why do plants

8/9/2019 Why Do Plants

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Why Do Plants

Have Leaves?

The picture at the right shows the International

Space Station as it will look when finished.People will live and work only in the relativelysmall, slender cylinders in the middle of theconstruction. The rest of the station -- the flat,black things -- are solar panels. These panelsconvert sunlight energy into energy usable bypeople inside the station.

Well, those solar panels are doing exactly themain thing done by mostleaves on mostkindsof green plants: They are capturing sunlightenergy, which is something needed by all livingthings, from bacteria to backyard trees topeople living in space. The problem of acquiringenergy is always one of the most important considerations any living thing has to deal with.

Therefore, a leaf's main job is usually this:

To present its broadsurface to the sun so that inside the leaf's' green tissue the almost-magicalprocess ofphotosynthesiscan take place.

PHOTOSYNTHESIS

Now, during photosynthesis, sunlight energy is used to power chemical reactions that combinewater with the air's carbon dioxide to form starchy carbohydrate. In other words, duringphotosynthesis, sunlight energy is stored in carbohydrate for later use. Here's the simplifiedchemical formula for photosynthesis:

6CO2 + 12H2O + sunlight ---> 6O 2 + C6 H12O6 + 6H2 O

or...carbon dioxide + water + sunlight --->

oxygen + carbohydrate + water

Some carbohydrate remains in the leaf but mostly it's transported elsewhere in the plant,perhaps into the stem, or maybe into special underground storage areas, such as the potatoplant's potatoes.

What the finished International Space Station will looklike

Image courtesy ofNASA


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THOUGHTS BEYOND THE CHEMISTRY

Don't lose sight of the fact that according to that wonderful chemicalformula above, during photosynthesis leaves such as the Red Mapleleaves at the left take two essentially invisible items -- carbon dioxide gas

and water -- and with energy from sunlight combine them into thesubstance making up the largest part of any plant you look at. Look at thebiggest tree and by golly it's mostly carbohydrate and water, and thatcarbohydrate has been produced from gas and water, with the processbeing fueled by sunlight.

Surely this is one of the most amazing facts in the Universe, but we seldomthink about it.

Also, notice that when we burn wood, basically we're doing thephotosynthesis formula in reverse. We're breaking down the carbohydrateand producing carbon dioxide gas and water, plus energy, which, likesunlight, feels hot and looks bright. In a real sense, first sunlight was

captured in the carbohydrate, then the carbohydrate was kept for a while inthe plant, and now as the plant burns the sunlight's energy is beingreleased again.

OTHER THINGS LEAVES DO

Leaves do serve other purposes, including the following:

y On the tree:o they create shade that cools the forest in the summero they keep wind from drying out the forest ecosystemo they provide food for a complex community of herbivorous organisms, including deer and many

kinds of insects, which in turn serve as food for insectivorous birds

y Fallen onto on the ground:o they are like a blanket that keeps the soil from temperature extremeso they keep the soil from drying outo they decay and enrich the soil with organic matter and nutrients that can be recycled

SOMETHING WORTH THINKING ABOUT

It's worth reflecting about the fact that if we humans were put into a room filled with nothing but carbon dioxide, we'd

die pretty quickly. In the same way, if all photosynthesizing plants were removed from the Earth, before long weoxygen-needing animals would die.

Therefore, when we speak of leaves photosynthesizing, we're referring to something profoundly important andspecial.

Now consider this: Right now humans are destroying Earth's photosynthesizing plant communities, especiallyrainforests and algae in the oceans (agricultural herbicides and many kinds of pollution drain into the oceans) as ifour lives did notdepend on them...


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Photosynthesis Equation

In photosynthesis, solar energy is converted to chemical energy. The chemical energy is stored inthe form of glucose (sugar). Carbon dioxide, water, and sunlight are used to produce glucose,

oxygen, and water. The chemical equation for this process is:

6CO2 + 12H2O + light C6H12O6 + 6O2 + 6H2O

6 molecules of carbon dioxide (6CO2) and 12 molecules of water (12H2O) are consumed in theprocess, while glucose (C6H12O6), six molecules of oxygen (6O2), and six molecules of water

(6H2O) are produced.

