Cellular Function: Membrane Transport and Cellular Respiration

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Cellular Function

Overview

Cellular function encompasses the essential processes that sustain life at the cellular level. These include membrane transport, cellular respiration, cell division, and genetic expression and regulation. Understanding these processes is fundamental to the study of Anatomy & Physiology.

Cell transport – how cells move substances in and out
Cellular respiration – how cells harvest energy
Cell division – how cells reproduce
Genetic expression & regulation – how proteins are made and regulated

Membrane Transport

Introduction

Membrane transport refers to the movement of substances across the plasma membrane. This process is vital for maintaining cellular homeostasis and involves two main mechanisms: passive and active transport.

Passive transport: No energy required
Active transport: Cell must provide energy (usually ATP)

Key Terms

Solution: A homogeneous mixture of two or more components
Solvent: The dissolving medium (usually water in biological systems)
Solute: The components that dissolve in the solvent
Intracellular fluid: Fluid inside the cell
Extracellular fluid: Fluid outside the cell

Selective Permeability

The plasma membrane is selectively permeable, allowing some materials to pass while excluding others. This property is crucial for regulating the internal environment of the cell.

Passive Transport

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

Diffusion: Movement of molecules from regions of high concentration to regions of low concentration (down the concentration gradient).
Simple diffusion: Lipid-soluble materials or small molecules pass directly through the membrane.
Osmosis: Simple diffusion of water across a selectively permeable membrane.
Facilitated diffusion: Substances require a protein channel or carrier to move across the membrane.
Filtration: Water and solutes are forced through a membrane by hydrostatic pressure, moving from high to low pressure areas.

Tonicity

Tonicity describes the concentration of solutes dissolved in a solution, which determines the direction and extent of diffusion.

Isotonic: Equal solute concentration inside and outside the cell; no net movement of water.
Hypertonic: Higher solute concentration outside the cell; water moves out, causing the cell to shrink.
Hypotonic: Lower solute concentration outside the cell; water moves in, causing the cell to swell.

Solution Type	Solute Concentration	Water Movement	Cell Effect
Isotonic	Equal inside/outside	No net movement	Normal
Hypertonic	Higher outside	Out of cell	Shrinks
Hypotonic	Lower outside	Into cell	Swells

Active Transport

Active transport requires cellular energy (usually ATP) to move substances against their concentration gradient or to transport large molecules.

Solute pumping: ATP energizes protein carriers to move substances (e.g., amino acids, some sugars, ions) against their concentration gradient.
Bulk transport: Used to move large quantities of material at once, including:
- Exocytosis: Moves materials out of the cell via vesicles that fuse with the plasma membrane.
- Endocytosis: Moves materials into the cell by engulfing them in vesicles. Includes:
  - Phagocytosis: "Cell eating" – engulfs solid particles.
  - Pinocytosis: "Cell drinking" – engulfs liquid droplets.
  - Receptor-mediated endocytosis: Specific uptake of target molecules via membrane receptors.

Cellular Respiration

Introduction

Cellular respiration is the process by which cells extract energy from food molecules, primarily glucose, to produce ATP, the energy currency of the cell. This process occurs in all living cells and is essential for cellular activities.

Stages of Cellular Respiration

Glycolysis: Occurs in the cytoplasm; 1 molecule of glucose is split into 2 molecules of pyruvate, producing 2 ATP and high-energy electrons carried by NADH.
Pyruvate Oxidation: Pyruvate is transported into the mitochondria and converted into Acetyl-CoA, releasing CO2 and more NADH.
Krebs Cycle (Citric Acid Cycle): Acetyl-CoA enters a cycle of reactions in the mitochondria, producing 2 ATP, releasing CO2, and generating high-energy electrons carried by NADH and FADH2.
Electron Transport Chain (ETC): High-energy electrons from NADH and FADH2 are transferred through a series of proteins in the inner mitochondrial membrane, creating a proton gradient that drives ATP synthesis via ATP synthase. Oxygen acts as the final electron acceptor, forming water.

Total ATP yield per glucose molecule: Approximately 36 ATP

Equations

Overall equation for aerobic cellular respiration:

Aerobic vs. Anaerobic Respiration

Aerobic respiration: Requires oxygen; produces more ATP.
Anaerobic respiration: Occurs without oxygen; only glycolysis occurs, producing less ATP and resulting in byproducts such as lactic acid.

Versatility of Cellular Respiration

Other organic molecules (carbohydrates, lipids, proteins) can also be used as fuel, but they enter the pathway at different points and are less efficient than glucose for ATP production.

Summary Table: Membrane Transport Mechanisms

Type	Energy Required?	Direction	Examples
Simple Diffusion	No	High to Low	O2, CO2
Osmosis	No	High to Low (water)	Water
Facilitated Diffusion	No	High to Low	Glucose, ions
Filtration	No	High to Low (pressure)	Kidney filtration
Active Transport (Pumping)	Yes (ATP)	Low to High	Na+/K+ pump
Bulk Transport	Yes (ATP)	Varies	Endocytosis, Exocytosis

Example Application

Example: The Na+/K+ pump is an example of active transport, moving sodium ions out of the cell and potassium ions into the cell against their concentration gradients, which is essential for nerve impulse transmission.

Additional info: These notes cover content relevant to Chapter 3 (Cells and Tissues) and aspects of Chapter 2 (Basic Chemistry) from a standard Anatomy & Physiology curriculum, focusing on cellular processes and membrane dynamics.