Biology E/M

By: Evrhett Davis

 The Discovery of Cells

Most cells are too small to be observed with the naked eye. For this reason, even the existence of cells escaped notice until scientists first learned to harness the magnifying power of lenses in the second half of the seventeenth century. At that time a Dutch clothing dealer named Antonie van Leeuwenhoek (1632–1723) fashioned extraordinarily accurate single-lens microscopes. Gazing into the lens of these microscopes, he discovered single-celled organisms, which he called “animalcules” and which, today, we call bacteria and protists.

Englishman Robert Hooke (1635–1703) expanded on Leeuwenhoek’s observations with the newly developed compound microscope, which uses two or more aligned lenses to increase magnification while reducing blurring. When Hooke turned the microscope on a piece of cork, he noticed that the tiny, boxlike compartments of the wood resembled the cells of a monastery. The term “cell” was born.

Cell Theory Emerges

As microscope technology improved, scientists were able to study cells in ever-greater detail. Hooke had no way to tell if cells were living things, but later researchers who could see the nucleus and the swirling motion of the cytoplasm were convinced that cells were indeed alive. By 1839, enough evidence had accumulated for German biologists Matthias Schleiden and Theodore Schwann to proclaim that cells are “the elementary particles of organisms.” But many researchers still did not believe that cells arose from other cells until 1855, when famous German pathologist Rudolph Virchow pronounced, “All cells come from cells.” Nearly 200 years after the discovery of cells, the observations of Virchow, Schleiden, and Schwann established the cell theory:

• All living things are made of cells.
• All cells arise from preexisting cells.

These two tenets made clear that the cell is the fundamental unit of life.

Cell Size

Cells could not be studied until the microscope was developed because they are very small. This fact raises two questions: why are cells so small, and why are living things made up of millions of tiny cells?

Cells are small because their surface area and volume must be balanced. In order to stay alive, cells with a larger volume need to carry out more chemical activity than smaller cells do. However, for metabolic activity to take place, the cell must also have enough surface area to allow an adequate supply of nutrients and waste products to move in and out of the cell. Because surface area increases at a slower rate than volume as objects get bigger, the surface area-to-volume ratio in a cell decreases dramatically as the cell gets larger. It turns out that a size of 10  provides the surface area-to-volume ratio necessary for the survival of most cells. ( the micrometer, is one thousandth of a millimeter.)

Microscopes

Two major types of microscopes allow scientists to study the miniature world of the cell.

The Light Microscope

Light microscopes use light and lenses to magnify their subjects. The most common of these used in the laboratory is the compound microscope, which creates high magnification by combining two relatively low-power lenses. The total power of a compound microscope is the power of the ocular lens, located in the eyepiece, multiplied by the power of the objective lens, located near the slide. For example, an ocular lens of 10x and an objective lens of 11x yield a total magnification of 110x. Typical high school microscopes offer magnifications of up to about 430x.

From time to time the SAT II Biology tests your knowledge of the various parts of the compound microscope, usually by showing you an image and asking you to identify the parts.

Many parts of the cell are hard to see under microscopes because they are colorless. In order to view them, scientists sometimes employ stains that mark various cell parts differently. One alternative to staining is a technique called phase contrast microscopy, which uses filters to emphasize the contrast between different parts of the cell.

The Electron Microscope

At high magnifications, light microscopes produce blurry images. In the 1950s, scientists invented a new type of microscope called the electron microscope, which offers increased image clarity, or resolving power. Electron microscopes are powerful enough to resolve individual fats and proteins. Light microscopes are still widely used, however, because electron microscopes are expensive and can only be used to view matter that is not living.

Types of Cells

There are two major types of cells: prokaryotes and eukaryotes. Eukaryotic cells, whose name derives from the Greek eu, meaning “good,” and karyon, “kernel” or “nucleus,” have a nucleus and membrane-bound organelles. Prokaryotic cells, whose name derives from the Greek pro, meaning “before,” contain neither nucleus nor organelles. As the names imply, prokaryotic cells are less evolutionarily advanced than eukaryotic cells.

Prokaryotes

Prokaryotes include some of the most primitive forms of life: bacteria and blue-green algae (also known as cyanobacteria). Prokaryotic organisms are generally single-celled.

Prokaryotes have a cell membrane, and they are made up of generally undifferentiated fluid, called the cytoplasm, in which floats a circular ring of DNA that controls the functioning of the cell. Prokaryotes maintain their shape through a cytoskeleton and have ribosomes that float in the cytoplasm. In addition, some prokaryotes have a special type of cell wall made of a protein-sugar combination called peptidoglycan. A few prokaryotes possess whiplike tails called flagella that help propel the cells through water.

Though less complex and less efficient than eukaryotes, prokaryotes are hardy because of their simplicity. They are able to survive environmental extremes that would kill higher life forms.

Eukaryotes

All living things besides bacteria and cyanobacteria consist of eukaryotic cells, which are larger and structurally more complex than prokaryotic cells. Like prokaryotes, eukaryotes are surrounded by a lipid bilayer cell membrane and have cytoplasm and ribosomes. However, unlike prokaryotes, eukaryotes also contain organelles and a defined nucleus containing DNA.

