Zelda L. Kravitz
All individuals are made of cells. Every human being begins as a single fertilized cell that divides over and over to make billions of cells in a mature human being. The process of cellular reproduction holds the key to heredity in all living things.
The cell theory states simply that:
1. all living matter is composed of cells,
2. all cells arise from other cells,
3. all the metabolic reactions of living organisms, including all energy exchanges and all processes, take place within the cells. Metabolism is simply the sum of all the chemical reactions taking place in a living system.
It also states that cells contain the hereditary information of the organisms of which they are a part, and that this information is passed from parent cell to daughter cell.
Organization of the cell
. The cell is the smallest part of the living organism which carries on life functions. There are many different kinds of cells. Within our bodies there are more than 100 different and distinct types. Thus the first remarkable fact about cells is their diversity.
The second, even more remarkable fact is their similarity. Every cell is a self contained unit surrounded by an outer membrane that controls the passage of materials in and out of the cell; and so makes it possible for the cell to differ biochemically and structurally from its surroundings. All cells also, at least sometime in their existence, contain control centers, or nuclei. Many have a variety of internal structure, the organelles, which are similar or identical from one cell to another throughout a wide range of cell types. Also, all are composed of the same remarkably few kinds of atoms and molecules.
. All cells are basically very similar combining many of the same structures, the same types of enzyme systems, and same kind of genetic material. An outer membrane, apparently conforming to the same biochemical design, in all cells, surrounds the cells. The cell membrane holds the cells together and separates one cell from other cells. The cell membrane is selective and allows materials in solution to pass in and out of the cell, by a process called diffusion. The living materials bounded by the membrane, sometimes called protoplasm, consists of the cytoplasm and the nucleus.
. The nucleus is a large, spherical body, usually the most prominent structure within the cell. It is surrounded by two unit membranes which together make up the nuclear envelope. Penetrating the surface of the envelope are nuclear pores, which are covered by a thin single membrane. These pores apparently permit only specific large molecules to go through, thus keeping the chemical composition of the nuclear material different from that of the cytoplasm. The chromosomes, which are composed of DNA (deoxyribonucleic acid) and protein, are found within the nucleus. When the cell is not dividing, the chromosomes are visible only as a tangle of fine threads, called chromatin. The most conspicuous body within the nucleus is the nucleolus. The nucleolus, which is composed of DNA and protein, like the chromosomes; and is formed from a portion of a chromosome, is the site at which a particular kind of RNA (ribonucleic acid) is formed. RNA is a messenger that takes orders from the DNA and delivers them to the cell.
Functions of the nucleus
. The nucleus performs two crucial functions for the cell:.
1. It carries the heredity information for the cell—the instructions that determine whether a particular organism will develop to become a paramecium, one celled bacteria , oak tree, or a human being—and not just any paramecium, oak tree, or human being, but one that resembles the parent or parents of that particular unique organism.
2. The nucleus directs the ongoing activities of the cell, ensuring that the complex molecules the cell requires are synthesized in the number and of the kind needed. The cell nucleus is usually the largest structure in the cell.
. The cytoplasm is inside the cell. Water makes up 75-90% of it. The cytoplasm moves constantly and with it carries particles that are suspended called organelles. Each organelle has at least one definite job or function. Each organelle has a definite form.
. In some cells are vacuoles which are gas or liquid filled. They regulate the water content, store food, and give off waste.
. This substance is a double membrane network in the cytoplasm. It also connects the outer layer of the nuclear membrane with the inner layer of the cell membrane.
. Ribosomes are made of RNA and produce protein. They are organelles of beadlike structure located along the endoplasmic reticulum. These proteins are needed for growth, reproduction, and cell part repair. Certain antibiotics stop the action of Ribosomes.
. They take care of processing secretory functions of protein.
take care of the digestion within cells.
. The mitochondria contain enzymes that transfer Hydrogen to Molecular Oxygen. They also convert sugar and starch to energy, trigger the energy release, or cellular respiration.
At this point students can make their cell model according to the directions in the activity page. Overlays on the projector are also helpful.
All living matter is composed of cells. All cells arise from other cells. The metabolism of living systems—the sum total of their biochemical activities take place within cells and as a result of cellular activities.
Cells are units of protoplasm—living matter—separated from their environment by an outer cell membrane which restricts the passage of materials in and out of the cell and so protects the cell’s structural and functional integrity. A living cell either reproduces or dies, and each new daughter cell is an identical copy of the parent. Cells divide when they are ready, and they are ready only when they have completed certain preparations for divisions. Something limits cell size and halts the growth process. The size of a cell is limited by its nucleus to support growth. Hidden in the cell is some process that makes it possible for the cell to enter each phase.
(A quiz might be given at this point, and there is one included in the activity sheet).