Biol 2305
Cell Membrane, Transport, and Communication
Membranes and Cell Transport
• All cells are surrounded by a plasma membrane.
• Cell membranes are composed of a lipid bilayer with globular proteins embedded in the
bilayer.
• On the external surface, carbohydrate groups join with lipids to form glycolipids, and with
proteins to form glycoproteins. These function as cell identity markers.
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Fluid Mosaic Model
In 1972, S. Singer and G. Nicolson proposed the Fluid Mosaic Model of membrane structure
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Phospholipid Bilayer
Mainly 2 layers of phospholipids; the non-polar tails point inward and the polar heads are on
the surface..
• Is fluid, allowing proteins to move around within the bilayer.
• Phospholipids
o In phospholipids, two of the –OH groups on glycerol are joined to fatty acids. The
third –OH joins to a phosphate group which joins, in turn, to another polar group of
atoms.
o The phosphate and polar groups are hydrophilic (polar head) while the hydrocarbon
chains of the 2 fatty acids are hydrophobic (nonpolar tails).
Membrane Components
• Membrane carbohydrates
o Interact with the surface molecules of other cells, facilitating cell-cell recognition
o Cell-cell recognition is a cell’s ability to distinguish one type of neighboring cell from
another
• Steroid Cholesterol
o Wedged between phospholipid molecules in the plasma membrane of animal cells.
o At warm temperatures (such as 37°C), cholesterol restrains the movement of
phospholipids and reduces fluidity.
o At cool temperatures, it maintains fluidity by preventing tight packing.
o Thus, cholesterol acts as a “temperature buffer” for the membrane, resisting changes
in membrane fluidity as temperature changes.
• Membrane Proteins
o A membrane is a collage of different proteins embedded in the fluid matrix of the
lipid bilayer
o Peripheral proteins are appendages loosely bound to the surface of the membrane
o Integral proteins penetrate the hydrophobic core of the lipid bilayer
o Many are transmembrane proteins, completely spanning the membrane
Functions of Cell Membranes
• Regulate the passage of substance into and out of cells and between cell organelles and
cytosol
• Detect chemical messengers arriving at the surface
• Link adjacent cells together by membrane junctions
• Anchor cells to the extracellular matrix
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Major Functions Of Membrane Proteins
Transport - A protein that spans the membrane may provide a hydrophilic channel across the
membrane that is selective for a particular solute. (right) Other transport proteins shuttle a
substance from one side to the other by changing shape. Some of these proteins hydrolyze
ATP as an energy source to actively pump substances across the membrane
Enzymatic activity - A protein built into the membrane may be an enzyme with its active site
exposed to substances in the adjacent solution. In some cases, several enzymes in a
membrane are organized as a team that carries out sequential steps of a metabolic pathway.
Signal transduction - A membrane protein may have a binding site with a specific shape that
fits the shape of a chemical messenger, such as a hormone. The external messenger (signal)
may cause a conformational change in the protein (receptor) that relays the message to the
inside of the cell.
Cell-cell recognition - Some glycoproteins serve as identification tags that are specifically
recognized by other cells.
Cell Junctions - Long-lasting or permanent connections between adjacent cells, 3 types of
cell junctions:
o Tight Junctions - Connect cells into sheets. Because these junctions form a tight
seal between cells, in order to cross the sheet, substances must pass through the cells,
they cannot pass between the cells.
o Anchoring Junctions - Attach the cytoskeleton of a cell to the matrix surrounding
the cell, or to the cytoskeleton of an adjacent cell.
o Communicating (Gap) Junctions - Link the cytoplasms of 2 cells together,
permitting the controlled passage of small molecules or ions between them.
Membrane Transport
• The plasma membrane is the boundary that separates the living cell from its nonliving
surroundings
• In order to survive, A cell must exchange materials with its surroundings, a process
controlled by the plasma membrane
• Materials must enter and leave the cell through the plasma membrane.
