Διάλεξη 2_Η ζωή ως ομαδικό άθλημα.pdf

# The human microbiome and holobionts The human microbiome and its symbiotic relationship with the host represent a co-evolved entity known as a holobiont, fundamentally reshaping our understanding of evolution and health [5](#page=5). ### 1.1 Defining the human microbiome The human microbiome refers to the vast and diverse population of microorganisms, predominantly prokaryotes, residing within the human body. Even seemingly healthy individuals host an immense variety of bacteria, with prokaryotic cells constituting approximately 90% of human cells [3](#page=3). ### 1.2 The dynamic nature of the microbiome Crucially, microbiomes are not static entities. Their composition changes over time in response to environmental factors, including alterations in diet, the use of medications, and atmospheric conditions. This dynamic interplay underscores the intricate relationship between the host and its microbial inhabitants [4](#page=4). ### 1.3 Symbiotic relationship and the concept of holobionts The relationship between humans and their resident bacteria is considered symbiotic rather than infectious, as these microbes are essential for maintaining human health. This co-evolution has led to the formation of a eukaryotic-prokaryotic consortium termed the holobiont [4](#page=4) [5](#page=5). > **Tip:** Understanding the microbiome as an integral part of our being, rather than just a collection of external organisms, is key to grasping the concept of holobionts. ### 1.4 The holobiont as a "superorganism" and the hologenome The recognition of holobionts has revolutionized scientific perspectives on evolution. The human host and its microbiome can now be viewed as a "superorganism" that possesses the capacity to evolve as a single genetic unit, collectively referred to as the hologenome. This hologenome encompasses the genetic material from both the host and its associated microbes [6](#page=6). ### 1.5 Genetic contributions of the microbiome The human microbiome significantly contributes to the genetic diversity of this superorganism. While human cells possess approximately 22,000 different genes, with remarkable genetic similarity among individuals (99.9%), current estimates suggest that the microbiome harbors around 10 million genes. Humans effectively contain genetic material detectable in bacteria and even viruses, highlighting the profound genetic contribution of the microbiome [6](#page=6) [7](#page=7). > **Example:** Imagine the human genome as a single book, and the microbiome's genome as a vast library containing millions of unique volumes. The holobiont's capabilities are a combination of both. ### 1.6 The microbiome's role in health and disease Changes in the microbiome are increasingly implicated in human health and disease. A growing body of evidence suggests that a variety of illnesses, including inflammatory and autoimmune disorders, may be linked to the loss of specific components within our microbiome [8](#page=8). ### 1.7 Microbiome influence during pregnancy and early life The influence of the microbiome is evident from early stages of life. For instance, during pregnancy, bacteria like *Lactobacillus* colonize the vaginal tract, creating a highly acidic environment that prevents the entry of potentially pathogenic microorganisms [9](#page=9). During childbirth, newborns are exposed to microbes that help establish their initial microbial colonies. This initial colonization can be altered in infants born via Cesarean section, who may miss out on this early exposure through the vaginal canal. Furthermore, breastfeeding mothers can provide nutrients in their milk that support the growth of specific beneficial bacteria in the infant's gut, thereby preventing the colonization by harmful pathogens [10](#page=10) [11](#page=11). ### 1.8 Therapeutic applications: Fecal Microbiota Transplant (FMT) The understanding of the microbiome's impact has led to novel therapeutic interventions. Physicians are now leveraging the concept of "transplanting" microbes from one individual to another to treat diseases [12](#page=12). #### 1.8.1 Clostridium difficile infection (CDI) A prime example of this therapeutic approach is the treatment of *Clostridium difficile* infection (CDI). *Clostridium difficile* is a common bacterium in the human digestive tract, usually harmless. However, under certain conditions, it can cause CDI, characterized by