The Working Cell: Membranes, Energy, and Cellular Respiration

Chapter 5: The Working Cell

Overview

The plasma membrane and its proteins enable cells to survive and function. This chapter addresses how working cells use membranes, energy, and enzymes.

• Membranes

• Energy

• Enzymes

Big Ideas

• Cellular respiration

• ATP

• Membrane structure and function

• Energy and the cell

• How enzymes function

Membrane Structure and Function

Fluid Mosaic Model

• Definition: The fluid mosaic model describes a membrane’s structure as diverse protein molecules suspended in a fluid phospholipid bilayer.

• The plasma membrane exhibits selective permeability, allowing certain substances to pass while blocking others.

• Various proteins embedded in the membrane perform multiple functions, such as:

  • Transport

  • Reaction catalysis

  • Communication between cells

  • Cell recognition

Membrane Components

• Phospholipids: Form the fundamental structure of the cell membrane, consisting of hydrophilic heads and hydrophobic tails.

• Proteins: Various types include:

  • Transport Proteins: Allow specific ions or molecules to enter or exit the cell.

  • Enzymatic Proteins: Catalyze biochemical reactions.

  • Receptor Proteins: Bind signaling molecules and relay messages into the cell.

  • Attachment Proteins: Help support the membrane by attaching to the extracellular matrix and cytoskeleton.

Membrane Transport

Passive Transport

• Definition: The movement of molecules across a cell membrane without the use of energy (ATP).

• Diffusion: Tendency of particles to spread out evenly in available space.

Types of Passive Transport

1. Simple Diffusion: Movement of small nonpolar molecules (e.g., O2, CO2) across a membrane.

2. Facilitated Diffusion: Movement of polar or charged substances through a membrane with the help of specific transport proteins; still passive, relies on concentration gradients.

3. Osmosis: Diffusion of water across a selectively permeable membrane. Water moves down its own concentration gradient.

Tonicity

• Tonicity: Describes the ability of a surrounding solution to cause a cell to gain or lose water.

  • Hypertonic Solution: Higher solute concentration compared to the cell; results in cell shrinkage.

  • Hypotonic Solution: Lower solute concentration compared to the cell; results in cell swelling.

  • Isotonic Solution: Equal solute concentration; animal cells remain normal, plant cells become flaccid.

Active Transport

• Definition: The movement of solutes against their concentration gradient, requiring energy (ATP).

Mechanism

1. Solute binding: On the cytoplasmic side, the solute binds to the transport protein.

2. Phosphorylation: ATP transfers a phosphate group to the transport protein.

3. Transport: The protein changes shape, releasing the solute on the other side.

4. Resetting protein: The transport protein returns to its original shape, allowing more solute to bind.

Bulk Transport

• Cells utilize exocytosis and endocytosis to move large molecules across membranes.

• Exocytosis: Exports bulky molecules (e.g., proteins or polysaccharides). The material is packaged in vesicles that fuse with the plasma membrane.

• Endocytosis: Takes in large molecules, including two types:

  • Phagocytosis: Engulfment of particles forming vacuoles.

  • Receptor-mediated endocytosis: Uses receptors to transport specific solutes; the material is also packaged in vesicles.

Energy and the Cell

Energy Definitions

• Energy: The capacity to cause change (work).

• Types of Energy:

  • Kinetic Energy: Energy of motion.

  • Potential Energy: Stored energy, including chemical energy.

Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Energy transfer increases disorder (entropy) and loss of usable energy, often as heat.

Cellular Work

• ATP (Adenosine Triphosphate): Powers nearly all forms of cellular work (biochemical, transport, mechanical).

• Work Types:

  • Chemical Work: Making and breaking chemical bonds.

  • Transport Work: Pumping molecules across membranes against concentration gradients.

  • Mechanical Work: Movement of structures within cells.

Enzymes

General Function

• Enzymes: Biological catalysts that lower the activation energy needed for reactions to occur, without being consumed in the process.

Mechanism of Action

1. Active Site: The specific region of the enzyme where the substrate binds.

2. Enzyme-Substrate Complex: Enzyme holds the substrate with an induced fit, causing a chemical reaction.

3. Product Formation: The substrate is transformed into products which are released.

Inhibition Types

• Competitive Inhibition: Inhibitor competes with substrate for the active site.

• Noncompetitive Inhibition: Inhibitor binds elsewhere on the enzyme, altering its shape and function.

• Feedback Inhibition: Regulatory mechanism to control metabolic pathways; prevents overproduction by inhibiting early-stage enzymes.

Key Takeaways

• Understand the structure and functions of the plasma membrane.

• Differentiate between different types of transport (passive vs. active).

• Grasp the concepts of energy transformation, cellular energy management using ATP, and the fundamentals of enzymatic activity.

• Recognize the significance of enzyme inhibitors in regulation, as well as their implications in health and industry (e.g., drugs, pesticides).

Introduction to Cellular Respiration

• Cellular respiration is a vital biochemical process in organisms.

• Oxygen serves as a reactant in this process, facilitating the breakdown of sugars and other food molecules.

• The result of successful respiration is the generation of ATP (adenosine triphosphate), which is recognized as the energy currency of the cell, as well as the release of heat.

• Brown fat cells have a unique mechanism where their cellular respiration is altered to produce heat directly, without forming ATP.

• This chapter outlines the stages of cellular respiration and examines how ATP is produced in the presence of oxygen.

Cellular Respiration and Photosynthesis

• In nearly all ecosystems, the primary source of energy is the sun.

• Photosynthesis: Sunlight energy is harnessed to rearrange the atoms of carbon dioxide (CO2) and water (H2O), producing organic molecules and releasing oxygen (O2).

• Cellular respiration: Oxygen (O2) is consumed, and organic molecules are degraded to carbon dioxide (CO2) and water (H2O), releasing ATP.