• Membrane structure results in selective permeability, it allows some substances to cross it
more easily than others
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Passive Transport - Passive transport is diffusion of a substance across a membrane with no
energy investment
o Simple diffusion
o Dialysis
o Osmosis
o Facilitated diffusion
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Diffusion
The net movement of a substance from an area of higher concentration to an area of lower
concentration; down a concentration gradient
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o Caused by the constant random motion of all atoms and molecules
o Movement of individual atoms & molecules is random, but each substance moves
down its own concentration gradient.
Solutions and Transport
• Solution – homogeneous mixture of two or more components
o Solvent – dissolving medium
o Solutes – components in smaller quantities within a solution
• Intracellular fluid – nucleoplasm and cytosol
• Extracellular fluid
o Interstitial fluid – fluid on the exterior of the cell within tissues
o Plasma – fluid component of blood
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Diffusion Across a Membrane
o The membrane has pores large enough for the molecules to pass through.
o Random movement of the molecules will cause some to pass through the pores; this
will happen more often on the side with more molecules. The dye diffuses from
where it is more concentrated to where it is less concentrated
o This leads to a dynamic equilibrium: The solute molecules continue to cross the
membrane, but at equal rates in both directions.
o Two different solutes are separated by a membrane that is permeable to both
o Each solute diffuses down its own concentration gradient.
o There will be a net diffusion of the purple molecules toward the left, even though the
total solute concentration was initially greater on the left side
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The Permeability of the Lipid Bilayer
o Permeability Factors
o Lipid solubility
o Size
o Charge
o Presence of channels and transporters
o Hydrophobic molecules are lipid soluble and can pass through the membrane rapidly
o Polar molecules do not cross the membrane rapidly
o Transport proteins allow passage of hydrophilic substances across the membrane
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Passive Transport Processes
Osmosis - Diffusion of the solvent across a semipermeable membrane. In living systems the
solvent is always water, so biologists generally define osmosis as the diffusion of water
across a semipermeable membrane:
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Osmotic Pressure - Osmotic pressure of a solution is the pressure needed to keep it in
equilibrium with pure H20. The higher the [solutes] in a solution, the higher its osmotic
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pressure. Tonicity is the ability of a solution to cause a cell to gain or lose water – based on
the concentration of solutes
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Fofi’s Definition of Osmosis
Osmosis is the diffusion of water across a semipermeable membrane from a hypotonic
solution to a hypertonic solution
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Tonicity
o If 2 solutions have equal [solutes], they are called isotonic
o If one has a higher [solute], and lower [solvent], is hypertonic
o The one with a lower [solute], and higher [solvent], is hypotonic
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Facilitated Diffusion - Diffusion of solutes through a semipermeable membrane with the
help of special transport proteins i.e. large polar molecules and ions that cannot pass through
phospholipid bilayer. Two types of transport proteins can help ions and large polar molecules
diffuse through cell membranes:
o Channel proteins – provide a narrow channel for the substance to pass through.
o Carrier proteins – physically bind to the substance on one side of membrane and
release it on the other.
o Charateristics:
 Specific – each channel or carrier transports certain ions or molecules only
 Passive – direction of net movement is always down the concentration
gradient
 Saturates – once all transport proteins are in use, rate of diffusion cannot be
increased further
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Active Transport
o Uses energy (from ATP) to move a substance against its natural tendency e.g. up a
concentration gradient.
o Requires the use of carrier proteins (transport proteins that physically bind to the
substance being transported).
 2 types:
 Membrane pump (protein-mediated active transport) A carrier protein uses
energy from ATP to move a substance across a membrane, up its
concentration gradient
 Coupled transport (cotransport). 2 stages:
 Carrier protein uses ATP to move a substance across the membrane
against its concentration gradient. Storing energy.
 Coupled transport protein allows the substance to move down its
concentration gradient using the stored energy to move a second
substance up its concentration gradient
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Sodium-potassium Pump – a key pump in this course
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Bulk Transport
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Allows small particles, or groups of molecules to enter or leave a cell without actually
passing through the membrane.
