Download Cell Membrane Structure & Function

Cell Membrane
Structure and
Function
Membrane Functions

Isolate the cell’s contents from the
external environment

Regulate traffic in and out of the cell

Communicate with other cells
II. Plasma membrane
structure and functions
The phospholipid bilayer and isolation
1. Impermeable to water-soluble and polar
molecules, ions
2. Permeable to small and nonpolar molecules
3. Lipids oriented with polar heads facing out
tails
(hydrophobic)
head
(hydrophilic)
extracellular fluid
(watery environment)
phospholipid
hydrophilic
heads
hydrophobic
tails
bilayer
hydrophilic
heads
cytoplasm
(watery environment)
Membrane Structure and
Function

Membranes are “fluid mosaics” with
proteins embedded in or attached to
the membrane

Proteins can move within the fluid
lipid bilayer
extracellular fluid (outside)
recognition protein
receptor protein
transport protein
binding site
phospholipid
bilayer
carbohydrate
phospholipid
cholesterol
protein filaments
cytoplasm (inside)
Types of Membrane Proteins
1.
Transport proteins
•
regulate the movement of water-soluble
molecules across the membrane
•
Channel proteins
•
Carrier proteins
Types of Membrane Proteins
2. Receptor Proteins
•
trigger cellular response when
specific molecules bind to them
Nervous system
Endocrine system
Types of Membrane Proteins
3. Recognition proteins
•
•
act as ID tags and cell surface
attachment sites
the immune system
Transport across membranes

Passive transport is a function of
molecular size, lipid solubility, and size
of the concentration gradient
1. Simple diffusion
1 A drop of dye is
placed in water.
drop of dye
pure water
2 Dye molecules
diffuse into the
water; water
molecules diffuse
into the dye.
3 Both dye molecules
and water molecules
are evenly dispersed.
(a) simple diffusion
(extracellular fluid)
(cytoplasm)
Transport across membranes
Passive transport…(cont.)
•
2. Osmosis
• a. Isotonic
• b. Hypertonic
• c. Hypotonic
(a)
selectively
permeable
membrane
H2O
free water
molecule: can
fit through pore
sugar
bound water molecules
clustered around sugar:
cannot fit through pore
pore
(b)
selectively permeable
membrane
sugar molecule
water molecule
pure water
bag
bursts
10 micrometers
(a) isotonic solution
equal movement of water
into and out of cells
(b) hypertonic solution
net water movement
out of cells
(c) hypotonic solution
net water movement
into cells
Transport across membranes
Passive transport…(cont.)
•
3. Facilitated diffusion
(b) facilitated diffusion through a channel
ions
proteins forming
permanent
hydrophilic channel
channel
protein
(c) facilitated diffusion through a carrier
amino acids,
sugars,
small proteins
carrier
protein
Carrier protein
has binding
site for
molecule.
(extracellular fluid)
(cytoplasm)
Molecule
enters binding
site.
Carrier protein changes
Carrier protein
shape, transporting molecule resumes original
shape.
across membrane.
Transport across membranes
Energy-requiring transport
1. Active transport
• Ion gradients and energy
production
2.
Endocytosis
3.
Exocytosis
(extracellular fluid)
transport protein
ATP ATP
recognition
binding Transport protein
site
site
binds ATP and
Ca2+
Ca2+.
(cytoplasm)
Transport protein
Transport protein uses
resumes original
energy from ATP to
change shape and move shape.
ion across membrane.
(a)
pinocytosis
(extracellular fluid)
1
2
3
vesicle containing
extracellular
fluid
(cytoplasm)
cell
(b)
phagocytosis
food particle
pseudopod
1
2
3
particle
enclosed in vesicle
secreted
material
(extracellular fluid)
plasma membrane
plasma membrane
2
3
1
vesicle
(cytoplasm)
0.2 micrometer