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Describe how signals move between cells using gap junctions (animals) and plasmodesmata (plants)
Plant cells have cell walls, so the plasma membranes of neighbouring cells do not come into direct contact. Plasmodesmata are holes in the cell wall that allow cytosol to pass between plant cells, and allow water and small solutes to pass freely from cell to cell by passive diffusion.
Gap junctions, also known as communicating junctions, provide cytoplasmic channels between adjacent cells, similar to plasmodesmata in plant cells. Ions, water, and other small molecules can pass through these gap junctions.
Signals move between cells using gap junctions or plasmodesmata during juxtacrine signaling. Ligands are sent from the signaling cell to the target cell through gap junctions or plasmodesmata.
Describe the forms of signaling: paracrine, autocrine, endocrine, juxtacrine
Paracrine
Ligands diffuse through the space between cells that are near each other
Allows cells to coordinate activities with neighbouring cells
Autocrine
A cell releases a ligand that binds to its own surface (or to a receptor inside the cell)
Helps cells take on and reinforce their identities
Endocrine
Cells use the circulatory system to distribute ligands (hormones) to target cells in distant parts of the body
These hormones are made in endocrine glands
Juxtacrine
Ligands are sent through gap junctions (animals) or plasmodesmata (plants)
Can also be when cells bind to each other’s surface proteins
Allows cells to coordinate their response to a signal only one cell may have received
Describe quorum sensing
Quorum sensing is a process of cell to cell communication where autoinducers inform bacteria about the density of their population. When signaling by autoinducers (chemical signal molecules bacteria produce) reaches a threshold, all bacteria in the population will change their behaviour or gene expression at the same time.
What is a hormone? Compare steroid hormones and peptide hormones
Hormones are chemical messengers that travel through the bloodstream to different parts of the body, where they help control how cells and organs function.
Steroid Hormones
Steroid hormones are lipid-based, can pass through the plasma membrane and act on intracellular receptors in the cytoplasm or the nucleus, and need a transport protein to travel through the bloodstream as they are hydrophobic and insoluble in blood
Peptide Hormones
Peptide hormones are protein-based, can not pass through the plasma membrane and act on cell-surface receptors, and can travel through the bloodstream without a transport protein as they are hydrophilic and soluble in blood
Compare positive and negative feedback loops.
Feedback loops are a mechanism to maintain homeostasis.
Positive Feedback Loops
Positive feedback loops amplify or reinforce changes within a system
Positive feedback loops are helpful in maintaining homeostasis as some changes within a system need to be amplified for the system to reach homeostasis
Negative Feedback Loops
Negative feedback loops counteract or oppose changes within a system
A single negative feedback loop would not be helpful in maintaining homeostasis as many feedback loops need to work together for a system to reach homeostasis
Describe a ligand-gated ion channel
Ligand-gated ion channels are ion channel proteins that open or close in response to a ligand binding. A ligand will bind to a ligand-gated ion channel receptor, opening the gate to allow ions to enter the cell. Changes in ion levels inside the cell can change the activity of other molecules to produce a response
Describe a phosphorylation cascade
A phosphorylation cascade is a series of chemical reactions within a cell where one enzyme phosphorylates another by adding a phosphate group, usually from ATP. A phosphorylation cascade amplifies a signal, so a small initial stimulus triggers a large response. For example, in response to a signal like a growth factor hormone, a receptor activates a kinase, which phosphorylates another kinase, and so on, until the final proteins are activated to carry out a specific cell function, like gene expression or metabolism regulation.
Describe how cell signaling pathways can amplify a signal received by a single ligand
Many cell signaling pathways amplify the initial signal, so that one molecule of ligand can lead to the activation of many molecules of a downstream target. After reception and during transduction in cell signaling, a cell signaling pathway will amplify a signal using phosphorylation or second messengers, leading to the creation of a response.
What is a growth factor? How can problems with growth factors result in cancer?
A growth factor is a protein that stimulates cell growth, division, or survival by binding to specific receptors on a cell’s surface and activating signaling pathways.
Problems with growth factors, such as overproduction or mutations in their receptors, can lead to uncontrolled cell division. This can result in tumours or cancerous growths.
Describe the structure of a neuron, where are signals received? Where are they transmitted from?
A neuron has a cell body (soma), dendrites, and an axon.
Signals are received into the dendrites and cell bodies.
Signals are transmitted from the axon terminals.
Compare electrical and chemical synapses
Electrical Synapses
Electrical synapses use gap junctions to allow the electrical current or cations to flow directly from one cell to the next. They transmit signals very quickly, can carry the electrical current both ways, and allow groups of connected cells to carry out synchronized activities. Because electrical synapses do not have mechanisms to block or reduce activity, they are purely excitatory.