2 mechanisms of bulk transport: endocytosis and exocytosis.
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Endocytosis
The plasma membrane envelops small particles or fluid, then seals on itself to form a
vesicle or vacuole which enters the cell:
Phagocytosis: The substance engulfed is a solid particle
Pinocytosis: The substance engulfed is a liquid
Exocytosis
The reverse of endocytosis
During this process, the membrane of a vesicle fuses with the plasma membrane and
its contents are released outside the cell:
Cells Communication
• Direct contact
• Cells touch each other and signal molecules travel through special connections called
communicating junctions
• Communicating Junctions link the cytoplasms of 2 cells together, permitting the controlled
passage of small molecules or ions between them.
• Paracrine signaling
• Endocrine signaling
• Synaptic signaling
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Local and long-distance cell communication in animals
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Cell Signaling
The cells of a organism communicate with each other by releasing signal molecules that bind
to receptor proteins located either on or inside of target cells.
Three stages of cell signaling:
• Reception - each target cell has receptors that detect a specific signal molecule and binds to it
o A signal molecule binds to a receptor protein, causing it to change shape
o The binding between signal molecule (ligand) and receptor is highly specific
o A conformational change in a receptor
o Often the initial transduction of the signal
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Transduction – binding of the signal molecule changes the receptor protein in some way that
initiates transduction or conversion of the signal to a form that can bring about a specific
cellular response
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Response – transduced signal triggers a specific cellular response, any cell activity
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Receptors
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o Cell surface receptors - Signal molecules that cannot pass through the plasma
membrane bind to receptors located on the surface of the membrane
o Intracellular receptors - Some signal molecules that are small or hydrophobic can
pass through the plasma membrane and bind to receptors located inside the cell
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Intracellular Receptors
o Gene Regulators - Signal molecule joins to the receptor, the receptor changes shape
and a DNA binding site is exposed. The DNA binding site joins to a specific segment
of DNA and activates (or suppresses) a particular gene
o Enzyme Receptor - These receptors function as enzymes – proteins that catalyze
(speed up) specific chemical reactions. When a signal molecule joins to the receptor,
the receptor’s catalytic domain is activated (or deactivated).
Signal Pathways: Membrane Receptors
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Chemically Gated Ion Channels - Open or close when the signal molecule binds to the
channel.
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Enzymatic Receptors - Embedded in the plasma membrane, with their catalytic site exposed
inside the cell. Catalytic site activated when the signal molecule joins to the receptor.
Function as protein kinases (enzymes that phosphorylate proteins.)
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G-protein-linked Receptors - Signal molecule joins to a receptor, the receptor activates a G
protein. The activated G protein can then activate an ion channel or enzyme in the plasma
membrane.
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Second Messengers - Some enzymatic receptors and most G-protein-linked receptors relay
their message into the cell by activating other molecules or ions inside the cell. These
molecules and ions, called second messengers, transmit the message within the cell. The 2
most common second messengers are cAMP and Ca++
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cAMP Second Messenger
o G-protein-signaling pathway
o Signal molecule binds to surface receptor
o Surface receptor activates a G protein
o G protein activates the membrane-bound enzyme, adenylyl cyclase
o Adenylyl cyclase catalyzes synthesis of cAMP, which binds to a target protein
o Target protein initiates cellular change
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Cyclic AMP - cAMP recycling takes place in the mitochondria as a part of ATP synthesis
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Signal Pathways: Signal Amplification
Transducers convert extracellular signals into intracellular messages which create a response
Signal Amplification - Stimulation of glycogen breakdown in a liver cell by epinephrine
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Calcium and IP3 in signaling pathways
o Signal molecule binds to surface receptor
o Surface receptor activates a G protein
o G protein activates the membrane-bound enzyme, phospholipase C
o Phospholipase C catalyzes synthesis of inositol triphosphate (IP3), which stimulates
release of Ca++ from ER
o Released Ca++ initiates cellular change
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