You Must Know (Chapter 3.2) Describe the structure and function of the cell membrane. How are cells organized into living things?

Cell Membrane http://learn.genetics.utah.edu/content/cells/membranes/

Cell Membrane The plasma membrane is selectively permeable Allows some substances to cross more easily than others Fluid Mosaic Model Fluid: membrane held together by weak interactions Mosaic: phospholipids, proteins, carbs

Early membrane model (1935) Davson/Danielli – Sandwich model Problems: 9/15/2018 Early membrane model (1935) Davson/Danielli – Sandwich model phospholipid bilayer between 2 protein layers Problems: varying chemical composition of membrane, hydrophobic protein parts

The freeze-fracture method Revealed the structure of membrane’s interior

Fluid Mosaic Model Hydrophobic – water-fearing Hydrophilic – water-loving

Phospholipids Bilayer hydrophilic head, hydrophobic tail Hydrophobic barrier keeps hydrophilic molecules out Selective permeability

Selective Permeability Small molecules (polar or nonpolar) cross easily Fats and oils, gases, CO2, O2) Hydrophobic core prevents passage of ions, large polar molecules

Membrane Proteins Integral Proteins Peripheral Proteins Embedded in membrane Transmembrane with hydrophilic heads/tails and hydrophobic middles Extracellular or cytoplasmic sides of membrane NOT embedded Held in place by the cytoskeleton or ECM Provides stronger framework

Integral & Peripheral proteins

Some functions of membrane proteins

Membrane fluidity Cholesterol hinder close packing at low temps 9/15/2018 Membrane fluidity Cholesterol limit fluidity at high temps hinder close packing at low temps Adaptations: bacteria in hot springs (unusual lipids); winter wheat ( unsaturated phospholipids)

Carbohydrates Function: cell-cell recognition; developing organisms Glycolipids, glycoproteins E.g. blood transfusions are type-specific

Cellular organization

Review questions 1. Describe the composition of the plasma membrane. 2. Explain why hydrophobic (“water-hating”) molecules can easily cross the plasma membrane, while hydrophilic (“water-loving”) molecules cannot. Cells carry out all the functions of life, and they use nutrients and oxygen and produce wastes. These substances must cross the plasma membrane. 3. How do you think substances cross the plasma membrane to enter or leave the cell? 4. What if the substances are large? Protein molecules, for example, are very large. How do they enter or leave the cell?