AP BIO UNIT 4 Flashcards
Cell Communication
Cell-to-cell communication is critical for the function and survival of cells. It is responsible for the growth and development of multicellular organisms.
3 Ways Cells Communicate
Direct Contact
Local Signaling
Long-Distance Signaling
Direct Contact
Communication through cell junctions. Signaling substances and other materials dissolved in the cytoplasm can pass freely between adjacent cells.
Direct Contact in Animal Cells
Occurs through gap junctions
Direct Contact in Plant Cells
Occurs through plasmodesmata
Direct Contact in Immune Cells
Antigen-presenting cells (APCs) communicate to T-cells through direct contact.
Ligand
Chemical message
Local Regulators
A secreting cell will release chemical messages (local regulators/ligands) that travel a short distance through extracellular fluid. The chemical messages will cause a response in a target cell
2 Types of Local Regulators
Paracrine Signaling
Synaptic Signaling
Paracrine Signaling
Secretory cells release local regulators (ie. growth factors) via exocytosis to an adjacent cell
Synaptic Signaling
Occurs in animal nervous systems. Neurons secrete neurotransmitters. They diffuse across the synaptic cleft space between the nerve cell and the target cell.
Long-Distance Signaling
Animals and plants use hormones for long-distance signaling.
Long-Distance Signaling in Plants
Release hormones that travel in the plant vascular tissue (Xylem and phloem) or through the air to reach target tissues.
Long-Distance Signaling in Animals
Use endocrine signaling. Specialized cells release hormones into the circulatory system where they reach target cells.
Long-Distance Signaling Example - Insulin
Insulin is released by the pancreas into the bloodstream where it circulates through the body and binds to target cells.
What type of communication involves a cell secreting a substance to an adjacent target cell?
Local Regulators (Paracrine Signaling)
Plant cells in direct contact with each other can diffuse substances through these structures to communicate. What are they?
Plasmodesmata
Cell-to-Cell Signaling Overview
Reception: ligand binds to receptor
Transduction: signal is converted
Response: a cell’s response alters the cell process
Reception
The detection and receiving of a ligand by a receptor in the target cell.
Receptor
Macromolecule that binds to a signal molecule (ligand). All receptors have an area that interacts with the ligand and an area that transmits a signal to another protein. Binding between ligand and receptor is highly specific. When the ligand binds to the receptor, the receptor activates (via a conformational change). This allows the receptor to interact with other cellular molecules. Initiates transduction signal. Receptors can be in the plasma membrane or intracellular.
Plasma Membrane Receptors
Most common type of receptor involved in signal pathways. Bind to ligands that are polar, water-soluble, large.
Plasma Membrane Receptor Examples
G-protein coupled receptors (GPCRs)
Ligand-gated ion channels
Intracellular Receptors
Found in the cytoplasm of nucleus of target cells. Bind to ligands that can pass through the plasma membrane
Intracellular Receptors Examples
Hydrophobic molecules.
Steroid and thyroid hormones.
Gasses like nitric oxide.
Transduction
The conversion of an extracellular signal to an intracellular signal will bring about a cellular response. Requires a sequence of changes in a series of molecules known as a signal transduction pathway.
Signal Transduction Pathway
Regulates protein activity through phosphorylation and dephosphorylation. (Change in shape means change in function). During transduction, the signal is amplified.
Phosphorylation in Signal Transduction Pathway
Phosphorylation by the enzyme protein kinase relays signals inside the cell.
Dephosphorylation in Signal Transduction Pathway
Dephosphorylation by the enzyme protein phosphatase shuts off pathways.
Second Messengers
Small, nonpolar molecules and ions help relay the message and amplify the response. Cyclic AMP (cAMP) is a common second messenger.
Response
The final molecule in the signaling pathway converts the signal to a response that will alter a cellular process.
Response Examples
Protein that can alter membrane permeability. Enzymes that will change a metabolic process. Protein that turns genes on or off.
Signal Transduction Pathways can influence…
How a cell responds to its environment. They can result in changes in gene expression & cell function. Can alter phenotypes or result in cell death.
Changes in Signal Transduction Pathway
Mutations to receptor proteins or to any component of the signaling pathway will result in a change to the transduction of the signal.
In Eukaryotic Organisms, there are 2 main categories of cell membrane receptors
G protein-coupled receptors
Ion Channels
G Protein-Coupled Receptors
The largest category of cell surface receptors. Important in animal sensory systems. Binds to a G protein that can bind to GTP, which is an energy molecule similar to ATP. The GPCR, enzyme, and G protein are inactive until ligand binding to GPCR on the extracellular side. Ligand binding causes cytoplasmic side to change shape. Allows for G protein to bind to GPCR. Activates the GPCR and G protein. GDP becomes GTP. Part of the activated G protein can then bind to the enzyme which activates the enzyme and amplifies the signal if needed, leading to a cellular response.
