Tutorial: The Cell
Anton van Leeuwenhoek | Robert Hooke | Schleiden & Schwann | Rudolf Virchow | Cell Theory | Microscopes | Various Cells | Eukaryotic Organelles | Cell Membrane | Cytoplasm | Nucleus | Animal Cell | Plant Cell | Cilia and Flagella | Ciliary/Flagellar Motion | Diffusion | Plasmolysis | Lysis | Osmosis | Facilitated Diffusion | Active Transport | Phagocytosis Animated | Exocytosis Animated | Quiz

Copyright © Steve Kuensting, 2004, All Rights Reserved.
This web tutorial may not be distributed by any means
without the expressed permission of the author!


The cell, a tiny "bag of chemicals", is the foundation of life on earth. They take in energy, are structured, reproduce themselves, store genetic information, and are adapted to their environment. Nothing more primitive or less structured is "alive".

Cell introduction

Anton van Leeuwenhoek

Cells were first observed in the 1600's when the microscope was first invented. The invention of the microscope as a biological tool is credited to a Dutch biologist named Anton van Leeuwenhoek. He observed many different types of unicellular life forms in pond water and wrote detailed accounts of his observations. He is also the first to observe human sperm - his own! He was also the first to use stains to enhance the views of magnified cells.


Robert Hooke

Cells have existed since life began on the planet, man has only known of them since the late 1600's. Their discovery depended on the invention of the microscope. An English scientist, Robert Hooke, observed thousands of tiny compartments in samples of cork under his scope. The little compartments reminded him of rooms so he named them "cells".


Schleiden & Schwann

After Hooke, more scientists began to observe pieces of organisms of all types, from liver cells to root cells. By 1838, Matthias Schleiden (botanist) proposed a theory: "All plants are composed of cells, and the plant cells are the basis of a plant's structure and function." In 1839, Theodor Schwann (zoologist) proposed that "All animals are composed of cells and the animal cells are the basis of animal structure and function."

Schleiden and Schwann

Rudolf Virchow

Later, in 1855, enough research had been done to conclude that the origin of all cells was some preexisting cell --- cells come only from other cells. This was the conclusion of Rudolf Virchow.


Cell Theory

The work of Schleiden, Schwann, and Virchow was combined into one general theory: The Cell Theory. It states:

1. All life forms are composed of cells.
2. Cells are the basis of all structure and function
of all life forms.
3. Cells only come from preexisting cells.


What is known about cells today is primarily made possible by the microscope. Light microscopes are the most common due to their low cost. They will magnify up to 1000X magnification. Specimens can be live or dead. Electron microscopes are very expensive and the viewed object is projected onto a TV screen. They will magnify up to 500,000X. Specimens are dead and placed in a vacuum chamber for viewing.

Light Microscope vs. Electron Microscope
Cost$300 to $2000$100,000+
Magnificationup to 1000Xup to 500,000X
ViewingThrough lensesOn a TV screen
Focusing materialglassmagnets
Prepcan view live objectsobjects dead in a vacuum

Monocular vs Binocular

Light microscopes are generally divided into 2 categories: monocular and binocular. Monocular light microscopes can magnify to approximately 1000X as light passes through a thin specimen or slice. Binocular light microscopes magnify up to 200 times as light bounces off of a whole specimen. While monocular light scopes produce a 2-dimensional image, binocular light scopes produce a 3-D image.

Light microscopes


Electron microscopes can be generally divided into 2 categories: transmission and scanning. TEM's (transmission electron micro scopes) look through thin slices. SEM's (scanning electron microscopes) look at the surface of small objects. While a TEM produces a 2-D image, an SEM produces a 3-D image. TEM's will magnify to 500,000X while SEM's generally magnify to 100,000X.

Electron microscopes

Various Cells

The remainder of this program is an exploration of the internal structure and activities of cells. Since cells vary greatly in size, shape, and internal structure, the descriptions that follow are meant to describe the general characteristics of cells.

