Chapter 5 BSCS

Cell Respiration: Releasing Chemical Energy

All chemical bonds contain chemical energy depending on the “energy level” of the shared valence electrons, some bonds have more energy than others. When bonds are broken, energy is usually released and when bonds are made, energy is usually required.

All cells have enzymes (proteins) which can help make (anabolism) or break (catabolism) bonds. Together, the building (anabolic reactions) and the breakdown (catabolic reactions), are called metabolism.

To get the activation energy for a cell’s reactions, organisms break ATP ADP + P. To reverse that, to recycle the ATP, cells must change ADP + P ATP. To do this, cells will typically break the bonds of carbohydrates (usually glucose), then lipids and finally proteins (in that order). All of those nutrients are being used at the same time but in those mentioned amounts. These reactions that remake ATP are called cellular respiration.

The breakdown of large molecules into smaller molecules, releases energy, free or useful energy this is used by cells to add a phosphate group to ADP making it ATP. Your book refers to useful energy as free energy. The remainder of the energy is converted to heat. The ratio is roughly 40% free energy and 60% heat energy.

This process of using the energy in organic molecules/food (glucose) to produce more ATP from ADP and P is called CELLULAR RESPIRATION. In an ATP molecule, the last two phosphate groups are held by, what can be thought as, very special high energy covalent bonds. Cells usually only break off the last P of ATP. As before, some of the released energy is useful causing chemical reactions (activation energy) but again most is changed into heat.

In other words, organisms constantly break down ATP to ADP and P for some activation energy. SLOW DOWN, REREAD THIS! DO YOU HAVE QUESTIONS? ASK ME.

Living cells can only do chemical reactions. No more, no less. All cellular work: growth, repair, cell division,cellular movement, synthesis of macromolecules (Biosynthesis) such as: carbohydrates, proteins, lipids and nucleic acids, are the result of chemical reactions. The activation energy for all these biochemical reactions comes ONLY from the breakdown of ATP.

Within cells two possible fates await glucose/ 2 PGAL. It will be either used during cellular respiration in order to remake ATP or it will be rearranged and/or have other elements/atoms added to it, resulting in the synthesis of the organic compounds (biosynthesis). Your book refers to these pieces of former glucose molecules that are used to make other organic molecules as “carbon skeletons”.

Organisms can obtain their original source of glucose in one of two ways: autotrophy or heterotrophy.

Complete cellular respiration requires oxygen, O₂ and the entire process is divided into three pathways. The pathways occur where the cell stores the required enzymes that catalyze those particular reactions.

The three pathways are called: GLYCOLYSIS, KREB CYCLE AND THE ELECTRON TRANSPORT SYSTEM. The last two (KREB CYCLE AND THE ELECTRON TRANSPORT SYSTEM)are referred to as AEROBIC because they require oxygen (O₂) to be present.

Glycolysis occurs in the cytosol, the 10 glycolytic enzymes are located throughout this cellular goo. Reactants are Glucose, or its equivalent, 2 NAD+ and 2 ATP for the activation energy to start the process. Products are 2 pyruvic acids, sometimes called pyruvate, 4 ATP (there is a net gain of 2 ATP) and 2 NADH.

Kreb cycle enzymes are in the matrix of the mitochondria. The 2 pyruvates will be able to be transported inside the mitochondria if there is sufficient O₂ present. Here both pyruvates will each enter the “intermediate step”. Once they each lose a CO₂ and transfer hydrogen to NAD+, they are carried into the Kreb cycle by an enzyme called Co-enzyme A, often abbreviated as CO-A. Reactants are the 2 pyruvates, 8 NAD+, 6 H2O, 2 FAD+ and 2 ADP + 2 P. The products are 6 CO₂, 8 NADH, 2 FADH2 and 2 ATP.

