Cellular Transport (Part V) Transport LIFE, ORGANELLES, CELL MEMBRANE, CELLULAR TRANSPORT

Part 4 Cellular Transport

1. Diffusion A. Recall that particles are always in motion, moving randomly. The amount of substance in a given area is called concentration. Without using any energy, particles will always move from an area of high concentration to low concentration until equilibrium is met, meaning the particle concentration is even through an area. Dynamic equilibrium is met when particles are still moving but there is no overall net change to the concentration.

1. Diffusion B. Some things can change the rate of diffusion. The higher the temperature, concentration, and/or pressure, the faster diffusion will occur because the particles collide more.

1. Diffusion C. Diffusion Across the Plasma Membrane a. Some items cannot automatically diffuse across the plasma membrane. They need help, like a facilitator. Facilitated diffusion uses proteins in the cell membrane to act like a tunnel, allowing only certain particles through. b. This requires no energy, since particles are moving naturally from high to low concentration.

1. Diffusion

2. Osmosis: Diffusion of Water A. When water diffuses through the cell membrane, it is called osmosis. This natural movement of water from areas of high to low concentration is crucial for maintaining homeostasis.

2. Osmosis: Diffusion of Water B. How osmosis works: a. Recall that water is the universal solvent and the particles in water are the solute. Water will move through cell membrane barriers until the concentration of solute reaches equilibrium both in and out of the cell.

B. How Osmosis Works b. Once equilibrium is met, water will still move back and forth, but no net amounts of water will change.

B. How Osmosis Works c. Cells in an Isotonic Solution: “Iso” is Greek for equal. When the concentration of water and solutes is equal both in and out of the cell, the cell is in an isotonic solution. Water enters and leaves the cell at an equal rate. Cells maintain their normal shape.

B. How Osmosis Works d. Cells in a Hypotonic Solution: “Hypo” is Greek for under. When the concentration of solutes outside of the cell is lower than the concentration inside the cell, the cell is in a hypotonic solution. Water will rush into the cell. The plasma membrane will expand until equilibrium is reached or the cell bursts.

B. How Osmosis Works e. Cells in a Hypertonic Solution: “Hyper” is Greek for above. When the concentration of solutes outside of the cell is higher than the concentration inside the cell, the cell is in a hypertonic solution. Water will rush out of the cell. The cell will shrink. Plants lose water mostly from their vacuoles, shriveling the membrane away from the cell wall. This is what causes plants to wilt.

B. How Osmosis Works

3. Active Transport A. All forms of diffusion occur without additional energy, as particles go from high to low concentration. But sometimes particles must go from low to high concentration, which is called moving against the concentration gradient. This requires energy, and is called active transport.

3. Active Transport B. Proteins can be used as transport pumps, allowing certain molecules or ions to enter or leave a cell against the concentration gradient. This requires energy in the form of ATP (cell energy currency).

4. Transport of Large Particles A. Some particles are too big or clunky to diffuse through the membrane or go through a protein channel.

4. Transport of Large Particles B. When a large molecule needs to enter a cell, endocytosis takes place. The substance pushes against the cell membrane until the membrane wraps around the substance, pinching off the membrane and creating a vacuole that can move inside the cell.

4. Transport of Large Particles C. When a large substance needs to exit a cell, exocytosis takes place. The vesicle that contains the substance pushes against the membrane until the membrane merges with the vesicle membrane, releasing the substance out of the cell.