CELLS - MEMBRANE PROTEINS #3

Membrane Proteins - Bumpy Services We spoke a little about the cell membrane and its structure. We also discussed the lipid bilayer. That lipid bilayer is not smooth around the entire cell. You will find thousands (millions?) of proteins throughout the cell membrane. Some are just on the inside of the cell and some on the outside. A special few cross the cell membrane. Each type of protein has a specific purpose. There are also embedded proteins in the other membranes for cell organelles. A Tale of Two Types There are two types of proteins in the cell membrane -- peripheral proteins and integral proteins. As you can guess from the name, integral membrane proteins are permanently connected to the cell membrane. They have large sections embedded in the hydrophobic (middle) layer of the membrane. Peripheral proteins are not bonded as strongly to the membrane. They may just sit on the surface of the membrane, anchored with a few hydrogen (H) bonds. Integral proteins are the hard workers of the cell membrane. Some integral proteins cross the membrane and act as pathways for ions and molecules. Some of the ion movement may not require work (passive transport), but other processes require a lot of energy and pumping action (active transport). When you look at the whole membrane, there are very few integral proteins when compared to the number of peripheral ones. Discovering Structures This structure of the membrane with embedded proteins and a lipid bilayer was discovered in the early 1970's. Two scientists, Singer and Nicolson, first developed the theory of the "Fluid Mosaic Model." They used several different methods, such as the freeze-fracture technique and electron micrographs, to look closely at the cell membrane and its structure. They identified the proteins that sat on the surface, were sunk into the membrane, and the others that crossed the membrane. CELL MEMBRANES - QUIZ

CELLS - CELL WALL #4

Cell Wall - What's it for?

While cell membranes might be around every cell, cell walls made of cellulose are only found around plant cells. Cell walls are made of specialized sugars called cellulose. Cellulose provides a protected framework for a plant cell to survive. It's like taking a water balloon and putting it in a cardboard box. The balloon is protected from the outside world. Cellulose is called a structural carbohydrate (complex sugar) because it is used in protection and support. 

Cell walls also help a plant keep its shape. While they do protect the cells, cell walls and cellulose also allow plants to grow to great heights. While you have a skeleton to hold you up, a 100-foot tall redwood tree does not. It uses the strong cell walls to maintain its shape. For smaller plants, cell walls are slightly elastic. Wind can push them over and then they bounce back. Big redwoods need strength in high winds and sway very little (except at the top). 

Another Hole in the Wall

A cell wall is not a fortress around the delicate plant cell. There are small holes in the wall that let nutrients, waste, and ions pass through. Those holes are called plasmodesmata. These holes have a problem: water can also be lost. But even when the plant cell loses water, the basic shape is maintained by the cell walls. So if a plant is drooping because it needs water, it can recover when water is added. It will look just the same as when it started. 

More Than Walls in Plants

You may hear about cell walls in other areas of biology. Bacteria also have a structure called a cell wall. Fungi and some ptotozoa also have cell walls. They are not the same. Only plant cell walls are made out of cellulose. The other walls might be made from proteins or a substance called chitin. They all serve the same purpose of protecting and maintaining structure, but they are very different molecules.