symptoms such as frequent watery diarrhea, severe abdominal pain, nausea, loss of appetite, fever, and potentially blood or pus in stool, which can be life-threatening [13](#page=13). #### 1.8.2 FMT as a treatment for CDI While oral antibiotics are the standard treatment for CDI, they can sometimes be ineffective. In such cases, a more complex treatment called Fecal Microbiota Transplant (FMT) may be employed. FMT involves transferring a fecal sample from a healthy donor to a patient with CDI, typically via colonoscopy or an orally administered capsule. This procedure has proven to be highly effective, with over 95% success rates in treating CDI [14](#page=14) [15](#page=15). > **Tip:** Researching FMT, CDI, and the role of fecal microbial flora provides valuable insights into how manipulating the microbiome can restore health [19](#page=19). ### 1.9 Reasons for CDI susceptibility Despite *Clostridium difficile* being common in the human digestive tract, only a small number of individuals develop CDI. This is primarily because a healthy, diverse gut microbiome typically keeps *C. difficile* populations in check, preventing them from proliferating and causing disease. The most common causes of CDI are disruptions to this balance, often triggered by the use of broad-spectrum antibiotics, which can kill off beneficial bacteria, allowing *C. difficile* to overgrow. Other factors, such as advanced age, compromised immune systems, and prolonged hospitalization, can also increase susceptibility [19](#page=19). --- # Prokaryotic and eukaryotic cell structures Prokaryotic and eukaryotic cells represent fundamental distinctions in cellular organization, with prokaryotes being simpler, unicellular organisms and eukaryotes exhibiting greater complexity through compartmentalization by organelles. ### 2.1 Prokaryotic cells Prokaryotes are characterized by their simplicity and adaptability, capable of thriving in extreme environments due to their evolutionary history as descendants of early Earth cells [25](#page=25). #### 2.1.1 Basic structure * Prokaryotic cells possess only a single membrane, the cytoplasmic membrane. All intracellular chemical reactions occur within this single compartment, the cytosol, limiting their specialization [20](#page=20). * While generally lacking complex internal membranes, some prokaryotes have evolved elaborate modifications of their cytoplasmic membrane, forming stacks of membranous folds that provide a degree of compartmentalization [22](#page=22). * The prokaryotic cytosol, despite the cell's simplicity, appears heterogeneous under electron microscopy, suggesting some level of internal organization [22](#page=22). #### 2.1.2 Unicellular nature and division * All prokaryotic organisms are unicellular. They do not form multicellular organisms, although some may aggregate into large symbiotic structures called biofilms [23](#page=23). * Prokaryotes do not undergo mitotic division; instead, they divide by binary fission [24](#page=24). * The majority of genetic information in prokaryotic cells is contained in a single circular DNA molecule, in contrast to the numerous linear chromosomes found in eukaryotic DNA [24](#page=24). #### 2.1.3 Environmental resilience and abundance * Despite their relatively simple structure, prokaryotes inhabit some of Earth's most hostile environments, including extreme temperatures, high atmospheric pressure, oxygen-deficient areas, and a wide pH range (2 to 12) [25](#page=25). * This resilience stems from their origin as early Earth cells adapted to vastly different conditions [26](#page=26). * Due to their adaptability, prokaryotes are the most abundant organisms on Earth [26](#page=26). #### 2.1.4 Protective cell wall * Most prokaryotes are protected by an additional layer outside the cell membrane, known as a cell wall, typically composed of peptidoglycan [27](#page=27). * The cell wall is primarily made of sugar molecules linked together in a dense mesh [27](#page=27). * Beyond protection from physical damage, the cell wall helps retain water, ensuring proper cell hydration [27](#page=27). ### 2.2 Eukaryotic cells Eukaryotic cells are significantly more complex, characterized by their organized cytoplasm and the presence of membrane-bound subcellular organelles [29](#page=29). #### 2.2.1 Compartmentalization and organelles * The most striking feature of eukaryotic cells under microscopic observation is their highly