Cells - The Basic Unit of Life

• Unicellular organisms


  Organisms consisting of one cell only
• Multicellular organisms


  Organisms consisting of many cells, which are specialised to perform different functions
• Cells are too small to see with the naked eye. Microscopes are used to study and observe the structure of cells.
• Key terminology related to microscopy

  • Microscopy
  • Microscopes
  • Compound light microscope
  • Electron microscope
  • Scanning electron microscope (SEM)
  • Transmission electron microscope (TEM)
  • Magnify
  • Mount
  • Micrograph

  
• Historical timeline of the development of the microscope

  1. 1590: Zacharias and Hans Jansen invented the first simple microscope

  2. 1609: Galileo Galilei developed a microscope with a focusing device

  3. 1665: Robert Hooke studied cork under the microscope and observed "cells"

  4. 1674: Antonie van Leeuwenhoek developed microscopes with lenses that magnified up to 270x and discovered bacteria

  5. 1839: Theodor Schwann and Matthias Schleiden formulated the Cell Theory

  6. 1855: Rudolf Virchow

  7. 1931: Max Knott and Ernst Ruska invented the electron microscope
• Differences between a light microscope and an electron microscope

  • Light microscope uses a beam of light, can magnify up to 1500x

  Electron microscope uses a parallel beam of electrons, can magnify up to a million times

  
• The Cell Theory states that all living organisms consist of cells, cells are the basic units of life, all cells originate from pre-existing cells, cells contain hereditary information, and all cells show the same basic chemical composition.
• Parts of a compound light microscope and their functions

  • Eyepiece/ocular
  • Nosepiece (rotating)
  • Objectives (three lenses)
  • Coarse adjustment knob
  • Fine adjustment knob
  • Stage
  • Clamps/clips
  • Condenser (lens)
  • Diaphragm
  • Light source/illuminator
  • Base
  • Arm
  • Microscope tube

  
• Handling and care of the microscope

  Carry the microscope upright, place on a firm surface, plug in and turn on light, start with lowest magnification, adjust diaphragm and condenser, use coarse and fine adjustment knobs to focus, move to higher magnifications carefully, return to lowest magnification before removing slide
• Magnification


  Total magnification = eyepiece magnification x objective magnification
• Practical investigation: observing cells


  Prepare a wet mount of onion epidermis cells, study under microscope, draw and label cells at 100x and 400x magnification
• Measuring the size of cells

  Cells are measured in micrometres (μm), 1m = 1000mm, 1mm = 1000μm, 1μm = 1000nm

  To calculate actual size: use scale line or magnification
• All cells, both plant and animal, have the same basic cell structure with a variety of organelles performing specific functions.
• Cell structures common to plant and animal cells

  • Cell membrane
  • Cytoplasm
  • Nucleus
  • Mitochondria
  • Endoplasmic reticulum
  • Golgi body
  • Ribosomes

  
• Structures found in plant cells but not animal cells

  • Cell wall
  • Plastids
  • Large vacuole

  
• Structures found in animal cells but not plant cells

  • Vesicles (e.g. lysosomes)
  • Centrosomes
• Protoplasm


  Living part of plant and animal cells
• Cytoplasm


  Jelly-like transparent substance between cell membrane and nucleus

  
• Cell membrane


  Outer boundary of cytoplasm; a living, selectively permeable structure

  
• Organelles


  Membrane-bound structures found in the cytoplasm that perform specific functions

  
• Cell wall


  Rigid, outer layer of plant cells made of cellulose; non-living and completely permeable

  
• Active transport


  Movement of substances through a selectively permeable membrane against a concentration gradient, requires energy
• Passive transport


  Movement of substances through a selectively permeable membrane down a gradient, does not require energy
• Diffusion


  Movement of particles of a gas or liquid from high to low concentration, down a concentration gradient
• Organelles


  Membrane-bound structures found in the cytoplasm; they perform specific functions

  
• Cell wall


  Rigid, outer layer of plant cells made of cellulose; it is non-living and completely permeable

  
• Active transport


  Movement of substances through a selectively permeable membrane against a concentration gradient; requires energy
• Passive transport


  Movement of substances through a selectively permeable membrane down a gradient; does not require energy
• Diffusion


  Movement of particles of a gas or a liquid, from a high- to a low concentration, down a concentration gradient, until equilibrium is reached

  
• Osmosis


  Movement of particles of a liquid(water) from a high- to a low concentration through a selectively permeable membrane

  
• Selectively permeable


  Allows certain substances to enter or leave the cell; controls movement of substances into and out of the cell
• Parts of the cell wall

  • Primary cell wall
  • Secondary cell wall
  • Middle lamella

  
• Primary cell wall


  • Thin, made up of cellulose; has small openings: pits which contain plasmodesmata; elastic

  
• Secondary cell wall


  • Consists of thick cellulose fibres with lignin; inelastic
• Middle lamella


  • Made up of pectin; connects the cell walls of adjacent plant cells

  
• The cell wall protects the living cell contents, gives the plant cells a more uniform and regular shape, and provides support
• Cell membrane


  Surrounds the cytoplasm; made up of a phospholipid bilayer with large protein molecules embedded in between

  
• The cell membrane is selectively permeable and controls the movement of substances into and out of the cell
• Transport across membranes


  1. Diffusion

  2. Osmosis

  3. Active transport

  
• Diffusion


  Spontaneous, random movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached