8th Class Science Cell - Structure and Functions Cell Structure and Function

Cell Structure and Function

Cell Structure

Cells fall into one of the two categories: prokaryotic or eukaryotic. In a prokaryotic cell, found only in bacteria and archaebacteria, all the components, including the DNA, mingle freely in the cell's interior, a single compartment. Eukaryotic cells, which make up the plants, animals, fungi and all other life forms contain numerous compartments or organelles within each cell. The DNA in eukaryotic cells is enclosed in a special organelle called the nucleus, which serves as the cell's command centre and information library. The term prokaryote comes from Greek word that mean "before nucleus" or "prenucleus," while eukaryote means "true nucleus."

Prokaryotic Cells

Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003mm in diameter. These cells, which can be rodlike, spherical or spiral in shape are surrounded by a protective cell wall.  

Eukaryotic Cells

Eukaryotic cells are typically about ten times larger than the prokaryotic cells. In animal cells, the plasma membrane rather than a cell wall, forms the cell's outer boundary. With a design similar to the plasma membrane of prokaryotic cells, it separates the cell from its surroundings and regulates the traffic across the membrane.

The eukaryotic cell cytoplasm is similar to that of the prokaryotic cell except for one major difference. Eukaryotic cells have a nucleus and numerous other membrane-enclosed organelles. Like separate rooms of a house, these organelles enable specialized functions to be carried out efficiently. For example, the building of proteins and lipids takes in separate organelles where specialized enzymes geared for each job are located.

The nucleus is the largest organelle in an animal cell. It contains numerous strands of DNA, the length of each strand being many times the diameter of the cell. Unlike the circular prokaryotic DNA, long sections of eukaryotic DNA pack into the nucleus by wrapping around proteins. As a cell begins to divide, each DNA strand folds over on to itself several times, forming a rod-shaped chromosome.

Cell Functions

To stay alive, cells must be able to carry out a variety of functions. Some cells must beable to move and most cells must be able to divide. All cells must maintain the right concentration of chemicals in their cytoplasm, ingest food and use it for energy, recycle molecules, expel wastes and construct proteins. Cells must also be able to respond to changes in their environment.

Movement

Many unicellular organisms swim, glide, thrash or crawl to search for food and escapes enemies. Swimming organisms often move by means of a flagellum, a long tail-like structure made of protein. For example, many bacteria have one, two or many flagella that rotate like propellers to drive the organism along. Some single-celled eukaryotic organisms such as euglena also have a flagellum, but it is longer and thicker than the eukaryotic flagellum. The eukaryotic flagellum works by waving up and down like a whip. In higher animals, the sperm cell uses a flagellum to swim toward the female egg for fertilization.

Energy

Cells require energy for a variety of functions, including moving, building up and breaking down molecules and transporting substances across the plasma membrane. Nutrients contain energy, but cells must convert the energy locked in nutrients to another form specifically, the ATP molecule. In single-celled eukaryotic organisms, such as the paramecium and in multicellular eukaryotic organisms, such as plants, animals and fungi mitochondria are responsible for this task. The interior of each mitochondria consists of an inner membrane that is folded into a mazelike arrangement of separate compartments called cristae. Within the cristae, enzymes form an assembly line where the energy in glucose and other energy-rich nutrients is harnessed to build ATP, thousands of ATP molecules are constructed each second in a typical cell. In most eukaryotic cells, this process requires oxygen and is known as aerobic respiration.

Cell Division

Organisms rely on cell division for reproduction, growth, repair and replacement of damaged or worn out cells. There are three types of cell division: binary fission, mitosis

And meiosis. Binary fission, the method used by prokaryotes, produces two identical cells from one cell. The mere complex process of mitosis, which also produces two genetically identical cells from a single cell, is used by many unicellular eukaryotic organisms for reproduction. Multicellular organisms use mitosis for growth, cell repair, and cell replacement.

The type of cell division required for sexual reproduction is meiosis. Sexually reproducing organisms include seaweeds, fungi, plants, and animals—including human beings. Meiosis differs from mitosis in that cell division begins with a cell that has a full complement of chromosomes and ends with gamete cells, such as sperm and eggs, that have only half the complement of chromosomes. When a sperm and egg unite during fertilization, the cell -suiting from the union, called a zygote, contains the full number of chromosomes.