Cellular Energy: Photosynthesis and Respiration

Objectives:• Compare and contrast the processes of photosynthesis and respiration• Describe the structure and function of leaf and chloroplast components• Distinguish between and list the reactants and end products for: glycolysis, photosynthesis, fermentation and aerobic respiration• Explain the need for certain vitamins/minerals as well as proteins, lipids, and carbohydrates in energy production• Discuss the role of the sun’s electromagnetic spectrum on life.• Identify the 2 major steps of cellular respiration.

Vocabulary:Photosynthesis * pigments * Respiration * ATP * NADH * NADPH * cytochromes * stomata * light/dark reactions * chlorophyll

FADPH * FADH2 * reduction * oxidation * grana * stroma thylakoid membranes * kilocalories * aerobic respiration * xylemFermentation * glycolysis * lactic acid & alcoholic fermentation Chloroplast * Mitochondria * basal metabolic rate * anaerobic * aerobic * Kreb’s or Citric Acid cycle * electron transport chain * autotroph * heterotroph * ultraviolet * infrared * phloem

All life requires energy to survive. Ultimately, all energy comes

either directly or indirectly from the sun. The nuclear fusion of

hydrogen atoms as they form helium, etc. releases energy in the form

of electromagnetic waves. These include, in order from lowest to

highest energy : radio, infrared, optic (light), ultraviolet, x-ray, and

gamma rays (Rats I Owe U {you} X {ten} Grand). Although ultraviolet

light is essential for stimulating the pineal gland and therefore

controlling melatonin production (sleep/wake cycles) and for the

conversion of cholesterol to vitamin D, in humans, and, for most other

animals, to make vitamin C, it can also be harmful (carcinogenic) with

prolonged exposure due to its high energy vibrations. Infrared rays are

lower energy “heat” waves, which are also obviously needed for life.

But, in between infrared and ultraviolet lie the optic, or light, waves

needed for photosynthesis - the primary food source for all life.

Organisms that can make their own food through photosynthesis are

called autotrophs. Those that can’t, are called heterotrophs.

Within the spectrum of visible light waves, fall the colors. Starting with the color closest to infrared (low energy) and running to the

highest energy light wave (just below ultraviolet waves), they are: red, orange, yellow, green, blue, indigo, violet (ROY G BIV). These are the electro-magnetic waves used by plants for photosynthesis.

PhotosynthesisAll photosynthesis occurs within chloroplasts (except in the case of true blue-green bacteria). Here, plants convert sunlight, carbon dioxide (from the air) and water into sugar (glucose). Note that almost the entire mass of even the largest tree comes from the products made through photosynthesis. Only a few grams of soil mass disappears into a tree and this is only because plants cannot manufacture minerals. The mineral they require the most is magnesium. It is extremely important in the primary photosynthetic pigment called chlorophyll. A pigment is a light absorbing, colored compound

Photosynthesis involves two sets of processes: the light reactions and the dark reactions (although these often occur in the light, they don’t

directly require light) also called the Calvin cycle, or C3 cycle (3 carbons). The light reactions trap the sun’s energy and release oxygen formed from the splitting of water. These reactions also produce ATP (the cell’s energy currency) and NADPH. NADPH (nicotinamide adenine dinucleotide)is a hydrogen carrying molecule needed for energy production in the cells of both plants and animals. It requires nicotinic acid (the vitamin niacin), hence the “N” in NADPH. During the dark reactions, the ATP, NADPH, and CO2 from the air are used to form glucose (a carbohydrate). The TRUE, FULL equationfor photosynthesis is:

6 CO2 + 12 H2O + (light energy) C6H12O6 + 6 O2 + 6 H2O (transpiration)

12 water molecules are needed since it is their splitting that produces the 6 oxygen molecules and the hydrogen ions are essential for the energy gradient that drives photosynthesis. The simplified equation which doesn’t actually occur in nature but is found in most textbooks and on most tests is:

6 CO2 + 6 H2O + (light energy) C6H12O6 + 6 O2

Unfortunately, this shortened equation does not show why

transpiration occurs (the 6 water in the product of the true equation) or

why dehydration slows energy production and/or weight loss (The true

cellular respiration equation is the reverse of the true photosynthesis

equation. The oxygen from the water is part of the oxygen used to form

carbon dioxide. So, water is essential in order for the reaction to

occur).

Photosynthesis occurs in the chloroplasts. However, there are several different, locations WITHIN the chloroplast for the different reactions.

The light reactions occur in the thylakoid membranes. The thylakoid membranes form stacks called grana. These are part of the inner membrane of the chloroplast and contain the photosynthetic pigments. Notice that most leaves appear green. That is because chlorophyll pigment REFLECTS green but absorbs the energy wavelengths of the other colors. Secondary, or accessory, pigments (yellow - xanthophyll, red - anthocyanin/phycobilins, oranges - carotenoids, etc.) specialize in absorbing specific light energy waves. Algae living deep under water and plants in shady environments often are red in color. This is because red pigment can absorb all but red light which doesn’t penetrate deep into the water, etc, due to its lower energy. Therefore, the remaining light waves are not wasted due to reflection off of other pigments. Generally, accessory pigments pass their energy (electrons) to chlorophyll for use in the electron transport chain.

