MCAT Study Guide/Structure and Function of the Cell
The cell is the basic unit of structure and function of all living organisms. All present cells are produced by the replication of preexisting cells.
Cells may be categorized according to their size. Cell size is typically measured in the units of the micrometer (μm) and Ångström unit (Å). Conversions to these units of measure are as follows: 1 m = 100 cm; 1 cm = 10 mm; 1 mm = 1000 μm; 1 μm = 10,000 Å.
Cells may vary in size; eukaryotic cells typically range from about 10 μm to 100 μm. Here are some commonly identified structures and their estimated relative sizes:
- eukaryotic cells: 10 μm (100,000 Å)
- mitochondria: 1 μm (10,000 Å)
- bacteria: 1 μm (10,000 Å)
- viruses: 0.1 μm (1,000 Å)
- macromolecules: 0.01 μm (100 Å)
- molecules: 0.001 μm (10 Å)
- hydrogen ion: 0.0001 μm (1 Å)
Resolution is a measure of the sharpness of an image, often defined as the ability to discriminate two points even if they are very close together. This measure is dependent on the wavelength of a light source; it could be calculated as roughly one-half of the wavelength.
Here are some rough approximates of resolving power:
- human eye: 0.1 mm (100 μm)
- light microscope: 0.2 μm (2,000 Å)
- transmission electron microscope: 2-5 Å
Protoplasm is the content of living cells, which is surrounded by a plasma membrane.
Here is a breakdown of its approximate elemental composition:
- oxygen: 75+%
- carbon: 10+%
- hydrogen: 10%
- nitrogen: 2+%
- sulfur: ~0.2%
- phosphorus: ~0.3%
- potassium: 0.3%
- chlorine: ~0.1%
- other elements (sodium, calcium, magnesium, iron, etc.): less than 0.1%
Cell Membrane Structure
The plasmalemma, or plasma membrane, is the barrier of permeability between the living cell and its environment. Within a eukaryotic cell, there are also other membranes that separate its various compartments.
All membranes consist of lipid bilayers with their associated proteins. Some of the proteins are associated with the surface, while others extend across the lipid bilayer. Eukaryotic cell membrane surfaces that do not face the cytoplasm may have many polymers of sugars, known as oligosaccharide groups, attached to proteins and lipids, especially on the external surface of the plasma membrane.
Most membranes, when seen by transmission electron microscopy at high magnification, appear as two dark lines separated by a light zone. The complex is about 10 nm (100 Å) thick.
The lipids in the lipid bilayers of cell membranes are mainly phospholipids. When mixed with water, phospholipids and glycolipids form bilipid structures spontaneously. This is a result of the hydrophilic and strongly polar 'head' of each lipid molecule, which contains sugar or phosphate, associating with water; and conversely, the non-polar alkyl 'tail' of the fatty acyl groups aggregating by hydrophobic interactions.
Two types of phospholipids are glycerolphospholipids and sphingolipids.
Glycerophospholipids are composed fo a glycerol linked to two fatty acyl groups and one phosphate group, which could in turn be linked to other groups, including ethanolamine, choline, inositol, or serine, which are conventionally named, in these examples, as "phosphatidyl [other group name]," respectively.
Sphingolipids are composed of a sphingosine linked to one fatty acyl group. They are usually linked to a phosphate plus choline (sphingomyelin), sugar (ceramides), or complex oligosaccharide (gangliosides). These lipids are absent from most prokaryotic cells, and are primarily found in the outer face of the plasma membrane in eukaryotic cells.
Lysophospholipids have one fatty acyl group removed, halving the ratio of nonpolar to polar structure. Destabilization may occur with the conversion of phospholipids to lysophospholipids, which promotes the conversion of phospholipid bilayers into micelles.
The consequences of substituting cis-unsaturated fatty acids for saturated fatty acids include decreased closeness of the packing in the nonpolar layer, increased fluidity in that layer, and increased movement of small polar molecules across the bilayer.
The consequences of the addition of cholesterol to a lipid bilayer include reduced fluidity of the lipids and reduced penetration of small polar molecules across the bilayer.