University of Balamand BIOL 205 Principles of Human Biology PDF

University of Balamand Faculty of Health Sciences BIOL 205– Principles of Human Biology CHAPTER 3 – ORGANIZATION OF THE CELL Dr. Espérance Debs  The cell theory 2  According to the cell theory: 1. All living things are composed of one or...

University of Balamand Faculty of Health Sciences BIOL 205– Principles of Human Biology CHAPTER 3 – ORGANIZATION OF THE CELL Dr. Espérance Debs  The cell theory 2  According to the cell theory: 1. All living things are composed of one or more cells 2. Cells are the basic living units of organization and function in all organisms 3. All cells come from preexisting cells Organization of the cell  Homeostasis 3  Cells first described in 1665 by Robert Hooke  Cells maintain an internal environment that is appropriate and supportive to life, known as homeostasis  Cells work continuously to overcome changes in pH, temperature, and other environmental changes  In order to maintain homeostasis, living cells are enclosed structurally by a special membrane known as the plasma membrane  The plasma membrane is a selective barrier between the cell and its environment  Cells have internal structures, called organelles, each with a specific function Organization of the cell  Cells vary in size 4 Organization of the cell  Cell size 5  Cell size may be variable but is the same for most cells  Cells are in the order of microns (10-100 millionth of a meter)  Cells are small for good reasons: 1. Plasma membrane must be large enough relative to cell volume to keep up with its demands: surface/volume ratio 2. Molecules converted must travel a distance, made short, to be converted into other forms Organization of the cell  Cell surface are-to-volume ratio 6 Organization of the cell  Function-dependent size 7  Sizes and shapes of cells are related to function:  WBC can change their shape as they pass through the capillaries  Sperm cells have long tails known as flagella for locomotion  Nerve cells possess long, thin extensions to conduct electrical messages  Epithelial cells are almost rectangular in shape  Cells could be seen with different types of microscopes Organization of the cell  Prokaryotic cells 8  Only bacteria and archaea-bacteria are prokaryotic  Prokaryotes lack internal compartments  No nucleus; DNA without membrane- bound area known as nucleoid  No membrane-bound organelles  Prokaryotes have cell walls  They have ribosomes smaller than those of eukaryotes  They have storage granules Organization of the cell  Eukaryotic cells : Plant cell 9 Cristae Cell wall Plasma membrane Membranous sacs Vacuole Mitochondrion Plants have a cell wall, large Golgi complex vacuoles for storage, chloroplasts for photosynthesis Organization of the cell  Eukaryotic cells : Plant cell 10 Nuclear Cell wall envelope Plasma membrane Nucleolus Granum Stroma Nuclear pores Chromatin Vacuole Nucleus Smooth ER Rough ER Ribosomes Rough and smooth endoplasmic reticulum (ER) Organization of the cell Chloroplast  Eukaryotic cells : Animal cell 11 Eukaryotic cells have Plasma membrane specialized compartments Nuclear They have a nucleus and envelope membrane-bound Nucleolus organelles Lysosome Nuclear envelope Chromatin Membranous sacs of Golgi Nuclear pores Golgi complex Organization of the cell Nucleus  Eukaryotic cells : Animal cell 12 Cristae Lysosome Plasma membrane Nuclear envelope Rough ER Ribosomes Mitochondrion Centrioles Smooth ER Rough and smooth Organization of the cell endoplasmic reticulum (ER)  Cell organization 13  Structure of an animal cell Organization of the cell  The cellular components 14 1. Nucleus 2. Ribosomes 3. Endoplasmic Reticulum 4. Golgi Complex 5. Lysosomes & Vacuoles 6. Peroxisomes 7. Mitochondria & Chloroplasts 8. Cytoskeleton: microtubules & filaments 9. Cell Coverings Organization of the cell  1 – Nucleus 15  The nucleus is the most prominent organelle in the cell occupying a fixed position at the center  The nucleus is only present in eukaryotic cells, and is the location of genetic material DNA (chromosomes)  The nuclear envelope consists of 2 concentric membranes 20-40 nm separation  Passage of certain material may be regulated through nuclear pores  The nucleolus is an area of the nucleus (usually 2 nucleoli per nucleus not membrane bound) where