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Cellular Energetics

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Cellular Energetics

Have you ever felt so hungry that you couldn't do anything? Our bodies need a constant amount of energy to function. Every action that we take from walking, running, sitting down, breathing, and even sleeping all require energy (Fig. 1). In fact, it is estimated an average human of 70 kilograms consumes around 70 kilograms of energy each day. That means we consume our own weight in energy every single day. In this section, we will cover cellular energetics which is how energy is used by living systems.

Cellular Energetics Figure 1: All activities from sleeping to running require energy,  Fabio Comparelli StudySmarterFigure 1: All activities from sleeping to running require energy, Fabio Comparelli

Cellular energetics definition

Cellular energetics is the study of how energy is captured and then used by living systems. In this overview, we will discuss the processes by which a cell uses energy and how these processes can change depending on the environment.

Cellular energetics function

Cellular energy

All living systems require constant energy to function. As we mentioned before, cellular energetics is the study of how energy is captured and then used by living systems. The function of energy in a living system is broad. For example, cells can use energy for repair, to divide mitotically, to bring in or expel molecules from the cell, and to power metabolism. Essentially, energy is necessary to power all the biological reactions within the cell. Cellularly energetics helps to provide that energy by breaking apart macromolecules. Importantly, cellular energetics expands beyond humans to plants and even bacteria as well.

To further place the importance of cellular energetics, imagine that a living cell is like a refrigerator. A refrigerator needs to be constantly powered to keep its contents colder than the outside environment. Following the second law of thermodynamics, we know that heat will flow from a hot object to a cold object. In our analogy, heat will naturally flow from the outside environment to inside the refrigerator. To counter this natural flow, a refrigerator needs to be constantly powered using electrical energy. Similarly, a cell needs to be constantly powered by energy to prevent natural energy leakage. If a cell does not have enough energy to overcome the natural energy leakage, the cell will die. Therefore, cellular energetics is critical for keeping cells alive.

To first understand how energy is acquired and used in biological reactions, a foundational understanding of enzymes is needed. Enzymes are proteins that can be found in nearly every biological reaction and without enzymes, survival is impossible.

Enzyme structure and function

Our bodies perform thousands of biological and chemical reactions each second. Each one of these reactions is critical for keeping us alive, however, without any assistance, they may take too long. Enzymes are biological catalysts that accelerate chemical reactions at a much faster rate. While enzymes speed up the reaction, enzymes themselves are not consumed during the reaction meaning that they remain unchanged throughout the reaction.

Each enzyme has a unique structure that allows it to specifically interact with its binding partners called substrates. The part of the enzyme that directly interacts with the substrate is called the active site. For an enzyme to catalyze a chemical reaction, the shape and charge of the substrate must be compatible with the active site of the enzyme. Similar to how a key cannot open every lock, an enzyme cannot catalyze every reaction. It can only catalyze reactions where the active site and substrates are compatible (Fig. 2).

A key concept is the structure-function relationship which states that the structure or shape of any molecule

is directly connected to its function. Molecules like enzymes and substrates interact by binding with each other and if these two structures are incompatible, they will not bind and therefore not interact.

Activation energy is the energy that is required for a chemical reaction to occur. Imagine trying to light a match on fire. Simply holding the match against the striking strip does not light the match because there is not enough energy. Some amount of force is necessary to generate friction and heat to reach the activation energy threshold of lighting the match. Enzymes catalyze reactions by lowering the activation energy threshold. Therefore, an enzyme-catalyzed reaction would need a smaller amount of force to light the match compared to an uncatalyzed reaction. Activation energy can be shown using the graph below (Fig. 3) with reactants on the left moving to products on the right. The "hill" is the activation energy that must be overcome for the reaction to occur.

Cellular energetics examples

One key component of cellular energetics is obtaining energy. Complex pathways involving multiple reactions are called cellular processes. Two examples of cellular processes that can be used to obtain energy are photosynthesis and cellular respiration. These two processes utilize a process called energy transfer to efficiently power multiple chemical reactions sequentially.

Energy transfer

A cellular process involves a chain of multiple different chemical reactions each with its own unique enzymes. While some chemical reactions release energy, other reactions require energy. Cellular processes are considered sequential meaning that a product of one reaction becomes the reactant in the next reaction. This means that a reaction that releases energy can be coupled with a reaction that requires energy so that energy released from the first reaction can fuel the energy required for the second reaction. In this way, energy is efficiently transferred in a controlled manner.

Photosynthesis

Photosynthesis is the cellular process by which organisms capture and store energy from the sun to produce sugar. While we often think that only plants undergo photosynthesis, it is a misconception that photosynthesis is exclusive to plants. In fact, photosynthesis is thought to have evolved from prokaryotic organisms, like cyanobacteria, that used photosynthesis to capture energy from the sun.

Taking a look at the net reaction for photosynthesis below, we can see that photosynthetic organisms convert carbon dioxide, water, and light into simple sugars and oxygen.

Importantly, the above equation is the net reaction and several intermediate reactions also take place that involves producing energy-rich molecules such as adenosine 5'-triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). In general, ATP is the currency of energy. Many biological reactions break apart ATP to generate enough energy to fuel the chemical reaction forward.

