Biology 101 Chapter 4
Cells as the Basic Unit of Life
The Cell Theory
Even though this is not a history class, a little history is necessary to understand the significance
of cells. Here are some of the most important contributors to the cell theory.
Major Contributors:
Galileo = first observations made with a microscope
Robert Hooke = first to observe small compartments in dead plant tissue, cells, coined the term
"cell"
Antony van Leeuwenhoek = first to observe living, mobile cells, single celled organisms and
bacteria
Robert Brown = first to observe the nucleus of the cell, first observation of an organelle
Rudolf Virchow = first to record cell division, noted every new cell comes from a pre-existing
cell
Schleiden and Schwann = plants and animals are composed of cells and cell products
Tenements of the Cell Theory:
A tenement is a specific part or phrase to a theory. There are three to the Cell Theory. All the
previous observations made by scientists were used to formulate the Cell Theory.
1) All organisms are composed of one or more cells. The smallest living thing is a single
unicellular (one-cell) organism. Anything larger is composed of two or more cells and is
called multicellular.
2) The cell is the smallest unit having the properties of life. The cell is the smallest unit of a
living thing that carries out its own metabolism.
3) The continuity of life arises directly from the growth and division of single cells.
Cell size and cell function: Surface area to volume ratio!
All cells are very small. Even the largest cell is microscopic to most people. Why are cells so
small and why aren’t there gigantic single celled organisms like the “blob?” Cells like to have a
lot of surface area compared to their internal volume. Cells absorb nutrients and oxygen through
their surface and excrete wastes out through their surface. A cell’s metabolism occurs in its
volume. The greater the volume, the greater the amount of nutrients required. However, cells
are limited because the only source for nutrition is through their surface, and, unfortunately, a
cell’s surface area does not increase as fast as its volume. Imagine a cube 2 inches to a side. Its
surface area is determined by length x width x number of sides (2 x 2 x 6). This equals 24 in2.
The volume is determined by length x width x height (2 x 2 x 2). This equals 8 in3. This gives a
ratio of 3:1 (this is good). Now let’s double the dimensions of the cube to 4 in a side. The
surface area is 96 in2 and the volume is 64 in3. This gives a ratio of 3:2. We want a larger ration
(bigger difference between the numbers, not bringing them closer. The cell is actually worse off
now. The bigger the ratio, the happier the cell.
- The larger the ratio, the better off the cell!
- Impact of surface area to metabolism
What is the largest cell in the human body? The smallest?
Cell Structures and Their Functions
All cells are placed in one of 2 classes: These are the first divisions in classification.
Prokaryotic = lack a nucleus (bacteria)
Eukaryotic = have a nucleus (protists, fungi, plants, and animals)
PROKARYOTIC CELLS
 very, very small; much smaller than a eukaryotic cell
 very simple structure, lack internal parts or chambers, no organelles
Parts: This is a list of the general parts of a prokaryotic cell.
A) Plasma (cell) membrane = the membrane that all cells posses that encloses the
cytoplasm, or internal fluid, of the cell (The membrane basically divides the cell into exterior
and interior, defining what is inside the cell and what is outside the cell. It also has the
capacity to pick and choose what will pass across itself, restricting movement of material into
and out of the cell. This ability is referred to as Selective Permeability)
B) Nucleoid Region = where DNA is at (not a nucleus) (The nucleoid region just
happens to be anywhere the DNA is at that time. It is NOT a specific structure.)
C) Ribosomes = assembles proteins with info from DNA (Ribosomes are generally
referred to as the “factories” of the cell. They manufacture all proteins needed by the cell.)
D) Bacterial Cell Wall = a rigid outer layer that surrounds the cell membrane, protects
the cell, maintains shape
E) Capsule = a sticky outer layer over cell wall, helps with adherence to surfaces and
protection (A substance very similar to melted sugar. Glue-like material used by
bacteria.)
F) Pili and Fimbriae = numerous short projections that help with adherence to surfaces
and some reproduction
G) Prokaryotic Flagella = longer projections that help with motility, the cells ability to
move
H) Plasmids = extra-chromosomal pieces of DNA with just a few genes on them
EUKARYOTIC CELLS
 have a nucleus
 very, very large; sometimes huge
 complex internal organization
 compartmentalized
 membrane bound organelles
Eukaryotic cells are quite large and very complex and sophisticated. They are the advanced cell.
All advanced or higher life forms on earth are composed of eukaryotic cells; chiefly 4 of the 5
Kingdoms are referred to as being eukaryotic. The big thing with these cells is that are
compartmentalized by a series of membranes that enclose small bubble like pockets of material
inside the cell and are called organelles. Organelles allow self-contained, separate environments
for chemical reactions of severe types and storage of vital material. There are many different
types of organelles in eukaryotic cells. Text books typically only list the most common.
Organelle = "small organ", membrane enclosed structures found inside the cell, each for a
specialized function. All chemical activities of the cell occur within organelles.
Benefits of Organelles:
1) Separate environments for chemical reactions
2) Increased membrane surface area
Surface area is vital to cells. The more surface area, in the form of extra membrane anywhere in
the cell, the happier the cell is. This principle also determines the maximum size of cells. All
cells are roughly the same size, regardless of organism. The only reason why larger organisms
are larger is because they are composed of more cells.
Eukaryotic Cells Broken Up into 3 Regions:
1. Cell Membrane
2. Cytoplasm (cytosol and organelles)
3. Nucleus
Organelles:
1. Nucleus * (The nucleus is a major part of discussion and a organelle)
2. Endoplasmic Reticulum (ER, smooth and rough)
3. Golgi Apparatus (or Body)
4. Vesicles (lysosomes and peroxisomes)
5. Mitochondria
6. Chloroplasts (only in plants) (and algae)
7. Storage Vacuole (mainly in plants)
8. Centriolus (only in animal cells)
Other Structures:
1. Ribosomes
2. Cell Wall (in plants)*
3. Cell Membrane
4. Cytoskeleton
a. Microtubules
b. Microfilaments
c. Intermediate Filaments
5. Flagella and Cilia (mainly in animals)
6. Nucleolus
Structures are other listed parts inside the cell, usually solid (as opposed to hollow like
organelles) and not enclosed in any membrane. Note! The ribosome is listed as a structure, not
as an organelle. Some high school and junior high school text books list ribosomes as
organelles. This is inaccurate. A ribosome is a structure. This is why bacteria can have
ribosomes and still claim to not have any organelles.
 The nucleus is a region of the cell and also the number one organelle
 This is a misleading statement. In reality plants, fungi and protists all have cell walls.
Therefore, it is more accurate to say animals lack a cell wall than to claim a plant possesses
one. But since most school textbooks list it as plants have a cell wall, I will keep it the same
here for simplicity’s sake.
The Nucleus








