stored in ATPbut, how is Oxygen
  involved?

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Electrons fall from food to oxygen during
cellular respiration

• Cellular respiration is an aerobic processit
  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

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• Overall equation for cellular respiration

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Why does the process of cellular respiration
release energy?

• An atoms positively charged nucleus exerts an
  electrical pull on negatively charged electrons
• When an electron falls toward the nucleus,
  potential energy is released

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• Oxygen attracts electrons very stronglyit is
  sometimes called an electron grabber
• Carbon and hydrogen atoms exert much less pull on
  electrons

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• Cellular respiration is a controlled fall of
  electronslike a step-by-step walk of electrons
  down an energy staircase
• Cellular respiration unlocks the energy in
  glucose in small, manageable amountsthe
  formation of ATP molecules
• Oxygen only comes in as an electron acceptor at
  the end

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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 cells metabolismcellular
  respiration is one type of chemical process
• Cellular respiration consists of a series of
  reactions and is referred to as a metabolic
  pathway

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Road Map of cellular respiration
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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 produceda net gain of
  two ATP molecules (payment)

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Fermentation

• Fermentation cellular process of making ATP
  without oxygen
• Makes ATP entirely from glycolysis remember that
  glycolysis does not use oxygen
• Doesnt seem very efficient, but if enough sugar
  is burned, fermentation can regenerate enough ATP
  molecules for short bursts of activity

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• 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

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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.

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• 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

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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 moleculethe 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

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• 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
• Butelectron 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

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• 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

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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

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• 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

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• 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!

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Adding up the ATP molecules

• Glycolysis 2 ATP
• Krebs cycle 2 ATP
• ATP synthase 34 ATP
• Total 38 ATP

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Cellular respiration
has three stages
generates
oxidizes
uses
ATP
glucose and organic fuels
(a)
produce some
C6H12O6
(b)
(d)
produces many
energy for
to pull electrons down
to
(c)
cellular work
(f)
by process called
uses
H diffuse through ATP synthase
(e)
chemiosmosis
uses
pumps H to create
H gradient