Photosynthesis is not one reaction, it involves multiple coupled reactions where the products of one reaction become the reactant of the next reaction.

For eukaryotic organisms, many of these intermediate reactions take place in an organelle called the chloroplast (Fig. 4). Chloroplast is a membrane organelle that houses a green pigment called chlorophyll. Chlorophyll itself contains electrons that become excited when hit by solar energy from the sun. Proteins in the chloroplast membrane called electron transport chain (ETC) proteins will then use these excited electrons as an energy source to drive the formation of a proton gradient. Eventually, this proton gradient will be used to generate ATP, an energy-dense molecule. ATP will then be transferred to a region in the chloroplast called the stroma where the energy will be used to create simple sugars in a process called the Calvin cycle.

Photosynthesis is an example of energy transfer. Solar energy is transferred to electrons in chlorophyll. These electrons will then be used to establish a proton gradient. The protein gradient will be used to generate ATP. And ATP will be used to make simple sugars.

Cellular respiration

Cellular respiration and fermentation are cellular processes performed by all living organisms that convert energy from biological macromolecules into energy in the form of ATP that can be used by the cell. While in the past, cellular respiration may have been expressed using the net equation below, it is important to note that like photosynthesis, cellular respiration and fermentation both involve multiple enzyme-catalyzed reactions.

Since energy is formed by breaking apart large macromolecules, like glucose, into smaller molecules, cellular respiration is called a catabolic process. The main steps in cellular respiration include glycolysis, pyruvate oxidation, the Kreb's Cycle, and oxidative phosphorylation.

Glycolysis is the breakdown of glucose into ATP, NADH, and pyruvate. Pyruvate is then shuttled from the cytosol into the mitochondria where pyruvate oxidation occurs. Within the mitochondria, Kreb's cycle involves a series of enzyme-catalyzed reactions involving organic intermediate to form carbon dioxide, ATP, NADH, and FADH2. The last set of cellular respiration is oxidative phosphorylation which takes place in the inner mitochondrial membrane. During oxidative phosphorylation, an electron transport chain (ETC) reaction occurs where electrons stored from glycolysis and the Kreb's cycle in the form of NADH and FADH2 are passed down ETC proteins in the inner mitochondrial membrane (Fig. 5). As electrons are being passed, energy is being released which is used to fuel the formation of a proton gradient across the inner mitochondrial membrane. The final stop for de-energized electrons is oxygen. Finally, protons are transported down its gradient through membrane-bound ATP synthase which can fuel the energy required to make ATP from ADP. ATP can then be used to provide energy to all the processes of the cell.

While cellular respiration is typically performed aerobically meaning that oxygen is present, in the case of extreme exercise where the cell does not have enough oxygen, glycolysis can take place anaerobically, or without oxygen, in a process called fermentation. Fermentation produces ATP as well as alcohol and lactic acid as waste products.

Aerobic means "with oxygen" and anaerobic means "without oxygen."

Deregulating cellular energetics

There are certain cases when cellular energetics is deregulated. This means that energy is not properly captured or energy is not used effectively by the cell. The two main scenarios for deregulating cellular energetics are environmental changes to temperature and pH and cancer.

Environmental changes

Recall that enzymes are present in nearly every biological reaction and without enzymes, we would not be able to survive. Enzymes are proteins meaning that they are highly sensitive to changes in temperature and pH. Changes in temperature will cause the protein to denature which means that the protein will unfold and the 3D shape will change. Since compatibility of the active site of the enzyme and its substrate is critical for enzymatic function, a denatured enzyme will no longer be able to catalyze a reaction. Similarly, changes in pH can also denature an enzyme by breaking apart the hydrogen bonds that hold together the shape of the enzyme.

Cancer

A hallmark of cancer is deregulated cellular energetics. Cancer is uncontrolled cell growth that can then spread to other sites of the body. Since growth requires energy, cancer takes up more glucose than normal cells. Additionally, it can use other energy sources like glutamate to provide energy to cancer cells. Furthermore, some cancer cells have mutations of specific enzymes of the cellular respiration pathway that allows cellular respiration to continue without stopping thereby providing an unending source of energy.

Cellular energetics summary

In the above, we covered the mechanism of cellular energetics in great detail, as well as the importance of cellular energetics within biological organisms. We know that photosynthesis, which is the process of gathering food and energy from the sun, is only done in organisms that contain special organelles known as chloroplasts. Cellular respiration is another form of cellular energetics that most biological organisms including humans use. The main steps in cellular respiration include glycolysis, pyruvate oxidation, the Kreb's Cycle, and oxidative phosphorylation. Each of these processes play crucial roles in maintaining homeostasis and many factors like environmental changes and cancer can lead to the disruption of homeostasis via the dysregulation of cellular energetics.

Cellular Energetics - Key takeaways

  • Cellular energetics is the study of how energy is captured and then used by living systems.
  • Enzymes are biological catalysts that accelerate chemical reactions at a much faster rate.
  • Two examples of cellular processes are photosynthesis and cellular respiration.
  • Electron transport chain (ETC) reactions occur across the chloroplast membrane in photosynthesis and across the inner mitochondrial membrane in cellular respiration.
  • The steps of cellular respiration are glycolysis, pyruvate oxidation, the Kreb's Cycle, and oxidative phosphorylation.

