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An overview of how cells manage energy resources through coupled reactions and ATP.
Every second, your body's cells consume and regenerate millions of molecules of a tiny chemical 'battery' just to keep you alive. If you couldn't recycle this molecule, you would need to consume your body weight in it every single day—how does your body manage this incredible energy feat?
Metabolism is the sum of all chemical reactions within an organism. These reactions are organized into two opposing pathways. Catabolic pathways break down complex molecules (like carbohydrates) into simpler ones, releasing stored energy. This is an exergonic process where the change in free energy () is negative. Conversely, anabolic pathways consume energy to build complex molecules from simpler ones, such as synthesizing proteins from amino acids. These are endergonic processes with a positive . To survive, cells must constantly balance these two 'halves' of metabolism, using the energy released from catabolism to fuel the demands of anabolism.
1. You eat a piece of bread (complex starch). 2. Enzymes break the starch into glucose molecules (Catabolism). 3. Energy is released and captured by the cell. 4. The cell uses that energy to build new muscle fibers from amino acids (Anabolism).
Quick Check
If a reaction has a and requires an input of energy, is it catabolic or anabolic?
Answer
It is anabolic.
Think of ATP as a fully charged battery and ADP as a partially depleted one. 1. To 'use' the battery, the cell breaks the third phosphate bond, releasing of energy. 2. To 'recharge' the battery, the cell must perform cellular respiration to add a phosphate back onto ADP. 3. This cycle happens roughly times per day for a single ATP molecule!
Quick Check
What specifically makes the bonds between phosphate groups in ATP so 'high-energy'?
Answer
The mutual repulsion of the negative charges on the phosphate groups makes the bonds unstable and ready to release energy when broken.
Cells perform work by energy coupling: using an exergonic process (ATP hydrolysis) to drive an endergonic one. The key mechanism is phosphorylation, the transfer of a phosphate group from ATP to another molecule. The recipient molecule is now called a phosphorylated intermediate. This intermediate is less stable (more reactive) than the original molecule, providing the 'kick' needed for the reaction to proceed. For example, ATP drives active transport by phosphorylating transport proteins, causing them to change shape and move solutes across the cell membrane against their concentration gradient.
The conversion of Glutamic acid to Glutamine is endergonic (). 1. ATP transfers a phosphate to Glutamic acid, creating a phosphorylated intermediate. 2. This intermediate is high-energy and unstable. 3. Ammonia () displaces the phosphate group easily. 4. Total coupled reaction: . Because the net is negative, the reaction now occurs spontaneously.
Which of the following is the best definition of energy coupling?
When ATP is hydrolyzed to ADP, what happens to the third phosphate group?
Anabolic pathways are generally exergonic and release energy.
Review Tomorrow
In 24 hours, try to sketch the ATP molecule and explain why the bond between the second and third phosphate is like a 'coiled spring'.
Practice Activity
Look up the 'Sodium-Potassium Pump' and identify which step involves phosphorylation and how it changes the protein's shape.