Membrane Structure and
Function
Membrane Function
 Membranes
organize the chemical
activities of cells.
 The outer plasma membrane


forms a boundary between a living cell and its
surroundings
Exhibits selective permeability
• Controls traffic of molecules in and out
Membrane Function
 Internal
membranes provide structural
order for metabolism
• Form the cell's organelles
• Compartmentalize chemical reactions
Fluid Mosaic Model of the PM
A

A

membrane is a mosaic
Proteins and other molecules are embedded
in a framework of phospholipids
membrane is fluid
Most protein and phospholipid molecules can
move laterally
Membrane Structure
Phospholipid
Phospholipids are
the major structural
component of
membranes.
Membrane Structure
All membranes are phospholipid bilayers
with embedded proteins.
Phospholipid Bilayer
Label the:
Hydrophilic heads
Hydrophobic tails
 Embedded

in the bilayer are proteins
Most of the membrane’s functions are
accomplished by the embedded
proteins.
• Integral proteins span the membrane
• Peripheral proteins are on one side or the other of
the membrane
Plasma Membrane Components
 Glycoproteins
and glycolipids are
proteins/lipids with short chain
carbohydrates attached on the
extracellular side of the membrane.
Plasma Membrane
Types of Membrane Proteins
Cell-cell recognition proteins
Integrins
Intercellular junction proteins
Enzymes
Signal transduction proteins
1.
2.
3.
4.
5.
•
6.
Aka - Receptor proteins
Transport proteins
– Passive and active
 Cell-cell
recognition proteins - identify type
of cell and identify a cell as “self” versus
foreign

Most are glycoproteins
• Carbohydrate chains vary between species,
individuals, and even between cell types in a given
individual.
• Glycolipids also play a role in cell recognition
 Integrins


are a type of integral protein
The cytoskeleton attaches to integrins on the
cytoplasmic side of the membrane
Integrins strengthen the membrane
 Intercellular

junction proteins
help like cells stick together to form tissues
• Many include these as a type of integrin
 Many

membrane proteins are enzymes
This is especially important on the
membranes of organelles.
 Signal
transduction (receptor) proteins
bind hormones and other substances on
the outside of the cell.

Binding triggers a change inside the cell.
• Called signal transduction
• Example: The binding of insulin to insulin receptors
causes the cell to put glucose transport proteins
into the membrane.
Fig. 5-1c
Messenger molecule
Receptor
Activated
molecule
Transport Proteins
Passive Transport Proteins


allow water soluble substances (small polar
molecules and ions) to pass through the
membrane without any energy cost
Active Transport Proteins


The cell expends energy to transport water
soluble substances against their
concentration gradient
Fig. 5-1d
Transport Proteins
Transport of Substances Across
the Plasma Membrane (PM)
Passive Transport
1.



(Simple) Diffusion (5.3)
Facilitated diffusion (5.6)
Osmosis (5.4, 5.5)
Active Transport (5.8)
Bulk Flow (5.9)
2.
3.


Endocytosis – 3 types
Exocytosis
Passive Transport
 In
passive transport substances cross
the membrane by diffusion

Diffusion - net movement of substances from
an area of high concentration to low
concentration
• no energy required
Factors Affecting Diffusion Rate

Steepness of concentration gradient


Molecular size


Steeper gradient, faster diffusion
Smaller molecules, faster diffusion
Temperature

Higher temperature, faster diffusion
Simple Diffusion
 Nonpolar,
hydrophobic molecules diffuse
directly through the lipid bilayer


Simple diffusion does not require the use of
transport proteins.
Examples: O2, CO2, steroids
 Polar,
hydrophilic substances cannot pass
directly through the lipid bilayer

Examples: water, ions, carbohydrates
Simple Diffusion
Polar molecules
(ex. Glucose, water)
small, nonpolar molecules
ions
(ex. O2, CO2)
(ex. H+, Na+, K+)
LIPID-SOLUBLE
LIPID-SOLUBLE
WATER-SOLUBLE
Facilitated Diffusion
 In
facilitated diffusion small polar
molecules and ions diffuse through
passive transport proteins.

No energy needed
• Most passive transport proteins are solute
specific
• Example: glucose enter/leaves cells
through facilitated diffusion
Facilitated Diffusion
Higher concentration of
Passive transport
protein
Lower
concentration
Osmosis
– diffusion of water across a
selectively permeable membrane
 Water moves from an area of _______
water concentration to an area of _____
water conc.
 Osmosis

Is energy required ?
 Water
travels in/out of the cell through
aquaporins
Osmosis Terms
Consider two solutions separated
by a plasma membrane.
 Hypertonic


