Transcript Cellular Energy: Photosynthesis and Respiration

Cellular Energy: Photosynthesis and Respiration
Objectives:
• Compare and contrast the processes of photosynthesis and respiration
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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 * xylem
Fermentation * 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 electromagnetic waves used by plants for photosynthesis.
Photosynthesis
All 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 equation
for 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?)