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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 membrance 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