Knowt - Topic 7
Topic 7: Energy and Life
Chapter 5 - The Dynamic Cell
What Is Energy?
Energy is defined as the capacity to do work.
The source of energy for our biosphere is primarily the sun.
Basic Forms of Energy
Two types of energy:
Potential Energy: This is stored energy.
Kinetic Energy: This is the energy of motion.
Conversion of Energy: Energy can be converted back and forth between potential and kinetic forms.
During energy conversions, some energy is lost as heat.
Potential Energy Versus Kinetic Energy
Measurement of Energy:
Energy in food is measured in calories.
Calorie: The amount of heat required to raise the temperature of 1 gram of water by 1°C.
Kilocalorie: Equivalent to 1,000 calories.
Energy Conversion in Nature:
Plants convert solar energy into potential energy, e.g., stored in apples.
All solar energy eventually dissipated as heat.
Humans convert the potential energy in food (apples) into kinetic energy during physical activities such as walking.
Energy Laws
First Law of Thermodynamics
Conservation of Energy:
Energy cannot be created or destroyed, only transformed from one form to another.
Second Law of Thermodynamics
Energy transformation is never 100% efficient; some energy is lost as unusable heat.
With each energy transformation, there is an increase in disorganization or disorder within a system.
Entropy: Describes the relative amount of disorder.
To maintain or create order within a system, energy must be added.
The universe is a closed system; all energy transformations lead to increased total entropy.
For example, energy from the sun allows plants to synthesize glucose from more disorganized substances like water and carbon dioxide, with some energy lost as heat.
Understanding Entropy through Real-World Comparisons
Less Stable System: Tidy room (less entropy) is more organized.
More Stable System: Messy room (more entropy) is less organized.
Measuring Hydrogen Ion Distribution
Unequal Distribution of Hydrogen Ions: Less stable, indicative of lower entropy.
Equal Distribution of Hydrogen Ions: More stable, indicative of higher entropy.
ATP: Energy for Cells
Overview of ATP
Adenosine Triphosphate (ATP): Known as the energy currency of cells.
Composed of one nucleotide and three phosphate groups, its structure makes it unstable and prone to losing a phosphate group, becoming adenosine diphosphate (ADP).
The ATP Cycle
ATP releases energy quickly during its breakdown and the energy release is typically just sufficient to meet biological demands.
Coupled Reactions: ATP breakdown drives energy-requiring reactions.
Example: Myosin binds with ATP, and as ATP breaks down, ADP + P is released, enabling myosin to change shape and exert force on actin filaments.
Importance of ATP
ATP is essential for muscle movement through energy coupling with ATP breakdown.
The Flow of Energy
Photosynthesis and cellular respiration processes enable energy transfer from the sun through living organisms.
Photosynthesis: Converts solar energy into carbohydrates (food).
Cellular Respiration: Breaks down carbohydrates to produce ATP, utilizing the energy stored in food.
Living organisms rely on a constant influx of solar energy for ongoing metabolic processes.
Human Energy Dynamics
Humans play a significant role in the cycling of energy by breathing oxygen and consuming food (plants and animals) to produce ATP necessary for various bodily functions.
Metabolic Pathways and Enzymes
Understanding Metabolic Pathways
A metabolic pathway is a series of linked biochemical reactions.
Enzymes, represented as E1 - E6, are proteins that act as organic catalysts to accelerate these reactions.
Enzyme Functionality
Enzymes act on specific substrates, facilitating both breakdown and synthesis reactions.
Lock and Key Model: Enzymes are specific to their substrates, fitting like a key in a lock.
Induced Fit Model: The enzyme may undergo a slight change in shape to better accommodate the substrate, facilitating the chemical reaction.
Active Site Dynamics
After the product is formed, the enzyme's active site returns to its original shape, ensuring the enzyme can act on new substrates.
Enzyme Inhibition: When an active enzyme is prevented from binding with a substrate, this inhibition can be due to toxins like cyanide, which targets cytochrome c oxidase, or penicillin, which inhibits bacterial enzymes.
Feedback Inhibition
Feedback inhibition occurs when an end product of a metabolic pathway increases, competing with the substrate for the enzyme's active site.
This competitive inhibition can shut down the entire metabolic pathway by causing structural changes in the enzyme.
Energy of Activation
Energy of Activation (Ea): The energy threshold required for a reaction to occur.
Enzymes lower the energy of activation, facilitating reactions by enabling substrates to come into contact and sometimes participating in the reaction itself.
A lower energy of activation simplifies and quickens reactions, akin to lowering a hurdle to make it easier to go over.
Cell Transport Mechanisms
The plasma membrane selectively regulates the entry and exit of substances in a cell and is characterized by its selective permeability.
Types of Cell Transport
Passive Transport: Movement of substances down their concentration gradient without the need for energy.
Examples:
Simple Diffusion: Movement of small or nonpolar molecules.
Osmosis: The diffusion of water across a semipermeable membrane.
Facilitated Diffusion: Requires transport proteins specific to particular molecules (e.g., aquaporins for water).
Active Transport: Movement against a concentration gradient requiring energy.
