Warm Up

What is the Surface Area to

Volume Ratio of a sphere with a

radius of 5mm? Of 10 mm? What

sphere can eliminate wastes and

move materials quicker?

Cell Membrane and FunctionChapter 7

• Big Idea #2: Biological systems use energy to

grow, reproduce, and maintain dynamic

homeostasis.

Essential Knowledge• 2B1:Cell membranes are selectively

permeable due to their structure

• 2B2: Growth and dynamic homeostasis are

maintained by the constant movement of

molecules across membranes

• 2B3: Eukaryotic cells maintain internal

membranes that partition the cell into

specialized regions

• Separates internal and external env.

• Is selectively permeable: some

substances can cross

Semi-Permeability

Due to structure

Has phosholipids, proteins, cholesterol,

glycoproteins, and glycolipids

Fig. 7-2

Hydrophilichead

WATER

Hydrophobictail

WATER

Made of a double

phospholipid layer that is

amphipathic (hydrophobic

and hydrophilic areas)

Fig. 7-3

Phospholipid

bilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

• Can be hydrophilic w/ charged and polar side

groups.

• Can be hydrophobic w/ nonpolar side groups

Embedded Proteins

Fig. 7-8

N-terminus

C-terminus

Helix

CYTOPLASMICSIDE

EXTRACELLULARSIDE

Hydrophobic area ex: coiled,

nonpolar alpha helices

Cholesterol• Steroid found in b/t

phospholipids.

• In high temps

less fluid

membrane.

• In low temps

Stops solidification

of membrane

Fig. 7-5a

(a) Movement of phospholipids

Lateral movement

(107 times per second)

Flip-flop

( once per month)

Membrane is fluid Moves

When Cold, Moves Less!!

Cell to Cell Recognition

• Cells “talk” w/ membrane carbs may be

covalently bonded to lipids (glycolipids) or to

proteins (glycoproteins)

Permeability of Lipid Bilayer

• Pass: Small, uncharged molecules and

small nonpolar molecules (N2)

• Others need help with channels or pumps

Aquaporins

Channel protein that allows water to

move across

Binds to molecules and changes shape

to shuttle them across

membrane…extremely specific

Transport Proteins

Cell Walls

Provides a structural boundary

Cell Wall

• Bacteria Cell Wall: Made of

peptidoglycan

• Plant Cell Wall: Made of Cellulose

• Fungi Cell Wall: Made of Chitin

• Movement of molecules in and out of

cell w/out using energy.

• AKA Diffusion

Passive Transport

• High to low

concentrations!

• Import

resources/

export of wastes

Concentration Gradient

Osmosis• Passive transport: movement of H2O

across membrane.

• High to Low!

Lower

concentrationof solute (sugar)

Fig. 7-12

H2O

Higher

concentrationof sugar

Selectivelypermeable

membrane

Same concentration

of sugar

Osmosis

Osmosis

Video Clip!

Tonicity• Ability of a solution to cause a cell to gain or

lose water osmoregulation

• Isotonic solution: Solute concentration is the

same as that inside the cell; no net water

movement across the plasma membrane

• Hypertonic solution: Solute concentration is

greater out of cell; cell loses water

• Hypotonic solution: Solute concentration is

high inside cell; cell gains water

Fig. 7-13

Hypotonic solution

(a) Animal

cell

(b) Plant

cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

Facilitated Diffusion• Channel protein

carries molecules

across (high to low

conc.)

• Polar molecules

Fig. 7-15

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

Fa

cil

ita

te

d D

iff

us

ion

Active Transport

• Energy-requiring:

move across cell

membrane from

low to high conc.

• Uses ATP

• Embedded

proteins

• Allows cells to maintain concentration gradients

that differ from their surrounding

Ex: Sodium-Potassium Pump

http://hig

hered.mcg

raw-

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tes/007249

5855/stud

ent_view0

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animation

__how_th

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Fig. 7-17Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

Exocytosis

• Internal vesicles fuse w/ plasma mebrane to

secrete large macromolecules out of cell.

• Requires ATP!

• Cell takes in macromolecules by forming

new vesicles derived from plasma

membrane.

Endocytosis

Eukaryotes Compartmentalize…

• Minimizes competing interactions and increases

surface area.

• Keep reactions and enzymes localized

• Bacteria and Archaea (ancient) can’t do this