The Cell and Its Surroundings Cell Membrane Transport Cell Junctions

Cell Membrane Provides protection and support Regulates what enters and leaves Composed of a double-layered sheet called a lipid bilayer

Cell Membrane Structure Composed mostly of phospholipids and proteins Phospholipids have hydrophilic “water- loving” region (heads) and hydrophobic “water-fearing” region (tails) Proteins are embedded throughout

Cell Membrane Structure Peripheral proteins sit on the surface Integral proteins are embedded in the middle layer Many proteins have carbohydrate molecules attached

Cell Membrane Structure Fluid Mosaic Model

Transport The cytoplasm is a solution of many substances in water This creates a concentration gradient – different concentrations inside and out  Concentration = mass solute/volume solvent Remember: Molecules will naturally move from high to low concentration – “downhill”

Diffusion Particles move from high concentration to low concentration Equilibrium – when concentration is the same inside and out  Particles still move at this point, but if one goes in, one usually goes out at the same time. (no directional movement)

Osmosis Some substances are too large to pass through the membrane (impermeable) Water passes quite easily (permeable) through a membrane Osmosis is the diffusion of water through a selectively permeable membrane

Osmosis As the water moves from high to low, it causes the right side to rise Remember: Salt Sucks  Draws water in its direction  “Salt” can be any Solute!

Solutions and Osmosis Isotonic – concentrations of water and solute are the same Hypertonic – higher concentration of solute (low water) Hypotonic – lower concentration of solute (high water) (These terms always refer to solute!)

Passive Transport Does not require energy  Includes diffusion and osmosis Also includes facilitated diffusion – carrier proteins in the membrane help larger molecules pass through

Facilitated Diffusion

Active Transport Sometimes cells must move materials in the opposite direction – against the concentration gradient (low to high) - “uphill” This requires energy Carried out by protein “pumps” in the cell membrane

Active Molecular Transport Proteins are used to move small molecules such as calcium, potassium, and sodium ions across the membrane Use of energy enables cells to concentrate substances in a particular area

Other Types of Active Transport Endocytosis – cell takes in large molecules by creating pockets and engulfing them  Phagocytosis – “cell eating;” extensions of cytoplasm surround food particle, packages it within a food vacuole and cell engulfs it Used by ameobas  Pinocytosis – tiny pockets along the membrane fill with liquid and pinch off; used to take in liquid Exocytosis – opposite of endocytosis; cell removes materials

Cell Junctions Connect one cell to another Three major types in vertebrates:  Desmosomes – spot welds that hold cells together  Tight junctions – seals around cells to prevent leakage  Gap junctions – very small channels that allow ions and other small substances to pass from cell to cell

Cell Junctions Plasmodesmata – (plant cells only) extend through pores in the cell wall connecting the cytoplasm of each cell to neighboring cells.