ribosomal RNA is made Organization of the cell  The Nucleus structure 16  The nucleus controls protein synthesis  The nucleus contains the chromatin which constitutes chromosomes Organization of the cell  Nuclear pores 17  Numerous pores occur in the envelope, allowing RNA and other molecules to pass  DNA cannot pass Organization of the cell  2- Ribosomes 18  Ribosomes are the sites of protein synthesis  They are found in both prokaryotes and eukaryotes  Eukaryotic ribosomes are slightly larger than prokaryotic ones  Structurally, the ribosome consists of a small and a larger subunit  Biochemically, each subunit consists of 3 ribosomal RNA strands (rRNA) and 75 structural proteins  In eukaryotes, ribosomes cluster on the endoplasmic reticulum Organization of the cell  Ribosomes 19  Structure of ribosomes: They contain the enzyme necessary to form a peptide bond Organization of the cell  3- Endoplasmic Reticulum 20  The endoplasmic reticulum (ER) is a maze of interconnected membranes that encircle the nucleus and extend into many regions of the cytoplasm  The RER plays a central role in synthesis and assembly of proteins and their transport  ER membranes consist of a series of tightly packed saclike structures that form interconnected compartments in the cytoplasm  The internal space enclosed by the membranes forms a single continuous compartment called the ER lumen  The ER membranes on both surfaces, and the lumen, contain a large variety of enzymes involved in different chemical reactions Organization of the cell  Endoplasmic Reticulum 21  Two distinct regions of the ER exist: SER & RER  The two regions are connected and their internal spaces are continuous, but their functions are different  Rough endoplasmic reticulum (RER) is so-named because of its rough appearance due to the numerous ribosomes that occur along it  RER connects to the nuclear envelope through which mRNA is translocated to the ribosomes where it is translated into proteins.  mRNA >> proteins (ribosomes) = translation Organization of the cell  Endoplasmic Reticulum 22  Molecular Chaperones in the ER lumen mediate folding of proteins into proper conformations  The final protein product buds out of the ER membrane in small transport vesicles and is transported to other compartments  Smooth ER (SER) is more tubular but lacks the ribosomes characteristic of RER  The SER is the primary site of phospholipids, steroids and fatty acids metabolism (SER involved in lipid metabolism)  SER is a site for enzymatic detoxification Organization of the cell  Endoplasmic Reticulum 23 RER SER Organization of the cell  4 – Golgi Complex 24  Golgi complex consists of flattened stacks of membrane- bound sacs called cisternae  Each of the flattened sacs has an internal space or lumen  Unlike the ER, Golgi complex has most cisternae that constitute separate compartments  It functions as a packaging plant, processing, sorting, and modifying proteins that arrive in vesicles from the RER  Golgi complex may be located in one area of the cell or is dispersed in a number of locations Organization of the cell  Golgi complex 25  Each Golgi stack has 3 distinct areas: the cis and the trans faces, and the medial region  Thecis face is located nearest the nucleus to receive materials from transport vesicles coming from ER  Thetrans face, nearest to the plasma membrane, packages molecules in vesicles towards the outside  Golgi complex is an organelle specialized in modifying proteins in general, and synthesize glycoproteins and some polysaccharides Organization of the cell  Golgi complex 26 Golgi complex synthesize some Golgi has 3 areas: cis, medial, and trans polysaccharide for plant cell wall Organization of the cell  Golgi 27 complex Organization of the cell  Flow of material direction 28 Protein synthesized on ribosome  ER lumen  transport vesicle takes it to cis Golgi  Modification in Golgi  Vesicle transport from trans Golgi to plasma membrane  content released Organization of the cell  5 - Lysosomes 29  Lysosomes are small sacs dispersed in the cytoplasm of most eukaryotic cells  They contain about 40 different hydrolytic or digestive enzymes that could destroy the cell  These enzymes are synthesized in the ER, and are mostly active under acidic conditions (pH 5)  Lysosomal contents function in the