Cell Membranes A Cellular Boundary All cells contain a cell membrane that regulates what enters and leaves the cell and also protects and supports the cell. The composition of nearly all cell membranes is a double-layered sheet called a lipid bilayer, which gives cell membranes a flexible structure and forms a strong barrier between the cell and its surroundings.

The Properties of Lipids Many lipids have oily fatty acid chains attached to chemical groups that interact strongly with water. The fatty acid portions of such a lipid are hydrophobic, or “water-hating,” while the opposite end of the molecule is hydrophilic, or “water-loving.” When such lipids are mixed with water, their hydrophobic fatty acid “tails” cluster together while their hydrophilic “heads” are attracted to water. A lipid bilayer is the result. The head groups of lipids in a bilayer are exposed to water, while the fatty acid tails form an oily layer inside the membrane from which water is excluded.

http://www. youtube. com/watch http://www.youtube.com/watch?annotation_id=annotation_307447&feature=iv&src_vid=9UvlqAVCoqY&v=cj8dDTHGJBY

7.3 Cell Transport

Passive Transport Every living cell exists in a liquid environment. One of the most important functions of the cell membrane is to keep the cell’s internal conditions relatively constant. It does this by regulating the movement of molecules from one side of the membrane to the other side. Passive Transport – the movement of materials across the cell membrane without using cellular energy. Diffusion – particles move from an area of high concentration to an area of low concentration. Driving force behind the movement of many substances across the cell membrane. Facilitated Diffusion – process in which molecules that cannot directly diffuse across the membrane pass through protein channels Does not require any additional use of cell’s energy

Osmosis: An Example of Facilitated Diffusion The inside of a cell’s lipid bilayer is hydrophobic—or “water-hating.” Because of this, water molecules have a tough time passing through the cell membrane. Many cells contain water channel proteins, known as aquaporins, that allow water to pass right through them. Without aquaporins, water would diffuse in and out of cells very slowly. Osmosis - The movement of water through cell membranes by facilitated diffusion.

Osmosis: An Example of Facilitated Diffusion Osmosis is the diffusion of water through a selectively permeable membrane. Osmosis involves the movement of water molecules from an area of higher concentration to an area of lower concentration. In the experimental setup below, the barrier is permeable to water but not to sugar. This means that water molecules can pass through the barrier, but the solute, sugar, cannot.

NO ENERGY USED!!!!! IT’S PASSIVE!! Passive Person – Passive Transport – NO ENERGY REQUIRED Diffusion – High to low Facilitated Diffusion – Needs a helper (facilitator) Protein channel used Osmosis is an example – movement through aquaporin without energy NO ENERGY USED!!!!! IT’S PASSIVE!!

Active Transport Active Transport - The movement of material against a concentration difference Requires energy. The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins, or protein “pumps,” that are found in the membrane itself. Many cells use such proteins to move calcium, potassium, and sodium ions across cell membranes. A LOT of energy used by cells in their daily activities is devoted to providing the energy to keep this form of active transport working.

Endocytosis Bulk Transport Endocytosis is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane. The pocket that results breaks loose from the outer portion of the cell membrane and forms a vesicle or vacuole within the cytoplasm. Large molecules, clumps of food, and even whole cells can be taken up by endocytosis. Two examples of endocytosis are phagocytosis and pinocytosis.

Endocytosis Phagocytosis - extensions of cytoplasm surround a particle and package it within a food vacuole. The cell then engulfs it. Amoebas use this method for taking in food. Engulfing material in this way requires a considerable amount of energy and, therefore, is a form of active transport. Pinocytosis, cells take up liquid from the surrounding environment by forming tiny pockets along the cell membrane. The pockets fill with liquid and pinch off to form vacuoles within the cell.

Exocytosis Many cells also release large amounts of material from the cell, a process known as exocytosis. During exocytosis, the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell.

Active Transport USES ENERGY – ATP!!!!!!!!!!!!!!!!!!!!!!!!!

Cell membrane proteins are involved in the movement of some substances across the cell membrane. Which type of molecular movement requires cellular energy in addition to the membrane proteins? A. osmosis B. endocytosis C. active transport D. facilitated diffusion

Salt concentration in the body fluids of most saltwater fish is low compared to their surroundings. Which ability helps these fish to live in their high salt concentration environment? A. passing concentrated urine B. taking in water through their gills C. active transport of salt out of their bodies D. avoiding intake of water by closing their mouths

Which transport mechanisms require the formation of a vesicle to transport material into or out of a cell? A. diffusion and osmosis B. exocytosis and endocytosis C. exocytosis and calcium pumps D. diffusion and facilitated diffusion