Principles of Biology
By
Frank H. Osborne, Ph. D.
Cells
THE CELL THEORY
• The word theory in science means
"explanation." The cell theory is the
accepted explanation about cells. It was
developed after many observations of living
things by biologists.
The Cell Theory
The Cell Theory (Explanation)
• All living things are made of cells.
• All cells arise from pre-existing cells
Exceptions
• Viruses are considered to be non-living.
• Mitochondria and chloroplasts can replicate
on their own.
Basic Types of Cells
There are two basic types of cells. The cells
that came first are prokaryotic (pro means
"before" and karyo means "nucleus").
These cells came before there was a nucleus.
The cells with a nucleus are eukaryotic (eu
means "true"). So, eukaryotic cells have a
"true nucleus" meaning that the cellular
nucleus has a membrane around it.
Biology of Cells
The word cell was used to describe these
objects because the first ones seen in cork
reminded the observer of the cells in a
monastery. Cells are too small to see with
the unaided eye so we use microscopes.
Biology of Cells
Microscopes are used to see cells because they
are very small. There are two types of
microscopes.
• Simple microscope--one lens like a
magnifier.
• Compound microscope--two lenses working
in series to make a magnified image of the
specimen.
Biology of Cells
Simple microscope Compound microscope
Biology of Cells
Cellular structures include:
• Cell membrane--made of two layers of
phospholipid and surrounds the cytoplasm.
• Nucleus--control center of the cell surrounded
by a nuclear membrane.
• Cytoplasmic structures--perform various
cellular functions: mitochondria, chloroplasts,
and others.
Biology of Cells
When viewed with the light microscope, only
the nucleus and cytoplasm are visible. In
plant cells chloroplasts are also visible.
With the electron microscope, many more fine
details are visible. These are referred to as
ultrastructure.
GENERALIZED CELLS
A generalized cell is a drawing that contains
all parts of the cell. In reality, there are
many cells that lack one or more parts. A
red blood cell, for example, contains
cytoplasm and a cell membrane but the
other parts are missing.
Cellular Ultrastructure
Diagram of Cellular Ultrastructure
Eukaryotic/Prokaryotic Cells
Eukaryotic cells are found in all higher
organisms. They contain a well-defined
nucleus surrounded by a membrane.
The prokaryotic organisms, Bacteria and
Archaea, do not have a membrane-bounded
nucleus.
Cell Membrane
The cell membrane is a structure that surrounds
each cell.
• All living cells have a cell membrane
• It is the boundary between the cell
and its environment.
• It has the same function in both plant
and animal cells.
Proteins of the Cell Membrane
Transport proteins assist passage of large
molecules or ions into the cell.
Integral proteins help maintain the structure of
the cell membrane. Some also serve as
signal transmitters.
Peripheral proteins are associated with the
integral proteins. They assist in
transmitting messages into the cell.
Cell Membrane Structure
Cell Membrane Structure
In the electron microscope, a single
membrane, as illustrated, appears as two
parallel lines 80 Angstrom units apart.
The "Fluid Mosaic" Model
The proteins in the membrane can move
around inside the two layers of lipids and
even protrude through the surface. This
gives the surface of the membrane the
appearance of a mosaic. So, the cell
membrane is called a "fluid mosaic."
The "Fluid Mosaic" Model
The Nucleus
The nucleus is the central organelle that
controls the cell. It is surrounded by a
membrane known as the nuclear membrane.
It contains chromosomes and a nucleolus.
Cellular Ultrastructure
The nucleus controls the cell.
The Nucleus
Parts of the Nucleus
• The nuclear membrane surrounds the
nucleus. It contains pores and channels that
connect to the endoplasmic reticulum, a
network of channels that is found in the
cytoplasm .
• The chromosomes are structures that contain
DNA and control the activities of the cell.
• The nucleolus is a large, round structure
within the cell. It is responsible for the
formation of ribosomes.
Cellular Ultrastructure
The nuclear membrane surrounds the nucleus.
Cellular Ultrastructure
Chromatin contains the DNA.
Cellular Ultrastructure
The nucleolus helps form ribosomal parts.
The Cytoplasm
The cytoplasm is the part of the cell that is
found outside the nucleus. It is bounded on
the outside by the cell membrane. The
cytoplasm contains many different
organelles that perform various functions
for the cell.
Cellular Ultrastructure
The cytoplasmic membrane surrounds the cell.
Mitochondria
Mitochondria are the structures that produce
energy for the cell. They contain an internal
membrane that is folded to form cristae.
Each single crista is an individual fold of this
membrane where cellular respiration occurs.
Mitochondria
Cellular Ultrastructure
Mitochondria provide energy for the cell.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a series of
channels in the cytoplasm. It provides rapid
movement of materials within the cell.
Endoplasmic Reticulum
Rough ER has ribosomes attached to it. Its
appearance in the electron microscope is that
of sandpaper, hence "rough."
Smooth ER does not have ribosomes attached
to its membranes. It forms part of the
internal membrane system of the cell.
Cellular Ultrastructure
Endoplasmic reticulum is for communications.
Cellular Ultrastructure
It has ribosomes for protein synthesis.
Ribosomes
• Ribosomes are the places where
proteins are made. Each ribosome has a
small subunit and a large subunit.
• When a protein is being synthesized, you
find ribosomes attached to a piece of
messenger RNA. This structure is sometimes
called a polysome.
