Cell Ultrastructure Membranes Plasma membrane z Outermost layer
Cell Ultrastructure Membranes
Plasma membrane z. Outermost layer of cytoplasm z. Serves as a barrier to prevent cell contains from mixing with the surroundings z. Allows molecules to enter and leave z. Contains protein sensors that allow cell to interact with environment z. Allows cell growth and movement z. Many other functions as well
Some Membrane Functions
Membrane Distribution z. Prokaryotes have a single membrane - the plasma membrane z. Eukaryotes y. Plasma membrane y. Many internal membranes that enclose organelles
Prokaryotes and Eukaryotes
Membranes in a Eukaryotic Cell
Membrane composition z 1890 s - thought membranes largely lipid z 1930 s - membranes determined to be lipid AND protein z. Various models developed over the years z. Today biologists accept the fluid mosaic model of membrane structure ylipid bilayer with scattered proteins
Fluid Mosaic Model
Membrane Lipids
Phospholipids
Lipid Bilayer
In water a phospholipid bilayer will form a closed spherical vesicle
Lipid Bilayer is Fluid z Lipid molecules in each monolayer can yrotate ymove laterally change position with neighbor z Fluidity due to unsaturated FA chains and cholesterol z Flip-flop does not occur
Cholesterol
Cholesterol and unsaturated FA prevent packing of phospholipids
Components of Model
Protein Properties
Integral Proteins z Integral Proteins have a hydrophobic region z Nonpolar side chains immersed in lipid bilayer
Various Protein Shapes
Examples of Plasma Membrane Proteins and Their Functions
Membrane Proteins
Membrane Proteins
Glycocalyx
Membrane Asymmetry
Cell fusion experiment proves membrane is fluid
Membranes Are Selective Barriers z. Membranes have a variety of functions but the most basic function is that of a permeability barrier z. Membranes serve as selective barriers that regulate the passage of materials into and out of cells (or organelles) z. Any substance that enters or leaves a cell must pass through the membrane
Membrane Barriers z. For a material to cross a membrane y. The membrane must be permeable to that substance y. OR the material is transported across the membrane
Membrane Permeability z. If a membrane is permeable to a substance that means that the substance is able to diffuse across or through the membrane
Kinetic Energy z. All particles (all matter in the universe) possess a certain amount of kinetic energy - results in random motion of particles (Brownian Movement) z. Largely as a result of temperature (temp is a measure of average kinetic energy) z. Because particles are in constant motion, there are several effects - diffusion is one
Diffusion z. Net movement of particles from a region of higher concentration of the substance to a region of lower concentration z. Net movement because there is always some in all directions because of the random movement z. Particles of a substance tend to become distributed within the available space
Diffusion z. Diffusion is fastest in gases y. Slower in liquids y. Infinitely slow in solids z. Net effect is always from a region of high concentration to lower concentration as long as temperature and pressure are equal y. Or from a region of high free energy to lower free energy
Diffusion
Diffusion and Osmosis A B Membrane is permeable to water but not to glucose molecules Osmosis is the diffusion of water across a differentially permeable membrane
Measuring Osmotic Pressure
Osmosis in Animal Cells Area outside cell is Hypotonic or Hypo-osmotic
Osmosis in Animal Cells Equal concentrations of solutes inside and outside Concentration of solutes lower inside the cell - water will leave Isotonic or Iso-osmotic concentrations Area outside is hypertonic or hyperosmotic - Plasmolyzed
Plant Cells Iso-osmotic Hyperosmotic
- Slides: 37