CONCEPTUAL LIFE SCIENCE

CELLS
THE CELL THEORY
The word th~ory in science means "explanation." The ceU theory is the accepted
explanation about cells. The cell theory was developed after many observations ofliving
things by different biologists. The cell theory has two parts.

1. All living things are made ofcells.

2. All cells arise from pre-existing cens.

Exceptions to the cell theory.

1. Mitochondria and chloroplasts are self-replicating.

2. Viruses are not true cens.

There are two basic types of cells. The cells that came first are called prokaryotic cens.
The prefix pro means "before" and the bryo stem means "nucleus." These celIs came
before the cells that have a nucleus. The celIs with a nucleus are called euluJryotic cells.
The prefix eu means "tnie." So, eukaryotic celIs have a "true nucleus" me~g that the
cellular nucleus has a membrane around it. .

The word ceU 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.

MlCROSCOPES

Simple microscope. The simple microscope consists ofonly one lens. The magnifying
glass is an example ofa simple microscope.

Compound microscope. The compound microscope has two lenses working in series.
The objective lens is the lens nearest the slide. It magnifies the object on the slide to
produce a real image ofthe object inside the body of the microscope.

The ocUlar lens is the lens nearest the eye. It produces a virtual·image ofthe real
image produced by the objective lens. The virtual image appears on the retina of yom
eye. You look into the ocular lens to see the magnified image of the specimen. When
observing living material with the compound microscope it is possible to see the nucleus,
the cytoplasm and, in plant cells, the chloroplasts. This is pretty much all that can be
seen with optical microscopes. For further detail, much higher magnificatiOns are
possible using the electron microscope.

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Electron microscope

The electron microscope uses a beam ofelectrons instead oflight to magnify the
specimen. Very high magnifications are possible using the electron microscope. These
reveal the ultrastructure ofthe cell.

Ultrastructme and organelles

Ultrastructme is the fine detail observed with the electron microscope. Itis
possible to see such structures as membranes which 8re not visible in the light
microscope because they are too thin to see. The tiny structures observed in cells are
known as organelles because they are small and they serve the functions ofcellular
organs.

GENERALIZED CELLS AND CELL STRUCTURES

A generalized cell is a drawing that contains all parts ofthe 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 ~ut the other parts are missing.

Figure 4-1. Typical ultrastructure of an animal cell. .

Eukaryotic cells are found in all higher organisms. They contain a Well-defmed
nucleus surrounded by a membrane. The prokaryotic organisms, Baeteriaand An:haea,
do not have a membrane-bounded nucleus.· .,.
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Cell membrane

The cell membrane is a structure that surrolDlds each cell. All living cells have a
cell membrane. As shown in Figure 4-2, the cell membrane consists oftwo layers of
lipid molecules with the polar heads toward the outside and the non-polar fatty acid tails
in the center ofthe membrane. This prevents nearly all molecules from passing through
the membrane. Only water and a few small neutral molecules can pass through the cell
membrane directly. It is the bOlDldary between the living cell and its environment. It has
the same function in both plant and animal cells.

The cell membrane has transport proteins to assist the passage of larger
molecules or ionized molecUles. There are a variety ofother proteins embedded in the
cell membrane. These are known as integralproteins. They help to maintain the
structure ofthe cell membrane and other important functions. Sometimes the integral
proteins have auxiliary proteins associated with them known as peripheralproteins.
These are important for other cellular functions, such as tran~tting messages to parts of
the cell.

Twa I.,ers at lipid

molecUle.

Transport
Protein

Figure 4.2. Diagram ofcell membrane structure.

In the electron microscope, a single membrane, such as illustrated in Figure 4.3,

appears as twoparaJlel lines at a distance of 80 Angstrom lDlits apart. This is shown in
Figure 4-3.

===::i1 IDA

Figure 4-3. Width of membrane.

