Lecture 13 - Cell Communication PDF

**Lecture 13 -- Cell Communication** **Endocrine Signal** -- An endocrine signal is a type of long-distance signaling in animals where signaling molecules, called hormones, are secreted by endocrine cells and travel through the circulatory system to reach target cells throughout the body. This allo...

**Lecture 13 -- Cell Communication** **Endocrine Signal** -- An endocrine signal is a type of long-distance signaling in animals where signaling molecules, called hormones, are secreted by endocrine cells and travel through the circulatory system to reach target cells throughout the body. This allows for the coordination of physiological processes across various tissues and organs. For instance, leptin, a hormone secreted by fat cells, travels through the bloodstream to the brain, where it regulates appetite and energy expenditure. **G-Protein Coupled Receptor** -- G-protein Coupled receptors (CPCRs) constitute a large family of cell surface receptors found in animals, with over 800 different GPCR genes identified in the human genome. These receptors are involved in various physiological processes, including vision, taste, smell, and the responses to neurotransmitters and peptide hormones. Structurally GPCRs are characterized by seven transmembrane domains, meaning they span the cell membrane seven times. GPCRs are coupled to G-proteins, intracellular proteins that bind to guanine nucleotides (GDP or GTP). When a ligand binds to a GPCR, it causes confrontational change in the receptor, activating the associated G-protein. The activated G-protein then interacts with effector proteins, triggering a cascade of intracellular events that lead to a cellular response. **Hormone** -- A hormone is a chemical messenger that is secreted by endocrine cells and travels long distances through the circulatory system to target cells in other parts of the body. Hormones play a crucial role in regulating various physiological processes, including growth, development, metabolism, and reproduction. Examples of hormones include leptin, which regulates appetite, and cortisol, which is involved in the stress response. **Ligand** -- A ligand is a signaling molecule that binds to a specific receptor protein, triggering a cellular response. Ligands can be diverse in nature, including small molecules, peptides, proteins, and even gases. The binding of a ligand to its receptor is highly specific, often involving a shape change in the receptor that initiates the transduction of the signal. For example, insulin, a peptide hormone, acts as a ligand that binds to its receptor on target cells, leading to glucose uptake and utilization. **Ligand-Gated Channel** -- A ligand-gated ion channel is a type of membrane receptor that acts as a gate for ions, allowing them to pass through the cell membrane. These channels are typically found in nerve and muscle cells, where they play a critical role in signal transmission. When a ligand, often a neurotransmitter, binds to the receptor, it causes a conformational change that opens the channel, allowing specific ions such as sodium (Na+) or calcium (Ca~2~+) to flow through. The movement of these ions alters the electrical potential across the membrane, triggering a cellular response, such as a muscle contraction. For instance, acetylcholine, a neurotransmitter, binds to acetylcholine-gated sodium channels at the neuromuscular junction, causing sodium influx into muscle cells and triggering muscle contraction. **Paracrine Signal** -- A paracrine signal is a type of local signaling where a signaling molecule is secreted by a cell and travels to nearby target cells. Paracrine signaling allows for communication between cells within a tissue or organ, coordinating local responses. **Receptor** -- A receptor is a protein molecule, often located on the cell surface or inside the cell, that binds to a specific signaling molecule called a ligand. The binding of a ligand to its receptor initiates a cellular response by triggering a series of intracellular events known as signal transduction. Receptors are highly specific for their ligands, ensuring that cells respond only to the appropriate signals. For example, GPCRs are a type of cell surface receptor that bind to a wide range of ligands, while nuclear receptors are intracellular receptors that bind to steroid hormones. **Signal Transduction** -- Signal transduction is the process by which a cell converts an extracellular signal into a cellular response. It involves a series of steps that relay and amplify the signal, often involving a cascade of protein-protein interactions and second messenger molecules. Signal transduction pathways can lead to various cellular responses such as changes in gene expression, enzyme activity, or cell behaviour. For example, the binding of a ligand to a GPCR initiates a signal transduction pathway that involves the activation of G-proteins, the production of second messengers, and the modulation of downstream effector proteins. **What are the main signaling types?