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Chapter 5: The Dynamic Cell — Energy, Metabolism, and Transport

5.1 What Is Energy?

  • Energy is the capacity to do work.
  • Energy exists in many forms, including sonic, gravitational, nuclear, and thermal.
  • The biosphere gets its energy from the sun; solar energy powers activities from charging phones to powering homes.
  • Summary: Energy comes in various forms and flows through living systems, starting with the sun.

5.1b Types of Energy

  • Mechanical energy
  • Thermal energy
  • Nuclear energy
  • Chemical energy
  • Electromagnetic energy
  • Sonic energy
  • Gravitational energy
  • Kinetic energy: energy of motion
  • Potential energy: stored energy
  • Ionization energy (listed as a form of energy)
  • Note: These forms can be converted from one to another in biological processes.

5.1c Basic Forms of Energy

  • Two basic forms:
    • Potential energy — stored energy
    • Kinetic energy — energy of motion
  • These two forms are interconvertible, but during conversions some energy is lost as heat.
  • Statement to remember: energy transformations are not 100% efficient; some usable energy becomes heat.

5.1d Potential vs Kinetic − Figure reference concepts

  • Potential energy is stored energy before a change; kinetic energy is energy during motion.
  • Energy flow includes conversion between potential and kinetic, with heat as a byproduct.

5.1e Calorie and Kilocalorie

  • Energy in foods is measured in Calories (Cal).
  • Definitions:
    • A calorie is the amount of heat required to raise the temperature of 1 g of water by 1°C.
    • A kilocalorie (kcal) equals 1000 calories.
  • Food energy is measured in Calories; the commonly seen value on packages represents kilocalories.
  • Key relationship: 1\,\text{Cal (food Calorie)} = 1\,\text{kcal} = 1000\,\text{cal}

5.2 Energy Laws

  • First Law (conservation of energy): energy cannot be created or destroyed, but it can be transformed from one form to another.
  • Second Law: energy cannot be transformed from one form to another without a loss of usable energy; heat is the least usable form of energy.

5.2a Entropy (Entropía)

  • Entropy measures the disorganization or disorder of energy transformations.
  • Entropy increases with every energy transformation.
  • To maintain or create order, energy must be added.

5.2b Entropy and the Universe

  • Our universe is treated as a closed system; all energy transformations increase the total entropy of the universe.
  • The sun provides energy that allows plants to make glucose from water and carbon dioxide, though some solar energy is lost as heat.

5.2c Cells and Entropy − Figure concepts

  • Entropy relates to how energy is distributed within cells; energy flow maintains order and function.

5.2d ATP: Energy for Cells

  • ATP: Adenosine triphosphate; energy currency of cells.
  • Cells use ATP to carry out nearly all activities.
  • Structure: one nucleotide with three phosphate groups makes ATP unstable; it easily releases a phosphate group to become ADP.
  • Continual cycle of breakdown and regeneration: \text{ATP} \rightarrow \text{ADP} + \text{P}i\,. (Pi = inorganic phosphate)

5.3 The ATP Cycle

  • ATP releases energy quickly; the amount of energy released is typically just enough for a biological purpose.
  • ATP breakdown energy can be easily coupled to an energy-requiring reaction.
  • The cycle: ATP stores energy, releases energy when hydrolyzed to ADP + P, and is regenerated.
  • Figure reference: ATP cycle shows the continual conversion between ATP and ADP + P_i.

5.3a Coupled Reactions

  • Coupled reactions: An energy-releasing reaction drives an energy-requiring reaction.
  • Typical energy-releasing step: ATP breakdown provides energy for other cellular processes.

5.3b Coupled Reaction Example – Muscle Movement

  • An example: Myosin binds ATP; ATP breaks down; release of ADP + P leads to a conformational change in myosin, pulling on actin and generating movement.
  • Summary: ATP hydrolysis provides the mechanical work for muscle contraction.
  • Key process: energy release from ATP breakdown enables another reaction to proceed.

5.4 The Flow of Energy

  • The flow of energy in organisms is driven by chloroplasts and mitochondria.
  • Photosynthesis: solar energy converts water and carbon dioxide into carbohydrates; this food energy supports plants and other organisms.
  • Cellular respiration: carbohydrates are broken down to release energy used to build ATP.
  • Useful energy is lost with each transformation; living things depend on a constant input of solar energy to sustain energy flow.

5.4a Flow of Energy – Figure concept

  • Diagrammatic depiction: energy from sun enters photosynthesis, becomes chemical energy in carbohydrates, which are then used by organisms to generate ATP via cellular respiration; heat is released throughout.

5.4b Humans and Energy

  • Humans participate in molecular cycling of nutrients between plants and animals.
  • We inhale oxygen and eat plants/animals; energy-rich foods provide ATP required for bodily functions and activities.

5.3 Metabolic Pathways and Enzymes

  • Metabolic pathway: a series of linked reactions.
  • Enzymes: protein molecules that act as organic catalysts to speed up reactions; they facilitate otherwise possible reactions.
  • Active enzyme and active pathway vs inactive enzyme and inactive pathway.
  • A pathway requires a series of enzymes (represented as E1, E2, E3, E4) acting on substrates to form end product P.