breakdown of complex molecules in bacteria and debris that scavenger cells ingest Organization of the cell  Lysosomes 30 Primary lysosomes  They are formed by budding from Golgi complex  Containing hydrolytic enzymes synthesized in ER then modified with a sugar to be sorted toward lysosomes Secondary lysosomes  Consist of a primary lysosome that has fused with a vesicle containing a foreign substance  Foreign particles such as bacterial debris come in contact with the hydrolytic enzymes and are degraded  Lysosomes may breakdown organelles for recycling Organization of the cell  Lysosomes 31 Secondary lysosome Primary lysosome 5µm Organization of the cell  Lysosomes 32  In certain genetic diseases, known as Lysosomal Storage Disease, one hydrolytic enzyme is absent leading to accumulation of substrate  Tay-Sachs: accumulation of lipids in the brain leads to retardation & death Lysosomes budding from Golgi Organization of the cell  5 - Vacuoles 33  Vacuoles are large single-membrane organelles that are part of the plant cell  They look like a big vesicle and perform functions carried out by lysosomes in animal cells  The single membrane surrounding them, known as a tonoplast, is part of the endomembrane system.  Many organisms will use vacuoles as storage  Vacuoles play an essential role in plant growth and development and may join to make a central vacuole  A plant cell may increase in size by adding water to the central vacuole Organization of the cell  Functions of vacuoles 34 1. Central vacuoles in plant cells have various functions including disposing of toxic metabolic waste products & breakdown of unneeded organelles & maintain turgor pressure 2. Food vacuoles in some animal cells and single-celled protozoa fuse with lysosomes for digestion 3. Contractile vacuoles in protozoa to Plant Cell: remove excess water and maintain Central Vacuole turgor pressure Organization of the cell  Functions of vacuoles 35 Food vacuoles containing diatoms 15µm Organization of the cell  6- Peroxisomes 36  Peroxisomes are membrane-bound organelles that contain families of enzymes involved in detoxification  They may resemble a lysosome, however, they are not formed in the Golgi complex  Components accumulate at a given site and they can be assembled into a peroxisome  They are self replicating, like the mitochondria Organization of the cell  Peroxisomes 37  Usually a hydrogen is transferred from various compounds to oxygen and a byproduct which is Hydrogen Peroxide H2O2 is formed  Excess H2O2 is toxic, detoxified by a peroxisome enzyme known as catalase, and peroxidase  Peroxisomes detoxify a number of compounds including ethanol  In plants specialized peroxisomes are called glyoxysomes that converts fatty acids to sugar  Human cells are unable to do such conversion Organization of the cell  Peroxisomes 38  Peroxisomes are found in large numbers in cells that synthesize, store, or degrade lipids  In humans they are found in large amounts in liver and nerve cells  They are distinguished from lysosomes by having a crystalline look Organization of the cell  7- Mitochondria and Chloroplasts 39  Convert energy: chemical energy is most commonly stored in ATP (Adenosine Triphosphate)  Mitochondria & chloroplasts can split to grow and reproduce  They contain their own circular DNA and ribosomes similar to prokaryotes  Provide support for the Endosymbiont Theory  Endosymbiont Theory: mitochondria and chloroplasts evolved from prokaryotes that took residence in larger cells and lost autonomy Organization of the cell  Mitochondria and Chloroplasts 40 Aerobic respiration Photosynthesis Mitochondria (most eukaryotic cells) Chloroplasts (some plant and algal cells) Light CO2 CO2 Glucose O2 ATP ATP O2 Glucose H2O H2O Organization of the cell  Mitochondria 41  All eukaryotes have mitochondria  Mitochondrial division is remarkably similar to the prokaryotic methods  Mitochondria function as the site of aerobic respiration and ATP formation  The mitochondrion has been termed the powerhouse of the cell since it transforms food molecules into energy  They vary in size and are numerous in cells of high energy requirement (1000 mitochondria in liver cells)  Mitochondria are bounded by two membranes Organization of the cell  Mitochondria 42  The Inner Membrane folds into a series