1. Lecture 21 Cell Discovery and Theory Ozgur Unal
NIS - BIOLOGY
Lecture 21
Cell Discovery and Theory
Ozgur Unal

2. Cell How many cells do you think you have in your body?
Observe the cork given to you and try to describe what you see.
Using the microscope, look at the cork and describe what you see.
A cell is the basic structural and functional unit of all living things.
Cells are the building blocks of organisms.
How small do you think cells are?
How many cells do you think you have in your body?

3. History of the Cell Theory
In 1590, Hans and Zacharias Janssen invented the first compound microscope.
In 1655, Robert Hooke observed cork and
named the tiny chambers that he saw cells.
In 1683, Dutch biologist Anton
van Leeuwenhoek discovered
single-celled organisms.
In , scientists
discovered that plants and
animals have cells and all cells are produced from the division of existing cells.
Figure 7.1

4. Cell Theory
All these discoveries lead to the cell theory with three principles:
All living organisms are composed of one or more cells.
Cells are the basic unit of structure and organization of all living organisms.
Cells arise only from previously existing cells, with cells passing copies of their genetic material on to their daughter cells.
How were scientists able to observe cells in the past?
What tools do they use to study cells today?

5. Microscope Technology
The discovery of cells and the development of the cell theory would not be possible without microscopes.
Miscroscopes help us see very small objects.
There are two important characteristics of microscopes:
Magnification
Resolution
Higher magnification helps you see smaller objects.
Higher resolution helps you see two objects very close to each other (help us to see the details)

6. Microscope Technology
Compound light microscopes:
Consists of a series of lenses and
uses visible light.
Contains an eyepiece and an
objective lens.
Maximum magnification around 1000x.
Electron microscopes:
Use electrons to produce magnified images.
Transmission Electron microscopes (TEM) magnify up to 500,000x.
Scanning electron microscopes (SEM) produce 3-D images.
TEM and SEM can be used only for dead cells.

7. Electron Microscopes Scanning Tunneling Electron
Microscopes (STM) can be used
for live specimens.
STMs produce 3-D images for
very small objects.

8. Basic Cell Types
You have probably realized that the cells that make up organisms must have different types.
For example, the cells in a plant are different than the ones in a animal.
Scientist have grouped cells into two broad category:
Prokaryotic cells and Eukaryotic cells.
You see images of two
different cells. List the
similarities and differences
between each.

9. Basic Cell Types Both types of cell have a boundary.
A plasma membrane is a special boundary that helps control what enters and leaves the cell.
Plasma membrane exists
in all cells.
All cells contain genetic
material that provides
instructions.

10. Basic Cell Types Eukaryotic Cells:
These cells have internal structures called organelles.
Each organelle carries out specific cell functions.
Nucleus is a central organelle that contains cell’s genetic material in the form of DNA.
Most organisms are made up
of eukaryotic cells and are
called eukaryotes.
There are some unicellular
organisms that are eukaryotes.

11. Basic Cell Types Prokaryotic Cells:
These cells are defined as cells without a nucleus or other membrane-bound organelles.
Many scientists think that prokaryotes are similar to the first organsism on Earth.
Up to 100 times smaller
than eukaryotic cells.

12. Origin of Cell Diversity
Why are there two basic types of cells?
Eukaryotic cells probably evolved from prokaryotic cells millions of years ago.
According to endosymbiont theory, a symbiotic mutual relationship involved one prokaryotic cell living inside another. (This will be discussed in Chapter 14)
Eukaryotic cells are larger and have distinct organelles.
These cells have developed specific functions and this lead to the cell diversity, such as nerve cells, bone cells, leaf cells, stem cells etc.

13. Lecture 22 Plasma Membrane Ozgur Unal
NIS - BIOLOGY
Lecture 22
Plasma Membrane
Ozgur Unal

14. Function of the Plasma Membrane
Look at the filter paper as I pour muddy water on it.
Answer the following questions.
What did you just observe?
What is the purpose of the filter here?
What type of particles are selected by the filter paper to pass?
What type of particles are not selected by the filter paper to pass?

15. Function of the Plasma Membrane
How about this fishnet?
What is the purpose of this fishnet?

16. Function of the Plasma Membrane
Do you think cells need such a filtering system?
What controls what enters and leaves a cell?
Remember that plasma membrane is a special boundary that helps control what
enters and leaves a cell.
Plasma membrane is a
thin and and flexible
boundary between a cell
and its environment.

17. Function of the Plasma Membrane
Homeostasis is important for cells.
Plasma membrane allows nutrients into the cell and allows waste and other products leave the cell.
A key property of the plasma membrane is selective permeability.
By selective permeability, a membrane allows some substances to pass through while keeping others out.
The structure of plasma membrane controls how, when and how much of these substances enter and leave a cell.

18. Structure of the Plasma Membrane
Most of the molecules in the plasma membrane are lipids.
Plasma membrane is composed of phospholipid bilayer, in which two layers of phospholipids are arranged tail to tail.
Figure 7.6
Phospholipids have a polar
head and a non-polar tail.
What is the advantage of
having non-polar tails in a
plasma membrane?

19. Structure of the Plasma Membrane
Plasma membrane animations:
Do you think vitamin C can enter cells?
No, although vitamin C is water-soluble it acnnot diffuse through the plasma membrane because it is a charged, polar molecule.

20. Structure of the Plasma Membrane
Moving with and among the phospholipids in the plasma membrane are cholesterol, proteins and carbohydrates.
Proteins at the inner surface anchor the plasma membrane to the cell’s internal structure, giving the cell its shape.
Transport proteins move needed substances or wastes through the plasma membrane,
contributing to the selective
permeability.

21. Structure of the Plasma Membrane
Nonpolar cholesterol is repelled by water and it is positioned among phospholipids.
Cholesterol prevents phospholipids bilayer tails from sticking together. This contributes to the fluidity of the plasma membrane.
Carbohydrates in the membrane are attached to proteins adn they stick out to define cell’s characteristics and help identify chemical signals.
For example, carbohydrates in the membrane help disease fighting cells recognize and attack potentially harmful cells.

22. Structure of the Plasma Membrane
Phospholipids bilayer in the plasma membrane create a “sea” in which other molecules can float.
Phospholipids can move sideways in the membrane.
Components of the plasma membrane are in constant motion, sliding past each other.
Fluid-mosaic model of the plasma membrane explains this characteristics.