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Unformatted text preview: Chapter 5 Chapter 5 The Working Cell Introduction: Turning on the Lights to Be Invisible *Some organisms use energy-converting reactions to produce light *Examples are organisms that live in the ocean and use light to hide themselves from predators *Energy conversion involves not only energy but also membranes and enzymes *So, production of light involves all of the topics covered in this chapter MEMBRANE STRUCTURE AND FUNCTION 5.1 Membranes are a fluid mosaic of phospholipids and proteins *Membranes are composed of phospholipids and proteins *Membranes are commonly described as a fluid mosaic *This means that the surface appears mosaic because of the proteins embedded in the phospholipids and fluid because the proteins can drift about in the phospholipids *Many phospholipids are made from unsaturated fatty acids that have kinks in their tails *This prevents them from packing tightly together, which keeps them liquid *This is aided by cholesterol wedged into the bilayer to help keep it liquid at lower temperatures *Membranes contain integrins, which give the membrane a stronger framework *Integrins attach to the extracellular matrix on the outside of the cell as well as span the membrane to attach to the cytoskeleton *Some glycoproteins in the membrane serve as identification tags that are specifically recognized by membrane proteins of other cells *For example, cell-cell recognition enables cells of the immune system to recognize and reject foreign cells, such as infectious bacteria *Carbohydrates that are part of the extracellular matrix are significantly involved in cell-cell recognition *Many membrane proteins function as enzymes , others in signal transduction , while others are important in transport *Because membranes allow some substances to cross or be transported more easily than others, they exhibit selectively permeability *Nonpolar molecules (carbon dioxide and oxygen) cross easily *Polar molecules (glucose and other sugars) do not cross easily 5.3 Passive transport is diffusion across a membrane with no energy investment * Diffusion is a process in which particles spread out evenly in an available space *Particles move from an area of more concentrated particles to an area where they are less concentrated *This means that particles diffuse down their concentration gradient *Eventually, the particles reach equilibrium where the concentration of particles is the same throughout *Diffusion across a cell membrane does not require energy, so it is called passive transport *The concentration gradient itself represents potential energy for diffusion 5.4 Osmosis is the diffusion of water across a membrane *It is crucial for cells that water moves across their membrane...
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- Spring '08