Cell Structure & Functions PDF

General Principles of Physiology Physiology by Dr Yusuff Dimeji Igbayilola Department of Human Physiology,, Baze University, Abuja.  Lecture -1 Cell-Structure & functions Introduction  Good day, everyone! Today, we’re going to ex...

General Principles of Physiology Physiology by Dr Yusuff Dimeji Igbayilola Department of Human Physiology,, Baze University, Abuja.  Lecture -1 Cell-Structure & functions Introduction  Good day, everyone! Today, we’re going to explore the fascinating world ofCell Physiology—the study of how cells function, interact, and maintain life.  Cells are the basic building blocks of all living organisms, and understanding their inner workings helps us grasp how the body performs everything from energy production to communication and repair.  In this session, we’ll dive into key processes like membrane transport, cell signaling, and energy metabolism, all of which play critical roles in keeping our bodies healthy and functioning. By the end, you'll have a deeper appreciation for the amazing complexity happening inside each cell! What is Cell? The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life.  Unicellular and multicellular Organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. Humans have about 100 trillion cells Discovery of cell The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in. Types of Cell There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are those cells which have nuclear material without nuclear membrane. For ex- bacteria and blue green algae. The cell having well- organized nucleus with a nuclear membrane are called eukaryotic Types of cell white blood cell Amoeba red blood cell muscle sper cell cheek m cells nerve Parameciu cell m Shape of cells Generally, cells are round, spherical or elongated some cells are long and pointed at both ends. They exhibit a spindle shape. Cells sometimes are quite long. Some are branched like nerve cells or a neuron. Some are sphere like RBC. Organelles of Cell Very small size Can only be observed under a microscope Have specific functions Found throughout cytoplasm Endoplasmic reticulum (rough & smooth) ; Golgi Bodies;Nucleolus; Lysosomes; Ribosomes Cytoplasm of a Cell Jelly-like substance enclosed by cell membrane Provides a medium for chemical reactions to take place Cytoplasm Cell nucleus The cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope. Nucleolus  Cell may have 1 to 3 nucleoli  Inside nucleus  Disappears when cell divides  Makes ribosomes that make proteins Mitochondria Mitochondria are present in eukaryotes only. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Endoplasmic reticulum The ER has two forms: the rough ER, which has ribosomes on its surface and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER plays a role in calcium sequestration and release. Endoplasmic reticulum The ribosome The ribosome is a large complex of RNA and protein molecules. They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesize proteins from amino acids. Ribosomes can be found Ribosomes Lysosomes and Peroxisomes Lysosomes and Peroxisomes are present in eukaryotes only. Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. Lysosomes and Peroxisomes Golgi Bodies  Stacks of flattened sacs  Have a shipping side & a receiving side  Receive & modify proteins made by ER  Transport vesicles with modified proteins pinch off the ends Transport vesicle Image of Cell Membrane   It is made up of a double layer of phospholipids that separates the cell from the outside world.  The cell membrane’s main mission is to serve as a barrier between the cell (which might also be a single-celled organism) and the world; so the cell needs to have a structure which allows it to interact with both.  A cell’s membrane is primarily made up of a double layer of phospholipids (fatlike, phosphorus-containing substances).  Each layer is composed of phospholipid molecules that contain a hydrophilic (water-loving) head and a hydrophobic (water-repellent) tail.  The heads in the outermost layer face and interact with the watery external environment, while the heads of those in the interior layer point inward and interact with the cell’s watery cytoplasm.  The region between the two layers is fluid repellent, which has the effect of separating the inside of the cell from the outside world. The cell membrane is semipermeable, which allows selected molecules to pass into or out of the cell. Cell Membrane  It contains proteins that provide a number of critical functions.  Since proper cell functioning depends on the movement of nutrients and useful materials into the cell and the removal of waste products from the cell, the cell membrane also contains proteins and other molecules that perform a wide variety of these duties.  Some proteins are attached to these mats of phospholipids to help move nutrients (such as oxygen and water) and wastes (such as carbon dioxide); some help the cell connect with and attach to the right kinds of materials (as well as other cells); and some proteins keep the cell from linking up with toxic materials as well as the wrong kinds of cells, foreign or otherwise.  