Chemiosmosis in Photosynthesis & Respiration

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  • 0:02 ATP and Molecular Currency
  • 1:22 Chemiosmosis
  • 3:01 Cellular Respiration
  • 4:07 Photosynthesis
  • 5:31 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Cells need energy to survive, but they can't use the energy from nutrients or light unless processed into certain molecules. In this lesson, we are going to explore the process of creating these molecules both in cells that rely on respiration as well as those that rely on photosynthesis.

ATP and Molecular Currency

Every culture has some kind of currency. We use money, but some people use…donkeys or shells or IOUs. Currency means that something of value is being exchanged for something else, and as it turns out, cells have their own systems of economy. So, what is valuable to a cell? Energy. The exchange of energy is what keeps a cell running, and that means that cells have their own form of currency. It's called adenosine triphosphate, or ATP, and it is the molecule that transports energy within a cell.

There is energy all around, but cells can only use it when converted into ATP, and so the exchange of ATP keeps various organelles running. So, just as we need to print money to keep our economy running, the cell needs to continually produce ATP, otherwise the enzymes go on strike, organelles shut down; it's a whole mess.

One way to do this is through chemiosmosis, the movement of ions across a selectively permeable membrane down an electrochemical gradient. Want to see more? Well, grab your traveler's checks, and let's go exchange some molecular currency.


So, let's look at this process of chemiosmosis a little more closely. Here's how this works. Within ions is a lot of potential energy. Within the cell, chemiosmosis harnesses that energy by actively transporting hydrogen ions across this selectively permeable membrane, so called because it only lets some things through. Why hydrogen ions? Because hydrogen only has one proton and is very stable. And, how do we move it? The enzyme ATP synthase actively transports the hydrogen protons across the membrane along an electrochemical gradient, meaning from areas of high concentration to low concentration.

On this side of the membrane are lots of ions, and on this side there are few. Chemiosmosis is like almost every other osmotic reaction in that it is always moving towards an equal amount on both sides; that's what motivates this transport of ions. So, as ATP synthase is moving the protons of hydrogen ions across this membrane, the action of moving it along the electrochemical gradient creates kinetic energy, or energy processed by motion. This energy is then attached to the organic molecule ADP, making ATP molecules that contain energy the cell can use. So, ATP is created by chemiosmosis as the enzyme ATP synthase moves hydrogen ions from areas of high to low concentration along the electrochemical gradient. See? Simple.

Cellular Respiration

So, where do we actually see chemiosmosis taking place? In living organisms, the process of converting nutrients into useable energy is called cellular respiration. This is where chemiosmosis comes into play, and in cells that rely on respiration, this is where chemiosmosis occurs. This is a mitochondrion, an organelle with double-membranes.

These two membranes are what allow energy production to occur. The inner membrane is the selectively permeable one with channels of ATP synthase to transport hydrogen ions, but how do we create the electrochemical gradient? Through a series of chemical reactions, mitochondria actively pump hydrogen ions into the space between the two membranes. This creates a space that has a much higher concentration of ions than the space within the mitochondrion, and these ions are passed down the gradient, across the inner membrane by ATP synthase, and chemiosmosis occurs, generating ATP that can be transported throughout the cell.

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