Chapter 4 Cell Structure and Function: An Overview

I. Pastures of the Seas
A. Vast populations of single-celled organisms (phytoplankton) carry on photosynthesis.
B. These cells also respond to changes in their oceanic environment.

II. Generalized Picture of the Cell
A. Emergence of the Cell Theory
1. In the early seventeenth century, Galileo Galilei first used a microscope to observe a biological specimen (an insect eye).
2. By mid-seventeenth century, Robert Hooke first used the term “cell” to refer to the compartments in a slice of cork.
3. Anton van Leeuwenhoek observed a bacterium with the use of microscope.
4. In the 1820s, Robert Brown found a structure in every plant cell and called it the nucleus.
5. In 1839, Theodor Schwann noted cells in animal tissues.
6. Matthias Schleiden concluded that all plant tissues are composed of cells.
7. Schwann first stated two aspects of the cell theory: All life is composed of cells, and the cell is the basic unit of life.
8. A decade later Rudolf Virchow added another part to the cell theory: All cells arise from preexisting cells.
B. Basic Aspects of Cell Structure and Function
1. All cells have a nucleus (or nucleoid), cytoplasm, and a plasma membrane.
2. A plasma membrane separates each cell from the environment, permits the flow of molecules across the membrane, and contains receptors that can affect the cell’s activities.
3. A nucleus is membrane bound and contains DNA. A bacterial cell’s DNA is found in the nucleoid region that is not membrane bound.
4. The cytoplasm contains membrane systems, particles, filaments (the cytoskeleton), and a semifluid substance.
C. Cell Size and Shape
1. Most cells are too small to be seen without a microscope.
2. Light microscopes can explore details of 0.2 micrometer, and electron microscopes can observe much smaller details.
3. The small size of cells permits efficient diffusion across the plasma membrane and within the cell.
4. As the surface area of a cell increases to the square of the diameter, the volume increases to the cube of the diameter.

III. Prokaryotic Cells—The Bacteria
A. Bacteria are some of the smallest and simplest cells.
1. A somewhat rigid cell wall supports the cell and surrounds the plasma membrane, which regulates transport into and out of the cell.
2. Ribosomes, protein assembly sites, are dispersed throughout the cytoplasm.
B. The term prokaryotic (“before the nucleus”) indicates existence of bacteria before evolution of cells with a nucleus; bacterial DNA is clustered in a distinct region of the cytoplasm.

IV. Eukaryotic Cells
A. Functions of Organelles
1. All eukaryotic cells contain organelles.
2. Organelles form compartmentalized portions of the cytoplasm.
3. Organelles separate reactions with respect to time (allowing proper sequencing) and space (allowing incompatible reactions to occur in close proximity).
B. Typical Components of Eukaryotic Cells
1. The nucleus controls access to DNA and permits easier packing of DNA during cell division.
2. The endoplasmic reticulum (ER) modifies proteins and is also involved with lipid synthesis.
3. Golgi bodies also modify proteins, sort and ship proteins, and play a role in the biology of lipids for secretion or internal use.
4. Lysosomes are involved with intracellular digestion.
5. Transport vesicles transport material between organelles and to and from the cell surface.
6. Mitochondria have enzymes capable of ATP formation.
7. Ribosomes are “free” or attached to membranes.
8. The cytoskeleton determines cell shape and provides for motility.
9. Plastids are present in photosynthetic cells and function in food production and storage.
10. Central vacuoles and a cell wall are found in many protistans, fungi, and plants.

V. The Nucleus
A. The nucleus isolates DNA, which contains the code for protein assembly, from the sites (ribosomes in cytoplasm) where proteins will be assembled.
B. The nucleus has the following components:
1. The nuclear envelope consists of two lipid bilayers with pores and a ribosome-studded outer surface.
2. The nucleolus is a region where subunits of ribosomes are prefabricated before shipment out of the nucleus.
3. Chromosomes are composed of DNA and associated proteins (some serve as enzymes, others for support); DNA is duplicated and condensed before cell division occurs.