Photosynthesis in Plants

In plants, photosynthesis occurs mainly within the leaves. Since photosynthesis requires carbondioxide, water, and sunlight, all of these substances must be obtained by or transported to the

leaves. Carbon dioxide is obtained through tiny pores in plant leaves called stomata. Oxygen isalso released through the stomata. Water is obtained by the plant through the roots and delivered

to the leaves through vascular plant tissue systems. Sunlight is absorbed by chlorophyll, a greenpigment located in plant cell structures called chloroplasts. Chloroplasts are the sites of

photosynthesis. Chloroplasts contain several structures, each having specific functions:

Outer and inner membranes: protective coverings that keep chloroplast structures

enclosed.

Stroma: dense fluid within the chloroplast. Site of conversion of carbon dioxide to sugar.

Thylakoid: flattened sac-like membrane structures. Site of conversion of light energy tochemical energy.

Grana: dense layered stacks of thylakoid sacs. Sites of conversion of light energy to

chemical energy.

Chlorophyll: a green pigment within the chloroplast. Absorbs light energy.

Stages of Photosynthesis

Photosynthesis occurs in two stages. These stages are called the light reactions and the dark

reactions. The light reactions take place in the presence of light. The dark reactions do notrequire direct light, however dark reactions in most plants occur during the day.

Light reactions occur mostly in the thylakoid stacks of the grana. Here, sunlight is converted to

chemical energy in the form of ATP (free energy containing molecule) and NADPH (high

energy electron carrying molecule). Chlorophyll absorbs light energy and starts a chain of steps

that result in the production of ATP, NADPH, and oxygen (through the splitting of water).


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Oxygen is released through the stomata. Both ATP and NADPH are used in the dark reactions to

produce sugar.

Dark reactions occur in the stroma. Carbon dioxide is converted to sugar using ATP and

NADPH. This process is known as carbon fixation or the Calvin cycle. Carbon dioxide is

combined with a 5-carbon sugar creating a 6-carbon sugar. The 6-carbon sugar is eventually

broken-down into two molecules, glucose and fructose. These two molecules make sucrose or

sugar.

Photosynthesis Summary

In summary, photosynthesis is a process in which light energy is converted to chemical energy

and used to produce organic compounds. In plants, photosynthesis occurs within the

chloroplasts. Photosynthesis consists of two stages, the light reactions and the dark reactions.

The light reactions convert light into energy (ATP and NADHP) and the dark reactions use theenergy and carbon dioxide to produce sugar.

y In plants, photosynthesis is used to convert light energy from sunlight into chemicalenergy (glucose). Carbon dioxide, water, and light are used to make glucose, oxygen, andwater.

y Photosynthesis is not a single chemical reaction, but rather a set of chemical reactions.The overall reaction is:

6CO2 + 12H2O + light -> C6H12O6 + 6O2 + 6H2Oy The reactions of photosynthesis can be categorized as light-dependent reactions and dark

reactions.

y Chlorophyll is a key molecule for photosynthesis, though other cartenoid pigments alsoparticipate. There are four (4) types of chlorophyll: a, b, c, and d. Although we normallythink of plants as having chlorophyll and performing photosynthesis, many

microorganisms use this molecule, including some prokaryotic cells. In plants,chlorophyll is found in a special structure, which is called a chloroplast.

y The reactions for photosynthesis take place in different areas of the chloroplast. Thechloroplast has three membranes (inner, outer, thylakoid) and is divided into three

compartments (stroma, thylakoid space, inter-membrane space). Dark reactions occur inthe stroma. Light reactions occur the thylakoid membranes.

y There is more than one form of photosynthesis. In addition, other organisms convertenergy into food using non-photosynthetic reactions (e.g. lithotroph and methanogen

bacteria)y Overall Equation of Photosynthesis

y Definition : Photosynthesis is the intracellular anabolic process, characteristic of thegreen cells of plants in which carbohydrates are synthesized from carbon dioxide and

water in the presence of light and chlorophyll. In this process, light energy is convertedinto chemical energy and stored in carbohydrate molecules, while oxygen is liberated.

y The overall process of photosynthesis is represented by the following general chemicalequation.


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y

Photosynthesis (yucky chemistry details)

Hi. You are here to expand or reinforce your understanding of the

chemistry behind the biology, right?

(If not, get out while the gettin' is good.)

Like most "life processes", photosynthesis can be studied at various

levels ---

y the global level (how it's important to ecosystems),

y the plant part level (how leaves are organized for the process),

y and/or the cellular level (the chemical reactions occurring in

the chloroplasts of the plant cells in the leaves of the plant).

