Chapter 9-Energy in a Cell

Chapter Overview

•In this Chapter we will discuss two main concepts:•Photosynthesis-when autotrophs/producers make sugar using sunlight.•Cellular Respiration-when any organism turns a sugar into energy

ATP

• In our cells, we need energy in order to carry out vital cell processes (and live).•Most of our energy is in the form of a molecule called

Adenosine TriPhosphate (ATP).•ATP has a related molecule called Adenosine

DiPhosphate (ADP). ADP can store energy by adding a phosphate group which makes ATP. •When ATP loses a phosphate group, this releases

energy. It works like a battery that can be recharged.

•ATP is only used for short bursts of energy, so only a small amount is found in cells. To store energy long term, cells use the basic sugar glucose (C6H12O6).•The glucose can then be broken down as needed and used to recharge ADP into ATP.

Photosynthesis

•Photosynthesis is the process of changing light energy into storable and usable energy for a cell. ATP is a good storage molecule for the day, but what about longer periods without light (like night)?•Photosynthesis turns light energy into glucose, which can then be turned into starches for longer-term energy storage.

•Photosynthesis uses carbon dioxide and water to turn light from the sun into glucose and oxygen gas.

•The formula for photosynthesis is:

6CO2 + 6H2O C6H12O6 + 6O2

Light

•How do plants do this? They use a pigment called chlorophyll which exists in chloroplasts.•Sunlight looks white to us, but is actually a mix of all the different colors (visible and invisible to us).•Plants gather the sun’s energy with light absorbing molecules called pigments (which are lipids).•The three types of pigments in plants are chlorophylls, xanthophylls and carotenes.

•Chlorophylls are the major pigment in plants, but do not absorb green well. •What happens if a color of light is not absorbed? It is _____________.•This is why most plants appear green to us.•This is also why we see different colored leaves in the fall…the chlorophyll is no longer produced and we see the other pigments that still remain (for a time).

•There are two main types of Chlorophyll:•Chlorophyll a•Chlorophyll b

When plants absorb light, much of the energy is transferred directly to electrons, which allows photosynthesis to work.

Inside a Chloroplast•Chloroplasts, you should remember, only exist inside plant cells and the cells of other producers.• Inside the chloroplast are sac-like membranes called thylakoids. They are arranged in stacks called grana. (Granum for one.)•Proteins in the thylakoids arrange the pigments into clusters known as photosystems. These are what collect light inside of a chloroplast.

•There are two parts to Photosynthesis:•The Light-Dependent Reactions which take place within the thylakoids.•The Calvin Cycle (or Dark Reactions or Light-Independent Reactions) which takes place in the stroma. The stroma is the space inside the chloroplast that is not taken up by thylakoids.

•When the electrons within chlorophyll are excited, they gain a lot of energy. •Think of it as a hot charcoal briquette from a fire. If you wanted to move it, you wouldn’t pick it up with your hands, you would use a carrier. Cells treat them the same way.•Cells use electrons carriers to transport them. The process is called electron transport (duh) and the carriers are known as the electron transport chain.

•Some of the notable carriers:•NADP+ holds two electrons and a Hydrogen ion (H+).

This changes it into NADPH.•NAD+ holds two electrons and an H+ ion. This changes

into NADH.• FAD holds two electrons and two H+ ions. This changes

into FADH2.•These carriers can then be used to carry electrons throughout the cell to be used where needed.

The Light-Dependent Reactions (or the Light Reactions)

•These reactions need light. This is why plants need light.•The light reactions produce oxygen gas and turn ADP and NADP+ into ATP and NADPH respectively. It happens in 5 main steps:

• First, pigments in Photosystem II absorbs light. This excites the electrons and raises their energy levels. These electrons are then passed on to the electron transport chain.•New electrons are gained from H2O molecules. Enzymes

inside of the thylakoid break each H2O into 2 electrons, 2 H+ ions, and 1 oxygen atom.• The two electrons replace the electrons lost by

chlorophyll to the electron transport chain.•Oxygen is paired up and released into the air. This is the

source of almost all of our oxygen on earth.• The extra hydrogen ions from water are released within

the thylakoid.

•Second, the excited electrons move through the electron transport chain to Photosystem I. Energy from the electrons is used to transport H+ ions from the stroma to inside the thylakoids.

•Third, pigments in Photosystem I use energy from light to re-energize the electrons. NADP+ picks them and H+ up and becomes NADPH.

•Fourth, as electrons are passed from chlorophyll to NADP+ more H+ ions are pumped across the thylakoid membrane. The inside of the thylakoid fills up with positively charged H+ ions.•This makes the outside membrane negatively charged. The difference between the inside and outside in charge provides the energy to make ATP.

