Energy from Food
All living things need energy in order to survive.
The Sun Powers Life
All organisms need food for energy, and those living things are classified based on what they
eat.
 Autotrophs (aka producers) – use photosynthesis to make their own food.
 Heterotrophs (aka consumers) – depend on autotrophs for food; they are unable to
make their own food.
Types of Energy
 Energy is the ability to do work.
 Kinetic Energy – the energy of motion.
 Thermal Energy – random molecular motion.
 Example: heat – thermal energy transferred from a warmer object to a
cooler one.
 Potential Energy – energy that is stored due to an objects position or arrangement.
 Chemical energy – potential to do work due to the arrangement of atoms within
molecules.
 Example: food – proteins, carbohydrates, etc – energy is released when
the body or cell rearranges the atoms in the molecules.
Energy from Food
Cells convert 40% of energy from food into useful work, the remaining 60% is lost in
the form of heat.
Calories – units of energy
 calorie – the amount of energy required to raise 1 gram of water 1ºCelsius.
 A Calorie as it is written on the nutritional labels of food is actually equal to a kilocalorie
(kcal) or 1000 calories.
ATP
 Adenosine Triphosphate
 Adenosine part of molecule composed of adenine and ribose (a 5 carbon sugar).
 Triphosphate tail of molecule contains 3 phosphate groups attached to one another.
 Energy is released from ATP when it is converted to ADP.
 A phosphate group is broken off of the end of the tail and only Adenosine Diphosphate
remains.
ATP/ADP Cycle
Within cells, ATP is constantly used and converted into ADP, and as long as chemical
energy (food) is present in the cells, ADP can be converted back into ATP by adding a
phosphate group.
Uses of ATP
Chemical work – building larger molecules.
Mechanical work – moving a muscle protein.
Transport work – pumping solutes across a membrane.
Harvesting Energy from Glucose
There are two possible paths:
- Cellular Respiration
- Fermentation
 Both paths to the formation of ATP energy from Glucose begin with GLYCOLYSIS.
 Glycolysis occurs in the cytoplasm of cells.
 Glycolysis means the splitting of sugar; during this stage, a glucose molecule is split into
2 pyruvic acid (aka: pyruvate molecules).
 It takes 2 ATP molecules to initiate this step, however in the end, 4 ATP molecules are
produced = a net gain of 2 ATP.
 2 NADH molecules are also produced.
 Electron carriers
When Oxygen is Present
If oxygen is present, Glycolysis leads to Cellular Respiration, which consists of two more steps:
 The Krebs Cycle (Citric Acid Cycle)
 The Electron Transport Chain
Aerobic Respiration – in the presence of oxygen
Occurs in the mitochondria of the cell.
-
The mitochondria is considered the powerhouse of the cell.
It is the site of ATP synthesis.
Krebs Cycle
-
Also known as the Citric Acid Cycle.
Breaks pyruvic acid made during Glycolysis into 4 CO2 molecules and 2 ATP molecules as
well as NADH and FADH2
 Requires two turns of the cycle.
-
Occurs inside the mitochondria within the fluid matrix.
Electron Transport Chain
-
Occur on the inner membrane (cristae) of the mitochondria.
The electron carrier molecules NADH and FADH2 made during Glycolysis and The Krebs
Cycle are used to pump protons (H+ ions) from the mitochondrial matrix into the outer
compartment of the mitochondria.
 They will then diffuse back across the membrane (cristae) producing 32-34 ATP and H20.
Cellular Respiration
Chemical equation:
C6H12O6 + 6O2  6CO2 + 6H2O + 36-38ATP
-
Most ATP production occurs after glycolysis and requires oxygen.
Consists of 3 steps:
 Glycolysis
 Krebs Cycle
 Electron Transport Chain
When Oxygen Is Not Present
-
Fermentation occurs following Glycolysis.
Anaerobic Respiration – without oxygen
Only ATP produced is that of Glycolysis stage (net gain 2 ATP).
There are 2 types of fermentation:
 Alcoholic fermentation
 Occurs in microorganisms such as yeast.
 Produces ethyl alcohol and CO2.
 Used to make breads, wine, cheese, beer, etc.
 Lactic Acid fermentation
 Occurs in human muscle cells when they run out of oxygen.
 Produces lactic acid.
 Both types of fermentation recycle NADH for glycolysis.
Alcoholic Fermentation
Lactic Acid Fermentation