I did my best to illustrate the first two levels on the "big picture"photosynthesis page which you probably have looked at already. If

you haven't, take a gander now (or later) by clicking here .

OK, let's get to some detailed details.

Detail

#1

Photosynthesis occurs in organelles

called CHLOROPLASTS.

More specifically, half of the

photosynthesis chemistry stuff occursin a liquid inside the chloroplasts called

STROMA, & the other half of

photosynthesis occurs in stacks ofmembranes inside the chloroplast

The inside of a chloroplast

(seen through an electron microscope).


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Brain

Refresher

Before we get into more details, please recall that the overall chemical

equation for photosynthesis is the following:

6CO2 + 12H2O + light energy ---> C6H12O6 + 6O2 + 6H2O

or, in words:

carbon dioxide + water + light energy ---> glucose + oxygen + water

OK, onward ....

Detail#4

Detailed details about the light

reactions.

Of the substances in the overall

equation, the light reactions use light &water, & produce the oxygen "waste

product".

Remember that name "photolysis"?

That translates as light (photo-) splits

(-lysis). Turns out that the light energyis used to split the water molecule,

which produces H+ & O- ions. The

oxygen is released from the plant into

the air.

ATP & NADPH2 are energy molecules.

They will be used up during the dark

reactions, as will the H+ ions.

the light reactions

Footnote toDetail

#4

Upon close inspection of the

photosynthesis equation,

6CO2 + 12H2O + light energy --->

C6H12O6 + 6O2 + 6H2O

one may notice that two reactants (CO2

& H2O) contain oxygen. So, theskeptical folks out there would ask,

"How do you know that the oxygen we

breathe comes from the water

The answer:Scientists have used an isotope of

oxygen called O-18 to follow oxygen

atoms through the chemical maze of

photosynthesis. When they tagged the"O" of the CO2 none of the oxygen

being released from the plant contained

O-18. But when they tagged the "O"

in the H2O, all of the oxygen being

produced contained O-18.

So the O we breathe originates from

water molecules used by the plant


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molecule that is split in the light

reactions & NOT from the CO2?"

during photosynthesis.

Detail

#5

Don't be afraid of the dark ... reactions, a.k.a. "carbon fixation".

These reactions use the energy of the ATP & NADPH2 (produced in thelight reactions) to combine the H+ (also produced in the light reactions)

with CO2.

Now the end product we're after here is glucose. To build glucose weneed C, H, & O, which we have in our H+ & CO2. Nifty, huh? So there's a

bunch of reactions from which & a 3-carbon compound called PGAL is

produced. Think of PGAL as 1/2 of a glucose. Then, 2 PGAL's are

combined & WA-LA! A glucose is born!

SimpleSummar

y?

The overall equation (again) & what happens to what during this fun

chemical reaction ............

Simple, right?

Hints, clues & stuff to remember:

y The names for the 1st phase (light reactions, photochemical reactions,

photolysis) all contain clues that light is involved. So remember the names, &then ask yourself, "Self, how is light involved?" And then answer yourself,

"The energy in light is use to split water".


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y The only things you can get by splitting water (H2O) is hydrogen & oxygen.

The oxygen exits (is released from the plant). The hydrogen is used in thedark reactions.

y The dark reactions, or carbon fixation, must involve carbon (duh). The

reactant with the carbon is CO2. The CO2 is combined with the hydrogenleft over from the water split in the light reactions. First they form PGAL

(a 3-C compound), & then 2 of those PGAL's are converted to our favorite 6-

C compound ... glucose (C6H12O6). Some water molecules are produced as awaste product during this step.

y The dark reactions are powered by energy compounds formed during the

light reactions, namely ATP & NADPH2.

y P.S. PGAL stands for phosphoglyceraldehyde.

Review & Practice Questions

1. Which equation summarizes photosynthesis?

a) water + starch ---> glucose + glucose +

glucose

b) water + carbon dioxide ---> oxygen +

glucose + water

c) glucose + oxygen ---> water + carbon

dioxide + ATP

d) glucose + glucose ---> maltose + water

2. In what organelle does photosynthesis

occur?

a) the nucleus

b) chloroplasts

c) the vacuole

d) the cell wall

3. Four identical plants are grown under

different colored light bulbs. Under which

color will the release of oxygen gas be

slowest?

a) blue

b) green

c) orange

6. In what part of photosynthesis would the

tracer carbon-14 be used to study the path

of carbon through the chemical reactions?

