NN

AP Bio Unit 4 Review Flashcards

Cell Communication, Cell Signaling, and Signal Transduction

  • Cells communicate constantly, a basic feature of life.
  • Cells are in populations or multicellular organisms, not truly isolated.
  • Direct cell to cell communication:
    • Junctions between adjacent cells allow molecule passage.
    • Enables one cell to change another's behavior.
  • Communication via signals:
    • Cells secrete molecules into the bloodstream or extracellular fluid.
    • Target cells pick up these messages.
    • Signals are called ligands.
    • Two types of ligands:
      • Hormones: Long-distance signals traveling via the bloodstream from glands.
      • Local Regulators: Short-distance cell-to-cell communication.
  • Ligands are signaling molecules that bind to receptors based on complementary shape (like enzymes and substrates).
  • Binding leads to a cellular response.
  • Quorum Sensing:
    • Cell communication in biofilm formation in bacteria (e.g., plaque on teeth).
    • Bacteria release signaling molecules that bind to cytoplasmic receptors.
    • When the signal intensity exceeds a threshold (quorum), genes are activated.
    • Gene activation leads to biofilm production (polysaccharide).
    • Takeaway: All cells communicate, even bacteria, and brush your teeth.
  • Cell signaling phases:
    1. Reception: Ligand binds to receptor.
    2. Transduction: Initial message converted and amplified, often involving membrane proteins and second messengers.
    3. Response: Cellular change occurs due to activated enzymes or gene activation.
  • Reception Phase:
    • Signal molecule (ligand) binds with the receptor molecule.
    • Binding is based on complementary shape.
    • Receptor is embedded in the cell membrane's phospholipid bilayer.
  • Transduction and Response Phases:
    • Receptor interacts with membrane proteins to produce a second messenger.
    • Second messenger and relay molecules carry the message to the cytoplasm.
    • This activates enzymes or the nucleus, leading to gene activation.
  • Steroid (Nonpolar) Hormones vs. Water-Soluble Hormones:
    • Nonpolar hormones (e.g., estrogen, testosterone) diffuse through the phospholipid bilayer.
    • They bind with cytoplasmic receptors, forming a receptor-hormone complex.
    • The complex diffuses into the nucleus and acts as a transcription factor, activating genes which then make RNA, which ribosomes create proteins from.
    • Water-soluble hormones bind with receptors and interact with second messengers.
    • Steroid hormone responses are generally slower but longer-lasting, while water-soluble hormone responses are quicker.

Epinephrine and G Protein-Coupled Receptor Systems

  • Cellular communication works through three phases.

  • Fight or flight response context:

    • Adrenal glands produce epinephrine (adrenaline).
    • Epinephrine acts on the liver to produce glucose that goes into the blood.

  • Epinephrine (adrenaline) is a polar, water-soluble hormone that binds at the membrane.

    • Epinephrine's \, structure \, contains \, hydroxyl \, groups \, (polar)

  • Epinephrine's effects are widespread but tissue-specific.

  • Only tissues with receptors respond, response differs based on tissue types.

  • Adaptations during fight or flight:

    • Decreased digestion.
    • Increased heart rate.
    • Pupil dilation.
    • Conversion of glycogen to glucose.
    • Bronchial dilation.

  • Epinephrine interacts with liver cells causing them to hydrolyze stored glycogen into glucose monomers.

  • Glucose diffuses into the bloodstream, providing energy for fight or flight.

  • Off state (before epinephrine):

    • Receptor is unbound.
    • Nearby G protein is inactive, bound to GDP (low energy form).
    • Membrane-embedded enzyme adenylyl cyclase is also in the off state.

  • Epinephrine enters the system:

    • Epinephrine binds with a G protein-coupled receptor, causing an allosteric change.
    • Nearby G protein interacts with that part of the receptor, receptors change induces G protein to discharge GDP and bind with GTP (high energy form).
    • G protein is activated.