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The "fluid mosaic" model of the cell membrane

The proteins in the membrane can move around inside the two layers oflipicls and
even protnJde through the SUlface. This gives the smface of the membrane the
appearance of a mosaic, a piece of art made of little tiles glued together. The cell
membrane therefore is referred to as a "fluid mosaic."

The Nucleus

The nucleus is the central organelle that controls the cell. It is surrounded by a
membrane known as the nuclear membrane. It coptains chromosomes and a-~cleolus.

r: ~---Nuclear Membrene

r/ *f,~ :~::matln

~
~
» NucleolllB

~)-------Endoplasmic Reticulum

~~;:!-------- Ribosome

Figure 4-4. Nucleus and Endoplasmic Reticulum

Nuclear membrane

The nuclear membrane surrounds the nucleus as shown in Figure 4-4. It contains
pores and channels that connect to the endoplasmic reticulum, a network ofchannels that
is found· in the cytoplasm..

Chromosomes

The chromosomes are structures that contain DNA and are found inside the .
nucleus ofthe cell. They control the activities ofthe cell.

Nucleolus

The nucleolus is a large, round structure within· the nucleus. It is responsible for
the fonnation ofthe components ofribosomes. .

... '~.
 , .

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The Cytoplasm

The cytoplasm is the part ofthe cell that is found outside ofthe nucleus. It is
bounded on the outside by the cell membrane. The cytoplasm contains many different
organelles that perfonn various fimctions for the cell.

Mitochondria

Mitochondria are the structures that produce useful energy for the cell. They
contain an internal membrane that is folded to form cristae. Each single crista is an
individual fold ofthis inner membrane. See Figure 4-5.

IIl}!r---- Outer Membrane

H~I+--- Crist.

It.fli-----lnnerMembrane

Figuie 4-5. Struetme ofthe mitochondrion.

The folds ofthe cristae ofthe mitochondria contain enzymes that perfonn cellular
respiration. Cellular respiration is the process by which useful energy is provided to the
cell.

Endoplasmic reticulum

The endoplasmic reticulum (ER) is a series of channels in the cytopl8$ID~

. "Reticulum" means net or netWOrk. "Endo" means inside. So, "endoplasmic ~culum"
means the network within the cytoplasm. It provides rapid molecular movement within .
the cell. There are two kinds: rough ER and smooth ER.

Rough ER has ribosomeS attached to it. Its appearance in the electron microscope
is that ofsandpaper. So it is known as rough because it looks rough. Smooth ER does
not have ribosomes attached to its membranes.

. . Ribosomes

Ribosomes are the places where proteins are mad~. Each ribosome has a small
. sUbunit and a large subunit. When a protein is being synthesized, you find riboSomes

attached to a piece ofmessenger RNA. This structure is sometimes called a

polyribosome or palys0'r'e; .See Figure 4-6.

. ':.;: .'.

.'
.',. '."
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,..---Small Subunit

'6 6 6 6 :: Subunit

Figure 4-6. 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 move toward the cell membrane. They are sometimes called
dietyosomes. See Figme 4-7. .

rr--f- Flat membrane

~--- Filled membrane

_ _",,--Cellmembrane

.Figure 4-7. Golgi body.

Lysosomes

Lysosomes are structures that are membrane-bounded and contain digestive .

. enzymes. The concept of lysis in biology implies breaking down, bursting, or otherWise
being taken apart in some way. The digestive enzymes digest or breakdown chemically
the particles that are taken into the lysosome. This process ofdigestion is the reverse of
dehydration synthesis.

Vacuoles

Vacuoles are areas in the cytoplasm that have a membrane surrounding them.
The term is derived from vacuum, which implies empty. Long ago it was thought that
vacuoles were empty and contained no contents. It is now known that they are filled with
cytoplasmic liquids or storage materials of various kinds.
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The cytoskeleton

The cell has a netWork offlexible fibers within the cytoplasm. This network of
fibers is called the cytoskeleton. The fibers are made ofpolymers 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 in Figure 4-8. There are some structures that
are found only in plant cells. These include the cell wall and the plastids.