•Fifth (and last), H+ ions can’t just cross the membrane. However, they can pass through a protein called ATP synthase which is a carrier protein.•As the H+ ions pass through ATP synthase the protein is turned like a turbine and it binds ADP and a phosphate group together to make ATP.•This means that the light reactions produce H+ ions and ATP.•But what do we do with all this ATP and NADPH?

The Dark Reactions

•No…not the Dark Side!• The Dark Reactions…

•The Dark Reactions are more commonly called the Calvin Cycle.•They occur in the stroma of the chloroplast.

•Not that Calvin, Melvin Calvin! (Nerdy old guy.)

•ATP and NADPH can hold energy, but are not stable enough to hold onto that energy for more than a few minutes.•During the Calvin Cycle, plants (and other producers) use the ATP and NADPH to build high-energy compounds that can stored for a long time. It does this with high-energy sugars such as glucose.•It happens in 4 parts:

•First, 6 carbon molecules enter the cycle from the atmosphere (courtesy of CO2). These each combine with a 5-carbon molecule called RuBP (six total). The result is 12 molecules of 3 carbons each .

•Second, our twelve 3-carbon molecules are converted into higher energy forms. This energy comes from ATP and NADPH made in the light reactions.•This leaves us with ADP and NADP+ to return to the thylakoids and be used again. The end product is called PGAL.

•Third, two of the twelve 3-carbon molecules are removed from the cycle. •The cell uses these to make sugars, lipids, amino acids, and other compounds that are needed for plant metabolism and growth.

•Fourth, the remaining ten 3-carbon molecules are converted back into six 5-carbon molecules (PGAL). •They begin the cycle again.

•The Calvin Cycle needs 6 molecules of CO2 to produce one glucose (C6H12O6).•The plant then uses glucose to make starch (storage), cellulose (structure), and just to break down for energy in cellular respiration.•When other organisms eat plants, they get the energy from these carbohydrates.

Rates of Photosynthesis•Amount of water available.•Temperature (the essential enzymes must be between 0°C and 35°C (32-95 degrees F).•The intensity of light.

•Because of this different producers are adapted to different conditions! Think of a cactus, vs. an oak tree vs. a pine tree.

Stop!

Cellular Respiration•Organisms (including us) need food for:• Synthesizing new molecules• Energy

•Glucose is the main source of energy for living organisms. One gram of glucose (burned with oxygen) provides 3811 calories of heat energy. (That is 3.811 kilocalories…or the Calories you see on the side of a food package.)•A calorie is the amount of energy needed to raise 1

gram of water 1°C.

•Cellular Respiration is made up of three parts:•Glycolysis• (If oxygen is present) The Krebs Cycle• (If oxygen is present) The Electron Transport Chain

•There is also an alternative pathway if oxygen is not present that follows glycolysis:•Fermentation

•Cellular Respiration is the process of breaking down glucose and other food molecules in the presence of oxygen in order to release or gain energy.•The formula is:

602 + C6H12O6 6H2O + 6CO2 + Energy

Oxygen gas and glucose yields carbon dioxide, water, and Energy

•Although it looks very simple, it must take place in several steps or most of the energy would be released as heat and/or light.

Glycolysis•Glycolysis occurs in the cytoplasm of a cell.•Glycolysis only releases a small amount of energy, but it

prepares the essential components for any subsequent stages.• In glycolysis, one molecule of glucose is broken in half,

producing two molecules of pyruvic acid. Pyruvic Acid is a 3-carbon molecule.• The cell needs some energy in order to begin glycolysis so 2

molecules of ATP are used. This is because activation energy is needed to start any reaction.

•At the end of glycolysis, 4 ATP molecules are produced. This is a net gain of 2 ATP.•Glycolysis also removes 4 high-energy electrons and gives

them to the carrier NAD+. NAD+ accepts 2 electrons and a H+ ion and becomes NADH.• This is a small yield, but is very fast so the cell can produce

thousands of ATP molecules in a few milliseconds. It also does not require oxygen.•However, the cell has only a limited amount of NAD+ carriers

which are all full after a few seconds, so the cell cannot continue glycolysis for long.

•At the end of glycolysis only 10% of the energy in glucose has been released. To release the rest the cell uses Oxygen gas (O2) which is a very powerful electron acceptor.•Because cellular respiration needs oxygen, we call it an aerobic process.

The Krebs Cycle(I held off with a Mr. Krabs joke FYI).•In the Krebs Cycle pyruvic acid is broken down into CO2 to release energy.•It is also called the Citric Acid Cycle because the first compound formed in the cycle is citric acid.•It has two main steps (but a lot goes on in each).

