In the first lecture of the semester, we discussed some of the characteristics of living systems. One of these characteristics was the ability of all living things to metabolize.

Metabolism: ability of cells to acquire energy and to use this energy to build, store, break apart and eliminate substances in a controlled fashion.

To understand metabolism, we need to understand energy because metabolism involves obtaining and using energy to do the various things that cells do.

"Energy" is another one of those terms where just about everyone has sort of a vague idea of what it is.

For our purposes, energy can be defined as "heat or anything that can be transformed into heat."

In fact, we often measure energy in units called calories -- 1 calorie is the amount of heat energy required to raise the temperature of 1 gram of water from 14.5C to 15.5C -- dietitians use kilocalories (= 1,000 calories) to measure the energy content of food -- e.g. a bottle of domestic beer has about 150 kcals.

Where in this substance called "beer" are these calories (energy) located? The answer is in the chemical bonds holding atoms together in the compounds that make up beer (e.g. carbohydrates).

Here’s an important concept: when chemical bonds are broken, energy is released. Conversely, energy must be supplied for chemical bonds to be formed.

Metabolic pathways: orderly sequences of chemical reactions in cells.

1. biosynthetic pathways -- small molecules assembled into larger molecules -- requires energy to form new chemical bonds -- e.g. protein synthesis where amino acids are assembled into proteins.

2. degradative pathways -- large molecules broken down into smaller ones -- energy released as bonds are broken -- e.g. cellular respiration.

Before we talk about specific metabolic processes, we need to consider a special molecule that is of central importance to living cells and metabolic reactions.

We said that energy is present in the chemical bonds of various compounds (e.g. carbohydrates) and that cells use this energy for the various things that they do (grow, repair, move) -- however, before cells can use the energy stored in these compounds, they must convert it to a biologically useable form.

For cells, biologically usable energy is found in molecules of a compound called adenosine triphosphate (ATP).

ATP can be viewed as the "energy currency" of the cell -- here’s an analogy that might help: You can’t simply walk into a store and say, "look, I have $1,000 on deposit in the bank and I’d like this CD player." Money on deposit in the bank is not in a usable form -- you must convert your "stored" money into a usable form by going to the ATM and getting cash or writing a check for the CD player -- likewise, energy in the form of carbohydrates and fats is not in a form that cells can use directly.

ATP is a nucleotide -- consists of a 5-carbon sugar (ribose), a nitrogenous base (adenine) and 3 phosphate groups (each PO₃) -- ribose + adenine = adenosine; adenosine + 3 PO₃ = adenosine triphosphate.

Important thing about ATP is the three phosphate groups -- the last two phosphates are linked by what are called "high-energy phosphate bonds" -- bonds that are readily broken to release lots of energy.

Can think of the life of an ATP molecule as being a "cycle:"

1. When energy is needed by the cell, the last high energy phosphate bond is broken and the energy released is used by the cell -- a phosphate group is freed and ATP becomes ADP (adenosine diphosphate).

2. Cells use chemical energy released from breakdown of food molecules to put phosphates back on ADP to make ATP. 

 

All processes in organisms that require energy get that energy from the conversion of ATP to ADP + PO_3.

Organisms can be classified into two major groups depending upon how they acquire energy from their environment:

1. autotrophs -- organisms that are able to make their own food.

2. heterotrophs -- organisms that must obtain their food by eating other organisms (living or dead).

Many autotrophs obtain energy from their environment through the metabolic pathway called photosynthesis.

Photosynthesis -- process in which cells capture energy contained in sunlight and convert it to chemical energy in the form of the carbohydrate called glucose.

Only green plants and certain bacteria are able to carryout photosynthesis -- in plant cells this process takes place in the organelle called the chloroplast .

While the process of photosynthesis is complicated, we can describe the overall process with a simple equation:

6H₂O + 6 CO₂ + energy (sunlight) 6O₂ + 1 C₆H₁₂O₆

In words, water, carbon dioxide, and sunlight energy are involved in series of chemical reactions that produce oxygen and glucose.

We can distinguish two separate sets of steps that makeup photosynthesis:

1. the light-dependent reactions -- sunlight needed for these reactions --sunlight captured by molecules of chlorophyll and energy in light used to make ATP from ADP and phosphate through a process called photophosphorylation -- also, water molecules are split in the light reactions -- process called photolysis -- H+ ions and O₂ are released.

2. the light-independent reactions (Calvin Cycle) -- reactions do not require sunlight to occur -- also take place in the chloroplasts of plant cells -- carbon removed from atmosphere (CO₂ fixation) and energy from the ATP's produced during the light reactions used to rearrange them into glucose precursor called PGAL.

Molecules of PGAL can be converted to a number of different organic molecules that the plant can use:

a. glucose -- "food" -- more on this later.

b. polysaccharides -- e.g. starch, cellulose.

c. amino acids, lipids, nucleotides.

Some miscellaneous points about photosynthesis:

1. about 30% of the light energy absorbed by chloroplasts ends up being stored as PGAL -- process has 30% efficiency rate.

2. producing 1 glucose (2 PGAL's) by photosynthesis requires the expenditure of 18 ATPs -- recall that these are produced by the light reactions.

Now, let’s consider the other major metabolic pathway. This is called cellular respiration.

Cellular respiration -- can be thought of as the reverse of photosynthesis -- carbohydrates are broken down so that energy is released for the cell to use -- this energy is in the form of ATP -- general equation for respiration is the reverse of photosynthesis:

C₆H₁₂O6 + 6O₂ 6H₂0 + 6CO₂ + energy (ATP)

Note: this is virtually the same equation as the one for photosynthesis except backwards!

