Chapter 4: Cell Structure and Function
Animalcules and the First Microscopes 

A. Early Microscopists

1. Galileo saw details of insect eyes with two crude lenses.

2. Robert Hooke used simple lenses to observe cork in which he saw tiny compartments he called cells (cellulae).

3. van Leeuwenhoek saw protistans, sperm, and bacteria with his lenses and microscopes.

B. The Cell Theory

1. Schleiden (a botanist) and Schwann (a zoologist): believed that all plants and animals consist of cells.

2. Virchow: cells come from preexisting cells.

C. The Cell Theory: three generalizations:

1. All organisms are composed of one or more cells.

2. The cell is the smallest unit having the properties of life.

3. The continuity of life arises directly from the growth and division of single cells.

I. Basic Aspects of Cell Structure and Function

A. Structural Organization of Cells

1. The cell is the smallest entity that still retains the characteristics of life.

2. All cells have three basic parts:

a. 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.

b. A DNA-containing region occupies a portion of the interior.

c. The cytoplasm contains membrane-bound compartments (except bacteria), particles, and filaments&endash;all bathed in a semifluid substance.

3. Eukaryotic cells are defined by their possession of a membrane-bound nucleus.

4. Prokaryotic cells have no defined nucleus; the only representatives are bacteria.

B. Fluid Mosaic Model of Cell Membranes

1. The "fluid" portion of the cell membrane is made of phospholipids.
a. A phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails.

b. If phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and a bilayer results; hydrophilic heads are at the outer faces of a two-layer sheet.

c. Bilayers of phospholipids are the structural foundation for all cell membranes.

2. Within a bilayer, phospholipids show quite a bit of movement; they diffuse sideways, spin, and flex their tails to prevent close packing and promote fluidity, which also results from short-tailed lipids and unsaturated tails (kink at double bonds).

C. Overview of Membrane Proteins

1. A variety of different proteins are embedded in the bilayer or positioned at its two surfaces.

2. Membrane proteins serve as transport proteins, receptor proteins, recognition proteins, and adhesion proteins.

II. Cell Size and Cell Shape

A. Because of their small size, most cells can only be seen by using light and electron microscopes.

B. Cell size must be small; remember surface-to-volume ratio!

A cell that is too large will not be able to move materials into and out of the cell.
III. Focus On Science: Microscopes: Gateways to Cells

IV. The Defining Features of Eukaryotic Cells

A. Major Cellular Components

1. Organelles form compartmentalized portions of the cytoplasm.

2. All eukaryotic cells contain organelles.

a. The nucleus controls access to DNA and permits easier packing of DNA during cell division.

b. The endoplasmic reticulum (ER) modifies newly formed polypeptide chains and is also involved with lipid synthesis.

c. The Golgi body modifies, sorts, and ships proteins; they also play a role in the synthesis of lipids for secretion or internal use.

d. Vesicles transport material between organelles and function in intracellular digestion.

e. Mitochondria are efficient factories of ATP production.

3. Cells also contain non-membranous structures:

b. Ribosomes, "free" or attached to membranes, participate in assembly of polypeptide chains.

g. The cytoskeleton helps to determine cell shape, internal organization, and movements.

4. Organelles separate reactions with respect to time (allowing proper sequencing) and space (allowing incompatible reactions to occur in close proximity).

B. Which Organelles Are Typical of Plants?

1. Figure 4.7a gives the locations of plant cell parts.

2. Although it is labeled "typical," no one diagram can speak for all variations in plant cells.

C. Which Organelles Are Typical of Animals?

1. Figure 4.7b gives the locations of animal cell parts.

2. Although it is labeled "typical," no one diagram can speak for all variations in animal cells.

3. Also notice the differences between plant and animal cells, particularly the cell wall and large central vacuole of plant cells.

 

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.

1. Localization of the DNA makes it easier to sort out hereditary instructions when the time comes for a cell to divide.

2. The membranous boundary of the nucleus helps control the exchange of signals and substances between the nucleus and the cytoplasm.

B. Nuclear Envelope

1. The nuclear envelope consists of two lipid bilayers with pores.

2. It surrounds the nucleoplasm within.

3. On the inner surface are attachment sites for protein filaments that anchor the DNA molecules and keep them organized.

C. Nucleolus

1. Located within the nucleus, the nucleolus appears as a darker globular mass.

2. It is a region where subunits of ribosomes are prefabricated before shipment out of the nucleus.

D. Chromosomes

1. Chromatin refers to the cell’s total collection of DNA and associated proteins.

2. A chromosome is an individual DNA molecule and its associated proteins.

3. DNA is duplicated and condensed before cell division occurs.

E. What Happens to the Proteins Specified by DNA?

1. Some of the polypeptide chains assembled on the ribosomes are stockpiled in the cytoplasm.

2. Others pass through the cytomembrane system, where they take on their final form and become packaged in vesicles for use within the cell or for export.