Eukaryotes benefit enormously from the presence of membrane-bound organelles. Each organelle creates an additional compartment in the cell that can specialize in particular activities or processes, increasing productivity as a result. The structure of eukaryotic cells and the specific functions of the various organelles are often tested by the SAT II Biology.

Cytoplasm

The cytoplasm refers to the entire area of the cell outside of the nucleus. The cytoplasm has two parts, the organelles and the cytosol, a grayish gel-like liquid that fills the interior of the cell. The cytosol provides a home for the nucleus and organelles as well as a location for protein synthesis and other fundamental chemical reactions.

Cytoskeleton

The cytoskeleton is a protein structure that maintains cell shape and helps move organelles around the cell. There are two types of cytoskeleton proteins: microtubules and microfilaments. Microtubules are thick, hollow rods that provide a strong scaffold for the cell. The smaller microfilaments are thin rods made of a protein called actin; they are strung around the perimeter of the cell to help it withstand strain. In some organisms, the microtubules power limbs called cilia and flagella, creating movement. Contraction of the microfilaments powers muscle movement in animals and facilitates the creeping motion of creatures like amoebas. The microtubules also form protein tracks on which organelles can slide around the cell.

The Organelles

Floating in the cytoplasm are the many membrane-bound organelles, each with a distinct structure and an important function in the processes of the cell.

NUCLEUS:

stores the cell’s genetic material in strands of DNA and choreographs life functions by sending detailed messages to the rest of the cell. The interior of the nucleus is separated from the cytosol by a membrane called the nuclear envelope, which lets only select molecules in and out. The DNA itself is wrapped around proteins known as histones in an entangled fibrous network called chromatin. When the nucleus is about to split in two, this amorphous mass coils more tightly, forming distinct structures called chromosomes. The nucleus also houses a small, dark structure called the nucleolus, which helps manufacture ribosomes.

RIBOSOMES:

synthesize proteins for the cell. Some ribosomes are mounted on the surface of the endoplasmic reticulum (see below), and others float freely in the cytoplasm. All ribosomes have two unequally sized subunits made of proteins and a substance called RNA. All living cells, prokaryotic and eukaryotic alike, have ribosomes. Ribosomes are explained in more detail in the chapter on Cell Processes as part of the larger discussion about the way the cell manufactures proteins.

MITOCHONDRIA:

produces energy for the cell through a process called cellular respiration (see the chapter on Cell Processes). The mitochondria has two membranes; the inside membrane has many folds, called cristae. Many of the key cell-respiration enzymes are embedded in this second membrane. The chemical reactions of respiration take place in the compartment formed by the second membrane, a region called the mitochondrial matrix.

ENDOPLASMIC RETICULUM:

an extensive network of flattened membrane sacs that manufactures proteins. These proteins are transferred to the Golgi apparatus, from which they will be exported from the cell. There are two types of endoplasmic reticulum: rough and smooth. Rough endoplasmic reticulum is studded by ribosomes covering its exterior. These ribosomes make the rough endoplasmic reticulum a prime location for protein synthesis. The smooth endoplasmic reticulum moves the proteins around the cell and then packages them into small containers called vesicles that travel to the Golgi apparatus. The smooth endoplasmic reticulum also functions in the synthesis of fats and lipids.

GOLGI APPARATUS:

a complex of membrane-bound sacs that package proteins for export from the cell. Proteins enter the Golgi complex from the endoplasmic reticulum and proceed through the stacks, where they are modified and stored before secretion. When proteins are ready for export, pieces of the Golgi membrane bud off, forming vesicles that send them to the cell membrane.

LYSOSOMES:

small membrane-bound packages of acidic enzymes that digest compounds and worn-out cellular components that the cell no longer needs.

Plant Cell Organelles

The organelles described above are found in both animal and plant eukaryotic cells. But plants have additional organelles—chloroplasts, vacuoles, and cell walls—that support their unique life cycles.

CHLOROPLASTS:

Animal cells break down the food that they ingest to produce energy. Plants do not need to ingest food; they manufacture their own from sunlight, using the process of photosynthesis (covered in the chapter on Plant Structure and Function). Chloroplasts are the organelles in which photosynthesis takes place. They are large oval-shaped structures containing a green pigment called chlorophyll that absorbs sunlight. Chloroplasts, like mitochondria, are built from two membranes: an external membrane forming the boundary of the organelle and a stacked inner membrane within the organelle.

VACUOLES:

large liquid-filled storage containers found in plant cells. Plant cells can put virtually anything in their vacuoles, from nutrients to wastes to water to pigments. Vacuoles can be quite large, allowing plant cells to grow to substantial -volumes without making new cytoplasm. Some animal cells in freshwater microorganisms have specialized contractile vacuoles that pump water out of the cell to prevent bursting.

CELL WALL:

Plant cells have a rigid cell wall surrounding their cell membrane. This wall is made of a compound called cellulose. The tough wall gives the plant cell added stability and protection from harm.

