Cell Membrane and Fluid Mosaic Model Section 3.3

Objectives SWBAT describe the structure of the cell membrane. SWBAT summarize how chemical signals are transmitted across the cell membrane.

Vocabulary (3.3) Cell membrane Phospholipid Fluid mosaic model Selective permeability Receptor Ligand Hydrophobic Hydrophilic

Cell Membrane The cell membrane (also called the plasma membrane) forms a boundary between the cell and the outside environment. It controls the passage of materials into and out of the cell. As you should remember, it is made up of a double layer of phospholipids. There are also other molecules interspersed among the phospholipids.

Cell Membrane A phospholipid is a molecule composed of 3 basic parts: A charged phosphate group. Glycerol – which is a solvent. Two fatty acid chains. Together, the glycerol and the phosphate groups form the “head,” which bears a charge and thus is polar. The polar head of the phospholipid forms hydrogen bonds with water molecules. We say they are HYDROPHILIC (attracted to water).

Cell Membrane In contrast, the fatty acid tail is HYDROPHOBIC. It is a nonpolar molecule and cannot form hydrogen bonds. The tails are attracted to one another and repelled by water.

Cell Membrane Proteins The phosphate heads are attracted to the water environment outside the cell and the cytoplasm inside the cell. Therefore, the phosphate heads point out of the cell and inside the cell, arranging themselves in a double layer, “like a sandwich.” Outside cell Cytoplasm Protein Channel

Cell Membrane The other Molecules in the cell membrane give it a diversity of functions. For example: Carbohydrate Chain Cholesterol molecules give the cell membrane strength. Proteins can extend through one or both layers of the membrane, helping important materials cross it. Carbohydrate tags serve to identify and distinguish different types of cells. Cholesterol Protein Channel Cytoskeleton

Fluid Mosaic Model The fluid mosaic model – describes the arrangement of molecules that make up a cell membrane. The model is named due to two characteristics: The cell membrane is flexible, not rigid. The other molecules in the cell membrane give it different patterns and textures – like tiles in a mosaic.

Fluid Mosaic Model The cell membrane is flexible, not rigid. The phospholipids in each layer can move from side to side and slide past one another. Thus, the membrane behaves like a fluid – similar to “a film of oil on the surface of water.”

Selective Permeability Our egg has a selectively permeable membrane:

Selective Permeability Some molecules can cross the membrane while others cannot.

Selective Permeability (semipermeable) Selective permeability – the cell membrane allows some but not all materials to cross. This property allows cells to maintain homeostasis – despite changing conditions outside the cell. The cell must maintain a stable environment or else it does not function properly – or might die. Selective permeability allows the cell to control its inner environment in the face of a changing exterior environment.

Selective Permeability How a molecule crosses the cell membrane depends on several factors: Molecule’s size Polarity An concentration inside the cell versus outside the cell. Small nonpolar molecules typically pass easily through the cell membrane. Small polar molecules are transported by proteins Large molecules are moved by vesicles.

Cellular Receptors Cellular receptor – is a protein that detects a signal molecule and performs an action in response. Receptors recognize and bind to only certain molecules. This ensures that the right cell gets the right signal at the right time. Ligand – a specific molecule, the ligand, that binds to a specific receptor.

Receptors When a receptor and a ligand bind, they both change shape. The shape change is critical because it affects how a receptor molecule interacts with other molecules. There are Intracellular Receptors and Membrane Receptors.

Intracellular Receptor Intracellular means within, or inside the cell. A ligand (a specific molecule) may cross the cell membrane and bind to an intracellular receptor.

Intracellular Receptor An example of a ligand of this type would be a hormone (in this case we will use aldosterone). Aldosterone, the ligand, is small and non-polar. When it enters a kidney cell, it binds to an intracellular receptor. The aldosterone ligand and intracellular receptor enter the nucleus of the cell – turning on certain genes.

Intracellular Receptor As a result of the intracellular receptor and aldosterone (the ligand) entering the nucleus of the kidney cell, proteins are made that help the kidneys absorb sodium ions and retain water. This process is important in helping the body to maintain normal blood pressure.

Membrane Receptor For molecules that cannot cross the cell membrane, they may bind to a receptor in the membrane.

Membrane Receptor The ligand binds to the receptor molecule in the cell membrane. The receptor sends a message to the cell’s interior. The entire receptor changes shape. The change in the receptor cell causes molecules in the cell to respond – telling the cell what to do.

Membrane Receptor Example: “band 3 protein” in red blood cells, when activated, tell the cells to transfer carbon dioxide from body tissues to the lungs (for being exhaled from the body).