How cells communicate
- Cells live in environments with numerous signals.
- Cels take up materials from the environments across plasma membranes.
Transport across membranes
- Two factors determine whether materials can cross the plasma membrane.
- Size: Small molecules can cross the plasma membrane faster than larger molecules.
- Attraction to water: hydrophobic molecules can easily cross the plasma membrane compared to hydrophilic molecules.
Crossing the border
- Molecules that cross the plasma membrane diffuse across the plasma membrane until they reach equilibrium.
- Diffusion can occur with or without involving transport proteins.
- Diffusion is divided into two categories:
- Simple diffusion: diffusion across membranes without help from transport protein.
- Facilitated diffusion occurs when transport proteins facilitate diffusion of molecules across membranes. This can further be divided into active or passive transport
- Channel proteins are built like straws of soda.
- Channel proteins inserted in membranes form water-filled channels through the membranes. Channel proteins that remain all the time are called gated channels.
- Carrier proteins: they transport certain molecules across the membranes.
- Each carrier protein has a certain binding site specific for the molecule to be transported.
Going with the flow
- Osmosis is the process by which water molecules move from a highly concentrated region to a region of low concentration.
- Osmosis is a passive process that does not require energy input from the cell.
- Membrane relative concentration is divided into 3 terms:
- Hypertonic solutions have a greater solute concentration.
- Hypotonic solutions; have a lower solute concentration.
- Isotonic solutions; have an equal solute concentration.
It is an uphill battle
- Cells use active transport to move molecules against their concentration gradient.
- Active transport needs energy input from the cell which is available in the form of adenosine triphosphate (ATP).
- Carrier proteins that perform active transport are called pumps.
- One common active transport protein is the sodium-potassium pump.
Cell-cell interactions
- Animal cells are connected and to the extracellular matrix around them.
- The cells are connected depending on their functions.
- The types of cellular connections include:
- Tight junctions which bring cells tightly together. They provide strength and structure to the tissues. Proteins pass through both cell membranes making them appear like they are sown together. Tight junctions are important in mucous membranes and skin tissues.
- Anchoring junctions hold cells tightly together but allow movement of materials through intracellular space.
- There are three anchoring junction proteins:
- Desmosomes
- Hemidesmosomes
- Adherens junctions.
Sending and receiving signals
- Signals can be produced in one cell and initiate a response in another cell.
- Signals that travel to target cells over long distances are called hormones.
- To respond to signals, cells receive signals through receptors.
- There are two types of receptors:
- Cell surface receptors
- Intracellular receptors
Signal transduction
- Signal transduction is the process by which ligands bind to receptors on the cell surface and cause desired changes within the cell.
- Signal transduction steps include the following:
- The ligand recognizes and binds to its receptor.
- The receptor changes shape and becomes ready to cause intracellular changes.
- Signal amplification is activated by second messengers when the receptor induces intracellular changes.
- The second messengers enable a cell response leading changes in cellular behavior.
Enzyme-linked receptors
- These are receptors on the cell surface that have an intracellular domain linked to an enzyme to catalyze intracellular changes.
- The commonly studied enzyme-linked receptor is a tyrosine kinase.
- Signal transduction steps for tyrosine kinase include the following:
- The ligand binds to the tyrosine receptor.
- The receptor undergoes phosphorylation and activates the intracellular enzyme.
- The receptor then activates the membrane-associated protein called Ras.
- Ras begins the phosphorylation cascade.
G proteins
- This is a signaling pathway that involves the activation of G proteins that are associated with GTP.
- G protein signal transduction steps include the following:
- The receptor binds the primary messenger.
- The G protein on the receptor is activated.
- A section of the G protein travels across the membrane and associates with an enzyme.
- The enzyme stimulates the generation of second messenger molecules.
Deactivating the signal
- Several molecules downregulate signaling pathways;
- Phosphatases are enzymes that cleave phosphates from molecules. This hence inactivates the molecules.
- Activated Ras and G proteins. They convert GTP to GDP hence causing inactivation.
- Short-lived second messengers. These messengers are degraded faster hence deactivating the signaling pathway.
