Energy in the Cell

1
Energy in the Cell

• Energy is in two basic forms
• 1. potential energy, which is energy stored up,
  ready to use, like a coiled spring, the capacity
  to do work. 2. kinetic energy, which is energy of
  motion, actually doing work.
• Food molecules contain potential energy in their
  chemical bonds.
• calories are a measure of energy. Some foods
  contain more energy per gram than others, because
  their chemical bonds store more energy. For
  instance, carbohydrates and proteins store 4
  calories per gram, while fats store 9 calories
  per gram.
• Cells convert the chemical bond energy in food
  molecules to chemical bond energy stored in ATP
  molecules.
• ATP energy is used to run metabolism and all
  other bodily processes.

2
Thermodynamics

• First Law the total mount of energy in the
  Universe is constant. Energy is neither created
  not destroyed, it just changes form.
• When energy is expended, part of it goes to do
  useful work, and the rest ends up as heat. None
  of it is lost, but it changes forms.
• Second Law disorder (entropy) increases. Energy
  goes from useful forms to useless heat.
• Every energy transformation step is inefficient
  (as a consequence of the Second Law), meaning
  that some of the energy is converted to waste
  heat at every step, and the amount of useful work
  decreases with every step.
• Life is very orderly compared to non-living
  things. Living things are able to locally
  reverse the overall direction of entropy by using
  a lot of energy.
• The energy of living cells comes from the Sun,
  and it ends up as waste heat.

3
ATP

• In living cells, energy for immediate use is
  stored as molecules of ATP, adenosine
  triphosphate. When the energy is used, one of
  the phosphates attached to ATP is released,
  giving ADP, adenosine diphosphate.
• The 3 phosphates each have a negative charge, and
  so they repel each other. When the bond holding
  them together is broken, the phosphates fly
  apart, like a spring being released. The cell
  can use this energy in many different ways.

4
Metabolic Reactions

• A metabolic reaction is the conversion of one
  chemical compound into another one inside a
  living cell.
• For every metabolic reaction, you start with
  reactants and convert them to products. An
  enzyme does the conversion. Each reaction uses a
  different enzyme.
• The basic rule reactions run downhill more
  energetic reactants are converted to less
  energetic products.
• If a reaction needs to run uphill, creating
  products that contain more energy than the
  reactants, energy in the form of ATP must be
  added.
• Reactants are also called substrates.

5
Enzymes

• Enzymes are proteins that cause specific chemical
  reactions to occur.
• Enzymes act as catalysts they help the reaction
  occur, but they arent used up in the reaction.
• All reactions require an input of energy to get
  them started the activation energy. Think of
  touching a match to a piece of paper to start a
  fire the match is supplying the activation
  energy.
• Enzymes work by lowering the activation energy
  for a reaction. The reaction occurs thousands
  or millions of times faster than without the
  enzyme. The little bit of activation energy
  needed is supplied by the collision of the
  molecules involved.
• Enzymes are very specific for their substrates
  they work on only a very limited number of
  similar molecules.

6
Enzyme-Substrate Interactions

• Each enzyme has an active site, a special region
  that holds the substrates together and causes
  them to react
• The active site promotes the reaction by
  orienting the substrates properly, straining
  their bonds so they break more easily, and by
  providing acidic or basic amino acids to help the
  reaction along.
• Enzymes often use small accessory molecules
  called coenzymes to help carry out the reaction.
  Most vitamins are coenzymes. .
• Enzymes often have small molecules that act as
  inhibitors or activators of their activity.
  These molecules alter the active site so the
  enzyme reacts differently to the substrate.
• Enzyme activity is strongly influenced by
  temperature. They have an optimum temperature
  to hot or too cold slows them down. Most of the
  enzymes in humans have an optimum temperature
  near body temperature.
• pH and salt level also influence enzyme activity,
  with optimum values for each.

7
Generating ATP from Food

• ATP (adenosine triphosphate) is made from ADP
  (adenosine diphosphate) plus a phosphate ion
  (symbolized by Pi). Making ATP requires energy,
  which comes form the potential energy stored in
  food molecules.
• More specifically, electrons from glucose or
  other food molecules are passed through a series
  of steps, releasing part of their energy in each
  step, and ultimately ending up attached to
  oxygen. The energy from the electrons going down
  the energy hill is used to create ATP from ADP
  and phosphate.
• Similarly, high energy electrons are carried by
  the molecule NADH. When NADH uses its high
  energy electrons, it is converted to NAD.
  Electrons in the cell are often accompanied by an
  H (hydrogen).

