AP Biology Unit 4: Cell Communication, Feedback and Homeostasis, the Cell Cycle

plasmodesmata

• Gaps in the cell walls of adjacent cells that form an open channel through which cytoplasm can flow, allowing for the passage of molecules and ions from cell to cell “communicating”

Cell communication

• Cells detecting and responding to signals (ligands) via a three-stage pathway: reception, transduction, and response

• Cell communication occurs through:

  • Direct cell-to-cell contact via junctions, allowing molecules to pass between adjacent cells.

  • Ligands: signaling molecules that bind to receptors in bloodstream/extracellular fluid based on complementary shapes, leading to a cellular response

    • Classified into 2 types:

      • Hormones: Long-distance signals traveling through the bloodstream.

      • Local regulators: Short-distance signals for nearby cells

Quorum Sensing

• Cell communication in bacteria involves the release of signaling molecules that bind to cytoplasmic receptors

• When the bacterial population density is high (a quorum), these signals activate genes.

• Biofilm formation (which causes plaque on teeth)  is an example of bacterial quorum sensing.

Reception

• A ligand binds to a receptor protein, typically embedded in the cell membrane.

• Binding is based on complementary shape (as is the binding between enzyme and substrate, codon and anticodon, adenine and thymine, etc.)

Transduction

• The receptor interacts with membrane proteins to produce a second messenger.

• Relay molecules amplify the signal and transmit it to the cytoplasm or nucleus.

Response

• Activation of cytoplasmic enzymes or activation of gene expression in the nucleus.

Water-Soluble (Polar) Hormones

• Ligands that:

  • Bind to membrane receptors.

  • Rely on second messengers and relay molecules.

  • Responses are faster but typically short-lived.

Steroid (Non-Polar) Hormones:

• Diffuse through the phospholipid bilayer due to their non-polar nature and bind to cytoplasmic receptors to form a receptor-hormone complex.

• The complex diffuses into the nucleus, acts as a transcription factor, and activates genes, leading to slower responses but longer lasting.

Illustrative example: Epinephrine (Adrenaline) activation of liver cells through a G Protein-Coupled Receptor System

• Context: Epinephrine (adrenaline) plays a key role in the fight-or-flight response, affecting multiple tissues in the body, such as the liver, heart, and lungs.

• Main Response: In liver cells, epinephrine triggers the conversion of glycogen (a polysaccharide) into glucose (a monosaccharide), providing energy for the fight-or-flight response. Other effects of Epinephrine include

  • Increased heart rate.

  • Pupil dilation.

  • Decreased digestion.

  • Bronchial dilation for better oxygen intake.

Phases of Epinephrine Action in G Protein-Coupled Receptor Systems

1. Reception

   1. Epinephrine, a polar, water-soluble hormone, binds to a G protein-coupled receptor on the cell membrane causing a conformational change enabling interaction

2. Activation of the G Protein

   1. The receptor interacts with the dormant G protein, causing it to release GDP (low-energy similar to ATP) and bind with GTP (high energy molecule) that activates the G protein who binds to a membrane enzyme (adenylyl cyclase)

3. Signal Transduction

   1. Adenylyl cyclase converts ATP into cyclic AMP (cAMP), a second messenger that initiates a phosphoyrlation cascade where multiple kinases are activated = millions of enzymes activated

4. Cellular Response

   1. The enzyme glycogen phosphorylase is activated, breaking down glycogen into glucose that diffuses into the bloodstream

Homeostasis

• The tendency of living systems to maintain internal conditions at a relatively constant, optimal level.

• Examples:

  • Maintaining body temperature around 37°C (98.6°F).

  • Keeping blood glucose within a relatively narrow range (about 90 mg/dL (milligrams per deciliter).

Feedback Mechanisms:

• Systems where the output is also an input, influencing further activity.

• Two types:

  • Negative Feedback: Stabilizes a system by returning it to a set point.

  • Positive Feedback: Accelerates a process toward a conclusion.

Negative Feedback

• Regulatory systems that maintain homeostasis by reversing a deviation to return a system to its set point, returning the system to its target value by counteracting initial stimuli

Biological Examples of Negative Feedback

• Blood Glucose Regulation

  • Blood glucose is maintained by negative feedback using insulin and glucagon. High blood sugar triggers insulin release, causing cells to take in and store glucose, lowering levels. Low blood sugar triggers glucagon release, causing the liver to release glucose, raising levels back to homeostasis.