Finally the Electron Transport System happens, it uses the 10 NADH and 2 FADH2 products from the first two pathways (glycolysis and Kreb cycle). The enzymes and other proteins of this system are stuck to the cristae of the mitochondria. Here the hydrogens from the 10 NADH and 2 FADH₂ are separated into electrons and protons. It is a similar strategy as was used in Photosynthesis’s Photosystem II where a high concentration of protons on one side of a membrane was used to “power” the enzyme ATP synthetase. Each NADH will be responsible for remaking 3 ATP and each FADH₂ can remake 2 ATP. When the electrons and protons are reunited they will combine with an Oxygen atom, O from O₂ forming water H₂O. Oxygen is the “final electron acceptor”. The reactants here are 10 NADH, 2 FADH2, 6 O₂ and 34 ADP + P. The products are 10 NAD+, 2 FAD+, 12 H2O and 34 ATP.

So for every glucose molecule that starts in glycolysis 38 ATP can be remade.

C6H12O6 + 6 O₂ + 6 H2O 6 CO₂ + 12 H2O

Glycolysis ETS Kreb 38 ADP 38 ATP Kreb ETS

+ 38 P

Every cell in your body must make its own supply of ATP for continuous use, obviously a busy cell such as a brain cell or a muscle cell has greater needs than a skin or a bone cell. All cells must make ATP even when they are at “rest”. Any serious diminishment in this mandatory and continuous production will lead to the death of that cell.

Every cell then needs a continuous supply of the Cellular Respiration’s reactants and equally important a continuous removal of its potentially toxic products. The faster ATP is needed, the faster the delivery of the reactants to the cell and the removal of the products.

In humans and many higher animals, the cardiovascular system: which is the heart, blood vessels and blood, is the internal transport system.

Nutrients, toxins, gases are constantly circulating around an animal’s body inside a variety of connected “pipes” or blood vessels. These blood vessels are like freeways (interstate 8) = ARTERY/VEIN, major streets (Linda Vista Road)= ARTERIOLE/VENULE and residential streets (your street) = CAPILLARY. Delivery of cellular goodies and trash pick up can only happen when the blood is moving slow enough and the blood vessel wall is thin enough to allow stuff to diffuse into or out of the houses/cells (muscle cells, cheek cells, etc.) and into or out of that type of nearby blood vessel (capillary). This exchange of good stuff and trash is only accomplished along the narrow and slow moving residential streets (capillary). The larger streets and freeways (arteries, veins, arterioles, venules) are necessary to move large amounts of blood to the essential residential streets (capillaries). Keep in mind, all the streets and highways (blood vessels) ultimately form a continuous loop, blood just keeps going around and around. The heart is responsible for pumping the blood around and around.

To summarize:

Your heart pumps the blood into a few but large diameter vessels the thick walled, “freeways”, the arteries , from here the blood flows into more numerous but smaller diameter vessels the arterioles and then into the most numerous but smallest diameter and thinnest walled vessels, the capillaries, here the capillary walls are so thin that materials(glucose, gases, toxins, water, etc.) can diffuse right through these walls and into/out of the cells of your tissues. The blood leaving the capillaries flows into the larger diameter but fewer venules, these then join into larger diameter,but fewer vessels the veins. The blood in the veins returns to the pump, the heart, and around it goes again.

The “houses” are your body’s cells. Every house/cell not only needs a constant delivery of nutrients, especially glucose and O₂ gas but also the pick up, removal of metabolic toxins, including the CO₂ which was produced during the Kreb cycle. Without these deliveries and “trash removal” no cellular respiration will occur, insufficient ATP supplies are produced and the cell dies.

What causes these gases to do “what they should do”, as always, the answer is diffusion! If there is more O₂ in the bloodstream than in the cell, then O₂ will diffuse out of the capillary’s blood and into the cell, if there is more CO₂ in the cell than in the bloodstream then CO₂ will diffuse out of the cell and into the bloodstream.

So the cardiovascular system is responsible for the pick up and drop off of all materials to the cells, how does the stuff get into the bloodstream in the first place?