organized cytoplasm [29](#page=29). * The presence of membranes allows eukaryotic cells to create specialized compartments within the cytoplasm dedicated to specific cellular tasks under optimal conditions [30](#page=30). * These internal membranes, like the plasma membrane, are selectively permeable, contributing to a unique internal environment suitable for the contained molecules [30](#page=30). * A **subcellular organelle** is defined as a microscopic cellular structure that performs specific functions within a cell [31](#page=31). * Organelles are often enclosed by their own membranes, dividing the cell into multiple compartments for different biochemical reactions [31](#page=31). * Subcellular organelles are responsible for a wide range of duties, from energy production to controlling cell growth and reproduction [31](#page=31). #### 2.2.2 Key subcellular organelles ##### 2.2.2.1 Nucleus * The nucleus is the cellular organelle that houses the chromosomes [32](#page=32) [33](#page=33). * It features a nuclear membrane with pores that selectively allow certain molecules, such as proteins and nucleic acids, to pass in and out [33](#page=33). * The nucleolus, located within the nucleus, is the site of ribosome production [36](#page=36). * **Example:** In humans and all mammals, mature red blood cells lack a nucleus, allowing for more space to store oxygen-binding hemoglobin, thus increasing oxygen transport capacity. These cells also have a biconcave shape to maximize surface area for oxygen diffusion. In non-mammalian vertebrates, mature red blood cells do possess a nucleus [37](#page=37). ##### 2.2.2.2 Mitochondria * Mitochondria are known as the "powerhouses" of the cell, converting energy from food into adenosine triphosphate (ATP), the body's primary energy currency [39](#page=39) [40](#page=40). * They are composed of distinct compartments: an outer membrane, an inner membrane that folds into cristae, and an intermembrane space between them [42](#page=42). * The outer membrane contains porins, protein complexes that allow free diffusion of small to medium-sized molecules [43](#page=43). * The intermembrane space generally resembles the cytosol, except for larger molecules like proteins that cannot pass through the outer membrane [43](#page=43). * Mitochondrial DNA is found within the matrix, the innermost compartment [43](#page=43). * Mitochondria are considered semi-autonomous organelles [43](#page=43). * **Origin hypotheses:** * **Endosymbiotic hypothesis:** Suggests mitochondria were once prokaryotic cells engulfed by an early eukaryotic cell approximately 2 billion years ago [44](#page=44) [46](#page=46). * **Autogenous hypothesis:** Proposes that DNA from a progenitor eukaryote fragmented and exited the nucleus [44](#page=44). * **Evidence supporting the endosymbiotic hypothesis:** 1. Mitochondria are similar in size to average bacteria [47](#page=47). 2. Mitochondria possess their own DNA, can synthesize some RNA, ribosomes, and proteins. Mitochondrial DNA sequences show closer kinship to bacteria than to eukaryotes. This DNA is organized into multiple copies of identical, circular chromosomes, unlike the linear chromosomes of eukaryotes [47](#page=47) [48](#page=48). 3. Mitochondria have a double membrane; the inner membrane is believed to be from the original prokaryote, and the outer membrane is thought to be from the host eukaryote's engulfing membrane [49](#page=49). 4. The lipid composition of the inner mitochondrial membrane resembles that of bacteria, while the outer membrane's lipid composition is more similar to eukaryotes [49](#page=49). * Mitochondria vary in size (0.5 to 3 micrometers), shape, and number depending on the cell type's energy requirements. Cells with high energy demands, like muscle, liver, and brain cells, contain more mitochondria. Hepatocytes can have 1,000-2,000 mitochondria, while heart, sperm, and muscle cells may have around 5,000 per cell [50](#page=50). * Mitochondria are sensitive indicators of cell health, and metabolic disturbances can lead to morphological changes, including an increase in size to become megamitochondria [51](#page=51). ##### 2.2.2.3 Ribosomes * Ribosomes are described as molecular protein production factories [53](#page=53). * They are composed of protein and ribonucleic acid (RNA) and are the site of translation, the process of protein synthesis [54](#page=54). * All eukaryotic ribosomes are 