8
Oxidation

• Energy from chemical bonds is transferred in the
  form of electrons. Oxidation means removing
  electrons. Its opposite is reduction, which is
  gaining electrons. LEO Lose Electrons
  Oxidation GER Gain Electrons Reduction.
• Some common forms of oxidation burning and
  rusting.
• Cells oxidize glucose to form carbon dioxide and
  water. The cell removes electrons from glucose
  (in a series of steps), which converts it to
  carbon dioxide. The energy stored in the
  electrons is used to make ATP. Finally, the
  electrons are given to oxygen molecules,
  converting them to water.
• By passing the electrons through a series of
  steps before their final destination in water,
  the cell can harvest the energy efficiently. In
  contrast, burning releases the energy all at
  once, so it cant be captured easily.
• The electrons are often accompanied by a hydrogen
  (H), and they are usually carried in the cell by
  the molecule NADH (or its close relative NADPH).

9
Aerobic and Anaerobic Respiration

• Respiration is generating energy by breaking down
  food molecules, converting the energy in their
  chemical bonds to ATP energy.
• Before oxygen was present in the atmosphere, all
  cells used anaerobic respiration, which means
  generating energy in the absence of oxygen.
• Many bacteria only have anaerobic respiration.
  Some are even poisoned by the presence of oxygen
  the bacteria that cause gangrene, for example.
• Most eukaryotes use aerobic respiration,
  generating energy with the use of oxygen , in
  addition to anaerobic respiration. We use
  anaerobic respiration to start the process, but
  finish it with aerobic. Aerobic respiration is
  much more efficient than anaerobic.
• The anaerobic pathway is called glycolysis, which
  means breaking down glucose. It occurs in the
  cytoplasm.
• The aerobic pathway occurs in the mitochondria.

10
Summary of Respiration

• 1. glycolysis (anaerobic) breaks glucose (a 6
  carbon chain) into 2 molecules of pyruvate (3
  carbons each). This require 2 ATPs as input, and
  yields 4 ATPs. Glycolysis thus nets 2 ATPs for
  each glucose.
• intermediate step between glycolysis and the
  Krebs cycle conversion of pyruvate to acetyl
  CoA.
• 2. Krebs cycle acetyl CoA converted to carbon
  dioxide (aerobic).
• 3. electron transport high energy electrons
  converted to ATP (aerobic).
• Aerobic respiration yields 34 more ATPs per
  glucose, giving a total of 36 ATPs generated from
  each glucose. All but 2 of them come from
  aerobic respiration.

11
Glycolysis

• Occurs in the cytoplasm, not in mitochondria
• Does not use oxygen.
• Almost all living things use this pathway.
• Basic process add phosphates (from ATP) to each
  end of the glucose, then split it in half, using
  that chemical bond energy to generate 4 ATPs.
  Final 3-carbon products pyruvate.
• Also releases 2 electrons, which are carried by
  NADH. These electrons can be converted to energy
  if oxygen is present, but they cause problems if
  not.
• What to do with excess electrons? Need to
  regenerate NAD so it can take up more electrons,
  so give the electrons back to pyruvate in some
  way
• In yeast, the pyruvate gets converted to ethanol
  when the electrons are added back.
• In humans and many bacteria, pyruvate gets
  converted to lactate (lactic acid). Causes
  muscle pain during intense exercise when not
  enough oxygen gets to the muscle cells.

12
Aerobic Pathway

• Requires oxygen, occurs in the mitochondria
• Conversion of pyruvate (from glycolysis) to
  carbon dioxide, with generation of high energy
  electrons and ATP.
• Preliminary steps before starting the Krebs
  cycle 3 carbon pyruvate to 2 carbon acetyl CoA
  third carbon lost as carbon dioxide. Generates
  high energy electrons carried by NADH.
• Krebs cycle add 2 carbon acetyl CoA to 4 carbon
  sugar, remove the 2 extra carbons one at a time
  as carbon dioxide, generate several high energy
  electrons on NADH plus some ATP.

13
Electron Transport

• The final stage in aerobic respiration
• Krebs cycle generates many high energy electrons
  (carried by NADH). Also some from glycolysis.
  These need to be converted to ATP so the cell can
  use them.
• Electron transport pumps electrons from the inner
  compartment to the outer compartment of the
  mitochondria.
• Electrons are passed from NADH through 3 proteins
  which use the electron energy to pump H ions
  through the membrane. Each protein pump drains
  energy from the electrons, so by the end of the
  process, the electrons are low energy.
• The final protein pump adds the electrons (plus
  hydrogens) to oxygen, producing water.
• The H level builds up between the membranes. It
  flows back into the inside through a special
  protein channel called ATP synthase, which uses
  the energy of their flow to combine ADP and Pi
  into ATP. This is the main way energy is
  generated in the cell.
• Cyanide blocks electron transport chainno more
  ATP is made
• Brown fat runs electron transport chain without
  generating ATP, just to produce heat.