Cell's genetic control center
Double membrane
Nucleoplasm
Nuclear envelope (collective term for the double membrane layer of the nucleus)
Nuclear pores (small opening in the nuclear envelope itself, review over their purpose)
Chromatin = DNA + associated proteins
Chromatin vs. Chromosome
Nucleolus = internal structure of nucleus, site of ribosome assembly
The Cytomembrane System
 Function: internal transport, importing and exporting of cell
 3 parts:
1. ER
2. Golgi Apparatus
3. Vesicles (vesicles are pretty generic, there are specialized versions of vesicles though)
Endoplasmic Reticulum
This is a tubular transportation network inside the cell for more efficient movement of material
inside it. Similar to an elaborate and sophisticated subway system or plumbing network.





Single, continuous membrane
Pipes, tubes and tunnels in cell
Continuous with nuclear envelope
Superhighway of the cell
2 kinds: Rough ER + Smooth ER
Rough ER
- Flattened connected sacs
- Studded, or covered, with ribosomes
- Major site of protein synthesis
- Synthesis of new membrane
Smooth ER
- Lacks ribosomes
- Continuous with rough ER
- Functions:
1. Transport
2. Synthesis of lipids
3. detoxification
4. Storage of calcium ions
Golgi Apparatus




Stack of flattened, pancake looking sacs located near cell membrane
Handles export and import of material for cell
Not continuous with ER, NOT physically connected
Functions:
1. Storage, packaging, sorting and final touches and modification of proteins before
exportation
2. The UPS of the cell
Vesicles