Specialized proteins called enzymes help break down larger nutrients or help combine different nutrients with one another into more useable forms. Cell Membrane  Depending upon their design and function, protein molecules may be attached to the surface of one of the cell membrane’s layers or they may be fully embedded within the layer residing alongside the phospholipids.  Some proteins tasked with funneling nutrients into and out of the space between the cell membrane’s inner and outer layer cross only one of the phospholipid layers.  Others, which are designed to transport nutrients into the cell itself or funnel wastes away from the cell, are large enough to span both. There are also proteins that help the cell maintain its shape.   It contains carbohydrates that help to identify the cell and link the cell to others.  Carbohydrates, compounds of carbon, hydrogen, and oxygen (such as sugars, starches, and celluloses), are found along the surface of the outermost layer of the cell membrane.  Carbohydrates form glycolipids after linking with lipids, and glycoproteins after linking with proteins.  Depending upon their design, glycolipid and glycoprotein molecules may act as chemical markers or receptors that help identify the cell or assist in linking the cell to other cells.  Glycoproteins also bind with other proteins to make enzymes and other substances that, depending on the molecule’s purpose, could be involved in blood clotting, capturing foreign bacteria, protecting against diseases, and other activities.  Singer and Nicolson’s fluid mosaic model is often used to describe the cell membrane’s structure. It can be difficult to envision how the cell membrane functions. After all, the cell, cell membrane, and all the activities the cell engages in occur at levels too small for the naked eye to see. In 1972, two American scientists, S.J. Singer and G.L. Nicolson, developed the fluid mosaic model to describe the structure and functions of the cell membrane. The model notes that the membrane itself is fluid, in the sense that it is constantly changing. Individual phospholipids move about laterally (in the same layer); however, one or more lipids may flip to the other layer on occasion.  Lipids are drawn to one another through weak hydrophobic attractions, so while they do stick to one another, the bonds are routinely broken.  The membrane’s proteins also move about within this sea of lipids—as do cholesterols (which occur only in animal cells).  Cholesterols increase the membrane’s rigidity and firmness at moderate and higher temperatures by making the membrane less soluble.  At lower temperatures, however, cholesterols separate phospholipids from one another so that the membrane does not become too rigid.  The fluid mosaic model also describes how nutrients are transported into and out of the cell.  Nutrient and waste transport may be passive (that is, it does not require energy) or active (that is, energy is required) to move molecules across the cell membrane.  Passive transport can occur through diffusion, where molecules flow from a region of high concentration to a region of low concentration (down a concentration gradient).  If molecules diffuse through a semipermeable membrane, the process is called osmosis.  However, in cells, a type of assisted passive transport called facilitated diffusion works because of transport proteins, which create membrane-spanning portals for specific kinds of molecules and ions or attach to a specific molecule on one side of the membrane, carry it to the other side, and release it.  In contrast, active transport is fueled by a coenzyme called adenosine triphosphate (ATP)—which delivers chemical energy captured from the breakdown of food to other parts of the cell—to move molecules up a concentration gradient.  Among other things, active transport allows the cell to expel waste ions, such as sodium (Na+), from the cell even though the concentration of sodium ions outside the cell may be higher than the concentration inside.  It’s You! Thank You Cellular Transport Yusuff Dimeji Igbayilola, PhD Department of Human Physiology Baze University Abuja  Students will describe the structure and function of cells, tissues, organs, and organ systems. A. Explain that cells take in nutrients in order to grow and divide and to make needed materials. B. Relate cell structures (cell membrane, nucleus, cytoplasm, chloroplasts, mitochondria) to basic cell functions. C. Explain that cells are organized into tissues, tissues into organs, organs into systems, and systems into organisms. D. Explain that tissues, organs, and organ systems serve the needs cells have for oxygen, food, and waste removal.  