CELLS: THE BASIC UNITS OF LIFE

This section will introduce you to cells, their discovery and their diversity. You will also learn the parts of the cell and their functions.  

Why weren´t cells discovered until 1665?
What invention made their discovery possible?

Cells weren´t discoverd until 1665 because almost all cells are too small to be seen with the naked eye. The microscope is the invention that made their discovery possible. 

Robert Hook was the first person to describe cells. In 1665 he built up a microscope to look tiny objects and one day he looked at a thin slice of coork, which looked like if it was made of little boxes. As they reminded him of the small rooms monks lived in at monastey
He named this boxes: cells. In 1673 Anton van Leeuwenhoek, made his own microscopes and used to look at pond scum. He saw small organisms moving all around in the water and named animalcules. Today we called them protists.

200 years later, around 1839 two scientists : Matthias Schleiden and Teodor Schwann concluded that cells are present in all living things. Soon after they wrote what is nowadays called: 

The Cell Theory:

1. All organisms are made of one or more cells.

2. The cell is the basic unit of all living things.

3. All cells come from existing cells. 

Remember the first subject we reviwed together. I told you
Living things are made of cells and that these cells were the basic building blocks of life. 

But how small really are they? how much a micrsocpe is enhancing their size? Remember I told you to think of a sandcastle. If you see it from the distance it will look like a smooth building. But as you get closer you begin to see is made of millions of tiny sand grains.  

Let´s see how small cells really are: 

Interactive scale to cells

Click start and look at the size of different things you know. Wonder about the tiny size of cells, including protists such as amoeba and parmecium or the bacteria E.coli or even look at the size of the carbon.

Light microscopes magnify 1500 times an object. Nowadays we have electron micorscopes which may magnify 500,000 times the actuak size if an object.

bacterial cell size  is around 1 micrometer (see below an E. coli under the electron microscope)

•1 micrometer=  0.001 mm

We now eucaryotic cells are generaly from one to two hundred times bigger than prokaryotic cells: see below an electron micrography of an amoeba.

With microscopes scientists observed that all cells contain certain small specialized structures: a memebrane, cytoplasm and Genetic material (DNA), and ribosomes. Althouugh, only some of them were having their DNA cover in a membrane, we nowadays called nucleus. So cells were divided in two different types:

Procaryotes: cells without nucleus, unicellular organisms.

Eukaryotes: cells with nucleus, this cells may belong to unicellular or multicellular organisms.

Also scientists observed that eukaryotic cells shown some other specialized structures, we called them organelles and each one, has specific functions.

Organelles: 

Nucleus: The nucleus controls the cell activity.

It contains the cell´s DNA or genetic material

Ribosomes: organelle composed of RNA and proteinsThe site where proteins are made

Endoplasmic Reticulum: composed of a

system of membranes.  

Helps the production and processing of proteins.

Mitochondria. Main power source of cell

The site of cellular respiration.

Lysosomes. Similar to kidneys

The organelle that digest food particles, wastes, cell 

parts for recycling and foreign invaders

Golgy´s apparatus. Where proteins are 

transported

Where proteins are processed and transported out the

cell

Vacuole: is a vesicule
They are useful to storage materials

Chloroplasts: Where photosynthesis take place
The organelle that uses the energy of sunlight to make food. Chloroplast are green because they contain chlorophyll

Now, click on the link below to see the different organelles in two kinds of eykaryotic cells: animal and plant cells.

Types of cells

Scrool your mouse down the cells alive page to found out about the specific function of each organelle in each cell type. Compare them! What is similar and what is different between plant and animal cells? Let´s watch the videos below and found out.

Now watch this video on cell structure to understand the different functions of each oragnelle. 

cell structure

Comparition bwteween Animal and Plant Cells

Then there are two main types or categories of cells: prokaryotic cells and eukaryotic cells. Both of these types of cells have several things in common. All cells are surrounded by a plasma membrane, whithin this membrane, is the cytoplasm which is composed of the fluid and organelles of the cell.

Bacteria (Kingdom Monera) are prokaryotes. They do have DNA, but it is not organized into a true nucleus with a nuclear envelope around it. Also, they lack many other internal organelles such as mitochondria and chloroplasts. 

Organisms in the other four kingdoms are eukaryotes. Their DNA is organized into a true nucleus surrounded by a nuclear envelope. The nucleus of eukaryotic cells contains the genetic material which chemically directs all of the cell’s activities. 

Plant Cell Diagram

Different Cell types: 

cell specialitiation (cell/tissues / organs / Organ Systems)

• Once you had learned that the cell is the smallest unit of life
•  let´s focus on multicellular organisms.
• 
  
• Organisms are like machines, some have just one part like bacterias and others have many parts, like us: we have got trillions of cells

Being multicellular have some benefit:

• Lager size
• Long life: life span is not limited to a single cell
• Specialization: each type of cell has a particular job. 

Lets´watch this video to see how it works

Cells, tissues, organs and organ systems

So cells work together. 

A tissue is a group of cells that work together to perform a specific job. For example Cardiac muscle tissue is just one type of tissue in a heart. 

Animals have four basic types of tissues: nerve tissue, muscle tissue, connective tissue and protective tissue.

Plants have three types of tissues: transport tissue, protective tissue and ground tissue (where photosynthesis takes place)

Then tissues work together

an organ is formed when two or more tissues work together to perform a specific function

And Finally a group of organs working together to perform a particular function is called an organ system. For example your digestive system.