Energy Flow and Metabolism in Cells
Unit 2: Chapters 6 and 7 - Energy and Cellular Respiration
Course Logistics and Important Dates
Assignments:
One assignment for Chapters 6 and 7.
One assignment for Chapter 8.
Midterm Exam:
Date: Tuesday, October 14th.
Class Cancellation: No class on October 7th.
Labs:
Topics: Biomolecules, Calorimetry, and Photosynthesis.
Lab Midterm: Thursday, October 16th.
Class Start Time: 5:30 PM.
Attendance Policy:
Students who anticipate or have already missed 2+ labs must contact the instructor immediately.
Missing more than 2 labs will result in course failure.
Students should communicate with the instructor regarding any missed classes to discuss making up content.
6.1 Life and the Flow of Energy
Energy: The ability to do work or bring about a change.
Cellular and Organismal Energy Needs: Cells and organisms require a constant supply of energy for:
Growth
Response to stimuli
Metabolism
Reproduction
Origin of Life's Energy: Life on Earth is fundamentally dependent on solar energy.
Photosynthesis: Converts solar energy into chemical energy (nutrients) for the majority of organisms.
Forms of Energy
Kinetic Energy: The energy of motion.
Example: A ball rolling down a hill.
Potential Energy: Stored energy.
Example: Food contains stored chemical energy.
Types of Energy:
Chemical Energy: Stored in chemical bonds.
Mechanical Energy: Energy of motion, e.g., walking.
Two Laws of Thermodynamics
First Law of Thermodynamics (Law of Conservation of Energy):
Energy cannot be created or destroyed.
Energy can only be changed from one form to another.
Example in Ecosystems: A leaf cell performing photosynthesis uses solar energy to form carbohydrates but does not create the energy itself. Some energy is lost as heat, but none is destroyed.
Second Law of Thermodynamics:
Energy cannot be changed from one form to another without a loss of usable energy (often as heat).
No energy conversion process is 100\% efficient.
Heat released from cellular respiration or photosynthesis dissipates into the environment, becoming unusable for work.
This heat cannot be captured and converted into more useful forms of energy.
Example in Ecosystems: Plants and organisms utilize cellular respiration to convert chemical energy from carbohydrates into mechanical energy for cell activities and growth. Some energy is inevitably lost as heat, but none is destroyed.
Cells and Entropy
Alternative Statement of the Second Law of Thermodynamics: Every energy transformation increases the disorganization (disorder) of the universe.
Entropy: Refers to the relative amount of disorganization or randomness.
Cellular Processes and Entropy: All energy transformations occurring in cells increase the total entropy of the universe.
Energy Input: Cellular processes (e.g., transport, biosynthesis) require a continuous input of energy from an outside source.
Ultimately, living organisms depend on a constant supply of energy from the sun.
Entropy and Stability:
More organized systems: More potential energy, less stable (lower entropy).
Less organized systems: Less potential energy, more stable (higher entropy).
Example: Glucose (C6H{12}O6) is more organized (more potential energy, less stable) than carbon dioxide (CO2) and water (H_2O) (less organized, less potential energy, more stable).
Example: An unequal distribution of hydrogen ions (H^+) across a membrane represents a more organized state with higher potential energy (less stable) than an equal distribution (less organized, less potential energy, more stable).
6.2 Energy Transformations and Metabolism
Metabolism: The sum of all chemical reactions that occur within a cell.
Components of Metabolism:
Catabolism: Breaking down molecules.
Anabolism: Building up molecules.
Chemical Reactions:
Reactants: Substances that participate in a reaction.
Products: Substances that are formed as a result of a reaction.
General Equation: A + B \rightarrow C + D
Free Energy (\Delta G): The amount of energy available to do work.
Calculated as the free energy of products minus the free energy of reactants.
A negative \Delta G indicates a reaction will proceed spontaneously.
Types of Reactions based on Free Energy:
Exergonic Reactions: Spontaneous reactions that release energy.
Products have less free energy than reactants (\Delta G is negative).
Endergonic Reactions: Reactions that require an input of energy to proceed.
Products have more free energy than reactants (\Delta G is positive).
ATP: Energy for Cells
ATP (Adenosine Triphosphate): The common energy currency for cells.
ATP Generation: Generated from ADP (adenosine diphosphate) and an inorganic phosphate molecule (P_i).
