6.1 Energy and Metabolism

Energy and Metabolism

Energy and metabolism cover all the chemical reactions happening inside living cells. These reactions break down molecules to release energy and build new molecules that cells need for growth, repair, and everyday function. Understanding metabolism gives you the foundation for everything else in cellular biology, from photosynthesis to cell division.

Anabolic vs. Catabolic Pathways

Metabolism refers to the complete set of chemical reactions occurring in an organism. These reactions fall into two broad categories that work in opposite directions.

Catabolic pathways break down complex molecules into simpler ones and release energy in the process. Think of catabolism as "breaking down." Key examples:

• Glycolysis breaks glucose (a 6-carbon sugar) into two molecules of pyruvate (3 carbons each)
• The citric acid cycle oxidizes acetyl-CoA, generating ATP and electron carriers (NADH and $FADH_2$)
• Fatty acid oxidation (beta-oxidation) chops fatty acids into acetyl-CoA units that feed into the citric acid cycle

Anabolic pathways build complex molecules from simpler ones and require energy input. Think of anabolism as "building up." Key examples:

• Protein synthesis assembles amino acids into proteins
• Gluconeogenesis generates glucose from non-carbohydrate sources like amino acids, lactate, or glycerol
• Lipogenesis synthesizes fatty acids from acetyl-CoA

The relationship between these two is direct: catabolic reactions release the energy that anabolic reactions consume. They're constantly running in parallel to keep cells functioning.

ATP as Cellular Energy Currency

ATP (adenosine triphosphate) is the primary energy currency of cells. Nearly every energy-requiring process in your body is powered by ATP.

Structurally, ATP is made of three parts: an adenine base, a ribose sugar, and three phosphate groups linked in a chain. The energy is stored in the bonds between those phosphate groups, particularly the bond between the second and third phosphate.

When a cell needs energy, it breaks that terminal phosphate bond through hydrolysis:

$ATP + H_2O \rightarrow ADP + P_i + \text{Energy}$

This converts ATP into ADP (adenosine diphosphate) plus an inorganic phosphate ($P_i$), releasing energy the cell can use. ATP is then regenerated by reattaching a phosphate group to ADP, which requires energy input from catabolic pathways. This cycle of breaking down and rebuilding ATP happens constantly.

ATP powers a wide range of cellular work:

• Muscle contraction for movement
• Active transport of molecules against their concentration gradients
• Biosynthesis of complex molecules like proteins, nucleic acids, and lipids
• Cell division during mitosis and meiosis

Enzymes in Metabolic Reactions

Enzymes are biological catalysts, meaning they speed up chemical reactions without being consumed or permanently altered. Without enzymes, most metabolic reactions would occur far too slowly to sustain life.

How Enzymes Work

Enzymes work by lowering the activation energy of a reaction. Activation energy is the minimum energy needed to get a reaction started. The enzyme doesn't change whether a reaction releases or absorbs energy overall; it just makes the reaction happen faster.

Each enzyme is specific to particular substrates (the molecules it acts on). The substrate binds to the enzyme's active site, a region with a shape and chemical environment that fits the substrate. The induced fit model describes how the enzyme slightly changes shape when the substrate binds, creating a tighter and more effective fit.

Factors Affecting Enzyme Activity

• Temperature

  1. Most human enzymes work best around 37°C (body temperature)
  2. High temperatures denature enzymes by disrupting their 3D structure, permanently reducing activity

• pH

  1. Each enzyme has an optimal pH range (e.g., pepsin in the stomach works best at pH ~2, while trypsin in the small intestine prefers pH ~8)
  2. Extreme pH values can denature enzymes, similar to high temperature

• Substrate concentration — increasing substrate concentration speeds up the reaction rate, but only until all enzyme active sites are occupied (enzyme saturation). After that point, adding more substrate has no effect.

• Enzyme concentration — more enzyme molecules means more active sites available, which increases the reaction rate (assuming sufficient substrate is present).

Enzyme Regulation

Cells don't just let enzymes run freely. They regulate enzyme activity to match the cell's needs.

• Allosteric regulation — molecules bind to a site on the enzyme other than the active site, changing the enzyme's shape and activity.
  • Activators increase activity (e.g., fructose-2,6-bisphosphate activates phosphofructokinase)
  • Inhibitors decrease activity (e.g., ATP inhibits phosphofructokinase)
• Covalent modification — chemical groups are added to or removed from the enzyme. Phosphorylation (adding a phosphate group) and dephosphorylation (removing it) are the most common examples. Glycogen phosphorylase, for instance, is activated by phosphorylation.
• Feedback inhibition — the end product of a metabolic pathway inhibits an enzyme earlier in that same pathway. This prevents the cell from overproducing a product it already has enough of. For example, ATP inhibits citrate synthase in the citric acid cycle when energy levels are already high.

Thermodynamics and Energy in Biological Systems

The laws of thermodynamics govern how energy moves through living systems, just as they govern energy everywhere else.

Free energy (G) is the energy in a system that's available to do work. The change in free energy ($\Delta G$) during a reaction tells you whether that reaction is spontaneous:

• If $\Delta G < 0$, the reaction is exergonic (spontaneous, releases free energy). Catabolic reactions are typically exergonic.
• If $\Delta G > 0$, the reaction is endergonic (not spontaneous, requires energy input). Anabolic reactions are typically endergonic.

Cells often couple exergonic and endergonic reactions together so that the energy released by one drives the other. ATP hydrolysis is the most common energy source for this coupling.

Entropy (S) measures the degree of disorder in a system. The second law of thermodynamics says that entropy in the universe always tends to increase. Living organisms seem to defy this by maintaining highly organized, low-entropy structures, but they do so by constantly taking in energy and releasing heat and waste, which increases entropy in their surroundings.

Two critical processes tie thermodynamics to ATP production:

• The electron transport chain passes electrons through a series of protein complexes embedded in the inner mitochondrial membrane, releasing energy at each step
• Oxidative phosphorylation uses that released energy to pump protons across the membrane, creating a gradient that drives ATP synthase to produce ATP

These processes together account for the majority of ATP generated during cellular respiration.