Cell Membrane and Transport

Overview

This lecture explains the structure and function of the cell membrane, focusing on different types of transport—passive and active—used by cells to maintain homeostasis.

Cell Structure and Homeostasis

• All cells have genetic material, cytoplasm, ribosomes, and a cell membrane.
• Eukaryotic cells also contain membrane-bound organelles with specialized functions.
• The cell membrane regulates what enters and exits the cell to maintain homeostasis.

Cell Membrane Structure

• The cell membrane consists of a phospholipid bilayer—two layers of lipids.
• The phospholipid head is polar (attracts water); the tail is nonpolar (repels water).

Passive Transport

• Small, non-polar molecules like oxygen and carbon dioxide diffuse directly through the membrane (simple diffusion).
• Simple diffusion moves molecules from high to low concentration (with the gradient) and requires no energy.
• Facilitated diffusion uses transport proteins to help larger or polar molecules cross the membrane.
• Facilitated diffusion still moves with the concentration gradient and does not require energy.
• Examples include ions using protein channels, glucose using carrier proteins, and water using aquaporins (osmosis).

Active Transport

• Active transport moves molecules from low to high concentration (against the gradient) and requires energy, usually from ATP.
• ATP (adenosine triphosphate) releases energy when its third phosphate bond breaks.
• Transport proteins powered by ATP can force molecules against the gradient.
• Example: The sodium-potassium pump uses ATP for active transport.

Bulk Transport: Endocytosis and Exocytosis

• Endocytosis is the process of taking large molecules into the cell by forming vesicles from the membrane.
• Types of endocytosis include phagocytosis (engulfing solids), receptor-mediated endocytosis (specific uptake), and pinocytosis (fluid uptake).
• Exocytosis is when vesicles fuse with the membrane to release substances out of the cell.
• Exocytosis is used for waste removal and exporting large molecules, such as plant cell wall components.

Key Terms & Definitions

• Homeostasis — maintaining a stable internal environment in the cell.
• Phospholipid Bilayer — double layer of lipids forming the cell membrane.
• Simple Diffusion — passive movement from high to low concentration without energy or proteins.
• Facilitated Diffusion — passive movement from high to low concentration with the help of transport proteins.
• Active Transport — movement from low to high concentration requiring ATP energy.
• ATP (Adenosine Triphosphate) — energy-carrying molecule used in cells.
• Endocytosis — process of taking substances into the cell by engulfing them.
• Exocytosis — process of expelling substances from the cell via vesicles.

Action Items / Next Steps

• Research different types of endocytosis: phagocytosis, receptor-mediated endocytosis, and pinocytosis.
• Review the sodium-potassium pump as an example of active transport.