Cells must continuously receive nutrients and rid themselves of waste products—one of the characteristics of life. Many of the proteins that are associated with the plasma membrane are involved in moving molecules across the membrane. Some proteins are capable of moving from one side of the plasma membrane to the other and shuttle certain molecules across the membrane. Others extend from one side of the membrane to the other and form channels through which substances can travel. Some of these channels operate like border checkpoints, which open and close when circumstances dictate. Some molecules pass through the membrane passively, whereas others are assisted by metabolic activities within the membrane.
MicroscopesMicroscopes allow us to study cells in detail. The ones that use visible light to illuminate objects are called light microscopes. There are two types: Simple and Compound. A more powerful microscope is the Electron microscopes use electrons instead of visible light to illuminate samples. Because electrons travel in wavelengths that are much shorter than those of visible light, electron microscopes can resolve details that are much smaller than you can see with light microscopes. Electron microscopes use magnetic fields to focus beams of electrons onto a sample.
• Wavelengths of light are 400-750 nm
• If a structure is less than one-half of a wavelength long, it will not be visible
• Light microscopes can resolve objects down to about 200 nm in size
• Uses streams of accelerated electrons rather than light
• Electrons are focused by magnets rather than glass lenses
• Can resolve structures down to 0.5 nm