Energy is stored in the chemical bonds holding the C, H, and O atoms together in glucose -- breaking these bonds releases energy -- must be done in a stepwise fashion to avoid excess heat -- if all bonds were broken at once, cell would burn up!!

As energy in glucose is released, it is used to convert ADP and phosphate into ATP (phosphorylation) -- about 40% of energy in glucose ends up in ATP through respiration.

There are two types of cellular respiration -- most cells are able to do both depending on the environment:

1. aerobic respiration -- preferred pathway -- when O₂ is available.

2. anaerobic respiration -- not as efficient -- when O₂ is not available.

Aerobic respiration -- results in 1 glucose molecule generating 36 ATP molecules -- 3 separate processes : glycolysis, Krebs cycle, and oxidative phosphorylation.

I. Glycolysis -- stepwise breakdown of 1 glucose molecule (C6) into 2 molecules of pyruvate (C3) -- occurs in the cytoplasm of the cell -- 2 ATP are generated.

H+ and electrons released that are picked up by NAD+ (hydrogen acceptor) to become NADH.

II. Krebs cycle -- pyruvate enters a cyclic series of reactions in which the 6 original carbon atoms in the glucose are converted into CO2 and released -- this occurs in the mitochondria of the cell -- 2 more ATP are generated -- more NADHs produced.

III. oxidative phosphorylation -- occurs in electron transport system consisting of cytochrome molecules in the mitochondria -- uses energy in electrons and hydrogen atoms released during glycolysis and Krebs Cycle (NADHs) to phosphorylate ADP into ATP -- 32 ATPs generated -- leftover hydrogens and electrons combined with O₂ (hence the name aerobic respiration) to form H2O ("metabolic water").

32 ATP are generated by oxidative phosphorylation from each glucose that enters glycolysis -- 4 ATP produced by glycolysis and Krebs cycle for a total of 4 + 32 = 36 ATP/glucose.

Figure 7.3 in your text gives a general overview of aerobic respiration:

Some miscellaneous points about aerobic respiration:

1. O₂ must be present to accept H+ and electrons -- otherwise, electron transport in respiratory chain stops -- analogous to a traffic jam -- no ATP produced -- this is precisely what happens during asphyxiation -- cells actually starve because ATP and its energy are used up and not replenished.

2. cytochromes in electron transport system have affinities for other molecules (e.g. cyanide gas) -- used in gas chambers -- cyanide molecules bind with cytochromes to block electron transport -- result is again "ATP starvation".

3. some organisms, especially desert dwellers are able to use "metabolic water" to meet all their needs -- kangaroo rats go through their entire lives without ever drinking free water -- rely on seeds for food -- seeds are high in carbohydrates.

4. not all cells can perform aerobic respiration -- prokaryotes (e.g. bacteria) lack mitochondria -- are able to meet their energy demands solely by the 2 net ATP that comes from glycolysis or from anaerobic respiration (see below).

5. ATP generated by respiration is used to meet the various energy demands of the cell.

6. While only green plant cells can photosynthesize, all eukaroyte cells are capable of aerobic respiration -- could even have a plant cell doing both photosynthesis and respiration simultaneously.

Anaerobic respiration -- respiration in the absence of oxygen -- much less efficient than aerobic respiration. in terms of ATP's per glucose -- efficiency rate of only 2.1%.

Anaerobic respiration is little more than a means of allowing the organism to continue glycolysis -- there are two different types of anaerobic respiration:

1. alcohol fermentation -- done by yeast and plants.

2. lactate fermentation -- done by most animals.

Both types begin with glycolysis: glucose (C6) converted to 2 molecules of pyruvate (2C3), with 2 net ATP's produced and 2 NAD+'s being converted to 2 NADH -- glycolysis will only continue as long as there is NAD+ available to pickup hydrogen and electrons --stated another: continued glycolysis limited by the availability of NAD+ -- conversion of NADH back to NAD is the function of anaerobic respiration.

alcohol fermentation: 2 pyruvates (2C3) converted to 2 ethanols (2C2) and 2 carbon dioxides (2C1) -- in the process, 2 NADH are oxidized to 2 NAD+ -- these NAD+ are then used in glycolysis to convert more glucose to pyruvate, generating a net of 2 ATP.

lactate acid fermentation: 2 pyruvates (2C3) converted to 2 lactates (2C3) -- again, in the process, 2 NADH give up their hydrogens to become NAD+ that can be used again in glycolysis.

1. each molecule of glucose oxidized anaerobically produces only 2 ATP's (compare with 36 ATP for aerobic respiration).

2. alcohol fermentation has been known to man for thousands of years -- commercially important in brewing, winemaking -- CO₂ produced by yeast is the "head on the beer" and the "bubbly" in champagne.

3. sour milk results from anaerobic respiration of lactobacillus bacteria -- bitter taste is lactic acid.

4. strenuous exercise may result in your muscles having to respire anaerobically to meet energy demand -- unable to supply oxygen to muscles fast enough to meet demand -- "oxygen debt" -- lactate builds up in tissues -- produces symptoms of fatigue including cramps -- when oxygen becomes available, we repay the "oxygen debt" -- lactate acid converted back to pyruvic acid.

NEXT TIME: Cell Division: Mitosis and Meiosis

Top of Page

Home