Photosynthesis

Like it or not, chemistry continues to sneak itself into our beloved biology course. This is because what distinguishes living things from nonliving things is the presence of cells, and cells are nothing but bags of chemicals with a multitude of chemical reactions occurring inside them. And furthermore, all the metabolic activities in cells are being directed by a famous bunch of chemicals we refer to as DNA. Compounds, chemicals, chemical reactions ... these are what produce the structures & functions within the basic units of structure & function for living things (cells).

Anyhoo ... our topic right now, namely photosynthesis, is arguably one of the most important chemical reactions occurring on the planet. Let's see why.

Let's begin with some basic questions & answers about photosynthesis.

QUESTION	ANSWER
What is photosynthesis?	Photosynthesis is a chemical reaction in which light energy is converted to chemical energy in glucose. It is the means by which the energy in sunlight becomes usable to living things. Living things can eat glucose, we can't eat sunlight.
Exactly why is photosynthesis so important?	Two big reasons. One product of photosynthesis is glucose (sugar), which provides the basis for most food chains. The second product of photosynthesis is oxygen which comes in handy if your happen to be an aerobic organism that requires oxygen for survival.
Where does photosythesis occur?	This question can be answered on various levels. On the planetary level, most photosynthesis occurs in the oceans, because the oceans occupy the vast majority (70%) of the earth's surface. In terms of organisms, photosynthesis occurs in autotrophes that contain the green pigment chlorophyll. This would include the entire plant kingdom, "half" of the Protista (the algae), & "half" the Moneran Kingdom (the blue-green algae). On the multicellular level, photosynthesis occurs in the leaves of plants. We will take an up-close & personal look at these in a bit ... On the cellular level, the reactions for photosynthesis occur in organelles called chloroplasts (in eukaryotic cells). Blue-green algae (which are prokaryotic) carry-out the photosythesis reactions in the cytoplasm.
What raw materials are needed for the process of photosynthesis?	As mentioned in the above box, chlorophyll must be present. (Chlorophyll absorbs solar energy). Sunlight is necessary. That's why we call it PHOTOsynthesis. And from the environment, photosynthetic organisms must get water & carbon dioxide.
To which "life function" is photosynthesis related?	Photosynthesis falls under nutrition, autotrophic nutrition to be exact. Nutrition has to do with how organisms obtain the materials necessary for energy, growth, etc. We call these necessary materials nutients. "Autotrophic" means that the organism makes the nutrients on its own using materials in the environment (see the above box).
How can we summarize photosynthesis ?	In words, remember this: carbon dioixde + water + light energy ---> glucose + oxygen + water As a chemical reaction it's like so: CO₂ + H₂O + light energy ---> C₆H₁₂O₆ + O₂ + H₂O We'll get back to this equation later ...
What else should we remember about photosynthesis?	Well, it's an important component of the Oxygen-Carbon Dioxide Cycle. Organisms that carry-out photosynthesis are like oxygen factories. At the same time they are removing CO₂ from the atmosphere. Increasing levels of CO₂ appear to go hand in hand with increasing global temperature ("Global Warming"). So damaging the ecosystems where photosynthesis occurs (i.e. the rainforests, the oceans) reduces the amount of oxygen being pumped into the air (that's a bad thing) & slows the rate at which carbon dioxide is removed from the atmosphere (which also appears to be a bad thing).

For the most part, when thinking of organisms that carry-out photosynthesis, it is safe to picture plants. It's not that we have any right to ignore the photosynthetic Protists & Monerans, but, well, it sure is easier to picture a tree than a Euglena. Don't you agree?
Remember, it turns out that most photosynthesis on Earth is occurring in the oceans, & the organisms in the oceans doing the job happen to be classified into the Protist Kingdom --- algae (seaweeds), diatoms, etc. So let's not forget them completely.

Having said that, let's take a look at plants & how they are adapted for photosynthesis.
The leaves of plants are the photosynthesis factories. The structure of a "typical" leaf is illustrated in the following table.

LEAF LAYERS

This pic shows a leaf cross section with only the layers labelled.
Please note that each roundish shape is a single plant cell. So each layer is a plant tissue (a group of cells).
The top & bottom layers are both referred to as an epidermis (outer layer). The upper epidermis (colored purple) & the lower epidermis (colored red) basically are protective layers. They are coated with a waxy surface, the cuticle, which further protects the leaf & prevents excess water loss.
The area sandwiched in the middle of the leaf is referred to as "mesophyll". There are two layers in the mesophyll:
1) the palisade layer - packed tight w/ oval cells
2) the spongy layer - this area contains circular cells that are spread out --- allowing gases to circulate within the leaf

THE VASCULAR BUNDLE (VEIN)

If you've ever looked closely at a leaf you've noticed some pattern of lines running through it.
These lines are veins, or "vascular bundles" (the side of one bundle is colored light green).
Inside the vascular bundle are tubes that transport materials from place to place in the plant.

