Cell Structure and Function Biology Lecture Notes PDF

Cell Structure and function Biology Lecture 5 Prof. Abu Salim Mustafa PhD, FRCPath (UK) Department of Microbiology Faculty of Medicine Kuwait University Tel. 24636505 E-mail: [email protected] 1 Objectives To explain the structure and function of 1. 2. 3. 4. Cytoplasm The energy-related org...

Cell Structure and function Biology Lecture 5 Prof. Abu Salim Mustafa PhD, FRCPath (UK) Department of Microbiology Faculty of Medicine Kuwait University Tel. 24636505 E-mail: [email protected] 1 Objectives To explain the structure and function of 1. 2. 3. 4. Cytoplasm The energy-related organelles: Chloroplasts Mitochondria The cytoskeleton: Actin filaments Intermediate filaments Microtubules Cell wall • Reading: Lecture notes and Biology by Sylvia S. Mader 2 Cytoplasm/Cytosol Cell membrane • A semifluid (jelly-like) solution between the nucleus and cell membrane. • Cell organelles are suspended in the cytoplasm. • Contains water (80%) and inorganic (salts, ions) and organic molecules. • Many cellular reactions Cytoplasm occur in the cytoplasm, e.g. Protein synthesis, glycolysis, gluconeogenesis, etc. Nucleus 3 The Energy-related organelles O2 O2 Chloroplasts capture solar energy and convert CO2 and H2O to glucose and release O2 In mitochondria, glucose is oxidized to carbon dioxide, and oxygen is reduced to water. ATP is produced ATP= Adenosine triphosphate Functions of ATP in Cells • Chemical work: ATP supplies the energy needed to synthesize macromolecules (anabolism) for the cell. • Transport work: ATP supplies the energy needed to pump substances across the plasma membrane. • Mechanical work: ATP supplies the energy needed to permit muscles to contract, cilia and flagella to beat, chromosomes to move, etc. Chloroplast: structure A double membrane surrounds a chloroplast, and its semifluid interior is called the stroma. Thylakoid A different membrane system within the stroma forms flattened sacs called thylakoids, which are stacked to form grana. The space of each thylakoid is connected to the space of every other thylakoid within a chloroplast, forming an inner compartment within chloroplasts called the thylakoid space. The thylakoid membrane contains chlorophyll and other pigments that absorb the solar energy that drives photosynthesis. Chloroplast function: Photosynthesis NAD = Nicotinamide adenine dinucleotide phosphate Photosynthesis consists of light reactions and the Calvin cycle reactions. The light reactions occur on thylakoids and produce ATP and NADPH, which are used in the Calvin cycle reactions. The Calvin cycle reactions occur in the stroma and use carbon dioxide to synthesize carbohydrates. Nicotinamide adenine dinucleotide phosphate Photosynthesis Involves oxidation-reduction reactions. Carbon dioxide is reduced and water is oxidized. (CH2O) represents carbohydrates and it is the food produced by plants, algae, and cyanobacteria that feeds the biosphere. If the equation is multiplied by 6, the carbohydrate would be C6H12O6, or glucose. Mitochondria: Cell powerhouse, the site for energy production have two membranes, the outer membrane, and the inner membrane. The inner membrane encloses A semifluid matrix, which contains mitochondrial DNA and ribosomes. The inner membrane is highly convoluted into folds called cristae that project into the matrix. Function of mitochondria: Cellular respiration Mitochondrial Diseases Many genetic mitochondrial diseases that affect different body tissues have been identified. • The common factor among these genetic diseases is that the patient’s mitochondria are unable to completely metabolize organic molecules to produce ATP. • As a result, harmful toxins accumulate inside the mitochondria and the body and cause damage. Cytoskeleton: A network of protein fibers in the cytoplasm. Maintains cell shape and assists in the movement of cell parts. Microtubules: protein cylinders that move organelles Intermediate filaments: Protein fibers that provide shape stability Actin filaments: protein fibers that play a role in cell shape & division Centrioles: short cylinders of microtubules Centrosome: microtubule organizing center that contains a pair of centrioles Actin Filaments: formerly called microfilaments • Long, extremely thin, flexible fibers. • Each actin filament contains two chains of actin monomers twisted aaround one another. Actin filaments move the cell and its organelles by interacting with motor molecules (myosin), which are proteins that can attach, detach, and reattach farther along an actin