Terms to Know Concentration – the amount of solute in a solution. Solute – the dissolved substance in a solution. Solution – a mixture in which two or more substances are mixed evenly. Concentration gradient - the gradual difference in the concentration of solutes in a solution between two regions.  Cell Membrane (Transport) Notes Cell Membrane and Cell Wall: ALL cells have a cell membrane made of proteins and lipids protein channel Layer Cell 1 Membran Layer e 2 lipid protein bilayer pump SOME cells have cell membranes and cell walls – ex: plants, fungi and bacteria Cell Membrane Cell Wall  cellulose – that cellulose is fiber in our diet Bacteria and fungi also have cell walls, but they do not contain cellulose Cell membranes and cell walls are porous allowing water, carbon dioxide, oxygen and Function of the Cell Membrane: Cell membrane separates the components of a cell from its environment—surrounds the cell “Gatekeeper” of the cell—regulates the flow of materials into and out of cell— selectively permeable Cell membrane helps cells maintain homeostasis—stable internal balance Types of Cellular Animations of Active Transport & Transport Passive Transport Weeee!  Passive Transport !! cell doesn’t use energy 1. Diffusion 2. Facilitated Diffusion high 3. Osmosis low  Active Transport cell does use energy 1. Protein Pumps This is gonna 2. Endocytosis be hard 3. Exocytosis work!! high low  Diffusion is the movement of small particles across a selectively permeable membrane like the cell membrane until equilibrium is reached. These particles move from an area of high concentration to an area of low concentration. outside of cell inside of cell  DIFFUSION IGH to LOW concentratio  Osmosis is the diffusion of water through a selectively permeable membrane like the cell membrane Water diffuses across a membrane from an area of high concentration to an area of low concentration. Semi-permeable membrane is permeable to water, but not to sugar  Facilitated Diffusion is the movement of larger molecules like glucose through the cell membrane – larger molecules must be “helped” Proteins in the cell membrane form channels for large molecules to pass through Proteins that form channels (pores) are called protein channels Glucose outside of cell molecules inside of cell Clic concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel. Hypotonic Solutions: contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode. Isotonic Solutions: contain the same concentration of solute as another solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into nteractive Red Blood Ce Clic Active Transport Active transport is the movement of molecules from LOW to HIGH concentration. Energy is required as molecules must be pumped against the concentration gradient. Proteins that work as pumps are called protein pumps. Ex: Body cells must pump carbon dioxide out into the surrounding blood vessels to be carried to the lungs for exhale. Blood vessels are high in carbon dioxide compared to the cells, so energy is required to move the carbon dioxide across the cell membrane from LOW tooutside HIGHofconcentration. cell Carbon Dioxide molecules inside of cell ANALOG Y: ENERGY NEEDED: Active Transport NO ENERGY NEEDED: Diffusion Osmosis Facilitated Diffusion  Endocytosis and Exocytosis is the mechanism by which very large molecules (such as food and wastes) get into and out of the cell Food is moved into the cell by Endocytosis Wastes are moved out of the cell by Exocytosis Ex: White Blood Cells, which are part of the immune system, surround and engulf bacteria by endocytosis.  Types of Active Transport Forces 3. Exocytosis: Endocytosis & Exocytosis material out of cell in animations bulk membrane surrounding the material fuses with cell membrane Cell changes shape – requires energy EX: Hormones or wastes released from cell Osmosis—Elodea Leaf Effects of Osmosis on Life  Osmosis- diffusion of water through a selectively permeable membrane  Water is so small and there is so much of it the cell can’t control it’s movement through the cell membrane.  Osmosis Animations for Hypotonic Solution isotonic, hypertonic, and hypotonic solutions Hypotonic: The solution has a lower concentration of solutes and a higher concentration of water than inside the cell. (Low solute; High water) Result: Water moves from the solution to inside the cell): Cell Swells and bursts open (cytolysis)!  Osmosis Animations for Hypertonic Solution isotonic, hypertonic, and hypotonic solutions Hypertonic: The solution has a higher concentration of solutes and a lower concentration of water than inside the cell. (High solute; Low water) shrink s Result: Water moves from inside the cell into the solution: Cell shrinks (Plasmolysis)!  