hello everyone my name's iman welcome back to my youtube channel i'm so glad you're here today marks the start of a fully revised and updated mcat biology series if you followed along before you know i'm always refining these videos to make them more structured more clear and of course more comprehensive and your feedback has played a big role in shaping these updates ultimately i just want you to feel supported and confident as you study for the mcat now everything i post here is completely free because i believe that access to highquality education should never depend on your ability to pay so in the description box below you'll find a link to the notes as well as full transcripts of each lesson with that let's start with chapter 1 which is titled the cell in this chapter we'll be covering the following objectives first we'll start with cell theory which is a set of principles that define what a cell is and how it relates to life itself these foundational ideas are essential to everything else we'll learn in biology next we'll focus on ukareotic cells these are the types of cells that make up plants animals fungi and protests we're going to look at their internal structures like their membrane bound organels and we're going to discuss how the cytokeleton helps maintain their shape and organize their contents we're also going to talk about how ukareotic cells come together to form tissues after that we'll turn our attention to proarotic cells these are simpler cells like bacteria but they're incredibly diverse and important we'll talk about the domains they belong to how they're classified by shape and how they differ in their use of oxygen we'll also go over the basic structure of a typical proarotic cell from there we're going to move into how proaryotic cells grow and reproduce even though they don't divide the way our cells do they still pass on genetic information and they have fascinating ways of increasing their genetic diversity we're going to talk about bionary fision genetic recombination and the stages of population growth finally we'll take a closer look at viruses and subviral particles these aren't considered living cells but they still play a major role in biology and in human health we're going to examine what viruses are made of how they replicate and how they infect both proarotic and ukareotic cells will also introduce prons and vyroids which are even smaller infectious agents that disrupt biological systems in unique ways so with that introduction in mind let's begin by discussing the core principles of cell theory before the invention of the microscope people believed that living organisms were complete and indivisible which means that they couldn't be broken down into smaller components but all of that changed in the 1600s when the microscope opened an entirely new world of observation in 1665 robert hook built a crude compound microscope and he used it to examine a slice of cork what he saw reminded him of small rooms that he called cells just a few years later in 1674 anton von louuven hook became the first person to observe a living cell under a microscope this was a groundbreaking moment because it showed that cells were dynamic and alive then in 1850 rudolph vchau added another key insight he demonstrated that diseased cells could arise from normal cells in healthy tissue this suggested that cells don't just appear randomly they come from other cells and these discoveries led to the original version of cell theory which includes three main tenants first all living things are composed of cells second the cell is the basic functional unit of life and third all cells arise from preexisting cells later as molecular biology advanced scientists added a fourth tenant to the theory this final principle states that cells carry genetic information in the form of dna and importantly this genetic material is passed on from parent cells to daughter cells during cell division so together these four principles form the foundation of our understanding of what cells are and how life operates at the microscopic level but we'll see in our last objective that viruses challenge some of these ideas and that's where things start to get a bit tricky but really interesting now that we've established the core ideas of cell theory we're going to begin exploring the structure and function of cells in more detail one of the first distinctions we make among living organisms is whether they're composed of proarotic or ukareotic cells this chapter is going to cover both cell types but we'll start with ukareotic cells which are the kinds found in animals plants fungi and protests now proarotic organisms they are always single cell but ukareotic organisms they can exist as single cells or as multi-ellular systems with specialized tissues and one of the key differences between them is that ukareotic cells contain a true nucleus this nucleus is enclosed in a membrane and it houses the cell's dna proarotic cells do not have a nucleus instead their genetic material is just found freely floating in the cytoplasm in addition to the nucleus ukareotic cells contain membranebound organels which compartmentalize specific functions and allow the cell to carry out multiple processes simultaneously without interference these organels are suspended in the cytool which is a semifluid substance that facilitates the movement of molecules throughout the cell the membranes that enclose these compartments those are composed of phospholipid billayers which regulate what enters and exits each space and this structural organization gives ukareotic cells a high level of internal complexity and coordination in this objective again we will focus first on ukareotic cells and what we're going to do is we're going to start with a broad overview of the important cellular components we need to know for the mcap so i'm going to introduce them in this summary table here and then we're going to spend more time going over the nucleus the mitochondria