A Tour of the Cell BIOL120- S2023 PDF

A TOUR OF THE CELL M.Sc. Vinicius Azevedo Cells Fundamental units of life Within the levels of biological organization, cells are the simplest collection of matter that is alive As stated in the cell theory… all organisms consist of cells all cells are derived from pre-existing cells Cells and Micro...

A TOUR OF THE CELL M.Sc. Vinicius Azevedo Cells Fundamental units of life Within the levels of biological organization, cells are the simplest collection of matter that is alive As stated in the cell theory… all organisms consist of cells all cells are derived from pre-existing cells Cells and Microscopy Most cells are microscopic Small To look at üMost cells can’t be seen with the naked eye. üScientists use microscopes to study cells Types of microscopes Ø Light Microscope ü1,000x magnification Ø Electron Microscope ü100,000x magnification Light Microscopy Conventional microscope üUses visible light (400-700 nm) üWavelengths between 400 and 700 nm üSamples are usually stained Gill of Poecilia vivipara (guppies) Fluorescence microscope üUses much higher intensity light source üLight excites (excitation wavelength) the fluorochrome (or fluorophore) in the sample, that then emits (emission wavelength) light captured by the microscope to build the image Neuron Electron Microscopy Transmission Microscopy (TEM) üUsed to see thin specimens (tissue sections, molecules, etc…) üElectrons pass through, generating the image. Plant Cell Scanning Microscopy (SEM) üUse whole samples dried and covered with gold. üProvides detailed images of surfaces of cells and whole organisms. Gill of Poecilia vivipara Types of Cells Prokaryotic Cell Domain Bacteria Kingdom Eubacteria Domain Archaea Kingdom Archaebacteria Eukaryotic Cell Domain Eukarya Kingdom Animalia (animals) Kingdom Plantae (plants) Kingdom Fungi (molds, mushrooms) Kingdom Protist (paramecium, algae) Cell’s Size Eukaryote cells are 10x larger than prokaryote cells Prokaryotic Cells No membrane-bound organelles ü Only some have membrane-bound organelles that perform specialized tasks like storing calcium ions and organizing enzymes for building organic compounds ü However, they do not have the typical membranebound organelles found in eukaryotic cells. No nucleus ü DNA is found in a non-membrane bound region called the nucleoid region DNA packaged as a single, circular chromosome Cytoplasm bound by the plasma membrane Binary Fission ü Type of asexual reproduction (cell division) ü DNA is copied and cells splits into two genetically identical cells Eukaryotic Cells Membrane-bound organelles ü Eukaryotic cells have membrane-bound organelles to perform specialized function such as cellular respiration, photosynthesis, etc… They have a nucleus ü DNA is found in a nucleus bounded by a membranous nuclear envelope DNA packaged as a multiple, linear chromosomes Cytoplasm in region between plasma membrane and nucleus Mitosis ü Typical of asexual reproduction (cell division) Prokaryotic Cells X Eukaryotic Cells The Nucleus House the genetic instructions The nucleus contains most of the DNA in a eukaryotic cell (a.k.a. “information central”) Nuclear envelope encloses nucleus, separating it from the cytoplasm. This envelope is a double membrane; each membrane consists of a lipid bilayer. Pores regulate entry and exit of molecules from nucleus. DNA and proteins form genetic material called chromatin. Nucleolus is located within nucleus and is site of ribosomal RNA (rRNA) synthesis along with the large and small ribosomal subunits. Attached to the inner nuclear membrane on the nucleoplasm side is the nuclear lamina. This is a sheet of proteins that provides support for and strengthens the nuclear envelope. The nuclear lamina also attaches to and anchors chromatin. Nucleus 1 µm Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Surface of nuclear envelope Rough ER Ribosome 1 µm 0.25 µm Close-up of nuclear envelope Pore complexes (TEM) Nuclear lamina (TEM) Ribosomes: protein factories Synthesis of protein Ribosomes are particles made of ribosomal RNA and protein Ribosomes are formed by two subunits; a large and a small. Uses the information from the DNA to make proteins during translation Carry out protein synthesis in two locations: – In the cytosol (free ribosomes) – On outside of endoplasmic reticulum or nuclear envelope (bound ribosomes) Endomembrane System Composed of different membranes suspended in the cytoplasm of eukaryotic cell Components : ü Nuclear envelope ü Endoplasmic reticulum (rough and smooth) ü Golgi apparatus ü Lysosomes ü Plasma membrane The components work together to modify, package and transport lipids and proteins. Endoplasmic reticulum biosynthetic factory The endoplasmic reticulum (ER) accounts for about 50% of the total membrane in many eukaryotic cells ER membrane is continuous with (or connected to) nuclear envelope There are two distinct regions of ER: ü Smooth ER, which lacks ribosomes ü Rough ER, which has ribosomes on its surface Functions of Smooth and Rough ER Mainly involved in the synthesis of lipids (i.e., phospholipids and steroids). Detoxification (enzymes are embedded in smooth ER to convert toxic organic, lipidsoluble chemicals to safer water-soluble products so that they can be easily excreted from the body). Stores calcium ions in muscle cells (these ions are found in the sarcoplasmic reticulum, a type of smooth ER). Has bound ribosomes, which are involved in production of proteins (so mainly involved in protein synthesis) Golgi apparatus Shipping and receiving center Consists of flattened membranous sacs (cisternae) Functions of the Golgi apparatus: ü Modifies, sorts and packages different macromolecules for cell secretion (exocytosis) or use within the cell ü Sorts and packages materials into transport vesicles Lysosomes Digestive compartments A lysosome is a membrane-bound organelle that contains hydrolytic enzymes involved in digesting proteins, fats, nucleic acids, and carbohydrates Note – lysosomes can also use these enzymes to break down the cell’s own organelles in order to be recycled (called autophagy) Hydrolytic enzymes are active at acidic pH (about 5) and maintained within lysosomes but not at neutral pH (about 7), seen with the rest of the cytoplasm. ü If the lysosomal membrane were to break down, the released