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Homeostasis maintenance of a relatively stable internal environment in the face of changes in external or internal conditions. Maintained through negative feedback mechanisms
Stimulus-Response Model part 1 The changes in internal or external environments serve as stimuli (the change) and is detected by receptors.
Stimulus-Response Model part 2 The intensity of the stimulus must be sufficient to reach the threshold of the receptor, which is the weakest stimulus to which a receptor can respond. Receptors then stimulate effectors to produce a response
Stimulus A change in internal or external conditions that elicits a response from a cell. Stimuli are varied in nature; changes in pressure, light, temperature or chemical molecules are all examples of stimuli
Receptor specialised structure that can detect a specific stimulus and initiate a response
Signal transduction Signal is received by a specific receptor at the effector cell surface, and within the cell, is converted via a signal transduction pathway into effector molecules that produce the response
Effector cell cell that responds to a stimulus
Response physical, biochemical or physiological changes brought about by effector proteins in the cell
Negative feedback systems stimulus-response mechanisms that act to restore the original state. The response produced reduces the effect of the original state, that is, the response provides feedback that has a negative (opposite direction) effect on the stimulus
Signalling molecules Not all cells are capable of detecting or responding to all stimuli. Cells able to detect stimuli can pass this info to other cells by producing and releasing signalling molecules. Effector cells respond to signalling molecules
Animal hormones A hormone is a signaling molecule produced in small amounts that can relatively long-lasting effects on target cells. This endocrine system is made up of many glands and organs within the body that synthesise and secrete hormones into the bloodstream
Steroid hormones Class of hydrophobic signalling molecules from cholesterol Hydrophobic and lipophilic Transported in blood with aid from carrier proteins Diffuse across PM Bind to intracellular receptors Directly regulate gene expression Long lasting response
Peptide and protein hormones a class of hydrophilic signalling molecules composed of shorter polypeptide chains (peptides)
Amino-acid-derived hormones Class of hydrophilic signalling molecules derived from amino acids tyrosine and tryptophan Hydrophilic and lipophobic Transported in solution in blood plasma Unable to cross PM Indirectly acts on genes Shorter response period
Plant hormones produced in low concentrations but have a significant effect on plant development and growth. Each plant cell is able to produce many different types of hormone. Plant hormones also have a variety of modes of transmission
Pheromones signalling molecules that are excreted externally of an organism. They influence the behaviour or physiology of another individual, commonly triggering alarm and aggressive responses, marking territory, marking food trails and attracting mates in animals
Neurotransmitters group of hydrophilic signalling molecules secreted by neurons in the nervous system. When a neuron is stimulated, gated sodium and potassium ion channels on its membrane are opened which initiates the action potential
Cytokines group of hydrophilic signalling molecules that are involved in communication between immune cells. Cytokines bind to specific receptors on the surface of target cells, where they trigger a variety of cellular responses, depending on the receptor
Animal hormones - source glands and organs
Animal hormones - mode of transmission * autocrine * paracrine - through interstitial fluid between cells * endocrine - via the blood circulation
Animal hormones - hydrophilic or hydrophobic * hydrophobic steroid hormones * hydrophobic peptide hormones * hydrophobic or hydrophilic amino-acid-derived hormones
Plant hormones - source most plant cells are capable of producing a variety of plant hormones
Plant hormones - mode of transmission * various, including diffusion
Plant hormones - hydrophilic or hydrophobic * hydrophobic phytohormones * hydrophilic phytohormones
Neurotransmitters - source neurons
Neurotransmitters - mode of transmission * paracrine - through exocytosis into the synaptic gap and diffusion across the gap
Neurotransmitters - hydrophilic or hydrophobic hydrophilic
Cytokines- source many cells, including immune cells such as macrophages, B- and T- lymphocytes and mast cells
Cytokines- mode of transmission * autocrine * paracrine * endocrine
Cytokines- hydrophilic or hydrophobic hydrophilic
Pheromones - source various cells depending of the species
Pheromones - mode of transmission * communication between organisms by diffusion outside the organism
Pheromones - hydrophilic or hydrophobic * hydrophobic pheromones * hydrophilic pheromones
Autocrine signalling Signalling molecules act on the actual cell or the same type of cell that secreted them
Paracrine signalling Signalling molecules act on cells that are close to the secreting cell. For example, neurotransmitters are secreted by a neuron and target the next neuron in the neural pathway or a neighbouring effector cell.