Primarily generated through glucose breakdown (cellular respiration).
The creation of ATP from ADP and P_i is an endergonic reaction, requiring energy input (\Delta G is positive).
ATP Utilization: ATP breakdown (hydrolysis) releases stored energy, which can power endergonic reactions (\Delta G is negative).
Structure of ATP: A nucleotide composed of:
Adenine (a nitrogen-containing base).
Ribose (a 5-carbon sugar).
Three phosphate groups.
Energy is stored in the high-energy chemical bonds between the phosphate groups.
Stability: ATP is unstable and has high potential energy. ADP is more stable and has lower potential energy.
Function of ATP
Cellular Use of ATP: ATP provides energy for various cellular activities:
Chemical Work: Energy for synthesizing macromolecules (anabolism).
Transport Work: Energy to pump substances across membranes against their concentration gradients.
Mechanical Work: Energy for muscle contraction, cilia and flagella beat, and other movements.
Coupled Reactions: A mechanism where the energy released by an exergonic reaction is used to drive an endergonic reaction.
ATP breakdown is frequently coupled with reactions that require energy input.
Mechanisms of Coupling ATP Hydrolysis to Endergonic Reactions:
ATP energizes a reactant: By transferring a phosphate group to it.
ATP changes the shape of a reactant: Also typically through phosphorylation.
Phosphorylation: The transfer of a phosphate group to a reactant.
Examples of ATP-Powered Reactions:
Macromolecule synthesis.
Organization of molecules.
Maintenance of internal conditions (homeostasis).
Movement of cellular organelles and the organism.
Muscle Contraction Example: In muscle contraction, ATP hydrolysis changes the shape of myosin filaments, enabling them to bind and pull on actin filaments, generating force and motion.
Myosin binds with ATP, assuming its resting shape.
ATP splits into ADP and P_i, causing myosin to change shape and attach to actin.
Release of ADP and P_i causes myosin to again change shape and pull against actin, generating force and motion.
6.3 Enzymes and Metabolic Pathways
Metabolic Pathways: A series of interconnected reactions within a cell.
Reactions are not random; they follow specific sequences, starting with a reactant and ending with a product.
Energy is captured and utilized more efficiently because it is released in controlled increments.
The product of one reaction often serves as the reactant for the next reaction in the pathway.
Enzymes:
Typically proteins that function as biological catalysts, accelerating the rate of chemical reactions.
Ribozymes: RNA molecules that can also exhibit catalytic activity.
Enzymes participate in chemical reactions but are not consumed or used up during the reaction.
Enzymes do not determine if a reaction will proceed; the change in free energy (\Delta G) dictates reaction spontaneity.
Metabolic Pathway Example: If A is converted to D through intermediates B and C, then A \xrightarrow{E1} B \xrightarrow{E2} C \xrightarrow{E_3} D
Reactants (substrates): A, B, C.
Products: B, C, D.
Enzymes: E1, E2, E_3.
Energy of Activation (E_a)
Energy of Activation: The minimum amount of energy that must be added to cause reactants to initiate a chemical reaction.
Even exergonic reactions (which have a negative \Delta G) require an initial input of energy to overcome the energy of activation. (e.g., a match to start wood burning).
Enzyme Function: Enzymes lower the energy of activation.
They do not alter the overall \Delta G or the final equilibrium of the reaction.
By lowering the \Delta G, enzymes significantly increase the reaction rate.
How Enzymes Function
Enzyme-Substrate Complex: An enzyme binds specifically with its substrate(s) to form an enzyme-substrate complex.
Active Site: A specific, small region on the enzyme where the substrate(s) bind.
Sequence of an Enzyme-Catalyzed Reaction:
E \; (enzyme) + S \; (substrate) \rightarrow ES \; (enzyme-substrate \; complex) \rightarrow E \; (enzyme) + P \; (product)Reaction at the Active Site: The substrate(s) and enzyme fit together at the active site, facilitating the chemical reaction.
Product Release and Enzyme Recycling: After the reaction, products are released, and the enzyme is free to catalyze subsequent reactions.
Types of Enzymatic Reactions:
Degradation: An enzymatic reaction breaks down a single substrate into multiple smaller products.
Synthesis: An enzymatic reaction combines multiple substrates to produce a larger product.
Induced Fit Model:
The active site and substrate are not perfectly complementary (like a