filament. The motor molecule myosin uses ATP to pull actin filaments along in this way. Myosin has both a head and a tail. Actin Filaments: formerly called microfilaments • Actin filaments provide structural support as a dense, complex web just under the plasma membrane, to which they are anchored by special proteins. • Actin filaments can rearrange themselves and facilitate cellular movement, e.g. when an amoeba moves over a surface with pseudopods. Intermediate Filaments • They form a ropelike assembly of fibrous polypeptides. Some intermediate filaments support the nuclear envelope, whereas others support the plasma membrane and take part in the formation of cell-to-cell junctions. In the skin, intermediate filaments made of the protein keratin give great mechanical strength to skin cells. Microtubules Small, hollow cylinders. They are made of a globular protein called tubulin (dimers of alpha and beta tubulin). The motor molecules kinesin and dynein are associated with microtubules. Microtubules help to maintain the shape of the cell and act as tracks along which organelles can be moved. ATP is required. Microtubules: Centrioles and Centrosome • Centrioles are short cylinders with a 9 + 0 pattern of microtubule triplets. • Centrosome is the major microtubule-organizing center for the cell and contains two centrioles lying at right angles to each other. Cilia and flagella are microtubule-based projections of the plasma membrane that are responsible for the movement of a variety of eukaryotic cells. Animal Cell vs Plant Cell Animal Cell Central Vacuole Chloroplast Cell Wall Plant Cell Cell Wall - Differentiates plant cells from animal cells - A rigid extracellular structure -The major part of plant cell wall is Cellulose - Maintains the shape of the plant cell - Prevents too much water from getting into the plant cell Page 118 campbell Outside : layer between two plant cells Inside Lipids+ proteins +Sugars Cellulose: a polymer of glucose Thank you Thank you 21 Cell Structure and function Biology Lecture 6 Prof. Abu Salim Mustafa PhD, FRCPath (UK) Department of Microbiology Faculty of Medicine Kuwait University Tel. 24636505 E-mail: [email protected] 22 Objectives To explain the structure of 1. Cell membrane To explain the function of cell membrane Passive transport Active transport • Reading: Lecture notes and Biology by Sylvia S. Mader 23 Cell/Plasma Membrane Forms a barrier between intracellular and extracellular side Composed of lipids, proteins, and carbohydrates Fluid-mosaic model Lipids in the cell membrane Phospholipid bilayer: The hydrophilic heads of the phospholipids are at the surfaces of the membrane. The hydrophobic tails make the interior of the membrane. Cholesterol molecules are present in the hydrophobic region of phospholipids and provide stability to the cell membrane. Phospholipid bilayer: is thin, only two molecules thick and continuous over the entire cell surface. Phospholipids have a glycerol molecule to which attached is a polar phosphate group and two nonpolar fatty acid chains. The polar (hydrophilic) head is soluble in water, whereas the two hydrophobic (nonpolar) tails are not. Amphipathic molecule: hydrophilic and hydrophobic regions -permeable to small non-polar molecules and fat-soluble substances (O2, CO2, and alcohols). -impermeable to water-soluble substances (glucose, urea, various ions). Carbohydrates in cell membrane Carbohydrates in the cell membrane are usually on the outside of the membrane attached to the protein (glycoprotein) and lipid (glycolipid) molecules, which make the cell belong to a particular individual and tissue. Carbohydrates play an important role in immune reactions and also act as receptors on the surface of the cell. Proteins in Cell Membrane Are mostly glycoproteins (proteins+carbohydrates). Peripheral Protein Integral Protein Proteins are scattered throughout the membrane in an irregular pattern, and this pattern can vary from membrane to membrane. - Two types of membrane proteins: i) Integral proteins: protrude all the way through the cell. These proteins provide pathways through which water and water-soluble substances can cross. ii) Peripheral proteins: occur on the membrane, and normally attached to the integral proteins. These proteins function as enzymes to control the chemical reactions inside the cell. Cell/plasma membrane: Structure Plasma Membrane function: Permeability • The plasma membrane regulates the passage of molecules into and out of the cell. • It is selectively permeable, allowing only certain substances into