Download The Plasma Membrane: Structure and Function

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
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
Membrane Structure
• 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
• Glycoproteins and glycolipids are
proteins/lipids with short chain
carbohydrates attached on the
extracellular side of the membrane.
Carbohydrate of
glycoprotein
Glycoprotein
Glycolipid
Integrin
Phospholipid
Microfilaments
of cytoskeleton
Cholesterol
Membrane Proteins
Types of Membrane Proteins
1. Recognition proteins
2. Integrins
3. Cell junction proteins
4. Enzymes
5. Receptor proteins
6. Transport proteins
– Passive and active
• 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
• Integrins are a type of integral protein
– The cytoskeleton attaches to integrins on the
cytoplasmic side of the membrane
– Integrins strengthen the membrane
• Cell Junction proteins - help like cells stick
together to form tissues
• Many membrane proteins are enzymes
– This is especially important
– on the membranes of organelles.
Receptor Proteins
• 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.
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
Transport of Substances Across the Plasma
Membrane (PM)
1. Passive Transport
– (Simple) Diffusion
– Facilitated diffusion
– Osmosis
2. Active Transport
3. Bulk Flow - endocytosis and 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
– Steeper gradient, faster diffusion
• Molecular size
– 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
• Osmosis – diffusion of water across a
selectively permeable membrane
• Water moves from an area of _______ water
concentration to an area of _____ water conc.
– Is energy required ?
Osmosis Terms
Consider two solutions separated
by a plasma membrane.
• Hypertonic
– solution with a relatively high concentration of solute
• Hypotonic
– solution with a relatively low concentration of solute
• Isotonic
– solutions with the same solute concentration
Lower
concentration
of solute
Higher
concentration
of solute
Equal
concentration
of solute
H2O
Solute
molecule
Selectively
permeable
membrane
Water
molecule
Solute molecule with
cluster of water molecules
Net flow of water
Osmosis and Animal Cells
Osmosis and Plant Cells
Osmosis
• 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.
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)
See page 83
Osmosis Summary
•When a cell is placed in a Hypotonic solution:
– Cell gains water through osmosis
– Animal cell lyses; plant cell becomes turgid (firm)
•When 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
Active Transport of “X”
–
Active transport proteins span the plasma
membrane
– They have openings for “X” on only one
side of the membrane
– “X” enters the channel and binds to
functional groups inside the transport
protein.
– Cytoplasmic ATP binds to the transport
protein
Active Transport of “X”
–
A phosphate group is transferred from ATP to
the transport protein
–
protein is energized by the added –P.
–
The energized transport protein changes
shape and releases “X” on the other side of
the cell.
–
The phosphate group is released from the
transport protein and it resumes its original
shape.
–
Process repeats.
Transport
protein
Solute
1 Solute binding
Transport
protein
Solute
1 Solute binding
2 Phosphorylation
Transport
protein
Protein
changes shape
Solute
1 Solute binding
2 Phosphorylation
3 Transport
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
Bulk Flow
• Vesicles are used to transport large particles
across the PM.
– Requires energy
• Types:
– Exocytosis
– Endocytosis
• Phagocytosis, pinocytosis, receptor-mediated
Exocytosis
Fluid outside cell
Vesicle
Protein
Cytoplasm
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
Endocytosis
Vesicle forming
Endocytosis can occur in three ways
• Phagocytosis ("cell eating")
• Pinocytosis ("cell drinking")
• Receptor-mediated endocytosis
Endocytosis
• Endocytosis
– PM sinks inward, pinches off and forms a vesicle
– Vesicle often merges with Golgi for processing and
sorting of its contents
Endocytosis - terms
• Phagocytosis – cell eating
– Membrane sinks in and captures solid particles for
transport into the cell
– Examples:
• Solid particles often include: bacteria, cell
debris, or food
• Pinocytosis – 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…
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. Receptor proteins on PM bind specific
substances (vitamins, hormones..)
2. 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
Phagocytosis
EXTRACELLULAR
FLUID
CYTOPLASM
Pseudopodium
Food
being
ingested
“Food” or
other particle
Food
vacuole
Pinocytosis
Plasma
membrane
Vesicle
Plasma membrane
Receptor-mediated endocytosis
Coat protein
Receptor
Coated
vesicle
Coated
pit
Coated
pit
Specific
molecule
Material bound
to receptor proteins
Plasma Membrane Notes Continued…
What Are Three Forms of Transport Across the Membrane ?
Objective:
1. Compare active transport with passive
transport.
2. Distinquish between endocytosis and
exocytosis
57
Review…
Passive Transport
is a type of Simple Diffusion which…
1. Doesn’t require energy
2. Moves solids, liquids and
gases from high to low
concentration
 Biological connection
 Oxygen or water diffusing into lung cells and
carbon dioxide diffusing out.
Passive Transport can
also occur as …
Facilitated diffusion which…
Doesn’t require energy
Uses transport proteins
to move high to low
concentration
Biological connection:
Glucose or Protein in the form of
amino acids move from blood
into a cell.
In cells proteins (amino acids) are Critical
to Membrane Function because of 4 main
reasons which are…
2 Types of Transport Proteins are…
• Channel proteins are embedded in the cell
membrane & have a pore for materials to
cross
• Carrier proteins can change shape to move
material from one side of the membrane to
the other
Facilitated Diffusion
Molecules will randomly move
through the pores in Channel
Proteins.
Facilitated Diffusion
• Some Carrier
proteins do not
extend through the
membrane.
• They bond and drag
molecules through
the lipid bilayer and
release them on the
opposite side.
Carrier Proteins
• Other carrier
proteins change
shape to move
materials across
the cell
membrane
Active Transport
Requires energy or
ATP
Moves materials from
LOW to HIGH
concentration
AGAINST
concentration gradient
What is the name of this organelle?
• What is the mitochondria function?
• How does it transport the materials it needs to function?
• Why is this important?
Moving the “Big Stuff”
Exocytosis-
moving
things
out.
Molecules are moved out of the cell by vesicles that fuse with the
plasma membrane.
This is how many hormones are secreted and how nerve cells
communicate with one another.
Exocytosis
Exocytic vesicle
immediately after
fusion with plasma
membrane.
Moving the “Big Stuff”
Large molecules move materials into the cell by
one of three forms of endocytosis.
Pinocytosis
Most common form of endocytosis.
Takes in dissolved molecules as a vesicle.
Pinocytosis
• Cell forms an
invagination
• Materials dissolve
in water to be
brought into cell
• Called “Cell
Drinking”
Example of
pinocytic vesiclesPinocytosis
forming
mature transport vesicle
Transport across a capillary cell (blue).
Receptor-Mediated Endocytosis
Some integral proteins have receptors on their
surface to recognize & take in hormones,
cholesterol, etc.
Receptor-Mediated Endocytosis
Endocytosis – Phagocytosis
Used to engulf large particles such as
food, bacteria, etc. into vesicles
Called “Cell Eating”
Phagocytosis About to Occur
This is a picture of a
White Blood Cell…
about to flow around and
eat a Prokaryotic
Bacteria
Phagocytosis Capture of a
Yeast Cell (round
yellow object ) by
the White Blood
Cells (BLUE)
extending found in
the Immune System
Cell
Exocytosis
The opposite of endocytosis is exocytosis. Large molecules that are
manufactured in the cell are released through the cell membrane.
From inside the
Cell out to the …
cells environment which will then
distribute the molecule to the
proper system to get rid of the
material