Osmosis Animations for Isotonic Solution isotonic, hypertonic, and hypotonic solutions Isotonic: The concentration of solutes in the solution is equal to the concentration of solutes inside the cell. Result: Water moves equally in both directions and the cell remains same size! (Dynamic Equilibrium) What type of solution are these cells in? A B C Hypertoni Isotoni Hypotonic c c  How Organisms Deal  Paramecium (pr otist) removing excess water vi with Osmotic Pressure deo Bacteria and plants have cell walls that prevent them from over-expanding. In plants the pressure exerted on the cell wall is called tugor pressure. A protist like paramecium has contractile vacuoles that collect water flowing in and pump it out to prevent them from over-expanding. Salt water fish pump salt out of their specialized gills so they do not dehydrate. Animal cells are bathed in blood. Kidneys keep the blood isotonic by remove excess salt and water. Intercellular Junctions  The extracellular matrix allows cellular communication within tissues through conformational changes that induce chemical signals, which ultimately transform activities within the cell.  However, cells are also capable of communicating with each other via direct contact through intercellular junctions.  There are some differences in the ways that plant and animal cells communicate directly.  Plasmodesmata are junctions between plant cells, whereas animal cell contacts are carried out through tight junctions, gap junctions, and desmosomes. Intercellular Junctions Junctions in Plant Cells  In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell wall that surrounds each cell.  How then can a plant transfer water and other soil nutrients from its roots, through its stems, and to its leaves?  This transport primarily uses the vascular tissues (xylem and phloem); however, there are also structural modifications called plasmodesmata (singular: plasmodesma) that facilitate direct communication in plant cells.  Plasmodesmata are numerous channels that pass between cell walls of adjacent plant cells and connect their cytoplasm; thereby, enabling materials to be transported from cell to cell, and thus throughout the plant. Plasmodesmata: A plasmodesma is a channel between the cell walls of two adjacent plant cells. Plasmodesmata allow materials to pass from the cytoplasm of one plant cell to the cytoplasm of an adjacent cell. Junctions in Animal Cells  Communication between animal cells can be carried out through three types of junctions.  The first, a tight junction, is a watertight seal between two adjacent animal cells. The cells are held tightly against each other by proteins (predominantly two proteins called claudins and occludins).  This tight adherence prevents materials from leaking between the cells.  These junctions are typically found in epithelial tissues that line internal organs and cavities and comprise most of the skin.  For example, the tight junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular space. Tight Junctions: Tight junctions form watertight connections between adjacent animal cells. Proteins create tight junction adherence.   Also found only in animal cells are desmosomes, the second type of intercellular junctions in these cell types.  Desmosomes act like spot welds between adjacent epithelial cells, connecting them. Short proteins called cadherins in the plasma membrane connect to intermediate filaments to create desmosomes.  The cadherins join two adjacent cells together and maintain the cells in a sheet-like formation in organs and tissues that stretch, such as the skin, heart, and muscles. Desmosomes: A desmosome forms a very strong spot weld between cells. It is created by the linkage of cadherins and intermediate filaments.   Lastly, similar to plasmodesmata in plant cells, gap junctions are the third type of direct junction found within animal cells.  These junctions are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate.  Structurally, however, gap junctions and plasmodesmata differ.  Gap junctions develop when a set of six proteins (called connexins) in the plasma membrane arrange themselves in an elongated doughnut-like configuration called a connexon.  When the pores (“doughnut holes”) of connexons in adjacent animal cells align, a channel between the two cells forms.  Gap junctions are particularly important in cardiac muscle.  The electrical signal for the muscle to contract is passed efficiently through gap junctions, which allows the heart muscle cells to contract in tandem. Key Points  Plasmodesmata are intercellular junctions between plant cells that enable the transportation of materials between cells.  A tight junction is a watertight seal between two adjacent animal cells, which prevents materials from leaking out of cells.  Desmosomes connect adjacent cells when cadherins in the plasma membrane connect to intermediate filaments.  Similar to plasmodesmata, gap junctions are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances.  Alright class, let's dive into the proton pump, which is a fascinating example of active transport.  Active transport, as you know, is the process where cells move substances against their concentration gradient, from low concentration to high concentration.  This requires energy, and the proton pump is a great example of how cells use energy to move protons (H⁺ ions) across a membrane.  