The Working Cell: Membrane Structure, Transport, and Enzymes

Chapter 5: The Working Cell

Introduction

The plasma membrane and its proteins are essential for cell survival and function. This chapter explores how cells utilize membranes, energy, and enzymes to perform vital processes.

• Plasma Membrane: Acts as a selective barrier, regulating the movement of substances in and out of the cell.

• Energy: Required for various cellular activities, including transport and metabolism.

• Enzymes: Catalyze biochemical reactions, enabling efficient cellular metabolism.

Membrane Structure and Function

Fluid Mosaic Model

Biologists describe the structure of cell membranes using the fluid mosaic model. This model depicts the membrane as a dynamic arrangement of diverse protein molecules suspended in a fluid phospholipid bilayer.

• Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, forming a semi-permeable barrier.

• Membrane Proteins: Embedded within the bilayer, performing various functions such as transport, signaling, and structural support.

• Selective Permeability: The membrane allows certain substances to pass while restricting others.

Functions of Membrane Proteins

• Transport Proteins: Facilitate the movement of specific ions or molecules across the membrane.

• Enzymatic Activity: Some proteins act as enzymes, catalyzing sequential reactions.

• Attachment Proteins: Anchor the membrane to the cytoskeleton and extracellular matrix, providing structural support and facilitating communication.

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

• Junction Proteins: Form intercellular junctions, connecting adjacent cells.

• Glycoproteins: Serve as identification tags recognized by other cells.

Membrane Transport

Passive Transport

Passive transport is the movement of substances across a membrane without the expenditure of cellular energy.

• Diffusion: The tendency of particles to spread out evenly in an available space, moving from areas of higher to lower concentration.

• Osmosis: The diffusion of water across a selectively permeable membrane.

Key Equation:

$\text{Net movement of water} = \text{from lower solute concentration to higher solute concentration}$

Tonicity and Water Balance

Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water.

• Hypertonic Solution: Higher solute concentration outside the cell; cell loses water and shrinks.

• Hypotonic Solution: Lower solute concentration outside the cell; cell gains water and swells.

• Isotonic Solution: Equal solute concentration; no net water movement.

Effects on Cells:

Solution Type	Animal Cell	Plant Cell
Hypotonic	Lysed (bursts)	Turgid (normal)
Isotonic	Normal	Flaccid
Hypertonic	Crenated (shriveled)	Plasmolyzed

Facilitated Diffusion

Polar or charged substances cannot easily cross the lipid bilayer. Facilitated diffusion uses transport proteins to move these substances down their concentration gradient without energy input.

• Aquaporins: Specialized channel proteins that facilitate rapid water transport.

Active Transport

Active transport requires energy (usually from ATP) to move solutes against their concentration gradient.

1. Solute binds to a specific transport protein.

2. ATP provides energy for the protein to change shape.

3. Solute is released on the other side of the membrane.

4. Protein returns to its original shape.

Key Equation:

$\text{Active Transport:} \quad \text{Energy (ATP)} + \text{Transport Protein} + \text{Solute}_{\text{low}} \rightarrow \text{Solute}_{\text{high}}$

Bulk Transport: Exocytosis and Endocytosis

Cells move large molecules or particles via vesicles in two main processes:

• Exocytosis: Exports bulky materials (e.g., proteins, polysaccharides) by fusing vesicles with the plasma membrane.

• Endocytosis: Imports materials by engulfing them in vesicles. Includes:

  • Phagocytosis: "Cell eating"; engulfs particles into a vacuole.

  • Receptor-mediated endocytosis: Uses membrane receptors to selectively take in specific molecules.

Energy and the Cell

Forms of Energy

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy due to position or structure; includes chemical energy stored in bonds.

• Heat: Energy transferred due to temperature difference.

Laws of Thermodynamics

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

• Second Law: Energy transformations increase disorder (entropy); some energy is lost as heat.

Chemical Reactions in Cells

• Exergonic Reactions: Release energy (e.g., cellular respiration).

• Endergonic Reactions: Require energy input (e.g., photosynthesis).

• Metabolism: The sum of all chemical reactions in a cell.

Key Equation:

$\text{Exergonic:} \quad \text{Reactants} \rightarrow \text{Products} + \text{Energy}$

$\text{Endergonic:} \quad \text{Reactants} + \text{Energy} \rightarrow \text{Products}$

ATP and Energy Coupling

ATP (Adenosine Triphosphate) is the cell's main energy currency. It powers cellular work by transferring a phosphate group to other molecules (phosphorylation).

• ATP Hydrolysis: Releases energy by converting ATP to ADP and a phosphate group.

Key Equation:

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

Enzymes and Cellular Metabolism

Enzyme Function

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required. They are not consumed in the reaction.

• Active Site: The region on the enzyme where the substrate binds.

• Substrate: The specific reactant acted upon by the enzyme.

• Enzyme-Substrate Complex: Temporary association during the reaction.

Enzyme Specificity and Catalytic Cycle

1. Enzyme is available with an empty active site.

2. Substrate binds to the active site, forming the enzyme-substrate complex.

3. Enzyme catalyzes the conversion of substrate to product.

4. Products are released; enzyme is free to catalyze another reaction.

Enzyme Inhibition

• Competitive Inhibitors: Bind to the active site, blocking substrate access.

• Noncompetitive Inhibitors: Bind elsewhere on the enzyme, altering its shape and function.

Applications: Many drugs, pesticides, and poisons act as enzyme inhibitors.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Osmosis	No	Water down gradient	Water via aquaporins
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Exocytosis	Yes (ATP)	Out of cell	Secretion of proteins
Endocytosis	Yes (ATP)	Into cell	Phagocytosis

Course Competencies

• Predict the effect of solutions with different tonicities on plant and animal cells.

• Relate the role of enzymes in biochemical pathways and cellular metabolism.