LECTURE 8
The Anatomy of Cells
Anatomy: from the Greek word for dissection
The cell is the basic unit of life: The Cell Theory is summarized by 2 statements.
All organisms are composed of cells
All cells come from preexisting cells
Cells are entities that have these 3 characteristics of living systems:
1. reproduction
2. transformation of matter and energy
3. response to their environment
Viruses cannot independently express these characteristics.
Why are cells so small?
The exchange of materials (waste, nutrients) with the external environment is favored by a large surface area-to-volume ratio.
surface area depends on r2; volume depends on r3
Thus, as r (size) increases, the surface area-to volume ratio decreases.
All cells have a plasma membrane boundary through which all traffic of nutrients and waste must pass.
The efficiency of this transfer limits cells to a relatively small size.
Cells fall into 2 broad organizational patterns: Prokaryotic and Eukaryotic
Prokaryotic:"prior to" nucleus (small cells: 0.25 to 4 m m)
includes the Eubacteria and Archaebacteria
These 2 groups were originally distinguished on the basis of their metabolism. Archaea form 3 distinctive metabolic groups:
2. halophiles
3. thermoacidophiles
All prokaryotic cells have a plasma membrane, a nucleoid region, and cytoplasm filled with ribosomes (Figure 4.3).
Shape typically maintained by a external cell wall.
Chromosome is typically a circular DNA molecule.
Eukaryotic:"true" nucleus (large cells: 5 - 100 m m)
includes the Protista, Plantae, Fungi, and Animalia
Features:
1. The DNA of eukaryotic cells is enclosed in a special double-membrane enclosed compartment called the nucleus.
2. Eukaryotic cells contain additional, metabolically specialized, membrane-enclosed compartments (organelles).
Internal comparmentation in eukaryotic cells has allowed them to overcome some of the problems of nutrient and waste traffic and achieve a relatively larger size.
Features of Eukaryotic Cells (Figure 4.7)
The abundance of internal membranes is a striking feature.
Nucleus: the most conspicuous organelle (5 m m diameter)
surrounded by nuclear envelope (a double membrane) with pores composed of a cluster of 8 proteins (the nuclear pore complex) - Figure 4.8
nuclear envelope: 2 phospholipid bilayers separated by 20-40 nm.
nuclear pores: small molecules pass freely in and out. Large molecules (proteins, RNA) are actively transported in and out under the control of the nuclear pore complex.
Nearly all the DNA in the cell is in the nucleus, organized as chromatin (DNA + protein) that is usually too dispersed to see. At times of cell division, chromatin condenses to form visible chromosomes. Each chromosome contains a single, linear DNA molecule.
Nucleolus: the most visible structure in the nucleus. Roughly spherical, densely staining, sometimes 2 or more nucleoli per cell. Site of assembly of ribosomal subunits from rRNA + protein.
Nuclear lamina: a protein meshwork attached to the nuclear envelope and to the chromatin. The nuclear lamina maintains nuclear shape.
Ribosomes Are ribosomes really organelles? They are not membrane-enclosed structures.
Thousands per prokaryotic cell; millions per eukaryotic cell.
The sites of protein synthesis, as directed by the genetic information delivered in the form of an mRNA.
Assembled in the nucleolus from rRNA synthesized in the nucleolus and a set of more than 50 proteins which are imported from the cytoplasm.
Ribosomal subunits (40S [1 million daltons] and 60S [1.8 million daltons]) are exported from the nucleus to the cytoplasm where they assemble into ribosomes (80S [2.8 million daltons], » 30 nm diameter)
Ribosomes are either: (though exchangeable between these 2 sites):
1. Free - in the cytosol, unattached to any membranes, making cytoplasmic proteins
2. Associated with the endoplasmic reticulum membrane (rough ER), making membrane proteins or proteins destined for export from the cell (secretion).
The Endomembrane System
The many different membranes of the eukaryotic cell are actually all part of an endomembrane system either directly connected to one another or related through the transfer of membrane vesicles. The total surface area of the endomembrane system is about 10 to 25 times that of the surface area of the plasma membrane.
The endomembrane system includes:
nuclear envelope
endoplasmic reticulum (ER)
Golgi complex
lysosomes
peroxisomes
vacuoles
and the plasma membrane (NOT endo, but related)
Endoplasmic Reticulum (extensive; » half the total membrane) Figure 4.16
Encloses a lumen (about 10% of the total cytoplasmic volume)
A network of interconnected compartments, each » 30 nm wide
Continuous with the nuclear envelope
Rough versus Smooth ER
Rough: flattened sacs studded with ribosomes
secretory protein synthesis
membrane biosynthesis
Smooth: fine tubules free of ribosomes
formation of transport vesicles:
functions in the transfer of products of the rough ER by budding off transport vesicles
drug-detoxifying enzymes
steroid hormone biosynthesis
The rough ER and smooth ER are connected to one another.