The energy for this process comes from ATP, which is often referred to as the energy currency of the cell.  In the proton pump mechanism, ATP is used to drive the movement of protons from the inside of the cell (or organelle) to the outside.  Specifically, in cells like plant cells, fungi, and some bacteria, the proton pump pushes protons across the plasma membrane, creating a gradient where there are more protons outside the cell than inside.  This creates two effects: a difference in proton concentration (called the proton gradient) and a difference in charge across the membrane (called the electrochemical gradient).  Now, why is this important?  Well, the buildup of protons outside the cell generates a form of stored energy, similar to water being held behind a dam.  When the protons flow back down their concentration gradient through specialized proteins called channels, this energy can be harnessed for various cellular processes.  One key process is the production of ATP itself, which happens in cellular structures like mitochondria and chloroplasts through a related system called ATP synthase.  Now, why is this important?  This proton pump system is central to many biological processes.  For example, in plants, the proton pump is used to power the transport of nutrients and other substances into the cell.  In the stomach lining of animals, proton pumps play a critical role in secreting stomach acid, which is essential for digestion.  Without these pumps, the body wouldn't be able to break down food as effectively.  Interestingly, the proton pump is a target for certain medications, such as proton pump inhibitors (PPIs), which are used to reduce stomach acid production in people suffering from conditions like acid reflux or ulcers.  These drugs block the activity of the proton pump in the stomach lining, reducing the amount of acid produced and providing relief from symptoms.  In summary, the proton pump is an essential component of active transport, helping cells maintain proper balance, produce energy, and carry out critical functions in both plants and animals.  It highlights the importance of energy in moving substances against their natural flow, ensuring that cells can maintain the right conditions for life to thrive. Proton Pump Cell Signalling (Introduction)  In order to survive, a cell must be able to understand its environment.  This is true whether the cell is a single-celled organism or part of a larger, more complex multicellular organism.  Cells communicate with their environment through a process called signaling.  Cell signaling is how the cell collects information and then responds with an action at the correct time.  Signaling is the initial event associated with many key cellular functions, from the correct timing of cell division, to the decision to migrate in a particular direction, and even to whether a cell needs to go through programmed cell death. Cell Signalling (Introduction)  So many of the cellular events we explore in biology are dependent on signaling to happen correctly.  Not only that, but many of the concepts covered in upper-level biology courses are, at their hearts, studies of how the cell receives information and responds to it.  For example, developmental biology, sensory perception, endocrinology, and even physiology will make much more sense if you have a foundational understanding of how signaling works. General Principles of Signaling Learning Goals  Explain the three stages of a general signaling cascade.  Provide examples of common protein families involved in each stage.  List and explain the different types of intracellular responses possible for a given signal.  Distinguish among relay, amplification, integration, and distribution steps in a signaling cascade.  Correctly interpret schematic representations of signaling cascades. What is Cell signaling?  Cell signaling is the process by which cells communicate with each other to coordinate various functions in the body.  It involves sending and receiving signals through molecules like hormones, neurotransmitters, or proteins, which bind to specific receptors on the surface of target cells.  Once a signal is received, it triggers a cascade of reactions inside the cell, leading to a response such as growth, movement, or changes in metabolism.  This communication is crucial for maintaining the body's balance and ensuring that tissues and organs work together properly. Types of Cell Signaling: There are five different types of signaling that are common in cells:  Endocrine  neuronal  paracrine  autocrine  juxtacrine Types of Cell Signaling: There are five different types of signaling that are common in cells:  Long-Distance Signaling: Endocrine (Slow)  Endocrine signaling is considered to be slooow, as the signal must be produced and secreted into the bloodstream, moved throughout the body, and then picked up by another cell that is likely very far from the site of ligand release.  