The nutrients/food; carbohydrates, lipids, proteins, nucleic acids are eaten, yum! Then your digestive system, primarily the small intestine does the majority of the digestion making the food molecules small enough to be transported/diffused into the blood, they are now simple sugars, fatty acids & glycerol, amino acids and nucleotides. The small intestine also has a fantastically rich supply of those “residential streets” the capillaries. It is here in the small intestinal capillaries, that the digested nutrients are absorbed into the bloodstream. Once they are in the transport system they will be eventually pumped past every cell of the body. As that blood in the capillaries, goes by the cells, diffusion/active transport of nutrients from the blood and into cells occurs and the diffusion/active transport of metabolic wastes out of the cells into the passing blood occurs.

There are over 300,000,000 microscopic elastic “balloon like” structures called alveoli, that make up the functional part of your lungs. Each alveoli is wrapped in a net of capillaries. Some of your blood supply is always being pumped to the alveoli and then on to your other tissues.

Gases such as, O₂ and CO₂ diffuse into and out of the capillaries blood from the interior of the alveoli. Remember that diffusion only follows the concentration gradient of each particular gas.

If the blood entering these capillaries has less O₂ and more CO₂ than the inhaled air in the alveoli, then O₂ will diffuse into the bloodstream and CO2 will diffuse into the alveoli. This is the normal situation, as the blood sent to the lungs has just come from your body’s cells, such as the those of the muscles, kidneys, bones, nerves etc. If that same blood has more CO₂ (produced during the Kreb cycle) than the inhaled air, then CO₂ will diffuse from the bloodstream and into the lung’s alveoli.

Note: If you are inhaling air too low in O₂ and/or too high in CO₂, then the blood leaving the lungs will have lost O2 and gained CO2, that would be a fatal condition.

Once the blood leaving the lungs is enriched with O₂ and lost CO₂, then it is pumped by the heart to all of the body’s cells.

So the digestive system is responsible for obtaining your supply of glucose, the respiratory system is responsible for delivering O₂ and expelling CO₂. And the cardiovascular system is responsible for the transport of all things to and from all cells.

Animals are either endotherms or ectotherms. Endotherms keep (within limits) their internal temperature constant regardless of the environmental temperature. Internal temperatures influence an organism’s metabolic rate. There are only two groups of animals that are endothermic. The birds and mammals are the only examples of endotherms. All other animals such as such as fish, amphibians, reptiles, insects, crustaceans etc. are ectotherms. The ectotherms waste very little of their glucose on heat generation but instead rely on the environment to determine their internal temperature and therefore the metabolic rate. Environment hot, organism hot. Environment cold, organism cold.

There are advantages and disadvantages to each strategy. The advantages to endothermy is the ability to be active in a wide range of climates and habitat temperatures. The disadvantages are major. The disadvantages are it takes a huge amount of food to maintain endothermy. Not only are they using glucose to recycle their supplies of ATP but for the heat produced during these reactions. The greatest challenge is finding enough food, especially when the climate/habitat is cold. Not only are they rapidly losing heat in colder environments but typically there is less food available (at least in terrestrial ecosystems, colder aquatic ecosystems are often more bountiful). The advantages of ectothermy are the opposite of endothermy. The ectotherm needs very little glucose compared to a similar sized endotherm. The great majority of their glucose goes for ATP production to drive their cellular energy needs and not for heat production. How often and how much do you have to feed a one kilogram rat (endotherm)? How often and how much would you have to feed a similar sized snake (ectotherm)? Think about it. Another advantage is that in colder climates when food is hard to find, the ectotherm’s metabolism also slows down, therefore requiring less ATP production. Some ectotherms and even endotherms greatly slow their metabolism by hibernating thus requiring very, very little cellular respiration. The greatest disadvantage to ectothermy is the opposite of the advantage of endothermy. The terrestrial ectotherms can not successfully out compete the endotherms in the polar habitats.