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