Chemical Synapses
Chemical synapses store neurotransmitters in vesicles, and these vesicles fuse with the axon terminal membrane of the pre-synaptic cell after an action potential opens voltage-gated Ca2+ channels, or after the pre-synaptic neuron is excited by the action potential. The neurotransmitters are released into the synaptic cleft, where they bind with the ligand-gated ion channel receptors on the post-synaptic cell, causing different types of ion channels to open. Some neurotransmitters can cause an excitatory response by allowing positive ions like Na+ to flow in, while other neurotransmitters will cause an inhibitory response by allowing negative ions like Cl- to flow in or positive ions like K+ to flow out.
Describe a neuron at rest, which ions are located within/outside the neuron, and the membrane potential
Neurons at rest are polarized, meaning the inside of the neuron is more negative than the outside. The resting membrane potential of a neuron is -70mV. There is a high concentration of K+ ions inside the neuron at rest and a high concentration of Na+ and Ca2+ ions outside the neuron at rest.
Describe an action potential, both in terms of a membrane potential graph and in terms of what is happening with the ions in and out of a neuron
An action potential is a rapid change in a neuron’s membrane potential that allows the neuron the transmit a signal. At the neuron’s resting state, the membrane potential is -70mV and Na+, K+, and Ca2+ channels are closed, the sodium-potassium pump is maintaining the gradient. Neurotransmitters bind to receptors on the dendrites of a neuron, which depolarizes the neuron, causing the neuron to reach its action potential threshold, -55mV. Once the threshold is reached, a large number of Na+ channels open, allowing Na+ ions to enter the neuron, causing the neuron to depolarize further and the membrane potential to be +30mV. At +30mV, Na+ channels close and K+ and Ca2+ channels open to allow K+ ions to leave the neuron and Ca2+ ions to enter the neuron. The Ca2+ ions fuse the vesicles with the neuron membrane, releasing neurotransmitters into the synaptic cleft. The K+ ions flow out of the neuron, repolarizing the neuron to a membrane potential of -90mV. When the membrane potential is -90mV, the K+ and Ca2+ channels close. The sodium potassium pump, leaky sodium channels, and leaky potassium channels will return the -90mV membrane potential to the resting membrane potential of -70mV.
Describe the epinephrine signaling pathway that results in glycogen breakdown
Epinephrine binds to a G-protein-coupled receptor, which undergoes a conformational change, activating the G protein. The activated G protein activates adenylyl cyclase, which catalyzes the conversion of ATP to cAMP. cAMP activates protein kinase A, which starts a phosphorylation cascade. Eventually glycogen phosphorylase is activated, which breaks down glycogen into glucose
Describe the roles of insulin and glucagon in maintaining blood sugar
Low blood glucose stimulates the release of glucagon, which increases rates of glycogen breakdown and glucose release by liver.
High blood glucose stimulates the release of insulin, which increases rates of glucose uptake by body cells, and synthesis of glycogen in the liver and skeletal muscles.
Describe the TRH/TSH/Thyroxine pathway involving the hypothalamus, anterior pituitary, and thyroid glands
Sensory neurons note that the thyroid hormone is below the normal range and sends nerve impulses to the hypothalamus. The hypothalamus releases TRH into the blood, which goes to the anterior pituitary gland. TRH causes the anterior pituitary gland to release TSH into the blood. TSH stimulates the thyroid gland to secrete thyroid hormones (T3 and T4) into the blood. Thyroid hormone levels increase and when levels return to normal, the thyroid hormone blocks TRH and TSH.
Describe the negative feedback loop involving ADH and water reabsorption
When blood osmolarity rises (too many solutes, not enough water) increased ADH is released into the bloodstream, the epithelium of the distal tubules and collecting ducts become more permeable to water, and water reabsorption increases (lower urine volume).
When blood osmolarity drops (not enough solutes, too much water), decreased ADH is released into the bloodstream, the epithelium of the distal tubules and collecting ducts become less permeable to water, and water reabsorption decreases (greater urine volume).
Describe the endocrine signaling pathway involving GnRH, LH, and FSH that results in ovulation
The hypothalamus secretes GnRH, which causes the pituitary gland to release FSH and LH. FSH stimulates a follicle to develop and begin producing estrogen. As the follicles mature, they release estrogen which stimulates the hypothalamus and pituitary gland to secrete hormones. LH induces the final maturation of the follicle and ovulation, where the egg cell is released from the ovary. When the egg is released from the ovary, the remaining cells of the follicle develop into the corpus luteum.
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