Overview of metabolism
Overview of metabolic pathways, energy flow in a cell, and anabolism and catabolism.
What’s going on in your body right now? Your first answer might be that you’re hungry, or that your muscles are sore from a run, or that you feel tired. But let’s go even deeper, moving past the layer of your consciousness and looking at what’s going in your cells.
If you could peek inside of any cell in your body, you’d find that it was a remarkable hub of activity, more like a busy open-air market than a quiet room. Whether you are awake or sleeping, running or watching TV, energy is being transformed inside your cells, changing forms as molecules undergo the connected chemical reactions that keep you alive and functional.
Overview of metabolism
Cells are constantly carrying out thousands of chemical reactions needed to keep the cell, and your body as a whole, alive and healthy. These chemical reactions are often linked together in chains, or pathways. All of the chemical reactions that take place inside of a cell are collectively called the cell’s metabolism.
To get a sense of the complexity of metabolism, let's take a look at the metabolic diagram below. To me, this mess of lines looks like a map of a very large subway system, or possibly a fancy circuit board. In fact, it's a diagram of the core metabolic pathways in a eukaryotic cell, such as the cells that make up the human body. Each line is a reaction, and each circle is a reactant or product.
Abstract diagram representing core eukaryotic metabolic networks. The main point of the diagram is to indicate that metabolism is complex and highly interconnected, with many different pathways that feed into one another.
In the metabolic web of the cell, some of the chemical reactions release energy and can happen spontaneously (without energy input). However, others need added energy in order to take place. Just as you must continually eat food to replace what your body uses, so cells need a continual inflow of energy to power their energy-requiring chemical reactions. In fact, the food you eat is the source of the energy used by your cells!
To make the idea of metabolism more concrete, let's look at two metabolic processes that are crucial to life on earth: those that build sugars, and those that break them down.
Breaking down glucose: Cellular respiration
As an example of an energy-releasing pathway, let’s see how one of your cells might break down a sugar molecule (say, from that candy you had for dessert).
Many cells, including most of the cells in your body, get energy from glucose () in a process called cellular respiration. During this process, a glucose molecule is broken down gradually, in many small steps. However, the process has an overall reaction of:
+ + +
Breaking down glucose releases energy, which is captured by the cell in the form of adenosine triphosphate, or ATP. ATP is a small molecule that gives cells a convenient way to briefly store energy.
Once it's made, ATP can be used by other reactions in the cell as an energy source. Much as we humans use money because it’s easier than bartering each time we need something, so the cell uses ATP to have a standardized way to transfer energy. Because of this, ATP is sometimes described as the “energy currency” of the cell.
Building up glucose: Photosynthesis
As an example of an energy-requiring metabolic pathway, let's flip that last example around and see how a sugar molecule is built.
Sugars like glucose are made by plants in a process called photosynthesis. In photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas into sugar molecules. Photosynthesis takes place in many small steps, but its overall reaction is just the cellular respiration reaction flipped backwards:
+ + +
Like us, plants need energy to power their cellular processes, so some of the sugars are used by the plant itself. They can also provide a food source for animals that eat the plant, like the squirrel below. In both cases, the glucose will be broken down through cellular respiration, generating ATP to keep cells running.
Left: image of a tree with acorns growing on it. Right: image of a squirrel eating an acorn.
Anabolic and catabolic pathways
The processes of making and breaking down glucose molecules are both examples of metabolic pathways. A metabolic pathway is a series of connected chemical reactions that feed one another. The pathway takes in one or more starting molecules and, through a series of intermediates, converts them into products.
Metabolic pathways can be broadly divided into two categories based on their effects. Photosynthesis, which builds sugars out of smaller molecules, is a "building up," or anabolic, pathway. In contrast, cellular respiration breaks sugar down into smaller molecules and is a "breaking down," or catabolic, pathway.
Anabolic pathway: small molecules are assembled into larger ones. Energy is typically required.
Catabolic pathway: large molecules are broken down into small ones. Energy is typically released.
Anabolic pathways build complex molecules from simpler ones and typically need an input of energy. Building glucose from carbon dioxide is one example. Other examples include the synthesis of proteins from amino acids, or of DNA strands from nucleic acid building blocks (nucleotides). These biosynthetic processes are critical to the life of the cell, take place constantly, and use energy carried by ATP and other short-term energy storage molecules.