Examples:
Solute Pumping.
Vesicular Transport:
Endocytosis: Movement into the cell.
Exocytosis: Movement out of the cell.
Receptor-Mediated Endocytosis: Specific and efficient uptake mechanism.
Osmosis in Different Environments
Isotonic Solution:
No net gain or loss of water, cells remain stable.
Hypotonic Solution:
Water concentration is higher outside the cell, leading to cell swelling (lysis in animal cells and turgidity in plant cells).
Hypertonic Solution:
Water concentration is higher inside the cell, leading to cell shrinkage (plasmolysis in plant cells).
Active Transport Mechanisms
Active transport requires energy and transport proteins to maintain ionic gradients essential for processes like nerve impulse conduction (e.g., sodium-potassium pump).
Bulk Transport
Bulk Transport: Macromolecules may be too large to pass through protein transporters, resulting in vesicle formation for transport.
Endocytosis: Engulfs substances into the cell (e.g., phagocytosis, pinocytosis, receptor-mediated).
Exocytosis: Secretion of materials out of the cell.
Chapter 6 – Energy of Life
Overview of Photosynthesis
Photosynthesis transforms solar energy into the chemical energy of carbohydrates, primarily occurring in plants, algae, and cyanobacteria.
Producers: Organisms that can feed themselves and serve as food for other organisms, with photosynthesis occurring mainly in the green parts of plants (chloroplast).
Chloroplast Structure and Function
Chloroplasts: Specialized organelles in plant cells for photosynthesis, contain:
Thylakoids: Membrane disks containing chlorophyll for light absorption.
Grana: Stacked thylakoids.
Stroma: Space surrounding thylakoids where carbohydrates are synthesized.
The Photosynthetic Process
Photosynthesis utilizes carbon dioxide and water, driven by solar energy, with oxygen released as a by-product.
The end product is glucose, represented as CH2O or C6H12O6.
Oxidation-Reduction Reactions in Photosynthesis
Oxidation: Loss of electrons and energy.
Reduction: Gain of electrons and energy.
These reactions always occur in pairs (redox reactions).
Photosynthetic Reactions
Two sets of reactions:
Light Reactions: Occur in thylakoid membranes, converting solar energy to chemical energy (ATP and NADPH).
Water is oxidized to provide electrons, hydrogen ions, and oxygen.
ATP produced via electron transport chain.
Calvin Cycle: Occurs in the stroma, utilizes ATP and NADPH to convert carbon dioxide into carbohydrates.
Light Reactions Details
Photosystems (PSI and PSII) capture light energy, generating ATP and NADPH.
Electron Transport Chain: Takes energized electrons from PSII, creating a hydrogen ion gradient that facilitates ATP production.
The Calvin Cycle: Making Sugars
Reactions: 3 main steps in the Calvin Cycle include:
Carbon Dioxide Fixation: RuBP combines with CO2 to form 6-carbon molecules, which split into two 3-carbon molecules (3-PGA).
Reduction of Carbon Dioxide: ATP and NADPH convert PGA into G3P.
Regeneration of RuBP: G3P is used to reform RuBP using ATP.
G3P: Can be transformed into glucose or other essential molecules (structure, energy storage).
Chapter 7 – Energy for Cells
Cellular Respiration Overview
Cellular Respiration: The metabolic process that converts glucose and oxygen into ATP, carbon dioxide, and water, fundamentally the reverse of photosynthesis.
Phases of Glucose Breakdown
Glucose breakdown is a multi-step process to maximize energy capture for ATP synthesis.
Breakdown involves coenzymes (e.g., NAD⁺ → NADH, FAD⁺ → FADH2).
Glycolysis
Location: Occurs in the cytoplasm of eukaryotic cells.
Process: Breaks down glucose into two pyruvate molecules.
Divided into:
Energy-investment steps (ATP utilization).
Energy-harvesting steps (ATP and NADH production).
Anaerobic Breakdown: Fermentation
In absence of oxygen, pyruvate can be reduced to lactate for a brief energy source.
Microorganisms utilize fermentation pathways to produce organic compounds such as alcohol or acids.
Inside the Mitochondria
Preparatory Reaction: Converts pyruvate into acetyl-CoA, producing NADH and releasing CO2.
Citric Acid Cycle: Acetyl-CoA is processed, further generating NADH and FADH2 while producing ATP.
Electron Transport Chain (ETC)
The ETC in the mitochondria captures energy from NADH and FADH2 by transferring electrons, generating a hydrogen ion gradient.
Oxygen acts as the final electron acceptor, forming water, and ATP synthase utilizes this gradient for ATP synthesis.
Energy Yield from Glucose Metabolism
Up to 38 ATP can be generated from complete glucose metabolism, with some variability in energy capture based on cell type.
Approximately 36-38 ATP represents about 40% of the energy in a glucose molecule; remainder is dissipated as heat.
Alternative Metabolic Pathways
Cells can also use fats and proteins as energy sources, with fats yielding higher ATP due to longer carbon chains.
Intermediate metabolites from glucose can be converted into various biomolecules, emphasizing cellular flexibility in energy utilization and storage.