Tuesday, November 2, 2021

Cellular Respiration Overview

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Model and understand aerobic respiration demonstrating the flow of matter and energy out of a cell and explain energy transfer systems. Also, compare aerobic respiration to alternative processes of glucose metabolism.

Article includes ideas, images, and content from Troy Smigielski (2021-10)

Cellular Respiration Overview
(Wait! Are we talking about phones again?)

While idea of breathing smartphones is intriguing… nah…

"Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from oxygen molecules or nutrients into adenosine triphosphate, and then release waste products (Source, 2021-11).

What does that mean?

Alright, think about a few things and let's see where we end up!

Glucose is created in photosynthesis.

Humans don’t necessarily use that exact glucose, but we do consume glucose as a basic unit of energy.

To gain energy (ATP is the energy cells need) from the glucose, it needs to be broken down.

Thinking about photosynthesis, we can summarize:

6CO2 + 6H2O   ---->   C6H12O6 + 6O2

The products of photosynthesis are the reactants of cellular respiration.

C6H12O6 + 6O2   ---->   6CO2 + 6H2O + ATP

In essences, ATP is used up in photosynthesis to make glucose and glucose is used up in cellular respiration to make ATP. And… ATP is the substance that powers cellular functions (including muscle contractions).

Photosynthesis provides
oxygen for cellular respiration to happen. Photosynthesis also produces glucose, but this glucose is not always the direct source of the glucose needed in C.R.

Cellular respiration provides carbon dioxide, CO2, for photosynthesis to happen. Cellular respiration also produces water, but this water is not the direct source of the water needed in photosynthesis.

Thus, we can say that Photosynthesis and cellular respiration work together in a cycle.

Once more, think a little…

What is the main goal of photosynthesis?

If photosynthesis and cellular respiration are the opposites, what do you think the main goal of cellular respiration is? 

The main goal of photosynthesis is to build sugars. The main goal of cellular respiration is to extract energy in the form of ATP by breaking down sugars. That is to say, cellular respiration creates ATP by breaking down sugars.

In cellular respiration, we create:

  • CO2 which is exhaled
  • water
    (these are byproducts) 

  • and the real goal, ATP as a source of energy.

Most of this process happens in the mitochondrion.

There are three major steps in cellular respiration.

The 3 steps in cellular respiration are:
  1. Glycolysis
  2. Krebs cycle (which is also called the Citric Acid Cycle)
  3. Electron transport chain (ETC)
Glycolysis happens outside of the mitochondrion in the cytosol. The remaining steps—the Krebs cycle and the Electron transport change take place in the mitochondrion. Regardless of where they happen, all 3 steps of cellular respiration create ATP (energy).

Reactants and Products
    Inputs       and     Outputs

Look at the diagram (above and below) and consider what goes in and what comes out?


C6H12O6 + 6O2   ---->

Going in (reactants) are Glucose (C6H12O6) and Oxygen (O2)
  • Glucose is used in the glycolysis step
  • Oxygen goes in to the Electron transport chain step
---->   6CO2 + 6H2+ ATP

Coming out (products) are carbon dioxide (CO2,) water (H2O), and ATP:
  • Carbon dioxide is a product of the Krebs cycle step. (Krebs cycle is also called the citric acid cycle)
  • Water is a product of electron transport chain step.
  • ATP is a product of all three steps.

In-between substances include Pyruvate and the compounds dealing with the electron movement, NADH and FADH2.

Pyruvate is produced through glycolysis and is passed on to the mitochondrion.

You should also notice that there are some electrons e- and hydrogen ions H+ that move around…

The electron carriers in cellular respiration are NADH and FADH2.

More information on NADH can be found here. Likewise more information on FADH2 can be found here.

Even more, fancier?

Enough already! Let's get on with Cellular Respiration, okay? 

Let's have a look at the flow of… stuff… through cellular respiration… One more time!

In the beginning, there was photosynthesis that produced glucose…


1. Glycolysis (occurring in the cytosol)… (Wait… what is cytosol?) See More Notes at the end of the page.

Source, 2021-11
  • takes in glucose and breaks it down
  • producing a little ATP, some NADH, and Pyruvic Acid
  • and passing the Pyruvic acid on to the Krebs Cycle
  • and passing the NADH on to the Electron Transport System
  • and then provides the ATP to the cell for other cellular functions.

2. Krebs Cycle (occurring in the mitochondria)… (also called citric acid cycle)

  • takes in Pyruvic Acid and
  • producing ATP and NADH
  • and passing the NADH on to the Electron Transport System
  • and passing the NADH on to the Electron Transport System
  • and producing a waste byproduct (carbon dioxide) 
  • and then provides the ATP to the cell for other cellular functions.

3. Electron Transport System (occurring in the mitochondria)…

  • takes in NADH and oxygen
  • producing ATP
  • and producing a waste byproduct (water) 
  • and then provides the ATP to the cell for other cellular functions.

Source, 2021-11

Wow… just… wow!

Okay, so we have three steps. Normally, the three steps are looked at according to a specific classification.

The 3 steps are classified by whether they require oxygen or not.

If they do require oxygen, they are called aerobic.

If they do not require oxygen, they are called anaerobic.

Glycolysis is anaerobic, which means it happens without oxygen. The Krebs cycle and ETC are aerobic, which means they need oxygen in order to function.

 If you are undergoing anaerobic respiration, you cannot gain the ATP from the processes that are aerobic.


What is Cytosol?

Source, 2021-03

The cytosol is the liquid medium contained within a cell. The cytosol is a component of the cytoplasm. The cytoplasm includes the cytosol, all the organelles, and the liquid contents inside the organelles. ... The main component of cytosol is water. (Source, 2021-03)

The Whole Process!

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