Photosynthesis

I. Chemical Energy and ATP

A. Energy is the ability to do work. Nearly every

activity, whether in society or organisms, depends on

energy. When a car runs out of gas (chemical energy),

it comes to a stop. When the electricity in our homes is

out, we come to realize just how many things depend

on that electrical energy. Organisms are no different –

nearly every activity requires energy. Without energy,

life would cease to exist.

1. Energy comes in many forms – light, heat,

chemical, electrical.

2. Cells use many different chemical compounds for

energy. Chemical energy is stored in the bonds

between the atoms of compounds. When the

bonds are broken and rearranged, energy can be

released as new bonds are formed that are at a

lower energy state than the original bonds.

3. One of the most important energy compounds used

by cells is ATP, which is adenosine triphosphate.

a. ATP consists of adenine, a 5 carbon sugar called

ribose, and three phosphate groups.

b.

c.

d.

e.

f.

g.

Adenine Ribose 3 phosphate groups

b. Energy is released from ATP by breaking

the bond between the last two phosphate

groups, which results in the compound ADP

(adenosine diphosphate).

c. In this way, ATP is like a rechargeable

battery. As the cell has energy available, it

can store it by adding phosphates onto ADP

molecules, making ATP. When it needs

energy, it can remove a phosphate from

ATP to release energy.

d. This makes ATP exceptionally useful as a

basic energy source for all cells.

4. What are some of the activities that require ATP?

a. Active transport – For example, ATP drives

the sodium-potassium ion pump.

b. Movement –ATP provides the energy for

motor proteins that contract muscles and

power cilia and flagella.

Adding a phosphate to

ADP to make ATP is like

recharging a battery

c. Making Molecules – ATP powers the

production of proteins and other

macromolecules; it also powers other chemical

responses in the cell.

d. Light – fireflies, bioluminescence

5. Ironically, cells aren’t jammed with huge amounts

of ATP at once. It’s not really great at storing

large amounts of energy over long periods of time.

Other molecules, like sugars, are better at that

task. So it’s more efficient to keep a small amount

of ATP on hand and keep cycling between ATP

and ADP as needed by using energy from those

sugar molecules.

B. Where do you get ATP?

1. Cells must produce ATP. They don’t have it to

start. So how do you get ATP? From the chemical

energy stored in the food we eat.

a. If you are a heterotroph, you eat other

organisms, either plants or animals or both, to

get the chemical energy you need. Fungi and

many bacteria decompose the remains of

organisms to get chemical energy.

Remember: Food = chemical energy

b. If you are an autotroph, you are able to make

your own food, usually from the energy of the

sun. Plants, algae, and some bacteria are able

to use photosynthesis to convert energy from

the sun and store it in molecules that make up

food (glucose). This is a major achievement:

solar energy (sunlight) is converted to

chemical energy (food molecules).

c. Think about the word photosynthesis. From

the Greek, photo means “light” and synthesis

means “putting together” or making

something. So photosynthesis means “using

light to put something together.”

1. Not all autotrophs use photosynthesis.

Some organisms (so far, mostly some

bacteria) use a process called

chemosynthesis. What do you think the

word chemosynthesis means?

2. Chemosynthetic organisms

(chemoautotrophs) are able to produce

carbohydrates from inorganic molecules

such as hydrogen sulfide.

3. Many of these organisms are found in

very extreme environments – bottom of

the ocean, deep sea volcanic vents, acidic

hot springs. But they’ve also been found

in tidal marshes and in the bottom of

swamps.

d. Remember, all energy in food, whether it’s

made in plants or consumed, originates from

the sun. It’s the producers’ ability to capture

that energy and convert it into food that

makes all other life forms dependent on them.

II. Photosynthesis: An Overview

A. Chlorophyll and Chloroplasts – How is light energy

captured?

1. Energy from the sun travels to the Earth in the

form of light. White light, or what we call

sunlight, is actually a mixture of many different

wavelengths of energy. Some of these wavelengths

are visible to us – ROYGBIV. Others are not –

infrared and ultraviolet for example.

2. Pigment: molecules that absorb certain

wavelengths of light and reflect others

a. Whatever wavelength of light that is reflected

is the color that you see

b. Plants use different pigments to absorb light

and capture the energy.

3. The main pigment used by plants in

photosynthesis is chlorophyll. There are two types

of chlorophyll found in plants, a and b.

a. Chlorophyll a absorbs light very well in the

violet and orange-red areas of the light

spectrum, and a little bit in the blue areas.

b. Chlorophyll b absorbs light very well in the

blue and orange-red areas, and a little bit in

the violet areas.

c. What’s missing? Neither type of chlorophyll

absorbs light very well in the green region of

the spectrum.

d. This gives plants their green color –

remember, what isn’t absorbed is reflected,

and that is what you see.

e. What happens in the fall to give us the

beautiful colors? Chlorophyll breaks down

first, and now the other pigments in the plants,

called carotenoids, can be seen. Carotenoids

are typically red, orange, or yellow pigments.

They are usually masked by the abundant

amount of chlorophyll in the plants.

