Plasma Membrane & Homeostasis All living cells, prokaryotic and eukaryotic, have a plasma membrane. The plasma membrane is selectively permeable – it will allow nutrients and other essential elements to enter the cell and waste materials to leave the cell. Homeostasis, a stable internal environment, is maintained.

Structure of the Plasma Membrane – Fluid Mosaic Model Lipid bilayer – two sheets of lipids (phospholipids). Found around the cell, the nucleus, vacuoles, mitochondria, and chloroplasts. Embedded with proteins and strengthened with cholesterol molecules.

What’s a Phospholipid? It’s a pair of fatty acid chains and a phosphate group attached to a glycerol backbone. Polar (water-soluble) heads face out and the nonpolar fatty acids hang inside.

Membrane Proteins Determine what particles can pass through the membrane. Serve as enzymes (may speed reactions). Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell (glycoproteins act in immune response).

Diagram representing the cell membrane. The above image is from http://www.biosci.uga.edu/almanac/bio_103/notes/may_15.html. But I found it at An On-Line Biology Book: http://gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookCELL2.html

Passive Transport: Osmosis (NO ATP Required) Diffusion of water across a selectively permeable membrane. Water molecules move from high concentration to low concentration. Occurs until water is balanced on both sides of the membrane.

Osmosis & Cell Concentrations Hypertonic solutions – more dissolved solute outside the cell. Water leaves the cell. Cell shrinks. Ex. Saltwater solution Hypotonic solutions – more dissolved solute inside the cell. Water enters the cell. Cell swells. Ex. grocers spritz veggies. Isotonic solutions – same amount of dissolved solute inside & outside the cell. No osmosis. Ex. immunizations

Overcoming Osmosis Contractile vacuoles – expel excess water from bacterial cells that live in water. Turgor pressure – water pressure in a plant cell. Loss of turgor pressure causes wilting (plasmolysis).

Passive Transport: DIFFUSION (NO ATP Required) Diffusion – movement of particles from an area of high concentration to an area of low concentration. Caused by Brownian motion (movement of particles because of the movement of their atoms). Continues until an equilibrium is reached (no gradient). Dynamic equilibrium – particles move freely and are evenly distributed.

Cellular Transport Small molecules, such as oxygen, carbon dioxide, and water, are able to freely diffuse across the membrane, The passage of larger molecules, such as amino acids and sugars, is carefully regulated and use facilitated diffusion.

Passive Transport: FACILITATED DIFFUSION Passive transport –particles move from high concentration to low concentration. Facilitated diffusion – channel proteins embedded in the membrane allow larger molecules like glucose to pass through the cell membrane.

Active Transport: Requires ATP Active transport –particles move from low concentration to high concentration. Protein Pumps – embedded proteins change shape to open and close passages across the membrane. Cells use protein pumps to move calcium, potassium and sodium ions across cell membranes from areas of low concentration to high concentration areas.

How the Sodium Potassium Pump Works http://www. youtube. com/watch

Active Transport: Endocytosis – taking something into the cell. Phagocytosis- packaged in food vacuole (WBC “eat” damaged cells) Pinocytosis-taking up liquid into a vacuole Exocytosis – expelling material from the cell.

Active Transport: Endocytosis and Exocytosis

Compare & Contrast Passive Transport vs. Active Transport