Lecture Exam 1: Chapter 3.2 pdf

Chapter 03: Cells – The Living Units

Introduction

This chapter focuses on the mechanisms by which cells transport substances across their plasma membranes, emphasizing the importance of active membrane transport processes in maintaining cellular function and homeostasis.

Active Membrane Transport

Overview of Active Membrane Transport

Active membrane transport refers to cellular processes that require energy (usually in the form of ATP) to move substances across the plasma membrane. These processes are essential when:

• The solute is too large for membrane channels

• The solute is not lipid soluble

• The solute cannot move down its concentration gradient

There are two major types of active membrane transport:

• Active transport

• Vesicular transport

Active Transport

Carrier Proteins and Solute Pumps

Active transport requires specialized carrier proteins, also known as solute pumps, which:

• Bind specifically and reversibly to the substances being transported

• May transport more than one substance at a time

• Include antiporters (move substances in opposite directions) and symporters (move substances in the same direction)

Active transport moves solutes against their concentration gradient (from low to high concentration), which requires energy input from ATP.

Types of Active Transport

• Primary active transport: Energy comes directly from ATP hydrolysis.

• Secondary active transport: Energy is obtained indirectly from ionic gradients created by primary active transport.

Primary Active Transport

In primary active transport:

• Hydrolysis of ATP causes a conformational change in the transport protein

• This change allows solutes (often ions) bound to the protein to be pumped across the membrane

• Examples of primary active transport pumps include calcium pumps, hydrogen (proton) pumps, and the sodium-potassium (Na+-K+) pump

Sodium-Potassium Pump (Na+-K+ ATPase)

• Most studied active transport pump

• Functions as an enzyme (Na+-K+ ATPase) that pumps Na+ out of the cell and K+ into the cell

• Located in all plasma membranes, especially active in excitable cells such as nerves and muscles

Role in Electrochemical Gradients

• Leakage channels allow Na+ to enter and K+ to exit the cell down their concentration gradients

• The Na+-K+ pump works as an antiporter, pumping Na+ out and K+ in against their gradients

• This maintains electrochemical gradients, which are crucial for muscle and nerve function

Secondary Active Transport

Secondary active transport depends on the ion gradient established by primary active transport:

• Energy stored in these gradients is used indirectly to drive the transport of other solutes

• Low intracellular Na+ concentration (maintained by the Na+-K+ pump) increases the drive for Na+ to enter the cell

• As Na+ flows into the cell, it can drag other molecules (such as glucose or amino acids) with it via symporters

• Commonly transports sugars, amino acids, and ions into cells

Example: Sodium-Glucose Symporter

• Na+ gradient drives glucose uptake into the cell against its concentration gradient

Vesicular Transport

Overview of Vesicular Transport

Vesicular transport involves the movement of large particles, macromolecules, and fluids across the plasma membrane within membranous sacs called vesicles. This process requires cellular energy, usually in the form of ATP.

• Essential for transporting substances that cannot pass through membrane channels or carriers

Types of Vesicular Transport

• Endocytosis: Transport into the cell

• Exocytosis: Transport out of the cell

• Transcytosis: Transport into, across, and then out of the cell

• Vesicular trafficking: Transport from one area or organelle in the cell to another

Endocytosis

• Involves formation of protein-coated vesicles

• Usually receptor-mediated, making it a selective process

• Substances must bind to specific receptors to be internalized

• Pathogens may exploit this process to enter cells

• Internalized vesicles may fuse with lysosomes or undergo transcytosis

Types of Endocytosis

• Phagocytosis: 'Cell eating'; uptake of large particles by engulfment

• Pinocytosis: 'Cell drinking'; uptake of extracellular fluid and dissolved solutes

• Receptor-mediated endocytosis: Highly selective uptake of specific molecules via receptor binding

Exocytosis

• Process by which materials are ejected from the cell

• Triggered by cellular signals such as changes in membrane voltage or calcium influx

• Substances are enclosed in secretory vesicles that fuse with the plasma membrane

• Examples include secretion of hormones, neurotransmitters, and waste products

Summary Table: Types of Active Membrane Transport

Transport Type	Energy Source	Direction	Examples
Primary Active Transport	Directly from ATP hydrolysis	Against gradient	Na+-K+ pump, Ca2+ pump
Secondary Active Transport	Indirectly from ion gradients	Against gradient (for co-transported solute)	Na+-glucose symporter
Vesicular Transport	ATP	Bulk transport (in/out/across cell)	Endocytosis, Exocytosis, Transcytosis

Key Terms and Definitions

• Carrier protein (solute pump): Membrane protein that actively transports substances across the membrane

• Antiporter: Carrier protein that moves two substances in opposite directions

• Symporter: Carrier protein that moves two substances in the same direction

• ATP (Adenosine Triphosphate): Main energy currency of the cell

• Electrochemical gradient: Combined effect of concentration and electrical charge differences across a membrane

• Vesicle: Membranous sac used for transport within cells

Relevant Equations

ATP Hydrolysis

The hydrolysis of ATP provides energy for active transport:

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

Sodium-Potassium Pump Stoichiometry

For each cycle:

$3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}}$

Examples and Applications

• Na+-K+ pump: Maintains resting membrane potential in nerve and muscle cells

• Glucose absorption: In the intestine, glucose is absorbed into cells via Na+-glucose symporters

• Phagocytosis: White blood cells engulf bacteria as part of the immune response

Additional info:

• Resting membrane potential (RMP) is established primarily by K+ gradients, with Na+ also contributing

• Vesicular transport is critical for processes such as neurotransmitter release and hormone secretion