CLOZE Cellular Membranes
Cellular Membranes
Membrane Composition and Structure
Cell Recognition and Adhesion
Passive ________ of Membrane Transport
Active Transport
Endocytosis and Exocytosis
Membranes Are Not Simply Barriers
Membranes Are Dynamic
Membrane Composition and Structure
Cell membranes are ________ dynamic structures that:
§ Perform vital physiological roles
§ Form boundaries between cells and their environments
§ Regulate movement of molecules into and out of cells
Lipids, proteins, and carbohydrates in various combinations make these tasks
possible.
Membrane Composition and Structure
The lipid portion of a cellular membrane provides a ________ for water-soluble
molecules.
Lipids are like the water of a lake in which proteins "float." This general
design is called the fluid ________ model.
Membrane proteins are embedded in the lipid bilayer.
Carbohydrates attach to lipid or protein molecules on the membrane, generally
on the outer surface.
Membrane Composition and Structure
Most of the lipid molecules found in biological membranes are phospholipids.
Each has a hydrophilic region, where the phosphate groups are located, and a
hydrophobic region, the fatty acid "tails."
The phospholipids organize themselves into a bilayer.
The interior of the membrane is fluid, which allows some molecules to move
laterally in the membrane.
Membrane Composition and Structure
Although all biological membranes are structurally similar, some have quite
different compositions of lipids and proteins.
________ may increase or decrease fluidity depending on other factors, such as
the fatty acid composition of the other lipids found in the membrane.
For any given membrane, fluidity also decreases with declining temperature.
The membranes of cells that live at low temperatures tend to be high in ________
and short-chain fatty acids.
Membrane Composition and Structure
All biological membranes contain proteins.
The ratio of protein to phospholipid molecules varies depending on membrane
function.
Many membrane proteins have ________ and ________ regions.
The association of protein molecules with lipid molecules is not covalent;
both are free to move around laterally, according to the fluid mosaic model.
Membrane Composition and Structure
Integral membrane proteins have hydrophobic regions of amino acids that
penetrate or entirely cross the phospholipid bilayer.
§ Transmembrane proteins have a specific ________ showing different "faces" on
the two sides of the membrane.
Peripheral membrane proteins lack hydrophobic regions and are not embedded in
the bilayer.
Membrane Composition and Structure
Some of the proteins and lipids can move around in the membrane.
Experiments have demonstrated that when two cells are fused, a single
continuous membrane forms around both cells and membrane proteins distribute
themselves uniformly around this membrane.
Membrane Composition and Structure
Some proteins are restricted in movement because they are ________ to
components of the cytoskeleton or are trapped within regions of lipid rafts.
This causes an unequal distribution of proteins, allowing for ________ of
certain regions of the cell membrane.
Membrane Composition and Structure
Some cells have carbohydrates associated with their external surfaces.
Carbohydrate-bound lipid is called glycolipid.
Most of the carbohydrate in the membrane is covalently bonded to proteins,
forming glycoproteins.
Plasma membrane ________ enable cells to be recognized by other cells and
proteins.
Cell Recognition and Adhesion
Cells are able to arrange themselves into groups by two processes:
§ In cell recognition, one cell specifically binds to another cell of a certain
type.
§ In cell adhesion, the relationship between the two cells is "cemented".
Tissue-specific and species-specific aggregation occur because of plasma
membrane recognition proteins.
Cell Recognition and Adhesion
There are two general ways that cell adhesion molecules work:
§ Homotypic binding occurs when both cells possess the same type of cell surface
receptor and their interaction causes them to stick together.
§ Heterotypic binding occurs between two different but complementary proteins
and resembles a plug and socket.
Cell Recognition and Adhesion
Specialized cell junctions form between cells in a tissue.
Animals have three types of cell junctions: tight junctions, ________ and gap
junctions.
Cell Recognition and Adhesion
Tight junctions are specialized structures at the plasma membrane that link
adjacent epithelial cells.