Hypotonic


solution with a relatively high concentration of solute
solution with a relatively low concentration of solute
Isotonic

solutions with the same solute concentration
Osmosis and Animal Cells
Osmosis and Plant Cells
Isotonic solution
Hypotonic solution
H2O
H2O
H2O
Hypertonic solution
H2O
Animal
cell
(2) Lysed
(1) Normal
H2O
H2O
(3) Shriveled
Plasma
membrane
H2O
H2O
Plant
cell
(4) Flaccid
(5) Turgid
(6) Shriveled
(plasmolyzed)
Osmosis Practice
 When
a Cell is Placed in a Hypotonic
Solution


Water concentration is _________ the cell.
Water flows ___________ the cell.
Osmosis
 When
a Cell is Placed in a Hypertonic
Solution


Water concentration is _________ the cell.
Water flows ___________ the cell.
Osmosis Summary
When


Cell gains water through osmosis
Animal cell lyses; plant cell becomes turgid (firm)
When


a cell is placed in a Hypotonic solution:
a cell is placed a Hypertonic solution:
Cell loses water through osmosis
Animal cell shrivels; plant cell plasmolyzes
Active Transport
 Active
transport proteins move substances
across the PM against their concentration
gradient.




Requires energy (ATP)
Active transport proteins are highly selective
Active transport is needed for proper
functioning of nerves and muscles
Open to page 78
Active Transport of “X”
Active transport proteins span the
plasma membrane

–
They have openings for “X” on only one
side of the membrane
(1) “X” enters the channel and binds to
functional groups inside the transport
protein.
(2) Cytoplasmic ATP binds to the protein and
transfers a P group to the transport protein
- protein is energized by the added –P.
Active Transport of “X”
–
(3) The energized transport protein
changes shape and releases “X” on the
other side of the cell.
–
(4) The phosphate group is released from
the transport protein and it resumes its
original shape.
–
Process repeats.
Fig. 5-8-1
Transport
protein
Solute
1 Solute binding
Fig. 5-8-2
Transport
protein
Solute
1 Solute binding
2 Phosphorylation
Fig. 5-8-3
Transport
protein
Protein
changes shape
Solute
1 Solute binding
2 Phosphorylation
3 Transport
Fig. 5-8-4
Transport
protein
Protein
changes shape
Solute
1 Solute binding
2 Phosphorylation
3 Transport
Phosphate
detaches
4 Protein reversion
Active Transport
tell the story…
ATP
P
ADP
http://www.mhhe.com/biosci/bio_animations/
05_MH_MembraneTransport_Web/index.ht
ml
http://www.youtube.com/watch?v=2UPqLmuDnI
Bulk Flow (5.9)
 Vesicles
are used to transport large
particles across the PM.

Requires energy
 Types:


Exocytosis
Endocytosis
• Phagocytosis, pinocytosis, receptor-mediated
Bulk Flow
 Exocytosis


Cytoplasmic vesicle merges with the PM
and releases its contents
Example:
• Golgi body vesicles merge with the PM an
release their contents
• How nerve cells release neurotransmittors
Exocytosis
Fluid outside cell
Vesicle
Protein
Cytoplasm
Endocytosis
 Endocytosis


PM sinks inward, pinches off and forms a
vesicle
Vesicle often merges with Golgi for
processing and sorting of its contents
Endocytosis
Vesicle forming
Endocytosis can occur in three ways
• Phagocytosis ("cell eating")
• Pinocytosis ("cell drinking")
• Receptor-mediated endocytosis
Endocytosis - terms
 Phagocytosis


Membrane sinks in and captures solid
particles for transport into the cell
Examples:
• Solid particles often include: bacteria, cell
debris, or food
 Pinocytosis

– cell eating
– cell drinking
Cell brings in a liquid
Endocytosis - comments
 Phagocytosis
and pinocytosis are not
selective


Membrane sinks inward and captures
whatever particles/fluid present.
Vesicle forms and merges with the Golgi
body…
Fig. 5-9, pg 79
Phagocytosis
EXTRACELLULAR
FLUID
CYTOPLASM
Pseudopodium
Food
being
ingested
Phagocytosis
“Food” or
other particle
Food
vacuole
Pinocytosis
Plasma
membrane
Pinocytosis
Vesicle
Plasma membrane
Receptor-mediated endocytosis
Coat protein
Receptor
Receptormediated
endocytosis
Coated
vesicle
Coated
pit
Coated
pit
Specific
molecule
Material bound
to receptor proteins
Receptor Mediated Endocytosis
Receptor Mediated Endocytosis is a
highly specific form of endocytosis.


Receptor proteins on the outside of the cell
bind specific substances and bring them into
the cell by endocytosis
Receptor Mediated Endocytosis
1.
2.
Receptor proteins on PM bind specific
substances (vitamins, hormones, LDL..)
Membrane sinks in and forms a pit
– Called a coated pit
3.
Pit pinches closed to form a vesicle around
bound substances
•
Cytoskeleton aids in pulling in the membrane and
vesicle formation
Plasma membrane
Receptor-mediated endocytosis
Coat protein
Receptor
Coated
vesicle
Coated
pit
Coated
pit
Specific
molecule
Material bound
to receptor proteins
Endo and exocytosis video
Another animation