Unit 4: Cell Communication and Signaling

Explain juxtacrine signaling

establishes the communication between cells that are in direct contact

Give examples of juxtacrine signaling

• Immune cells: direct contact occurs during binding of immune cells, such as T helper cells and T killer cells, to specific antigens displayed on the surface of antigen presenting cells 

  • Antigen presenting cells can be phagocytes that have engulfed pathogens or infected body cells 

  • This binding activatesthe immune cells and changes their activities, such as causing them to differentiate into memory cells

Explain paracrine signaling

occurs between cells that are a short distance apart, using ligands, known as local regulators, that travel only within the immediate area. Local regulators diffuse through spaces between cells and only affect cells in immediate vicinity of source

Explain process of nerve impulse

neurotransmitters are released at the synapses of neurons and diffuse across tiny gaps between neurones, binding to receptors on the new cell and enable the continued transmission of a nerve impulse

Explain quorum sensing

bacteria release signaling molecules into their environment, where the concentration of these chemicals allow individual bacteria to determine the population density and change their activities accordingly

What are morphogens?

signaling molecules that provide information to cells during early development of an individual

Explaining signaling in spinal cord

Spinal cord in an embryo releases a ligand known as Shh, which diffuses to nearby cells, causing the concentration of Shh to decrease with distance from its source, determining the development of different neurone types

Explain plant immune response.

pathogens release chemicals that are detected by receptors on plant cell surface, triggering a cascade response within the cell, and signaling molecules are also passed to neighboring cells, warning them of infection

Explain distant signaling.

occurs between cells of a long distance apart, communicating using ligands, known as hormones, that can travel a long way from their source

Explain how insulin reproduces signals

Insulin is a hormone released by cells of pancreas in response to increased blood glucose levels, travelling in the blood to the liver, where it binds to receptors on the surface of liver cells in order to increase uptake of glucose from blood

Explain the different hormonal communication

• Human growth hormone released by pituitary gland in the brain and regulates growth and metabolism 

• Thyroid hormones are released by the thyroid gland in the neck and regulates metabolism, body temperature, bone health, and digestion 

• Testosterone hormone is released by testes or ovaries, and regulates development of male secondary sexual characteristics and bone and muscle health 

• Estrogen is released by the testes or ovaries, and regulates the development of female secondary sexual characteristics, their menstrual cycle, and bone health

Explain signal transduction pathway

sequence of events that links the receipt of a cell signal, provided by a chemical signaling molecule (ligand), with a cellular response 

• Ligands bind to receptors on or in specific target cells

• The cellular response might involve changes to cell processes, such as changes in gene expression 

  • Protein modification

  • Phosphorylation cascades

Explain reception in signal transduction pathway

• Cells receive a signal when a ligand binds to the ligand-binding domain of a receptor proteins, which are specific to each type of ligand 

  • No specific receptors that can bind to the ligand causes the ligand to have no effect on the cell 

  • Receptors can be located on the cell surface membrane or within the cytosol of target cells 

    • Hydrophilic ligands cannot cross cell surface membrane, so they function by binding to extracellular membrane receptors which then relay the signal across the membrane 

    • Hydrophobic ligands can pass between the phospholipids of the cell surface membrane and bind directly to intracellular receptors 

  • Binding of a ligand triggers a conformational change in receptor, triggering transduction 

    • Binding of a ligand to extracellular receptors triggers a conformational change in the intracellular domain of the receptor 

    • Binding of ligands to ligand-gated ion channels can cause channel proteins to open or close, altering the passage of substances across the cell membrane, resulting in a chemical change inside the cell 

Explain transduction in signal transduction pathway

• Series of chemical changes that occur inside a cell which result in an eventual cellular response 

  • Activation of kinase enzymes, which transfer phosphate groups to other molecules in order to activate them 

  • Activation of phosphate enzymes, which remove phosphate groups from other molecules to activate them 

  • Production of secondary messenger molecules, which relay and amplify the intracellular signal after the binding of an extracellular ligand

