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PAP and Biology
A Look at Cells—Cell Theory, Structure, & Function NOTES
Cells are the smallest unit of life that can carry out all the functions of a living organism; not discovered until the
invention of the microscope.
Organisms can be very broadly classified on the number of cells they are made of. Unicellular organisms are made of
only 1 cell; they are the smallest of all living organisms. Ex) some bacteria, many protists
Other living organisms are made of many cells and are called multicellular Ex) Plants, animals, and fungi
Shape is related to function
Blood cells (hemocytes) - round to move through vessels
Nerve - long to carry message to spinal cord
Muscle cells - for contracting and relaxing
Categories/Types of Cells
Cells can be separated into two broad categories or types based on their internal structures:
Prokaryotic cells and Eukaryotic Cells

Prokaryotic cells- do not have a nucleus or other membrane-bound organelles.
- cell’s DNA is suspended in the cytoplasm
- most prokaryotes are microscopic, single-celled organisms
- Bacteria are the ONLY prokaryotes!

Eukaryotic cells – have a nucleus and other membrane bound organelles
- cell’s DNA is enclosed in the nucleus
- Eukaryotes can be unicellular or multicellular organisms
- Animals (including humans), plants, fungi, and protists are ALL eukaryotes!
Discoveries
In 1665 Robert Hooke observed a thin slice of cork from an oak tree
What he saw reminded him of the rooms in which monks lived so he called what he saw under the
microscope a “cell”; Hooke is credited with naming the “cell”
In 1670 Anton van Leeuwenhoek used the first “real” microscope to view pond water
He saw living creatures that we now call singled celled organisms
In 1840, Robert Brown discovered the nucleus of a cell; He observed the epidermal cells of orchids
and the anthers of Tradescantia (spiderwort plant)
Looking at Cells
-Cells are the smallest unit of life that can carry out all the functions of a living organism
-Not discovered until the invention of the microscope
Cell Theory
The Cell Theory was established by 3 men:
• Schleiden 1838, botanist, “all plants are made of cells”
• Schwann 1839, zoologist, “all animals/organisms are made of cells”
• Virchow 1855, physician, “all new cells come from other cells” (biogenesis)
Cell Theory
• Cells are the basic units of all
life
• All organisms are made of 1
or more cells
• All cells come from other cells
Cell Structure
•
•
•
Many components make up a cell and carry out specific activities
Each component works together, like a factory, to keep the cell functioning
These components are called organelles
Typical Animal Cell:
Typical Plant Cell:
5 structures in ALL cells (prokaryotes and eukaryotes)
1. Cell Membrane
2. Cytoplasm
3. Cytoskeleton
4. DNA
5. Ribosomes
When comparing just plants and animal cells:
Only plants
-
Only Animals
Central Vacuole
Chloroplast
Cell Wall
-
lysosome*
centriole*
Cell Organelles Structure & Function
Cell Membrane:
Structure: Made of phospholipids and proteins
-2 phospholipid layers called a bilayer
- Semipermeable (a.k.a. selectively permeable)
- Hydrophilic (H2O loving) head
- Hydrophobic (H2O fearing) tail
- Thin layer of lipids embedded with protein
Function: maintain homeostasis
1. Controls what enters and leaves
2.Boundary separating the cell from its
environment
Cell Wall
Structure:
- Found in plant, algae, fungi and most bacterial cells
- In plants and algae made of cellulose
- In fungi made of chitin
- In eubacteria made of peptidoglycan
- In archaebacteria made of lipids
Function:
- Provides support and protection
- Helps maintain square shape
Cytoplasm:
Structure: Jelly like base throughout the cell
- consists of mainly water and cytosol (an organic compound)
- contains the cytoskeleton
Function:
-Contains or “suspends” organelles inside the cell
- Site of many chemical reactions where water is the solvent
Polar
Non-polar
Cytoskeleton
Structure-composed of flexible protein fibers
3 types:
• microtubules –act as tracks for movement of organelles (largest)
• intermediate filaments – give the cell its strength (medium)
• microfilaments – tiny threads that allow cells to move and divide
(smallest)
Function- provides support and a place to “anchor” other structures
Centrioles
Structure
-Found in animal cells and some algae
- made of microtubules
- shaped like a cylinder
Function
- Aid in the division of chromosomes in mitosis.
Nucleus
Structure:
- Found in the cytoplasm
- Contains the genetic material (chromosomes made of DNA &
proteins)
- Surrounded by a nuclear envelope (double membrane)
-Contains the nucleolus that makes ribosomes
Function:
Control headquarters for the cell (brain)
Nucleolus
-Found inside of nucleus
Structure
- Dense knot of chromatin
Function
- Produces ribosomes
Nuclear Envelope
Structure- double lipid bilayer. It
has small holes called nuclear
pores.
Function- encloses genetic
material (DNA) in eukaryotic cells
and the holes allow mRNA to
leave during transcription
Ribosomes
Structure- spheres composed of RNA and protein (assembled in the nucleolus)
Function-site of protein synthesis
• Free ribosomes make proteins that remain inside the cell
• Attached ribosomes make proteins that become part of the cell membrane or
are exported or secreted from the cell (usually attached to the ER)
Endoplasmic Reticulum (ER)
Two types:
1. The rough ER
- has ribosomes attached
2. The smooth ER
- does not have ribosomes
1. Rough ER
Structure:
- Network of folded membranes near nucleus
- Contains ribosomes on surface
Function:
-Transports proteins that were made by the attached ribosomes
through the cell to the golgi.
2. Smooth ER
Structure:
- Network of folded membranes throughout cell
- Contains no ribosomes on surface
Function:
- Makes lipids and breaks down toxic substances, (such as
drugs and alcohol)
Golgi Apparatus
Structure
- Closely stacked, flattened membrane sacs
- Found in the cytoplasm
- Near nucleus
Function
- Stores, packages, and transports products inside for use outside
-Releases vesicles(a bit of the golgi pinched off) for transport of stored protein.
Vesicles
Structure
- bits of membrane from the plasma membrane or from the golgi
apparatus
- found inside or outside of the plasma/cell membrane
Function
- Vesicles fuse with the cell membrane
- may be used to release or transport chemicals in or out of the cells:
o Exocytosis out of the cell
o Endocytosis into the cell
- Transports chemicals to lysosomes or vacuole
Lysosomes
Structure
- found in animal cells
- saclike
- contain digestive enzymes
Function
- Digests food particles
- Digests old organelles
- Defends cells from bacteria and viruses
Vacuole
Structure
- Found in cytoplasm
- Sac of fluid surrounded by a membrane
Plants: Large in size (have one central vacuole)
Animals: small in size (have many)
Function
- Sites of storage of water, ions and some nutrients.
- Plants: allows plants to maintain turgor pressure; plant wilts when large central vacuole is
empty
Mitochondria
Structure
- Oval
- Inside is a complex matrix of folded membranes to increase
surface area.
- Has own ribosomes and DNA
Function
- Changes energy stored in food into a more useful form
- ATP is formed inside as a product of cellular respiration
- powerhouse of the cell
Chloroplast
Structure
- Found in plant cells (and some protists)
- Double phospholipid bilayer.
Inner layer
 Disc like structure containing chlorophyll = thylakoid
 Stacks of thylakoids = grana
 Non stacked area = stroma
Function
- Capturing radiant energy and converting it to chemical energy during photosynthesis.
- Chlorophyll is the green pigment that traps the energy from the sun
Cilia
Structure
- Found outside of the cell
- short hair-like
Function
Locomotion (movement)
• tiny, flutters of movement.
Flagella
Structure
- Found outside of the cell
- long whip-like
Function
Locomotion (movement)
• one big wave motion will cause a large amount of movement