Catabolic pathways involve the breakdown of complex molecules into simpler ones and typically release energy. Energy stored in the bonds of complex molecules, such as glucose and fats, is released in catabolic pathways. It's then harvested in forms that can power the work of the cell (for instance, through the synthesis of ATP).
One final but important note: the chemical reactions in metabolic pathways don’t take place automatically, without guidance. Instead, each reaction step in a pathway is facilitated, or catalyzed, by a protein called an enzyme. You can learn more about enzymes and how they control biochemical reactions in the enzymes topic.
Want to join the conversation?
- I'm curious about how ATP ended up being the energy currency for both plants and animals, why the same molecule? Is because of a common ancestor?
Is there any cell that doesn't use ATP as its "energy currency"?(33 votes)
- Yes, it is because of the common ancestor. If there was a different, more efficient molecule then this would have been used instead. Keep in mind that in the long run only the most effective processes and molecules can transferred by generations.(35 votes)
- So basically, Metabolism is the core of a cell. It's where all the work happens right?(3 votes)
- Metabolism is the process used to store or release energy for use in the cell. It allows other essential chemical reactions to happen. it is the basis for all the work in cell. Try to think of it as a process not an area where reactions happen(21 votes)
- How can a molecule be "worn out"...? Does he mean they've outgrown their usefulness, or that they actually lose hydrogens or their groups come apart somehow over time?(5 votes)
- Good question... they don't truly mean "worn out" as I think you are thinking... I think what they mean is that a molecule such as glucose gets broken down a few times to harvest some energy in the form of ATP... and then another molecule such as pyruvate, for instance, enters another metabolic process for recycling, harvesting both energy, and the use of the carbons for other purposes. See the citric acid cycle and this will start to make more sense ( https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/pyruvate-oxidation-and-the-citric-acid-cycle/v/krebs-citric-acid-cycle )... but you are right... "worn-down" is confusing wording.(6 votes)
- How energy is transfered from cellular respiration to the process that formats ATP. Is it in heat? If so doesnt it affect other molecules in the area? Thanks(3 votes)
- The majority of ATP is generated through ATP synthase at the end of the electron transport chain. In this process, a concentration gradient of protons (H+) is what is used to drive ATP synthase, not heat.(9 votes)
- Does metabolism vary widely between people? I have heard that it does not, but it would seem that it would be highly dependent on the weight of an individual.(5 votes)
- yes, it does, because you could have an illness and because of this illness one of your hormones gets produced more or less. And so your metabolism would be regulated as faster or slower.(4 votes)
- Are ADP/ATP reusable? What I mean is, once ATP released its energy, does it transform back to ADP? If yes can this ADP be used again to form back ATP?(4 votes)
- This is an essential cycle that never stops until you die: ADP gets phosphorylated in the mitochondria, storing energy in ATP, and the ATP gets used to perform cellular work, releasing its energy in coupled reactions, and transforming back to ADP.(4 votes)
- What is ADP (adenosine diphosphate)? How is it different from ATP?(3 votes)
- ADP is adenosine diphosphate
ATP is adenosine triphosphate
In ADP there is 2 phosphate molecules
In ATP there is 3 phosphate molecules(4 votes)
- Is the convergence of glucose to glycogen considered anabolism ?(3 votes)
- Yes - this is an anabolic process, promoted by the action of insulin on the hepatocyte or myocyte. The reverse - i.e. the hydrolysis of glycogen back into glucose-1-phosphate - is a catabolic process.(4 votes)
- This says that an anabolic reaction builds larger molecules from smaller ones, and needs an input of energy. But when oxygen reacts with carbon to create CO2, a larger is made but this reaction, release energy. So is this reaction anabolic or catabolic?(2 votes)
- Good question... but I think that anabolic and catabolic are talking mostly about monomers becoming polymers... or side chains being added or removed... when you get down to the level of elemental oxygen and carbon forming carbon dioxide, I don't think this terminology of anabolic / catabolic exactly applies.(6 votes)
- Why is it that ATP happens to resemble an adenine base in DNA? Are they related in any way beyond structure? Is the adenine base special? Is there another energy currency molecule like ATP? Can we artificially create another energy currency molecule?(4 votes)