4. Chloroplasts – remember that this is the organelle

where photosynthesis will take place.

a. Thylakoids: saclike membranes inside

chloroplasts

b. Thylakoids are interconnected and arranged

in stacks called grana (singular stack is a

granum).

c. Chlorophyll is located in the thylakoid

membranes.

d. Outside of the thylakoids is a fluid called the

stroma.

5. Since chlorophyll absorbs light, it is absorbing

energy. When it absorbs the light, some of that

energy is transferred directly to electrons in the

chlorophyll molecule itself. This raises the energy

level of the electrons, so light energy can be used to

supply a steady supply of high-energy electrons.

THIS IS WHAT MAKES PHOTOSYNTHESIS

WORK. If those electrons didn’t jump energy

levels and become “excited,” photosynthesis

wouldn’t happen and life on the planet would be

very different indeed!

B. You wouldn’t grab a hot metal pan right out of the

oven with your hands – the pan is too hot for your

hands to handle and you would be burned. So you use

an oven mitt to handle the heat of the pan to protect

your hand. In the same way, high energy electrons

made by chlorophyll are too “hot” to handle and

require a special “carrier” to move them from place to

place. The plant cell’s “oven mitts” are called electron

carriers.

1. Electron carrier: a compound that can accept

a pair of high energy electrons and transfer

them, along with most of their energy, to

another molecule

2. NADP+ (nicotinamide adenine dinucleotide

phosphate) is one of these electron carriers.

NADP+ accepts and holds 2 high energy

electrons, along with a hydrogen ion (H+).

3. This converts NADP+ to NADPH. This is one

way to trap some of the energy from sunlight

into chemical form. NADPH can then carry

the high-energy electrons (produced when

chlorophyll absorbed light) to chemical

reactions elsewhere in the cell.

4. High-energy electron carriers are used to help

build a variety of molecules the cell needs,

including carbohydrates like glucose.

C. Photosynthesis is complicated. There are many steps.

But the overall result is not. In a nutshell:

1. Photosynthesis uses the energy of sunlight to

convert water and carbon dioxide into high

energy sugars (glucose) and oxygen.

2. The reaction of photosynthesis is below.

Please note which compounds are the products

and which are the reactants.

6 CO2 + 6H2O light

C6H12O6 + 6O2

carbon dioxide water glucose oxygen

3. Photosynthesis consists of two sets of

reactions, the light-dependent reactions and

the light-independent reactions.

a. The light-dependent reactions require the

direct involvement of light and the light-

absorbing pigments. They use energy

from sunlight to produce energy rich

compounds like ATP and NADPH. Light-

dependent reactions happen in the

thylakoid membranes of the chloroplast

and require water.

b. The light-independent reactions use the

ATP and NADPH molecules from the

light-dependent reactions to produce high

energy sugars from carbon dioxide. No

light is required, and these reactions take

place in the stroma.

As you can see, the two sets of reactions work together to capture the

energy of the sunlight and transform it into energy rich carbohydrates.

III. Details of Photosynthesis : The Light Dependent

Reactions – What happens?

A. The light dependent reactions use the energy of

sunlight to produce oxygen and convert ADP and

NADP+ to the energy carriers ATP and NADPH.

1. Thylakoids contain clusters of chlorophyll and

proteins known as photosystems.

2. Photosystems absorb sunlight and generate high-

energy electrons. The electrons are then passed to

a series of electron carriers embedded in the

thylakoid membrane.

B. There are two photosystems (I and II). They are

named in the order that they were discovered. This

part of photosynthesis is all about following the

electrons. Ironically, we begin with photosystem II.

1. As light is absorbed by photosytem II, electrons

are energized in the chlorophyll. More and more

high energy electrons are passed to the electron

transport chain.

2. Electron transport chain: a series of electron

carrier proteins that shuttle high energy electrons

during ATP generating reactions

3. Why doesn’t the chlorophyll in photosystem II run

out of electrons? At the same time these high

energy electrons are passed onto the electron

transport chain, enzymes of photosystem II are

splitting water molecules.

a. Water is split into hydrogen ions (H+),

electrons, and oxygen.

b. The electrons replace the high energy

electrons that have been passed from the

chlorophyll to the electron transport chain.

c. As the electrons are taken from the water, the

oxygen that is left behind is released into the

atmosphere. (This is the main source of

oxygen in the atmosphere and we need it to

survive.)

d. What about the H+ ions from the water? They

are released inside the thylakoid.

4. As electrons move down the electron transport

chain, their energy is used by the protein

molecules to pump H+ ions from the stroma into

the thylakoid space.

5. At the end of the electron transport chain, the

electrons are then passed to a second photosytem

called photosystem I.

6. When the electrons get to photosystem I, they

don’t have as much energy as they did before since

some was used to actively transport hydrogen ions

into the thylakoid space. So the pigments in

photosystem I use the energy of the sun to

reenergize the electrons.

7. The reenergized electrons enter a second electron

transport chain that is very short. At the outer

surface of the thylakoid membrane, this chain

transfers the electrons to NADP+ which is in the

stroma. In addition to the electrons, NADP+ picks

up H+ ions in the stroma, making NADPH. This

NADPH (full of energy now) becomes very

important in the second part of photosynthesis, the

light-independent reactions.

