Chapter 6

Biology and Society:  Marathoners versus Sprinters

           Sprinters do not usually compete at short and long distances.

           Natural differences in the muscles of these athletes favor sprinting or long-distance running.

           The muscles that move our legs contain two main types of muscle fibers:

             slow-twitch and

             fast-twitch.

           Slow-twitch fibers

           last longer,

           do not generate a lot of quick power, and

           generate ATP using oxygen (aerobically).

           Fast-twitch fibers

           contract more quickly and powerfully,

           fatigue more quickly, and

           can generate ATP without using oxygen (anaerobically).

           All human muscles contain both types of fibers but in different ratios.

 

ENERGY FLOW AND CHEMICAL CYCLING IN THE BIOSPHERE

           Animals depend on plants to convert the energy of sunlight to

           chemical energy of sugars and

           other organic molecules we consume as food.

           Photosynthesis uses light energy from the sun to

           power a chemical process and

           make organic molecules.

 

Producers and Consumers

           Plants and other autotrophs (self-feeders)

           make their own organic matter from inorganic nutrients.

           Heterotrophs (other-feeders)

           include humans and other animals that cannot make organic molecules from inorganic ones.

           Autotrophs are producers because ecosystems depend upon them for food.

           Heterotrophs are consumers because they eat plants or other animals.

 

Chemical Cycling between Photosynthesis and Cellular Respiration

           The ingredients for photosynthesis are carbon dioxide (CO2) and water (H2O).

           CO2 is obtained from the air by a plants leaves.

           H2O is obtained from the damp soil by a plants roots.

           Chloroplasts in the cells of leaves use light energy to rearrange the atoms of CO2 and H2O, which produces

           sugars (such as glucose),

           other organic molecules, and

           oxygen gas.

           Plant and animal cells perform cellular respiration, a chemical process that

           primarily occurs in mitochondria,

           harvests energy stored in organic molecules,

           uses oxygen, and

           generates ATP.

           The waste products of cellular respiration are

           CO2 and H2O,

           used in photosynthesis.

           Animals perform only cellular respiration.

           Plants perform

           photosynthesis and

           cellular respiration.

           Plants usually make more organic molecules than they need for fuel. This surplus provides material that can be

           used for the plant to grow or

           stored as starch in potatoes.

 

CELLULAR RESPIRATION: AEROBIC HARVEST OF FOOD ENERGY

           Cellular respiration is

           the main way that chemical energy is harvested from food and converted to ATP and

           an aerobic process—it requires oxygen.

           Cellular respiration and breathing are closely related.

           Cellular respiration requires a cell to exchange gases with its surroundings.

          Cells take in oxygen gas.
          Cells release waste carbon dioxide gas.

           Breathing exchanges these same gases between the blood and outside air.

 

The Simplified Equation for Cellular Respiration

           A common fuel molecule for cellular respiration is glucose.

           Cellular respiration can produce up to 32 ATP molecules for each glucose molecule consumed.

           The overall equation for what happens to glucose during cellular respiration is

           glucose & oxygen -> CO2, H2O, & a release of energy.

 

Redox Reactions

           Chemical reactions that transfer electrons from one substance to another are called

           oxidation-reduction reactions or

           redox reactions for short.

           The loss of electrons during a redox reaction is oxidation.

           The acceptance of electrons during a redox reaction is reduction.

           During cellular respiration

           glucose is oxidized and

           oxygen is reduced.

           Why does electron transfer to oxygen release energy?

           When electrons move from glucose to oxygen, it is as though the electrons were falling.

           This fall of electrons releases energy during cellular respiration.

 

           Cellular respiration is

           a controlled fall of electrons and

           a stepwise cascade much like going down a staircase.

           The path that electrons take on their way down from glucose to oxygen involves many steps.

           The first step is an electron acceptor called NAD+.

           NAD is made by cells from niacin, a B vitamin.

           The transfer of electrons from organic fuel to NAD+ reduces it to NADH.

           The rest of the path consists of an electron transport chain, which

           involves a series of redox reactions and

           ultimately leads to the production of large amounts of ATP.

 

An Overview of Cellular Respiration

           Cellular respiration is an example of a metabolic pathway, which is a series of chemical reactions in cells.

