Download Chapter 2: Basic Biological Principles Lesson 2.2: Structural and

Chapter 2: Basic Biological Principles
Lesson 2.2: Structural and Functional Relationships at Biological Levels of
Organization
Cells may be small in size, but they are extremely important to life. Like all other living things,
you are made of cells. Cells are the basis of life, all organisms are made up of one or more cells. Cells of
all living organisms have many of the same structures and carry out the same basic life processes.
Knowing the structures of cells and the processes they carry out is necessary to understanding life itself.
You will learn more about these amazing building blocks of life, like the dividing cell pictured above,
when you read this chapter.
Lesson Objectives
•
•
•
•
•
•

Describe the diversity of cell shapes, and explain why cells are so small.
Identify the parts that all cells have in common.
Describe the structure and function of the plasma membrane.
Outline the form and function of the nucleus and other organelles.
Compare and contrast prokaryotic and eukaryotic cells.
Explain how cells are organized in living things.
Describe the relationship between structure and function at various levels of biological organization.
Vocabulary
• endoplasmic reticulum
 endosymbiosis
 extracellular
 Golgi apparatus
 intracellular
 mitochondria (mitochondrion, singular)
 multicellular
 nucleus
 organ
• organelle
 organism
 organ system
• plasma membrane
• ribosome
 tissue
 unicellular
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INTRODUCTION
Your body is made up of trillions of cells, but all of them perform the same basic life functions.
They all obtain and use energy, respond to the environment, and reproduce. How do your cells carry out
these basic functions and keep themselves—and you—alive? To answer these questions, you need to
know more about the structures that make up cells and how they function.
DIVERSITY OF CELLS
Today, we know that all living cells have certain things in common. For example, all cells share
functions such as obtaining and using energy, responding to the environment, and reproducing. The
function a cell must carry out influences its physical features and its internal organization. We also know
that different types of cells—even within the same organism—may have their own unique functions as
well. Cells with different functions generally have different shapes that suit them for their particular job.
Cells vary in size as well as shape, but all cells are very small. In fact, most cells are much smaller than
the period at the end of this sentence. If cells have such an important role in living organisms, why are
they so small? Even the largest organisms have microscopic cells. What limits cell size?
Cell Size
The answer to these questions is clear once you know how a cell functions. To carry out life
processes, a cell must be able to quickly pass substances into and out of the cell. For example, it must be
able to pass nutrients and oxygen into the cell and waste products out of the cell. Anything that enters
or leaves a cell must cross its outer surface. It is this need to pass substances across the surface that
limits how large a cell can be. Look at the two cubes in Figure 2.7. As this figure shows, a larger cube has
less surface area relative to its volume than a smaller cube. This relationship also applies to cells; a larger
cell has less surface area relative to its volume than a smaller cell. A cell with a larger volume also needs
more nutrients and oxygen and produces more wastes. Because all of these substances must pass
through the surface of the cell, a cell with a large volume will not have enough surface area to allow it to
meet its needs. The larger the cell is, the smaller its ratio of surface area to volume, and the harder it
will be for the cell to get rid of its wastes and take in necessary substances. This is what limits the size of
the cell.
Figure 2.7 Surface Area to Volume Comparison. A larger cube has a smaller surface area (SA) to
volume (V) ratio than a smaller cube. This holds true for cells and limits how large they can be.
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Cell Shape
Cells with different functions often have different shapes. The cells pictured in Figure 2.8 are
just a few examples of the many different shapes that cells may have. Each type of cell in the figure has
a shape that helps it do its job. For example, the job of the nerve cell is to carry messages to other cells.
The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to
many other cells at once. Do you see the tail-like projections on the algae cells? Algae live in water, and
their tails help them swim. Pollen grains have spikes that help them stick to insects such as bees. How do
you think the spikes help the pollen grains do their job? (Hint: Insects pollinate flowers.)
Figure 2.8 As these pictures show, cells come in many different shapes. Clockwise from the upper left photo are a
nerve cell, red blood cells, bacteria, pollen grains, and algae. How are the shapes of these cells related
to their functions?
PARTS OF THE CELL COMMON TO ALL ORGANISMS
Although cells are diverse, all cells have certain parts in common. The parts include a plasma
membrane, cytoplasm, ribosomes, and DNA.
1. The plasma membrane (also called the cell membrane) is a thin coat of phospholipid and
protein molecule bilayer that surrounds a cell . It forms the physical boundary between the
cell and its environment, so you can think of it as the ‘‘skin” of the cell. It controls the
movement of materials in and out of the cell through either active or passive transport
mechanisms.
2. Cytoplasm refers to all of the cellular material inside the plasma membrane. Cytoplasm is
made up of a watery substance called cytosol and contains other cell structures such as
ribosomes.
3. Ribosomes are cellular structures either in the cytoplasm or attached to the rough
endoplasmic reticulum composed of RNA (ribonucleic acid) and proteins where proteins are
made in eukaryotic and prokaryotic cells.