It can take minutes for the signal to be received, which is quite a long time in the world of cells and signaling. Types of Cell Signaling: There are five different types of signaling that are common in cells:  On the other hand, since the hormone will become very dilute as it moves through the body and the receptor must be able to find and bind the ligand even in these low-concentration conditions, the receptors for endocrine signals are quite sensitive.  In some cases, a single molecule from the bloodstream can be detected.  A receptor that can be activated by a small, dilute dose of signal is said to have high affinity for its ligand. Types of Cell Signaling: There are five different types of signaling that are common in cells:  Examples of endocrine signaling ligands include most hormones (i.e., insulin, adrenaline, estrogen, growth hormones, etc.). Types of Cell Signaling: There are five different types of signaling that are common in cells: Neuronal (Fast) Signaling Neuronal signaling is the type of signaling used by the nervous system. In a nutshell, an electrochemical signal is sent through our neurons, across large distances, to elicit a response: This kind of signaling is quite fast, which is good, considering that neuronal signaling is what makes you move your hand when you touch a hot surface by accident. The signal is passed along and received in a matter of milliseconds. Types of Cell Signaling: There are five different types of signaling that are common in cells: Quite a bit of this process happens within the nerve cell. Nerves are very long cells—as an example, your sciatic nerve starts at the base of your spine and ends in your foot and is about 1 m long! Keeping the signal inside the cell for as long as possible makes it much easier for the signal to move quickly. At some point the signal will need to exit the neuron and be passed to the muscle or other tissue that is expected to respond. Multiple chemical signals are used to help with this part. Collectively, we call these signals neurotransmitters. Types of Cell Signaling: There are five different types of signaling that are common in cells: You likely have heard of many of them: dopamine, epinephrine (also known as adrenaline), and acetylcholine, for example. Some recreational drugs act by affecting the ability of neurons to send and/or receive neurotransmitter signals. Neurotransmitters help the signal move from the neuron to the target cell (like a muscle cell). The gap between the nerve cell and the muscle cell is really small, so the signal doesn’t have to diffuse very far. To increase the chances that the signal will be received as quickly as possible, the nerve cell floods the entire area with the neurotransmitter. As such, these receptors are not as sensitive as those used for endocrine signaling…they don’t really have to be. Thus, we consider the receptors in the synapse to be low affinity. Types of Cell Signaling: There are five different types of signaling that are common in cells:  Medium- to Short-Distance Signaling: Paracrine (Diffusion Based), Juxtacrine (Contact Dependent)  Paracrine signaling is considered to be a local signaling mechanism.  The ligand is released into the extracellular space, and it diffuses through the extracellular matrix to be picked up by nearby receptors.  As a result, cells that are farther from the source are expected to be exposed to a lower dose of the ligand.  This kind of gradient-dependent ligand is heavily used throughout development, as different doses of the signal can be detected, and responded to, differently.  Medium- to Short-Distance Signaling: Paracrine (Diffusion Based), Juxtacrine (Contact Dependent)  Compared to endocrine and neuronal signaling, described above, the affinity for the ligand is likely to be more moderate  Examples of paracrine signaling include the synaptic signaling we discussed earlier, where the neurotransmitter is released into the synapse and received by the receptors on the other cell in the synapse.  Also, many of the growth hormones used in development will have dose-dependent responses, thus allowing multiple responses from the same growth hormone.  Autocrine signaling is when the signal is both released and received by the same cell.  It is considered a type of paracrine signaling, since the signal must diffuse to the receptor through the extracellular environment.  This form of signaling is often used by the immune system in order to help it ramp up the immune response when activated.  It is also something that happens in cells that have become cancerous; it helps the cancerous cells break free of the normal regulatory controls so that they can grow and divide without restriction. Finally, juxtacrine signaling isn’t very common overall compared to the other kinds of signaling. It’s also known as contact-dependent signaling, which gives you an idea of what is involved in this form of signaling. In this case, the cell that receives the signal comes into direct contact with the one that is sending out the signal