80S ribosomes, consisting of a small 40S subunit and a large 60S subunit. The "S" refers to Svedberg units, a measure of a particle's sedimentation coefficient [54](#page=54). * During protein synthesis, a ribosome attaches to an mRNA template and reads its code. Transfer RNA (tRNA) molecules carrying amino acids bind to the ribosome, leaving the amino acid behind as the ribosome moves along the mRNA, thus building the protein chain [55](#page=55). ##### 2.2.2.4 Endoplasmic reticulum (ER) * The ER is a network of interconnected sacs involved in processing newly synthesized secreted and membrane proteins (rough ER) and in lipid production and toxin detoxification (smooth ER) [59](#page=59). * Rough ER (RER) appears as flat membrane sacs called cisternae with ribosomes attached to the cytoplasmic face; its quantity reflects the cell's protein synthesis level [57](#page=57). * Smooth ER (SER) is involved in lipid synthesis and detoxification [58](#page=58). ##### 2.2.2.5 Vesicles * Vesicles are organelles containing fluid enclosed by a phospholipid bilayer [60](#page=60). * They perform various functions, including transporting substances into and out of the cell, and serving as storage units [60](#page=60). * During endocytosis, the cell membrane actively takes in larger molecules, forming pockets of the phospholipid bilayer that pinch off to become vesicles [62](#page=62). ##### 2.2.2.6 Vacuoles * Vacuoles are specialized vesicles primarily containing water [63](#page=63). * In most plant cells, vacuoles maintain osmotic balance and store nutrients [63](#page=63). * They can also store pigments in colorful plant organs like petals and store proteins and lipids in plant seeds, serving as an energy source for developing seeds [63](#page=63). ##### 2.2.2.7 Golgi apparatus * The Golgi apparatus receives proteins and lipids from the ER, packages them, and sends them to their destinations within or outside the cell [65](#page=65). * It is a collection of flattened membranes called cisternae where synthesized proteins are further processed and modified [67](#page=67). * Proteins are tagged with recognition labels to ensure proper delivery [68](#page=68). * The Golgi apparatus can be visualized as a cellular post office, sorting, packaging, and dispatching proteins to their correct locations. It was first identified by Camillo Golgi in 1898 [69](#page=69) [70](#page=70). ##### 2.2.2.8 Lysosomes * Lysosomes are specialized vesicles involved in cellular digestion [73](#page=73) [74](#page=74). * They contain acid hydrolase enzymes to break down waste materials, cellular debris, and ingested food particles [72](#page=72) [74](#page=74). * During phagocytosis, vesicles containing food merge with lysosomes, releasing digestive enzymes that break down biomolecules into their fundamental monomers [74](#page=74). * These monomers can be recycled by the cell to synthesize new biomolecules and organelles, a process known as autophagy [75](#page=75). * Lysosomes are also believed to be involved in programmed cell death (apoptosis) in multicellular organisms [76](#page=76). * **Lysosomal storage diseases (LSDs)** are inherited metabolic disorders characterized by the accumulation of substrates within cells due to faulty lysosomal function, leading to organ dysfunction. Most LSDs are caused by mutations in genes encoding lysosomal enzymes, specifically hydrolases, which break down chemical bonds using water [77](#page=77) [78](#page=78). ##### 2.2.2.9 Peroxisomes * Peroxisomes are organelles that use molecular oxygen and hydrogen peroxide to perform oxidative reactions [81](#page=81). * They contain oxidative enzymes, such as catalase, which breaks down hydrogen peroxide into water and oxygen [81](#page=81). ##### 2.2.2.10 Cytoskeleton * The cytoskeleton is a structure that helps cells maintain their shape and internal organization, providing mechanical support for essential functions like division and movement [83](#page=83). * It is an interconnected network of protein filaments present in all cells, including prokaryotes and eukaryotes [85](#page=85). * Its primary function is to resist compression, allowing the cell to maintain its overall shape. In multicellular organisms, it stabilizes tissues and provides structural integrity [85](#page=85). * The cytoskeleton is also involved in cell movement. Its protein filaments can contract and relax, changing cell shape to facilitate