CELL MEMBRANECELL MEMBRANE

Cell Membrane Structure

Cell Membrane Structure

Cell BoundariesEvery cell is surrounded by a liquid environmentliquid environment

and has a liquid inside (cytoplasm)liquid inside (cytoplasm)

Two types of boundaries separate:

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

– Cell wall (outer)• Strong supporting layer outside the cell membrane made

of carbohydrate (cellulose, chitin)• Only in some organisms (plants, algae, fungi, prokaryotes)• PorousPorous

Cell Membrane Structure

Phospholipids• Cell membranes are

mostly built from fat molecules called phospholipids– Have a polar head and

two nonpolar tails• Polar heads glycerol

with phosphate attached– attracted to water =

hydrophilic• Nonpolar tails

– repelled by water =

hydrophobic

Glycerol with phosphate group attached

2 fatty acid tails

Lipid Bilayer

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

• Polar heads facing the polar liquid environments, nonpolar tails on the inside away from the polar liquid environment

Cell Membrane Structure

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 antennae/signals to other cells

- If attached to proteins – glycoproteins- If attached to phospholipids - glycolipids

• Contains cholesterol- Stabilizes the membrane

Cell Membrane Structure

Cell Membrane

Outsideof cell

Insideof cell(cytoplasm)

Cellmembrane

Proteins

Proteinchannel Lipid bilayer

Carbohydratechains

Section 7-3

Figure 7-12 The Structure of the Cell Membrane

Cell Membrane Structure

Inside of cell(cytoplasm)

Cellmembrane

Proteins

Proteinchannel Lipid bilayer

Carbohydratechains

Section 7-3

Figure 7-12 The Structure of the Cell Membrane

Fluid Mosaic Model

• Moveable/fluid

• Has proteins throughout – Can have carbohydrate “ID tags”

http://www.susanahalpine.com/anim/Life/memb.htm

Cell Membrane Structure

Cell Membrane Structure

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

Cell Membrane Structure

FUNCTIONSFUNCTIONS of Cell Membranes

• Regulates what goes in and what goes out of a cell– Must be able to take in raw materials and

eliminate waste products- Like a bouncer at a club

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

Cell Membrane Structure

PermeablePermeable – substance can pass through membraneImpermeableImpermeable – substance can not pass through membrane

PERMEABILITY CREATES SPECIFICITY PERMEABILITY CREATES SPECIFICITY OF THE CELLOF THE CELL

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

it lets some things in, but not other things

Transport depends on…

size (smaller get through more easily)

concentrations,

& type of molecule trying to get through

(lipid and nonpolar get through

more easily)• Substances keep moving till there are ~ equal

amounts of them on both sides of the membrane DYNAMIC EQUILIBRIUMDYNAMIC EQUILIBRIUM

Cell Membrane Structure

To Move or Not To Move Through… Fig 3.2 p 79

Can Move Across Through the Phospholipids

• Small, hydrophobic molecules– Steroids, O2, CO2

• Small, uncharged polar molecules– H2O

Can Move Across The Membrane Either With HELP or Through Active Transport (endocytosis or

exocystosis)

• Larger, uncharged polar molecules– Think monomers

• Ions