  • Chemical changes form part of signaling cascades, which are the sequences of chemical reactions where one change triggers another

    • Signaling cascades amplify signals, resulting in significant cellular response

Explain responses that can occur in signal transduction pathways

• Regulation of gene expression through control of transcription or translation 

• Changes in metabolic activity 

• Secretion of molecules (enzymes or hormones) 

• Cell death/growth

Explain quorum sensing in bacteria

• Bacteria constantly releases chemical signals into environment, where individual bacteria can detect levels of chemical signals to use as measure of population density to adjust cellular processes on the basis of population density 

  • May increase expression of chemicals that make them more effective competitors at high population density

Explain epinephrine and glycogen’s role in signal transduction

• Epinephrine is a hormone (AKA adrenaline) that can act to increase blood glucose concentration, binding to receptors on the membrane of liver cells, activating enzyme adenylyl cyclase by changing shape of enzyme

• Active adenylyl cyclase catalyses conversion of ATP to secondary messenger cAMP, which binds to and activates protein kinase A enzymes, that phosphorylate phosphorylase kinase enzymes, that activate glycogen phosphorylase enzymes, that catalyse the breakdown of glycogen to glucose

Explain how mutations affect signal transduction pathways

Mutations in genes that code for ligand-binding domain of receptor proteins may result in receptors to which ligands can no longer bind

Explain how mutations in genes that code for adenylyl cyclase affect production of cyclase AMP

Mutations in genes that code for adenylyl cyclase may prevent the formation of seondary messenger cAMP, preventing signals from extracellular receptors reaching the inside of cell

Explain how cytosines affect signal transduction pathways

Cytokines regulate gene expression to allow for cell replication and division, such as during coordination of the immune response

Explain how ethylene affect signal transduction pathways

Ethylene (ethene) gas is a plant hormone that binds to receptors on the surface of fruit cells, initiating a signaling cascade that results in altered expression of genes that regulate fruit ripening

Explain how hox genes affect signal transduction pathways

Hox genes code for proteins which regulate transcription during development of embryo, ensuring different body parts develop in the right part of an organism, regulating expression of genes involved with cell division, differentiation, and apoptosis

Explain how chemicals in the environment can affect signal transduction pathways

chemicals interacting with a component of signaling pathway can activate or inhabit pathway

Explain how cholera toxin affects signal transduction pathways

Cholera toxin modifies the G protein so that it stays permanently active, disrupting signaling cascade that regulates chloride ion channels in cell membranes of intestinal cells, resulting in severe diarrhoea

Explain how lead affects signal transduction pathways

Lead is a metal that can imitate the action of calcium ions in signal transduction pathways, affecting cellular processes

Explain how pheromones in yeast cells affects signal transduction pathways

Yeast cells release chemicals called pheromones, which bind to G protein-coupled receptors, causing a signaling cascade that leads to transcription of mating-specific genes

What is homeostasis?

maintenance of constant internal conditions within suitable limits

What are feedback mechanisms?

mechanisms by which organisms detect and respond to change, involving receiving information about environment and initiating an internal mechanism to allow an appropriate response to the change 

What are negative feedback mechanisms?

• allow for organisms to maintain a constant internal environment, achieved by reducing initial stimulus to regulate physiological processes 

  • When any deviation from normal levels occur, negative feedback mechanisms act to return systems to a target set point, achieved by reversing the direction of change, operating at molecular, cellular and whole organism levels 

Explain how blood sugar regulation works

• Blood sugar is regulated by the hormones insulin and glucagon

• A rise in blood glucose causes insulin to be released by pancreas, where insulin binds to receptors on target cells resulting in an increase in the uptake of glucose from the blood, conversion of glucose into glycogen, and an increased glucose metabolism 

• A fall in blood glucose causes glucagon to be released by pancreas, where glucagon binds to receptors on target cells, resulting in conversio of glycogen to glucose, and conversion of other molecules into glucose

What are positive feedback mechanisms?

