Cell Membrane and Transport Maintaining homeostasis and providing

Cell Membrane and Transport Maintaining homeostasis and providing nutrients to cells

Cell Membrane Structure l The cell membrane is made of lipids and proteins. l The lipids are arranged in a bilayer. l The bilayer is a “barrier” that is impermeable to most molecules. l The proteins are embedded in the bilayer. l These help specific molecules across

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

Phospholipids, continued: l Have a hydrophilic “head” that “loves” water (both are polar) l l 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) l Hydrophobic tails face interior of bilayer so they can avoid water

Phospholipid Bilayer

Role of the Cell Membrane l The cell membrane is described as “SELECTIVELY PERMEABLE” Means it allows some things to enter while keeping others out l How does this feature relate to the job/function of the cell membrane? l l Cell l membrane acts as a “guard” Allows nutrients in & allows removal of waste products (also keeps harmful things out)_

Passive Transport l Passive Transport is the movement of substances without the use of any energy l Things go from high concentration to low concentration l Transport continues till equilibrium is achieved (same amount inside and out) l Molecules go “down the concentration gradient”

Factors that affect Passive Transport: l 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. ) l Molecules move by one of the following methods: diffusion, facilitated diffusion, or osmosis.

Net Movement

Types of passive transport 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 CHANNEL PROTEINS embedded in the membrane. 3. Osmosis: water molecules move through the membrane.

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

Example: Transport of Glucose HIGH Net movement LOW

Example: Osmosis l If the membrane is permeable to both water and solutes, both will diffuse to reach equilibrium. l 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)

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Hypertonic, Hypotonic, and Hypertonic, Hypotonic, & Isotonic

l Solution: l A mixture of solute and solvent l Solvent: l Something that does the dissolving (usually water) l Solute: l Something that gets dissolved in the solvent

Hypertonic solution: l Hypertonic Solution: a cell is surrounded by a solution that has a high concentration of solutes. Inside of Cell Outside of Cell

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

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

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

Example: Sodium-Potassium Pump l l l 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 Animation 1 Similar to facilitated diffusion: l l Different from facilitated diffusion: l l Uses a carrier protein, Animation 2 requires energy. Overall: Na+ (sodium) becomes concentrated on the outside of a cell. l Important in the proper functioning of neurons and the kidneys.

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