• Type 1 Diabetes

  • Type 1 diabetes is an autoimmune disorder where insulin-producing cells are destroyed, preventing glucose uptake by cells and causing high blood sugar. It is treated with insulin injections.

• Type 2 Diabetes

  • Type 2 diabetes occurs when cells become resistant to insulin, leading to high blood glucose levels and potential organ damage. It is managed with diet, exercise, and medication.

Positive Feedback

• The output of a system amplifies the system’s activity, driving a process to completion.

Biological Examples of Positive Feedback

• Oxytocin release during childbirth.

  • Growing baby activates stretch receptors in the cervix (the tip of the uterus), triggering oxytocin release.

  • Oxytocin causes stronger contractions, leading to more oxytocin release until the baby is born.

• Fruit Ripening:

  • Ethylene gas released by ripening fruit induces nearby fruit to release more ethylene.

  • Accelerates the ripening process for all fruit in proximity.

  • Controlled in fruit shipping by suppressing ethylene with carbon dioxide.

Mitosis

• The process of eukaryotic cell division when the nucleus divides, and the duplicated chromosomes are evenly distributed into two genetically identical daughter cells.

• It supports growth and development, tissue repair and replacement, and asexual reproduction in unicellular eukaryotes.

The Cell Cycle

• Divided into 2 main phases: interphase and M phase

Chromatin

• The relaxed form of DNA during interphase

Sister Chromatids

• Identical copies of a chromosome (before it’s pulled apart during mitosis). Sister chromatids are attached at a centromere.

Kinetochore

• A protein structure on sister chromatids where spindle fibers attach.

Interphase

• The cell prepares for division by growing and duplicating its DNA/chromosomes. Consists of:

  • G Phase: Growth phase 1, the cell increases in size

  • S Phase: Synthesis Phase, DNA replication occurs

  • G2 Phase: Growth phase 2, organelles and structures required for cell division are prepared

Prophase

• Chromatin condenses into visible chromosomes, consisting of two sister chromatids and the nuclear membrane disintegrates.

• Nucleolus (the site of ribosome assembly) disappears

• Spindle fibers begins to form from the centrosomes.

Metaphase

• Spindle fibers attach to chromosomes at their centromeres.

• Chromosomes are aligned at the cell’s equator (metaphase plate).

Anaphase

• Spindle fibers pull the sister chromatids apart, moving them to opposite poles.

• Non-kinetochore microtubules elongate the cell.

Telophase

• New nuclear membranes form around each set of chromosomes.

• Chromosomes relax into their interphase form (chromatin), and the nucleolus reappears.

Cytokinesis

• The cytoplasm divides, resulting in two genetically identical daughter cells.

  • In animal cells a ring of protein fibers called microfilaments pulls together the membrane between the two daughter nuclei, splitting the cell in half

  • In plant calls a new cell wall grows between the daughter nuclei.

G0 Phase

• A resting state where cells exit the cell cycle and stop dividing.

  • Example: Highly specialized cells like nerve and muscle cells enter G₀.

Checkpoints

• Regulate cell progression by checking internal conditions, with key checkpoints including: G1, G2, and M

• Regulation of the cell cycle is important because unregulated cell growth can lead to cancer

• If conditions are unfavorable, or if the cell’s fate is to develop into a highly specialized, non-dividing state, then the cell enters G₀

• Damaged cells can initiate (or be signaled to initiate) apoptosis (programmed cell death).

• Healthy cells in appropriate conditions will proceed through the cell cycle and divide.

Cyclins and Cyclin-Dependent Kinases (CDKs)

• Cyclins and CDKs control the timing of the cell cycle.

• Cyclins are proteins that increase and decrease in concentration during the cell cycle, while CDKs are enzymes that are always present but only become active when bound to cyclins.

• When cyclins bind to CDKs, they form Cyclin-CDK complexes that act like “go signals,” allowing the cell to pass checkpoints and move to the next phase

Cancer

• Caused by unregulated cell division due to genetic mutations

• Proto-oncogenes: Genes that promote cell division

• Tumor Suppressor Genes: Genes that prevent cell division

Specific Cancer Mutations that you should know: RAS Proto-Oncogene

• Normal RAS activates cell division only when growth signals are present.

• Mutated RAS (the oncogene) becomes constitutively active, promoting constant cell division without external signals (linked to ~30% of human cancers).

Specific Cancer Mutations that you should know: p53 Tumor Suppressor Gene

• Activated in response to DNA damage to either halt the cell cycle for repair or trigger apoptosis.

• Nonfunctional p53 leads to uncontrolled division of cells with damaged DNA, increasing cancer risk.