4. DNA is a nucleic acid molecule found in cells. It contains the genetic instructions that cells
need to make proteins, carry out live processes, and pass on inheritable characteristics.
These parts are common to all cells, from organisms as different as bacteria and human beings.
How did all known organisms come to have such similar cells? The similarities show that all life on Earth
has a common evolutionary history.
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The Plasma Membrane
The plasma membrane forms a barrier between the cytoplasm inside the cell (intracellular) and
the environment outside the cell (extracellular). It protects and supports the cell, controls everything
that enters and leaves the cell, and recognizes chemical signals. It allows only certain substances to pass
through, while keeping others in or out. The ability to allow only certain molecules in or out of the cell is
referred to as selective permeability or semi-permeability. To understand how the plasma membrane
controls what crosses into or out of the cell, you need to know its composition. The cell membrane
consists of two layers of phospholipids with proteins embedded within these layers. The surface of the
cell contains molecules which recognize other molecules which may attach to or enter the cell, see
Figure 2.9. The plasma membrane is discussed at http://www.youtube.com/watch?v=-aSfoB8Cmic .
Figure 2.9 Plasma membrane or cell membrane is often referred to as a lipid bilayer.
The Phospholipid Bilayer
The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and
alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid
bilayer. As shown in Figure 2.10, each phospholipid molecule has a head and two tails. The head ‘‘loves”
water (hydrophilic) and the tails ‘‘hate” water (hydrophobic). The water-hating tails are on the interior
of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the
fluid that surrounds the cell. Molecules that are hydrophobic can easily pass through the plasma
membrane, if they are small enough, because they are water-hating like the interior of the membrane.
Molecules that are hydrophilic, on the other hand, cannot pass through the plasma membrane—at least
not without help—because they are water-loving like the exterior of the membrane.
Figure 2.10 Phospholipid Bilayer. The phospholipid bilayer consists of two layers of phospholipids (left),
with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior. A
single phospholipid molecule is depicted on the right.
Other Molecules in the Plasma Membrane
The plasma membrane also contains other molecules, primarily other lipids and proteins. The
yellow molecules in Figure 2.9, for example, are the lipid cholesterol. Molecules of cholesterol help the
plasma membrane keep its shape. Many of the proteins, the blue molecules in Figure 2.9, in the plasma
membrane assist other substances in crossing the membrane. Glycoproteins and surface carbohydrates
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serve as cell receptors, points of attachment, for other cells, infectious bacteria, viruses, toxins,
hormones, and many other molecules.
Extensions of the Plasma Membrane
The plasma membrane may have extensions, such as whip-like flagella or brush-like cilia. In
single-celled organisms, like those shown in Figure 2.11 and Figure 2.12, the membrane extensions may
help the organisms move. In multicellular organisms, the extensions have other functions. For example,
the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose.
Figure 2.11 Flagella on bacteria cells aid in cell
movement.
Figure 2.12 Cilia are extensions of the plasma
membrane of many cells.
Cytoplasm
The cytoplasm consists of the fluid, the cytoskeleton, and all the membrane-bound organelles
except the nucleus. The part of the cytoplasm that contains molecules and small particles, like
ribosomes is called the cytosol. The water in the cytoplasm makes up about two thirds of the cell’s
weight and gives the cell many of its properties.
Functions of the Cytoplasm
The cytoplasm has several important functions, including:
1. suspending cell organelles
2. pushing against the plasma membrane to help the cell keep its shape
3. providing a site for many of the biochemical reactions of the cell
Cytoskeleton
Crisscrossing the cytoplasm is a structure called the cytoskeleton, which consists of threadlike
filaments and tubules. You can see these filaments and tubules in the cells in Figure 2.13. As its name
suggests, the cytoskeleton is like a cellular ‘‘skeleton.” It helps the cell maintain its shape and also holds
cell organelles in place within the cytoplasm of cells that do have organelles. Some unicellular organisms
do not have organelles, they do not have a cytoskeleton. The cytoskeleton is discussed in the following
video http://www.youtube.com/watch?v=5rqbmLiSkpk&.
Figure 2.13 The cytoskeleton gives the cell an internal structure, like the frame of a house. In this
photograph, filaments and tubules of the cytoskeleton are green and red, respectively.
The blue dots are cell nuclei.
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Ribosomes
Ribosomes are small organelles where proteins are made. They contain the nucleic acid RNA,
which assembles and joins amino acids to make proteins. Ribosomes can be found alone or in groups
within the cytoplasm as well as on the RER (rough endoplasmic reticulum), see Figure 2. 14.
Figure 2.14 Free ribosomes are found in the cytoplasm and others are attached to the RER.
PROKARYOTIC AND EUKARYOTIC CELLS
Two Types of Cells
Based on whether they have a nucleus, there are two basic types of cells, prokaryotic cells and
eukaryotic cells. A nucleus is a membrane‐bound organelle in eukaryotic cells that functions to maintain
the integrity of the genetic material and, through the expression of that material, controls and regulates
cellular activities. All eukaryotic cells have a nucleus but prokaryotic cells do not. The nucleus of a
eukaryotic cell is a structure in the cytoplasm that is surrounded by a membrane (the nuclear
membrane) and contains DNA. Prokaryotic cells also have DNA which is often concentrated in a part of
the cell called the nucleoid.
