Breakdown and Utilization of Sugars and Fats - Online Tutor, Practice Problems & Exam Prep

Metabolism encompasses all chemical reactions in a cell, divided into catabolism (breaking down macromolecules) and anabolism (synthesizing macromolecules). Cellular respiration, particularly the breakdown of glucose, is crucial for energy production, yielding ATP. This process involves glycolysis, converting glucose to pyruvate, followed by oxidative phosphorylation in mitochondria, which generates the majority of ATP. Oxygen availability determines the metabolic pathway: aerobic respiration (high energy yield) or anaerobic respiration (lower energy yield). Understanding these pathways is essential for grasping cellular energy dynamics.

Metabolism Review

Hi, in this video we're going to be talking about the breakdown and utilization of sugars. So, first let's overview metabolism. We know about metabolism because you talked about it before in your intro classes, but metabolism is just going to be defined as the sum of all chemical reactions in a cell. And so there are different reactions that happen that make up metabolism. The first that we are going to talk about is catabolism, which includes all reactions that break down something, so break down macromolecules. These are generally going to be energetically favorable because it is easier to break down stuff than it is to form things and because the energy is released from the bonds.

The second type we want to talk about is anabolism, which is going to be the opposite, so those are reactions that synthesize or form macromolecules. These are going to be unfavorable reactions because you have to put energy in to be able to create new macromolecules. Finally, the last term that I really want to review in this video is going to be metabolic pathways and what those are. Those are going to be a series of reactions where the product of one reaction is the substrate for the next. So, you can kind of imagine these as just like lines, so you have lines where you can see sometimes these before where you have some type of reaction and it forms a product, and that's used to form another product.

This is going to be a metabolic pathway where the product of one reaction is going to be the starting material for the next one. Another term associated with this is called metabolites, which are metabolic intermediates, which you may see. These are going to be compounds that are formed in the process of creating the end product. So different like chemicals or different products, they can all be metabolites, these are things that are made in the process of creating something else, some kind of end product.

As a review of the different forms of metabolism, we have catabolism here, which is going to be breaking apart something, and then we have anabolism which is going to be forming something. So you can see there are all these individual pieces and it gets formed and that anabolism or catabolism releases energy, but anabolism uses energy. And so, that's the difference and a review of metabolism. So now let's move on.

Glucose

So in this video, we're going to be talking specifically about glucose and how it's important in getting energy to the cell. So in order to survive, cells have to get energy from fuel molecules. And what I mean by fuel is usually some type of food. It doesn't have to be. It can be light, for instance, but energy has to be obtained from somewhere. And so, the first one that I want to talk about is cell respiration, and that's going to be the process of harvesting energy from bonds in sugar molecules. This is really important, especially for animal cells, in order to get the energy that they need to survive. So glucose is actually the most important sugar for respiration, and the reason is that it carries all this energy. So if you remember back to the ΔG that we talked about, I think, a long time ago if you don't, it's fine. You don't need to know this number. But if you do, the ΔG of glucose is negative 686 kilocalories per mole, which is a huge negative number, which means that it's really energetically favorable to break it apart. So the breakdown of sugars, things like glucose is energetically favorable, and generally the cell likes to couple this breakdown with the creation of other forms of energy. So usually this energy formation is ATP. So normally in a cell, the creation of ATP is really hard. It's not easy, it requires a lot of energy, and so it's not that favorable of a reaction. So what cells do to overcome that is they take sugar molecules that are really easily broken down, and they take that energy to form more usable energy in the form of ATP. And so, because they want to couple these reactions, the breakdown of sugar and the formation of other usable energy like ATP, the process has to be really controlled. And so, the reason it has to be is that it needs to be harvested in tiny steps, little by little, so that the energy can all be taken up by other molecules instead of just kind of like exploding on something that's not really able to handle that much energy in one go. So metabolism and cellular respiration is, and the breakdown of sugars is a really controlled process to harvest the energy from sugars to create usable forms of energy like ATP. So, if we just look here at this formula, you don't need to memorize this formula, especially not right at this second. You may need to in the future, but not right now. But essentially what this means is you take glucose here and respiration occurs with things like oxygen, which we're familiar with, you know, animals need oxygen. So glucose and oxygen undergo respiration to create some waste products, CO₂, water, but most importantly it couples this reaction with the formation of ATP. So the breakdown of glucose releases energy which is then used in this reaction to create ATP. It's a really, really important process. So with that, let's now move on.