Frequently Asked Questions about Cellular Energetics

Cellular energetics is the study of how energy is captured and then used by living systems 

All living systems require constant energy to function. Energy is necessary to power all the biological reactions within the cell. 

Cells get energy from either sunlight or energy stored in food. The energy can then be captured by cell through photosynthesis or cellular respiration. 

Deregulating cellular energetics means that energy is not properly captured or used effectively by the cell. Most commonly this occurs due to environmental changes in pH and temperature as well as cancer.   

Some cancer cells have mutations of specific enzymes of the cellular respiration pathway that allows cellular respiration to continue without stopping thereby providing an unending source of energy.   

Final Cellular Energetics Quiz

Question

What are enzymes

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Enzymes are biological catalysts in biochemical reactions. 

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Almost all enzymes are proteins

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True

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What are globular proteins?

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Globular proteins are enzymes, carriers, hormones, receptors, and more

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Enzyme names end in -ase

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True

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How do enzymes get their names?

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They get their names from the type of substrate or chemical reaction they catalyze 

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What substrate does lactase belong to?

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Lactose

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What is an active site?

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An active site is the functional part of an enzyme

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What are substrates?

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A molecule that can bind to the active site

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What is activation energy?

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The energy needed to start a reaction

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Enzymes are catalysts

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True

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Enzymes raise the activation energy

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False

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What factors affect enzyme activity?

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Temperature

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What is denaturation? 

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The way external factors can harm the enzyme structure and cause it to no longer function properly or at all

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Enzymes speed up catabolic reactions

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True

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Enzymes don't catalyse anabolic reactions

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False

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Where are enzymes found?

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All tissues and fluids in the body

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Enzymes are proteins

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True

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What is the Lock and Key Model?

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It is a model demonstrating how only the right sized enzyme can fit within the substrate's hole

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What is a brief description of the catalytic cycle?

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The substrate attaches itself to the enzyme's active site then the enzyme-product complex forms. Reaction products are then released, allowing the enzyme to repeat the cycle

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What is an active site?

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It is a small area where the catalytic process occurs on an enzyme

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What is the induced fit theory?

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A substrate binding to an enzyme causes the enzyme's shape to change

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The activity of an enzyme increases as the concentration of the enzyme increases. 


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True

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An enzyme’s activity decreases with the rise in substrate concentration. 

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False

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What type of shape represents a graph showing the relationship between pH and enzyme activity?

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Bell-shaped (the optimal pH will be the peak of the graph)

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What is denaturation?

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Denaturation is when an enzyme is damaged and is unable to function as well

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Higher temps increase the rate of reaction until it gets too hot and causes denaturation

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True

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What is the optimal temp range for most enzymes?

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98.6 - 113 F

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Enzymes are

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Proteins

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What is the difference between competitive and noncompetitive inhibitors? 

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Competitive inhibtors: Molecules bind to the active site in order to block substrates

Noncompetitive inhibitors: Bind somewhere other than the active site

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What type of relationship is there between the reaction rate and substrate concentration?

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Hyperbolic

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ATP is an essential energy-carrying molecule for all living organisms

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True

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There is excess ATP being stored in the body for later use

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True

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ATP is made up of

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Adenine

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How is energy released from ATP?

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A bond is broken between the 2nd and 3rd phosphate group 

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What happens when ATP is broken? 

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The reaction forms ADP, one Pi group (inorganic phosphate), and energy is released

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What are the three processes for ATP synthesis? 

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Oxidative phosphorylation, substrate-level phosphorylation, and photosynthesis. 


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Which process makes the most ATP?

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Oxidative phosphorylation 

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ATP during photosynthesis is made in the chloroplasts

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True

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What is substrate-level phosphorylation?

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Substrate-level phosphorylation is the process by which phosphate molecules are transferred to form ATP 

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ATP is better than glucose as an immediate energy source

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True

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What does ATP help with?

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ATP helps in muscle contraction, active transport, synthesis of nucleic acids, DNA and RNA, formation of the lysosomes, and synaptic signaling. It allows enzyme-catalyzed reactions to take place more quickly.

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How does ADP go to ATP?

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A phosphate molecule is added

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What is hydrolysis?

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The process of bonds being broken in order to release energy

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Where does oxidative phosphorylation take place?

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Oxidative phosphorylation takes place in the membrane of mitochondria 

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Why is ATP also known as "energy currency"?

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ATP is considered a medium of exchange (similar to money) because energy is carried from cell to cell

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What is photosynthesis?

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Photosynthesis is when plants take light energy from the sun to turn carbon dioxide and water into glucose and oxygen

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Photosynthesis is a light-driven reaction

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True

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How many stages of photosynthesis are there?

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Two

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What is the glucose formed by the plant used for?

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Energy

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What are the two stages of photosynthesis?

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The light-dependent reaction and light-independent reaction (aka Calvin cycle)

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