The Cell Membrane

The cells of all organisms, prokaryotic and eukaryotic alike, are surrounded by a thin sheet called the cell membrane. This barrier keeps cellular materials in and foreign objects out. The membrane is key to the life of the cell. By regulating what gets into and out of the cell, the membrane maintains the proper chemical composition, which is crucial to the life processes the cell carries out.

Structure of the Cell Membrane

The cell membrane is made up of two sheets of special fat molecules called phospholipids, placed on top of each other.

This arrangement is known as a phospholipid bilayer. Phospholipid molecules naturally arrange in bilayers because they have a unique structure. The long chains of carbon and hydrogen that form the tail of this molecule do not dissolve in water; they are said to be hydrophobic or “water fearing.” The hydrophilic phosphorous heads are attracted to water. Forming a bilayer satisfies the water preferences of both the “heads” and “tails” of phospholipids: the hydrophilic heads face the watery regions inside and outside the cell, and the hydrophobic tails face each other in a water-free junction. The bilayer forms spontaneously because this situation is so favorable.

The Fluid Mosaic Model

Phospholipids form the fundamental structure of the cell membrane, but they are not the only substance found there. According to the fluid-mosaic model of the cell membrane, special proteins called membrane proteins float in the phospholipid bilayer like icebergs in a sea.

The sea of phospholipid molecules and gatekeeper membrane proteins is in constant motion. The membrane’s fluidity keeps the cell from fracturing when placed under strain.

Transport Through the Cell Membrane

The most important property of the cell membrane is its selective permeability: some substances can pass through it freely, but others cannot. Small and nonpolar (hydrophobic) molecules can freely pass through the membrane, but charged ions and large molecules such as proteins and sugars are barred passage. The selective permeability of the cell membrane allows a cell to maintain its internal composition at necessary levels.

Molecules that can pass freely through the membrane follow concentration gradients, moving from the higher concentration area to the region of lower concentration. These processes take no energy and are called passive transport. The molecules that cannot pass freely across the phospholipid bilayer can be carried across the membrane in various processes that require energy and are therefore called active transport.

Passive Transport

There are three main types of passive transport: diffusion, facilitated diffusion, and osmosis. In fact, osmosis is simply the term given to the diffusion of water.

DIFFUSION

In the absence of other forces, substances dissolved in water move naturally from areas where they are abundant to areas where they are scarce—a process known as diffusion. If there is a higher concentration of carbon dioxide gas dissolved in the water inside the cell than in the water outside the cell, carbon dioxide will naturally flow out from the cell until its distribution is balanced, without any energy required from the cell.

Nonpolar and small polar molecules can pass through the cell membrane, so they diffuse across it in response to concentration gradients. Carbon dioxide and oxygen are two molecules that undergo this simple diffusion through the membrane.

The simple diffusion of water is known as osmosis. Because water is a small polar molecule, it undergoes simple diffusion. SAT II Biology problems on osmosis can be tricky: water moves from areas where it is in high concentration to areas where it is in low concentration. Remember, however, that water is found in low concentrations in places where there are many dissolved substances, called solutes. Therefore, water moves from places where there are few dissolved substances (known as hypotonic solutions) to places where there are many dissolved substances (hypertonic solutions). An isotonic solution is one in which the concentration is the same as that found inside a cell, meaning osmotic pressure in both sides is equal.

Immersing cells in unusually hypotonic or hypertonic solutions can be disastrous: water can rush into cells in hypotonic conditions, causing them to fill up so fast that they burst. To combat this possibility, many cells that need to survive in freshwater environments possess contractile vacuoles to pump out excess water.

FACILITATED DIFFUSION

Certain compounds important to the functioning of the cell, such as ions, cannot enter the cell through simple diffusion because they cannot pass through the cell membrane. As with water, these substances “want” to enter the cell if the concentration gradient demands it. For that reason, cells have developed a way for such compounds to bypass the cell membrane and flow into the cell on the basis of concentration. The cell has protein channels through the phospholipid membrane. The channels can open and close based on protein membranes. When closed, nothing can get through. When open, the protein channels allow compounds to pass through along the concentration gradient, which is diffusion.

Active Transport

Quite often, cells have to transport a substance across the cell membrane against the normal concentration gradient. In these cases, cells use another class of membrane proteins. Instead of relying on diffusion, these proteins actively pump compounds in the direction the cell wants them to go, a process that requires energy. Cells can turn active transport on and off as needed.

Endocytosis and Exocytosis

Cells use yet another type of transport to move large particles through the cell membrane. In exocytosis, waste products that need to be removed from the cell are placed in vesicles that then fuse with the cell membrane, releasing their contents into the space outside the cell. Endocytosis is the opposite of exocytosis: the cell membrane engulfs a substance the cell needs to import and then pinches off into a vesicle that is inside the cell.

There are two kinds of endocytosis: in phagocytosis the cell takes in large solid food particles that it then digests. In pinocytosis, the cell takes in drops of cellular fluid containing dissolved nutrients.