of cristae, allowing more surface on which ATP is generated  The Outer Membrane is less selective than the inner membrane; it allows many small molecules to pass into it  The space between the 2 membranes is called the intermembrane space Organization of the cell  Mitochondria 43  The inside of the mitochondria is the matrix which contains a lot of digestive enzymes  The mitochondrial matrix contains several of the enzymes necessary for cell respiration as well as ribosomes and DNA  Mitochondrial DNA has been associated with a number of genetic diseases including blindness, muscle degeneration, and aging Organization of the cell  Mitochondria 44  Mitochondria may leak electrons forming Free Radicals (acting as oxidants) that cause damage (e.g. mutation and aging) to the cell  Mitochondria play an important role in programmed cell death known as apoptosis (self destruction)  Apoptosis is initiated by mitochondria in several ways such as interference with energy metabolism or activation of proteolytic enzymes  When a mitochondrion is damaged, Cytochrome c is released and will trigger Caspases to initiate apoptosis  Failing to trigger apoptosis in cases of heavy damage may lead to diseases such as cancer, AIDS, Alzheimer’s. Organization of the cell  Mitochondria 45 Outer Inner mitochondrial mitochondrial membrane membrane Matrix Cristae 0.25µm Organization of the cell  Chloroplasts 46  Chloroplasts are also membrane-bound organelles that only occur in plants and photosynthetic eukaryotes and prokaryotes  Chloroplasts are the sites of photosynthesis  They are responsible for converting light energy into chemical bond energy of carbohydrates.  Chloroplasts are enclosed by an outer membrane and an inner membrane that folds inward to form disc-like sacs known as thylakoids.  Thylakoid membranes enclose the thylakoid lumen.  Sets of thylakoids are arranged in stacks called grana. Organization of the cell  Chloroplasts 47  Thylakoids contain chlorophyll, the green pigment that traps light energy necessary for photosynthesis to occur.  Other associated accessory pigments include carotenoids.  The inner membrane encloses a fluid-filled space termed the stroma.  Within the stroma, is an energy-dependent series of reactions where carbon dioxide, in the presence of sunlight, is converted into glucose and ATP.  Like mitochondria, chloroplasts contain their own ribosomes and DNA Organization of the cell  Chloroplasts 48 Granum Stroma 1µm Outer Inner Thylakoid Thylakoid membrane membrane lumen membrane Organization of the cell  Plastids 49  Chloroplasts belong to a group of organelles known as Plastids  All plastids develop from Proplastids, a versatile precursor organelle found in less specialized plant cells  Chloroplasts are produced when proplastids are stimulated with light  Leukoplasts include amyloplasts which store starch  Chromoplasts store pigments associated with the bright colors of flowers and/or fruits. They attract animals that serve as pollinators Organization of the cell  8 – The Cytoskeleton 50  The cytoskeleton is a dense network of protein fibers  It gives the cell mechanical strength, shape, and motility  It is highly dynamic  It is made of 3 types of protein filaments: a. Microtubules b. Microfilaments c. Intermediate filaments Organization of the cell  a. Microtubules 51  Microtubules are the thickest filaments of the cytoskeleton (25 nm diameter)  They are hollow, unbranched tubes that plays a role in the movement of chromosomes and of other substances  They are the major structural components of cilia and flagella  They consist of dimers of 2 similar proteins: alpha and beta tubulin  A microtubule elongates by the addition of tubulin dimers  Microtubules have polarity and its ends are referred to as Plus & Minus  Plus end elongates more rapidly Organization of the cell  Microtubules 52 α-Tubulin Dimer Plus end β-Tubulin Dimer on Microtubule in green Minus end Dimers off Organization of the cell  Microtubule-associated proteins (MAPs) 53  MAPs are classified into 2 groups: 1. Structural MAPs that help in assembly of microtubules 2. Motor MAPs that use ATP energy to produce movement  Scientists have found that organelles may be moved around the cell by attaching to microtubules  Kinesin, a motor MAP, moves toward the plus end of a