           All of the reactions involved in cellular respiration can be grouped into three main stages:

1. glycolysis,

2. the citric acid cycle, and

3. electron transport.

 

The Three Stages of Cellular Respiration

           With the big-picture view of cellular respiration in mind, lets examine the process in more detail.

 

Stage 1: Glycolysis

            A six-carbon glucose molecule is split in half to form two molecules of pyruvic acid.

            These two molecules then donate high energy electrons to NAD+, forming NADH.

Stage 2: The Citric Acid Cycle

           In the citric acid cycle, pyruvic acid from glycolysis is first groomed.

           Each pyruvic acid loses a carbon as CO2.

           The remaining fuel molecule, with only two carbons left, is acetic acid.

           Oxidation of the fuel generates NADH.

           Finally, each acetic acid is attached to a molecule called coenzyme A to form acetyl CoA.

           The CoA escorts the acetic acid into the first reaction of the citric acid cycle.

           The CoA is then stripped and recycled.

           The citric acid cycle

           extracts the energy of sugar by breaking the acetic acid molecules all the way down to CO2,

           uses some of this energy to make ATP, and

           forms NADH and FADH2.

Stage 3: Electron Transport

           Electron transport releases the energy your cells need to make the most of their ATP.

           The molecules of the electron transport chain are built into the inner membranes of mitochondria.

           The chain

           functions as a chemical machine, which

           uses energy released by the fall of electrons to pump hydrogen ions across the inner mitochondrial membrane, and

           uses these ions to store potential energy.

           When the hydrogen ions flow back through the membrane, they release energy.

           The hydrogen ions flow through ATP synthase.

           ATP synthase

           takes the energy from this flow and
           synthesizes ATP.

           Cyanide is a deadly poison that

           binds to one of the protein complexes in the electron transport chain,

           prevents the passage of electrons to oxygen, and

           stops the production of ATP.

 

The Results of Cellular Respiration

           Cellular respiration can generate up to 32 molecules of ATP per molecule of glucose.

           In addition to glucose, cellular respiration can burn

           diverse types of carbohydrates,

           fats, and

           proteins.

 

FERMENTATION: ANAEROBIC HARVEST OF FOOD ENERGY

           Some of your cells can actually work for short periods without oxygen.

           Fermentation is the anaerobic (without oxygen) harvest of food energy.

 

Fermentation in Human Muscle Cells

           After functioning anaerobically for about 15 seconds, muscle cells begin to generate ATP by the process of fermentation.

           Fermentation relies on glycolysis to produce ATP.

           Glycolysis

           does not require oxygen and

           produces two ATP molecules for each glucose broken down to pyruvic acid.

           Pyruvic acid, produced by glycolysis,

           is reduced by NADH,

           producing NAD+, which

           keeps glycolysis going.

           In human muscle cells, lactic acid is a by-product.

 

The Process of Science: What Causes Muscle Burn?

           Observation: Muscles produce lactic acid under anaerobic conditions.

           Question: Does the buildup of lactic acid cause muscle fatigue?

           Hypothesis: The buildup of lactic acid would cause muscle activity to stop.

           Experiment: Tested frog muscles under conditions when lactic acid

           could and

           could not diffuse away.

 

           Results: When lactic acid could diffuse away, performance improved greatly.

           Conclusion: Lactic acid accumulation is the primary cause of failure in muscle tissue.

           However, recent evidence suggests that the role of lactic acid in muscle function remains unclear.

 

Fermentation in Microorganisms

           Fermentation alone is able to sustain many types of microorganisms.

           The lactic acid produced by microbes using fermentation is used to produce

           cheese, sour cream, and yogurt,

           soy sauce, pickles, and olives, and

           sausage meat products.

           Yeast is a microscopic fungus that

           uses a different type of fermentation and

           produces CO2 and ethyl alcohol instead of lactic acid.

           This type of fermentation, called alcoholic fermentation, is used to produce

           beer,

           wine, and

           breads.

 

Evolution Connection: Life before and after Oxygen

           Glycolysis could be used by ancient bacteria to make ATP

           when little oxygen was available, and

           before organelles evolved.

           Today, glycolysis

           occurs in almost all organisms and

           is a metabolic heirloom of the first stage in the breakdown of organic molecules.