Membranes of the Cell Review

The Properties of Phospholipids -The fatty acid portion of phospholipids are hydrophobic, or “water-hating,” while the phosphate opposite end of the molecule is hydrophilic, or “water- loving.”

Cell Membrane Structure -The composition of a cell membrane is a double-layered sheet of phospholipids called a phospholipid bilayer.

What is wrong with this cell membrane?

What is wrong with this cell membrane?

A double layer of phospholipids

The structure of a lipid bilayer -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 Lipid bilayer -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.

The Lipid bilayer -Because the inside of the membrane is nonpolar, it repels ions and polar molecules from passing through. However, small nonpolar molecules like dissolved oxygen and carbon dioxide pass easily. Larger macromolecules are too large to pass. -Cholesterol can be found within the hydrophobic center. It helps the membrane with maintaining rigidity.

-Most cell membranes contain protein molecules that are embedded in the lipid bilayer.

Glycoproteins and Glycolipids -Carbohydrate molecules are attached to these proteins and lipids, creating glycoproteins and glycolipids. -Many of the carbohydrate molecules act like chemical identification cards, allowing individual cells to be identified. Ex> the immune system will not attack these cells

The proteins embedded in the bilayer can move around and “float” among the lipids.

-Some of the proteins form channels and pumps that help to move material across the cell membrane.

Plasma membrane protein functions: -stabilize and shape for the membrane -outer surface proteins act as identifiers for other cells, such as those of the immune system -outer surface proteins act as receptors for hormones and other molecules or ions to attach to; when attached, receptors respond by causing change inside the cell -help to transport ions and polar molecules in and out of the cell

Membrane proteins -different cells have different combinations of proteins in their membranes, which helps determine each cell’s specialized function Ex> a cell that makes up estrogen sensitive tissue will have estrogen receptors on its surface or a cell that lines the small intestine will have glucose transport proteins, allowing it to absorb glucose from digested food

Cell Membrane Permeability -Although many substances can cross biological membranes, some are too large or too strongly charged to cross the lipid bilayer. -If a substance is able to cross a membrane, the membrane is said to be permeable to it.

Cell membranes are selective -The cell membrane is selectively permeable because it selects what will pass in and out of the cell.

Cell membrane permeability -A membrane is impermeable to substances that cannot pass across it. - Selectively permeable membranes are also called semipermeable membranes.

Endosymbiosis -Organelles such as chloroplasts and mitochondria contain their own genetic information in the form of small DNA molecules and have a double membrane, an outer and an inner. This is evidence that these organelles were once free living prokaryotes that were “captured” by other prokaryotes, producing primitive eukaryotic cells. -The theory of endosymbiosis states that eukaryotic organelles evolved from simpler prokaryotes; bacteria were engulfed by other prokaryotes and formed a symbiotic relationship; over time the engulfed bacteria evolved into the organelles of eukaryotic cells.