Catabolism Stages

So in this video, we're going to focus on the 3 stages of catabolism, which, if you remember what catabolism is, that is going to be the breakdown of macromolecules. And specifically, what we're going to talk about is the 3 stages of breaking down food, usually sugar molecules of some kind. The first thing that happens after you eat a meal is digestion. You take your pizza, and your stomach can break that down into larger molecules of smaller subunits. Pizza is made up of lots of complex molecules and sugars and carbohydrates, and your stomach, in the process of digestion, will break those really large molecules into smaller subunits. This can occur both outside the cells in the stomach or in the intestine, but it also can occur inside cells if the molecules can get in, and that will really happen in lysosomes because the lysosome can also act as the stomach of the cell to break apart these really big molecules.

Once the molecules are broken into individual subunits, it's useful, but we still can't really use those subunits for anything. They have to be the energy needs to be taken out of them. What happens next is a step process called glycolysis, and this is going to break down glucose or other sugar molecules, these monomers or subunits, into a chemical called pyruvate. During this process, which occurs in the cytosol, so now we're inside cells 100% of the time in the cytosol, the sugar molecule is changed into pyruvate, and what the cell does is it actually can generate things like ATP, this molecule called NADH which is an electron carrier, and pyruvate. Eventually, pyruvate is used by the cell to turn into carbon dioxide as well as this other molecule that we're going to talk a lot about called acetyl CoA, which is an activated carrier. An activated carrier is a molecule that can carry electrons. Carrying electrons is really important because it allows us to carry these electrons that have high energy from one reaction or one area of the cell to another to have some other type of reaction occur.

Now we've eaten our pizza, it's been digested, those sugars have been broken down through glycolysis and so now we have some ATP, some NADH, and pyruvate, maybe some CO₂ and Acetyl CoA. The next thing that happens is oxidative phosphorylation, and this is the metabolic pathway that leads to the largest production of ATP. Now we have moved into the mitochondria, so we're still in the cell but no longer in the cytosol. This process creates CO₂, NADH, ATP, and also citrate, which is for another cycle that we're going to talk about called the citric acid cycle. This cycle is called oxidative phosphorylation because it consumes oxygen. This is where the oxygen that we breathe in goes to for the most part.

We are not going to go over digestion, but we are going to go over glycolysis, oxidative phosphorylation, and the citric acid cycle in really great detail. If you don't necessarily want to memorize all this, such as glycolysis creating pyruvate, that's fine, but we will need to memorize this in the future so be prepared for that. This is an image here just looking at different types of metabolic pathways, and it's really complicated. I don't want you to remember these steps or anything, so just step back and look at this just as an image and let me walk you through it. What we can see here is that we're in the cytoplasm here and this area is going to be the mitochondria. You can see some of the steps that I talked about. You have glycolysis here which is going to occur in the cytoplasm and it's going to create some ATP, some NADH, and then this travels through here to the mitochondria oxidative phosphorylation occurring. All of this is happening, and it's creating ATP in various ways. It's creating proton gradients. It's creating electron carriers. It's creating all these different molecules, and we're going to go through the steps of all of them. But you can just sort of understand that the process of this breakdown of sugar molecules isn't necessarily that easy. It's really complex and involves a lot of steps, but I'm going to walk you through them.

So with that, let's now turn the page.

Oxygen and Respiration

So in this video, I'm going to focus on the importance of oxygen and the breakdown process of sugars. The presence of oxygen actually dictates which metabolic pathway the cell will use to break down sugars or whatever it's going to break down for energy. There are really 2 types of respiration, completely dependent on whether oxygen is available. So if oxygen is not available, it's called anaerobic respiration, no oxygen, or it's called aerobic respiration with oxygen. You can kind of think of that as like "aerobic air" has oxygen whereas anaerobic has no oxygen. Aerobic respiration, especially in animal cells, has a really high energy yield, much higher than that of anaerobic respiration. Generally, the cell is going to prefer to use aerobic respiration, but it can only do that if oxygen is available and sometimes, especially in some cells like in your muscles during a workout, oxygen isn't always available in enough levels to do that for energy.

Organisms are classified based on their requirement for oxygen during metabolism. Obligate aerobes have an absolute requirement for oxygen. These are humans. Humans cannot survive without oxygen because our cells can't break down food molecules for energy without it. But there are also organisms called obligate anaerobes, and these are organisms that cannot under any circumstance use oxygen; oxygen will potentially be toxic to them. You can think of these as organisms living at hydrothermal vents or deep in the ocean. Sometimes oxygen is not only unused but actually might be toxic to them if they are exposed to it. Then you have facultative organisms, which can function with or without oxygen and can switch between the two forms when oxygen is available or when it's not. This mainly classifies bacteria.