The dark reactions also occur within the chloroplast but they occur in the protein-rich solution surrounding the thylakoid membranes. This area is known as the stroma. This is where glucose (which may be used immediately for energy or stored as starch for later energy needs) will ultimately be produced. The process of forming glucose is often

called carbon fixation. The Calvin, C3 cycle, or dark reactions (all the same thing) must go through the cycle 6 times to make glucose. Each “turn” adds one more carbon to the compound. The first stable compound exists when 3 carbons are finally joined together, hence the

C3 cycle name. Similar cycles occur in tropical plants (C4 cycle) and cacti (CAM pathway).

Leaf Structure

In order for CO2 to enter the leaf (to participate in the dark reactions) and for oxygen (a waste product from the light reactions) and water to exit, leaves have small openings called stoma, or stomata. (“stoma” and “stroma” are VERY different). In order to conserve water, leaves have a waxy coating, or cuticle, over a single cell layer called the epidermis. Just under the epidermis on the upper-side of the leaf is the palisade parenchyma. This tissue layer of cells contains the most chloroplasts so it is the site of most photosynthesis.

Beneath the palisade layer is the spongy parenchyma. These cells

contain fewer chloroplasts (that’s why the underside of a leaf is usually

a pale green compared to the top side). The spongy layer has large

gaps between its irregularly shaped cells that allow carbon dioxide to

enter, and water vapor and oxygen to exit the leaf easily as they travel

to or from the stomata. Most stomata are in the epidermis on the

bottom side of a leaf, close to the spongy layer. The opening or closing

of the stomata is controlled by a pair of guard cells. When water is

plentiful, they inflate like an inner tube. When in dry conditions, they

deflate, closing the stomata. Together, the palisade and spongy

parenchymas are called the “mesophyll”.

Water is brought up from the roots by the xylem and glucose is

transported out of the leaf to other needy areas via the phloem.

Together, they form the vascular bundle, or veins, in the leaf.

Cellular Respiration Heterotrophs, which can’t make their own food, and even autotrophs, like plants, that produce their own food through photosynthesis, need to convert it to ATP (adenine nitrogen base, 3 phosphates, and a ribose sugar - adenosine triphosphate. Note how similar it is to RNA in components.) the cell’s energy currency. The process that releases food energy, to convert it to ATP, is called respiration.

All respiration starts with glycolysis and ends with either:

1) Fermentation: (anaerobic respiration) produces no ATP directly but does produce NAD+ which is used later in glycolysis for ATP production. Fermentation occurs in 2 different forms:

a) Lactic Acid fermentation - occurs primarily in animal cells (humans included) when there is too little oxygen present to undergo aerobic respiration (In chemistry terms, oxygen is the limiting reactant.)

b) Alcoholic fermentation - occurs primarily in yeast cells and some plant cells. It produces ethanol used by breweries.

2) Aerobic respiration: together with glycolysis, produces 38 ATP. Requires oxygen.

Glycolysis Glycolysis (the “splitting of glucose”) occurs in the cytoplasm. At this point, cellular respiration does not require oxygen but does require 2 ATP molecules. There are basically 4 simplified steps to glycolysis: 1) Phosphate is added to glucose (phosphorylation) to form glucose-6-phosphate. This requires 2 ATP. Two ADP (adenosine diphosphate which can actually be stripped of another phosphate to produce AMP - adenosine monophosphate - when conditions warrant it) are left over. 2) Glucose-6-phosphate is oxidized into two 3-carbon molecules.

This forms 2 phosphoglyceraldehyde (PGAL) 3) Through a series of steps, Each PGAL is soon converted to

pyruvic acid or pyruvate ( the ionized form) and NAD+ (niacin containing compound) is reduced to NADH. Arsenic can block this step in glycolysis. Much of Wisconsin’s well water is Arsenic contaminated due to naturally occurring arsenic deposits!

4) As both PGALs are converted to pyruvic acid (2 total) by breaking phosphate bonds, a total of 4 ATP are produced (2 from each PGAL). This means a NET output of 2 ATP (because 2 were put into step 1) occurs during glycolysis of a single glucose molecule.

Fermentation If little or no oxygen is present after glycolysis, fermentation will occur. Together, these are often called anaerobic respiration. Like glycolysis, fermentation also occurs in the cytoplasm (cytosol). Fermentation does not produce any ATP but it does breakdown pyruvic acid into NAD+ which can then be used again in glycolysis for more ATP (2) production. Lactic acid fermentation in animal cells, some bacteria, etc:

Pyruvic acid + NADH + H+ lactic acid + NAD+

The accumulation of lactic acid in muscle causes soreness, cramps, and fatigue. This usually occurs after strenuous exercise because oxygen is quickly used up during aerobic respiration. Eventually, the lactic acid is carried to the liver and broken down. Lactic acid fermentation is also how yogurt and cheese are made. Alcoholic fermentation in yeasts, etc.:

Pyruvic acid + NADH + H+ ethanol + CO2 + NAD+

The ethyl alcohol (ethanol) from Brewer’s yeast is used for wine, beer,

etc. The CO2 from Baker’s yeast helps bread, etc. rise and stay “light”.