8. Wait a minute! What’s going on with all those H+

ions that were left behind when water split? And

didn’t we just pump a bunch more into the

thylakoid space?

a. All those H+ ions in the thylakoid space make

that area positively charged and the stroma is

negatively charged in comparison.

b. This gradient, both in charge (positive and

negative) and in H+ ion concentration, between

the stroma and the thylakoid space provides

the energy to make ATP.

c. H+ ions can’t cross the thylakoid membrane

directly. They must use a protein in the

membrane called ATP synthase which allows

H+ ions to pass through it.

1. When the H+ ions pass through, they

force the ATP synthase to rotate.

This rotation causes ATP synthase to

bind an ADP and a phosphate group

together, making ATP.

2. This means that at the end of the light

dependent reactions, you have both

ATP and NADPH. These energy rich

compounds will be needed to make

the carbohydrates during the light-

independent reactions.

IV. Photosynthesis: The Light-Independent Reactions –

Producing Sugars

FYI – Also known as the Calvin Cycle or sometimes the

Dark Reactions

A. Recall that the light-dependent reactions resulted in

ATP and NADPH. During the light-independent

reactions (Calvin Cycle) plants use the energy that

Light-Dependent Reactions: take place in the

thylakoid of the chloroplast. They use energy

from sunlight to make ATP and NADPH.

ATP and NADPH contain to build high energy

carbohydrate compounds (sugars).

1. Why not just use the APT and NADPH for energy

directly? Why bother making another compound?

a. ATP and NADPH are not stable enough to

store energy for a long period of time. They

are only good for a few minutes.

b. Carbohydrates like glucose and other sugars

are very stable and can be stored for a long

time.

B. The Calvin cycle begins with carbon dioxide entering

from the atmosphere. Recall that CO2 has one carbon

atom. This part is all about following the carbon.

1. An enzyme in the stroma combines 6 carbon

dioxide molecules with 6 other carbon compounds

that were already present in the chloroplast. Each

of these carbon compounds has 5 carbon atoms

(for a total of 36 carbon atoms).

2. The same enzyme rearranges all these atoms to

make 12 molecules that contain 3 carbons each

(still 36 carbon atoms).

3. Other enzymes are going to take these compounds

with 3 carbons each and convert them into higher

energy forms in the rest of the cycle. The energy

to do this comes from ATP and the high energy

electrons in NADPH.

4. In the middle of the cycle, 2 of the 12 carbon

compounds are removed. Why is this important to

mention? These removed carbon compounds will

be the building blocks that the plant cell will use to

make glucose, one of the main sugars used for

energy.

a. These same carbon compounds will be used to

make other sugars, lipids, amino acids, and

many other compounds.

b. In this way, the Calvin cycle contributes to all

the other products needed for the plant to grow

and perform the basic needs of metabolism.

5. The 10 remaining carbon molecules (with 3

carbons each, so a total of 30) are converted back

into 6 carbon molecules each with 5 carbons.

Remember them? We needed these molecules to

combine with CO2 at the beginning of the cycle to

get the process started. In this way, the next cycle

is begun again.

C. Summary of the Calvin Cycle

1. The Calvin cycle uses 6 molecules of carbon

dioxide to produce a single 6 carbon sugar

molecule, typically glucose.

2. Recall the overall reaction of photosynthesis:

6CO2 + 6H2O LIGHT

C6H12O6 + 6O2

carbon dioxide water glucose oxygen

3. The reactions involved are made possible from the

energy in the ATP and NADPH produced in the

light-dependent reactions.

4. When you eat a plant or something else that ate a

plant, you are getting the energy and raw

materials stored in these compounds.

V. Factors that Affect Photosynthesis

A. Many factors can impact the rate of photosynthesis.

The most important are temperature, light intensity,

and the availability of water.

1. Temperature: photosynthesis depends on

enzymes that work best between 0° C and 35° C.

a. If the temperature is above or below, the

enzymes may be affected and photosynthesis

can slow down. At extremely low

temperatures, photosynthesis may stop

completely.

b. Think about what can happen to enzyme

shape at temperatures outside their range –

can it function if shape changes?

2. Light Intensity: high light intensity increases the

rate of photosynthesis

a. There is a maximum rate that once

reached can’t be surpassed.

b. No light = no photosynthesis

3. Water: it’s a raw material, so a lack of water can

slow or even stop photosynthesis

a. water loss can damage plant tissues

b. plants in dry conditions, such as the desert,

have waxy coatings to reduce water loss

c. other biochemical adaptations impacting the

efficiency of photosynthesis are also possible

B. Extreme Conditions and Photosynthesis

1. Some plants under extreme conditions such as

intense light and high heat have further

adaptations that allow photosynthesis to happen in

these extreme conditions.

2. Some regulate the openings in the leaves (called

stomata) to remain shut during the day and open

at night (cooler conditions) to reduce water loss.

But it also reduces the availability of carbon

dioxide. So these plants have different chemical

pathways that allow them to perform

photosynthesis while still minimizing water loss.