The Cell: The Basic Unit of Life
Which came first, the chicken or the egg? This ancient conundrum applies equally at the cell chemistry level.
It takes the instructions found in DNA to make protein, and it takes protein to make DNA.
It takes a series of protein-enzyme catalyzed reactions to make lipid, but without lipid membrane, the compartmentalization necessary for the synthesis of the lipid would not exist. Moreover, without an existing membrane, new lipid and membrane-associated protein cannot organize itself as it is found in living cells.
See chapter 25 for a discussion of the origin of life
The cell theory states that all organisms are composed of cells, and all cells come from preexisting cells. It is the basis for studying all life, whether single-celled or multicellular.
Cell size is limited by the surface area-to-volume ratio
Most cells are tiny, with diameters in the range of 1 to 100 mm.
The surface of a cell is the area that interfaces with the cell’s environment. The larger the surface area of a cell, the faster a cell can take in substances and remove waste products.
The volume of a cell is a measure of the space inside a cell. The larger the volume of a cell, the more chemical activity it can have.
Surface area-to-volume ratio is defined as the surface area divided by the volume. For any given shape, increasing volume decreases the surface area-to-volume ratio (See Figure 4.2.)
Shape also influences surface area-to-volume ratios.
A sphere has the least surface area-to-volume ratio of any shape.
Imagine you have a lump of clay. Fashioning it into a sphere minimizes the surface area.
Flatten the ball of clay to make a pancake shape, and the surface area increases, while the volume remains the same. Cells like red blood cells flatten into a pancake shape to increase surface area.
Fashioning the clay into a thin string also increases the surface area without increasing the volume. Nerve cells have this shape, and this allows some of them to be a meter long or more.
If the clay is spherical but the surface is irregular with many fine projections coming off the surface, surface area is greatly increased. In epithelial cells, such projections are called microvilli.
Microscopes are needed to visualize cells
The small size of cells makes the use of microscopes necessary to view them. (Figure 4.1 shows the relative sizes of biological objects ranging from atoms to trees.)
See also: http://www.wordwizz.com/pwrsof10.htm
With normal human vision one can resolve objects about 200 mm (0.2 mm) in size. To resolve means to distinguish two separate things. If two objects are too close together, they start to look like one object.
Light microscopes use glass lenses and visible light, but have resolution limits. The resolving power of light microscopes is typically 0.2 mm (0.2 ´ 10-6 m), but this depends on the wavelength of the illuminating light. In general, resolution is about 1000 times better than an unaided human eye. Living or killed and fixed cells may be viewed with light microscopes.
Nanometer = 10-9 meter = 10 Angstroms
Angstrom = 10-10 meter
The structure in a cell glows because it is linked to a fluorophore (a substance that glows when it is hit with UV light) such as fluorescein (a small chemical compound). These linkages are made by creating an antibody to the structure you wish to study and then connecting the fluorescent compound (such as fluorescein or rhodamine) to the antibody.
http://dept.kent.edu/projects/cell/FLUORO.HTM#Background on Fluorescence Microscopy
In vivo reporter: GFP
Another method of creating a fluorescent cell structure is to genetically manipulate the cell so as to express structure protein that includes a fluorescent protein, GFP. Then living cells can ‘light up’ when hit with UV light.
Confocal Microscopy: tissue reconstruction
This is the optical equivalent of slicing through the tissue you are viewing. The microscope steps its focus down through the tissue, taking images at each stage. The computerized images may then be manipulated in 3 dimensions, allowing viewing angles and information that could not be seen in the original focal plane.
Drosophila: GFP label in nucleii of sensory neurons
Cells show two organizational patterns
Living organisms can be classified into one of two major categories based on where, within the cell, the most genetic material is stored. (See Figure 4.4.)
Organisms called prokaryotes have no nucleus or other membrane-bounded compartments. Prokaryotes lack distinct organelles, although some do have invaginous membrane structures.
Organisms called eukaryotes have a membrane-bounded nucleus. Eukaryotic cells usually have other membrane-bounded compartments or organelles as well.
Prokaryotes inhabit the widest range of environmental extremes.
They can be found living at temperatures above boiling at thermal vents deep in the ocean. They also occur in extremely salty environments.
Some have been found deep in Earth's crust, far away from the sun, photosynthesizing organisms, and oxygen. These prokaryotes use inorganic, reduced chemicals for an energy source.
All prokaryotic cells share certain features
All have a plasma membrane.
All have a region called the nucleoid where the DNA is concentrated.
The cytoplasm, the plasma-enclosed region of prokaryotes, consists of the nucleoid, ribosomes which are the molecular protein synthesis machines, and a liquid portion called the cytosol.
Some prokaryotic cells have specialized features
Most prokaryotic cells have a cell wall just outside the plasma membrane. (See Figure 4.4.)
The cell wall functions to prevent plasma membrane lysis (bursting) when cells are exposed to solutions with lower solute concentrations than the cell interior. It also protects the membrane.
In most bacteria, (but not in Archaea), the cell wall is made of a polymer of amino sugars called peptidoglycan, which is covalently cross-linked to form one giant molecule around the entire cell. In some cases these amino sugars include D-amino acids to prevent digestion from competitors.
Some bacteria have another outer membrane outside the cell wall, a polysaccharide-rich, phospholipid membrane. This membrane has proteins embedded that make it more permeable than the interior membrane.
Some bacteria have even another layer in addition to a plasma membrane, a cell wall, and an outer membrane. The outermost slimy layer is made of polysaccharides and is referred to as a capsule.
For some bacteria, this capsule provides a means to escape detection by the immune systems of the animals they infect: slime. The capsule can prevent drying out of the cell and help trap other cells for food. If the cell loses the capsule, it can survive. Therefore, it is not essential to cell life.
Some bacteria have flagella. These are locomotary structures that are shaped like a corkscrew. They spin like a propeller to move the bacteria. The flagella bear no structural commonality to the flagella found in eukaryotic cells, such as sperm cells. (See Figure 4.6.)
Simple model to explain motility of E. coli towards an attractant
Flagellar motor model: http://lucy.itc.virginia.edu/~wmr5a/molecular_vrml/title.htm
Motile bacteria have two basic methods of movement: movement towards an attractant (taxis), or random movement (tumbling). Experimentation shows taxis involves a clockwise spin of the bacteria, fusing multiple flagella into a single ropelike structure that propels it in a single direction. Tumbling involves a counter-clockwise spin that releases the multiple flagella to whip independently, causing a random 3-dimensional motion.
The motile bacterium senses concentration per unit time to “decide” upon tumbling or taxis.
Motile E. coli: path traced with tracking microscope: