Study of Cells

Historical Development of the Cell Theory

Many scientist working during the past 300 years have contributed to the cell theory. A historical development of the cell theory is briefly given below.

In 1665, Robert Hooke, an English scientist, examined a thin sliced of cork under a microscope. Hooke could see that the cork was composed of many small compartments. Because the compartments resembled the little rooms, or cells, of a monastery, Hooke named these compartments cells. The cells that Hooke observed were not living. Hooke did not pursue his discovery by investigating the structure or function of living cells.

Almost 200 years passed before biologists came to understand the importance of calls. In 1835, the French biologists Felix Dujardin determined that many microorganisms are composed of a single cell. Dujardin also observed that the internal substance of all living cells was similar.

Three years later, Matthias Schleiden and Schwann concluded that all animals are composed of cells. Schleiden and Schwann then suggested that cells are the basic living components of all organisms.

Twenty years later, Rudolf Virchow, another German biologist, wrote that the body is “a state in which every cell is a citizen.” From his observation of dividing cells, Virchow concluded that cells can arise only from other cells.

The observations of these and other scientists formed the basis of cell theory. The cell theory states that

a. The cell is the basic unit structure of living things. All living things are composed of cells or the products of cells.

b. The cell is the basic unit of function of living things.

c. All cells come from other cells by the process of cell division.

Since Hooke’s discovery of cells, scientists have developed a wide variety of techniques to study cells. The earliest tool used was the light microscope. Scientists discovered that treating cells with various chemicals, or stains, would color certain parts so they could be distinguished under the microscope. This allow the biologists to investigate the internal structure of cells.

More recently, the electron microscope and other tools have been used to study cells. Modern tools and techniques have allowed biologists to increase greatly their knowledge of cells. All current findings continue to support the cell theory

Differences between Prokaryotes and Euakaryotes

Parts of a Euakaryotic Cell

A. The Cell or Plasma Membrane

Surrounding a cell is a thin layer called the plasma membrane. The plasma membrane is more than just a boundary between the inside and outside of the cell. Among other functions the plasma membrane determines which molecules may enter or leave the cell.

Plasma membrane is made up of a double layer of phospholipid molecules. Each phospholipid is composed of a lipid and a phosphate group. Embedded in the phospholipids are protein molecules.

B. The Nucleus

The nucleus is a spherical structure that is usually located near the center of the cell. The nucleus directs the activity of the cell. Within the nucleus is a material called chromatin. Chromatin is readily stained than the rest of the nucleus. The chromatin contains the hereditary information of the cell. When a cell reproduces, the chromatin becomes visible as long strands called chromosomes.

1. Nuclear membrane - enclosed the nucleus and it contains pores through which only certain substances can pass. It also keeps the contents of the nucleus separate from the rest of the cell.

2. Nucleolus - is a chromatin condensed into a darker area. The nucleolus is involved in the production of ribosomes, which are organelles involved in protein synthesis.

C. Protoplasm and Its Organelles

The plasma membrane enclose a mass of jellylike material called protoplasm. The protoplasm is a colorless or somewhat grayish, translucent, viscous substance that is colorless or somewhat grayish that is capable of flowing.

Among them are:

a. Mitochondria - contain an enzymes that release the energy stored in food in the process of cellular respiration. For this reason, mitochondria are sometimes called powerhouse of the cell. It is surrounded by two membranes. The inner membrane is highly folded, forming cristae, which are sites of many enzymes of the respiratory processes of the cell.

b. Ribosomes - are small granules found within a specialized region of the nucleus called nucleolus and in the outer lining of the rough endoplasmic reticulum. Ribosomes are composed of nucleic acids and proteins. The synthesis of proteins occurs on the ribosomes.

c. Endoplasmic Reticulum - consists of a system of canals which form a continuous network throughout the cytoplasm. The canals serve as a path for the transport of materials throughout the cell. The canals are enclosed by membranes which have structure similar to the cell membrane and nuclear membrane. There is evidence that rough endoplasmic reticulum is involved both in producing proteins and preparing them for shipment out of the cell. Golgi bodies serve as packaging and distribution centers, especially for substances formed on the endoplasmic reticulum.

d. Golgi Bodies - are believed to prepare proteins for secretion after they are released from the endoplasmic reticulum.

e. Vacuoles - are fluid-filled “bubbles” in the cytoplasm. They are bordered by a unit membrane that is identical to the cell membrane. They serve as storage center of the cell.

f. Lysosomes - are also membrane-bound organelles. They contain enzymes that are capable of digesting the cell’s proteins.

g. Cytoskeleton - is a miniature internal support system is made up of microtubules and other tiny protein structures. Together these structural elements give the cell its shape.

h. Centrioles - consists of microtubules which is seldom found in plants. Near the nucleus of the animal cell is a pair of centrioles which are involved in cell replication where they function as centers for organization of the spindle. They also serve as template for the organization and development of cilia and flagella.