Various cells


All cells have smaller components inside of them that perform tasks essential to the life of the cell. These small parts are called ORGANELLES. Each organelle performs a task essential to the life of the entire cell, just as a watch gear is essential to the functioning of a watch. Each numbered part below is an organelle.


Eukaryotic vs Prokaryotic Cells

There are 2 vastly different types of cells on earth - eukaryotic and prokaryotic cells. Prokaryotic cells are the simplest cells on earth and are known as bacteria and blue-green bacteria. Eukaryotic cells are more complex than prokaryotic cells and include single celled organisms such as paramecia and the cells of plants and animals. The biggest difference between the 2 cell types is the lack of membrane-bound organelles in prokaryotic cells. Note the diagram below.

Eukaryote vs prokaryote

Unicellular vs Multicellular

All organisms can be grouped into 2 categories: unicellular and multicellular. Some unicellular organisms are actually larger than some multicellular organisms. All prokaryotic organisms are unicellular. Eukaryotic organisms may be either unicellular or multicellular.

Unicellular vs multicellular

Eukaryotic Organelles

All eukaryotic cells have the following three components:
(They also usually have other various organelles)

1. cell membrane 2. cytoplasm 3. nucleus

Eukaryotic organelles

Cell Membrane

The cell membrane is the barrier separating the internal organelles and fluids of a cell from the external environment. The membrane will allow some materials to pass through it and not others. For this reason it is called a SELECTIVELY PERMEABLE MEMBRANE. It is double-layered and composed of 2 types of chemicals: phospholipids and proteins.

Cell membrane

Cell Membrane Composition

The cell membrane is made up of approximately 50% phospholipid and 50% protein. Lipids generally do not contain phosphorus (P), so phospholipids are an exception to the general rule that lipids contain only C,H,O. They are not rare, since a human contains over 100 trillion cells and each has a cell membrane made of over one half phospholipid. A phospholipid is drawn below and then shown in its abbreviated form.

Membrane structure

Phospholipid Bilayer

In a cell membrane, the phospholipids are arranged in specific ways due to their unusual chemistry. The cell membrane is a double layer of phospholipids. The phosphate portion of the phospholipids faces to the inside of the cell on the interior layer, and the phospholipids on the outside face to the exterior on the outer layer. The fatty acid tails of the phospholipids face each other on the interior of the membrane. Draw the picture below in your notes.


Proteins in the Membrane

Proteins are wedged in the membrane to primarily help in the transportation of substances through the membrane - an essential activity if a cell is to remain alive. Some proteins are present in the membrane for other purposes, such as serving as cell markers so cells in multicellular organisms can tell each other apart. The structure of the cell membrane can only be revealed with an electron microscope.

Membrane structure


The cytoplasm is the jelly-like fluid that is found in the cell. All cells are fluid --- that is they are not solid. The fluid is not as thin as water but not as thick as "jello". Organelles and chemicals can move around inside the cell through the cytoplasm fluid. The cytoplasm is basically a solution composed of water (70%), salts, enzymes, and small organic molecules.



The nucleus is usually a spherical organelle located somewhere near the center of the cell. It is the cell's control center. It contains the genetic instructions, an entire set for the entire organism it is a part of. The nucleus contains the chemicals water, DNA, RNA, and enzymes. The nucleus is large enough to be viewed with a light microscope, but its details require electron microscopy.


Eukaryotic Cell Organelles

All animal and plant cells have a cell membrane, cytoplasm, and nucleus. They also have many other organelles in common but they differ in a few unique organelles they possess. Because of these differences, they will be covered separately.


Animal vs Plant Cell

The next 2 pictures are diagrams of animal and plant cells. On the diagrams, unnamed pointers label various cell parts. Click (specifically with the tip of the cursor) on the organelles or cell parts that are specifically labeled by pointers. When viewing an organelle, be sure to note whether it can be seen with a light microscope or electron scope only! Close the organelle window when you have finished studying it!!

