The Plasma Membrane and Homeostasis

I. Homeostasis – Maintaining a Balance i Cells must keep the proper concentration of nutrients and water and eliminate wastes. ii The plasma membrane is selectively permeable – it will allow some things to pass through, while blocking other things.

II. Structure of the Plasma Membrane i Lipid bilayer – two sheets of lipids (phospholipids). ii Found around the cell, the nucleus, vacuoles, mitochondria, and chloroplasts. iii Embedded with proteins and strengthened with cholesterol molecules. http://www.youtube.com/watch?v=ULR79TiUj80

Cell Membrane Micrograph

III. What’s a Phospholipid? i It’s a pair of fatty acid chains and a phosphate group attached to a glycerol backbone. ii Polar (water-soluble) heads face out and the nonpolar fatty acids hang inside.

IV. Membrane Proteins i. Determine what particles can pass through the membrane. ii. Serve as enzymes (may speed reactions). iii. Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell (the immune system). ie antigens

Draw Membrane Proteins Page 82 in text: Channel Proteins Carrier Proteins Cell Recognition Proteins (Glyco proteins) Receptor Proteins Enzyme Proteins Write their functions too!

outside the cell carbohydrate chains membrane protein interior of cell cytoskeleton cholesterol carbohydrate chains membrane protein This is my own drawing. outside the cell interior of cell

Diagram representing the cell membrane. The above image is from http://www.biosci.uga.edu/almanac/bio_103/notes/may_15.html. But I found it at An On-Line Biology Book: http://gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookCELL2.html

V. Cellular Transport i. Diffusion – movement of particles from an area of high concentration to an area of low concentration. ii. Caused by Brownian motion (movement of particles because of the movement of their atoms). iii. Continues until an equilibrium is reached (no gradient). iv. Dynamic equilibrium – particles move freely and are evenly distributed.

VI Factors Affecting Rate of Diffusion: i. Concentration gradient ii. Molecular size iii. Cytoplasmic streaming (stirring) iv. Temperature v. Viscosity of fluid vi. Particle Mass

Diffusion Animation

VII. Passive Transport i Passive transport – no energy is needed to move particles. ii. Facilitated diffusion – embedded proteins act as channels allowing particles to “fall” through. Diffusion drives the movement: high conc. to low conc.

http://www.youtube.com/watch?v=s0p1ztrbXPY

VIII Active Transport i. Active transport – energy is needed to move particles: cell uses ATP! ii. Usually done to move materials against the concentration gradient iii. Requires Carrier proteins – embedded proteins change shape to open and close passages across the membrane. Na+1/K +1pump example http://www.youtube.com/watch?v=STzOiRqzzL4

Active Transport cont’d iv. Endocytosis – bringing a large molecule (ie protein), or cell fragment, into the cell by pinching off a piece of the membrane and forming a vesicle. v. Exocytosis – removal of large items (especially proteins for secretion) by the emptying of vesicles at the cell membrane.

Endocytosis and Exocytosis

IX. Osmosis i. Diffusion of water across a selectively permeable membrane. ii. Occurs until water is balanced on both sides of the membrane. iii. Water moves to dilute the higher concentration of solute ie from a low solute concentration to a high solute concentration.

X. Cell Concentrations i. Hypertonic cell cytoplasm– more dissolved solute (salt) inside the cell: cell is saltier than its environment ii. Hypotonic cell cytoplasm – less dissolved solute. Cell less salty iii. Isotonic solutions – the same concentration of dissolved solute. Cell of equal saltiness to its outer environment.

Osmosis Problems: Animal cells placed in fresh water will gain water, swell and possibly burst (lysis) Animal cells placed in salty solutions will lose water, and shrink (crenation)

REVIEW http://www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html