Cell Membrane and Transport Maintaining homeostasis and providing nutrients to cells

Cell Membrane Structure The cell membrane is composed of lipids and proteins. The lipids are arranged in a bilayer. The bilayer is a “barrier” that is impermeable to most molecules. The proteins are embedded in the bilayer. Specific molecules can be helped across the membrane by these proteins.

Cell membranes contain special lipids known as phospholipids Phospholipids: Composed of two fatty acid chains attached to a glycerol molecule and phosphate group. Fatty Acid Chains Glycerol Phosphate Group

Phospholipids, continued: Have a hydrophilic “head” that “loves” water (both are polar) Hydrophilic heads form outside of layer so they can touch water inside and outside cell Have a hydrophobic “tail” (hydrocarbon chain) that “fears” water (is non-polar) Hydrophobic tails face interior of bilayer so they can avoid water

Phospholipid Bilayer

Role of the Cell Membrane The cell membrane is described as “selectively permeable” How does this feature relate to the job/function of the cell membrane? Cell membrane acts as a “guard” Allows nutrients into cell Allows for removal of wastes and release of substances made by the cell that are needed by other cells.

Factors that affect Passive Transport: Whether a molecule can move through the membrane depends on: 1) the size of the molecule 2) the type of molecule (polar or nonpolar, charged, etc.) Molecules move by one of the following methods: diffusion, facilitated diffusion, or osmosis.

All 3 types are passive transport Passive Transport: movement of substances without any energy input by a cell. 1. Diffusion: small, nonpolar molecules (O 2 and CO 2 ) move straight through the membrane. 2. Facilitated diffusion: small, polar molecules (glucose, amino acids), or ions move through protein channels embedded in the membrane. 3. Osmosis: water molecules move through the membrane. Methods of Transport

Direction of transport The net movement (direction) of the molecules depends on the relative concentrations inside/outside a cell. In passive transport, the net movement is always “down the concentration gradient”. Molecules move from an area of higher concentration to an area of lower concentration.

Net Movement Think of dye or sugar molecules in water. (Water particles NOT shown.) Dye or sugar molecules will DIFFUSE through the water.

Example: Transport of Oxygen Concentration gradient for O 2 HIGH LOW

Example: Transport of Glucose HIGH LOW Net movement

Example: Osmosis If the membrane is permeable to both water and solutes, both will diffuse to reach equilibrium. Often, the membrane is NOT permeable to the solute(s). In this case osmosis occurs; water diffuses (high to low) to balance the concentration on both sides. (egg & potato labs)

Osmosis, continued: Situation #1: a cell is surrounded by a solution that has a high concentration of solutes. Inside of Cell Outside of Cell

Due to the concentration gradient for water, what happens to the cell? High solutes = low water concentration The net movement of water is from inside the cell (high WATER conc.) to outside the cell (low WATER conc). Cell loses mass.

Egg in hypertonic solution Water moved out of the egg to try to dilute the solutes outside of the egg and make the external environment have a higher water concentration to match the inside. Chicken egg in syrup

Situation #2: a cell is surrounded by a solution that has a low concentration of solutes. Low solutes = high water concentration The net movement of water is from outside the cell (high WATER conc.) to inside the cell (low WATER conc). Cell gains mass.

Egg in hypotonic solution Water moved into the egg to reduce the water concentration compared to solutes outside of the egg and make the external environment have a lower water concentration to match the inside. Chicken egg in tap water (or vinegar)

Situation #3: a cell is surrounded by a solution that has the same concentration of solutes. Equal concentrations = equilibrium NO net movement of water Equal number of molecules move in and out Cells mass doesn’t change.

Change in Mass of Potato Cores in Different Solutions Change in Mass (g) + - No change in mass

Facilitated Diffusion: Integral Proteins can “facilitate” or assist in transporting a substance by: 1. acting as a channel or tunnel 2. acting as a carrier

Diffusion through a carrier protein

Active Transport Cells move molecules from an area of LOW concentration to an area of HIGH concentration Molecules move AGAINST the concentration gradient Requires the cell to use energy in the form of ATP Animation

Example: Sodium-Potassium Pump Three Na + ions in the cytoplasm bind to carrier protein Shape of protein is changed, allowing the three Na + out of cell Two K + ions outside of cell bind to protein Shape of the protein is changed The two K + are allowed into the cytoplasm Similar to facilitated diffusion: Uses a carrier protein, Different from facilitated diffusion: requires energy. Overall: Na+ (sodium) becomes concentrated on the outside of a cell. Important in the proper functioning of neurons and the kidneys. Animation 1 Animation 2

Other types of active transport Not all active transport moves molecules from a low concentration to a high concentration Active transport used in two other situations: Moving very large molecules through membrane Moving large quantities of smaller molecules through membrane

Other examples of active transport: Endocytosis: Process in which cells ingest fluids, macromolecules, and large particles that are outside the cell Animation 1 Animation 2

Other examples of active transport: Exocytosis: how cells release large molecules (proteins) or get rid of large amounts of wastes