Cellular Respiration and Diffusion

This chapter explains the basic concepts regarding oxygen, carbon dioxide, energy, diffusion and pressure. This is done initially in the context of unicellular organisms and later with multicellular organisms.

Keywords

Cellular respiration; oxygen; carbon dioxide; unicellular; multicellular; diffusion; pressure; gradient; energy; ATP

Introduction

Breathing in and out is key to staying alive. It’s so important that even when we forget to breathe, our nervous system picks up the slack and keeps going. The process of breathing provides oxygen and removes carbon dioxide from the body. This process is essential to sustaining each and every cellular task within our bodies. The focus of this book is how the body achieves this seemingly simple process. We will take you from a single cell and how it regulates oxygen and carbon dioxide to the large-scale gas transport and delivery in the body under normal and pathologic conditions. So, sit back, relax, and take a deep breath!

If indeed you take a breath right now, you will breathe in air. Air in the atmosphere is a simply a mixture of gases. Atmospheric air, as it exists today, consists of about 21% oxygen, 78% nitrogen, 0.04% carbon dioxide, and some other miscellaneous gases such as argon. (Carbon dioxide makes up so little of the atmospheric air that it even gets beat out by argon, which weighs in at 1%. Seriously!)

But it wasn’t always this way. In fact, over 2.5 billion years ago, things weren’t looking too good for our oxygen-loving brethren. There was almost no oxygen in the atmosphere, and there was very little food around. So, some opportunistic little buggers called cyanobacteria took the warmth of the sun and made sustainable energy out it, much like plants do today. In the process they gave off oxygen as “waste.”

Little by little cyanobacteria began filling up the oceans with oxygen. The dissolved oxygen began to diffuse throughout the water (hopefully you’ll remember the principles of diffusion from our last book “Back to Basics in Physiology: Fluids in the Cardiovascular and Renal Systems”), and as the oceans filled with this “waste product” it diffused into the atmosphere. Over the next two billion years, the concentration of oxygen in the air reached the 21% we know and enjoy today. As oxygen became more and more plentiful in the environment, creatures began using this oxygen to create energy from available food sources more efficiently, and were able to grow larger than their non-oxygen-consuming counterparts. With size came more food consumption and a greater need for mobility, and with mobility and size came more energy utilization. Over time, organisms migrated from the water to land. Cyanobacteria made room for plants in the sea and on land, which produced even more oxygen. As organisms developed ways to use this newfound energy (e.g., growing brains!), they developed a larger need for oxygen, produced more carbon dioxide, and along the way came up with some pretty ingenious mechanisms to ensure constant oxygen delivery and carbon dioxide removal.

In our bodies today, out of the millions of functions that need to be carried out minute by minute in order to allow for life to proceed “uneventfully,” oxygen (O2) and carbon dioxide (CO2) exchange are arguably two of the most important processes our bodies require to stay alive. If the human body is deprived of oxygen, it will die far quicker than if deprived of food or water. If someone removed your kidneys right now, you would live for potentially several days. If they removed your heart or your lungs, the main organs responsible for moving the oxygen and carbon dioxide around the body, you would die within minutes. In fact, doctors’ primary goals in the setting of any medical emergency always revolve around bringing back or “stabilizing” a patient’s oxygen delivery, and to a lesser extent, carbon dioxide clearance. In fact, the classic ABCs of patient care (what doctors need to worry about first!) stand for Airway, Breathing, and Circulation. But why exactly are these two items so important?

O2 is consumed and CO2 is produced by all living cells in the body every second of every day in a process called aerobic cellular respiration. This process is absolutely vital to creating the energy that keeps the cells alive. O2 and CO2 allow for the most efficient energy extraction from the food we eat. In order to keep creating energy, these cells need a system that will move new O2 in and take CO2 out. So, before we go on to understand exactly how O2 and CO2 move in and out of the body, we need to take a step “in” and first understand why O2 and CO2 are important, and how they help create energy at the cellular level. Then we can move on to how these vital gases get in and out of cells and why blood is specialized to help aid this process. In the subsequent chapters, we will apply these concepts to the lungs and the rest of the cardiovascular system. By understanding how O2 and CO2 are used and how they move, the form and function of the rest of the pulmonary and cardiovascular systems will make sense intuitively.

O2 and CO2 for One Cell: Mechanics of Single Cell Gas Exchange

A cell is the most basic unit of life (ignoring viruses, which are a bit of a gray area). As such, it needs to be able to grow and respond to threats in its environment long enough to reproduce before eventually dying. Biochemically speaking, this involves a myriad of complex tasks. However, in order to perform all of these incredibly complex tasks, one thing is key: energy! Energy is needed for every major process the cell undertakes: movement of ions, signaling, and reproduction. We need energy for everything. But where does this energy come from?

Role of Oxygen (O2) and ATP

Much like how money is used to allow us to survive in a modern economy, cells must have a form of “energy currency” that allows them to rapidly generate and store energy that can be used at a moment’s notice. In organisms, this energy is most commonly stored as ATP, or adenosine triphosphate. Adenosine is a nucleoside. Nucleosides (a nitrogenous base with a carbohydrate backbone) are some of the most ubiquitous chemical compounds found in life. They are the building blocks of DNA and RNA, so your body has loads of them on hand. If multiple phosphate molecules are added to them, they become increasingly energy rich. In short, it is energy in the form of ATP that fuels life. As we shall soon see, oxygen makes ATP formation a heck of a lot more efficient. And efficient is good!

Generally speaking, ATP can be made without the help of oxygen. Many microorganisms from many walks of life live in some of the most hostile and oxygen-poor environments on this earth, but they can still thrive. They need to worry about providing fuel for only one little cell, though. The human body, on the other hand, is made up of trillions of cells, and within it ATP is broken down and formed and broken down and formed over and over again, millions of times a day. This pathway is so active that the body effectively turns over its own body weight in ATP every day! You can imagine then that ATP production can become exceedingly expensive to produce. Thankfully, oxygen helps us make ATP creation a lot easier.

Let’s look at ATP fabrication and recycling a little bit more closely, shall we? As we just mentioned, oxygen allows for the efficient creation of energy in the form of ATP. In more general terms, energy is extracted from the food we eat. As such one of the key molecules in all the food we eat is glucose. The process through which oxygen is used to extract energy to make ATP from glucose is called cellular respiration (Figure 1.1):

+O2→CO2+H2O+ATP

Role of Carbon Dioxide (CO2)

The amount of CO2 that is in the air we breathe is relatively low, but inside the body the amount of CO2 is much, much higher. As O2 is actively being consumed during cellular respiration, CO2 is being produced as a byproduct of the same biochemical pathway (Figure 1.1). Remember: While O2 is being consumed, CO2 is being produced. Similar to what happens with O2, the production of CO2 by the cell is closely linked to metabolism; the higher the...