Breathing and Cellular Respiration

• Respiration is frequently equated with breathing, defined as the exchange of gases: O2 is acquired from the environment, and CO2 is expelled as waste.

• There is a close interrelation between breathing and cellular respiration.

Energy Capture and ATP Production

• Cellular respiration is characterized as an exergonic process, meaning it releases energy.

• The process transfers energy from glucose to synthesize ATP.

• Approximately 34% of the available energy originally housed in glucose is captured, while the remainder is dissipated as heat.

Connection of ATP to Human Activities

• The human body continuously requires energy for functionality.

• Cellular respiration contributes to energy for routine maintenance and voluntary actions.

• A balance between energy intake and expenditure is essential for maintaining a healthy weight.

Cellular Energy Extraction Principles

• Cellular energy extraction involves the transfer of electrons in chemical reactions.

• NAD+ is an important electron carrier in cellular respiration.

• The resulting NADH transfers electrons into an electron transport chain. As electrons move from one carrier to another, culminating in oxygen (O2), energy is systematically released.

Stages of Cellular Respiration

Overview of the Three Stages of Cellular Respiration

1. Glycolysis: Occurs in the cytosol. Consumes cellular respiration by breaking down glucose into two molecules of pyruvate (a three-carbon compound).

2. Pyruvate oxidation and the citric acid cycle: Takes place in the mitochondria. Completes the breakdown of glucose into carbon dioxide and water, supplying electrons for the following stage.

3. Oxidative phosphorylation: This phase encompasses electron transport and chemiosmosis. Most ATP from cellular respiration is synthesized here, with electrons resolved to oxygen that is reduced to water.

Detailed Examination of Glycolysis

• Glycolysis initiates with the activation of glucose by utilizing ATP, splitting glucose into two pyruvate molecules.

• This generates a net yield of 2 ATP and 2 NADH – highlighting the transfer of energy during the reaction.

• ATP is formed mainly by substrate-level phosphorylation, where a phosphate group is directly transferred from an organic molecule to ADP.

• Glycolysis includes a phase of energy investment, consuming two ATP molecules to add phosphate groups before it is split into smaller units.

• The subsequent energy payoff phase, comprising steps 5-9, yields further energy through reactions that produce pyruvate and additional ATP.

• Each step of glycolysis is intertwined with several specific chemical reactions, characterized by specific enzymes that aid the process.

Summary of Glycolysis and Related Questions

• Net products of glycolysis per glucose molecule:

  • 2 Pyruvate

  • 2 ATP

  • 2 NADH

Figures Overview

• Figures show the transcript illustrating key processes, including the pathways of cellular respiration, equations for processes involved, and metabolic pathways showing how glycolysis reacts.

• Figures serve to exemplify concepts discussed, enhancing the understanding of complex biochemical processes involved in cellular respiration.

Stage 2: The Citric Acid Cycle (CAC)

Completion of the Energy-Yielding Oxidation of Organic Molecules

• The oxidation of pyruvate results in the formation of:

  • Acetyl CoA

  • Carbon Dioxide (CO2)

  • Nicotinamide adenine dinucleotide (NADH)

• For each turn of the Citric Acid Cycle (CAC):

  • 2 carbons from Acetyl CoA are added to the cycle.

  • 2 CO2 are released as waste products.

  • 3 NADH and 1 FADH2 are produced as energy carriers.

CAC Major Steps

1. Formation of Citrate: Two carbons enter the cycle from Acetyl CoA and combine with Oxaloacetate, forming Citrate.

2. Overview of Steps 2-3: Involve redox reactions that generate NADH, release CO2 as waste, and produce ATP.

3. Step 4: Completion of the Cycle: Further redox reactions take place from Alpha-ketoglutarate to Succinate, generating NADH + H+ and FADH2.

Stage 3: Oxidative Phosphorylation

• Most ATP production occurs during Oxidative Phosphorylation, involving:

  • Electrons from NADH and FADH2 being transferred through the Electron Transport Chain (ETC) in the mitochondria.

  • Oxygen (O2) acts as the final electron acceptor, forming water (H2O).

  • Energy released during these redox reactions powers the pumping of protons (H+) into the intermembrane space, creating a high gradient.

  • Chemiosmosis utilizes this gradient to drive the flow back through ATP synthase, synthesizing ATP.

Maximum ATP Yield in Cellular Respiration

• Breakdown of one glucose molecule can yield a maximum of:

  • 2 ATP from substrate-level phosphorylation during glycolysis

  • About 28 ATP from oxidative phosphorylation

• Total estimated ATP production per glucose molecule is around 32 ATP.

Fermentation: Anaerobic Energy Production

• Under anaerobic conditions, cells such as muscle cells, yeasts, and some bacteria can produce ATP through glycolysis alone.

• Key process: Regenerating NAD+ as pyruvate is reduced to:

  • Lactate (in lactic acid fermentation)

  • Ethanol and CO2 (in alcohol fermentation)

Types of Fermentation

• Alcohol fermentation: Glucose → Glycolysis → 2 Pyruvate → 2 Ethanol + 2 CO2

• Lactic acid fermentation: Glucose → Glycolysis → 2 Pyruvate → 2 Lactate

Summary Table: Cellular Respiration Stages and ATP Yield

Stage	Main Location	Main Products	ATP Yield (per glucose)
Glycolysis	Cytosol	2 Pyruvate, 2 NADH	2
Citric Acid Cycle	Mitochondrial Matrix	6 NADH, 2 FADH2, 4 CO2	2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	H2O, ATP	~28
Total	-	-	~32

Key Equations

• Cellular Respiration Overall Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP} + \text{heat}$

• ATP Hydrolysis:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Tables and equations have been formatted for study purposes.