Revision
Adaptor Proteins
proteins which are accessory to main proteins in a signal transduction pathway.
Amplification
(electronics) the act of increasing voltage or power or current
Autophosphorylation
the phosphorylation of a kinase protein catalyzed by its own enzymatic activity
cAMP
a cyclic form of adenosine monophosphate (adenylic acid) that plays a major role in controlling many enzyme-catalyzed processes in living cells
Calcium
It plays an important role in signal transduction pathways, where it acts as a second messenger, in neurotransmitter release from neurons, contraction of all muscle cell types, and fertilization
Calmodulin
a protein that binds calcium and is involved in regulating a variety of activities in cells.
Cell Surface Receptors
Membrane or Transmembrane receptors are specialized integral membrane proteins that take part in communication between the cell and the outside world.
Dephosphorylation
the essential process of removing phosphate groups from an organic compound (as ATP) by hydrolysis
Diacylglycerol (DAG)
a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages.
Direct Contact Signaling
short distances, fastest, gap junctions
Effector
an organ (a gland or muscle) that becomes active in response to nerve impulses
Endocrine Signaling
Signaling in which endocrine cells release hormones that act on distant target cells; long distance, slowest
Enzymatic Receptor
a transmembrane receptor, where the binding of an extracellular ligand causes enzymatic reaction
Enzymes
any of several complex proteins that are produced by cells and act as catalysts in specific biochemical reactions
G protein-coupled receptors
comprise a large protein family of transmembrane receptors that sense molecules outside the cell
Gap Junctions
a specialized intercellular connection between a multitude of animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells
Gated Ion Channels
A gated channel for a specific ion. When ion channels are opened or closed, the membrane potential of the cell is altered.
Gene Regulators
The modulation of any of the stages of gene expression, hence, it encompasses the various systems that control and determine which genes are switched on and off, and when, how long, and to what extent the genes are expressed.
Intracellular Receptors
receptors located inside the cell rather than on its cell membrane.
IP3
binds to and open ligand-gated calcium channels on the ER; secondary messenger
Kinase
an an enzyme that adds a phosphate and activates a reaction (phosphorylation)
Ligand
an ion or molecule attached to a metal atom by coordinate bonding
Local Signaling
Growth Factors/Panacrine signaling and Synaptic signaling (not direct contact but also not a long distance like endocrine signaling)
Mitogen-Activated Protein Kinase
serine/threonine-specific protein kinases that respond to extracellular stimuli (mitogens, osmotic stress, heat shock and proinflammatory cytokines) and regulate various cellular activities, such as gene expression, mitosis, differentiation
Multi-step pathway
…
Nucleotide exchange
proteins that stimulate the exchange (replacement) of nucleoside diphosphates for nucleoside triphosphates bound to other proteins
Paracrine Signaling
a form of cell signaling in which the target cell is near (“para” = near) the signal-releasing cell.
Phosphatase
an enzyme that that removes a phosphate and deactivates (dephosporylation)
Phosphorylation
the addition of a phosphate (PO4) group to a protein or other organic molecule; activates or deactivates many protein enzymes
PIP2
a minor phospholipid component of cell membranes
Ras
the network in the reticular formation that serves an alerting or arousal function
Receptors
an organ having nerve endings (in the skin or viscera or eye or ear or nose or mouth) that respond to stimulation
Receptor Tyrosine Kinase (RTK)
the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones.
Response Protein
a cellular stress response related to the endoplasmic reticulum
Secondary Messengers
molecules that relay signals from receptors on the cell surface to target molecules inside the cell, in the cytoplasm or nucleus
Signal Molecule
a chemical involved in transmitting information between cells; molecules are released from the cell sending the signal, cross over the gap between cells, and interact with receptors in another cell, triggering a response in that cell
Signal Transduction
a mechanism that converts a mechanical/chemical stimulus to a cell into a specific cellular response
Synaptic Signaling
type of cell-cell communication that occurs across chemical synapses in the nervous system.
Target Proteins
functional biomolecules that are addressed and controlled by biologically active compounds