the endopplasmic reticulum and the gg apparatus in a lot more detail those are really important to master the nucleus acts as the control center of the cell it stores genetic information and it directs the synthesis of rna and proteins mitochondria are responsible for generating energy in the form of atp using the triricaroxylic acid cycle also known as the kb cycle and the electron transport chain lysosomes are organels filled with hydraytic enzymes that help break down waste damaged organels and macroolelecules they often get this material by fusing with endosomes which carry cargo either from outside the cell or from internal trafficking also beyond that lossomes can also trigger autotolyis where the cell intentionally releases these enzymes to break down its own components during programmed cell death or apoptosis mitochondria can do this too by the way then there's the endopplasmic reticulum this exists in two forms the rough endopplasmic reticulum which is covered in ribosomes and it assists in protein synthesis and processing and the other form is the smooth endopplasmic reticulum which is involved in lipid synthesis and detoxification the golgi apparatus modifies packages and sorts proteins and lipids for delivery to their final destinations it also contributes to the formation of lysosomes lastly peroxomes carry out oxidative reactions including the breakdown of fatty acids and it detoxifies harmful byproducts now that we know the main components of the ukareotic cell let's begin discussing the structure and function of the nucleus in a lot more detail the nucleus again is often described as the control center of the cell because it houses the genetic material that directs all cellular activities from protein synthesis to cell division the nucleus is enclosed by a double membrane structure known as the nuclear envelope also called the nuclear membrane this envelope separates the nuclear contents from the cytoplasm and it helps maintain the internal environment of the nucleus embedded in this envelope are structures called nuclear pores these pores are not just openings they are highly regulated channels that allow selective two-way exchange of materials between the cytoplasm and the nucleus so for example messenger rna or mrna it needs to exit the nucleus through these pores to reach the ribosomes while certain proteins and enzymes have to be imported imported into the nucleus to support dna replication or transcription now inside the nucleus the genetic material is organized as dna but this dna is not floating around loosely instead it's wrapped around proteins called histones together the dna and histone proteins form a complex called chromatin chromatin can be loosely packed in a form called uk chromatin which is accessible and transcriptionally active or it can be more densely packed as heterocchromatin which is generally less active when a cell prepares to divide the chromatin is going to condense into visible structures called chromosomes each chromosome is made of a single continuous molecule of dna tightly wound and organized for efficient separation during cell division and we're going to cover that in a lot more detail in chapter 2 within the nucleus there is also a dense membraneless region called the nucleololis this is not involved in storing dna but rather in producing ribosomal rna or rrna this type of rna is a key structural and functional component of the ribosome and the nucleololis assembles our rna with ribosomal proteins to form the subunits of ribosomes which are then exported to the cytoplasm where they become active in protein synthesis finally surrounding the dna and the nucleololis is the nucleoplasm this is a viscous gel-like substance that supports nuclear structures and it allows molecules like nucleotides and enzymes to diffuse throughout the nucleus so altogether the nucleus integrates genetic storage regulation and ribosome production into one well organized structure that makes it really essential for the proper functioning and survival of ukareotic cells next we want to cover mitochondria these organels are best known as the powerhouses of the cell and that's because they're the primary site of atp production but beyond energy generation mitochondria also have several unique structural and genetic features that make them especially important to understand mitochondria are enclosed by two membranes you have the outer membrane which separates the mitochondrian from the cytool and it acts as a boundary between the cell and the internal mitochondrial environment it's relatively porous and it allows small molecules and ions to pass freely into the space between the membranes this region is called the intermembrane space the inner membrane in contrast this is highly selective and much more specialized it is folded into numerous infoldings called christe these serve to increase the surface area available for energy producing reactions embedded in this inner membrane are the components of the electron transport chain this is the final step of aerobic respiration where atp is synthesized using the energy carried by electrons the folding of the christe provides more space for these reactions to occur efficiently enclosed by that inner membrane is the mitochondrial matrix the matrix contains enzymes of the citric acid cycle or kb cycle as well as mitochondrial dna and ribosomes this is where several metabolic reactions take place and it's also where mitochondrial replication and gene expression are coordinated what makes mitochondria especially interesting is that they are semi-aututonomous unlike most other organels mitochondria contain their own circular dna and ribosomes which allows them to transcribe and translate some of their own proteins they are able to replicate independently of the nucleus through a process called binary fision we're going to cover that in a later objective in this chapter because mitochondrial dna is separate from nuclear dna it is inherited through