Animal Cell - Click on the labeled parts.

cellmembrane microtubules rougher nucleus smoother nucleolus lysosome microfilaments ribosome mitochondrion golgiapparatus centriole Animal cell
Plant Cell - Click on the labeled parts.

ribosome smoother nucleolus nucleus rougher cellwall golgiapparatus cellmembrane vacuole mitochondrion microtubules leucoplast chloroplast Plant cell

Cilia and Flagella

Two last cell parts are cilia and flagella. Both are composed of microtubules and function in locomotion. They are common to unicellular organisms (protozoans) but are also found on the cells of animals. (Example: sperm cells possess flagella.) Cilia are short and many are found on the cells that have them, flagella are long and only one is usually found on a cell that has them. Both are attached to the cell via a microtubular anchor called the basal body.

Cilia and flagella

Ciliary/Flagellar Motion

Cilia are usually used to push a cell through the water while flagella are usually (not always) used to pull a cell through the water. Unicellular organisms with cilia are called CILIATES while those with flagella are called FLAGELLATES. Use the hyperlink below to note the differences between the mechanics behind the 2 methods of unicellular locomotion.

The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =

Use the controls above to control the animation. You can play forward, stop, play reverse, and adjust the speed (actually the delay between images). The higher the speed (delay) values, the slower the animation. Reasonable delay values fall between 200 and 2000.


Cells must receive and rid of chemicals. They cannot do this exclusively on their own and they rely on a process called diffusion. Diffusion is the movement of particles from areas of high concentration to areas of low concentration. It occurs anywhere - not only in living cells. It is caused by the continuous random motion of particles. Cells obtain oxygen and salts by diffusion, and rid of carbon dioxide and ammonia waste by diffusion. Below are models of gas molecules and the directions of their random motion.


Direction of Diffusion

Diffusion can only move chemicals with their concentration gradients. This means diffusion can only move chemicals from where they are plentiful to where they are few. It cannot move them backwards - from where they are few to where they are plentiful. This would be akin to oxygen gas separating on its own to the corner of the room - it just doesn't happen. Oxygen gas always disperses out - from plants to the entire atmosphere!


The most common chemical found within and around cells is water. Cells pay close attention to the movement of water through their membranes. If too much moves in or out the cell will easily burst or shrivel. Osmosis is the name given to the diffusion of water through (in or out) a cell membrane. The small particles below are water osmosing in and out of a plant cell. Water always moves from a higher water concentration to a lower water concentration - following the rules of diffusion.



When too much water osmoses out of a cell the cell will dehydrate. Plasmolysis is the shrinking of a cell membrane due to loss of water. It is caused by a greater concentration of water inside than outside, so water osmoses out of the cell. It can easily result in the death of the cell if it is prolonged. When a plant wilts from insufficient water, its cells are plasmolyzed. If it is watered before wilting is severe, the cells will reinflate as water osmoses back into the cells and the plant will survive. Below is a plant cell before and after plasmolysis. Notice that the cell wall does not shrink, only the cell membrane. This easily occurs when a freshwater plant cell is placed in saltwater.



The reason a freshwater cell will plasmolyze in salt water is that more water leaves the cell than enters. Saltwater has less water than freshwater, so more water strikes the inside of the membrane and leaves than strikes the outside and enters. Ultimately this happens because the salt cannot pass through the membrane fast enough to equalize its own concentration. Thus, the water moves instead and the cell dies.



If a cell is moved from its normal environment and placed into an environment where the concentration of water around it is greater than that within it, water will osmose into the cell and the cell membrane may rupture. Lysis is the rupture of a cell membrane due to overexpansion of the cell volume - caused by too much water osmosing into the cell. Below is shown a dermis skin cell as it appears in normal body fluid and after it is placed in tap water. The reason the cell lyses is that more water strikes the outside surface and enters than strikes the inside surface and leaves. Result: cell swells and ruptures


Solutes and the Membrane

Diffusion cannot move materials through a cell membrane that are too large to fit through the spaces between the phospholipids and proteins. For molecules the size of glucose, the membrane must transport them or form a pore large enough for them to pass through. This is the job of the transport proteins that are wedged in the cell membrane.