Ion Channels
Ligand-gated ion channels. Located in the plasma membrane. Important in the nervous system. Receptors that act as a “gate” for ions. When a ligand binds to the receptor, the “gate” opens or closes allowing the diffusion of specific ions. Initiates a series of events that lead to a cellular response.
Homeostatis Overview
The bod must be able to monitor its internal conditions at all times.
Set Points
Values for physiological conditions that the body tries to maintain. This set points has a normal range for which it can fluctuate Example: body temperature (Set Point: 98.6 F, Set Range: 97-99 F)`
Homeostasis
The state of relatively stable internal conditions. Organisms can detect and respond to a stimulus (balance) The body maintains homeostasis through feedback loops.
Types of Feedback Loops
Positive
Negative
Stimulus
A variable that will cause a response
Receptor/Sensor
Sensory organs that detect a stimulus. This information is sent to the control center (brain).
Effector
Muscle or gland that will respond
Response
Changes (decreases or increases) the effect of the stimulus
Negative Feedback
The most common feedback mechanism. This type of feedback reduces the effect of the stimulus. Examples (sweat, blood sugar, breathing rate).
Positive Feedback
This type of feedback increases the effect of a stimulus. Examples (child labor, blood clotting, fruit ripening).
Homeostatic Imbalances
There are many reasons fo why the body may not be able to regulate homeostasis. Examples (genetic disorders, drug or alcohol abuse, and intolerable conditions (extreme heat/cold)).
Disease
When the body is unable to maintain homeostasis. Example (cancer: the body cannot regulate cell growth. Diabetes: the body cannot regulate blood sugar levels).
Cell Signaling as a Means of Homeostasis
In order to maintain homeostasis, the cells ina. multicellular organisms must be able to communicate, Communication occurs through signal transduction pathways.
Regulation of the Cell Cycle
Throughout the cell cycle, there are checkpoints. Control points that regulate the cell cycles. Cells receive stop/go signals.
G1 Checkpoint
Most important checkpoint. Checks for cell size, growth factors, and DNA damage. “Go”: cell completes the whole cell cycle. “Stop”: the cell enters a nondividing (quiescent) state known as G0 phase.
G0
Some cells stay in G0 forever (muscle/nerve cells). Some cells can be called back into the cell cycle.
G2 Checkpoint
Checks for completion of DNA replication and DNA damage. “Go”: Cell proceeds to mitosis. “Stop”: Cell cycle stops and the cell will attempt to repair damage. If the damage cannot be repaired, the cell will undergo apoptosis (cell death).
M (Spindle Checkpoint)
Checks for microtubule attachment to chromosomes at the kinetochores at metaphase. “Go”: Cell proceeds to anaphase and completes mitosis. “Stop”: cell will pause mitosis to allow for spindles to finish attaching to chromosomes.
Internal Cell Cycle Regulators
Regulation of the cell cycle involves an internal control system that consists of proteins known as cyclins and enzymes known as cyclin-dependent kinases (CDKs).
Cyclins
Concentration of cyclin varies. Cyclins are synthesized and degraded at specific stages of the cell cycle.
Cyclin-Dependent Kinases
Concentration remains constant through each phase of the cell cycle. Active only when its specific cyclin is present. Each cyclin-CDK complex has a specific regulator effect. Active CDK complexes phosphorylate target proteins, which help regulate key events in the cell cycle.
External Cell Cycle Regulators
Growth Factors
Contact (Density) Inhibition
Anchorage Dependence
Growth Factors
Hormones released by cells that stimulate cell growth. The signal transduction pathway is initiated. CDKs are activated leading to progression through the cell cycle.
Contact (Density) Inhibition
Cell surface receptors recognize contact with other cells. Initiates signal transduction pathway that stops the cell cycle in G1 phase.
Anchorage Dependence
Cells rely on attachment to other cells or the extracellular matrix to divide
Cancer
Normal cells become cancerous through DNA mutations. DNA mutations: changes in the DNA. Cancer cells on average have accumulated 60 or more mutations on genes that regulate cell growth.
Normal Cells
Follow checkpoints, divide on average 20-50 times in culture, go through apoptosis when there are significant errors.
Cancer Cells
Do not follow checkpoints. Divide infinitely when in culture (consider “immortal”), evade apoptosis, and continue dividing even with errors.
Tumor
A mass of tissues formed by abnormal cells
Benign Tumor
Cells are abnormal, but not considered to be cancerous (yet). Cells remain at only the tumor site and are unable to spread elsewhere in the body.
Malignant Tumor
Mass of cancerous cells that lose their anchorage dependency and can leave the tumor site.
Metasis
When cells separate from the tumor and spread elsewhere in the body.