Endocrine signalling Signalling molecules act on cells that are far from the cell that secretes them
signal transduction The processes involved in a cell detecting and responding to a signaling molecule are together known as signal transduction. Signal transduction can be considered in terms of a stimulus–response model.
Step 1 reception detection of the signalling molecule by a receptor. Each type of signalling molecule is designated for certain cells (target cells). Receptors are specific and will only bind to a particular signalling molecule
Step 2 transduction the relay of the signal into the cell. Involves converting the signal into a form that can be relayed to reach its final destination within the cell and bring about a cellular response. May be a one-step process or two being a cascade of caspases
Step 3 cellular response the activation of a cellular activity. Following transduction, a response is initiated. Cellular responses include any cellular activity such as gene transcription, the activation of enzymes or the secretion of signalling molecules by the cell
A signalling molecule can initiate different responses in different cell types due to: Different receptors for the same signalling molecule exist in different cells, which may activate different transduction cascades. Different secondary messages or proteins can be specific to cells and will lead to a response only in cells they're in
Signal transduction of hydrophilic signalling molecules water soluble. Unable to diffuse through plasma membranes. interact with receptors on the external surface of the plasma membrane (transmembrane proteins) & is activated by the binding of molecules on the surface of the cell to induce cellular responses
Signal transduction of hydrophobic signalling molecules lipid-based molecules. Easily diffuse through the plasma membrane. Inside the target cell they bind to an intracellular receptor. The hormone-receptor protein complex created moves from the cytoplasm into the nucleus and activates certain genes
Apoptosis regulated and programmed cell death Enables a multicellular organism to regulate the number of cells in the body Highly regulated process Receptor-mediated response triggered by signalling molecules
Steps in apoptosis 1-5 1 separation from adjacent cells 2 collapse of the cell’s cytoskeleton 3 cellshrinkage 4 breakdown of organelles and nucleus 5 blebbing of the plasma membrane
Steps in apoptosis 6-7 6 budding of plasma-membrane-bound vesicles called apoptotic bodies which prevent toxic or immunogenic substances from leaking when the apoptotic bodies are phagocytosed. 7 phagocytosis of the apoptotic bodies by specialised cells
Intrinsic or mitochondrial pathway Signalled from within the cell Depends on factors released from the mitochondria Damage to the cell stimulates the transcription and translation of genes into proteins (signalling molecules) that activate the mitochondrial pathway of apoptosis
Extrinsic or death receptor pathway All cells have transmembrane proteins on their plasma membrane called death receptors Signalling molecules binds to the death receptor and signal transduction in the cell initiates a cascade of caspase activation that leads to apoptosis
Malfunctions in apoptosis - too little apoptosis example 1 Mutations present in cancer cells enable these cells to grow in an uncontrolled fashion, crowding out normal healthy cells, and forming tumours This situation arises because the cancer cells do not respond to the normal apoptotic signal to self-destruct
Malfunctions in apoptosis - too little apoptosis example 2 Autoimmune diseases appear to involve disruption of apoptosis For example, rheumatoid arthritis is characterised by an excessive proliferation of cells in the synovial tissue of the joints that leads to joint destruction.
Malfunctions in apoptosis - too much apoptosis example 1 Neurodegenerative disorders such as Parkinson’s, abnormal progressive loss of nerve cells occurs in particular regions of the brain, associated with unregulated activation of caspases and apoptosis that produces excessive cell death
Malfunctions in apoptosis - too much apoptosis example 2 In untreated acquired immune deficiency syndrome (AIDS), excessive unregulated apoptosis leads to the loss of a particular group of immune cells, known as T-helper cells
Created by: emmawalton05
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