Lecture 4: Cell Structure and Function (A Tour of the Cell)

Lecture 4: Cell Structure and Function (A Tour of the Cell) The Fundamental Units of Life (cells) • All organisms are made of cells. • Cells are the simplest collection of matter that can be considered to be alive. • In multicellular organisms, the cell forms the basic unit of structure and function of tissues and organs. • All cells arise from pre-existing cells: they grow, divide to give rise to new cells and die. Microscopy •Microscopes are used to visualize cells. •In a light microscope (LM), visible light is passed through a specimen and then through glass lenses. –The lenses refract (bends) the light which magnifies the image of the specimen. –The image is then projected into the eye or camera. •The quality of an image depends on: Magnification, The ratio of an object’s image size to its real size. Resolution Contrast The measure of the clarity of the image, the minimum distance two points can be separated, and still can be distinguished. visible differences in brightness between parts of the sample. •LMs can magnify effectively to about 1,000 times the size of the actual specimen. •Various techniques enhance contrast and enable cell components to be stained or labeled to be seen. •Most subcellular structures, including organelles (membrane-enclosed compartments), are too small to be resolved by an LM. •Recent advances in light microscopy: –Labeling individual cells with fluorescent markers improves the level of detail that can be seen. –Confocal microscopy and deconvolution microscopy provide sharper images of three-dimensional tissues and cells. Electron Microscope •Two basic types of electron microscopes (EMs): are used to study subcellular structures. •Scanning electron microscopes (SEMs): focus a beam of electrons onto the surface of a specimen, providing images that look 3-D. •Transmission electron microscopes (TEMs): focus a beam of electrons through a specimen. •TEMs are used mainly to study the internal structure of cells (i.e. organelles). There are only Two types of cells •The basic structural and functional unit of every organism is one of two types of cells: prokaryotic or eukaryotic. •Only organisms of the domains Bacteria and Archaea*consist of prokaryotic cells. •Protists, fungi, animals, and plants all consist of eukaryotic cells. prokaryotic cells Common features No nucleus Plasma membrane DNA in an unbound region called eukaryotic cells DNA in a nucleus that is bound by a membranous nuclear envelope Semifluid substance called cytosol Membrane-bound organelles the nucleoid No membrane-bound organelles Chromosomes (carry genes) Cytoplasm in the region between the plasma membrane and nucleus Cytoplasm bound by the plasma membrane Ribosomes (make proteins) larger than prokaryotic cells Difference between animal and plant cells •Presence of cell wall. •Large vacuoles. •Chloroplasts (photosynthesis). •Plasmodesmata versus intercellular junctions (cell signaling lecture). Plasma Membrane •The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell. •The general structure of a biological membrane is a double layer of phospholipids. The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes. •The nucleus contains most of the DNA in a eukaryotic cell (chromosomes/genes). •Ribosomes use the information from the DNA (mRNA) to make proteins. •The nuclear membrane is a double membrane; each membrane consists of a lipid bilayer. The Nucleus: Information Central •Pores regulate the entry and exit of molecules from the nucleus. •The shape of the nucleus is maintained by the nuclear lamina, which is composed of protein. •In the nucleus, DNA and proteins form genetic material called chromatin. •The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis. Ribosomes: Protein Factories •Ribosomes are particles made of ribosomal RNA and protein. •Ribosomes carry out protein synthesis in two locations: –In the cytosol (free ribosomes). –On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes). Lysosomes: Digestive Compartments •A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules. •Lysosomal enzymes work best in the acidic environment inside the lysosome. •Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids. Mitochondria and chloroplasts change energy from one form to another •Mitochondria are the sites of cellular respiration, a metabolic process that generates ATP. •Chloroplasts, found in plants and algae, are the sites of photosynthesis. •Mitochondria and chloroplasts –Have a double membrane. –Have proteins made by free ribosomes. –Contain their own DNA. Peroxisomes: Oxidation •Peroxisomes are oxidative organelles. •Peroxisomes are specialized metabolic compartments bounded by a single membrane. •Oxygen is used to break down different types of molecules. •Peroxisomes produce hydrogen peroxide (as a by producing of oxidation) and because it is toxic it is then converted to water. Functions of peroxisomes –Some use oxygen to break fatty acids into smaller molecules, eventually used for fuel for respiration. –In the liver, they detoxify alcohol and other harmful compounds. The Cytoskeleton: Support and Motility •The cytoskeleton is a network of fibers extending throughout the cytoplasm. •It helps to support the cell and maintain its shape. –It organizes the cell’s structures and activities, anchoring many organelles. •It interacts with motor proteins to allow cell motility. –Inside the cell, vesicles and other organelles can use motor protein “feet” to travel along tracks provided by the cytoskeleton. 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. 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. –The extracellular matrix (ECM) of animal cells. •These extracellular materials and structures are involved in many essential cellular functions. 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. •ECM proteins bind to receptor proteins in the plasma membrane called integrins. •Integrins: integrate signals between ECM (outside) and cytoskeleton (inside). The ECM has an influential role in the lives of cells •The ECM can regulate a cell’s behavior by communicating with a cell through integrins. •The ECM around a cell can influence the activity of genes in the nucleus. •Mechanical signaling may occur through cytoskeletal changes that trigger chemical signals in the cell. Thank you..