General, short term transport, some storage, single membrane
3 special types:
Transport Vesicles
1) Used to transport material from ER to Golgi Apparatus
2) Transport of finished product from Golgi to Cell Membrane for export (process reversed for
import) (Note how the vesicles are formed and incorporated into the next organelle.)
Lysosomes
1) Contain digestive (hydrolytic) enzymes
2) Breakdown cell's food and wastes
Peroxisomes
1) Breakdown lipids
2) Detox alcohols (which is a lipid) and hydrogen peroxide
Vacuoles
Vacuoles are kin to vesicles, but are not a vesicle. They are single membrane organelles of
immense size. They vary from vesicles in at least three important ways:
1. They are huge. Vesicles typically are very small relative to the cell, and number in the
hundreds, perhaps 0.012 to 0.015 the size of the cell. Vacuoles, on the other hand, can be
25% to 50% the volume of the cell.
2. Vesicles are short lived, lasting no more than a few seconds to a couple of minutes in
duration. Vacuoles can last weeks to years inside a cell.
3. Purpose: vesicles primary purpose is to transport material (ferry it) from the ER to the
Golgi, and from the Golgi to the cell membrane (and vice versa). Vacuoles purpose is to
store stuff for long periods of time, especially water.


Very large, single membrane sacs
Functions:
1. Work with lysosomes for digestion
2. Storage of food and water
3. Stores wastes, excess water
4. Turgor pressure in plants
Ex. Large Central Vacuole of plants
When dealing with mitochondria and chloroplasts, note their similarity in structure and function.
These are the main two, the only two, organelles involved in energy transformations. They
either make food from energy or burn food for energy. They are inexorably linked together.
Also note that the mitochondria are found in every eukaryotic cell, even plants.
Mitochondria






Found in all eukaryotic cells
Carry out cellular respiration to produce energy for the cell
Cell's "power house"
Composed of 2 membranes
Cristae = folds of inner membrane, site of energy production
Matrix = fluid inside mitochondria
Chloroplasts






Found only in green plant cells and algae
Site of photosynthesis
Contain the pigment chlorophyll
Composed of 3 membranes
Grana = stacks of discs of inner membrane, actual site of photosynthesis
Stroma = fluid inside chloroplast
Centriolus




Also referred to as basal bodies and MTOCs (Microtubule Organizing Centers)
Composed of two centrioles in a membrane
Used for anchoring, microtubule growth
Centrioles also used in cell reproduction
The centriolus is responsible for the formation of microtubules and their resulting structures,
such as cilia, flagella and the mitotic spindle (used in cell division). Think of the centriolus as
something similar to a hair follicle.
Structures Based on Microtubules
The Cytoskeleton

Framework of protein fibers inside cell


Support and movement (dynamic)
Composed of:
1. Microfilaments = thinnest (actin)
2. Intermediate filaments = (composition varies)
3. Microtubules = thickest (tubulin)
The cytoskeleton is a living, changing part of the cell that gives a cell its overall shape and
support. It is very similar to the steel framework inside a skyscraper, though it can change and is
dynamic. The cytoskeleton also supports and positions all the internal organelles of the cell.
Microtubules are like really thick beams and microfilaments are like relatively thin beams.
Intermediate filaments are made of different proteins, depending on the species involved.
Cilia & Flagella

Used in locomotion
1. Cilia = numerous, very short (These are often referred to as being hair-like.)
2. Flagella = few, very long (These are often referred to as being whip-like.)

"9 + 2" arrangement of microtubules
 9 outer pairs
 2 single central
Plant Cell Wall





Surrounds the cell external to cell membrane
Very stiff, rigid structure
Supports cell, gives it shape, protects it
Composed of a complex sugar called cellulose
Note: some protistans and all fungi also have a cell wall
Some notes on microscopes: You should read up on the electron microscopes in the textbook.
Stereomicroscopes (dissecting)
Compound light microscopes (These are the most common, basic microscope that we use in labs.
They rely on visible light.)
Scanning Electron Microscope (SEM)
Tunneling Electron Microscope (TEM)
Scanning-Tunneling Electron Microscope (STM)
Drawbacks on electron microscopes
Electron microscopes are very powerful and produce incredible images of the smallest and most
remote objects. However, they do have drawbacks. Electron microscopes use a concentrated
beam of electrons instead of visible light to illumine the object. A beam of electrons is basically
a lightning bolt. Though it sounds like something from Star Trek, you are shooting an energy
beam at the sample. Here are the problems:
1) We can never see anything alive with an electron microscope! All samples are dead.
2) The microscope has the habit of destroying the sample while viewing it (vaporizes).