Definition: Cell transport refers to the movement of substances across the cell membrane, which is vital for maintaining the cell's internal environment by allowing the entry of valuable substances and the removal of waste.
Key Concept: The cell membrane is selectively permeable, meaning it selectively controls what substances can pass through it, rather than being a completely impermeable wall.
Structure of the Cell Membrane
Phospholipid Bilayer:
Composed of phospholipids, which have a structure that includes:
Polar Head:
Hydrophilic (water-attracting), allowing interaction with water.
Nonpolar Tail:
Hydrophobic (water-repelling), interacting with nonpolar lipids.
The bilayer consists of two layers:
Outer Layer: Polar heads face the external environment.
Inner Layer: Polar heads face the inside of the cell.
Proteins embedded throughout facilitate movement across the membrane.
Factors Influencing Movement Across the Cell Membrane
Nature of the Membrane: The structure and chemical properties of the membrane influence what can pass through.
Properties of Molecules: Molecules crossing the membrane have distinct properties (e.g. hydrophilic, hydrophobic, size).
Concentration Gradient: The difference in concentration of substances between the intracellular and extracellular environments is critical for determining movement.
Concentration Gradient: Defined as the difference in concentration from high to low.
Passive Movement Mechanisms
Simple Diffusion
Definition: Movement of solute molecules from a higher concentration area to a lower concentration area without energy input (passive transport).
Importance: Efficient for cells as it conserves energy for other functions.
Examples:
Coffee Brewing: Aroma spreads from high concentration near the coffee maker to lower concentration throughout the house.
Swimming Pool Chlorine: Smell detected upon entering a building due to diffusion from high concentration near the pool.
Hand Sanitizer: Odor spreads through air due to diffusion in class.
Factors Affecting Diffusion Rate
State of Matter: Diffusion occurs more readily in gases than in liquids and solids.
Concentration Gradient: A steeper gradient results in a faster diffusion process.
Pressure: Increased pressure in gases leads to quicker molecular movement.
Size of Molecules: Smaller molecules diffuse faster than larger ones.
Osmosis
Definition: The diffusion of water across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
Importance: Essential for cell function as water is vital.
Understanding Concentration:
In a 10% salt solution (90% water) vs. a 30% salt solution (70% water), the 10% solution has a higher concentration