  • G protein bound to GTP:

    • Drifts in the membrane and binds with adenylyl cyclase, activating it.
    • Adenylyl cyclase converts ATP into cyclic AMP (cAMP), the second messenger.
      ATP \rightarrow cyclic \, AMP

  • Reception review:

    • Ligand (epinephrine) binds with G protein-coupled receptor.
    • Receptor changes shape on the cytoplasmic side.
    • G protein discharges GDP and binds with GTP, becoming activated.
    • G Protein activates adenylyl cyclase, which converts ATP into cyclic AMP.

  • Cellular response:

    • Cyclic AMP activates a chain of relay molecules called kinases (phosphorylation cascade).
    • Kinases are activated by phosphorylation, activating the next in the chain.
    • Signal amplification occurs.
    • One epinephrine molecule leads to millions of enzyme activations.
    • In liver cells, the terminal enzyme glycogen phosphorylase is activated which is responsible for converting glycogen into glucose.

  • System Shutdown (after threat):

    • Ligand diffuses away.
    • G protein drops the phosphate, binds to GDP, and becomes inactive.
    • Adenylyl cyclase stops creating cyclic AMP.
    • Kinase phosphorylation stops as protein phosphatases clip off phosphates.
    • Glycogen phosphorylase stops hydrolyzing glycogen, blood glucose normalizes.
      *L Liver cells return to their resting state.

Feedback and Homeostasis

  • Homeostasis: Tendency of a living system to maintain internal conditions at a relatively constant, optimal level.
  • Feedback: Output of a system is also an input.
    • Maintains homeostasis or accelerates internal changes.
    • Negative feedback maintains homeostasis.
    • Positive feedback accelerates change.
  • Set points: The value around which a homeostatic process fluctuates.
  • Negative Feedback:
    • Output of the system decreases the system's output.
    • Promotes homeostasis, returning a system to its set point.
    • Example: Home thermostat maintaining temperature at a set point.
  • Antagonistic Negative Feedback Loops:
    • Paired systems, one for cooling (air conditioner) and one for heating (furnace).
    • Maintain homeostasis by responding to conditions above or below the set point.
  • Blood Sugar Homeostasis:
    • Insulin is the main hormone.
    • Negative feedback system:
      • High blood glucose levels trigger insulin release from the pancreas.
      • Insulin binds at a receptor in the liver, activating a signaling cascade.
      • Glucose transporters open.
      • Glucose diffuses into liver cells and is converted into glycogen or fat.
      • Blood glucose levels decrease, restoring homeostasis.
  • Insulin and Glucagon:
    • Blood glucose set point is 90 \frac{mg}{100 \, mL}.
    • Above set point, pancreas releases insulin, glucose is absorbed and stored as glycogen in liver, fat, and muscle cells.
    • Below set point, pancreas releases glucagon, which induces the liver to convert glycogen into glucose.
  • Type 2 Diabetes:
    • Cells become insulin resistant.
    • Insulin binding does not lead to the signaling cascade.
    • Glucose channel remains closed.
    • Blood glucose level stays high, damaging organs and tissues.
  • Type 1 vs. Type 2 Diabetes:
    • Type 1 (juvenile): Autoimmune disorder where immune cells attack insulin-producing pancreas cells.
    • Type 2 (adult-onset): Insulin resistance where the receptor is insensitive to the insulin signal.
  • Positive Feedback:
    • Output of a system increases the system's activity and output.
    • Accelerates biological processes to a conclusion.
    • Childbirth:
      • Baby's growth activates uterine stretch receptors.
      • The brain releases oxytocin.
      • Oxytocin leads to more contraction.
      • Increased contraction leads to more oxytocin release until birth.
    • Fruit Ripening:
      • Ripening leads to the release of ethylene gas.
      • Ethylene induces additional ripening and more ethylene production.
      • Increased ethylene accelerates ripening until all fruit ripens.