VI. Cytomembrane System
A. Within the cytoplasm, newly formed polypeptide chains may be in solution or may enter the cytomembrane system (ER, Golgi, lysosomes, vesicles).
B. Endoplasmic Reticulum and Ribosomes
1. The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs.
2. Two kinds of ER may be found in a cell:
a. Smooth ER has no ribosomes; it is the area from which vesicles carrying proteins and lipids are budded; it also inactivates harmful chemicals.
b. Rough ER consists of stacked, flattened sacs with many ribosomes attached; polysaccharide groups are attached to polypeptides as they pass through on their way to other organelles or to secretory vesicles.
C. Golgi Bodies
1. Here proteins and lipids undergo final processing, sorting, and packaging.
2. A Golgi body consists of flattened sacs—resembling a stack of pancakes—whose edges break away as secretory vesicles.
D. Assorted Vesicles
1. Lysosomes are vesicles that bud from Golgi bodies; they carry powerful enzymes that can digest the contents of other vesicles, worn-out cell parts, or bacteria and foreign particles.
2. Peroxisomes contain enzymes that use oxygen to degrade fatty acids and amino acids; the harmful byproduct, hydrogen peroxide, is converted to water.
3. Glyoxisomes are abundant in certain seeds such as peanuts where enzymes in the vesicles convert fats and oils to sugars necessary for rapid growth.

VII. Mitochondria
A. Mitochondria are the primary organelles for transferring the energy in carbohydrates to ATP under oxygen-plentiful conditions.
B. Mitochondria may have originated from ancient bacteria engulfed by predatory cells.
C. Each mitochondrion has an outer membrane and an inner folded membrane (cristae).
1. Two compartments are formed by the membranes.
2. Hydrogen ions and electrons move between the compartments during ATP formation.

VIII. Specialized Plant Organelles
A. Chloroplasts and Other Plastids
1. Chloroplasts are oval or disk shaped, bounded by a double membrane, and are critical to the process of photosynthesis.
a. In the stacked disks (grana), pigments and enzymes trap sunlight energy to form ATP.
b. Sugars are formed in the fluid substance (stroma) surrounding the stacks.
c. Pigments such as chlorophyll (green) confer distinctive colors to the chloroplasts.
2. Chromoplasts store red and brown pigments that give color to petals, fruits, and roots.
3. Colorless amyloplasts store starch granules.
B. Central Vacuoles
1. In a mature plant, the central vacuole may occupy 50 to 90 percent of the cell interior.
a. Central vacuoles store amino acids, sugars, ions, and wastes.
b. The vacuole enlarges during growth and greatly increases the cell’s outer surface area.
2. The enlarged cell, with more surface area, has an enhanced ability to absorb nutrients.

IX. The Cytoskeleton
A. Components of the Cytoskeleton
1. An interconnected system of bundled fibers, slender threads, and lattices extends from the nucleus to the plasma membrane.
2. The main components are microtubules, microfilaments, and intermediate filaments—all assembled from protein subunits.
3. Some portions are transient, such as the “spindle” microtubules used in chromosome movement during cell division; others are permanent, such as filaments operational in muscle contraction.
B. Flagella and Cilia
1. Microtubular extensions of the plasma membrane have a 9 + 2 cross-sectional array and are useful in propulsion.
2. Flagella are quite long, not usually numerous, and found on one-celled protistans and animal sperm cells.
3. Cilia are shorter and more numerous and can provide locomotion for free-living cells or may move surrounding water and particles if the ciliated cell is anchored.
C. MTOCs and Centrioles
1. Microtubule organizing centers (MTOCs) are small masses of proteins in the cytoplasm.
2. An MTOC near the nucleus of animal cells includes a pair of centrioles that govern the plane of cell division.
3. Centrioles also serve as patterns for the assembly of basal bodies, which in turn organize flagella and cilia microtubules.

X. Cell Surface Specializations
A. Cell Walls
1. Most are carbohydrate frameworks for mechanical support in bacteria, protistans, fungi, and plants; cell walls are not found in animals.
2. They provide support and resist excessive expansion during growth.
3. Microscopic pores allow water and solute passage to and from underlying plasma membrane.
4. Cutin, suberin, and waxes are embedded in many plant cell walls for protection and to reduce water loss.
B. Extracellular Matrix and Cell Junctions
1. This is a meshwork that holds animal cells and tissues together and influences how the cells will divide and metabolize.
a. Its components include collagen, other fibrous proteins, glycoproteins, and polysaccharides that form the “ground substance” through which molecules diffuse from cell to cell.
b. In multicelled plants, adjacent cells are cemented together (middle lamella) by pectin compounds.
2. Cell junctions in animal tissues occur where cells link together to form a barrier between interior and exterior.
a. Tight junctions occur between cells of epithelial tissues in which cytoskeletal strands of one cell fuse with strands of neighboring cells.
b. Adhering junctions are like spot welds at the plasma membranes of two adjacent cells that need to be held together during stretching.
c. Gap junctions are small, open channels that directly link the cytoplasms of adjacent cells.
3. Cell junctions in plants link cell walls together but channels (plasmodesmata) connect cytoplasms.