Golgi Bodies
Golgi bodies serve as packaging organelles for
materials to be exported outside of the cell.
They start out as flat membranes that fill up
as the material enters them. They become
spherical and migrate toward the outside.
Cellular Ultrastructure
The Golgi apparatus is for external secretions.
Lysosomes
• Lysosomes are structures that are
membrane-bounded and contain
digestive enzymes.
• Lysosomes digest large particles
that are brought into the cell.
• The process of digestion is the
reverse of dehydration synthesis.
Vacuoles
• Vacuoles are areas in the cytoplasm that have a
membrane surrounding them.
• They are filled with cytoplasmic liquids or
storage materials of various kinds.
The Cytoskeleton
• The cell has a network of flexible fibers within
the cytoplasm. This network of fibers is called
the cytoskeleton.
• The fibers are made of polymers of proteins
known as microtubules. These fibers have
elastic properties that provide flexibility to the
cell. They act as little muscles inside the cell.
Plant Cell Structures
• A generalized plant cell is shown below. The
cell wall and plastids are found only in plants.
Cell wall
Chloroplast
Cell Wall
• The cell wall is made of cellulose.
• It is considered non-living.
• Cellulose fibers form primary (thin) and
secondary (thick) walls. Different types of plant
cells have different thicknesses of cell wall.
• The cell membrane is pushed up against the
inside of the cell wall by turgor pressure so it
cannot be seen.
Cell Wall Layers
The middle lamella is produced when the plant
cell divides.
Each cell produces a thin primary wall on its side
of the middle lamella. All plant cells generally
have a primary wall.
The secondary wall is much thicker. Not all plant
cells produce secondary walls.
Plastids
Plastids are structures found only in plant cells.
There are three kinds which are chloroplasts,
chromoplasts and leukoplasts.
Stroma
Diagram of chloroplast structure.
Chloroplast Structure
Grana
Stroma
Chloroplasts
• Chloroplasts are plant organelles that make
food by the process of photosynthesis.
• Photosynthesis is the synthesis of organic
materials from inorganic raw materials.
• Using light energy the chloroplast converts
carbon dioxide and water to glucose.
• The chloroplast contains layers of membranes
called grana that contain chlorophyll.
Chlorophyll traps the light energy from the
Sun.
Chromoplasts
Chromoplasts are plastids that are colored but do
not perform photosynthesis. Most contain
carotenoid pigments that are related to
carotene. These pigments are various shades of
orange or yellow. The pigments can be
separated from leaves by means of a technique
known as chromatography.
Leukoplasts
"Leuko" means lacking color or without color.
Leukoplasts are plastids that do not have any
color. They are used for storage of materials.
An example is the starch grain which is found
within the cells of potatoes.
Cilia and Flagella
Cilia and flagella are organelles of locomotion.
They contain contractile proteins. The proteins
allow cilia and flagella to move. Flagella are
much larger than cilia. Cilia are more
numerous on a cell than flagella. A cell will
have either cilia or flagella but not both.
The contractile proteins of cilia and flagella form
a characteristic arrangement. They come in
pairs.
Cilia and Flagella
There are nine pairs around the outside with two
fibers in the middle. This structural
configuration is known as the 9+2 arrangement.
Cilia and Flagella
Flagella are attached to the cell membrane.
Inside the cell membrane is a basal body, which
is anchored in the cytoplasm by rootlets. The
filament, is located outside the cell membrane.
Centrioles
Centrioles are animal cell structures
made of microtubules. The centriole is located
in the cytoplasm just outside of the nuclear
membrane. It is used by animal cells during
cell division.
Centrioles produce a network of fibers known as
the spindle. Spindle fibers pull the
chromosomes apart during the process of
mitosis.
Summary Outline of Cell Structures
Structures common to all eukaryotic cells
Cell membrane
Ribosomes
Nucleus
Golgi bodies
Mitochondria
Lysosomes
Endoplasmic reticulum Vacuoles
Summary Outline of Cell Structures
Structures found
only in plant cells
Cell wall
Chloroplasts
Plastids
Structures found
only in animal cells
Cilia and flagella
Centrioles
PRINCIPLES AND DEFINITIONS
Diffusion
• Diffusion is the net movement of the particles of
a particular substance from an area of higher
concentration of that substance to an area of
lower concentration of that substance, at
constant temperature and pressure.
PRINCIPLES AND DEFINITIONS
Osmosis
• Osmosis is the diffusion of water through a
differentially (semi-)permeable membrane.
• Generally water moves from the outside of the
cell to the inside of the cell because the
concentration of water is higher outside the cell.
• Osmosis is one of the most misunderstood
concepts in biology.
PRINCIPLES AND DEFINITIONS
Osmosis
• Osmosis must involve water passing through a
membrane. If not, it is not osmosis.
PRINCIPLES AND DEFINITIONS
Active Transport
• Active transport is the movement of a
substance across the cell membrane requiring
the use of energy from the cell.
• Cellular energy is produced by the
mitochondria and stored in the form of
adenosine triphosphate (ATP).
PRINCIPLES AND DEFINITIONS
Endocytosis and Phagocytosis
• Endocytosis is the movement of substances into
the cell. If the substances are very large, such
as bacteria, phagocytosis is used to allow them
to enter the cell.
• "Phago" refers to eating, so phagocytosis is the
process by which the cell eats.
PRINCIPLES AND DEFINITIONS
Phagocytosis
The End
Principles of Biology
Cells