The Working Cell: Membrane Structure, Function, and Cellular Energy

The Working Cell

Introduction

This chapter explores how cells function as dynamic systems, focusing on the roles of membranes, proteins, and energy transformations. Understanding these processes is essential for grasping how cells maintain life and perform work.

• Membranes and proteins are crucial for cell survival and function.

• The chapter addresses how working cells use membranes and enzymes to regulate internal conditions and energy use.

Big Ideas of Chapter 5

• Membrane Structure and Function: Understanding the composition and properties of cellular membranes.

• Energy and the Cell: How cells obtain and use energy, particularly through cellular respiration and ATP.

• How Enzymes Function: The role of enzymes in facilitating biochemical reactions.

Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of diverse protein molecules embedded in a fluid bilayer of phospholipids.

• Membranes are composed of a phospholipid bilayer with proteins suspended within it.

• Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Proteins are not fixed in place; they move laterally within the membrane, allowing flexibility and dynamic function.

Selective Permeability

Cell membranes are selectively permeable, meaning they allow some substances to cross more easily than others.

• Small, nonpolar molecules (e.g., O2, CO2, water) can diffuse freely across the membrane.

• Ions and large polar molecules are generally blocked unless transported by specific proteins.

• Membrane proteins perform various functions, including transport, signaling, and cell recognition.

Phospholipids and Membrane Assembly

• Phospholipids spontaneously form bilayers in aqueous environments, a process believed to be important in the origin of life.

• The bilayer separates the internal environment of the cell from the external environment.

• Glycolipids (lipids with carbohydrate groups) are found on the extracellular surface and contribute to the glycocalyx, a carbohydrate-rich coating important for cell recognition.

Membrane Proteins

• Integral proteins span the membrane and are involved in transport and communication.

• Peripheral proteins are attached to the membrane surface and play roles in signaling and maintaining cell shape.

Transport Across Membranes

Passive Transport

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

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

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

Osmosis and Tonicity

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

Solution Type	Relative Solute Concentration	Effect on Animal Cell	Effect on Plant Cell
Isotonic	Equal inside and outside	Normal	Flaccid
Hypotonic	Lower outside	Lysed (bursts)	Turgid (normal)
Hypertonic	Higher outside	Shriveled (crenation)	Plasmolyzed (shriveled)

Facilitated Diffusion

Some molecules (e.g., ions, polar molecules) cannot diffuse directly through the lipid bilayer and require transport proteins to facilitate their movement.

• Facilitated diffusion is still passive and does not require energy.

• Specific proteins (channels or carriers) help substances move down their concentration gradient.

• Aquaporins are channel proteins that facilitate the rapid diffusion of water.

Active Transport

Active transport moves substances against their concentration gradient and requires energy, usually from ATP.

• Transport proteins bind to the solute and use energy to change shape and move the solute across the membrane.

• Example: The sodium-potassium pump in animal cells.

Bulk Transport

• Endocytosis: The process of taking in large particles or fluids by engulfing them in vesicles.

• Phagocytosis: "Cellular eating"; engulfing large particles or cells.

• Pinocytosis: "Cellular drinking"; engulfing fluids and dissolved solutes.

• Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors on the cell surface.

• Exocytosis: The process of expelling materials from the cell by vesicle fusion with the membrane.

• Transcytosis: Movement of material into, across, and then out of a cell.

Energy and the Cell

Forms of Energy

• Energy is the capacity to cause change or do work.

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy due to position or structure, including chemical energy in bonds.

Laws of Thermodynamics

• First Law: Energy can be transferred and transformed, but cannot be created or destroyed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

ATP: The Energy Currency of the Cell

• ATP (adenosine triphosphate) stores and releases energy for cellular work.

• Energy is released when ATP is hydrolyzed to ADP and a phosphate group:

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

• ATP powers chemical, transport, and mechanical work in cells.

Enzymes and Cellular Reactions

Enzyme Function

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required.

• Enzymes are not consumed in the reaction.

• Each enzyme is specific to its substrate, which binds at the enzyme's active site.

• The enzyme-substrate complex undergoes a reaction to form products.

Enzyme Inhibition

• Competitive inhibitors bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors bind elsewhere on the enzyme, changing its shape and reducing activity.

• Feedback inhibition: The end product of a metabolic pathway inhibits an earlier step, regulating the pathway.

• Many drugs and poisons act as enzyme inhibitors.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose, ions via channels
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Endocytosis	Yes (ATP)	Into cell	Phagocytosis
Exocytosis	Yes (ATP)	Out of cell	Secretion of proteins

Additional info: Some context and terminology were inferred and expanded for clarity and completeness, such as the detailed explanation of membrane proteins, the ATP cycle, and the summary tables.