Chapter 4 – Cells and their EnvironmentMr. Lopez – Ag. Biology – Shandon

High School

California Content Standards: 1a, 1b, 10b, 10d, IE1d

Section 1 - Passive Transport• Homeostasis maintained by

controlling the movement of substances across the cell membrane

• Movement of substances can be accomplished in two ways: passive or actively.

Section 1 - Passive Transport (continued)

What is passive transport?

• Movement across the cell membrane that does not require energy

• Figure 1 on page 75 provides an example of passive transport.

Section 1 - Passive Transport (continued)

Section 1 - Passive Transport (continued)

Difference in concentration of substances is known as?

• Concentration gradient

When the concentration of a substance is equally distributed, it is know as?

• Equilibrium

Section 1 - Passive Transport (continued)

What is diffusion?

• The movement of a substance from an area of high concentration to an area of lower concentration due to random motion

Section 1 - Passive Transport (continued)

• Molecules and ions enter and leave the cell by diffusion

• Many times the concentrations of substances very between the cell and the outside

• Yet remember that the membrane is “selectively permeable” – Nonpolar lipid layer repel ions and most polar molecules

Section 1 - Passive Transport (continued)

• Osmosis

– Diffusion of water through a selectively permeable membrane

– Figure 2, shows how osmosis occurs across the membrane.

Section 1 - Passive Transport (continued)

• Hypertonic Solution– As water diffuses out of a cell, the cell

shrinks– Fluid outside the cell has a higher

concentration of dissolved particles

• Hypotonic Solution– As water diffuses into a cell, the cell swells– Fluid outside the cell has a lower

concentration of dissolved particles

Section 1 - Passive Transport (continued)

• Isotonic Solution– No changes in size– State of equilibrium is reached–Water diffuses in and out at the same

rate

Section 1 - Passive Transport (continued)

• Crossing the Cell Membrane– Ions and polar molecules are able to

enter cells via the aide of transport proteins

– Transport proteins known as channels provide polar passageways that allow specific substances to pass through the cell

– For example, ions like sodium, Na+, potassium, K+, calcium Ca2+, chloride, Cl-, cross into the cell via ion channels

Section 1 - Passive Transport (continued)

Section 1 - Passive Transport (continued)

–Movement of charged particles also influenced by the particle’s positive or negative electrical charge

– In general, the inside of cells are negatively charged

– Therefore positively charged ions are more likely to enter the cell

– Negatively charged ions are more likely to leave the cell

Section 1 - Passive Transport (continued)

• What are carrier proteins?– Binds to specific substances in

membrane– Carry the substance across the

membrane– Release the substance where it is

needed by concentration gradient– This “ride-along” of the substance is

known as facilitated diffusion

Section 1 - Passive Transport (continued)