a) the absorption of light by chlorophyll

b) the dark reactions

c) photolysisd) the light reactions

7. Atmospheric oxygen that is inhaled by

animals comes from:

a) carbon dioxide molecules split during the

light reactions

b) carbon dioxide split during the dark

reactions

c) water molecules split during the light

reactions

d) water molecules split during the darkreactions

8. What change occurs during photosynthesis?

a) solar energy is converted to chemical energy

b) kinetic energy is converted to chemical

energy

c) chemical energy is converted to radiant


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d) red

4. Which of the following would have the

smallest effect on the rate of photosynthesis

in a green plant?

a) carbon dioxide concentrationb) light intensity

c) oxygen concentration

d) water available

5. Which of the following is produced during

photosynthesis?

a) carbon dioxide

b) PGAL

c) DNA

d) lactic acid

energy

d) water is converted to chemical energy

For questions #9-18 choose the best

description from the list.

A - photochemical reactions only

B - carbon fixation reactions only

C - both the photochemical & carbon fixation

reactions

D - neither the photochemical & carbon

fixation reactions

9. Water molecules are split.

10. Carbon dioxide is used.11. PGAL is formed.

12. Light energy is absorbed by chlorophyll.

13. The reactions occur in the stroma of the

chloroplast.

14. The reactions occur in the grana of the

chloroplast.

15. Glucose is formed.

16. Enzymes are present.

17. Oxygen is released.

18. Oxygen gas is used.

Ihope that most of the details related

to this topic are a little clearer now.

Remember to visit the Photosynthesis"big picture" page when you get a

chance.

Stay happy & keep pluggin' away.

photosynthesis

"big picture"

main

index

Please e-mailquestions,

comments,

or siteproblems to

mr.lubey

Lubey'sBioHELP!

1999-today


11/12

1. Which equation summarizes photosynthesis?

a) water + starch ---> glucose + glucose +

glucose (HYDROLYSIS)

b) water + carbon dioxide ---> oxygen +

glucose + water

c) glucose + oxygen ---> water + carbondioxide + ATP (RESPIRATION)

d) glucose + glucose ---> maltose + water

(DEHYDRATION SYNTHESIS)

2. In what organelle does photosynthesis

occur?

a) the nucleus

b) chloroplasts

c) the vacuole

d) the cell wall

3. Four identical plants are grown under

different colored light bulbs. Under which

color will the release of oxygen gas be

slowest?

a) blue - fastest

b) green

c) orange - fast

d) red - faster

4. Which of the following would have the

smallest effect on the rate of photosynthesis

in a green plant?

a) carbon dioxide concentration

b) light intensity

c) oxygen concentration - a product does not

effect the rate of reaction, the other choices

are reactants

6. In what part of photosynthesis would the

tracer carbon-14 be used to study the path

of carbon through the chemical reactions?

a) the absorption of light by chlorophyll

b) the dark reactions - involve carbon dioxide

c) photolysisd) the light reactions

(note that c & d are the same thing so neither

is right)

7. Atmospheric oxygen that is inhaled by

animals comes from:

a) carbon dioxide molecules split during the

light reactions

b) carbon dioxide split during the dark

reactions

c) water molecules split during the light

reactions

d) water molecules split during the dark

reactions

8. What change occurs during photosynthesis?

a) solar energy is converted to chemical

energy - sunlight --> food energy

b) kinetic energy is converted to chemical

energy

c) chemical energy is converted to radiant

energy

d) water is converted to chemical energy


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d) water available

5. Which of the following is produced during

photosynthesis?

a) carbon dioxide - used

b) PGALc) DNA

d) lactic acid - in sore muscles do to anaerobic

respiration

For questions #9-18 choose the best

description from the list.

A - photochemical reactions only

B - carbon fixation reactions only

C - both the photochemical & carbon fixation

reactionsD - neither the photochemical & carbon

fixation reactions

9. Water molecules are split. A

10. Carbon dioxide is used. B - carbon

fixation, carbon dioxide

11. PGAL is formed. B

12. Light energy is absorbed by chlorophyll. A

- "photo" for light

13. The reactions occur in the stroma of the

chloroplast. B14. The reactions occur in the grana of the

chloroplast. A

15. Glucose is formed. B - last product,

formed from PGAL

16. Enzymes are present. C - all reactions

require enzymes

17. Oxygen is released. A - comes from the

split water

18. Oxygen gas is used. D - oxygen is a

product, not a reactant

why do plants

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