microtubule  Dynein & Dynactin, motor MAPs, transport organelles in opposite direction toward the minus end: known as Retrograde Transport Organization of the cell  Microtubule-associated proteins (MAPs) 54 Vesicle Kinesin receptor Kinesin ATP ATP Kinesin attaches to vesicle thru receptor and “walks” along microtubules Organization of the cell  Microtubules 55  In non-dividing cells, the minus part seems to be anchored in regions called Microtubules-Organizing Centers (MTOCs)  In animal cells, the main MTOC is the cell center or centrosome  Each centrosome consists of 2 structures called centrioles oriented at right angles to each other  Each centriole is made of a 9 X 3 microtubular structure forming a hollow cylinder. These duplicate before cell division  Most Plant cells have an MTOC but no centrioles  Many of the tubulin subunits organize into a structure called the mitotic spindle, which serves as a framework for distribution of chromosomes during cell division Organization of the cell  Microtubules 56 MTOC 0.25µm Centrioles Organization of the cell  Cilia and Flagella 57  Cilia and Flagella are motile organelles consisting of microtubules that allow the cell to move in liquid media  Flagella are fewer and longer  Cilia are numerous and shorter  Cilium & Flagellum are anchored in the cell by a basal body (9 x3 structure) and covered by an extension of the plasma membrane  Both cilia and flagella have similar structure consisting of 9 pairs surrounding 2 unpaired microtubules known as 9 + 2 arrangement  The movement takes place when these microtubules slide past each other mediated by a Dynein, translated into a bending motion Organization of the cell  Cilia and Flagella 58 Dynein Outer ATP microtubules Plasma membrane Inner microtubules 9 + 2 arrangement cilia 3 cilia Organization of the cell  b. Microfilaments 59  Microfilaments (7 nm diameter) consist of 2 chains of Actin polymers forming a double helix  Microfilaments form bundles of fibers to provide mechanical support for cell components  In muscle cells, the interaction of actin fibers with the protein Myosin is the primary cause of contraction  The assembly and disassembly of actin monomers in association with myosin is used in non-muscle cells for changing shape of cell or increasing surface area (microvilli) Organization of the cell  Microfilaments 60 Actin polymer Microfilaments in green 100µm Organization of the cell  c. Intermediate filaments 61  They are stable, tough flexible fibers made of polypeptides that can vary in size and composition  They help strengthen the cytoskeleton and stabilize cell shape.  Certain genetic mutations in genes coding for these filaments may cause neurodegenerative diseases such as Amyotrophic Lateral Sclerosis or Lou Gehrig’s in nerves that control muscles Protofilament Protein subunits Intermediate filament Organization of the cell  9- Cell Coverings 62  Most eukaryotic cells are surrounded by a glycocalyx, or cell coat, formed by polysaccharide side chains of proteins and lipids that are part of the plasma membrane  Glycocalyx is important in protection, keeping distance, and recognition  Many animal cells are also surrounded by Extracellular Matrix (ECM) secreted by the cell  ECM is composed of fibrous proteins such as collagen and carbohydrates  Certain glycoproteins of ECM such as fibronectins help organize the matrix and cell attachment to it  Integrins are proteins that serve as membrane receptors for ECM; involved in cellular signaling Organization of the cell  Extracellular Matrix ECM 63 Collagen Fibronectins Extracellular matrix Integrin Microfilaments Cytosol Organization of the cell  Plant Cell Wall 64  Most bacteria, fungi, and plant cells are surrounded by a cell wall (not animal cells)  Plant cells secrete cellulose and other polysaccharides that form rigid cell walls  A growing plant cell usually secretes a thin flexible primary cell wall, then after the cell stops growing a thick solid secondary cell wall (wood)  Between primary walls of adjacent cell is glue-like layer of polysaccharides called pectins, known as the middle lamella  Middle lamella make cells adhere to each other Organization of the cell  Plant Cell Wall 65 Cell 1 Middle lamella Primary cell wall Cell 2 Multiple layers of secondary cell wall Organization of the cell