Endosymbiotic Theory and Eukaryotic Cells

Endosymbiotic Theory

• Endosymbiotic theory explains how eukaryotes evolved
• Endo refers to Inside and Symbiotic refers to collaboration between organisms
• Involves sequential, cell-merging events between an ancient eukaryotic ancestor and certain prokaryotes
• Mitochondria evolved from an engulfed non-photosynthetic prokaryote
• Chloroplasts originated from an engulfed photosynthetic prokaryote, such as a cyanobacterium
• Mitochondria and chloroplasts possess their own circular DNA (like bacteria), 70S ribosomes (similar to those in bacteria), double-membrane structures and the ability to replicate by a process similar to binary fission
• Furthermore, their inner membrane resembles the bacterial plasma membrane

Eukaryotic vs Prokaryotic Cells

• Eukaryotic cells include plants, animals, protists, and fungi
• Eukaryotic cells are generally larger and more complex than prokaryotic cells, often containing more extensive genomes and multiple linear chromosomes
• Eukaryotic organisms can be unicellular (protists and yeast) or multicellular (animals, plants, and most fungi)
• Cell division occurs through asexual (mitosis) and sexual (meiosis) processes
• The plasma membrane often includes sterols
• Cell walls are present in plants, fungi, and certain protists
• These cells contain a nucleus, 80S ribosomes in the cytoplasm and rough endoplasmic reticulum, and 70S ribosomes in mitochondria and chloroplasts
• Their genetic material is DNA organized into multiple linear chromosomes, and they have membrane-bound organelles
• Prokaryotes include unicellular archaea and bacteria
• They are much smaller than eukaryotes
• Cell division occurs asexually through binary fission
• Plasma membranes rarely contain sterols
• Most prokaryotes (except Mycoplasma and L-forms) have a cell wall
• They lack a nucleus and possess only 70S ribosomes
• Their genetic material is DNA organized into a single circular chromosome, and they lack membrane-bound organelles

Membrane-Bound Organelles

• Eukaryotic cells have a defined nucleus and a variety of other membrane-bound organelles like mitochondria and chloroplasts

Cell Division in Eukaryotes

• Eukaryotic cells can undergo either sexual or asexual reproduction
• Mitosis generates two genetically identical offspring from one parent cell
• Offspring cells maintain the same number of chromosomes as the parent cell
• Outcome of mitosis is similar to binary fission, yet the process differs
• Meiosis is involved in sexual reproduction
• A parent cell produces four gametes (daughter cells)
• Gametes are haploid (one complete set of chromosomes)

Eukaryotic Cell Transport: Endocytosis and Exocytosis

• Involve the import and export of substances into and out of the cell
• Endocytosis imports substances into the cell, while exocytosis exports substances out of the cell
• The membrane folds around extracellular substances during endocytosis and pinches off to form endocytic vesicles containing those substances
• Pinocytosis, phagocytosis, and receptor-mediated endocytosis are types of endocytosis
• Phagocytosis ("cell eating") involves the endocytosis of undissolved substances
• Specialized immune system cells (e.g., macrophages) use phagocytosis
• Receptor-mediated endocytosis transports specific large substances
• Exocytosis uses vesicles to deliver their contents to the plasma membrane, mainly for removing toxic substances and secreting specific substances into the extracellular space

Eukaryotic Kingdoms

• Eukaryotic organisms fall into four different kingdoms: Animalia Includes Multicellular organisms
• Animalia include parasitic worms (helminths) and arthropods
• Helminths are parasitic worms (e.g., roundworms and flatworms)
• Plants carry out photosynthesis and contain chloroplasts to make their organic carbon using light energy
• Fungi grow as hyphae, tubular structures that are often hairlike

Fungi

• Hyphae include septate hyphae, which have divisions between cells, and aseptate hyphae, which have no divisions
• Fungal spores (reproductive cells) help classify fungi and can be either asexual or sexual
• Mycoses are diseases caused by fungi, most often in immunocompromised individuals or people with disrupted normal microbiota
• Some fungi are true pathogens that can infect typically healthy hosts and stimulate allergies or produce mycotoxins
• It is important to know that fungal diseases include opportunistic pathogens and true pathogens and can produce toxins causing allergies or disease
• Protists are grouped by means of motility in their mature form

Protozoans

• Amoeboid protozoans move with pseudopodia
• Flagellated protozoans move with flagella
• Ciliated protozoans move with cilia
• Spore-forming protozoans are apicomplexa that move by gliding and are obligate intracellular parasites
• The apicomplexa life cycle includes merogony (asexual reproduction), gamogony (sexual reproduction), and sporogony
• Sporogony refers to the zygote dividing to form sporozoites, which are typically the infective stage
• Examples of apicomplexans that are human pathogens include Toxoplasma gondii, Plasmodium (malaria), Cryptosporidium, and Microsporidium