locomotion. In muscle cells, actin filaments contract to shorten the cell [86](#page=86). * In some cells, the cytoskeleton extends beyond the cell membrane to form structures like cilia and flagella [86](#page=86). * **Cilia** are hair-like projections extending from the cell surface, acting as cellular antennae to receive signals from the extracellular environment. Motile cilia are found in various cell types and can facilitate movement, such as the cilia in human fallopian tubes moving an egg towards the uterus [87](#page=87) [91](#page=91). * **Flagella** are typically longer, thicker, and fewer than cilia. These whip-like extensions create efficient locomotion [88](#page=88). * In eukaryotes, the cytoskeleton also facilitates the movement of vesicles within the cell, guiding them along protein filaments to their destinations. It plays a role in endocytosis and organelle transport as well [90](#page=90). * The cytoskeleton is composed of three main types of protein filaments: * Microtubules * Microfilaments (actin filaments) * Intermediate filaments [84](#page=84). --- # Pathogen-host interactions and cellular mechanisms This topic explores the intricate ways pathogens interact with host cells, focusing on how intracellular pathogens exploit cellular machinery for their survival and propagation [93](#page=93) [94](#page=94) [95](#page=95) [96](#page=96) [97](#page=97). ### 3.1 Exploiting the host cytoskeleton Certain intracellular bacteria leverage the host cell's actin cytoskeleton to create their own transport systems. A prime example is *Listeria monocytogenes*, which utilizes a protein called ActA. ActA hijacks the host's actin polymerization machinery to build an actin tail behind the bacterium. This actin tail propels the bacterium and allows it to push against the host cell membrane, forming protrusions that can invade neighboring cells [93](#page=93). ### 3.2 Interacting with host membrane compartments Some intracellular bacteria, such as *Legionella pneumophila*, establish residence within host cell membrane-bound compartments. Upon entry, these bacteria engage with host membranes and secrete effector proteins that facilitate their control. *Legionella*, specifically, interacts with the Golgi apparatus and endoplasmic reticulum. It 'steals' proteins from these organelles and redirects vesicular traffic for its own purposes [94](#page=94). ### 3.3 Lysosome interplay with intracellular pathogens Lysosomal homeostasis within eukaryotic cells is frequently disrupted by various pathogens. Infections with *Mycobacterium tuberculosis* (Mtb) and *Staphylococcus aureus* lead to an increase in host cellular lysosomes compared to uninfected conditions. In contrast, *Salmonella* infections induce the formation of filamentous structures derived from late endosomes/lysosomes. In *Trypanosoma* infections, lysosomes migrate towards the cell periphery at the site of pathogen entry and fuse with the plasma membrane, thereby facilitating pathogen entry into the host cell. Under these infectious conditions (Mtb, Salmonella, Staphylococcus, and Trypanosoma), the transcription factor EB (TFEB) is translocated to the nucleus, a phenomenon not observed in uninfected states [95](#page=95) [96](#page=96). > **Tip:** Understanding how pathogens manipulate host organelles like lysosomes is crucial for developing effective therapeutic strategies that can restore cellular balance or disrupt pathogen survival mechanisms. ### 3.4 Bacterial reprogramming of host cells Bacteria can fundamentally alter host cell behavior. *Mycobacterium leprae*, the causative agent of leprosy, induces extreme cellular reprogramming. It can revert host Schwann cells to a state resembling stem cells. These reprogrammed cells can then differentiate into other cell types, such as muscle cells, potentially aiding the bacterium's spread to different tissues. Furthermore, these reprogrammed cells can transmit the infection to macrophages [97](#page=97). --- # The cell theory and cell types The cell theory provides fundamental principles for understanding life at its most basic structural and functional level, while the diversity of specialized cell types in multicellular organisms highlights the complexity and sophistication of biological organization [100](#page=100) [99](#page=99). ### 4.1 The cell theory Scientific theories are well-substantiated explanations of the natural world, acquired through the scientific method and extensively tested and confirmed. The cell theory is one of the two major scientific theories in biology. It is governed by three core principles [100](#page=100) [99](#page=99): 1. The cell is the fundamental unit of life [100](#page=100). 