Quaestio how do organisms obtain the energy stored in food
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Quaestio : How do organisms obtain the energy stored in food?. Nunc Agenda: List what foods you have eaten today and the types of molecules that compose them. Energy. Energy : the ability to do work. Can you think of examples? In what forms does energy exist ?

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Quaestio : How do organisms obtain the energy stored in food?

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Quaestio: How do organisms obtain the energy stored in food?

Nunc Agenda:

List what foods you have eaten today and the types of molecules that compose them.


  • Energy: the ability to do work.

  • Can you think of examples?

    • In what forms does energy exist?

    • How do we use energy on earth?

A cell does 3 main kinds of work:

  • Mechanical work: beating cilia, contraction of muscle cells, movement of chromosomes during reproduction

  • Transport work: moving substances across membranes

  • Chemical work: running chemical reactions, synthesis of polymers from monomers

The Law of Conservation of Energy

  • The Law of Conservation of Energy: Energy can neither be created nor destroyed; it can only change forms.

    • Remember, the sum of energy in the universe is constant.

  • Examples of energy conversions:

    • Photosynthesis

      • (Light E Electrical E  Chemical E)

    • Respiration

      • (Chemical E  Kinetic E  Thermal E)

    • Internal Combustion Engine

      • (Chemical E  Thermal E  Kinetic E).

Energy for Living Things

  • All living things need energy to carry out their life processes.

  • Nutrition: the life process in which organisms obtain energy in food for metabolic processes.

  • Energy must exist to run “cellular machinery.”

Examples of Energy Needs

  • 1. Locomotion. (Muscle Contractions)

  • 2. Building complex molecules from simple ones (Synthesis).

  • 3. Digestion.

  • 4. Breathing, Talking, Thinking, Existing!

A Heterotroph’s nutrition must supply the organism with enough chemical energy to fuel its life’s activities.







In fireflys, Energy in the form of ATP combines with an enzyme to run a chemical reaction to produce flashes of lights

Ctenophores (Comb Jellies),like fireflies, have bioluminescence using the power of ATP.

Energy from Food

  • Living things rely on the chemical energy stored in their food to survive.

  • Carbohydrates, lipids, and proteins all have chemical energy and all can be broken down to yield energy

    • known as cellular respiration

  • Carbohydrates are the foods most commonly broken down.

    • Created during photosynthesis

Introducing the major players and processes:



  • Cells use chemical energy in the form of ATP

    • The energy released during cellular respiration is “stored” in the form of ADP and ATP.

  • ADP: Adenosine diphosphate

    • Has two phosphate groups.

  • ATP: Adenosine triphosphate

    • Has three phosphate groups

Behind the Names

  • Adenosine is the combination of a molecule of the nitrogenous base adenine with a molecule of the sugar ribose.

    • Adenine + Ribose = Adenosine

  • Diphosphate = 2 phosphate groups attached to adenosine.

  • Triphosphate = 3 phosphate groups attached to adenosine.

ATP: C10H16N5O13P3

: Nitrogenous Base

: 5-carbon sugar

Molecular Similarities

  • ATP and ADP use the same subunits as the nucleic acids:

    • A nitrogenous base (adenine is present in DNA and RNA).

    • A 5-carbon sugar (ribose is present in RNA only).

      • Can you remember what DNA has?

    • Phosphate groups

What makes ADP and ATP so important?

  • ATP has more energy than ADP:

    • due to a high-energy bond between the 2nd and 3rd phosphate group

  • When the third phosphate group is removed from ATP, it forms ADP, and chemical energy is released.

    • ATP + H2O  ADP + P + Energy


  • Phosphorylation: the transfer of energy when a phosphate group is transferred among molecules.

  • Phosphorylation is a common way for chemical energy to be transferred in living cells.

    • ATP loses a phosphate to the molecule that becomes phosphyorylated.

ATP is recycled

  • ATP is used continuously by a cell, but it can be regenerated by adding a phosphate to ADP.

    • It’s a renewable resource!

  • If ATP could not be regenerated by the phosphorylation of ADP, humans would consume nearly their body weight in ATP each day


  • AMP stands for adenosine monophosphate. It has only one phosphate group attached.

  • AMP has lower energy than ADP (and ATP).