They have two primary functions:
§ To restrict the migration of membrane proteins and phospholipids from one
region of the cell to another
§ To prevent substances from moving through the intercellular space
Cell Recognition and Adhesion
Desmosomes act like spot welds on adjacent cells, holding them together.
Desmosomes have dense plaques that are attached both to cytoplasmic fibers and
to membrane cell adhesion proteins.
The membrane cell adhesion proteins bind to the proteins of an adjacent cell.
The cytoplasmic fibers are intermediate filaments of the cytoskeleton.
Cell Recognition and Adhesion
Gap junctions are connections that facilitate communication between cells.
Gap junctions are made up of specialized protein channels called connexons.
Connexons span the plasma membranes of two adjacent cells and protrude from
them slightly.
Connexons are made of proteins called connexins, which snap together to
generate a pore.
Passive Processes of Membrane Transport
Biological membranes are ________ permeable. They allow some substances to
pass, while others are restricted.
Some substances can move by simple diffusion through the phospholipid bilayer.
Some must travel through proteins to get in, but the driving force is still
diffusion. This process is called facilitated diffusion.
Passive Processes of Membrane Transport
Diffusion is the process of random movement toward the state of equilibrium.
Although individual particles move randomly, in diffusion the ________
movement is directional, from regions of greater concentrations to regions of
lesser concentrations, until equilibrium is reached.
Passive Processes of Membrane Transport
With all diffusion, it is the concentration and not the total number of
molecules that determines the net direction of movement, because the probability
of molecules moving from one point to another depends on how many molecules
there are per unit area.
Passive Processes of Membrane Transport
Diffusion over large distances is very slow.
In a solution, diffusion rates are determined by ________ size of the
molecule, electrical charge of the molecule, and concentration gradient.
The insertion of a biological membrane affects the movement of chemicals in
solution according to the membrane's properties. It may be ________ to some
molecules and ________ to others.
Passive Processes of Membrane Transport
Small molecules can move across the lipid bilayer by simple diffusion.
The more lipid-soluble the molecule, the more rapidly it diffuses.
An exception to this is water, which can pass through the lipid bilayer more
readily than its lipid solubility would predict.
Polar and charged molecules such as amino acids, sugars, and ions do not pass
readily across the lipid bilayer.
Passive Processes of Membrane Transport
Osmosis is the diffusion of ________ across membranes.
Osmosis is a completely passive process and requires no metabolic energy.
Water will diffuse from a region of its higher concentration (low
concentration of solutes) to a region of its lower concentration (higher
concentration of solutes).
Passive Processes of Membrane Transport
Isotonic solutions have equal solute concentrations.
A hypertonic solution has a greater total solute concentration than the
solution to which it is being compared.
A hypotonic solution has a lower total solute concentration than the solution
to which it is compared.
Passive Processes of Membrane Transport
Polar and charged substances do not ________ across lipid bilayers.
One way for these important raw materials to enter cells is through the
process of facilitated diffusion.
Facilitated diffusion depends on two type of membrane proteins: channel
proteins and carrier proteins.
Passive Processes of Membrane Transport
Channel proteins are integral membrane proteins that form channels lined with
polar amino acids.
Nonpolar (hydrophobic) amino acids face the outside of the channel, toward the
fatty acid tails of the lipid molecules.
Passive Processes of Membrane Transport
The best-studied protein channels are the ion channels.
Ion channels can be open or closed (i.e., they are "gated").
Ion channels are specific for one type of ion.
Specificity results from a tight fit between the ion and the funnel-shaped
stem of the channel, where oxygen atoms are located.
Passive Processes of Membrane Transport
Facilitated diffusion using ________ proteins involves not just opening a
channel but also binding the transported substance.
Carrier proteins allow diffusion in both directions.
The concentration gradient can be kept by metabolizing the transported
substance once it enters the cell.
If the limited number of carrier protein molecules are loaded with solute
molecules, the carrier proteins are said to be saturated.
