Cell Membrane and Function

Chapter 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 environment of cell from external environment.

• Is selectively permeable: some substances can cross more easily than others (small,

nonpolar, not charged particles)

Semi-Permeability

• Is due to structure of membrane• Contains phosholipids, embedded proteins,

cholesterol, glycoproteins, and glycolipids

Fig. 7-2

Hydrophilichead

WATER

Hydrophobictail

WATER

Made of a double phospholipid layer that has amphipathic molecules

(hydrophobic and hydrophilic areas)

Fig. 7-3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

• Can be hydrophilic w/ charged and polar side groups.

• Can be hydrophobic with nonpolar side groups

Embedded Proteins

Fig. 7-8

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

Hydrophobic regions of an integral protein: made of 1 or more stretches of nonpolar amino acids, often

coiled into alpha helices

Cholesterol• Steroid found in b/t

phospholipids.• Affects membrane

fluidity• High temps Less

fluid membrane.• Low temps Hinders

solidification of membrane by disrupting phospholipid packing

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 recognize each other by binding to surface molecules, often carbs, on membrane

• Membrane carbs may be covalently bonded to lipids (forming glycolipids) or to proteins

(forming glycoproteins)

Permeability of the Lipid Bilayer

• Pass: Small, uncharged molecules and small nonpolar molecules (N2)

• Large hydrophilic, polar molecules (sugars) can cross w/ help of embedded channel or

transport proteins

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 and an internal permeability 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!

• Often used for import of resources/ export of wastes

Concentration Gradient

Osmosis• Passive transport process: movement

of water across cell membrane.• Again, it follows the magic rule!

Lowerconcentrationof solute (sugar)

Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof 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• A channel/carrier

protein carries molecules across (area of high to low conc.)

• Often charged and polar molecules cross here

Fig. 7-15

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

Facilit

ate

d D

iffu

sio

n

Active Transport• Energy-

requiring: materials move across cell membrane from low to high conc.

• Uses ATP• Done by

embedded proteins

• Allows cells to maintain concentration gradients that differ from their surrounding

Ex: Sodium-Potassium Pump

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

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