Eukaryotic Plasma Membrane

• All eukaryotes have a plasma membrane with a phospholipid bilayer structure
• Certain eukaryotes also have a cell wall, which, in fungi, plants, and certain protists, lacks peptidoglycan
• Most eukaryotes have a sticky extracellular layer called the glycocalyx as their outermost layer

Flagella

• Eukaryotic flagella are built from tubulin protein with a 9 + 2 arrangement of microtubules, are membrane-enclosed, and sprout from a centriole
• They have a wave-like motion
• Eukaryotic flagella have a wavelike, back-and-forth motion, while prokaryotic flagella built of flagellin protein with rotary (propeller) movement
• Cilia are structurally similar to flagella but are shorter and more numerous in a cell

Ribosomes

• Important for making protein and are made of protein and ribosomal RNA (rRNA)
• Eukaryotic ribosomes can be free in the cytoplasm or bound to the endoplasmic reticulum
• Bound ribosomes make membrane proteins or proteins destined for secretion
• Free ribosomes produce cytosolic proteins
• Mitochondria and chloroplasts contain ribosomes that resemble 70S ribosomes found in prokaryotes

Cytoskeleton

• Is a dynamic and responsive intracellular network of proteins and fibers
• Functions include helping cells to maintain shape, facilitate direct transport (e.g., vesicles, organelles), and coordinate cell division
• The cytoskeleton is made up of three main fibers: microtubules, intermediate filaments, and microfilaments
• Microtubules are roadways of the cells and form the spindle that helps separate chromosomes during cell division
• Intermediate filaments contribute tensile strength and give cells size and shape
• Microfilaments are fine fibers which function in muscle contraction, pinch cells apart, and facilitate the movement of pseudopodia

Eukaryotic Cell Structures

• The plasma membrane, present in all cells, is a phospholipid bilayer
• It acts as a selective barrier and interfaces with the extracellular environment
• Cell walls, present in most prokaryotes and some eukaryotes, add rigidity and protect cells from mechanical and osmotic stress
• The glycocalyx is a sticky layer present in many prokaryotes and eukaryotes, important in cell adhesion, protection, and communication
• Flagella provide motility in certain prokaryotes and eukaryotes - eukaryotic and prokaryotic flagella have different general structures
• Cilia, found only in eukaryotes, are short, numerous hair-like extensions used for motility and found on the surface of various human cells like cells of the upper airway as part of the mucociliary escalator
• Ribosomes are found in prokaryotes and eukaryotes and build proteins
• In eukaryotes, 80S ribosomes are membrane-bound or free in the cytoplasm, while 70S ribosomes are in mitochondria and chloroplasts
• The cytoskeletonis made of microtubules, intermediate filaments and microfilaments and Provides cell shape, plays important roles in cell movement, facilitates cargo transport, and protects the cell against external mechanical stress forces
• Nucleus has its DNA and serves as the cell's command center
• The rough endoplasmic reticulum has ribosomes on its surface and is mainly involved in protein production and detoxification

Nucleus

• The site where ribosome assembly begins
• It has a nuclear envelope and nuclear pores that control movement of materials in and out of the nucleus
• The endoplasmic reticulum is continuous with the nuclear envelope and the nucleolus is enriched with rRNA

Endoplasmic Reticulum

• Plays essential roles in protein and lipid production
• Interconnected membranes that originate from the nuclear envelope
• Rough ER has millions of ribosomes on the outer surface and modifies proteins
• Smooth ER is not associated with ribosomes, is involved in lipid production, and plays an important role in detoxifying substances

Golgi Apparatus

• Transports cellular proteins, builds lipids, then sorts and distributes finished products

Vesicles and Vacuoles

• Transport vesicles move substances around the cell
• Secretory vesicles shuttle materials to the cell surface or discharge from the cell
• Lysosomes contain hydrolytic enzymes that break down substances engulfed by the cell
• Peroxisomes contain enzymes that break down fats and amino acids by oxidation and protect cells from toxic oxygen intermediates
• Vacuoles are collections of many vesicles that merge to form a large membranous sac, and are common in plants & fungi

Mitochondria and Chloroplasts

• Have double-membrane structures, 70S ribosomes, and a circular chromosome
• Mitochondria make adenosine triphosphate (ATP), carry out programmed cell death (apoptosis), and differ in number
• Only photosynthetic cells have chloroplasts
• Chloroplasts allow cells to harvest energy from sunlight using light collecting pigments
• Mitochrondria structure has two membranes
• Chroroplast structure consists of an inner and outer membrane