The living cell is a miniature chemical factory where thousands of reactions occur. The cell extracts energy stored in sugars and other fuels and applies energy to perform work. Some organisms even convert energy to light, as in the bioluminescence seen in this picture.
Metabolism is an emergent property of life that arises from orderly interaction between molecules. An organism's metabolism transforms matter and energy and is subject to the laws of thermodynamics. So when I say metabolism, I'm referring to all of an organism's chemical reactions. A metabolic pathway begins with a specific molecule and ends with a product.
Each step along the way is catalyzed by a specific enzyme. There are two main types of metabolic pathways. First, catabolic pathways are those that release energy by breaking down complex molecules into simpler compounds.
An example of this is cellular respiration, which we'll learn more about next chapter. I like to remember catabolic pathways breaking things down because cats like to break things. Anabolic pathways consume energy to build up complex molecules from similar ones, like when we learned about the synthesis of protein from amino acids.
I like to remember anabolic pathways because ants build mounds. This whole chapter could be called bioenergetics instead. Bioenergetics is the study of how energy flows through living organisms. Energy, of course, being the capacity to cause change.
Energy exists in various forms, some of which can perform work. So kinetic energy, the energy associated with motion, one example of this that we tend to think about is thermal energy, where kinetic energy drives the... the random movement of atoms or molecules. Then we have potential energy, the energy that matter possesses because of its location or structure. An example of this is chemical energy, potential energy available for release in a chemical reaction.
And energy can be converted from one form to another. So in the example on the slide here, The person climbing up the ladder is using... chemical energy in the form of probably glucose in their cells to power their movement up the ladder Which is creating potential energy?
by changing their location so moving to the top of Ladder onto the diving board. They're storing energy because of that location and then it doesn't take much energy for that person to move from the diving board back down into the water. Once they're in the water they have less potential energy because they have released it as kinetic energy as they dive down back into the water.
All right, so as I said, these energy conversions are subject to the laws of energy transformation. So here we're having a little bit of a lesson in thermodynamics, the study of energy transformations. The first law of thermodynamics is that the energy of the universe is constant.
Energy can be transferred and transformed, but cannot be created or destroyed. This first law is also called the principle of energy of conservation of energy. And the consequence for organisms is that they have to get energy to power their cells from somewhere, either from the sun or by taking in chemical energy. In the case of this bear, he's getting his energy from other organisms by eating this fish.
The second law of thermodynamics is that organisms must get their energy from the environment. Every energy transfer or transformation increases the entropy or disorder of the universe. And so during every metabolic energy transfer or transformation, some energy is unusable, often lost as heat.
For example, we eat a plant, we only receive about 10% of the energy digested, and the rest is lost as heat. The second law basically means that living cells unavoidably... avoidably convert organized forms of energy to heat and energy loses quality every time it's transferred or transformed and So spontaneous processes occur without energy input they can happen quickly or slowly But for a process to occur without energy input it must increase the entropy of the universe and so In a certain way, it sounds like organisms might violate the second law of thermodynamics, the idea that everything is moving towards entropy in the universe, disorder, right? Organisms are highly ordered.
But the way we get around this is that we're basically... large mechanisms of entropy. So in order to maintain our highly ordered state, we move through the environment taking ordered molecules and breaking them down into less ordered molecules through our metabolic pathways. All right, so biologists want to know which reactions occur spontaneously and which require input of energy.
To do so, they need to determine energy changes that occur in chemical reactions. So a living system's free energy... is that energy that can do work.
It's also a measure of the system's tendency to change to a more stable state, and it can tell us whether or not a reaction is spontaneous. So a change in free energy, we call this delta G, that triangle sign there is delta like in math, during a process is related to its change in enthalpy. delta H and its change in entropy, delta S, as well as its temperature in Kelvin units, T.
So we get this equation here for change in free energy or also called Gibbs free energy. You don't need to memorize this equation necessarily and we won't be doing mathematical work with it, but it is important to know how to interpret delta G because reactions where delta G is less than zero, that is negative, are spontaneous. Spontaneous processes are those that can be harnessed to perform work, so that's pretty important to be able to interpret. A process is spontaneous and can perform work only when it's moving toward equilibrium.
So during a spontaneous change, free energy decreases and the stability of a system increases. The only systems that can do work, again, are those that are moving towards equilibrium. So in our top three pictures here, we've got situations where...
where we have more free energy that is a higher G. So we've got high potential energy here because of this person's location and it's being converted to kinetic energy and released to the environment as they dive downwards. Once they're in the water, they're at a more stable state, less likely to fall downwards, and they have undergone a spontaneous change.
Here we have atoms that are clustered together and just through their own random movement. This is not a chemically or energetically stable situation. The molecules are strained because they're going to be bumping into each other here. And so as they move apart from each other, it's a more stable situation.
and they've released energy in the process of that movement. And then here we have a molecule that is made of several atoms bound together. The closer those atoms are to each other, the more strained things are.
It takes energy to hold them in place in the form of bonds. If If those bonds are broken, the energy between the atoms is released, and it can be released. released to a more chaotic or entropic state where it's taking less energy to hold those molecules together. So the concept of free energy can be applied to the chemistry of life's processes as well. Exergonic reactions are those that proceed with a net release of free energy and therefore are spontaneous.
It doesn't take a large input of energy from the environment to make them happen. Endergonic reactions are those that absorb free energy from the surroundings. and tend to be non-spontaneous. You often probably have heard these phrases said more as exothermic and endothermic because often exergonic reactions are associated with a release of energy which can sometimes be in the form of heat and endergonic reactions are associated with the absorption of energy again which can be in the form of absorbing heat So a process, as we've said, is spontaneous and can perform work only when it's moving towards equilibrium. So reactions in a closed system eventually reach equilibrium and then can do no work.
So here we have a model as an analogy. We have a closed hydroelectric system. We've got a water tank here coupled to a water tank here with a turbine in the middle and at And as water moves from this tank to this tank, it's lighting up this light through the movement of this water turbine.
The water in the upper tank has potential energy because of its location at a higher area. As it moves past the water turbine, it's converted to kinetic energy and is able to do work. delta G is less than zero this work is spontaneous however once we get to a point where the water is equal in both tanks there's no longer that potential energy due to the water's location it's no longer able to be converted to this kinetic energy and now there's no change in Gibbs free energy and no work able to be done so what would happen to a cell if it was at an equilibrium similar to this. If it could do no work, it would die. If your cells stopped, Moving towards equilibrium, if they reached equilibrium, your cells would die.
Cells are not in equilibrium, even though we talk about cells striving to maintain homeostasis, this balanced state, they really are open systems experiencing a constant flow of materials. And therefore, a defining feature of life is that metabolism is never at equilibrium. So perhaps a better model for the cell would be an open hydroelectric system where physiological processes are constantly replenishing the the water in our in our water tank here in this example to continue the movement away from equilibrium so that again the movement towards equilibrium can then cause work and by breaking this into many small steps the energy created in this process released in this process rather is more controlled and able to do more small chemical reactions in the cell So a catabolic pathway in a cell releases free energy in a similar way, a similar controlled series of reactions that we're going to learn about much more next chapter.