Cells- Part 2: The Cell’s Plasma Membrane Some of the slides contain teacher’s notes at the bottom! I use the hand-out in conjunction with the presentation so students can label and take short notes as you go through the parts of the plasma membrane. Enjoy using this presentation! All images in this presentation are public domain unless otherwise indicated. Photo credit is given where required.

What is the Plasma Membrane? The youtube video linked here is a great animation of the lipid bilayer. It illustrates the fluidity of the structure which is hard to describe and also shows and explains why the phospholipid molecules orientate themselves heads out and tails in.

The Plasma Membrane The Cell Forms the outer structure of the cell (like your skin in a way) Acts as a barrier between the inside and the outside of the cell. The structure (lipid bilayer) allows the membrane to be fluid.

We’ll look at each component of the plasma membrane We’ll look at each component of the plasma membrane. As we discuss it – label it on your diagram and add any notes you will find useful

The Phospholipid Molecule One single component of the plasma membrane is this molecule: The phospholipid. You noted how the phospholipids orientated themselves in the video. Teachers Notes : Hydrophilic (water loving) heads orientate themselves to either the watery environment inside or outside the cell while the hydrophobic (water fearing) tails orientate themselves to the middle away from the watery environment). I use the 4 dots at the bottom to represent heads and then have the students tell me where to place the tails. When you are in slide show view on powerpoint you can use the pen tool in the bottom left corner to draw in the tails. When you finish the show you will be prompted to save the ink or not – choose NO if you want to re-use the show. Wikipedia

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. Inside the cell Outside the cell

Fatty Acid Tails Flexibility The fatty acid tails are flexible, causing the lipid bilayer to be fluid. This makes the cells flexible. At body temperature, membranes are a liquid with a consistency that is similar to cooking oil. It’s important to understand that the membrane is a fluid – using the analogy of cooking oil seems to help the students understand the quality of consistency.

Membrane Structure Plasma Membranes consist of a phospholipid bilayer (and proteins) in a specific arrangement. The surfaces of cell membranes are hydrophilic (water-loving); the interiors are hydrophobic (water fearing). Hydrophilic molecules tend to interact with water and with each other. Hydrophobic molecules avoid interaction with water and tend to interact with other hydrophobic molecules. WATERY Outside the Cell All cells live in a watery environment Inside the Cell

Other components of the plasma membrane

Proteins Proteins are also embedded in the Membrane Proteins are scattered throughout the membrane. They may be attached to inner surface, embedded in the bilayer, or attached to the outer surface. Hydrophilic (polar) regions of the protein project from the inner or outer surface.   Hydrophobic (nonpolar) regions are embedded within the membrane. Proteins

Transport Proteins A few substances move freely across the cell membrane by passive diffusion. Most small molecules or ions require the assistance of specific protein carriers to transport them across the membrane. Note that we will discuss Transport to a greater extent in section 4 of this Cell unit

Cholesterol In animals, cholesterol is a major membrane lipid. It may be equal in amount to phospholipids. It is similar to phospholipids in that it one end is hydrophilic (water loving -polar), the other end is hydrophobic (water hating non-polar). Note that the polar head of the cholesterol is orientated the same way as the polar head of the phospholipids. Cholesterol makes the membrane less permeable to most biological molecules.

Cholesterol I use this slide so that students can understand the relative placement of the cholesterol and label it on their diagram and write a short note about the function. Cholesterol helps maintain the fluidity of cell membranes and by adding firmness to the plasma membrane also prevents it from becoming overly fluid so Cholesterol generally helps the plasma membrane maintain it’s structure

Carbohydrate Chains Lipids and proteins within the membrane may have a carbohydrate chain attached. These carbohydrates often function as cell identification markers, allowing cells to identify other cells.  This is particularly important in the immune system where cells patrolling the body's tissues identify and destroy foreign invaders such as bacteria or viruses.

Cytoskeleton The cytoskeleton is a network of protein elements that extend through the cytoplasm in eukaryotic cells. It provides for the distinctive shape of cells such as red blood cells, muscle cells, and nerve cells (neurons). It produces movement of cells and is associated with movement of materials within cells. The Green structure is the cytoskeleton of the cell

The Fluid Mosaic Model The phospholipids create what is called the Fluid Mosaic. A fluid can be described as a substance that can change positions without separating. A mosaic in art is a picture made up of many small pieces like little tiles or glass. This links to a video of the fluid mosaic model on Youtube. The clip is only about 1- 2 minutes long and is a great animation of the lipid bilayer. It illustrates the fluidity of the structure which is hard to describe and also shows and explains why the phospholipid molecules orientate themselves heads out and tails in. Watch this short video for a good explanation of why the phospholipid bilayer is called a fluid mosaic

For further study Here’s a good link to help you further understand the cellular membrane The Next lesson in this unit on the cell is The Cell – Part 3 Organelles The link leads to a good site which reinforces the content material. In addition to a good brief description of the concepts there is a little mini quiz that you can either do with the class or have them do independently in computer lab.