The subjects for today's lesson are the properties of living cells. You must follow the lesson, answer the questions, then complete the research investigations, if required.

"Let us begin .......

It was Hans and Zacccharias Janssen, the eyeglass makers, who put two glass lenses together and discovered the enormous magnifying power this combination produced.

"It was this property of light magnification that led to the invention of the first microscope, an instrument which was used by Robert Hooke to discover the world of living cells. This discovery was later advanced by Antonie van Leeuwenhoek.

"However, it was not until this century (the nineteenth century - remember!) that microscopes have been so improved that our scientists are discovering an amazing new array of properties for these 'tiny units of life'.

Life is Cellular - the basic unit of life.

"It appears that all living things are made up of one or more of these cellular units.

"With this discovery scientists had to develop whole new ways of measuring objects so small they cannot be seen with the naked eye. The metric system had just been invented by the Emperor Napoleon's people, and this was the perfect tool for the job.

"But this did not answer the question 'Why are living cells this small?'. It turns out that the answer has to do with the properties of surface area and volume, which change in different ways as cells grow in size.

Types of Cells - two basic plans.

"We are not completely sure where the first cell came from, but it must have been a long time ago, and it's structure must have been very simple; probably very similar to the tiny bacteria that live in, and around, us everywhere.

"Today, however, we recognize two main types of cell; prokaryotes and eukaryotes. Prokaryotic cells are the simplest systems that show all the well recognized 'signs of life'. But it is eukaryotic cells that have the ability to join together and make larger structures, like ourselves, which are multicellular.

Eukaryotic Cells - the basic plan.

"Eukaryotic cells come in an enormous variety of shape, form and function, but almost all eukaryotic cells share a common, basic construction plan. Separating the contents of the cell from its external environment is a plasma membrane, which acts as a very selective barrier, keeping wanted materials inside, and unwanted materials outside. Embedded within the membrane are specialized proteins, which allow the passage of needed molecules.

"Although interactions between the cell and the exterior take place by way of proteins embedded in the plasma membrane, there are specialized extensions, the cilia and flagella, which can be used to move the cells through the surrounding liquid, or move the liquid over the cells.

"Cells move in other ways as well. To move without the use of cilia or flagella, cells must change shape, and to change shape requires the force of shape changing molecules.

"Early microscopists saw that cells contained a fluid. The name for this fluid and the ideas about what it did changed quite a lot since the early years, but now it is recognized that interior of the cell is a complex, semifluid that functions as a reservoir of raw materials, and the site of a vast amount of macromolecular synthesis.

"Within the cytoplasm of each cell is a complex network of protein assemblies, the cytoskeleton, which plays many roles in transporting material around inside the cell, and providing structural support. Most of these elements are constructed of different types of proteins, each with its own special role to play.

"The part of the cell involved in protein synthesis is the endoplasmic reticulum (the "inside cell network") sometimes simply called the ER. The ER is a finely divided system of membrane-enclosed compartments with an interconnecting network of tubules.

"Most, but not all eukaryotic cells, plant and animal, contain a specialized internal membrane system called the Golgi apparatus. The major function of these organelles is to process and export of materials from cells and to make lysosomes, and other vacuoles.

"Mitochondria are small cytoplasmic organelles found in all eukaryotic cells that use oxygen in the release and conversion of energy. They contain a highly specialized and integrated system of enzymes and other proteins that progressively release energy from fuel molecules and convert it into a form that can be used for anabolism and movement.

"The largest and most clearly visible of the cell's constituents is the nucleus. Within the nucleus, DNA molecules, the cell's genetic machinery, are stored, repaired, transcribed and eventually replicated.

"Plant cells, in addition to all the other organelles found in eukaryotic cells, possess two other specialized cellular structures.

"The cell wall is a box-like layer of material synthesized by plant cells outside the plasma membrane. Hooke saw the cell walls of cork cells when he first looked through his microscope. Cellulose fibers form the basic skeletal foundations of cell walls.

"Plant cells can contain several different types of plastids. These are organelles such as the amyloplast, which is involved in starch storage, and chromoplasts which contain colored pigments.

"However the most easily recognizable plastid is the chloroplast, a green organelle that harvests light, then uses the trapped energy to synthesize sugar molecules, which are then stored as starch.

Cell Division - getting new cells from old cells.

"All living cells are produced by the division of pre-existing cells - this is the universal process of cell division. However, the mechanism of the division process is different in prokaryotic an eukaryotic cells.

"The cell division process in eukaryotic cells is often called Mitosis after the part of the cell cycle that can easily be seen taking place using a light microscope. It is a regularly repeating pattern of events that include the growth of the cell, the synthesis of new DNA molecules and then the packaging and delivery of these new chromosomes to the new daughter cells.

In and Out - getting things into and out of cells.

"One property that is important to all cells is getting things in and out of them. Random movement of substances from one region to the next is called diffusion, and is a property of all substances when dissolved in water.

"But cells are surrounded by a plasmamembrane, or cell membrane, which acts as a barrier to the free movement of substances by diffusion. Water molecules, however, move and diffuse freely though membranes, a property that gives rise to the phenomenon of osmosis. It is possible to use the property of water movement across a semi-permiable membrane to calculate the molecular weights of various molecules.

"But really important materials, that are needed by cells despite their scarcity, have to be actively moved across the cell membrane. There are two types of transport mechanism that involve protein carrier molecules. These shuttle materials from one side of the membrane to the other.

"Larger materials, such as chunks of food, cannot go directly through the membrane, but are taken inside the cell by wrapping it up in part of the outer membrane, which then becomes an internal package of material, called a vacuole.

Below the threshold - viruses.

"Not really alive, viruses are tiny particles of protein and nucleic acid that spend part of their lives outside living cells, and part of their cycle inside cells, where they take over the synthetic machinery and make more of themselves".

e-textbook
Table of Contents -|- The Cell Theory -|- The First Cells

Departments
Cellular Life -|- Microscopes