1. Chapter 3
Cell metabolism

2. Metabolism: Doing Cellular Work
ATP is the cell’s energy currency.
Metabolism refers to all of the chemical reactions that occur in cells; ATP links the whole of these reactions together.
ATP is composed of adenine, ribose, and three phosphate groups.
ATP transfers energy in many different chemical reactions; almost all metabolic pathways directly or indirectly run on energy supplied by ATP.
ATP can donate a phosphate group (phosphorylation) to another molecule, which then becomes primed and energized for specific reactions.

3. Metabolism: Doing Cellular Work
There are two main types of metabolic pathways.
Metabolic pathways form series of interconnected reactions that regulate the concentration of substances within cells.
In anabolism, small molecules are assembled into large molecules—for example, simple sugars are assembled into complex carbohydrates.
In catabolism, large molecules such as carbohydrates, lipids, and proteins are broken down to form products of lower energy, releasing energy for cellular work.

4. Metabolism: Doing Cellular Work
Enzymes play a vital role in metabolism.
Enzymes are proteins that serve as catalysts; they speed up reactions.
Enzymes have several features in common:
Enzymes do not make anything happen that could not happen on its own; they just make it happen faster.
Enzymes can be reused.
Enzymes act upon specific substrates, molecules which are recognized and bound at the enzyme’s active site.

5. Metabolism: Doing Cellular Work
Because enzymes operate best within defined temperature ranges, high temperatures decrease reaction rate by disrupting the bonds that maintain three-dimensional shape (denaturation occurs).
Most enzymes function best at a pH near 7; higher or lower values disrupt enzyme shape and halt function.
Coenzymes are large organic molecules such as NAD and FAD (both derived from vitamins), which transfer protons and electrons from one substrate to another to assist with many chemical reactions.

6. How Cells Make ATP Cellular respiration makes ATP.
Electrons acquired by the breakdown of carbohydrates, lipids, and proteins are used to form ATP.
Overall, the formation of ATP occurs by cellular respiration; in humans this is an aerobic process meaning it requires oxygen.

7. How Cells Make ATP Step 1: Glycolysis breaks glucose down to pyruvate.
Glycolysis reactions occur in the cytoplasm and result in the breakdown of glucose to pyruvate, generating small amounts of ATP.
Glucose is first phosphorylated in energy-requiring steps, then split to form two molecules of PGAL.
Four ATP are produced by phosphorylation in subsequent reactions; but because two ATP were used previously, there is a net gain of only two ATP by the end of glycolysis.
Glycolysis does not use oxygen.

8. How Cells Make ATP
Step 2: The Krebs cycle produces energy-rich transport molecules.
Pyruvate (produced in the cytoplasm) enters the mitochondria for the oxygen requiring steps of cellular respiration.
The pyruvate is converted to acetyl-CoA, which enters the Krebs cycle to eventually be converted to CO2.

9. How Cells Make ATP
Reactions within the mitochondria and the Krebs cycle serve three important functions:
Two molecules of ATP are produced by substrate-level phosphorylation.
Intermediate compounds are regenerated to keep the Krebs cycle going.
H+ and e- are transferred to NAD and FAD, generating NADH and FADH2.
These reduced electron carriers will become oxidized as they drop off their hydrogens on the ETC.

10. How Cells Make ATP
Step 3: Electron transport produces many ATP molecules.
The final stage of cellular
respiration occurs in the
electron transport
systems embedded in
the inner membranes
(cristae) of the mitochondrion.

11. How Cells Make ATP
NADH and FADH2 made during glycolysis and the Kreb’s cycle give up their electrons to transport (enzyme) systems embedded in the inner mitochondrial membrane (electron transport chain = ETC).
Electrons flow through the system eventually to oxygen, forming water; as they flow, H+ are pumped into the outer compartment of the mitochondrion to create a proton gradient.
Oxygen is the final electron acceptor in the ETC.
H+ ions move down their gradient, through a channel protein called ATP synthase, in the process driving the synthesis of ATP.

12. TYPICAL ENERGY YIELD: 36 ATP
CYTOPLASM
glucose 4
ATP 2
ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 NADH
2 pyruvate
MITOCHONDRION
e- + H+
2 CO2
2 NADH
e- + H+
8 NADH
4 CO2
KREBS CYCLE
e- + H+ 2
ATP
2 FADH2 e-
ELECTRON
TRANSPORT SYSTEM
Figure 3.27: Summary of aerobic cellular respiration. 32
ATP H+
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP

13. Alternative Energy Sources
How the body uses carbohydrates as fuel.
Excess carbohydrate intake is stored as glycogen in liver and muscle for future use.
Free glucose is used until it runs low; then glycogen reserves are tapped.
Under some conditions a process called lactate fermentation can be used to produce ATP; here, pyruvate is converted
directly to lactic acid with
production of quick, but
limited, energy.

14. Alternative Energy Sources
Fats and proteins also provide energy.
Lipids are used when carbohydrate supplies run low.
Excess fats are stored away in cells of adipose tissue.
Fats are digested into glycerol (which enters glycolysis) and fatty acids, which enter the Krebs cycle.
Because fatty acids have many more carbon and hydrogen atoms, they are degraded more slowly and yield greater amounts of ATP.

15. Alternative Energy Sources
Proteins are used as the last resort for supplying energy to the body.
Amino acids are released by enzymatic digestion of proteins; protein is never stored by the body.
After the amino group is removed, the amino acid remnant is fed into the Krebs cycle to produce energy (ATP), or is used to make fats and carbohydrates.
Ammonia (from the amino group) is excreted as waste.

17. Basal metabolic rate
When the chemical bonds of food molecules and ATP are broken, energy is released. This energy is measured in kilocalories (kcal).
One kcal = Calorie is the amount of energy needed to raise the temperature of one kilogram of water one degree celsius.
A Milky Way™ candy bar has 240 Calories.
The basal metabolic rate (BMR) is the minimum amount of energy your body needs to sustain its activity.
Body weight x 10 = the amount of Calories one needs to consume to maintain that weight if one is sedentary and between the ages of 20 and 34.

18. Body Mass Index
The body mass index (BMI) is a value based on weight and height.
BMI = (weight x 703) / (height)2
Example: A 5 foot (60 inches) woman weighing 100 pounds has a BMI of / 3600 = (healthy).
A 5 foot woman weighing 160 pounds has a BMI of (obese).
In general, a person is overweight if they have a BMI of between 25 and 29.9.
Obesity is an excess of body fat and corresponds to a BMI of ≥ 30.

19. Body Mass Index (BMI)