Transcript Chapter 4 Cell Membrane Structure & Function

Chapter 4
Cell Membrane Structure & Function
4.1 – How Is the Structure of a
Membrane Related to its Function?
• The plasma membrane isolates the cell while
allowing communication with its surroundings
• Three General Functions:
1. Selectively Permeable
2. Regulates exchange
3. Communication
Membranes are Fluid Mosaics in
Which Proteins Move
• Fluid Mosaic Model was developed in
1972
• Phospholipids act as grout for membrane
proteins, which represent tiles
Phospholipid Bilayer is the Fluid
Portion of the Membrane
• Phospholipid Review
– Have a polar head
• Hydrophilic
– Have 2 nonpolar tails
• Hydrophobic
• Double bonds in the tail increase
fluidity of membrane
The Plasma Membrane is a
Phospholipid Bilayer
• The tails point inward
– away from the watery environment
• The heads point outward
– toward the watery environment
• Membrane is “fluid” because the
phospholipds are not bonded together
• Phospholipids bilayer selectively isolates
internal environment from external
environment
– Most biological molecules are hydrophilic
and cannot pass through the membrane
easily
– Some molecules can freely pass through
the membrane
• Cholesterol in animal cell membranes
make the bilayer stronger, more flexible,
less fluid & less permeable to water
soluble substances
• Flexibility and fluid nature of the bilayer is
important to its function
The Plasma Membrane
A Mosaic of Proteins is Embedded
in the Membrane
• Proteins embedded within or attached to
the surface of the bilayer regulate the
movement of substance across the
membrane and communicate with the
environment
• Many membrane proteins are
glycoproteins
Three Categories of Membrane
Proteins
1. Transport Proteins – regulate the
movement of hydrophilic molecules
through the membrane
1. Channel Proteins – form pores and
channels for small water-soluble
molecules
2. Carrier Proteins – bind molecules and
move them across the membrane
Channel proteins do
not change shape
Carrier
proteins
change
shape
Three Categories of Membrane
Proteins (con’t)
2. Receptor Proteins – bind molecules in
the environment, triggering changes in
the metabolism of the cell
3. Recognition Proteins – serve as
identification tags and cell-surface
attachment sites
4.2 – How Do Substances Move
Across the Membrane
Molecules Move in Response to
Gradients
• Characteristics of a fluid
– Fluid – any substance that can move or
change shape in response to external
forces without breaking apart
– Concentration – number of molecules in a
given unit of volume
– Gradient – physical difference in properties
such as temperature, pressure, or
concentration
Diffusion
• The movement of molecules from regions
of high concentration to regions of low
concentration
• Movement down the concentration
gradient
Diffusion Con’t
•Molecules move randomly and continuously,
colliding with each other, until a dynamic
equilibrium exists in which there is no
concentration gradient
•The greater the concentration gradient, the
faster the rate of diffusion
Example of Diffusion
A drop of food coloring in a glass of water
Movement Across Membranes Occurs
by Both Passive & Active Transport
• There are significant concentration
gradients of ions and molecules across
the plasma membrane because the
cytoplasm is very different from the
extracellular fluid
Movement Across the Membrane
Occurs by:
• Passive Transport
– Substances move down conc. gradient
– No energy is required
• Active Transport
– Substance move up conc. gradient
– Energy is required
Types of Passive Transport
1. Simple Diffusion
2. Facilitated Diffusion
3. Osmosis
Simple Diffusion
• Membranes are selectively permeable to
diffusion of molecules
• Lipid-soluble molecules & very small
molecules can easily diffuse across the
membrane
• Rate of simple diffusion depends on
conc. gradient, the size of the molecule &
its lipid solubility
Facilitated Diffusion
• Molecules cross the membrane with the
help of membrane transport proteins
– Channel proteins
– Carrier proteins
• No energy required
– molecules move down the conc.
gradient
Osmosis
• Osmosis is the diffusion of water
• Water moves down the conc. gradient
across a selectively permeable
membrane
• Dissolved substances reduce the
concentration of water molecules in
solution
Osmosis is Important
in the Life of Cells
• Water balance between cells and their
surroundings is crucial to organisms
• Three environments exist due to varying
water concentrations
1. Isotonic
2. Hypertonic
3. Hypotonic
Isotonic Cell Environment
• Water concentration around the cell is the same
as the water concentration inside the cell
• No net movement of water occurs
• Cell remains the same size
– The type of dissolved particles does not have to
be the same, but the total concentration of all
dissolved particles is equal
Isotonic
Water is moving in to
and out of the cell at
an equal rate.
Hypertonic Cell Environment
• The solution outside the cell has a higher
concentration of solutes than the interior
of the cell
– Lower water concentration
• Water will flow out of the cell by osmosis
– Cells shrivel and shrink
Hypertonic
Net movement of
water out of the cell
Cell shrinks
Hypotonic Cell Environment
• The solution outside the cell has a lower
solute concentration than the solution
inside the cell
– Higher water concentration
• Water will flow into the cell by osmosis
– Cells will swell and sometimes burst
Hypotonic
Net movement of
water into the cell
Cells swell
Active Transport Uses Energy to
Move Molecules
• All cells need to move some substances
against their conc. gradient
• Membrane proteins that require energy are
used to move molecules against their conc.
gradient
• Active transport proteins are sometimes
called “pumps” because they move
substances uphill
Active Transport Proteins
• Active transport proteins span the width
of the membrane and have 2 active
sites
1. One site binds the substance to be
transported
2. Second site binds an energy carrier
molecule, usually ATP
Example of Active Transport
Cells Engulf Particles
or Fluids by Endocytosis
• Types of Endocytosis
1. Pinocytosis
2. Phagocytosis
3. Receptor-Mediated Endocytosis
Pinocytosis
• Moves liquids into the cell
• Means “cell drinking”
• A small patch of membrane dimples
inward to form a vesicle surrounding the
fluid
• The acquired material has the same
concentration as extracellular fluid
Pinocytosis
Phagocytosis
• Moves large particles into the cell
• Means “cell eating”
• Extensions of the membrane fuse around
the large particle and carry it to the
interior of the cell in a vacuole for
intracellular digestion
Phagocytosis
Cells Move Material Out
of the Cell by Exocytosis
• A membrane-enclosed vesicle carrying
the material to be expelled moves to the
cell surface
• The vesicle then fuses with the plasma
membrane and releases its contents
Exocytosis
4.3 – How Are Cell Surfaces
Specialized?
Various Specialized Junctions Allow
Cells to Connect and Communicate
• Four types of
connections occur
between cells,
depending on the
organism and cell
type
1.
2.
3.
4.
Desmosomes
Tight Junctions
Gap Junctions
Plasmodesmata
Desmosomes
• Membranes of
adjacent cells are held
together by proteins
and carbohydrates
• Further strengthened
by protein filaments
that extend from inside
the desmosome to the
interior of each cell
Tight Junctions
• Membranes of
adjacent cells are
fused together to
create leak-proof
junctions
Gap Junctions
• Cell-to-cell
cytoplasmic
connections found
in animal cells that
need to
communicate with
each other
Plasmodesmata
• Cell-to-cell
cytoplasmic
connections
between plant
cells