1. Cells and Their Environment
Chapter 4
Cells and Their Environment
Section 1: Passive Transport
Section 2: Active Transport 1

2. Passive Transport Objectives:
Section 1
Passive Transport
Objectives:
Relate concentration gradients, diffusion, and equilibrium.
Predict the direction of water movement into and out of cells.
Describe the importance of ion channels in passive transport.
Identify the role of carrier proteins in facilitated diffusion. 2

3. Passive Transport Diffusion
Section 1
Passive Transport
Diffusion
Passive Transport Passive transport is the movement of substances across the cell membrane without the use of energy by the cell.
Concentration A difference in the concentration of a substance across a space is called a concentration gradient.
Movement of Substances Diffusion is the movement of a substance from an area of high concentration to an area of lower concentration. 3

4. Passive Transport Osmosis
Section 1
Passive Transport
Osmosis
Osmosis Osmosis is the diffusion of free water molecules across a selectively permeable membrane.
Movement of Water Across Membranes Water moves out of cells in hypertonic solutions, water moves into cells in hypotonic solutions, and there is no net water movement across cells in isotonic solutions. 4

5. Passive Transport Crossing the Cell Membrane
Section 1
Passive Transport
Crossing the Cell Membrane
Diffusion Through Ion Channels Ion channels are proteins that have a pore through which ions can cross the cell membrane.
Electrical Charge and Ion Transport The movement of a particle across the cell membrane is influenced by the concentration gradient of the substance and the particle’s electrical charge.
Facilitated Diffusion In facilitated diffusion, a carrier protein transports a substance across the cell membrane down the concentration gradient of the substance. 5

6. Active Transport Objectives:
Section 2
Active Transport
Objectives:
Compare active transport with passive transport.
Describe the importance of the sodium-potassium pump.
Distinguish between endocytosis and exocytosis.
Identify three ways that receptor proteins can change the activity of a cell. 6

7. Active Transport Movement Across a Concentration Gradient
Section 2
Active Transport
Movement Across a Concentration Gradient
Active Transport Active transport is the movement of a substance against the concentration gradient of the substance. Active transport requires cells to use energy.
Sodium-Potassium Pump In animal cells, the sodium-potassium pump uses energy supplied by ATP to transport sodium ions out of the cell and potassium ions into the cell. 7

8. Active Transport Movement in Vesicles
Section 2
Active Transport
Movement in Vesicles
Endocytosis During endocytosis, substances are moved into a cell by a vesicle that pinches off from the cell membrane.
Exocytosis During exocytosis, substances inside a vesicle are released from a cell as the vesicle fuses with the cell membrane. 8

9. Active Transport Membrane Receptor Proteins
Section 2
Active Transport
Membrane Receptor Proteins
Receptor Proteins Communication between cells often involves signal molecules that bind to receptor proteins on cells.
Functions of Receptor Proteins A signal molecule that is bound by a receptor protein on a cell can change the activity of the cell in three ways: by enabling specific ions to cross the cell membrane, by causing formation of a second messenger, or by speeding up chemical reactions inside the cell. 9