  • ADP is rarely broken down into AMP for energy.

The Role of Glucose.

  • Glucose (a simple sugar) is broken down to supply the energy needed to add a phosphate group to ADP to form ATP.

  • One C6H12O6 molecule can be used to form 36 molecules of ATP.

More on Carbohydrates

  • Glucose is not usually present in its simple form in the foods we eat.

  • We need to break complex carbohydrates into glucose first.

  • Review: Our digestive system breaks down complex carbohydrates:

    • Starch  Maltose  Glucose

  • Can you remember what enzymes are involved and where?


  • If ATP is directly used for energy, why do we need glucose at all?


Glucose contains a lot more energy than ATP, but is actually a smaller molecule. Glucose is a good way to store chemical energy, while ATP is more appropriate for directly supplying immediate energy for cellular reactions.

More on ATP vs. Glucose

  • Glucose Chemical Formula

    • C6H12O6

    • Smaller Molecule with More Energy.

  • ATP Chemical Formula

    • C10H16N5O13P3

    • Larger Molecule with Less Energy.

Glucose holds more energy than ATP



Glucose vs. ATP

  • Like a gold bar

  • Cash!

Can you explain the analogy?

Glucose is smaller but holds more energy, and needs to be broken down or exchanged before you can purchase with it. A suitcase full of money may be larger, like ATP, but can be used immediately.


  • If ATP is used as the main source of energy in a cell, then why does a cell only keep a small amount of ATP present at any time?

    • ATP is constantly being recycled from ADP

Ways to transfer energy in the cell

  • Transfer phosphate groups

  • Transfer electrons

  • Transfer hydrogen

Oxidation-Reduction Reactions: the transfer of electrons

  • Oxidation: A chemical change in which an atom or a molecule loses electrons.

    • Example: When sodium combines with chlorine to form sodium chloride (NaCl), sodium loses an electron to become a sodium ion (Na+).

  • Reduction: A chemical change in which an atom or a molecule gains electrons.

    • Example: Chlorine gains the electron from sodium, becoming a chloride ion (Cl-).


  • Why did sodium (Na) become Na+?

  • Why did chlorine (Cl) become Cl-?

Answer: Sodium lost an electron and became a positive ion. It now has more protons than electrons. Sodium was oxidized.

Answer: Chlorine gained an electron and became a negative ion. It now has more electrons than protons. Chlorine was reduced.

Remember Oil Rig!



Loss (of electrons)


Is Gain (of electrons)

Another way to remember:LEO goes GER

  • Lose

  • Electrons

  • Oxidation

  • Gain

  • Electrons

  • Reduction

Oxidation-Reduction Reactions

  • When one substance is oxidized, another must be reduced.

  • Redox Reaction: (short for Reduction-Oxidation Reaction): A reaction that involves both oxidation and reduction.

Gaining and Losing Hydrogen

  • Occasionally, rather than exchanging electrons, molecules will exchange hydrogen atoms.

    • Recall: a hydrogen atom consists of one proton and one electron. It is the simplest element.

  • The molecule that loses the hydrogen is oxidized

    • called the oxidant.

  • The molecule that gains the hydrogen is reduced

    • called the reductant.

Hydrogen Ion =




Hydrogen was Oxidized


Redox Reactions, Cont’d

  • Redox reactions involve a transfer of energy.

  • The oxidant (the electron or hydrogen donor) normally loses energy and the reductant (the electron or hydrogen acceptor) gains energy.

Biochemical Pathway

  • Cellular Respiration follows a biochemical pathway: a sequence of chemical reactions that leads to a result.

  • This pathway is fueled by redox reactions.

  • *Remember – if a molecule loses a hydrogen (oxidation), another molecule must accept that hydrogen (reduction).

Hydrogen Acceptors

  • NAD and FAD are two coenzymes that serve as hydrogen and electron acceptors.

  • NAD = nicotinamide adenine dinucleotide.

  • FAD = flavin adenine dinucleotide.

  • To be reduced:

  • NAD + H  NADH(higher energy)

  • FAD + 2H  FADH2 (higher energy)

Hydrogen Acceptors, Cont’d

  • The extra energy (electrons) carried by NADH and FADH2 can be used to make ATP from ADP.

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