VI. The Cytomembrane System

A. Endoplasmic Reticulum

1. The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs that begin at the nucleus and ramble through the cytoplasm.

2. There are two types distinguished by the presence or absence of ribosomes:

a. Rough ER consists of stacked, flattened sacs with many ribosomes attached; oligosaccharide groups are attached to polypeptides as they pass through on their way to other organelles or to secretory vesicles.

b. Smooth ER has no ribosomes; it is the area from which vesicles carrying proteins and lipids are budded; it also inactivates harmful chemicals.

B. Golgi Bodies

1. In the Golgi bodies, proteins and lipids undergo final processing, sorting, and packaging.

2. The membranes of the Golgi are arranged in stacks of flattened sacs whose edges break away as vesicles.

C. A Variety of 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 are vesicles containing enzymes that break down fatty acids and amino acids; the hydrogen peroxide released is degraded by another enzyme.

VII. Mitochondria

A. Mitochondria are the primary organelles for transferring the energy in carbohydrates to ATP under oxygen-plentiful conditions.

B. Hundreds of thousands of mitochondria occur in cells.

1. It has two membranes, an inner folded membrane (cristae) surrounded by a smooth outer membrane.

2. Inner and outer compartments formed by the membranes are important in energy transformations.

3. Mitochondria have their own DNA and some ribosomes, a fact which points to the possibility that they were once independent entities.

VIII. Specialized Plant Organelles

A. Chloroplasts and Other Plastids

1. Chloroplasts are oval or disk shaped, bounded by a double membrane, and 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 have carotenoids, which impart red-to-yellow colors to plant parts, but no chlorophyll.

3. Amyloplasts have no pigments; they store starch grains in plant parts such as potato tubers.

B. Central Vacuole

1. In the mature plant, the central vacuole may occupy 50 to 90% of the cell interior!
a. stores amino acids, sugars, ions, and wastes.

b. enlarges during growth and greatly increases the cell’s outer surface area.

2. The cytoplasm is forced into a very narrow zone between the central vacuole and the plasma membrane.

IX. The Cytoskeleton

A. Main Components

1. The cytoskeleton is an interconnected system of fibers, threads, and lattices that extends between the nucleus and the plasma membrane.

2. It gives cells their internal organization, overall shape, and capacity to move.

3. The main components are microtubules, microfilaments, and intermediate filaments: all assembled from protein subunits.

4. 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. The Structural Basis of Cell Movements

1. Through the controlled assembly and disassembly of their subunits, microtubules and microfilaments grow or shrink in length (example: movement of chromosomes).

2. Microfilaments or microtubules actively slide past one another (example: muscle movement).

3. Microtubules or microfilaments shunt organelles from one location to another (example: cytoplasmic streaming).

C. Flagella and Cilia

1. Flagella are quite long, are usually not numerous, and are found on one-celled protistans and animal sperm cells.

2. 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.

3. Both of these extensions of the plasma membrane have a 9 + 2 cross-sectional array (arising from centrioles) and are useful in propulsion.

X. Cell Surface Specializations

A. Eukaryotic Cell Walls

1. Many single-celled eukaryotes have a cell wall, a supportive and protective structure outside the plasma membrane

2. Microscopic pores allow water and solute passage to and from underlying plasma membrane.

3. In plants, bundles of cellulose strands form the primary cell wall, which is more pliable than the more rigid secondary wall that is laid down inside it later.

4. Plasmodesmata are the channels that cross the adjacent walls to connect the cytoplasm of neighboring cells.

B. Matrices Between Animal Cells

1. This is a meshwork that holds animal cells and tissues together and influences how the cells will divide and metabolize.

2. Cartilage consists of cells and proteins (collagen and elastin) scattered in a ground substance (modified polysaccharides).

C. Cell-to-Cell Junctions

1. At tissue surfaces, cells link together to form a barrier between the interior and exterior.

2. Three cell-to-cell junctions are common.

a. Tight junctions link cells of epithelial tissues to form seals.

b. Adhering junctions are like spot welds in tissues subject to stretching.

c. Gap junctions link the cytoplasm of adjacent cells; they form communication channels.

XI. Prokaryotic Cells: The Bacteria

A. The term prokaryotic (literally, "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.

B. 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.

3. Bacterial flagella (without a 9+2 array) provide movement; pili on the cell surface help bacteria attach to surfaces and one another.