• • •
'O~'O 'O'O • •
•

~
.'O
.'O

•
'.' • •

• ~

I
•
•
• .•.
~

Figure 4-8. Generalized plant celL .

Cell wall

The cell wall is made of cellulose. It is considered non-living. The cellulose

fibers fonn primary and secondary walls. The primary wall is usually thin. The
secondary walls are thicker. Different types ofplant cells have different thicknesses-of
cell wall. The cell wall pictured in Figure 4-8 is a primary cell wall. The cell membrane
. ofthe cell is pushed up against the inside ofthe cell wall by turgor pressure inside the
Cell. .
Cell wall layers

The middle/amella is produced when a plant ceU1tivides. "Lamelli" means ...

layer. The middle lamelJais the first layer fonned between. two plant cells. It is actuiJly

.a membrane that the cells use to separate themselves at the end ofplant cell divisiC).L· .

;.... -..
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Each cell produces a primary wall on its side ofthe middle lamella. The primary
wall is thin. All plant cells generally have a primary wall.

The secondary wall is generally much thicker than the primary wall. Secondary
walls are found in plant cells that provide strength to structures in the plant. Not all plant
cells produce secondary walls.

_ _ _ _ •_ Secondary Wall
~I. l... • . ' z. -Middle Lamella'
Primary Wan
•••••

z:.: --:..: :.;:

Z _ ._._ Z ,

Figure 4-9. Plant cell wall structure.

Plastids

Plastids are structures found only in plant cells. There are three kinds, which are
chloroplasts, chromoplasts and leukoplasts.

Chloroplasts. ChloroplaSts are plant organelles that make food by performing the
process ofphotosynthesis. Photosynthesis is the synthesis oforganic 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 is responsible for trapping light energy from the Sun.

@~j'5 g~=~Membran.
Figure 4-10. Chloroplast structure.

Chromoplasts. Chromoplasts are plastids that are colored ("chromo" means colored) but
do not perfonn photosynthesis. Most contain carotenid pigments that are related to
carotene. These pigments are various shades oforange or yellow. The pigments can be
separated from leaves by means ofa technique known as chromatography.

Lellkoplasts. "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 found within the cells ofpotatoes.
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Animal Cell Structures

Cilia and flagella

Cilia and flagella are organelles of locomotion. They contain contractile proteins..
The proteins allow the 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.

Figure 4-11. Cross section of a flagellum

showing the 9+2 arrangemenl

The contractile proteins in a cilium or a flagelllDD have a characteristic

arrangement. As shown in Figure 4-11, the protein fibers are in pairs. There are nine
pairs around the outside with two fibers in the center. This structural configuration is
. known as the 9+2 arrangemenl~ .

~I-.
--_- ::::::brane
~ BasalBodJ
f ,-- Rootlet .
Figure 4-12. Flagellar attachment.

As shown in Figme 4-12, flageIJa are attached to the cell membrane. InSidethe
.cell membrane is a basal body, which is anchored in the cytoplasm by rootlets.. The
filament ofthe flagel1um, the partwith the 9+2 arrangement,.is located outside the cell
membrane.

. .' ~'.::...
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Centrioles
Centrioles are animal cell structures made of microtubules.. The centriole is
located in the cytoplasm just outside ofthe nuclear membrane. It is used by the animal
cell during cell division. As part ofthe cell division process in animal cells, the centriole
produces a network of fibers known as the spindle. The spindle fibers pull the
chromosomes apart during the process of mitosis.

Summary Outline ofCell Structures

Structures common to all eukaryotic cells

Cell membrane

Nucleus

Mitochondria

Endoplasmic reticulmn

Ribosomes

.Golgi bodies

Lysosomes

Vacuoles

Structures found only in plant cells

Cell wall

Chloroplasts

Plastids

Structures foUli~ only in animal cells

Cilia and flagella

Centrioles

PRINCIPLES AND DEFINITIONS RELATING TO CELL PHYSIOLOGY

Living cells are influenced by the environment in which they find themselves.
The solution or external environment SUJTOunding the cell has an impact on how the cell
will ftmction. The principles and definitions are given below..