2. All living organisms are composed of cells [100](#page=100). 3. All cells arise from pre-existing cells [100](#page=100). > **Tip:** Understanding these three tenets is crucial for grasping the foundational concepts of biology, as they underpin all discussions of life at the cellular level. ### 4.2 Diversity of cell types in multicellular organisms Multicellular organisms exhibit a remarkable diversity of specialized cell types, each adapted for specific functions. This specialization allows for the complex organization and efficient operation of tissues, organs, and organ systems . #### 4.2.1 Embryonic stem cells Embryonic stem cells (ESCs) are pluripotent cells, meaning they possess the ability to differentiate into any cell type. The specific cell type an ESC matures into is determined by the biochemical signals it receives. This characteristic makes ESCs a promising source for regenerative medicine, potentially used to repair damaged tissues in conditions such as Parkinson's disease and insulin-dependent diabetes . > **Example:** A biochemical signal indicating a need for muscle repair might trigger an ESC to differentiate into a cardiac muscle cell. #### 4.2.2 Cardiac muscle cells Cardiac muscle is one of the three types of muscle tissue found in the human body, alongside skeletal and smooth muscle. It is exclusively located in the heart. Cardiac muscle cells are responsible for the coordinated contractions that enable the heart to pump blood throughout the circulatory system . #### 4.2.3 Bone tissue Bone tissue, particularly cancellous (spongy) bone, is found in the interior of bones. It provides structural support within the skeletal system . #### 4.2.4 Sperm cells (spermatozoa) Sperm cells, also known as spermatozoa, are the male reproductive cells. These small cells are produced in the testes and are essential for the fertilization of the female egg (ovum). Each sperm cell has a head containing the male hereditary material (DNA). During ejaculation, approximately 300 million sperm are released, whereas a female typically produces one large egg at a time . #### 4.2.5 Ova (egg cells) The ovum, or egg cell, is the female reproductive gamete. Mature ova are released from the Graafian follicle during ovulation . #### 4.2.6 Nerve cells (neurons) Nerve cells, or neurons, are specialized cells that form the basis of the nervous system. They are responsible for transmitting electrical and chemical signals throughout the body, enabling communication, sensation, thought, and movement . #### 4.2.7 Fat cells (adipocytes) Fat cells, or adipocytes, are among the largest cells in the human body, with diameters typically ranging from 100 to 120 microns. These cells are surrounded by fine strands of supportive connective tissue and are responsible for storing energy in the form of fat . > **Tip:** The specialization observed in these diverse cell types is a direct consequence of gene expression, where specific genes are activated or silenced to produce proteins that dictate a cell's structure and function. This process is fundamental to differentiation and the development of complex organisms. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Microbiome | The collection of microorganisms, including bacteria, archaea, viruses, and fungi, that live in and on a particular environment, such as the human body. | | Holobiont | A biological unit consisting of a host and its symbiotic microorganisms, considered as a single ecological and evolutionary entity. | | Superorganism | An entity composed of multiple organisms that functions as a single unit, where the collective behavior or characteristics are distinct from those of the individual components. | | Ologenome | The complete set of genes contributed by a host and its associated microbiota, forming a collective genetic repertoire for the holobiont. | | Prokaryote | A single-celled organism that lacks a membrane-bound nucleus and other membrane-bound organelles. Their genetic material is typically found in a circular DNA molecule in the cytoplasm. | | Eukaryote | An organism whose cells contain a nucleus and other membrane-bound organelles. These cells are typically larger and more