Transcript Cell Basics
Chapter 4 The Cell Theory Living things are composed of cells. Cells are the basic units of structure and function in living things. All cells come from other cells. Important scientists: Schleiden, Schwann, Virchow Similarities between cells support the theory of evolution – which includes an explanation of the original cell’s origin. All cells share the same basic organization Plasma membrane: Separates the contents of the cell from the external environment Allows a difference in composition between inside and out Serves as a selective barrier so cells can exchange materials with external environment and accumulate necessary molecules Organelles: Internal structures that are specialized to perform particular functions: metabolic activities, synthesis of important compounds, reproduction DNA: Genetic instructions Limits to Cell Size Materials used or produced inside the cell must go in or out of the cell through the plasma membrane. As a cell increases in size the surface area to volume ratio decreases compared to the volume of the cell, the surface area is smaller This limits the cell size – large cells would not be efficient at transporting materials across the plasma membrane Exceptions: the shape of a cell (long and thin) can influence the SA/V ratio ex: some human nerve cells Microscopes Magnification – how large the image appears compared to its actual size Resolution – the ability of the eye to distinguish between 2 distinct points (clarity of image) Light microscopes – up to 1,000 x magnification Electron microscopes – up to 250,000 x magnification Compound light microscopes More than one lens – multiply the objective lens by the ocular lens to determine the total magnification Bright-field microscopy – light is transmitted through a cell. Little to no contrast makes internal structures difficult to see Dark-field microscopy – the cell is visible as a bright object against a dark background There are many types of stains and dyes in use that allow us to see internal cell structures Electron microscopes Image is formed by electrons striking the specimen. The image cannot be viewed directly – it must be focused on a photographic plate or fluorescent computer screen. Live cells cannot be viewed this way. Transmission electron microscopy (TEM) – extremely thin slices are made of the specimen Scanning electron microscopy (SEM) – specimen is coated with a thin film of gold and the surface is scanned Cell Fractionation Used to study cell organelles – allows researchers to purify different cell parts Uses a centrifuge – which spins rapidly and forms: The pellet – heavier materials, such as nuclei, form at the bottom of the tube And the supernatant – the liquid above the pellet which contains lighter particles as well as dissolved materials Organelles purified in this way can be studied to determine their chemical makeup and reactions Basic Cell Types: Prokaryotes “Before the nucleus” Differences from eukaryotes: On average, smaller DNA not located in a membrane-bound nucleus DNA is located in a nucleoid May have folded plasma membranes for cellular reactions Most have cell walls Many have flagella Have ribosomes for protein synthesis Basic Cell Types: Eukaryotes “True nucleus” Highly organized and complex with a membranebound nucleus and organelles Cytoplasm – outside of the nucleus; contains the organelles and the fluid they are suspended in – which is the cytosol Nucleoplasm – within the nucleus Organelles are specialized for different functions Cell Membranes Allow different parts of the cell to do different jobs: Reactants are more likely to contact each other Reactive compounds are isolated from other cell parts Many different activities can occur simultaneously Allow cells to store energy: A difference in concentration across the membrane represents potential energy Provide work surfaces for the cells: Chemical reactions are carried out by enzymes embedded in the membrane Endomembrane system The internal membrane system of cells. Mitochondria and chloroplasts are not part of this system – they function independently. Some organelles have direct contact with each other. Others transport materials throughout the cells by using vesicles. The Cell Nucleus Usually the largest organelle, located in the cell center. Most cells only have one. Surrounded by the nuclear envelope – a double membrane which contains nuclear pores. DNA is contained in the nucleus, with associated proteins which make up chromatin. During cell division the chromatin becomes compacted and forms chromosomes. Many also contain at least one nucleolus which is responsible for making ribosomes.