Cells and Their Environment

Section 1: Passive Transport Your body responds to external conditions to maintain stable INTERNAL CONDITIONS. This is called HOMEOSTASIS.

One way cells maintain homeostasis is by CONTROLLING THE MOVEMENT OF SUBSTANCES INTO AND ACROSS THEIR CELL MEMBRANE.

Random Motion and Concentration Movement across the cell membrane that does NOT require energy is called PASSIVE TRANSPORT. A DIFFERENCE in the concentration of a substance across a space is called a CONCENTRATION GRADIENT.

As a substance enters a second area the concentration of the second area INCREASES while the concentration of a substance DECREASES IN THE FIRST AREA.

Eventually the concentration of the substance in both areas will be EQUAL- this means the amount of the substance moving from the FIRST AREA TO THE SECOND AREA IS EQUAL to the amount of the substance moving from the SECOND AREA TO THE FIRST AREA.

EQUILIBRIUM is a condition in which the concentration of a substance is equal throughout a space. Figure 1, page 75

Movement of Substances Particles of a substance in a solution move around RANDOMLY. If there is a CONCENTRATION GRADIENT in the solution, the solution will move from an area of HIGH CONCENTRATION to an area of LOWER CONCENTRATION in a process called DIFFUSION.

If DIFFUSION is allowed to continue EQUILIBRIUM eventually results. MOLECULES and IONS dissolved in the CYTOPLASM and in the FLUID OUTSIDE THE CELLS enter and leave cells by diffusing across the CELL MEMBRANE.

The NONPOLAR INTERIOR of the cell membrane REPELS IONS AND MOST POLAR MOLECULES and are thus PREVENTED from diffusing across the cell membrane.

Molecules that are VERY SMALL or NONPOLAR can diffuse across the cell membrane down their CONCENTRATION GRADIENT. Diffusion of such molecules across the cell membrane is the SIMPLEST type of PASSIVE TRANSPORT.

Osmosis The diffusion of water through a SELECTIVELY PERMEABLE membrane is called OSMOSIS. Osmosis is a type of PASSIVE TRANSPORT. A solution is a SUBSTANCE dissolved in ANOTHER SUBSTANCE.

On either side of the membrane many IONS and POLAR MOLECULES are dissolved in water. If solutions on either side of the cell membrane have DIFFERENT concentrations of DISSOLVED PARTICLES they will also have DIFFERENT concentrations of FREE WATER molecules.

OSMOSIS will occur as FREE WATER MOLECULES move INTO the solution with the LOWER CONCENTRATION of free water molecules.

Figure 2 Page 76: Osmosis

3 possible directions for water movement: 1. water moves out: cell SHRINKS – HYPERTONIC SOLUTION

2. water moves in: cell SWELLS – HYPOTONIC SOLUTION

3. No net movement: NO CHANGE in cell VOLUME – ISOTONIC solution

If the fluid outside the cell has… Table 1 Page 77 If the fluid outside the cell has… Then outside fluid is… Water diffuses… Effect on cell …lower free water molecule concentration than cytoplasm …hypertonic … out of cell H2O moves out of the cell (shrinks) Cell shrinks (becomes smaller) …higher free water molecule concentration than cytoplasm …hypotonic …into cell. H2O moves into the cell (swells) Cell swells (becomes larger) …same free water molecule concentration as cytoplasm …isotonic …into and out of cell at equal rates Cell stays same size (no change)

Real Life Example Isotonic or Hypotonic Hypertonic

Swelling caused by HYPOTONIC solution could cause a cell to BURST. When the cell loses water caused by a HYPERTONIC solution the cell will SHRINK. When there is no change and water flow evenly across a cell, ISOTONIC solution is occurring.

Crossing the cell membrane IONS and polar molecules can cross the cell membrane when they are aided by TRANSPORT PROTEINS CALLED CHANNELS. Each channel allows only a SPECIFIC SUBSTANCE to pass through the cell membrane-this SELECTIVITY is one of the most important properties of the CELL MEMBRANE.

Diffusion Through Ion Channel Example: NERVE CELLS sending electrical signals throughout the body; MUSCLE CELLS in your heart making your heart beat. Diffusion through ion pores is a form of PASSIVE TRANSPORT.

Figure 3 Page 78

Electron Charge and Ion Transport The inside of a typical cell is NEGATIVELY CHARGED with respect to the OUTSIDE of the cell. OPPOSITE charges ATTRACT and LIKE charges REPEL. The direction of movement caused by the CONCENTRATION GRADIENT may oppose the direction of movement caused by the ION’S ELECTRICAL CHARGE. This is very important to the functioning of NERVE CELLS in animals.

Facilitated Diffusion TRANSPORT PROTEINS that can BIND to a specific substance on one side of the cell membrane and RELEASE it on the other side are called a CARRIER PROTEIN. When carrier proteins are used to transport specific substances down their concentration gradient the process is called FACILITATED DIFFUSION.

Figure 4 page 80

Active Transport The transport of a substance across the cell membrane against its concentration gradient is called ACTIVE TRANSPORT. Unlike PASSIVE TRANSPORT, active transport requires ENERGY which is supplied by ATP

Some active transport processes involve CARRIER PROTEINS. Carrier proteins in active transport function as “PUMPS” that move substance AGAINST their CONCENTRATION GRADIENT. Carrier proteins are often called MEMBRANE PUMPS.

Sodium-Potassium Pump: Figure 5 PG 82

The SODIUM-POTASSIUM PUMP transport 3 sodium ions, Na+, OUT of a cell and 2 potassium ions, K+, INTO the cell. Sodium ions are usually more concentrated INSIDE the cell than OUTSIDE the cell and potassium ions are usually more concentrated OUTSIDE the cell than INSIDE the cell. Thus, the sodium-potassium pump transport these ions AGAINST their CONCENTRATION GRADIENT. Energy needed to power the Sodium potassium pump is supplied by ATP.

2 reasons that the sodium potassium pump is important 1 – It keeps the cell from accumulating too many sodium ions which would cause the cell to fill with water by osmosis and swell or burst. 2- It helps maintain the concentration gradient of sodium ions and potassium ions across the cell membrane.

Movement in Vesicles Some substances are too LARGE to be transported by carrier proteins. Vesicles move these large substances INTO a cell by ENDOCYTOSIS and move substances OUT of the cells by EXOCYTOSIS.

Figure 6 Pg 83

Endocytosis Exocytosis

Membrane Receptor Proteins How do your cells communicate ?????? They communicate through SIGNAL MOLECULES. One familiar example of signal molecules are HORMONES. Cells can RECEIVE the MESSAGES carried by certain signal molecules because the cell membrane contains specialized PROTEINS that BIND these signal molecules.

Where are the receptor proteins??? Examples: Muscle cells could NOT CONTRACT or RELAX without receptor proteins and signal molecules. Where are the receptor proteins??? EMBEDDED IN THE LIPID BILAYER OF THE CELL MEMBRANE

Many drugs affect the BINDING of signal molecules to receptor proteins. Example: In heart-muscle cells, signal molecules STIMULATE the cells, causing the HEART RATE TO INCREASE.

BETA BLOCKERS are prescribed to patients with a RAPID heartbeat BETA BLOCKERS are prescribed to patients with a RAPID heartbeat. Beta-Blockers interfere with the binding of signal molecules to the receptor, preventing the heart rate from increasing too rapidly.