CELL MEMBRANE

Cell BoundariesEvery cell is surrounded by a liquid liquid environmentenvironment

and has a liquid insideliquid inside

Two types of boundaries separate:

– Cell membrane (inner)• Thin, flexible layer that surrounds all cells

– Cell wall (outer)• Strong supporting layer outside the cell membrane• Only in some organisms (plants, algae, fungi, prokaryotes)• Porous

Phospholipids

• Cell membranes are mostly built from fat molecules called phospholipids– Have a polar head and

two nonpolar tails• Polar heads

attracted to water• Nonpolar tails

repelled by water

• Movie #1

Lipid Bilayer

• Because there is water inside and outside the cell, the phospholipids form a double layer called the lipid bilayer.

• Polar heads on outside, nonpolar tails on the inside

• Movie #2

Membrane Components

• Membranes also have proteins embedded in the bilayer- act as tunnels or pumps across the membrane- others protect the cell membrane

• Contains carbohydrates, usually attached to the proteins- act as signals to other cells

• Contains cholesterol- Stabilizes the membrane

Cell Membrane

Outsideof cell

Insideof cell(cytoplasm)

Cellmembrane

Proteins

Proteinchannel Lipid bilayer

Carbohydratechains

Section 7-3

Figure 7-12 The Structure of the Cell Membranehttp://www.youtube.com/watch?v=ULR79TiUj80

Inside of cell(cytoplasm)

Cellmembrane

Proteins

Proteinchannel

Lipid bilayer

Carbohydratechains

Section 7-3

Figure 7-12 The Structure of the Cell MembraneFluid Mosaic Model

• Lipid bilayer – double layer of molecules– Lipids are a certain class of C-H molecules – Do not like water ( think of oil + water) – the “tails”– The “heads” are polar molecules

• Has proteins throughout • Can have carbohydrate “ID tags”• Moveable/fluid• Selectively Permeable

Cell Wall

• Cell wall – tough, porous boundary that lies outside the cell membrane; found in plant cells and in some bacteria but NOTNOT in animal cells- Allow water, gases, and other substances to pass

through easily

• Most cell walls are made of carbohydrates and proteins- Specifically composed of cellulose

FUNCTION of Cell Membrane

• Regulates what goes in and

what comes out of a cell– Like a fence or a window screen– Regulation depends on size, concentrations, and

the type of molecule trying to get through

• Protects cell• Supports cell, but also flexible• Recognizes foreign material• Communicates with other cells

What goes in and out? (Transport)Cell membrane is selectively permeable,

it lets some things in, but not other things

Selection depends on…

size (smaller get through easily)

concentrations,

& type of molecule trying to get through

(lipid and nonpolar)

• Substances keep moving till there are ~ equal amounts of them on both sides of the membrane EQUILIBRIUM

Cell Membranes

• Must be able to take in raw materials and eliminate waste products- Like a bouncer at a club

Permeabilityhttp://www.youtube.com/watch?v=GW0lqf4Fqpg&feature=related

Do these get through?• H+ or Ca++• CO2

• K+, Cl- • H2O• Glycerol or ethanol (-OH)• O2

• Amino acids• Glucose