Chapter 4

  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. The Cell Theory

  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 All cells share the same basic organization

  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 Limits to Cell Size

  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 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 Compound light 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 Electron microscopes

 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 membrane- bound 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.