The Cell Cycle

  • Mitosis duplicates chromosomes, transmitting a cell's entire genome to daughter cells.
  • Functions of Mitosis:
    • Growth and repair in multicellular organisms.
    • Reproduction in unicellular eukaryotes.
  • Cell Cycle Phases:
    • Interphase (G1, S, G2):
      • G1 (Growth Phase 1): Cell increases in size.
      • S (Synthesis): DNA replication/chromosome duplication.
      • G2 (Growth Phase 2): Growth of structures required for cell division.
    • M Phase (Mitosis): Separation of chromosomes, followed by cytokinesis.
    • Two daughter cells are created that are clones of the parent cell.
    • Phases of Mitosis:
      • Interphase: Cell grows and replicates DNA.
      • Prophase: Chromosomes condense, nuclear membrane disintegrates, spindle apparatus grows.
      • Metaphase: Spindle fibers align chromosomes at the cell equator, doubled chromosomes consist of two sister chromatids.
      • Anaphase: Sister chromatids are pulled apart and dragged to opposite ends of the cell, nonkinetochore microtubules elongate the cell.
      • Telophase: New nuclear membrane grows around each set of chromosomes, chromosomes spread out, nucleolus reappears.
      • Cytokinesis: Cell splits into two daughter cells.
  • G0 Phase:
    • Specialized cells (e.g., nerve or muscle cells) leave the cell cycle and enter G0.
    • In G0 cells typically do not divide.
    • Certain stimuli can induce cells in G0 to reenter the cell cycle.

Regulation of the Cell Cycle

  • Cell Cycle Checkpoints:
    • Moments when the cell checks internal conditions before progressing to the next phase.
    • If conditions are right, the cell cycle continues; if not, the cell moves into G0 or initiates apoptosis.
    • Primary checkpoints: G1, G2, and M checkpoints.
  • Apoptosis (Programmed Cell Death):
    • Signaling cascade involving mitochondria and the nucleus.
    • Highly regulated, unlike traumatic cell injury.
    • Cells are broken down into cytoplasmic fragments (blebs) consumed by immune cells.
  • Cyclins and Cyclin-Dependent Kinases (CDKs):
    • Internal regulators of the cell cycle.
    • Cyclins: Molecules whose concentration rises and falls throughout the cell cycle.
    • Cyclin-dependent kinases (CDKs): Kinases that respond to rising and falling cyclin levels.
  • Cyclin-CDK Interactions:
    • CDKs are present at a constant level throughout the cell cycle.
    • Cyclin levels rise and fall.
    • High cyclin levels cause cyclins to bind with CDKs to form a complex called maturation (or mitosis) promoting factor (MPF).
    • MPF allows the cell to pass through the G2 checkpoint and divide.
    • During M phase, cyclin is broken down, allowing the process to repeat.
    • Connection Between Cell Division and Cancer:
    • Cancer: Disease of unregulated cell division.
    • Cells become rogue players, pursuing their own destiny at the expense of the organism.
  • Genetic Mutations and Cancer:
    • Mutations in proto-oncogenes increase cell division by creating too many growth factors.
    • Mutations in tumor suppressor genes remove cell inhibitors/checkpoints.
    • Cancer occurs when cells have mutated tumor suppressors and overactive growth factors.
  • Mutated ROS Proto-Oncogene:
    • Normal ROS (G protein) is only active when an outside growth signal binds with ROS's coupled receptors.
    • Cancerous ROS (oncogene) is constitutively active, binding GTP even without a growth signal.
    • Leads to overproduction of growth factor and too much cell division.
    • Connected with about 30% of human cancers.
  • Mutated p53 Tumor Suppressor Gene:
    • Normal Function: DNA damage activates p53, which halts the cell cycle for DNA repair or initiates apoptosis if damage is too great.
    • Mutated p53: Cell continues to divide even with damaged DNA, increasing the chance of acquiring further mutations that can lead to cancer.