Active Transport
In contrast to diffusion, active transport requires the expenditure of energy.
Ions or molecules are moved across the membrane against the concentration
gradient.
ATP is the energy currency used either directly or indirectly to achieve
________ transport.
Active Transport
Three different protein-driven systems are involved in active transport:
§ Uniport transporters move a single type of solute, such as calcium ions, in
one direction.
§ Symport transporters move two solutes in the same direction.
§ Antiport transporters move two solutes in opposite directions, one into the
cell, and the other out of the cell.
Active Transport
If ATP is used directly for the pumping system, as in the sodiumpotassium
pump, the system is a primary active transport system.
Only cations, such as sodium, potassium, and calcium, are transported directly
by pumps that use a primary active transport system.
Active Transport
Secondary active transport systems use established gradients to move
substances.
This form of transport uses ATP indirectly. The ATP molecules are consumed to
establish the ________ gradient.
The gradient is then used to move a substance, as described for the symport
and antiport systems.
An example is the symport system found in intestinal cells, which moves
glucose up its concentration gradient, while moving sodium ions down its ion
concentration gradient.
Endocytosis and Exocytosis
The group of processes called endocytosis brings macromolecules, large
particles, small molecules, and even other cells into the eukaryotic cell.
There are three types of endocytosis: phagocytosis, pinocytosis, and
receptor-mediated endocytosis.
In all three, the plasma membrane ________ toward the cell interior while
surrounding the materials on the outside.
Endocytosis and Exocytosis
During phagocytosis, which involves the largest vesicles, entire cells can be
engulfed.
Phagocytosis is common among unicellular protists.
White blood cells in humans and other animals also use phagocytosis to defend
the body against invading foreign cells.
Endocytosis and Exocytosis
Pinocytosis, which means "cellular drinking," involves vesicle formation as
well, but the vesicles are far smaller.
Dissolved substances and fluids are brought into the cell.
In humans, the single layer of cells separating blood capillaries from
surrounding tissue uses pinocytotic vesicles to acquire fluids from the blood.
Endocytosis and Exocytosis
Receptor-mediated endocytosis is similar to pinocytosis, but it is highly
specific.
Receptor proteins are exposed on the outside of the cell in regions called
coated pits. Clathrin molecules form the "coat" of the pits.
Coated vesicles form with the macromolecules trapped inside.
Endocytosis and Exocytosis
Exocytosis is the process by which materials packaged in vesicles are secreted
from the cell.
The vesicle membranes fuse with the plasma membrane and release vesicle
contents (wastes, enzymes, hormones, etc.) into the environment.
Membranes Are Not Simply Barriers
Membranes have many functions, including:
§ ________ processing
§ Energy transformation
q The inner mitochondrial membrane helps convert the energy of fuel molecules to
the energy in ATP.
q The thylakoid membranes of chloroplasts are involved in the conversion of
light energy in photosynthesis.
Membranes are involved in organizing chemical reactions, allowing them to
proceed rapidly and efficiently.
Membranes Are Dynamic
Membranes actively participate in numerous cellular processes.
Membranes continually form, move, and fuse.
Eukaryotic cells form their membranes through a series of activities.
Within cells, segments of membrane move about, change their structures, and
fuse with other membranes.
Each organelle modifies its membranes to carry out specific functions.
Membranes Are Dynamic
Despite the similar appearance and interconvertibility of membranes, they show
major chemical differences depending on their location in the cell and the
functions they serve.
Dynamic in both structure and activity, ________ are central to life.
Animation 5.1 Passive Transport
Animation 5.2 Active Transport
Animation 5.3 Receptor-Mediated Endocytosis
Video 5.1 The spreading of mouse epithelial cells onto a slide
Video 5.2 Migration of an epithelial cell
Video 5.3 Pinocytosis and membrane ruffling in a mouse epithelial cell
Video 5.4 Exocytosis of coccoliths in a marine golden alga, Pleurochrysis