5.3a Enzymes and Action

  • Enzymes act on substrates and may facilitate breakdown or synthesis reactions.
  • Enzyme activity is governed by the active site.

5.3b Enzyme's Active Site and Specificity

  • Active site accommodates substrate; often described as lock-and-key specificity.
  • Induced fit: enzyme undergoes slight shape change to better fit the substrate.
  • After reaction, the active site returns to its original shape and is not consumed in the reaction.

5.3c Enzyme Inhibition

  • Enzyme inhibition occurs when an active enzyme is prevented from binding to its substrate.
  • Examples:
    • Cyanide binds to cytochrome c oxidase, inhibiting it.
    • Penicillin interferes with a bacterial enzyme, killing bacteria.

5.3d Feedback Inhibition

  • When a product is in abundance, it competes with the substrate for the active site.
  • End product of a pathway can inhibit the first enzyme in the pathway by binding to a site other than the active site (allosteric site), causing a shape change that shuts the entire pathway down.

5.3e Figures 5.8a–c (Feedback Inhibition)

  • a. Active enzyme and active pathway: substrate S binds to the active site of enzyme E1; subsequent enzymes E2, E3, E4 produce end product P.
  • b. Product binds to enzyme (allosteric regulation) causing a conformational change and reduced activity.
  • c. When product falls, enzyme returns to active form and pathway resumes.

5.3f Energy of Activation

  • Activation energy Ea: energy required to start a reaction; molecules often do not react unless activated.
  • Enzymes lower Ea, bringing substrates into contact and sometimes participating in the reaction.
  • Lower Ea makes a reaction easier and faster.
  • Notation: Ea is the energy of activation; with enzyme, Ea is reduced.

5.4 Cell Transport

  • The plasma membrane regulates traffic in and out of the cell and is selectively permeable.
  • Three ways substances move across membranes:
    • Passive transport (no energy): movement from high to low concentration along the gradient.
    • Active transport (requires energy): movement from low to high concentration against the gradient.
    • Bulk transport (requires energy): movement of large substances via vesicles independent of gradients.

5.4a Passive Transport

  • Simple diffusion: molecules move down their concentration gradient until equilibrium; no energy expenditure by the cell.
  • Some molecules pass directly through phospholipid bilayer.
  • Facilitated diffusion: uses transport proteins specific to the molecule; water moves via aquaporins, which can speed transport beyond simple diffusion.

5.4b Diffusion Demonstrations (Figure references)

  • Simple diffusion demonstration shows dye spreading through water until even distribution.
  • Facilitated diffusion demonstrates a carrier protein in the plasma membrane assisting solute entry.
  • Osmosis demonstrations show water movement across a semipermeable membrane under different solute concentrations.

5.4c Osmosis and Tonicity

  • Osmosis: diffusion of water across a semipermeable membrane.
  • Isotonic environment: no net water movement; concentrations of solute and water are balanced; example: 0.9% saline is isotonic to red blood cells.
  • Hypotonic environment: higher water concentration outside the cell than inside; cells gain water; animal cells may lyse; plant cells become turgid.
  • Hypertonic environment: lower water concentration outside the cell than inside; cells lose water; animal cells shrink; plant cells may plasmolyze and wilt.

5.4d Active Transport

  • Active transport moves substances against a concentration gradient using energy.
  • Requires a transport protein.
  • The sodium–potassium pump is crucial for maintaining ion gradients used in nerve impulse conduction.

5.4e The Sodium–Potassium Pump – Stepwise Mechanism

  • Step 1: Carrier protein has a shape that allows it to take up 3 Na+ ions from inside the cell.
  • Step 2: ATP is split; a phosphate group is added to the carrier (phosphorylation).
  • Step 3: Change in shape releases 3 Na+ ions to the outside.
  • Step 4: Carrier now takes up 2 K+ ions from outside the cell.
  • Step 5: Phosphate group is released from the carrier.
  • Step 6: Carrier returns to its original shape and releases 2 K+ ions inside the cell.
  • Result: Maintains electrochemical gradients essential for nerve impulses.

5.4f Bulk Transport – Exocytosis and Endocytosis

  • Bulk transport moves macromcules via vesicles rather than transporter proteins.
  • Exocytosis: vesicle fuses with plasma membrane to release contents outside the cell.
  • Endocytosis: vesicle forms inward and brings contents into the cell.
  • Types of endocytosis:
    • Phagocytosis: cell engulfs solid particles.
    • Pinocytosis: vesicle forms around extracellular fluid or small particles.
    • Receptor-mediated endocytosis: receptors in coated pits bind specific substances; highly selective and efficient.

5.4g Receptor-Mediated Endocytosis (Figure 5.17)

  • A coated pit with receptor proteins binds a target molecule and internalizes it via a vesicle.

Quick Connections and Relevance

  • The energy flow in biology begins with solar energy and moves through photosynthesis and cellular respiration to power ATP-dependent processes.
  • Enzymes lower activation energy, enabling biological reactions to proceed rapidly enough to sustain life.
  • Cellular transport mechanisms regulate internal environments, enabling cells to maintain homeostasis and communicate via electrical signals (e.g., nerve impulses).
  • The laws of thermodynamics impose limits on energy efficiency and explain why living systems require continuous energy input from the environment.