Robert Hooke (1665) used a primitive microscope to view cork and he named the box shaped structures he saw ‘cells.’ In the late 1600’s Dutch lens maker Antonie von Leeuwenhoek was the first to view "animalcules", single celled organisms living in pond water.

In the 1830’s two researchers, Theodor Schwann and Mattias Schleiden saw the universality of cells in living things. They developed a cell theory.

1. Every living organism is made up of one or more cells

2. Some organisms are single cells and other cells are the functional units of multicellular organisms.

3. All cells arise from pre-existing cells.

Light Microscope – uses light and glass lenses (magnification is ~1000X maximum) The resolving power of a microscope is the ability to distinguish two objects as separate entities.

Electron Microscopes – can resolve in Angstroms. They use magnets and beams of electrons in place of a visible light. There are two types of electron microscopes, transmission and scanning. Transmission electron microscopes are used for viewing internal structures of cells. Before a specimen is viewed, it must be fixed then sliced very thinly so that the electron beam can be passed through it. A Scanning electron microscope is used for viewing 3-D structures and is especially useful for viewing overall structure of a cell.. In this case, samples must be dehydrated and cemented to a slide and coated with a heavy metal. Electrons are deflected from the object and the pattern created can be viewed television monitor.

Plasma membrane – double layer of phospholipid. The membrane isolates the cell from the external environment. It regulates the flow of materials into and out of the cell. It also allows for interaction with other cells.

The nucleus (or nucleoid) contains genetic material (DNA).

The cytoplasm is everything between the membrane and the nucleus.

Although there are some examples of macroscopic cells, most cells are very small. This is because the surface area to volume ratio of the cell must be optimal because many molecules must diffuse into the cell. As distance to the center of a cell increases, diffusion takes much longer.

Cell Types

Prokaryotic – "Before the nucleus." Domains Bacteria and Archaea belong to this cell type.

Eukaryotic – "True nucleus". All other Kingdoms are classified under this cell type.

The various types of eukaryotic cells share many characteristics, but they also have characteristics which make them unique and causing them to be classified into different Kingdoms. Below, some of the characteristics which are generally unique to each Kingdom are listed.

We have already established that eukaryotic cells have many intracellular organelles, small organs with specific roles in the life of the cell. In eukaryotic cells, these organelles are all interconnected by a membrane system, the endomembrane system.

Cell Membrane-the membrane of a cell is composed primarily of a phospholipid bilayer. Phospholipids are types of lipids; they have a glycerol backbone, with fatty acid chains linked to two of the glycerol’s carbons, and a phosphate group is linked to the third. The phosphate group changes the lipid molecule from being a completely non-polar lipid molecule to a partly polar and partly non-polar.

Phospholipid membranes comprise the cell surface as well as surround the internal organelles.

Nucleus.

Chromatin – DNA and attached proteins

(A chromosome is each fiber of chromatin)

Nuclear envelope – membrane covering the nucleus.

Nucleolus-a region of the nucleus (containing RNA and chromatin) where ribosomes are made

Ribosomes-cellular structures composed of RNA and protein. Ribosomes are the site of protein synthesis.

Rough ER – dotted with ribosomes and has two main functions:

1. Make proteins that are secreted by the cell
2. Make more ER

Smooth ER – (no ribosomes), the major function of the smooth ER is synthesis of lipids, fatty acids, phospholipids and steroids in the cell. Smooth ER also stores ions like Calcium.

Golgi apparatus -stacked membranous discs that are not interconnected. The Golgi modifies products from the ER then re-packages it in transport vesicles which export the products.

Lysosomes – membrane bound compartments which store hydrolytic enzymes. They help the cell break down food particles, digest ‘infecting’ microorganisms, and recycle damaged cell parts.

Food vacuoles

Central vacuoles

Contractile vacuoles

Chloroplasts – organelles found in photosynthetic cells and which have a series of stacked discs (grana) surrounded by fluid called stroma. Complete structure of the chloroplast is covered in Chapter 7.

Mitochondria – ‘energy’ centers of the cell because they convert chemical energy into ATP. ATP carries energy in chemical bonds which fuel cellular work. Complete mitochondrial structure is covered in Chapter 6.

Cytoskeleton – involved in cell structure and movement.

Microfilaments (actin)

Intermediate filaments – hold nucleus in place

Microtubules – made of tubulin and provide anchorage for organelles; they are also important in cell division.

Plants – plasmadesmata

Animals – tight junctions

anchoring junctions

communicating junctions