You can view an animation of both types of cells at:
http://www.learnerstv.com/animation/animation.php?ani=162&amp.
Prokaryotic Cells
Most prokaryotes are made up of just a single cell (unicellular) but there are a few that are
made of collections of cells (multicellular). Scientists have divided the prokaryotes into two groups, the
Bacteria and the Archaea. A prokaryotic bacteria is shown in Figure 2.15. Organisms with prokaryotic
cells are called prokaryotes, they lack a nucleus and do not contain membrane-bound organelles.
Organelles, “little organs”, are subunits within a cell that have specialized functions. Prokaryotes were
the first type of organisms to evolve and are still the most common organisms today.
Figure 2.15 Prokaryotic Cell. This diagram shows the structure of a typical prokaryotic cell, a bacterium.
Like other prokaryotic cells, this bacterial cell lacks a nucleus but has other cell parts,
including a plasma membrane, cytoplasm, ribosomes, and DNA (concentrated in an area
called the nucleoid).
Eukaryotic Cells
Eukaryotic cells can be unicellular or multicellular. All eukaryotic cells contain a nucleus. A
typical eukaryotic cell is shown in Figure 2.16. Eukaryotic cells are usually larger than prokaryotic cells.
Organisms with eukaryotic cells are called eukaryotes, and they range from fungi to people. Eukaryotic
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cells also contain other organelles besides the nucleus. For example, organelles called mitochondria
provide energy to the cell, and organelles called vacuoles store substances in the cell. Organelles allow
eukaryotic cells to carry out more functions than prokaryotic cells can.
Figure 2.16 Eukaryotic Cell. Compare and contrast the eukaryotic cell shown here with the prokaryotic
cell. What similarities and differences do you see?
Compare and Contrast Prokaryotes versus Eukaryotes
Table 2.1: The following table provides a more detailed comparison of prokaryotic and eukaryotic cells
Characteristic
Prokaryote
Eukaryote
Cells are enclosed by a plasma (cell)
membrane
Membrane-bound organelles
Cells contain DNA
Cells contain ribosomes
Have a cell wall
Plants, most fungi,
and some protists
Cells contain a nucleus
Includes unicellular organisms
Includes multicellular organisms
All cells are able to perform all
functions necessary for life
Cells reproduce by binary fission
Cells reproduce through the cell
cycle (mitosis) and meiosis
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Viruses: Prokaryotes or Eukaryotes?
Viruses, like the one depicted in Figure 2.17 on the next page, are tiny particles that may cause
disease. Human diseases caused by viruses include the common cold and flu. Do you think viruses are
prokaryotes or eukaryotes? The answer may surprise you. Viruses are not cells at all, so they are neither
prokaryotes nor eukaryotes.
Viruses contain DNA but not much else. They lack the other parts shared by all cells, including a
plasma membrane, cytoplasm, and ribosomes. Therefore, viruses are not cells, but are they alive? All
living things not only have cells; they are also capable of reproduction. Viruses cannot reproduce by
themselves. Instead, they infect living hosts, and use the hosts’ cells to make copies of their own DNA.
Also, viruses cannot obtain energy on their own; they use living cells to carry out their life processes.
For these reasons, most scientists do not consider viruses to be living things.
Figure 2.17 Scanning electron micrograph of HIV viruses (green) budding from a cultured T-lymphocyte.
What is a virus? Is it a cell? Is it even alive?
STRUCTURES FOUND ONLY IN EUKARYOTIC CELLS
The Nucleus and Other Organelles
Eukaryotic cells contain a nucleus and several other types of membrane-bound organelles.
These structures are involved in many vital cell functions.
The Nucleus
The nucleus is filled with a jellylike liquid called nucleoplasm, which holds the contents of the
nucleus and is similar in function to a cell’s cytoplasm. The nucleus is the largest organelle in a
eukaryotic cell and is often considered to be the cell’s control center. One of the reasons that it is
considered the control center is because the nucleus houses the cell’s genetic information and controls
which proteins the cell makes.
The nucleus of a eukaryotic cell contains most of the cell’s DNA is the form of a threadlike
material called chromatin. When a cell gets ready to divide, the chromatin condenses to form
chromosomes. Chromosomes are structures in the nucleus made of DNA and protein.
The nucleus is the site where DNA is transcribed into mRNA (messenger ribonucleic acid). The
mRNA copies the genetic code from DNA for protein synthesis and carries it out of the nucleus through
nuclear pores into the cytoplasm. In the cytoplasm with the help of ribosomes, proteins are actually
synthesized. You will study this process in more detail when we look at cell growth and reproduction.
Nuclear Envelope or Nuclear Membrane
The nuclear envelope surrounds the nucleus and is a double membrane structure made up of
two phospholipid bilayers. The surface of the nuclear membrane is covered by tiny, protein-lined pores
called nuclear pores. These nuclear pores are passageways for mRNA and other materials to enter or
leave the nucleus.
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Nucleolus
Most nuclei contain a denser area within them called the nucleolus. The nucleolus is the site
where DNA is concentrated during the process of making ribosomal RNA. Ribosomal RNA or ribosomes
are organelles composed of RNA and protein. Ribosomes are where the genetic code for synthesizing
proteins is translated into amino acids to form protein chains in the cytoplasm.
Figure 2. 18 The nucleus is surrounded by a nuclear envelope, which is a double membrane. The
nucleus stores the cell’s DNA and the nucleolus is were ribosomes are made.
Mitochondria
The mitochondrion (plural, mitochondria) is an organelle that makes energy available to the cell.
This is why mitochondria are sometimes referred to as the ‘power plants’ of the cell. They use energy
from organic compounds (carbon containing compounds) such as glucose during cellular respiration to
make molecules of ATP (adenosine triphosphate), an energy-carrying molecule that is used almost
universally inside cells as the fuel for chemical reactions. Very active cells, like muscle cells can have
hundreds of mitochondria.
Mitochondria have an inner and outer phospholipid membrane as shown in Figure 2.19. The
outer membrane separates the mitochondria from the cytosol. The inner membrane has lots of folds
called cristae, these folds contain proteins that carry out energy-harvesting chemical reactions. What
functionality purpose do these folds provide to the mitochondria? Think surface area to volume ratio…
Mitochondrial DNA
Scientists think that mitochondria were once free-living organisms because they contain their
own DNA and can reproduce only by the division of preexisting mitochondria. They theorize,
endosymbiotc theory, that ancient prokaryotes infected (or were engulfed by) larger prokaryotic cells,
and the two organisms evolved an endosymbiotic relationship that benefited both of them. The larger
cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy
from the smaller prokaryotes. Eventually, the prokaryotes became permanent guests of the larger cells,
as organelles inside them.
Outer membrane
Inner membrane
Cristae
Figure 2.19 Mitochondria convert organic molecules into energy for the cell. Mitochondria have an inner
membrane and an outer membrane. The folds of the inner membrane are called cristae, the
site where energy conversion occurs.
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Endoplasmic Reticulum
The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and
lipids, it is an intracellular highway. There are two types of endoplasmic reticulum: rough endoplasmic
reticulum (RER) and smooth endoplasmic reticulum (SER). Both types are shown in Figure 2.20.
Rough Endoplasmic Reticulum (RER)
RER looks rough because it is studded with ribosomes that cover interconnected flattened sacs.
The RER produces phospholipids and proteins. The proteins that it makes are exported either out of the
cell or inserted into the cell’s own membrane. If they are exported out the proteins are encased in little
sacs or vesicles formed from pinched off ends of the RER. For example, ribosomes make some proteins
which act as enzymes, they will be stored into vesicles until they need to be exported to perform a
specific function. You will find a lot of RER in cells that produce large amounts of proteins for export, like
digestive glands and antibody producing cells.
Smooth Endoplasmic Reticulum (SER)
SER looks smooth because it does not have ribosomes. SER produces lipids such as cholesterol.
SER found in the sex cells produces steroid hormones, like estrogen and testosterone. In skeletal and
heart muscle cells, the SER releases calcium, which help to stimulate contractions. The liver and kidney
cells, have SER which help to detoxify drugs and poisons.
Golgi Apparatus
The Golgi apparatus is a large organelle composed of a system of flattened, membranous sacs
that processes proteins and prepares them for use both inside and outside the cell. It is shown in Figure
2.20. The Golgi apparatus is somewhat like a post office. The sacs of the Golgi nearest to the nucleus
receive vesicles from the ER containing proteins or lipids. These vesicles travel from one part of the Golgi
to the next transporting substances. As the substances are transported they are packaged and modified.
The protein substances are “labeled” and directed to various other parts of the cell where they are
needed. The Golgi apparatus can add carbohydrate labels to proteins or alter new lipids in a variety of
ways. At the link below, you can watch an animation showing how the Golgi apparatus does all these
jobs. http://www.johnkyrk.com/golgiAlone.html.
Figure 2.20 This drawing includes the nucleus, RER, SER, and Golgi apparatus. From the drawing, you can see
how all these organelles work together to make and transport proteins.
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Vesicles
Vesicles are small spherical shaped sacs surrounded by a single membrane and are classified by
their contents. Certain types of vesicles called transport vesicles are found only in eukaryotic cells.
These vesicles pinch off from the membranes of the ER and Golgi apparatus (see Figure 2.20, on the
previous page) in order to store and transport protein and lipid molecules until they merge with the
plasma membrane. When these vesicles merge with the plasma membrane they release their contents
to the outside of the cell. Some other types of vesicles are used as chambers for biochemical reactions.
These types of vesicles include lysosomes, peroxisomes, glyoxysomes, endosomes, food vacuoles, and
contractile vesicles. Their functions are described in the following sections.
Lysosomes
These are vesicles that bud off the Golgi and contain digestive enzymes. The enzymes in these
vesicles break down large molecules, like proteins, nucleic acids, carbohydrates, and phospholipids.