Facilitated Diffusion

Some proteins only form pores and thus make it possible for particles to diffuse in and out of the cell. They serve as a door and are continually open - so a molecule can diffuse in or out according to its concentration gradient. This is called facilitated diffusion. It is a form of passive transport since it requires no energy to occur.

Facilitated Diffusion

Active Transport

Some proteins actually serve as a pump and they move particles against their concentration gradient from areas of low concentration to areas of high concentration. These proteins do not form a door and they will not let the particles move in the wrong direction. This type of membrane passage is called active transport and it requires energy which is supplied by the mitochondria. Nerve cells actively transport sodium ions out to keep themselves polarized to conduct nerve impulses.

Active transport

Sodium-Potassium Pump

The sodium-potassium pump is the most studied of all of the active transport pumps. It is most famously found on neuron and muscle cell membranes. The pump pushes 3 sodium ions out and 2 potassium ions in while using cellular ATP energy to do so. The sodium leaks back into the neuron during an action potential. The potassium leaks back out to quickly restore the charge on the neuron membrane. The pump continually pushes sodium ions out and potassium ions in to maintain a positive charge outside and a negative charge inside at all times.

NaK pump
The animations are Copyright © 2013, Steve Kuensting, All Rights Reserved.


There are times when whole cells or very large molecules must pass through a cell membrane. For whole organisms (cellular "eating") or large groups of large molecules other membrane passages are employed. Phagocytosis is the ingestion of whole cells while exocytosis is the ejection of large proteins. Both processes are demonstrated in animations. The two processes demonstrated are ameboid phagocytosis of food and insulin secretion by a human pancreas cell. Click on a hyperlink below to study the processes.


Below is the first frame of the Insulin Secretion Animation.

The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =


Below is the first frame of the phagocytosis animation.

The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =

The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.


  1. Which organelle is known as the powerhouse of the cell?

  2. Which organelle produces proteins?

  3. Which organelle produces ribosomes?

  4. Which animal cell organelle contains enzymes for digestion?

  5. Which organelle packages proteins for secretion?

  6. Which cell part is made of protein and involved in muscle cell contraction?

  7. Which organelle serves as an intracellular highway?

  8. Which plant cell organelle stores enzymes and waste products?

  9. Which cellulose plant cell part forms a permeable barrier?

  10. Which organelle is the cell's control center?

  11. Which short external cell part functions in locomotion?

  12. Which green plant organelle functions in photosynthesis?

  13. What semipermeable thin barrier separates the cell from its external environment?

  14. Which long external cell part serves in locomotion?

  15. What tubular network, covered with ribosomes, do pancreas cells possess?

  16. What cell part made of protein is found in cilia and flagella?

  17. What is the name of the movement of particles from areas of high concentration to areas of low concentration?

  18. What is the name of the diffusion of water through a cell membrane?

  19. What is the name of the movement of particles through cell membranes via proteins that form pores and use no energy?

  20. What is the name of the movement of particles through cell membranes via proteins that form pumps and use energy?

  21. Name the scientist that invented the microscope.

  22. Which part of a phospholipid faces the cytoplasm of a cell?

  23. Name the act of a cell bursting due to excess excessive inward osmosis.

  24. Name the shrinking of a cell due to excessive water loss.

  25. What is the acronym of the electron microscope that looks through thin slices of specimens?

  26. What type of light microscope looks at the surfaces of organisms?

  27. What is the acronym of the electron microscope that looks at whole specimens and shows surfaces only?

  28. Who developed cell theory?

Diagram - Name the numbered organelles.

Organelles quiz