Solution. A solution is a homogeneous mixture oftwo or more-components in which the

particles are so small that they cannot be distinguished. The size ofthe particles in a
solution is less than 1 om.
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Colloid. A colloid is a homogeneous dispersion ofparticles too large to be considered a

solution, but which do not settle out appreciably on standing. The particle size in a
conoid is approximately between 1 and 100 DID. A colloid exists either as a sol or a gel.
The cytoplasm of cens resembles a conoid.

Suspension. A suspension is a mixture ofparticles that settle out appreciably on standing.

In a suspension, the particle size is greater than 100 nm.

Diffusion. Diffusion is the net movement of the particles of a particular substance from
an area ofhigher concentration ofthat subStance to an area oflower concentration ofthat
substance, at constant temperatme and pressure,

Osmosis. Osmosis is the diffusion of water through a differentially (semi-) penneable

membrane. In most biological systems, under normal conditions, water will move from
the outside of the cell to the inside ofthe cen because the cen bas less water inside than
there is outside. Osmosis is one ofthe most misunderstood biological concepts.
Whatever is being seen, it has to involve water through a membrane. Ifnot, it is not
osmosis.

. Outside

100%
Water

Figure 4-13. Osmosis. Water enters the cen through the semi-penneable cell membrane..

In Figure 4-13, water enters the cell by omlosis because it is passing through the
ceD membrane. This is because the concentration ofwater is higher outside the cell. If
the cell were placed in salt water, water would leave the cell and pass through the
membrane to the outside. This process is called plasmolysis.

Active Transport. Active transport is the movement ofsubstance across the cell
.-- -~ -- - -_. -- -------- membrane requiring the use ofenergy from the-celt- CelJular-eneriY isproduoed-by the
mitochondria and stored in the fann ofadenosine triphosphate (ATP).

Endocytosis. 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 cen. The
"phago" prefix refers to eating. So, phagocytosis is a process by which the cell eats. .
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Pseudopod Food Vacuole

----""""

A B c
Figure 4-14. Phagocytosis. A. Cell approaches the food particle.B. Cell surroWltts the
food particle using pseudopods~ C. Food particle is completely engulfed and is located
inside a food vacuole.

In an organisni' such as the Amoeba, the cell uses pseudopods to surround the food
particle. Pseudopods ("false feet'') are extensions ofthe cytoplasm ofthe cell. See
Figure 4-14. Once the food particle is surrounded, it becomes part of a food vacuole.
The material in a food vacuole is digested by combination ofthe food vacuole with a
lysosome. Certain white blood cells aiso can perform phasocytosis.

Another form of endocytosis is pinoeyto$is. Pinocytosis is sometimes refmed to

as "cell drinking." In pinocytosis, small droplets ofliquids or large molecules enter the
cell via an invagination ofthe cell membrane. Pinocytosis is like phagocytosis only on a
much smaller scale. Both processes require ATP as a source ofenergy.

Exocytosis. Exocytosis is the movement ofsubstances out ofthe cell. The substances to
be exported are usually surrounded by a membranous vesicle, wch as that prodU<:ed by
the Golgi bodies. When the membrane coalesces with the cell membrane, the material
.inside the vesicle is exported outside the cell.

Regulation. The cell membrane cannot completely regulate the entrance or exit of all
materials. When a cell is placed in a hyperosmotic medium, it tends to. lose water into the
medium outside the cell. This results in plasmolysis. When the cell is placed in a
hypoosmotic medium, water enters the cell and it swells. Red blood cells burst open in
water because water is a hypoosmotic medium to them. A cell in an isoosmotic medium
will not appreciably gain or lose water.