Lysosomes found in the liver break down glycogen into glucose to be released into the bloodstream.
White blood cells use lysosomes to break down bacteria. Lysosomes found intracellular digest worn-out
organelles in a process called autophagy. Lysosomes also break down cells when they die. The digestion
or breaking down of dead cells is called autolysis. By carrying out these roles lysosomes help in
maintaining an organism’s health.
Peroxisomes
Peroxisomes also contain enzymes but they are not produced by the Golgi apparatus. These
vesicles are found in the liver and kidney cells, where the neutralize free radicals (oxygen ions that can
damage cells), and detoxify alcohol and other drugs. Peroxisomes get their name from the hydrogen
peroxide, H2O2 that they produce when breaking down alcohol and killing bacteria. Additionally
peroxisomes break down fatty acids so that the mitochondria can use them as an energy source.
Glyoxysomes, Endosomes, Food Vacuoles, and Contractile Vesicles
Glyoxysomes are specialized peroxisomes found in the seeds of some plants and break down
fats to provide energy for developing plant embryos. Endosomes are formed when some cells engulf
material by surrounding it with plasma membrane forming a pocket that buds off inside the cell.
Lysosomes fuse with these endosomes and digest the engulfed materials. Food vacuoles are vesicles
that store nutrients for the cell. Contractile vacuoles are vesicles that contract and dispose of excess
water inside of a cell.
Centrioles
Centrioles are organelles that are involved in cell division and are found in the cytoplasm near
the nucleus. They help organize the chromosomes before cell division so that each daughter cell has the
correct number of chromosomes after the cell divides. Centrioles are found only in animal cells. Plant
cells do not have centrioles, they have basal bodies. Basal bodies are found at the base of cilia and
flagella and help to organize the development of cilia and flagella.
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Further information:
A major function of cells is the production of protein. The pathway taken by some proteins from
synthesis to export follows the steps labeled in Figure 2.21. (1) The code for making proteins is
copied/transcribed from DNA into mRNA in the nucleus then (2) proteins assembled by ribosomes on the
RER. (3) vesicles transport proteins to the Golgi apparatus. (4 &5) Golgi modifies proteins and packages
them in new vesicles. (6) the new vesicles release the proteins that have destinations outside the cell.
Vesicles containing enzymes remain inside the cell as lysosomes, peroxisomes, endosomes, and
other types of vesicles.
Figure 2.21 The rough ER, Golgi apparatus, and vesicles work together to transport proteins to their
destinations inside and outside the cell.
SPECIAL STRUCTURES IN EUKARYOTIC PLANT CELLS
Plant cells have several structures that are not found in animal cells, including a cell wall, a large
central vacuole, and organelles called plastids. You can see each of these structures in Figure 2.22. You
can view them in an interactive plant cell at the link:
http://www.cellsalive.com/cells/cell_model.htm .
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PLASTID
CENTRAL VACUOLE
CELL WALL
Figure 2.22 In addition to the organelles and other structures found inside animal cells, plant
cells also have a cell wall, a large central vacuole, and plastids such as chloroplasts.
Can you find each of these structures in the figure?
Cell Wall
The cell wall is a rigid layer that surrounds the plasma membrane of a plant cell. It supports and
protects the cell. Tiny holes, or pores, in the cell wall allow water, nutrients, and other substances to
move into and out of the cell. The cell wall is made up mainly of complex carbohydrates, called
cellulose.
Central Vacuole
Most mature plant cells have a large central vacuole. This vacuole can make up as much as 90%
of the cell’s volume. The central vacuole has a number of functions, including storing substances such as
water, enzymes, and salts. It also helps plant tissues, such as stems and leaves, stay rigid and hold their
shape, if the central vacuole loses water the plant tissues will wilt.
Some central vacuoles store toxic materials. Acacia tree vacuoles store poisons as a defense
against plant-eating animals. Tobacco plant vacuoles store the toxin nicotine. Other vacuoles store plant
pigments, like the pigments that give the flowers their beautiful colors. An acacia tree, tobacco plant,
and a red-flowering plant shown in Figure 2.23.
Figure 2.23 On the left is an acacia tree, in the middle is a tobacco plant, and on the right is a red-flowering
plant.
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Plastids
Plastids are organelles surrounded by a double membrane in plant cells. Plastids carry out a
variety of different functions. The main types of plastids and their functions are described below.
• Chloroplasts are plastids that contain the green pigment chlorophyll. They capture light energy
from the sun and use it to make food. A chloroplast is shown in Figure 2.22.
• Chromoplasts are plastids that make and store other pigments. The red pigment that colors
the flower petals in Figure 2.23 was made by chromoplasts.
• Leucoplasts are plastids that store substances such as starch or make small molecules such as
amino acids.
Like mitochondria, plastids contain their own DNA. Therefore, according to endosymbiotic
theory, plastids may also have evolved from ancient, free-living prokaryotes that invaded larger
prokaryotic cells. If so, they allowed early eukaryotes to make food and produce oxygen.