14
Energy from Other Foods

• Glucose is the primary food molecule
• Carbohydrates are broken down into glucose in the
  stomach. It enters the blood through the small
  intestine.
• Cells absorb glucose and trap it inside by adding
  a phosphate to it.
• It is then either used directly or converted into
  starch, to be used later.
• Fats are the primary energy storage molecules,
  containing more than twice as much energy per
  gram as carbohydrates or proteins. The fatty
  acids are converted to acetyl CoA (preliminary
  steps of aerobic metabolism). From there they
  enter the Krebs cycle. The glycerol molecules go
  into glycolysis.
• The liver converts excess starch into fat.
• Proteins are mostly broken down into amino acids
  which become parts of new proteins.
• When proteins are used for energy, their carbon
  backbones enter glycolysis or Krebs cycle at
  various points. The amino group becomes ammonia,
  which is poisonous. Ammonia gets converted to
  urea, which is a lot less toxic, and then gets
  excreted in urine.

15
Photosynthesis

• Photosynthesis means taking energy from sunlight
  and converting it to a form usable by living
  cells.
• Green plants do photosynthesis so do many
  bacteria and protists (which are single celled
  eukaryotes).
• Two parts to photosynthesis
• 1. the light-dependent reactions, in which
  sunlight is used to extract high energy electrons
  from water. These high energy electrons are then
  used to make ATP. Oxygen from the water is
  released into the atmosphere.
• 2. the light-independent reactions (or dark
  reactions, or Calvin cycle), in which that ATP
  energy is used to convert carbon dioxide into
  glucose.
• In plants, all of these reactions occur in the
  chloroplast, an organelle that contains its own
  DNA and is thought to be derived from an ancient
  symbiosis between a free-living photosynthetic
  bacterium and a primitive eukaryote (just like
  the mitochondria).
• All the food we animals eat comes from these
  reactions.

16
Light and Pigments

• Visible light is a form of electromagnetic
  radiation, along with X-rays, ultraviolet,
  infrared, microwaves, radio waves, etc. The only
  difference between these forms of radiation is
  the wavelength.
• Visible light is all frequencies between 400 and
  700 nanometers, with blue light at the 400 end
  and red at the 700 end.
• White light is a mixture of all these
  wavelengths colors appear when only some
  wavelengths are present.
• Chlorophyll is green because it absorbs the red
  and blue wavelengths, reflecting only the green
  wavelengths.
• Other plant pigments absorb different
  wavelengths, so they have different colors.
• Absorbing light puts chlorophyll into a high
  energy state. This energy is then harvested by a
  series of metabolic reactions.

17
Light-Dependent Reactions

• The chlorophyll molecules are arranged in groups
  of 200-300, called photosystems. Each
  photosystem acts like an antennaany of the
  molecules can capture a photon of sunlight, but
  then that energy is transferred to a central
  reaction center molecule, which passes the
  energy (excited electrons) out of the
  photosystem.
• In green plants, there are 2 separate ways of
  extracting electrons. The more heavily used
  pathway needs 2 photons to boost electrons up to
  a high enough energy to be bound to the plant
  cells energy carrying molecule, NADPH. The
  electrons on NADPH are then passed to the
  light-independent reactions to generate glucose.
• The electrons are initially extracted from water,
  carried along with the hydrogens. The waste
  product is oxygen, which goes into the
  atmosphere. This is the source of all the oxygen
  in the atmosphere.

18
Light-Independent Reactions

• The energy from sunlight is captured in the form
  of excited electrons, which are bound to the
  electron-carrying molecule NADPH.
• To form glucose, carbon dioxide (which has 1
  carbon atom) is attached to a 5-carbon sugar,
  then processed through a series of intermediates
  called the Calvin cycle.
• The Calvin cycle goes around 6 times to create
  the 6-carbon glucose from carbon dioxide. Each
  turn regenerates all the necessary intermediates.
  The cycle uses electrons from NADPH, and also
  energy from ATP.
• After glucose is synthesized, it is converted to
  starch for storage, and then the starch is
  converted to sucrose (a disaccharide) for
  transport to other parts of the plant.