** **Autocrine Signaling** -- In autocrine signaling, a cell secretes signaling molecules that bind to receptors on the same cell. This type of signaling allows a cell to regulate its own behaviour. **Paracrine Signaling** -- In paracrine signaling, a cell secretes signaling molecules that travel to nearby target cells. This type of signaling allows cells to communicate with each other over short distances. **Endocrine Signaling** -- In endocrine signaling, cells secrete signaling molecules called hormones that travel long distances through the circulatory system to target cells throughout the body. Endocrine signaling allows for the coordination of physiological processes across various tissues and organs. **What are the steps involved in sending and receiving cell signals?** **Signal Reception** -- The target cell detects a signaling molecule, known as ligand, which binds to a receptor protein on the cell surface or inside the cell. **Signal Transduction** -- The binding of the ligand to the receptor initiates a signal transduction pathway, which is a series of steps that relay and amplify the signal within the cell. This process often involves a cascade of protein-protein interactions and second messenger molecules. **Cell Response** -- The transduced signal ultimately triggers a specific response in the target cell. This response can include changes in gene expression, enzyme activity, or cell behaviour. **What are the main types of membrane receptors?** **G-protein Coupled Receptors** -- GPCRs are a large family of cell surface receptors that interact with intracellular proteins called G-proteins. When a ligand binds to a GPCR, it activates the associated G-protein, which then interacts with effector proteins, triggering a cellular response. **Receptor Tyrosine Kinases** -- RTKs are membrane receptors that attach phosphates to tyrosine amino acids on proteins in response to ligand binding. This phosphorylation event triggers a cascade of intracellular signalling events. **Ligand-gated Ion Channels** -- Ligand-gated ion channels are membrane receptors that act as gates for ions. When a ligand binds to the receptor, it causes a conformational change that opens the channel, allowing specific ions to flow through the membrane. This ion flow can trigger a variety of cellular responses. **What are some outcomes in response to receiving a signal?** **Changes in Gene Expression** -- Some signaling pathways lead to the activation of transcription factors, which are proteins that bind to DNA and regulate the expression of genes. For example, cortisol, a steroid hormone, binds to an intracellular receptor that acts as a transcription factor, turning on specific genes. **Enzyme Activation or Deactivation** -- Signalling pathways can also regulate the activity of enzymes which are proteins that catalyze chemical reactions. This regulation can lead to changes in metabolic processes or other cellular functions. **Changes in Cell Behaviour** -- Cell signaling can also influence cell behaviour, such as cell growth, division, differentiation or movement. **Lecture 14 -- Organism Development** **Angiosperm** -- Angiosperms are seed plants that produce flowers. Their seeds are enclosed in fruit. They have complex leaves and are divided into monocots and dicots. **Anther** -- The anther is a part of the stamen, the male reproductive organ in a flower. It contains pollen sacs where pollen develops. **Blastulation** -- Blastulation is the process of forming a blastocyst. In humans, a blastocyst is a structure that forms early in embryonic development. It consists of an inner cell mass, which will eventually develop into the embryo, and an outer layer of cells, called the trophoblast, which will contribute to the placenta. **Carpel** -- The carpel is the female reproductive organ in a flower. It is composed of the stigma (which receives pollen), the style (through which pollen migrates), and the ovary (which contains ovules). **Double Fertilization** -- Double fertilization is a unique process in angiosperms where both sperm cells delivered by the pollen tube are involved in fertilization. One sperm fertilizes the egg, forming a zygote, while the other sperm fuses with two polar nuclei to form the triploid endosperm. This process ensures that the endosperm, which provides nourishment to the developing embryo, only develops in ovules containing fertilized eggs. **Embryogenesis** -- Embryogenesis is the process by which a zygote develops into an embryo. In plants, embryogenesis begins with the asymmetrical division of the zygote within the embryo sac and results in the formation of a mature embryo and a suspensor that anchors the embryo. **Endosperm** -- The endosperm is a triploid (3n) tissue in angiosperms that serves as a food reserve for the developing embryo. It forms when a sperm cell fuses with two polar nuclei during double fertilization. **Flower** -- Flowers are the reproductive structures in angiosperms. They produce gametophytes and consist of four floral organs: carpels, stamens, petals, and sepals. Stamens and carpels are the reproductive organs, while petals and sepals are non-reproductive components. **Ovule** -- The ovule is a structure within the ovary of a flower that contains the female gametophyte (embryo sac) and