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Cellular respiration intro

Content objective:

Lots of things are always going on inside our cells.
All those things involve moving parts!
So how do cells turn the food we eat into power?
Let’s start by seeing what really is happening inside every cell in our bodies:

Vocabulary objective

respiration, circulation, cellular respiration, aerobic, anaerobic

Building on what we already know

We’ve already learned that all life on earth is made of cells, and inside every cell there are chemicals that move, turn, twist, and otherwise do things.


Bid idea: Cells are organic machines. Molecules are constantly being built, broken down, or transported. Every action takes energy. Where does this energy come from? The process of how a cell gets and uses energy is called cellular respiration.


respiration = breathing in and out air. Brings oxygen into your lungs, then into your bloodstream.

circulation = carries oxygenated blood from your lungs to all body cells

cellular respiration = how cells use oxygen to release chemical energy from food molecules

aerobic = a body process that requires O2 (oxygen gas)

anaerobic = a body process that doesn’t require O2

Main ideas: Why use ATP?

We eat food molecules with lots of energy – sugars, carbs, fats, and proteins. Yet when we look inside our cells, they don’t use these molecules.

And instead, each cell process uses a less powerful molecule, ATP,  for energy.

What’s going on?

atp molecule

Seems weird – why not just directly use the sugars, fats or proteins?

But wait, this does make sense: Imagine having only a $1000 bill, and you want to buy stuff at the store.

Is that practical? No. Instead, we do this:

We break the large bill down into smaller bills which are easier to use.

The same is true for activities inside out cell that take energy.

Each sugar or fat molecule contains a lot of energy, yet each cell process event only requires a tiny bit of energy.

So cells do the same thing; they break a big bill into many little bills, so to speak.

Here we see a cell breaking down a single (high energy) sugar molecule down into many little (lower energy) ATP molecules.
Glucose convert to ATP

So sugar is like a $1000 bill, and ATP is like a $20 bill.

ATP works like a rechargeable battery

In the low power state this has 2 phosphate parts – called ADP.

    The D is short for “di” (two)

In the high power state this has 3 phosphate parts- ATP

    The T is short for “tri” (three)

Here we see an ADP molecule being turned into an ADP molecule.

By definition, this means adding one more ‘p’.

These aren’t really letters, of course, they are molecules.

This doesn’t happen spontaneously – it requires energy.

ADP to ATP GIF cellular respiration

from giphy.com

Where does the energy come from? From the food we eat!

Hydrolysis of ADP

In this process water is used to split apart ATP into ADP and Pi. This process releases energy that the cell can use.

A chemical reaction in which we use water to split apart molecules is called hydrolysis.

ATP + H2O → ADP + Pi   [ + energy ]

( Pi = inorganic phosphate)



Ideally if you break a $1000 bill you should get fifty $20 bills.

No money lost. If you ended up with than $1000, you’d have lower efficiency.

But when we break down the energy in a sugar, when we add up all the ATP energy, we only have about 38% of that energy.

Where did the rest go?

The rest of the energy is lost as “waste heat.”

No real-world-process is 100% efficient. Every process loses some energy as heat. In fact, that is one of the major laws of the universe – the laws of thermodynamics.

But that’s okay for 2 reasons:

A. Some of this waste heat is useful – it keeps our bodies at 98.6 degrees F, which is necessary for life.

B. Hundreds of millions of years of animal evolution have adapted us to live within these conditions; it is expected and normal. As long as we get more calories each day, we’re fine.

Slide 1: Burning fuel to make energy


photosynthesis vs cellular respiration

from the Amoeba sisters

Slide 2: A body’s energy budget

Slide 3: Harvesting energy

Slide 4: The energy needs of life

Slide 5: What do we need to make energy?

Slide 6: What happens if oxygen is missing?

Slide 7: Anaerobic respirarion

Sample questions

Feb 2016 MCAS:  In cells, aerobic respiration (cellular respiration in the presence of oxygen) is more efficient than anaerobic respiration (cellular respiration in the absence of oxygen). This is because aerobic respiration produces more of which of the following substances?

A. ATP        B. DNA      C. glucose      D. protein

Learning Standards

Massachusetts Curriculum Frameworks: Biology

8.MS-LS1-7. Use informational text to describe that food molecules, including carbohydrates, proteins, and fats, are broken down and rearranged through chemical reactions forming new molecules that support cell growth and/or release of energy.

HS-LS1-7. Use a model to illustrate that aerobic cellular respiration is a chemical process
whereby the bonds of food molecules and oxygen molecules are broken and new
bonds form, resulting in new compounds and a net transfer of energy.

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