• amplify change in living organisms, such as when a change occurs during which levels move away from a set point, initiating further change, resulting in levels moving further from set point and stimulus being intensified 

Explain how fruit ripening is affected by positive feedback mechanisms

Ethylene is a gas produced by fruit during later stages of ripening, where the gas can diffuse from one fruit to neighbouring fruits, triggering further release of ethylene (ripening of one fruit causes ripening of others)

Explain how childbirth is affected by positive feedback mechanisms

• The hormone oxytocin is released by the pituitary gland in the brain, stimulating contractions of muscles in the uterus, which the stretch receptors in the cervix detect and signal the pituitary gland to increase oxytocin secretion 

• More oxytocin creates further contractions, which in turn signal for further release of oxytocin in positive feedback loop, increase contractions and results in childbirth

Explain how lactation in mammals is affected by positive feedback mechanisms

When a young mammal feeds from a mammary gland, the suckling action results in production of prolactin, which is a hormone, leading to milk production, resulting in increased suckling action

What are the phases of the cell cycle?

Interphase (G1, S, G2), Mitosis (Prophase, Metaphase, Anaphase, Telophase), Cytokinesis, or G0

Explain interphase within the cell cycle

• cell grows and carries out normal cellular functions 

  • G1: cell is metabollically active, duplicating organelles and cytosolic components 

  • S: DNA is in the form of chromatin and replicates to form two sister chromatids connected at a centromere 

  • G2: protein synthesis occurs, ATP is produced in large quantities and centrosomes replicate

Explain mitosis within the cell cycle

• nuclear division, ensuring the transfer of a complete genome from a parent cell to the new daughter cells 

  • Produces two, genetically identical daughter cells, essential for growth, tissue repair, and asexual reproduction

  • Prophase: nuclear membrane breaks down, the sister chromatids condense, spindle fibers (protein fibres that separate genetic material) behind to form, and centrosomes move to opposite poles of the cell 

  • Metaphase: spindle fibers attach to centromere (region of chromosome connecting two daughter cells) of each chromosome, and chromosomes line up along equator of the cell 

  • Anaphase: spindle fibers shorten, pulling sister chromatids to opposite poles of cell, sister chromatids are separated and become chromosomes in their own right 

  • Telophase: the mitotic spindle breaks down, new nuclear membranes form around separated groups of chromosomes 

Explain cytokinesis

• the cytoplasm and organelles are divided between new daughter cells 

  • In animal cells, this process involves formation of cleavage furrow (the pinching-in of an animal cell’s surface that physically divides cytoplasm) 

  • Rigid cell walls mean that plant cells cannot divide in this way, so a cell plate develops to separate the two daughter cells

What is G0 and what type of cells enter the phase?

Non-dividing cells enter phase G₀, during which cell division does not occur, which can occur at any stage of the cycle, but can re-enter cell cycle in response to chemical signals

What are cell-cycle checkpoints?

• ensure that cells are large enough, DNA has been replicated correctly, and necessary nutrients are available

  • Checkpoints can be controlled by interactions between cyclins and cyclin-dependent kinases 

    • Cyclins are proteins, which levels change throughout the cell cycle, binding to cyclin-dependent kinases to produce complexes that trigger the next stage of cell cycle 

    • Cyclin-dependent kinases are enzymes present at same concentration throughout the cell cycle, remaining inactive until they bind to cyclins, phosphorylating other molecules to activate them 

How does cell-cycle disruption lead to cancer?

• Cancer can develop due to uncontrolled cell division 

  • Mutations in protooncogenes can cause them to become oncogenes, resulting in increased expression of proteins that stimulate cell division 

  • Mutations in tumour suppressor genes can result in decreased expression of proteins that detect errors in DNA replication, leading to cell division in cells that contain DNA errors

How can cell-cycle disruption lead to apoptosis?

• Destruction of cells that contain dangerous mutations

• Removal of cells no longer needed during development 

  • In embryos, the fingers of the hands are joined together by webs of skin, but this skin is eventually removed by apoptosis