Cell Membrane: Structure and Function

Cell Membrane Structure and Function

• The topic focuses on the structure and function of cell membranes.
• Class activities include notes, a cell membrane demonstration, and a worksheet.
• Assigned work involves cell membrane worksheets.
• Upcoming assessments consist of an In-Class Assessment (ICA) on March 4 and a Lab Assessment on March 6.
• On the unit timeline, the cell membrane topic is scheduled for March 24, passive transport on March 25, cell size lab on March 26, osmosis on March 27, plasmolysis lab on March 28, dialysis tubing lab on March 3 and an In Class Assessment (ICA) on March 4th.
• Active transport is scheduled for March 5th, Lab Assessment on March 6 and unit test review on March 7.
• March break follows this unit plan.

Key Definitions

• Protoplasm: The colorless material comprising the living part of a cell, including the cytoplasm, nucleus, and other organelles.
• Plasma Membrane: Another name for the cell membrane.
• Homeostasis: The state of steady internal physical and chemical conditions maintained by living systems.
• Selective Permeability: Only substances that meet certain criteria can pass through the membrane unaided.

Cell Membrane Basics

• The cell membrane separates cells from the external environment.
• It functions like a fence, controlling what enters and exits the cell.
• Cell membranes are complex.
• The cell membrane separates living cells from their nonliving surroundings.
• It is an 8nm thick barrier, and a human hair is around 90,000nm.
• Two visible layers are seen with an electron microscope.
• An electron micrograph shows a cell membrane at approximately 240,000x magnification.
• The plasma membrane is a protective boundary that regulates substance movement in/out of a cell acting as a selectively permeable layer.
• Allows some substances to cross more easily than others based on hydrophobic and hydrophilic properties.
• Not all cells have the same permeability.
• Cell membranes consist of phospholipids, proteins, & other macromolecules.

Functions of the Cell Membrane

• Separates the cell's protoplasm from the external environment.
• Mediates substance entry and exit.
• Maintains homeostasis and the stability of the cell's internal environment.
• Cells must receive nutrition and gasses and excrete wastes.
• Cells would die of starvation or be poisoned by their metabolic wastes if material exchanges did not occur.

Cell Membrane Composition

• Phospholipids are present.
• Cholesterol can be found.
• Proteins include peripheral and integral proteins.
• Glycocalyx: Carbohydrate layer is found on the exterior surface of the cell.
• Cytoskeleton filaments are connected to the membrane.

Macromolecules in Membranes

• Lipids: Includes the phospholipid bilayer and cholesterol.
• Proteins: Consists of transport, receptor, and recognition proteins.
• Carbohydrates: Features oligosaccharides on glycoproteins.

Membrane Lipids

• Membranes consist of a special kind of lipid, phospholipids, with a "split personality," a hydrophilic head "water-loving" and hydrophobic tail "water-fearing", making it amphipathic.
• Fatty acid tails are hydrophobic and phosphate group heads are hydrophilic.
• Phospholipids are arranged as a bilayer

Membrane Components

• Membranes cannot operate efficiently with only phospholipids.
• In addition to phospholipids, proteins, cholesterol, and glycoproteins are also components of the membrane.
• Glycoproteins are the sugar attached to a protein.

Fluid Mosaic Model

• In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer.
• The Fluid Mosaic Model: Individual phospholipids and proteins can move side-to-side within the layer, like it's a liquid and the scattered protein molecules create a mosaic pattern when the membrane is viewed from above.

Functions of Membrane Macromolecules: Lipids

• They form a boundary to isolate cell contents from the environment and restrict the passage of hydrophilic substances across the membrane

Functions of Membrane Macromolecules: Cholesterol

• Cholesterol is found in between phospholipid membranes of animal cells and is not found in bacteria and most plants
• Increases bilayer strength and flexibility.
• Reduces membrane fluidity and permeability to water-soluble substances.

Functions of Membrane Macromolecules: Proteins

• Regulate the movement of water-soluble substances. -Channel proteins: These have pores to allow passage of ions and small water-soluble molecules. -Carrier proteins: These bind to molecules and change shape for delivery across the membrane.
• Receptor proteins are docking sites for molecules outside the cell and trigger internal cellular response.
• Glycoproteins (recognition proteins) are Sugars attached to a protein and provides identification to cells and serves as cell-surface attachment sites.
• Glycoproteins play a key role in cell-cell recognition to distinguish one cell from another due to antigens, are important in organ & tissue development, and are the basis for rejection of foreign cells by the immune system.
• Carbohydrate chains are often attached to lipids (glycolipids) and proteins (glycoproteins).