hydrolytic enzymes would be inactive (denatured) at the neutral pH of the cytoplasm The acidic internal pH of lysosomes results from the action of a proton pump in the lysosomal membrane, which imports protons from the cytosol coupled to ATP hydrolysis. Mitochondria and Chloroplasts Mitochondria and chloroplasts change energy from one form to another Mitochondria are sites of cellular respiration, a metabolic process that generates ATP Chloroplasts, found in plants and algae, are the sites of photosynthesis Mitochondria and chloroplasts… ü are not part of endomembrane system ü have a double membrane ü have proteins made by free ribosomes ü contain their own DNA Mitochondria Major site of cellular respiration to generate ATP They have a smooth outer membrane and an inner membrane folded into cristae Internal fluid is called matrix Number of mitochondria in a cell can vary widely ü For example, human red blood cells have no mitochondria while liver cells can have more than 2000!! Chloroplasts Site of conversion of light energy to chemical energy Major site of photosynthesis Chloroplast structure includes: ü Thylakoids (membranous sacs) stacked to form a granum ü Stroma (internal fluid) Chloroplasts are found in leaves and other green organs of plants, as well as in algae – Chloroplasts are green because they contain the pigment chlorophyll in their thylakoids granum Peroxisomes Breakdown of fatty acid molecules and alcohol as well as protection of cells from reactive oxygen species Not part of the endomembrane system Has plasma membrane Contains a crystalline core, which contains enzymes Some enzymes are involved in the chemical breakdown (metabolism) of long-chain fatty acids and alcohol Peroxisomes in dotted circles Antioxidant Enzymes Some of these enzymes help protect the cell from harmful reactive oxygen species (ROS) molecules. ROSs are molecules created as a product of normal cellular metabolism, but also by radiation, tobacco, and drugs They cause what is known as oxidative stress in the cell by reacting with and damaging DNA and lipid-based molecules like cell membranes causing damage. Enzymes in peroxisomes that protect cells from ROSs are catalases, superoxide dismutases, and peroxidases. Cytoskeleton Network of fibres that organizes structures and activities in the cell The cytoskeleton is a network of fibres made up of protein subunits extending throughout the cytoplasm They make up the cellular skeleton (scaffolding) contained within a cell's cytoplasm Cytoskeleton is composed of three types of molecular structures: Microtubules Microfilaments (or actin filaments) Intermediate filaments Components of the cytoskeleton Three main types of fibers make up the cytoskeleton: Microtubules are the thickest of the three components of the cytoskeleton Microfilaments, also called actin filaments, are the thinnest components Intermediate filaments are fibers with diameters in a middle range Some key roles of the cytoskeleton Maintenance of cell shape – involves microtubules, microfilaments and intermediate filaments Movement of organelles in a cell – involves microtubules ü Organelles can travel along “monorails” provided by microtubules (interacts with motor proteins to generate movement) Anchorage of the nucleus and certain other organelles in a cell – intermediate filaments (also involved in the formation of the nuclear lamina) Cytoplasmic streaming – involves microfilaments ü Cytoplasmic streaming is a circular flow of cytoplasm within cells to allow for rapid distribution of materials within the cell Cell motility Involves microtubules and microfilaments Microtubules control the beating of cilia and flagella, which are locomotor appendages of some cells. Pseudopodia (literally means “false foot”) are temporary cytoplasmic extensions that are powered by microfilaments near the plasma membrane. Cell division – involves microtubules (for mitosis and meiosis) and microfilaments (for cytokinesis of animal cells) In cytokinesis, the cytoplasm of a parental cell is equally distributed into two daughter cells. Centrosome A microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other Each centriole is a cylinder of nine triplets of microtubules. Centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the centriole's exact function in cell division isn’t clear, because cells that have had the centrosome removed can still divide, and plant cells, which lack centrosomes, are capable of cell division. Extracellular components and connections between cells help coordinate cellular activities Most cells synthesize and secrete materials that are external to the plasma membrane These extracellular structures include: Cell walls of plants Extracellular matrix (ECM) of animal cells Cell walls of plants The cell wall is an extracellular structure that distinguishes plant cells from animal cells Prokaryotes, fungi, and some protists also have cell walls The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein The extracellular matrix (ECM) of animal cells Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM) The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin Glycoproteins are proteins with covalently bonded carbohydrates Collagen is the most abundant glycoprotein in the ECM of most animal cells (~40% of the total protein in the human body) ECM proteins bind to receptor proteins in the plasma membrane called integrins Some key functions of the ECM include Structural support to cell, cell movement, and cell-to-cell signalling. Plant Cells X Animal Cells 1. Plant cells have cell walls while animal cells do not (animal cells have an extracellular matrix) 2. Plant cells have chloroplasts while animals cells do not 3. Plant cells have a large central vacuole while animal cells have small vacuoles 4. Plant cells use starch as energy source while animal cells use glycogen 5. Most (some have) plant cells do not have lysosomes while animal cells have lysosomes 6. Plant cells do not have centrosome while animal cells have centrosome Plant Cells X Animal Cells 7. Plant cells are primarily autotrophic while animal cells are heterotrophic – Autotrophs are capable of producing their own food – Heterotrophs cannot make their own food, so they must eat or absorb it. 8. During cytokinesis, plant cells form a cell plate while animal cells form a cleavage furrow