{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Chapter 8 - CHAPTER 8 MEMBRANE STUCTURE AND FUNCTION...

Info icon This preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
CHAPTER 8: MEMBRANE STUCTURE AND FUNCTION Introduction The plasma membrane separates the living cell from its nonliving surroundings. This thin barrier, 8 nm thick, controls traffic into and out of the cell. Like other membranes, the plasma membrane is selectively permeable , allowing some substances to cross more easily than others. A. Membrane Structure The main macromolecules in membranes are lipids and proteins, but include some carbohydrates. The most abundant lipids are phospholipids. Phospholipids and most other membrane constituents are amphipathic molecules . Amphipathic molecules have both hydrophobic regions and hydrophilic regions. The phospholipids and proteins in membranes create a unique physical environment, described by the fluid mosaic model . A membrane is a fluid structure with proteins embedded or attached to a double layer of phospholipids. 1. Membrane models have evolved to fit new data Models of membranes were developed long before membranes were first seen with electron microscopes in the 1950s. In 1895, Charles Overton hypothesized that membranes are made of lipids because substances that dissolve in lipids enter cells faster than those that are insoluble. Twenty years later, chemical analysis confirmed that membranes isolated from red blood cells are composed of lipids and proteins. Attempts to build artificial membranes provided insight into the structure of real membranes. In 1917, Irving Langmuir discovered that phospholipids dissolved in benzene would form a film on water when the benzene evaporated. The hydrophilic heads were immersed in water. In 1925, E. Gorter and F. Grendel reasoned that cell membranes must be a phospholipid bilayer two molecules thick. The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water. Actual membranes adhere more strongly to water than do artificial membranes composed only of phospholipids. One suggestion was that proteins on the surface increased adhesion.
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
In 1935, H. Davson and J. Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins. Early images from electron microscopes seemed to support the Davson-Danielli model and until the 1960s, it was considered the dominant model. Further investigation revealed two problems. First, not all membranes were alike, but differed in thickness, appearance when stained, and percentage of proteins. Second, measurements showed that membrane proteins are actually not very soluble in water. Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions.
Image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern