Cellular Organization and Organelles: Structure, Function, and Membranes

Cellular Organization and Organelles

Introduction

This study guide covers foundational concepts in cell biology, focusing on cellular organization, the structure and function of biological membranes, and the distinctions between prokaryotic and eukaryotic cells. Understanding these topics is essential for grasping how cells function as the basic units of life.

The Importance of Biological Membranes

Structure and Function of Membranes

• Biological membranes enclose the internal compartments of eukaryotic cells, forming organelles.

• Membranes are primarily composed of phospholipid molecules assembled into a bilayer.

• Phospholipids have:

  • Hydrophilic (water-loving) heads that face outward toward the aqueous environment.

  • Hydrophobic (water-hating) tails that face inward, away from water, forming the interior of the bilayer.

Key Equation:

$\text{Phospholipid bilayer} = \text{Hydrophilic heads (outer)} + \text{Hydrophobic tails (inner)}$

• This arrangement creates a selective barrier that regulates the movement of substances into and out of the cell and its organelles.

Membrane Composition

• Membranes are not composed of phospholipids alone; they also contain:

  • Proteins (e.g., transmembrane proteins) that facilitate the movement of ions and small molecules across the membrane.

  • Cholesterol, which controls membrane rigidity.

• The phospholipid bilayer is impermeable to water, ions, and most hydrophilic small molecules.

• Proteins embedded in the membrane allow specific ions and molecules to cross.

• Other membrane proteins attach the cell to other cells or extracellular polymers, or modify the cell's shape.

Prokaryotic Cells

Lack of Internal Membranes

• Prokaryotic cells do not have internal, membrane-bound organelles.

• The nucleoid is the region where the chromosome (DNA) is located, not separated by a nuclear membrane.

• Without a nuclear membrane, transcription and translation can occur simultaneously.

Cell Wall Structure

• The bacterial cell wall is composed of peptidoglycan (a complex of protein and oligosaccharides).

• There are two main types of bacteria based on cell wall structure:

  • Gram-positive bacteria: Thick cell wall, no outer membrane, retain Gram stain.

  • Gram-negative bacteria: Thin cell wall, outer membrane present, do not retain Gram stain.

Eukaryotic Cells

Internal Membrane-Bound Organelles

• Eukaryotic cells contain internal organelles, each surrounded by membranes and specialized for distinct functions.

• Major organelles include:

  • Nucleus: Contains genetic material (DNA) and is the site of transcription.

  • Mitochondria: Site of aerobic respiration and ATP production.

  • Endoplasmic Reticulum (ER): Involved in protein and lipid synthesis.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

  • Lysosomes and Peroxisomes: Involved in degradation and detoxification.

  • Chloroplasts: (in plants) Site of photosynthesis.

  • Vacuoles: Storage and structural support (especially in plant cells).

• Bacterial cells may have microcompartments made of protein, but these are not surrounded by phospholipid membranes.

Comparing Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Internal Membranes	Absent	Present (organelles)
Nucleus	No (nucleoid region)	Yes (membrane-bound)
Cell Wall	Peptidoglycan (bacteria)	Plants/fungi: cellulose/chitin; animals: absent
Size	Generally smaller	Generally larger
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Recap Questions

• How are biological membranes constructed?

  • Explain the orientation of phospholipid heads and tails in the bilayer.

• What components enable movement of substances across membranes?

• Describe the parts and functions in a prokaryotic cell.

• Compare and contrast the concept of compartments in prokaryotic vs. eukaryotic cells.

Simultaneous Transcription and Translation in Prokaryotes

• Due to the absence of a nuclear membrane, prokaryotes can transcribe DNA and translate mRNA simultaneously.

• Multiple ribosomes (polyribosomes) can bind to a single mRNA strand as it is being synthesized.

Study Strategies for Cell Biology

• Read assigned textbook pages alongside study guides.

• Fill in guides with your own wording for better understanding.

• Review and revise notes after lectures for improved retention.

• Collaborate with peers on practice and application problems.

Additional info: Drawing and relating information between topic areas (networks) are emphasized as effective study strategies in this course.