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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
Made of a double
phospholipid layer that has
amphipathic molecules
(hydrophobic and
hydrophilic areas)
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Fig. 7-3
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Embedded Proteins
• Can be hydrophilic w/ charged and polar side
groups.
• Can be hydrophobic with nonpolar side groups
Fig. 7-8
Hydrophobic regions of an integral protein: made of 1
or more stretches of nonpolar amino acids, often
coiled into alpha helices
N-terminus
C-terminus
 Helix
EXTRACELLULAR
SIDE
CYTOPLASMIC
SIDE
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
Membrane is fluid  Moves
Lateral movement
(107 times per second)
Flip-flop
( once per month)
(a) Movement of phospholipids
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
Transport Proteins
• Binds to molecules and changes shape
to shuttle them across
membrane…extremely specific
Cell Walls
Cell Wall
• Provides a structural boundary and
an internal permeability boundary.
• Bacteria Cell Wall: Made of
peptidoglycan
• Plant Cell Wall: Made of Cellulose
• Fungi Cell Wall: Made of Chitin
Passive Transport
• Movement of molecules in
and out of cell w/out
using energy.
• AKA Diffusion
Concentration Gradient
• High to low
concentrations!
• Often used for
import of
resources/
export of wastes
Osmosis
• Passive transport process: movement
of water across cell membrane.
• Again, it follows the magic rule!
Fig. 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
H2O
Selectively
permeable
membrane
Osmosis
Video Clip!
Osmosis
Same concentration
of sugar
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
H2O
Isotonic solution
H2O
H2O
Hypertonic solution
H2O
(a) Animal
cell
Lysed
H2O
Normal
H2O
Shriveled
H2O
H2O
(b) Plant
cell
Turgid (normal)
Flaccid
Plasmolyzed
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
Facilitated Diffusion
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Carrier protein
(b) A carrier protein
Solute
Active Transport
• Energyrequiring:
materials move
across cell
membrane from
low to high conc.
• Uses ATP
• Done by
embedded
proteins
Ex: Sodium-Potassium Pump
• Allows cells to maintain concentration gradients
that differ from their surrounding
http://hig
hered.mcg
rawhill.com/si
tes/007249
5855/stud
ent_view0
/chapter2/
animation
__how_th
e_sodium
_potassiu
m_pump_
works.ht
ml
Fig. 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
Exocytosis
• Internal vesicles fuse w/ plasma mebrane to
secrete large macromolecules out of cell.
• Requires ATP!
Endocytosis
• Cell takes in macromolecules by forming
new vesicles derived from plasma
membrane.
Eukaryotes Compartmentalize…
• Minimizes competing interactions and increases
surface area.
• Keep reactions and enzymes localized
• Bacteria and Archaea (ancient) can’t do this