COMPARISON CHART SUMMARIZING THE DIFFERENCES BETWEEN PROKARYOTIC AND
EUKARYOTIC CELLS
Table 2.2 Comparison of the characteristics and structures of prokaryotic and eukaryotic cells.
Prokaryotic Cell
Eukaryotic Cell
Membrane-bound organelles
Absent
Present
Nucleus
Absent
Present
Cytoskeleton
May be absent
Present
Endoplasmic reticulum
Absent
Present
Ribosomes
Present- smaller
Present- larger
Mitochondria
Absent
Present
Golgi apparatus
Absent
Present
Vesicles
Present- but fewer types
Present
Lysosomes and peroxisomes
Absent
Present
Cell wall
Present- usually chemically complex Only in plant cells and fungi
(chemically simpler)
Large Central Vacuoles
Absent- only small vacuoles
Present- plants only
Plastids
Absent, chlorophyll scattered in the Present- in plants
cytoplasm
CELLULAR ORGANIZATION
The simplest level of cell organization is a single-celled organism, and the most complex level is a
multicellular organism. In between these two levels are biofilms and colonies.
• A single-celled (unicellular) organism floats freely and lives independently. Its single-cell is able
to carry out all the processes of life without any help from other cells.
• A biofilm is a thin layer of bacteria that sticks to a surface. Cells in a biofilm are all alike, but
they may play different roles, such as taking in nutrients or making the ‘‘glue” that sticks the
biofilm to the surface. The sticky plaque that forms on teeth is a biofilm of bacterial cells.
• Some single-celled organisms, such as algae, live in colonies. A colony is an organized structure
composed of many cells, like the Volvox spheres in Figure 2.24. Volvox are algae that live in
colonies of hundreds of cells. All of the cells in the colony live and work cooperatively. For
example, they can coordinate the movement of their flagella, allowing them to swim together
through the water as though they were part of a single organism.
• A multicellular organism consists of many cells and has different types of cells that are
specialized for various functions. All the cells work together and depend on each other to carry
out the life processes of the organism. Individual cells in a multicellular organism are unable
to survive on their own.
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Figure 2.24 Volvox Colonies. Volvox cells live in a colony shaped like a hollow ball. The cells of the colony
may be connected by strands of cytoplasm and can function together. For example, the
whole colony can swim from one place to another as one.
Levels of Organization in Multicellular Organisms
Scientists think that multicellular organisms (eukaryotes) evolved when many single-celled
organisms (prokaryotes) of the same species started to work together and benefited from the
relationship. The relationship that developed between them is called endosymbiosis and forms the basis
of the endosymbiotic theory, which states that eukaryotes evolved from prokaryotes. We will discuss
this in more detail when we study evolution.
The oldest known multicellular organisms are algae that lived 1.2 billion years ago. As
multicellular organisms continued to evolve, they developed increasingly complex levels of organization.
Today there are multicellular organisms at all levels of organization, from the simplest, cell level of
organization to the most complex, organ-system level of organization. Consider these examples:
• Sponges have cell-level organization, in which different cells are specialized for different
functions, but each cell works alone. For example, some cells digest food, while other cells let
water pass through the sponge.
• Jellyfish have tissue-level organization, in which groups of cells of the same kind that do the
same job form tissues. For example, jellyfish have some tissues that digest food and other
tissues that sense the environment.
• Roundworms have organ-level organization, in which two or more types of tissues work
together to perform a particular function as an organ. For example, a roundworm has a
primitive brain that controls how the organism responds to the environment.
• Human beings have organ system-level organization, in which groups of organs work together
to do a certain job, with each organ doing part of the overall task. An example is the human
digestive system. Each digestive system organ—from the mouth to the small intestine—does
part of the overall task of breaking down food and absorbing nutrients.
The human body is also organized on several different levels, from cells to organ systems. Cells
are the basic unit of life and the smallest unit capable of carrying out the functions of life. Humans
have a variety of specialized cells, such as, blood cells, bone cells, and nerve cells that are suited to
perform different and very specific tasks. Groups of connected cells that perform similar functions
come together to form tissues. They are four basic types of tissues in the human body: connective
tissue connects different structures to form overall structure of the body, epithelial tissue covers
body surfaces to protect organs and it also secretes and absorbs substances, muscle tissue can
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contract to move bones, and nervous tissue carries electric nerve signals. Two or more groups of
tissues working together form organs like the heart, brain, lungs, and skin. Organs that work together
form organ systems that perform more complex jobs like respiration and digestion.
All the organizational levels of the body work together to help maintain homeostasis, or keep
internal conditions stable. Additionally hormones and the endocrine system play a large role in
maintaining homeostasis. Failure of homeostasis can lead to sickness or even death. The human body
organ system-level organization is summarized in the diagram shown in Figure 2.25.
Figure 2.25 The diagram above shows the organ system-level organization found in humans.