develops into a seed after fertilization. **Pollen** -- Pollen grains contain the male gametophyte (microgametophyte) in angiosperms. They develop within the pollen sacs of anthers after meiosis, and each contain two sperm cells. **Pollination** -- Pollination is the transfer of pollen from the anther to the stigma of a flower. It can be facilitated by winder, water, insects or animals. **Stamen** -- The stamen is the male reproductive organ in a flower. It is composed of the anther (which produces pollen) and the filament. **Zygote** -- A zygote is a diploid (2n) cell that forms when a sperm cell fertilizes an egg cell. In both plants and animals, the zygote is the first cell of a new individual and undergoes embryogenesis to develop into an embryo. **What are the flower structures involved in angiosperm reproduction?** **Stamen** -- This male reproductive organ is composed of the anther, containing pollen sacs where pollen grains develop, and a filament that supports the anther. **Carpel** -- This female reproductive organ is made up of three parts: - **Stigma** -- Located at the top, receives the pollen grains. - **Style** -- Connects the stigma to the ovary and provides a pathway for pollen tube growth. - **Ovary** -- Houses the ovules, which contains the female gametophytes. **What are the steps in the process of double fertilization in plants?** Double fertilization is a unique process in angiosperms involving two sperm cells from a pollen grain participating in fertilization events within the ovule. **Pollen Tube Growth** -- After a pollen grain lands on the stigma, it germinates and produces a pollen tube that extends down the style toward the ovary, delivering two sperm cells. **Double Fertilization** -- Inside the ovule, the two sperm cells carry out separate fertilization events. One sperm cell fuses with the egg cell to form the diploid zygote, which will develop into the embryo. The other sperm cell fuses with the two polar nuclei within the embryo sac to form the triploid endosperm, a nutrient rich tissue that nourishes the developing embryo. **How do plant embryos develop?** Following double fertilization, the plant embryo develops a series of steps called embryogenesis. **Asymmetrical Zygote Division** -- The zygote undergoes an asymmetrical division within the embryo sac, initiating the formation of the embryo. **Embryo and Suspensor Development** -- Further cell divisions lead to the formation of the mature embryo and a structure called the suspensor, which anchors the embryo to the embryo sac and facilitates nutrient transfer from the parent plant. **Dormancy** -- The embryo continues to develop until it reaches a mature stage, at which point it enters a state of dormancy within the seed. **What are the initial steps involved in human embryo development?** Human embryo development, also known as embryogenesis, begins with the fertilization of the egg cell by a sperm cell, resulting in a zygote. The initial steps in this process are: **Cleavage** -- The zygote undergoes rapid miotic divisions, a process called cleavage, producing a cluster of cells called blastomeres. **Morula Formation** -- Around four days after fertilization, the blastomeres form a solid ball of cells called the morula. **Blastocyst Formation** -- By day five, the morula transforms into a blastocyst, characterized by an inner cell mass (which will develop into the embryo) and an outer layer of cells called the trophoblast (which contributes to the placenta). The process of forming the blastocyst is called blastulation. **Implantation** -- The blastocyst then becomes embedded in the endometrial lining of the uterus, marking the beginning of implantation. **Lecture 15 -- Organs (Digestion)** **Amylase** -- Amylase is an enzyme that breaks down carbohydrates. Salivary glands secrete salivary α-amylase, which initiates carbohydrate breakdown in the mouth. Pancreatic α-amylase is added to the duodenum, the first part of the small intestine. The acidic environment of the stomach inactivates α-amylase. The pancreas secretes sodium bicarbonate (NaHCO~3~) to neutralize the hydrochloric acid (HCl) in the stomach. Amylase on the surface of enterocytes, absorptive epithelial cells in the small intestine, continues carbohydrate digestion. **Enterocyte** -- An enterocyte is an absorptive epithelial cell found in the small intestine. Enterocytes have finger-like projections called villi and smaller cellular microvilli that increase the surface area for absorption. Enterocytes import sugars from the intestinal lumen using a sodium/glucose transporter (SGLT1) on their apical membrane. Glucose then travels through the enterocyte to the interstitial fluid (ISF), capillaries, hepatic portal vein, and finally to liver hepatocytes. **Gastrula** -- A gastrula is an embryo that has undergone gastrulation. It has three germ layers -- ectoderm, mesoderm, and endoderm. **Gastrulation** -- Gastrulation is a process in embryonic development that involves major cell movements, leading to the formation of a three-layered body plan. The three germ layers established during gastrulation are: - **Ectoderm --** This outer layer will form