a process known as cytoplasmic or extrauclear inheritance in most organisms mitochondrial dna is passed down exclusively through the maternal line and that's because during fertilization the mitochondria from the sperm are typically not retained in the embryo now this unique feature of mitochondrial genetics is part of the evidence supporting the endo symbiotic theory and according to this theory mitochondria are thought to have originated when ancestral ukareotic cells engulfed an aerobic proarot and rather than being digested the two cells formed a mutually beneficial relationship they were friends with benefits and then over time the engulf proariot evolved into the mitochondria we see in modern cells today so those friends with benefits finally got married so in addition to being the center for energy metabolism mitochondria also offers some really important insight into the evolutionary history of ukareotic life now let's move on to the endopplasmic reticulum or er the er is a continuous membrane system that plays a central role in the synthesis folding modification and transport of cellular materials now structurally it consists of a network of interconnected membranebound sacks and tubules and it's actually physically connected to the outer membrane of the nuclear envelope now this continuity allows for direct movement of materials between the nucleus and the er there are two types of endopplasmic reticulum and each one has a distinct structure and function we have the rough er and we have the smooth er the rough er it gets its name from the ribosomes that are attached to the surface these ribosomes they're not permanent fixtures they bind when the cell is actively translating proteins that are meant to be secreted inserted into the membrane or delivered to organels and as proteins are synthesized on these ribosomes they're threaded directly into the lumen of the rough er where they undergo folding and some post-transational modifications in this way the rough er serves as a major hub or site for protein synthesis especially for proteins that are destined to leave the cell or be embedded in membranes now in contrast to that the smooth endopplasmic reticulum it lacks ribosomes as you can tell and there's no ribosomes on the surface it's therefore not involved in protein synthesis instead it serves other functions it's involved in lipid synthesis including the production of things like phospholipids and steroids it also plays a role in detoxifying drugs and poisons especially in liver cells and in certain cells it can help regulate calcium ion storage and release now while the smooth er does not synthesize proteins itself it does play a key role in transporting proteins that were synthesized by the rough er and these proteins are packaged and directed towards their next destination which you guessed it is the gg apparatus the golgi apparatus is a critical component of the cell's transport and modification system structurally the golgi apparatus consists of a series of flattened membranebound sacks called sisterine these sacks are stacked together and each one has a distinct role in processing cellular products the golgi also has a defined polarity the cyphace is the one that receives incoming vesicles and the transphase is where modified products are sent out vesicles from the smooth endopplasmic reticulum they fuse with the cyspace and they release their contents into the lumen of the golgi apparatus for modification inside the golgi proteins and lipids can undergo various modifications this includes glycoilation where carbohydrate groups are added to proteins as well as things like phosphorilation and sulfation which can affect how the molecules function and where they're targeted now once they're modified the products are repackaged into new vesicles that bud off from that transface of the gene these vesicles are going to be directed to specific destinations within the cell so some may be sent into the loss or to the plasma membrane while others are just going to be transported out of the cell entirely if a protein is destined for secretion the vesicle carrying it is going to merge with the plasma membrane in a process known as exocytosis that releases those contents into the extracellular space with the er and the ggi working together the cell is really able to efficiently synthesize modify sort and deliver a wide range of biomolecules now that we've covered how molecules are transported and processed within and inside the cell let's shift our focus to another key system that supports cellular structure and movement and that's going to be the cytokeleton the cytokeleton gives the cell its shape and its structural support but it doesn't just hold the cell together it also plays a very important role in things like transport and in movement and even cell division the cytokeleton it's made up of three main types of proteinbased filaments we have microfilaments microtubules and intermediate filaments each type has its own structure and function and we're going to go over each in detail we'll start with microfilaments these are the thinnest filaments in the cytokeleton and they're made of a protein called actin the actin molecules are polymerized into solid rods which are usually arranged in tight bundles just beneath the cell membrane these filaments help the cell keep its shape by resisting external pressure or compression now microfilaments aren't just static they're dynamic and they're involved in movement when they work together with a motor protein called measin they can use atp to generate force for movement and this actin meosin interaction is how muscle cells contract but it also plays a role in things like cell crawling or changing shape another important function of microfilaments is during cell division in the final step called cytochinesis actin filaments form a ring around the middle of the dividing cell and this ring tightens and eventually pinches the cell into two new daughter cells next