Relationship Between Structure and Function in Biological Levels of Organization
Organizational Levels
Important levels of organization for structure and function of living things include organelles,
cells, tissues, organs, organ systems, and whole organisms. Organelles present in unicellular organisms
often act in the same manner as the tissues and systems found in multi-cellular organisms. The
organelles in unicellular organisms perform all of the life processes needed to maintain homeostasis, by
using specialized cell organelles. The cells of multi-cellular organisms are of different kinds and are
grouped into tissues that help their function.
Groups of tissues working together to perform a common function are called organs. An
example of this would include the nervous, muscle, and other tissues which make up the heart. Groups
of organs working together to perform a common function are referred to as a system or organ system.
The blood vessels, blood, and the heart are organs which work together to form the circulatory
system. Many different systems function together to allow a complex organism to function.
Cell Structure
Cells have particular structures or organelles that perform specific jobs. These structures
perform the life activities within the cell. Just as body systems are coordinated and work together in
complex organisms, the cells making up those systems must also be coordinated and organized in a
cooperative manner so they can function efficiently together.
Inside the cell a variety of cell organelles, formed from many different molecules, carry out the
transport of materials, energy capture and release, protein building, waste disposal, and information
storage. Each cell is covered by a membrane that performs a number of important functions for the cell
as well.
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Cell Organelles
Cells have particular structures that perform specific jobs. These cell structures are
called organelles and perform the actual work of the cell. These organelles are formed from many
different molecules. Some functions carried out by organelles include the transport of materials, energy
capture and release, protein building, waste disposal, and information storage. Single celled organisms
also have organelles similar to those in more advanced organisms to complete their life
processes. Many enzymes are needed for the chemical reactions involved in cellular life processes to
occur. A summary of some of the main organelles found in cells and their particular functions can be
seen in Table 2.3 below.
Table 2.3 The table below summarizes the main functions of certain organelles.
Cell Organelle
Function
nucleus
control center of the cell; contains DNA which directs the synthesis of proteins by the
cell
mitochondrion
carries on the process of cell respiration converting glucose to ATP, a high-energy
molecule the cell can use
endoplasmic reticulum transport channels within the cell
ribosome
found on the endoplasmic reticulum and free within the cell responsible for the
synthesis of proteins for the cell
cell membrane
selectively regulates the materials moving to and from the cell
food vacuole
stores and digests food
contractile vacuole
found in many single celled aquatic organisms; pumps out wastes and excess water
from the cell
chloroplast
found in plant cells and algae; carries on the process of photosynthesis
cell wall
surrounds and supports plant cells
Life Functions of Organ Systems
Humans and many other organisms require multiple systems for digestion, respiration,
reproduction, circulation, excretion, movement, coordination, and immunity. The systems collectively
perform the life processes and help maintain homeostasis. A summary of human organ systems and
their main functions is presented below:




The digestive system breaks down food molecules into their simple components so that can
enter the cells and aids in the elimination of solid wastes.
The respiratory system aids in the process which converts the energy in food to ATP (the form of
energy which can be used by cells for homeostasis), transfers inhaled oxygen to the blood,
exhales carbon dioxide (homeostatic gas exchanges), regulates the acidity of body fluids
(keeping them in dynamic homeostatic equilibrium), and the air flowing out of the lungs through
vocal cords produces sound.
The reproductive system allows for the making of more organisms of one’s kind, not needed by
an individual living thing but is needed by its species. This system also releases hormones that
regulate reproduction and body processes associated with reproduction, for example, mammary
glands produce milk.
The cardiovascular system aids in the movement of oxygen throughout an organism’s blood,
carries waste like carbon dioxide away from cells (again homeostatic gas exchanges), regulates
acidity, temperature, and water content of body fluids, and helps to defend against disease.
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






The urinary system removes cellular waste products (wastes may include water, salt, urea)
during perspiration, and urine formation, regulates the volume and chemical composition of
blood, maintains an organism’s body mineral balance, and helps to regulate red blood cell
production, all mechanisms of homeostasis.
The muscular system aids in movement and posture, and it produces heat.
The nervous system regulates body activities through nerve impulses that help to maintain
homeostasis, coordinates response to environmental changes by muscular contractions and
glandular secretions.
The endocrine system coordinates the control mechanisms in an organism which help it to
maintain dynamic equilibrium or homeostasis through hormones transported by the blood to
various organ targets.
The skeletal system supports and protects the body, assists with movement, stores cells that
produce blood cells, and stores minerals and lipids.
The lymphatic and immune systems return fluids and proteins to the blood, and protects the
body against disease-causing organisms.
The integumentary system aids in homeostasis by regulating body temperature and detecting
sensations such as pressure, pain, warmth, and cold.
It is important to realize that cell organelles are involved in many of these life processes, as well as
the organ systems of complex organisms. However, all of these life processes could not occur without
cellular communication.
Cellular Communication
Neurotransmitters and hormones allow communication between nerve cells and other body
cells. If nerve or hormone signals are changed, the disruption of communication between the body cells
can adversely affect an organism’s ability to maintain homeostasis.
