Overview
This lecture reviews the structure and function of cell membranes, focusing on the role of phospholipids and the formation of the phospholipid bilayer in eukaryotic cells.
Eukaryotic Cell Membranes
- Eukaryotic cells have a cell surface membrane and internal membranes around organelles.
- Membrane-bound organelles include the nucleus, Golgi apparatus, vesicles, vacuole (tonoplast), mitochondria, chloroplasts, and endoplasmic reticulum.
- Membranes separate functions within the cell, preventing harmful enzymes from damaging other cell parts.
Structure of Phospholipids
- Phospholipids are composed of a polar phosphate head, glycerol, and two nonpolar fatty acid tails.
- The phosphate head is hydrophilic (water-attracting); the fatty acid tails are hydrophobic (water-repelling).
- Only the phosphate head is water-soluble; the tails are not.
Formation of Cell Membrane
- Phospholipids arrange themselves so heads face watery environments inside and outside the cell, while tails face each other.
- This arrangement is called hydrophobic interaction, leading to the formation of the phospholipid bilayer.
- The bilayer forms the basic structure of all cell membranes, both at the cell surface and within organelles.
- The membrane appears as a double layer (bilayer) in cross-section, with heads outward and tails inward.
- The width of the bilayer is about 7-8 nanometers, viewable only with an electron microscope.
Summary
- Cell membranes are formed by phospholipid molecules with hydrophilic heads and hydrophobic tails.
- In water, phospholipids automatically organize into a bilayer, forming the foundation of all cellular membranes.
Key Terms & Definitions
- Eukaryotic cell — a cell with membrane-bound organelles and a defined nucleus.
- Phospholipid — a lipid molecule with a hydrophilic phosphate head and two hydrophobic fatty acid tails.
- Hydrophilic — attracted to water.
- Hydrophobic — repelled by water.
- Phospholipid bilayer — double layer of phospholipids forming the core of cell membranes.
- Membrane-bound organelle — cellular structure surrounded by its own membrane.
- Tonoplast — the membrane around the central vacuole in plant cells.
Action Items / Next Steps
- Review Chapter 1 for differences between eukaryotic and prokaryotic cells.
- Understand and draw the structure of the phospholipid bilayer.
- Prepare for upcoming lessons on membrane transport.