The xylem (colored blue) transport water & minerals from the roots, through the stem, & into the leaves
(remember ... we need water for photosynthesis).

Phloem (colored orange) are tubes that transport "food" through the plant. This "food" is basically sugary water (sap). The sugars are synthesized (made) during photosynthesis.

STOMATES & GUARD CELLS

Scattered about the lower epidermis are small openings called stomata. It is through these holes that carbon dioxide enters the leaf, & oxygen & water vapor exit.

Each stomate is surrounded by a pair of guard cells. When guard cells swell, the stomates close, when the guard cells shrink, the stomates are open. Plants respond to changes in temperature & humidity by opening or closing their stomata (an example of maintaining ... homeostasis).

CHLOROPLASTS

All of the "black dots" in the picture represent chloroplasts, which are the cell organelles in plant cells where the chemical reactions of photosynthesis take place.

You should notice that the vast majority of chloroplasts are found in the cells of the mesophyll. Even more specifically, the palisade layer has the most chloroplasts, so the greatest amount of photosynthesis occurs in that leaf layer.

Now let's revisit the summary equation for photosynthesis & note how each of the raw materials end up in the chloroplasts so that the whole photosynthesis deal can go down.

Now let's do the same, except pay attention to what happens to the products of photosynthesis.

I should mention that glucose may be used for things other than energy. For example,
a whole bunch of glucoses could be combined (by dehydration synthesis) to form cellulose,
which is the structural material of the cell wall surrounding plant cells. It could also be
converted to starch (again by dehydration synthesis) & stored by the plant for future use.

Alright, that is "the big picture" --- what photosynthesis is, why it's important, & how plants are adapted to carry it out. I'm afraid there is more chemistry to it, I have placed that info on a separate page, the "yucky chemistry details". Check it out when you have a chance, but not on a full stomach.
Before you do anything else, have a go at these sample questions about the material we learned here. Answers & explanations can be found in the extremely secret answer area (no peeking!).

SAMPLE QUESTIONS
Write down your answers (old-fashioned I know), & then dip into the secret answer area.

MATCHING:
1. organic compound produced during photosynthesis
2. source of energy for photosynthesis
3. is both a reactant & product of photosynthesis
4. an organism that can synthesize organic materials using materials in its environment
5. the cell organelle where photosynthesis occurs
6. the green pigment in plant cells that absorbs sunlight
7. photosynthestic Protists
8. photosynthestic members of the Kingdom Monera

A. algae
B. autotrophe
C. blue-green algae
D. chlorophyll
E. chloroplast
F. glucose
G. sunlight
H. water

TOP SECRET ANSWER AREA

Answers to MC - correct choices in ORANGE
1. The waxy coating on the surface of a leaf is the ....
a) epidermis
b) cuticle
c) palisade layer
d) chlorophyll

2. Water is lost from the leaves of plants through openings called ...
a) root hairs - ON ROOTS, NOT NEAR LEAVES
b) xylem - TUBES THAT TRANSPORT WATER IN THE PLANT
c) lenticels - OPENING ON WOODY STEMS FOR GAS EXCHANGE
d) stomates

3. The conversion of light energy to chemical energy occurs in the cells of ...
a) algae - ANSWER MUST BE AN AUTOTROPHE
b) invertebrates - ANIMALS (HETEROTROHPIC)
c) fungi - HETEROTROPHIC
d) teachers - ;)

4. The raw materials needed for photosynthesis include ...
a) oxygen & water - THESE ARE PRODUCTS
b) carbon dioxide & water
c) glucose & oxygen
d) glucose & carbon dioxide

5. Which word 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

6. Autotrophic activity in plant cells occur in organelles called ...
a) cytoplasm - WATERY STUFF, NOT AN ORGANELLE
b) chloroplasts
c) ribosomes - WHERE PROTEINS ARE PRODUCED
d) nuclei - DIRECTS CELL ACTIVITIES, CONTAINS DNA

Answers to PIC QUESTIONS
Base the next 7 questions on the diagram below.

1. Write the number & name of the principle area of photosynthesis.
ANSWER: 3 - the palisade layer
2. Write the number & name of the structure(s) that regulate the opening & closing of stomates.
ANSWER: 8 - guard cells
3. Which number indicates where oxygen exits the leaf?
ANSWER: 9 (stomata) - same place carbon dioxide enters
4. Which numbers indicate vascular tissues, which transport materials to & from the leaf? What are the names of the vascular tissues?
ANSWER: 5 - xylem, & 6 - phloem
5. Write the number & function of the cuticle.
ANSWER: 1 - protects inner layers & prevents excess water loss
6. The structure of which area in the leaf allows for the diffusion of gases (carbon dioxide & oxygen)? Give the number & name.
ANSWER: 4 - spongy layer
7. What do the "black dots" represent?
ANSWER: chloroplaSts, the organelles where the photosynthesis boogie occurs

Answers to Matching
1. F 2. G	3. H 4. B	5. E 6. D	7. A 8. C