Here we have a variety of different metabolic pathways starting from the sugar of choice, which is going to be glucose. In the cell, glucose undergoes glycolysis, which turns it into pyruvate. Now, the presence of oxygen dictates what pyruvate does. If oxygen is not present, then the cell uses that pyruvate to undergo a process called fermentation. You probably heard about it, it's used to create beer and stuff like that. But fermentation is done without oxygen and doesn't create a lot of energy. Alternatively, if oxygen is present then the cell can use all these other pathways, such as oxidative phosphorylation, the Krebs cycle, and the electron transport chain. You don't necessarily know all these yet; we're going to go over them individually, but it creates a lot of ATP for every starting molecule of glucose and so this is going to be the preferred pathway but it can only happen if oxygen is present. So with that, let's now turn the page.

Which of the following definitions defines anabolism?

• A. All reactions that breakdown molecules
• B. All reactions that synthesize molecules
• C. A metabolic pathway that leads to the production of energy
• D. Process of harvesting useful energy from bonds in sugar molecules

Which of the following pathways causes the breakdown of glucose into pyruvate?

• A. Glycolysis
• B. Oxidative Phosphorylation
• C. Citric Acid Cycle
• D. Krebs Cycle

Oxygen is not required for metabolism in which type of organism?

• A. Obligate aerobes
• B. Obligate anaerobes
• C. Facultative organisms

Here’s what students ask on this topic:

What is the difference between catabolism and anabolism in metabolism?

Catabolism and anabolism are two key components of metabolism. Catabolism involves the breakdown of macromolecules into smaller units, releasing energy in the process. This energy is often captured in the form of ATP. An example of catabolism is the breakdown of glucose during cellular respiration. Anabolism, on the other hand, is the synthesis of macromolecules from smaller units, which requires an input of energy. This energy is typically derived from ATP. An example of anabolism is the synthesis of proteins from amino acids. In summary, catabolism releases energy by breaking down molecules, while anabolism consumes energy to build up molecules.

How does glycolysis contribute to cellular respiration?

Glycolysis is the first stage of cellular respiration and occurs in the cytosol of the cell. It involves the breakdown of one glucose molecule into two molecules of pyruvate. This process generates a net gain of 2 ATP molecules and 2 NADH molecules, which are crucial for the cell's energy needs. The pyruvate produced in glycolysis can then enter the mitochondria, where it is further processed in the citric acid cycle and oxidative phosphorylation to produce more ATP. Glycolysis is essential because it initiates the process of glucose breakdown, providing the necessary substrates and energy carriers for subsequent stages of cellular respiration.

What is the role of oxygen in cellular respiration?

Oxygen plays a critical role in cellular respiration, particularly in the process of oxidative phosphorylation, which occurs in the mitochondria. Oxygen acts as the final electron acceptor in the electron transport chain. As electrons are passed along the chain, they eventually combine with oxygen and hydrogen ions to form water. This process helps to create a proton gradient across the mitochondrial membrane, which drives the synthesis of ATP through ATP synthase. Without oxygen, the electron transport chain would halt, and the cell would have to rely on less efficient anaerobic pathways, such as fermentation, to produce ATP.

What are the stages of catabolism in the breakdown of glucose?

The breakdown of glucose in catabolism occurs in three main stages: digestion, glycolysis, and oxidative phosphorylation. During digestion, complex carbohydrates are broken down into simpler sugars like glucose. In glycolysis, glucose is further broken down into pyruvate in the cytosol, producing ATP and NADH. The pyruvate then enters the mitochondria, where it is converted into acetyl-CoA, which enters the citric acid cycle. The final stage, oxidative phosphorylation, involves the electron transport chain and chemiosmosis, producing the majority of ATP. This stage requires oxygen and generates water and carbon dioxide as byproducts.

What is the difference between aerobic and anaerobic respiration?

Aerobic respiration requires oxygen and produces a high yield of ATP. It involves glycolysis, the citric acid cycle, and oxidative phosphorylation. Oxygen acts as the final electron acceptor in the electron transport chain, allowing for efficient ATP production. Anaerobic respiration, on the other hand, occurs without oxygen and results in a lower ATP yield. It relies on processes like glycolysis followed by fermentation. In anaerobic respiration, pyruvate is converted into lactate or ethanol and CO2, depending on the organism. While aerobic respiration is more efficient, anaerobic respiration allows cells to generate energy when oxygen is scarce.