Additionally, DNA molecule contains the instructions that direct the cell’s behavior through the
synthesis of proteins that aid in cellular communication. Figure 2.26 shows receptors on the cell
membrane which respond to hormones.
Cell Membrane Receptors
Many cell membranes
have receptor
molecules on their
surface. These receptor
sites play an important
role in allowing cells
and organs to
communicate with one
another.
Figure 2.26 Cell membrane receptor sites help to enable communication between cells and organs.
Hormonal Regulation
Hormones provide a primary way for cells to communicate with each other. A hormone is a
chemical messenger with a specific shape that travels through the bloodstream influencing
another target cell or target organ. Upon reaching the cell the hormone is targeted for, the hormone
often activates a gene within a cell to make another necessary compound. One example of this is
53
provided by the pituitary gland. This gland at the base of the brain makes a hormone called LH
(luteinizing hormone). This hormone travels through the bloodstream and stimulates the ovary to
produce yellow tissue that produces the hormone progesterone, which maintains the thickness of the
uterus lining. The graphic below in Figure 2.27 illustrates how this kind of hormonal regulation can work
in a plant cell. Animal cell hormonal regulation involves a similar mechanism.
A Hormonal Feedback Mechanism
The diagram at the
right illustrates how a
hormone can bind to
receptors on a cell
membrane and trigger
that cell to produce a
needed compound.
Figure 2.27 A diagram illustrating hormonal feedback mechanisms on a plant cell.
Nervous Regulation
Nerve cells or neurons also allow cells to communicate with each other. Neuron
communications are one way organism can detect and respond to stimuli at both the cellular and
organism level. This detection and response to stimuli helps to maintain homeostasis in the cell or
organism. Neurons may stimulate other nerve cells or muscle cells, thus causing the later to contract
and produce movement. Figure 2.28 summarizes the function of each structure of the nerve cell.
Structure and Function of a (Neuron) Nerve Cell
Structures and their Functions
1. dendrite -- neuron branch which detects stimuli (changes in the environment).
2. cyton -- cell body of the neuron where normal metabolic activities occur.
3. axon -- longest dendrite covered by a myelin sheath which provides electrical insulation -- carries
nerve message or impulse to the terminal branches.
4. terminal branches -- release nerve chemicals called neurotransmitters which stimulate adjacent
dendrites on the next neuron or a muscle cell.
Figure 2.28 Description of the function of each structure found in a nerve cell.
Any change in nerve or hormone signals will change the communication between cells and
organs in an organism and thus may cause problems for organism’s stability and ability to maintain
homeostasis.
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Lesson Summary
• All cells are very small because they need to pass substances across their surface. Their small size
gives them a relatively large ratio of surface area to volume, facilitating the transfer of substances.
The shapes of cells may vary, and a cell’s shape generally suits its function.
• Cells are diverse, but all cells contain a plasma membrane, cytoplasm, ribosomes, and DNA.
• Prokaryotic cells are cells without a nucleus. They are found in single-celled organisms. Eukaryotic
cells are cells with a nucleus and other organelles. They are found mainly in multicellular organisms.
• The plasma membrane is a phospholipid bilayer that supports and protects a cell and controls what
enters and leaves it.
• The cytoplasm consists of everything inside the plasma membrane, including watery cytosol and
organelles. The cytoplasm suspends the organelles and does other jobs. The cytoskeleton crisscrosses
the cytoplasm and gives the cell an internal framework.
• The nucleus is the largest organelle in a eukaryotic cell and contains most of the cell’s DNA. Other
organelles in eukaryotic cells include the mitochondria, endoplasmic reticulum, ribosomes, Golgi
apparatus, vesicles, vacuoles, and centrioles (in animal cells only). Each type of organelle has
important functions in the cell.
• Plant cells have special structures that are not found in animal cells, including a cell wall, a large
central vacuole, and organelles called plastids.
• Cells can exist independently as single-celled organisms or with other cells as multicellular organisms.
Cells of a multicellular organism can be organized at the level of organelles, cells, tissues, organs,
organ systems, and organisms.
 The biological levels of organization found in multi-cellular organisms work together to maintain
homeostasis in living things.
 Organelles present in unicellular organisms often act in the same manner as the tissues and systems
found in multi-cellular organisms. The organelles in unicellular organisms perform all of the life
processes needed to maintain homeostasis, by using specialized cell organelles.
References/ Multimedia Resources
Opening image copyright by MIT News Office, 2012. Retrieved from
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"Cells - Cell Basics." Cells - Cell Basics. N.p., n.d. Web. 27 July 2013.
<http://regentsprep.org/regents/biology/2011%20Web%20Pages/Cells-%20Cell%20Basics.htm >
"Cell Structure Animation." Cell Structure Animation. N.p., n.d. Web. Summer 2013.
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"Golgi Apparatus." Golgi Apparatus. N.p., n.d. Web. Summer 2013.
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"The Basics of Biology DVD Series." The Basics of Biology DVD Series. 21 Oct. 2009. Web. Summer 2013.
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Tortora, Gerald J., and Sandra R. Grabowski. "Table 1.1 Components and Functions of the Eleven
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