Alcohol still holds quite a bit of energy. It has about 9 kilocalories (Calories) of energy per gram versus 8 kcals in fat and 4 kcal in carbs and protein. Unfortunately, alcohol consumption drastically reduces the amount of many of the B vitamins in the body as the liver processes the alcohol. Alcoholics are often deficient in these vitamins. Because of its low energy output, fermentation is not an efficient process in humans, however, it is efficient enough for unicellular organisms like yeast and bacteria. Larger organisms require . . .

Aerobic Respiration Mitochondria are extremely important to ATP production and life. In fact, mitochondria have their own DNA (inherited strictly from Mom). There are 5 to 10 identical, circular DNA molecules found in the mitochondria. These carry the information for 37 genes that encode for substances needed in the electron transport chain. These circular DNA (circular DNA have no telomeres) are similar to those found in bacteria, especially Rickettsias. This supports the theory of endosymbiosis.

Aerobic respiration takes place in the mitochondria and has 2 main stages: The Kreb’s Cycle, or the Citric Acid Cycle, and the Electron Transport Chain.

Simplified Respiration Equation: C6H12O6 + 6O2 6CO2 + 6H2O + ATP (energy)

The Kreb’s Cycle (Citric Acid Cycle) occurs in the cytosol of prokaryotes but in the inner mitochondrial membrane and matrix (the space inside the inner membrane) area in eukaryotes. The inner membrane is similar to the thylakoid membrane in a chloroplast since both are involved in the electron transport chain.

After glycolysis, pyruvic acid enters the mitochondria and is converted to acetyl coenzyme A (Vitamin B5 aka Pantethine is needed for CoA synthesis) and CO2, NADH (used later in the electron transport chain) and H+ are produced. Unfortunately, arsenic can block this conversion. The acetyl CoA then enters the Kreb’s cycle. The acetyl CoA is temporarily converted to citric acid (hence the alternate name for the cycle). Since 2 pyruvic acid molecules were formed during glycolysis from 1 glucose molecule, each glucose molecule causes 2 “turns” of the Kreb’s cycle. The net result of 2 turns of the Kreb’s cycle is: 6 NADH + H+, 2 FADH2 (Flavin adenine dinucleotide - needs riboflavin aka vitamin B2), 2 ATP, and 4 CO2.

After the Kreb’s (citric acid) cycle, NADH + H+ and FADH2 contribute hydrogen ions which generate energy for the electron transport chain (also sensitive to arsenic) as ATP is produced. Oxygen is the final electron receptor. It then joins with H+ to form water. If not enough oxygen is present to accept the electrons generated and to join with H+ to form water, the cell is forced into anaerobic fermentation (Lactic acid or alcoholic fermentation). Cyanide can block the enzyme that transfer electrons to oxygen and therefore can stop aerobic respiration. If all goes well, the combination of glycolysis with aerobic respiration (Kreb’s cycle and electron transport chain) will produce about 30 to 38 ATP. (Remember: most ATP will attach to Magnesium at some point) Carbs are not the only macronutrient that can be used for ATP production though. The amino acids from protein can be converted to either pyruvic acid or acetyl CoA and then follow the usual pathways.And, fats are a combination of glycerol and fatty acids. The glycerol can be converted to pyruvic acid and the fatty acids to acetyl COA.

p. 129 Figure 7.12

Other Important Facts One of the nutrients acting as an electron “shuttle” for ATP production during the

electron transport chain is ubiquinone, or Coenzyme Q10. Although our bodies can

make Co Q10, illness, stress, and lack of the nutrients needed for the manufacture of

Co Q10 can create situations where we have less than desirable quantities available.

In order to make adequate Co Q10, we need the amino acid tyrosine (also used in thyroid hormone which is another important player in energy production and body

temperature regulation), niacin, vitamin B6, vitamin B12, vitamin C, and folate.

Coenzyme Q10 can be taken as a preformed supplement. However, like vitamins A, D, E, and K, it is fat soluble so needs to be taken with fatty foods or oil to be absorbed. Many people suffering from heart failure have shown a deficiency in Co

Q10. In some cases, large dose supplementation has improved heart function so

dramatically that patients no longer needed a heart transplant! In addition, Co Q10 protects the heart from chemotherapy and radiation damage during cancer treatment. (Remember the heart is a very active muscle so it requires a whole lot of ATP and therefore the nutrients mentioned here. Furthermore, many heart failure patients have high cholesterol, which niacin, in the right form, can help and have

high homocysteine, which can be controlled by vitamin B6, vitamin B12 (which the elderly especially have a tough time absorbing due to declining intrinsic factor) , and folate. (Do we see a connection between these nutrients, cellular respiration/ATP and our health yet?)