# Cell Structure and Functions, Animation

https://www.youtube.com/watch?v=eK-NCfvTtIE

[00:01] All living organisms are composed of cells.
[00:14] Cells are responsible for all anatomical and physiological features of all body systems.
[00:19] Different cell types can vary greatly in shape and size but they all have a common structure and similar components.
[00:25] A typical cell is enclosed in a plasma membrane and contains a nucleus and a cytoplasm.
[00:32] The plasma membrane serves as the cell’s boundary, controlling the traffic of substances in and out of the cell.
[00:37] It is also the site of communication between the cell and its environment.
[00:43] The membrane consists mainly of 2 layers of phospholipids, with their hydrophilic heads - the phosphate groups - facing the aqueous environments inside and outside the cell; and their hydrophobic fatty acid tails facing in together.
[00:59] Other membrane lipids include: cholesterol, which is essential to membrane structure and fluidity;
[01:04] and glycolipids, which maintain membrane stability and facilitate cell to cell interactions.
[01:11] The lipid membrane is dotted with membrane proteins, of which there are 2 types:
[01:16] - integral, or transmembrane, proteins, which span across the membrane, some passing through multiple times.
[01:22] Some transmembrane proteins have a small carbohydrate chain on the outside of the cell.
[01:28] - and peripheral proteins, which attach to the membrane on the inside.
[01:34] A peripheral protein typically functions together with an integral protein.
[01:39] Membrane proteins fulfill a variety of functions: - As receptors (or receptor-associated proteins), they receive messages from outside the cell.
[01:45] For example, a non-steroid hormone must bind to a membrane receptor and act via several other membrane proteins to activate a cellular response.
[01:58] Each receptor is specific to a certain molecule.
[01:58] - As ion channels or transport proteins, they help
[02:05] Move charged particles and large uncharged polar molecules across the cell membrane.
[02:11] As adhesion molecules, they help cells adhere to each other and to the extracellular matrix.
[02:17] As enzymes, they catalyze reactions that are required outside the cell but in the vicinity of the cell membrane.
[02:29] Transmembrane glycoproteins also serve as surface antigens, determining the cell’s identity.
[02:36] On top of the cell membrane, some cells have surface extensions that carry out specialized functions.
[02:42] Examples include microvilli that increase surface area in the small intestine, cilia that move mucus in the respiratory tract, and flagella that are responsible for the movements of sperm cells.
[02:53] The nucleus contains genetic material, the DNA, and is where DNA replication, and transcription - the major step of gene expression - take place.
[02:59] Most cells have 1 nucleus, with the
[03:05] Exception of red blood cells which have none, and some other cells that have multiple nuclei.
[03:12] The nuclear envelope surrounding the nucleus consists of 2 membranes: inner and outer, each of which is a phospholipid bilayer.
[03:18] The envelope is dotted with nuclear pores - protein complexes that provide controlled passage between the nucleus and cytoplasm.
[03:29] Chromosomes are strands of DNA wrapped around proteins.
[03:35] Under a light microscope, chromosomes are only visible during cell divisions, when they are highly condensed.
[03:40] Instead, the most prominent feature of the nucleus is the nucleolus - the area around the clusters of ribosomal RNA genes.
[03:47] This is where ribosomal RNAs are made and ribosomes are assembled.
[03:55] Ribosomes then move to the cytoplasm to fulfill their function in protein synthesis.
[04:01] The cytoplasm includes a gel-like liquid called cytosol, various organelles and cytoskeleton.
[04:09] The endoplasmic reticulum (ER), Golgi apparatus, and vesicles constitute the intracellular membrane system.
[04:17] The ER is a network of connected flattened sacs called cisternae.
[04:24] Its membrane is continuous with the outer nuclear membrane.
[04:32] Part of the ER appears "rough" as it is covered with ribosomes.
[04:37] This is where the synthesis of secretory and transmembrane proteins takes place.
[04:42] These proteins have a signal sequence within their amino terminus, which, as soon as it emerges from the ribosome, targets the RNA-ribosome complex to the ER membrane where translation continues.
[04:50] The emerging polypeptide enters the ER membrane as it is being translated.
[04:57] Transmembrane proteins, identified by the presence of a hydrophobic stretch, stay in ER membrane, while secretory proteins are released into the ER lumen.
[05:02] The "smooth" part of the ER synthesizes lipids and
[05:10] Lipid components of cell membranes.
[05:10] As lipids are produced, they are inserted into the ER membrane.
[05:17] Membrane proteins, lipids, and secretory proteins are then packaged into vesicles to be transported to the Golgi, where proteins undergo post-translational modifications.
[05:29] Vesicles pinch off from ER membranes, travel to Golgi apparatus, fuse with Golgi membranes, and release their content.
[05:36] The Golgi is a stack of separated cisternae, each contains a set of enzymes responsible for a certain step in protein maturation.
[05:48] Similar vesicles transport lipids and proteins from one cisterna to another, and ultimately to their destinations - the plasma membrane, lysosomes, or storage vesicles.
[06:01] The destination of a protein is typically determined by a signal sequence, acting as an address tag, within the protein.
[06:07] The ER is also a major site for metabolism and storage of calcium, whose release...
[06:13] It is a trigger for many cellular processes.
[06:20] Lysosomes are vesicles containing hydrolases that break down macromolecules into their building units, which can then be recycled.
[06:25] The enzymes are activated by the acidic environment within lysosomes.
[06:31] In white blood cells, lysosomes digest phagocytized bacteria and play a role in immune response.
[06:38] Mitochondria are best known as the cell’s powerhouses.
[06:43] This is where energy is extracted from food compounds and stored in energy-rich molecules.
[06:49] A mitochondrion has 2 membranes.
[06:55] The inner membrane has multiple folds called cristae.
[06:59] Two of the three main steps of cellular respiration occur in the mitochondria: citric acid cycle in the matrix, and oxidative phosphorylation on the cristae.
[07:07] Cytoskeleton is a network of protein filaments that fulfill a variety of functions.
[07:12] There are 3 types of filaments: microfilaments, intermediate filaments, and microtubules.
[07:18] Microfilaments are made of the protein actin.
[07:24] They enable muscle contraction, provide support for microvilli, produce cell movements, and play a role in cell division.
[07:30] Intermediate filaments are made of different proteins in different cells.
[07:35] Their roles are mostly supportive.
[07:41] Microtubules are large tubes of 13 proto-filaments, each is a long chain of tubulin dimers.
[07:46] A centriole is a short cylinder of 9 triplets of microtubules.
[07:53] A cell typically has 2 centrioles lying perpendicular to each other forming a structure called centrosome.
[07:59] Centrosome serves as a microtubule organizing center, from which microtubules grow out into the cytoplasm.
[08:06] In non-dividing cells, microtubule network holds organelles in place.
[08:12] During cell division, they form the mitotic spindle that guides chromosome movements.
[08:17] Microtubules are also responsible for the movements of cilia and flagella.