the cell while keeping others out. • Substances that are hydrophobic and therefore similar to the phospholipid center of the membrane are able to diffuse across membranes at no energy cost. • Polar molecules are chemically incompatible with the center of the membrane and so require an expenditure of energy to drive their transport. Plasma membrane functions: permeability The long, back-and-forth arrows indicate that these substances can diffuse across the plasma membrane. The curved arrows indicate that these substances cannot passively cross the plasma membrane, and require energy for their transport. Passage of molecules across the cell membrane • Passive transport – Diffusion – Osmosis – Facilitated diffusion • Active transport – Transport of ions and polar molecules – Bulk transport Diffusion Passive transport: Small molecules • The molecules follow their concentration gradient as they move from an area of high concentration to an area of low concentration (diffusion). Diffusion is spontaneous, and no energy is required Diffusion Passive transport: Small molecules • The concentration of O2 is low in the blood and high in the alveoli, therefore O2 diffuses from the alveoli into the blood. • On the contrary, the concentration of CO2 is high inside the blood, but low in the alveoli, therefore it diffuses from the blood into the alveoli. Diffusion of water across cell membrane: Osmosis • The diffusion of water across a selectively permeable membrane from high to low concentration. Hypertonic Hypotonic Isotonic Facilitated Transport • Carrier proteins are involved but transport occurs down the concentration gradient and no energy is required. • Water moves through the membrane by using a channel protein called aquaporin. • Glucose and amino acids move through the membrane by combining with specific carrier proteins. The carrier proteins recognize particular shapes of molecules and combine with the molecules, such as glucose, before changing its shape and transporting the molecule across the membrane. Active Transport: Transport of ions and polar molecules • Active transport occurs against the concentration gradient and requires energy in the form of ATP. • The transport is assisted by carrier proteins. • Each carrier protein recognizes particular shapes of molecules and must combine with an ion, such as sodium (Na+), or a molecule, before changing its shape and transporting the molecule across the membrane. • Therefore, carrier proteins are specific for the substances they transport across the plasma membrane. • Proteins involved in active transport often are called pumps, e.g. sodium-potassium pump. The sodium-potassium pump The concentration of Na+ is higher outside the cell and the concentration of K+ is higher inside the cell Bulk Transport: large molecules and particles • The transport of large molecules such as proteins, polysaccharides, or nucleic acids and particles like bacteria across the cell membrane and requires energy. • These molecules require vesicles because they are too large to be transported by carrier proteins. • Exocytosis: fusion of a vesicle with the plasma membrane moves a particle outside the membrane. • Endocytosis: vesicle formation moves a particle inside the plasma membrane. • Vesicle formation is required for the movement of macromolecules or even for something larger, such as a virus or bacterium. Exocytosis: secretes or deposits substances on the outside of the cell Endocytosis: cells take in substances by forming vesicles around the material • Phagocytosis transports large substances, such as viruses and bacteria • Pinocytosis transports small substances, such as a macromolecule, into a cell. • Receptor-mediated endocytosis is a special form of pinocytosis. Molecules first bind to specific receptor proteins. a. Phagocytosis occurs for the transport of large substances into the cell b. Pinocytosis occurs when a macromolecule, such as a polypeptide, is transported into the cell. c,d. Receptor-mediated endocytosis is a form of pinocytosis. The coated vesicle contains the molecules and their receptors. Passage of molecules across the cell membrane: Summary Name Direction Requirement Examples Diffusion Toward lower concentration Concentration gradient Lipid soluble molecules, gases Facilitated transport Toward lower concentration Chemical or carrier and concentration gradient Some sugars, amino acids Active transport Toward higher concentration Carrier plus energy Sugars, amino acids, ions Bulk trans[ort Toward outside or inside Vesicle utilization Macromolecules 44