the nervous system, skin, pigment cells, and hair cells. - **Mesoderm --** This middle layer will form most body organs, including muscles, blood vessels, kidneys, the heart, and the skeleton. - **Endoderm --** This inner layer will form the respiratory and digestive tracts, as well as the liver and pancreas. **Glucagon** -- Glucagon is a hormone produced by the pancreas that regulates blood sugar levels. When blood glucose levels are low, glucagon stimulates the breakdown of glycogen in the liver, releasing glucose into the bloodstream. Glucagon binds to a G-protein coupled receptor (GPCR) on hepatocytes, activating a signaling pathway that stimulates glycogen breakdown. **Glycogen** -- Glycogen is a complex carbohydrate that serves as the primary storage form of glucose in animals. Glucose is stored as glycogen in the liver to maintain energy levels. Animals maintain a balance between glycogen synthesis and breakdown. When blood glucose is high, glycogen is synthesized in liver cells (hepatocytes). When blood glucose is low, glycogen is broken down, releasing glucose from hepatocytes. **Insulin** -- Insulin is a hormone produced by the pancreas that regulates blood sugar levels. After eating, when blood sugar is high, insulin is secreted from the pancreas. It stimulates glycogen synthesis in hepatocytes. Insulin binds to a receptor tyrosine kinase (RTK) on the surface of cells, triggering a signalling pathway that activates glycogen synthesis. **What are the basic germ layer structures of the gastrula?** The gastrula is an early embryonic stage characterized by the presence of three primary germ layers: the ectoderm, mesoderm and endoderm. These layers arise during gastrulation, a process involving significant cell rearrangements that establish the basic body plan. Each germ layer gives rise to specific tissues and organs later in development: **Ectoderm --** This outer layer will form the nervous system, skin, pigment cells, and hair cells. **Mesoderm --** This middle layer will form most body organs, including muscles, blood vessels, kidneys, the heart, and the skeleton. **Endoderm --** This inner layer will form the respiratory and digestive tracts, as well as the liver and pancreas. **Which organs are involved in breaking down carbohydrates in food?** **Salivary Glands** -- Initiate carbohydrate digestion in the mouth by secreting the salivary α-amylase that breaks down starch. **Pancreas** -- Secretes a pancreatic α-amylase in the duodenum (the first part of the small intestine) to continue carbohydrate breakdown. Produces sodium bicarbonate (NaHCO~3~) to neutralize the acidic chyme from the stomach, creating favorable pH for enzymatic activity. **Small Intestine** -- The brush order of the intestinal lining, formed by microvilli on enterocytes, contains enzymes like amylase, maltase, sucrase, and lactase that complete the digestion of carbohydrates into simple sugars. **How does your body absorb glucose?** Glucose absorption primarily occurs in the small intestine, specifically through the enterocytes lining the intestinal wall. The process involves: **Active Transport at the Apical Membrane** -- The sodium/glucose transporter (SGLT1), located on the apical membrane (facing the intestinal lumen), actively transports glucose into the enterocyte. This cotransporter utilises the sodium gradient established by the Na+/K+ pump to drive glucose uptake against its concentration gradient. **Facilitated Diffusion at the Basolateral Membrane** -- Once inside the enterocyte, glucose exits through the basolateral membrane (facing the interstitial fluid) via facilitated diffusion, mediated by the glucose transporter GLUT2. **Transport to the Liver** -- From the interstitial fluid, glucose diffuses into capillaries and is transported via the hepatic portal vein directly to the liver. **How does your body control blood glucose levels?** The body tightly regulates blood glucose levels through a complex interplay of hormones and metabolic pathways. Insulin and glucagon, both produced by the pancreas, play crucial roles in maintaining glucose homeostasis. - **Insulin** -- Secreted when blood glucose levels are high, typically after a meal. Promotes glucose uptake and storage in various tissues, particularly the liver and muscles. Activates glycogen synthase, an enzyme responsible for converting glucose int glycogen for storage in the liver. Insulin binds to a receptor tyrosine kinase (RTK) on target cells, initiating a signaling cascade that ultimately activates glycogen synthase. - **Glucagon** -- Released when blood glucose levels are low, such as between meals. Stimulates the breakdown of glycogen (glycogenesis) in the liver, releasing glucose into the bloodstream. Activates glycogen phosphorylase, the enzyme responsible for glycogen breakdown. Glucagon binds to a G-protein coupled receptor (GPCR) on target cells, activating a signaling pathway that stimulates glycogen phosphorylase. The balance between insulin and glucagon ensures that blood glucose levels remain within a narrow, healthy range, preventing dangerous fluctuations like hyperglycemia (high blood sugar) and hypoglycemia (low blood sugar).