•First, pyruvic acid enters the mitochondrion. One carbon atom becomes part of CO2 which is eventually released as a waste product. NADH is made from released electrons.•The other two carbons are joined to a compound called coenzyme-A to form acetyl CoA. •Acetyl CoA then adds the 2 carbon acetyl group to a 4-carbon compound to make a 6-carbon compound called citric acid.

•Next, citric acid is broken down into a 4-carbon molecule, more CO2, and electrons are transferred to carriers.•First, one carbon is removed (and made into CO2) and NADH is made.•Then a second carbon is removed (and made into CO2) and ATP and NADH are made.•Finally, other electrons and H+ ions are put with FAD to become FADH2.

•The ATP can be used for immediate cellular processes…but what to do with our NADH and FADH2?•Their electrons can be used to make more ATP in the electron transport chain.•The electron transport chain can be found on the inner membranes of the mitochondrion in eukaryotes and in the cell membrane in prokaryotes.•The electron transport chain works in the following way:

•First, high energy electrons are passed from one carrier protein to the next. At the end of the chain is an enzyme that combines the electrons with H+ ions and oxygen to form water.•As an electrons goes down the chain it gives off it’s energy (like a salt truck gradually spreading on a road).•Oxygen is the final electron acceptor on the chain. It takes the low-energy electrons.

•The electrons move down the chain in pairs. As they move down the chain they transport H+ ions across the membrane. They create a charge difference on each side of the membrane (like in Photosynthesis).

•ATP Synthases in the inner membranes of the mitochondria are then spun to turn ADP into ATP.•Every time 2 electrons pass down the chain, the H+ ions are pumped back across the membrane to charge it, and as they go through the ATP synthase more ATP is made.•Two electrons will provide enough energy for 3 ATP’s.

Totaling Up• So, we’ve thrown a whole bunch of numbers of ATP, NADH, and

FADH2 around, but what have we actually made from one glucose?• Glycolysis produced a net gain of 2 ATP. (4 made, but 2 used to start it.)• The Krebs Cycle and ETC produce 2 ATP, 8 NADH, and 2 FADH2. When

converted to ATP this adds up to 34 ATP.• This total of 36 ATP’s represents about 40% of the total energy in

glucose. This may not seem like a lot, but it is actually more efficient than an engine in a car.• The remaining 60% of the energy is released as heat. This is one

reason that you feel so hot after exercising.

Fermentation•But what if there is little or no oxygen present? How does

the cell gain energy if the Krebs's Cycle and the ETC can’t be used?• Fermentation can be used to produce ATP in the absence of

oxygen.•During fermentation, cells convert NADH to NAD+ but giving

the high-energy electrons back to pyruvic acid which produces a small but steady supply of ATP.• Since it does not need oxygen, fermentation is referred to as

an anaerobic process.

•There are two main types of fermentation:•Alcoholic Fermentation•Lactic Acid Fermentation

Alcoholic Fermentation•Yeast and other microorganisms use alcoholic fermentation

and form ethyl alcohol and CO2 as wastes. The formula for this is:

Pyruvic Acid + NADH Alcohol + CO2 + NAD+

• The carbon dioxide produced is what causes bread dough to rise. The small amount of alcohol produced evaporates when the bread is baked.

Lactic Acid Fermentation• In many cells, the pyruvic acid can be converted into lactic acid. It has almost the

same formula as alcoholic fermentation:

Pyruvic acid + NADH lactic acid + NAD+

• Lactic Acid is produced in your muscles during rapid exercise when your body cannot supply enough oxygen to your cells.• The buildup of lactic acid is what causes the burning sensation immediately after

vigorous exercise....like sprinting, biking, or swimming as fast as you can.

•Lactic acid is also used by many prokaryotes/bacteria. • It can be used in the production of cheese, yogurt, buttermilk, and sour cream.•Some popular ethnic foods produced using lactic acid fermentation are pickles, sauerkraut, and kimchi.

Overview of Your Energy Use• In vigorous physical activity, different methods of gaining energy are

used at different times:• Pretend that you are running:• For several seconds, the ATP already in your cells provides enough energy to

power you and your muscles.• From that point until about 90 seconds, muscles use lactic acid fermentation.• After 90 seconds, your body uses cellular respiration (glycolysis, the Krebs's

Cycle, and the ETC) to gain energy using oxygen. This energy is good for 15-20 minutes.• After that, your body begins to break down stored molecules such as glycogen

and fat for energy. This is why exercise helps with weight loss and weight control!

Comparison of Photosynthesis and Cellular Respiration

In Summary

•Photosynthesis and Cellular Respiration are essentially opposite processes.•Almost all organisms undergo cellular respiration, which produces energy and releases CO2 into the atmosphere.•Producers undergo photosynthesis which creates food (for themselves and consumers) and releases O2 into the atmosphere.