complex than prokaryotic cells. | | Cytoplasmic membrane | The selective barrier that encloses the cytoplasm of a cell, regulating the passage of substances into and out of the cell. | | Cytoplasm | The material or protoplasm within a living cell, excluding the nucleus. It comprises the cytosol and the organelles suspended within it. | | Mitosis | A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. | | Binary fission | A form of asexual reproduction and cell division used by prokaryotic organisms and some eukaryotic organelles. The cell divides into two equal or near-equal parts. | | Peptidoglycan cell wall | A rigid layer found outside the plasma membrane of most bacteria, composed of a polymer of N-acetylglucosamine and N-acetylmuramic acid, which provides structural support and protection. | | Subcellular organelle | A specialized subunit within a cell that has a specific function, often enclosed by its own membrane. Examples include the nucleus, mitochondria, and ribosomes. | | Nucleus | A membrane-bound organelle found in eukaryotic cells that contains the cell's genetic material in the form of chromosomes. | | Mitochondrion | An organelle found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur. It has a double membrane, its own DNA, and ribosomes. | | Endosymbiotic theory | A theory proposing that certain organelles of eukaryotic cells, such as mitochondria and chloroplasts, originated as free-living prokaryotes that were engulfed by an ancestral eukaryotic cell and established a symbiotic relationship. | | Ribosome | A cellular particle made of ribosomal RNA and protein that serves as the site of protein synthesis in the cell. It translates messenger RNA into protein. | | Endoplasmic reticulum (ER) | A network of membranes found throughout the cytoplasm of eukaryotic cells, involved in protein and lipid synthesis. Rough ER has ribosomes attached and is involved in protein synthesis, while smooth ER is involved in lipid synthesis and detoxification. | | Vesicle | A small sac or bladder-like structure within a cell, enclosed by a membrane, that can transport or store materials. | | Vacuole | A membrane-bound organelle found in eukaryotic cells, typically containing fluid. In plant cells, they are large and involved in maintaining turgor pressure and storage. | | Golgi apparatus | An organelle in eukaryotic cells that receives proteins and lipids from the endoplasmic reticulum, modifies, sorts, and packages them for secretion or delivery to other organelles. | | Lysosome | A membrane-bound organelle in eukaryotic cells containing digestive enzymes that break down waste materials and cellular debris. | | Peroxisome | A small organelle present in the cytoplasm of eukaryotic cells, containing a variety of enzymes, including those involved in the breakdown of fatty acids and detoxification. | | Cytoskeleton | A network of protein filaments and tubules in the cytoplasm of many living cells, giving them shape and internal organization, and enabling movement. | | Cytoplasmic inclusions | Granules or droplets found within the cytoplasm of cells that store various substances, such as glycogen, lipids, or pigments. | | Endocytosis | The process by which cells absorb molecules from outside the cell by engulfing them with their cell membrane. | | Exocytosis | The process by which cells transport molecules (e.g., neurotransmitters or hormones) out of the cell (exo- + cytosis). | | Cell theory | A fundamental biological theory stating that all living organisms are composed of cells, that cells are the basic unit of life, and that all cells arise from pre-existing cells. | | Pluripotent | A stem cell that has the potential to differentiate into any type of cell in the body. | | Cytosol | The aqueous component of the cytoplasm of a cell, within which various organelles and particles are suspended. | | Homeostasis | The maintenance of a stable internal environment in an organism, despite changes in external conditions. | | Transcription factor EB (TFEB) | A master regulator of lysosomal biogenesis and function, and autophagy. It translocates to the nucleus to activate genes involved in these processes. | | Schwann cell | A type of glial cell that forms the myelin sheath around nerve axons in the peripheral nervous system. | | Macrophage | A large phagocytic cell found in stationary form in the tissues or as a mobile white blood cell, especially at sites of infection. |