up is microtubules these are much larger than microfilaments and they're shaped like hollow tubes they're made from polymerized subunits of a protein called tubulant in microtubules they stretch throughout the cell and they kind of act like internal highways for transporting materials specialized motor proteins like kinosin and dinine they walk along these microtubial tracks and they carry vesicles and other cargo to where they're needed microtubules are also responsible for forming motile structures like psyia and flagagula these structures they both use the same internal framework a 9 +2 arrangement of microtubules so that means there are nine pairs of microtubules arranged in a ring with two additional microtubules in the center now psyia these are short projections that move fluids or particles along the surface of the cell so for example the respiratory tract it's lined with psyia to move mucus upward flagella in in contrast they're longer and they're used to move the cell itself so sperm cells are a good example of that during cell division microtubules can also organize into the mitoic spindle which is what separates the chromosomes so this organization starts in a region called the centrosome which contains a pair of structures called centrialsles and what happens here is the following each centriol it's made of nine triplets of microtubules arranged in a ring the centrialsles help organize the spindle fibers which then attach to chromosomes at specific sites called kineticores and this connection is essential for making sure each daughter cell gets the right number of chromosomes now this is a very quick explanation we're going to go into details about this in chapter 2 but keep in mind that microtubules are important for this process they're important for cell division finally we have intermediate filaments these filaments are a bit more stable and they're less dynamic than the other two types their main job is really just to provide mechanical strength they help the cell hold its shape they help the cell resist stress and keep internal structures in place they're also really important for anchoring organels and m maintaining connections between neighboring cells especially in tissues that experience mechanical strain what's really interesting about intermediate filaments is that their composition varies depending on the cell type and because of this variability intermediate filaments are often used as molecular markers to identify different tissue types and that brings us to the final topic for objective two which is tissue formation one of the defining features of ukareotic cells is their ability to organize into complex tissues this allows for division of labor where different cells within a tissue take on specialized roles so instead of every cell doing everything tasks are distributed and that makes multisellular organisms more efficient and more adaptable now there are four major types of tissue in the human body epithelial tissue connective tissue muscle tissue and nervous tissue and each plays a distinct role muscle and nervous tissues are going to be covered in later chapters so today we're just going to focus on epithelial and connective epithelial tissues cover the body and they line internal cavities the epithelial cells are tightly packed together and they rest on a thin supporting layer called the basement membrane this basement membrane provides structural support and it acts as a barrier between the epithelium and underlying tissues in many organs epithelial cells form the parnea this is the functional part of the organ the part that actually carries out its main physiological role so for example in the kidney the epithelial cells of the nephron are responsible for filtering blood and forming urine epithelia can also be classified based on both the number of layers and the shape of the cells if the tissue has only one layer of cells it's referred to as simple epithelium if it has multiple layers it's referred to as stratified epithelium there's also a special case called pseudoratified this type of epithelium appears to be layered when viewed under the microscope but actually all of the cells are in contact with the basement membrane so it's technically only one layer then in terms of shape epithelial cells can be cubuidal which is cube shaped they can be um columnar which are tall and more rectangular or they can be squamus which is flat and scale-like these structural differences influence how the epithelium functions so for example squamus epithelium is ideal for diffusion while columnar epithelium is more specialized for absorption and secretion now let's turn to connective tissue unlike epithelial tissue which forms coverings and linings connective tissue provides support and structure its primary role is to form the stroma which is the supportive framework that holds organs together and provides a place for other tissue types to anchor most connective tissue cells produce and secrete proteins like collagen and elastin and these proteins make up the extracellular matrix this is a network of fibers and ground substance that gives connective tissue its strength and its elasticity the extracellular matrix is what allows tendons to resist tension it's what allows skin to remain flexible and it's what allows cartilage to cushion joints so together epithelial and connective tissues work in tandem epithelial cells carry out essential functions while connective tissues provide the support and the infrastructure needed to keep those functions stable and understanding how these tissues are built and how they interact lays the groundwork for studying organ systems and diseases later in this playlist for now we're going to go ahead and wrap up the first part of this chapter in the next video we'll continue and finish chapter 1 please let me know if you have any questions comments concerns down below other than that good luck happy studying and have a beautiful beautiful day future doctors