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CELL STRUCTURE AND FUNCTION By: Elizabeth Wickham CHAPTER 4: A TOUR OF THE CELL "Biology is a software process. Our bodies are made up of trillions of cells, each governed by this process. You and I are walking around with outdated software running in our bodies, which evolved in a very different era." What is the Cell Theory? • All organisms are composed of one or more cells • The cell is the smallest unit having the properties of life • The continuity of life arises directly from the growth and division of single cells What do all cells have in common? • The cytoplasm contains semifluid substance, membrane systems, filaments, and other particles. • A nucleus or nucleoid region which contains the hereditary material. • A plasma membrane which separates each cell from the environment and allows the flow of molecules across the membrane. Surface-toVolume Ratio • Cell size is constrained by the surface-to-volume ratio. • It states that if a cell expands its diameter during growth, its volume will increase more rapidly than its surface area will. • The smaller the cell the more efficient it is but if the cell gets to large it will not be able to move materials into or out of the cell. The Cell Membrane • A lipid bilayer forms a boundary between the inside and the outside of the cell. • It subdivides the cytoplasm into compartments. • It also regulates the entry and exit of substances. • Proteins embedded in membrane serve as channels, pumps, or receptors. Microscopes • Light Microscopes use waves of light to make an image. – Example: Compound Light Microscope • Electron Microscopes use magnetic lenses to bend and diffract beams of electrons resolving details 100,000 times smaller than with Light Microscopes. – Transmission Electron Microscopes produce images of internal details. – Scanning Electron Microscopes produce images of a specimen's surface. Prokaryotes • Prokaryotic, meaning "before the nucleus," indicates the existence of bacteria before the evolution of cells with a nucleus. • A prokaryotic cell lacks a membrane-enclosed nucleus and membrane-enclosed organelles. • Two domains exist: Bacteria and Archaea. Biofilms • Biofilms occur when single-celled organism share a layer of polysaccharides and glycoproteins. • They form as different species sense the presence of other species. • Microenvironments help to support the different species of the community. Eukaryotes • Eukaryotes have a membrane-enclosed nucleus and membrane-enclosed organelles. • The interactions between these organelles help the cell to function and metabolize. • Organisms with eukaryotic cells include protists, plants, fungi, and animals and they are called eukaryotes. Plant Cell vs. Animal Cell • The organelles that are specific to a plant cell are: – Cell Wall – Chloroplasts – Central Vacuole • Centrioles are specific to animal cells in cell division. The Nucleus • The nuclear envelope is a double membrane that encloses the semifluid interior of the nucleus, called nucleoplasm. • In the nucleolus subunits of ribosomes (ribosomal RNA) are prefabricated before shipment out of the nucleus. • A chromosome is a double stranded DNA molecule. • Chromatin is the total collection of DNA and associated proteins. Endoplasmic Reticulum • The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs that begin at the nucleus and ramble throughout the cytoplasm. – Rough endoplasmic reticulum has stacked, flattened sacs with ribosomes attached. – Smooth endoplasmic reticulum has no ribosomes but is the area from which vesicles carrying proteins and lipids are budded. Vesicles • Vesicles are transporters to and from the plasma membrane. • Peroxisomes are vesicles containing enzymes that break down fatty acids and amino acids. Vacuoles • Animal vacuoles house debris and toxic materials in the cell. • The plant central vacuole accumulates a watery solution of ions, amino acids, sugars, and toxic substances. Golgi Body • In the Golgi body, proteins and lipids undergo final processing, sorting, and packaging. • Edges of the Golgi body break away as vesicles and form lysosomes. • Lysosomes digest contents of other vesicles, worn-out parts, or bacteria and foreign particles. Mitochondria • Mitochondria are primary organelles that transfer the energy in carbohydrates to ATP under oxygen-plentiful conditions. • They have their own DNA and ribosomes. • Endosymbiosis is the theory that explains how mitochondria may have been independent prokaryotic cells that were engulfed by another cell but became permanent, remaining with the host cell. Chloroplasts • Chloroplasts are specialized for photosynthesis. • The innermost membrane, stacked disks, pigments, and enzymes trap sunlight energy to form ATP and NADPH. • Chromoplasts store red and brown pigments for fall leaves whereas colorless amyloplasts lack pigments and store starch grains. The Cell Wall • The cell wall is the carbohydrate frameworks for mechanical support in bacteria, protistans, fungi, and plants. • Cellulose strands form the primary wall and more layers are deposited to form the secondary wall made of lignin. Extracellular Matrix • The extracellular matrix between animal cells includes cell secretions and materials drawn from the surroundings between cells such as carbohydrates and proteins. Cell Junctions • Plasmodesmata cross the adjacent primary walls and connect the cytoplasm. – Tight Junctions produce an effective seal when cytoskeleton strands of one cell fuse with strands of neighboring cells. – Adhering Junctions help cells that need to be held together during stretching. – Gap Junctions are small, open channels that directly link the cytoplasm of adjacent cells. Cytoskeleton • The cytoskeleton forms an interconnected tube system of bundled fivers, slender threads, and lattices that extends form the nucleus to the plasma membrane. – Microtubules are composed of long tubulin cylinders and the help in the movement of chromosomes during cell division. – Microfilaments consist of two helically twisted polypeptide chains assembled form actin monomers and are important in movements on cell surface and shape of cell. – Intermediate filaments are the most stable of the cytoskeletal elements. They occur only in animal cells of specific tissues and help to strengthen and maintain the shape of cells or cell parts. Lets get moving! • Cilia are short and provide locomotion for free-living cells or may move surrounding water and particles if the ciliated cell is anchored. • Flagella are long and found on one-celled protistans and animal sperm cells. • Pseudopods are temporary lobes that project form the cell and are used in locomotion and to capture food. CHAPTER 5: MEMBRANE TRANSPORT AND CELL SIGNALING "Life evolved under conditions of light and darkness, light and then darkness. And so plants and animals developed their own internal clocks so that they would be ready for these changes in light. These are chemical clocks, and they're found in every known being that has two or more cells and in some that only have one cell." Phospholipid Bilayer • If the phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and the hydrophilic heads face outward forming a bilayer. • Cell membranes are composed of: – Phospholipids – Steroids – Proteins • Carbohydrates attach to cell membranes in different ways for different cells. • Fatty acid tails of membrane phospholipids vary in length and saturation. Protein Orientation • Peripheral proteins are positioned at the surface of the membrane. • Integral proteins span the lipid bilayer, with their hydrophilic domains extending past both surfaces. Membrane Proteins • Adhesion proteins help cells stay connected to one another in a tissue. • Recognition proteins identify certain cell type, guide cells to becoming tissues, and cell to cell recognition and coordination. • Receptor proteins have binding sites for hormones that can trigger changes in cell action, as in growth process. • Enzymes help to accelerate reactions without being changed themselves. • Transport proteins passively allow water-soluble substances to move through their interior, which opens on both sides of the bilayer. – Passive transporters require no energy. – Active transporters use ATP to pump substances. A Permeable Membrane • Cells keep extracellular fluid contents separate form the contents of the cell with membranes that are selectively permeable. Concentration Gradient • A concentration gradient refers to the difference in the number of molecules (or ions) of a substance in a given volume of fluid between two adjoining regions. Diffusion • In the presence of a concentration or electrochemical gradient, diffusion results in the net movement of a substance form a region where it is more concentrated to a region where it is less concentrated. • Facilitated diffusion is the passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy. • Factors that influence the rate and direction of diffusion: – Size – Temperature – Steepness of the concentration gradient – Charge – Pressure • Dynamic equilibrium is reached when gradients no longer exist and there is no net movement. Passive and Active Transport • In passive transport a concentration gradient and/or electric gradient drive diffusion of a substance across a cell membrane through a transport protein – a passive process expending no energy that continues until concentration gradients are equal on both sides. • Active transport moves ions and large molecules across a membrane against a concentration gradient and special proteins are induced to change shape, but only with an energy boost form ATP. Exocytosis and Endocytosis • In exocytosis cytoplasmic vesicles move substances from the cytoplasm to plasma membrane where the membranes of the vesicles and cells fuse. • Endocytosis encloses particles in small portions of the plasma membrane to form vesicles that then move into the cytoplasm. – Phagocytosis – Receptor-mediated endocytosis – Bulk-phase endocytosis Osmosis • Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients or both. – Hypotonic Fluid – lower concentration of solutes than the fluid in the cell (cause it to swell). – Hypertonic Fluid – higher concentration of solutes than the fluid in the cell (cause it to shrink). – Isotonic Fluid – the same concentration of solutes as the fluid in the cell (no movement occurs). Pressure • Cells are either in a constant environment or adapted to a hypotonic or hypertonic one. • Hydrostatic pressure increases the greater the solute concentration is. • Hydrostatic pressure is countered by osmotic pressure which prevents any further increases in the volume of the solution. THE END is near...