1. Photosynthesis and Cellular Respiration Moly-Mod Lab Tahoma Jr. High
8th Grade Science
Maple Valley, WA

2. We learned how
hydrocarbon fuels
are put together
and have energy
that can be released.
But we can’t eat oil,
or breathe in methane
and break it down
for energy.
Instead we use a similar chemical: carbohydrates.
But how are they made?

3. (use the short bonds AND set them aside)
Oil comes from old partially decomposed plants, so let’s “be a plant” for a while.
What do plants need every few days or they die?
WATER! – make 6 H2O molecules
(use the short bonds AND set them aside)
They also need a carbon source to put sugar together. What common gas has carbon in it?
That’s right! – CO2.
Make 6 CO2 molecules
Now we need an energy source that will break the bonds to allow the atoms to re-arrange themselves into a sugar molecule. What is it?

4. O2 – put them back together
Yup… the Sun !!
The sunlight will release the carbon to make a “skeleton” for the glucose (sugar) molecule. Break your six CO2 molecules apart and remove the carbon on each of them. What’s left without the carbon?
O2 – put them back together
(technically, this isn’t what happens, but we’ll keep things simple in this model)

5. You know plants give off nice, clean, fresh oxygen – but it breaks things down, so is a waste gas to plants during photosynthesis. (Later they will need some to break down their own food).
But for now, let’s put the O2 molecules off to the side and forget about them – they are waste. DO NOT TOUCH THEM UNTIL TOLD TO !!

6. As an FYI – since we’re dealing with molecules now, here’s a CHLOROPHYLL molecule:

7. (hint: don’t have two oxygens bonded to each other)
Now we can make the glucose (sugar) molecule. We’ve got 6 carbons, 6 water molecules, light and chlorophyll to work with. There are many combinations possible, so we’ll help you by giving you the carbon skeleton layout.
From there, there’s only ONE way to finish the molecule without having any bond holes open or bonds hanging out in space. Finish a glucose molecule…
(hint: don’t have two oxygens bonded to each other) C C C O C
all bonds
now are single
and it doesn’t matter if
they are short or long C C

8. carbon, hydrogen, lots of oxygen
Have your teacher check when you think you’re done. You should not have any atoms left over
(except for the six O2 molecules sitting on the side that were waste gas)
Count the atoms – what is the formula for glucose?
C6H12O6
that’s the formula for glucose
sugar is a “carbohydrate”:
carbon, hydrogen, lots of oxygen

9. GLUCOSE IS THEN CONVERTED TO STARCH FOR LONG-TERM STORAGE
Photosynthesis is probably the MOST important chemical reaction on Earth !!
It take sunlight energy (which can’t be stored) and turns it into storable chemical energy (glucose).
GLUCOSE IS THEN CONVERTED TO STARCH FOR LONG-TERM STORAGE

10. HERE’S A FAMILY PORTRAIT OF WORK GROUPS WHO COULD NOT PUT THEIR GLUCOSE TOGETHER FAST ENOUGH
extinct
fossilized plants
get it ?

11. C6H12O6 NOW WE’VE MADE A GLUCOSE MOLECULE.
SO WHAT ARE WE GOING TO DO WITH IT ?
C6H12O6

12. That’s right! – O2. Get those oxygens back
You are now going to “be an animal” for a while – but you can’t make your own food. You are a “consumer” – not a “producer”.
So, what do animals need to break down the food they eat for energy?
That’s right! – O2. Get those oxygens back
(you breathe them in) and then break down your glucose molecule to individual atoms.
Now think about what you give off as waste after breaking down your food. If you breathe onto a mirror you will see one of the chemicals. It is…?

13. Yup… water! H2O (make 6 H2O molecules)
Now, what gas are you breathing out (it’s the “opposite” of oxygen)?
That’s right! – CO2. Bond those six carbons together with the 12 oxygens you have left. You should get 6 CO2 molecules – and you’re back where you started from when you started as a plant!
You have just done Cellular Respiration and released its wastes in the process of breathing
X 6
6 X

14. CELLULAR RESPIRATION OCCURS IN CELL ORGANELLES (structures) CALLED
MITOCHONDRIA
(you should have learned about these in 7th Grade)

15. and then release carbon dioxide and water.
Animals use oxygen (brought to the cells by red blood cells in the blood) to break down glucose (also brought by blood)…
and then release carbon dioxide and water.
This diagram shows 6 extra waters – but they simply go in and come right back out again – so many diagrams/equations don’t have them included.

16. OXIDIZING HYDROCARBONS (alkanes) AND
CARBOHYDRATES RELEASES SIMILAR PRODUCTS

17. BUT WITH PHOTOSYNTHESIS AND RESPIRATION
THERE IS A TIDY COMPLETE RECYCLING OF MATTER
“free” light energy to trap
stored chemical energy
immediate chemical energy
to do work

18. The plants don’t make the sugar for the animals
Now… although plants do PHOTOSYNTHESIS and animals do RESPIRATION, who did the plants make their sugar for, and how do they get their energy for life?
The plants don’t make the sugar for the animals
(most of the time at least - nectar and fruit are “bribes” to pollinate and carry seeds away).
Plants make the sugar for THEMSELVES – and therefore, they must also do RESPIRATION.
Don’t forget this when you see diagrams. Plants do both photosynthesis and cellular respiration, but the important one is photosynthesis to make the sugar in the first place. Animals came along later and found a way to exploit this chemical energy source by eating plants.

19. MAKE SURE NOTHING IS MISSING !!
YOUR TEACHER WILL NOW LEAD A CLASS DEBRIEF ON PHOTOSYNTHESIS AND RESPIRATION EQUATIONS
COUNT THE ATOMS AND BONDS CAREFULLY AND PUT THEM IN THE BAG ALONG WITH THE PAPER LIST
MAKE SURE NOTHING IS MISSING !!

20. On the Chemistry Part B Test you should know how to write a balanced chemical equation for:
photosynthesis
and
cellular respiration
end show