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Formed by Cells

This is the paper version of a Science Bits multimedia learning unit.

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Photo & Image Credits Brandon Laufenberg, ©iStock.com (butterfly, cover and page 6-7); Lcrumling, freeimages.com (eye, page 8); Marcelo Terraza, freeimages.com (microscope, page 10); Ian Sutton and Umberto Salvagnin, flickr.com (image a and b, page 11); Kriss Szkurlatowski, freeimages.com (image c, page 11); Beholding Eye, ©iStock.com (plant tissue, page 12); Jpogi, commons.wikimedia.org (animal tissue, page 12); Djmapleferryman, flickr.com (one-celled alga, page 12); Nehring, ©iStock.com (paramecium, page 13); Alan Phillips, ©iStock.com (cells, page 13); N. commons.wikimedia.org (bacteria, page 13); Kaibara87, flickr.com (eukaryotic cells, page 15); The-Tor, ©iStock.com (microscope, page 18); Kriss Szkurlatowski, freeimages.com (figure 1, page 18); Centers for Disease Control and Prevention Public Health Image Library (PHIL), commons.wikimedia.org (Bacillus cereus figure 2, page 18); Umberto Salvagnin, flickr.com (plant cells figure 4, page 18); Nat Tarbox, flickr.com (image 3, page 19); Karl Dolenc, ©iStock.com (image 4, page 19); Kriss Szkurlatowski, sxc.hu (image 5, page 19); David Ahn, ©iStock.com (microscope, page 19); Michael Faes, sxc.hu (mobile, page 20); Kamila Turton, sxc.hu (beetle, page 20); Christopher Futcher, ©iStock.com (persons, page 20); Ove Topfer, sxc.hu (stones, page 20); Yucel Tellici, sxc.hu (corn, page 20); Zsuzsanna Kilian, freeimages.com (books, page 20); Patrick Hajzler, freeimages.com (clouds, page 20); kslyesmith’s, freeimages.com (mushrooms, page 20); Billy Alexander, sxc.hu (leaves, page 20); Sarah Williams, sxc.hu (cat, page 20); Uros Kotnik, sxc.hu (milk, page 20); Pascal THAUVI, freeimages.com (potato, page 20); Alex Brown, flickr.com (dog, page 20); Omer Unlu, flickr.com (locust, page 20); Richard Fisher, flickr.com (whale, page 20); Andrew Malone, flickr.com (Lemon tree, page 20); Giuseppe Vago, flickr.com (paramecium, page 20); Moise Nicu, flickr.com (mouse, page 20); John Tann, flickr.com (mosquito, page 20); Lightfoot, morguefile.com (gorilla, page 20); Ian Sutton, flickr.com (protozoon, page 21); Zituba, commons.wikimedia.org (onion cells, page 21); Giuseppe Vago, flickr.com (one-celled alga, page 21); Leboski, flickr.com (bacteria, page 21); ©iStock.com (elephant, dogs and girl, page 22); Paul Caputo, sxc.hu (koala, page 23); Giuseppe Vago, flickr.com (Cothurnia sp., page 24); Djmappleferryman, flickr.com (Dunaliella sp., page 24).

Unit

Formed by Cells

Life Sciences

www.science-bits.com4

Unit Structure

Cover pageThe unit title is highlighted and the learn-ing objectives presented. An index of the contents according to the 5-E Model is also provided. The digital version also includes an activity to review the skills and knowledge required to undertake the unit.

EngageA video and a related activity are available with the digital version. Both versions have a summary on page one. Different activities are proposed for the video; some of these may be interactive in the digital version, but they all contain open-ended questions.

ExploreThis exploratory and discovery activity is devised to mobilize prior knowledge and check it against the results obtained from the experience. The digital version con-tains the necessary multimedia resources required for this activity: simulators, videos, interactivities, etc.

www.science-bits.com 5

Unit Structure

ExplainThe unit’s key concepts are formally and systematically addressed. The digital version includes a wide range of multimedia resourc-es to facilitate understanding of the ideas presented. The printable version includes all basic graphical elements and references to the digital resources available. The digital version may also include reinforcement or enrichment activities.

ElaborateIn this section, the newly acquired knowl-edge can be applied in a practical way. Thus, a problem-solving activity is proposed, one which requires the application of the con-cepts, attitudes, and procedures learned throughout the unit. The digital version may provide multimedia tools (such as simula-tors or videos) to help develop and solve the activity.

ExercisesEach of the concepts addressed in the “Explain” section is related to interactive or open-ended exercises with the aim of consolidating the topics covered through practice. The printable version includes all the exercises except for reinforcement or en-richment activities, and they are organized in accordance with the contents from the “Explain” section.


In this unit, you will learn:

That all living organisms are made up of cells of microscopic dimensions.

How to observe cells under a light mi-croscope.

That some organisms are formed by many cells whereas others are single-celled.

That there are two broad types of cells: eukaryotic and prokaryotic.

That many-celled organisms are formed by different types of eukaryotic cells or-ganized into tissues.

Index· Engage

· The Size of Life ....................................................................08· Explore

· Under the Microscope .......................................................10· Explain

· The Cell: Structural Unit of Life ...........................................12· How Many Cells? ..................................................................12· Cell Sizes ................................................................................13· A Matter of Quantity ...........................................................13· Cell Structure ........................................................................14· Cell Types .............................................................................. 14

· Procaryotes .....................................................................14· Eukaryotes ......................................................................15

· Animal and Plant Cells ........................................................ 15· Cell Diversity ...................................................................16· Organization Into Tissues .............................................16

· One-Celled Eukaryotic Organisms ...................................17· Elaborate

· Classifying Cells ....................................................................18· Exercises ................................................................................... 20

Formed by Cells


Formed by Cells


The Size of Life

What would happen if all living organisms were the same size?

Engage

Video

As you know, life on our plan-et is very diverse. Some of the living organisms we can find on Earth are true giants. Oth-ers are simply big. And then, some living organisms are smaller in size. But, what is the smallest liv-ing organism you have ever seen?

Thanks to the microscope, we’ve discovered that within the tiny boundaries a droplet of water there’s enough room for hundreds of thousands of living organisms. Here are some of the inhab-itants in a droplet of water: these microscopic organ-isms, called protozoa, move around, seek food, and re-produce like any other living organism. As you can see, a simple droplet of water can be a world itself within our world.

Make a list of 5 living organisms that are much bigger than a human being. Remember not to focus only on animal organisms!

Now list 5 living organisms of a similar size to human beings.

Finally, think about the smallest living organisms you know.

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a. The video showed two devices used to magnify images. Which ones?Write down their names and determine the situations in which you would use one or the other.

b. A droplet of water may contain hundreds of thousands of living orga-nisms. However, this doesn’t mean that just any droplet of water will be teeming with them.Would you expect to find many microorganisms in tap or bottled water? Why?

c. In what types of water would you look for these microscopic organisms? What is the reason for this?

d. The video showed a type of organism called protozoa. They are not, by any means, miniature animals or plants. They lack organs or differentiated parts—head, limbs, leaves, roots, etc.

The microscopic world is formed by simple organisms; there is no such thing as a microscopic whale or a microscopic redwood tree. Why do you think there are no miniature elephants or redwood trees?

Questions

Explore

Interactive animation displaying the different parts of a light microscope.


Under the Microscope

What Are Microscopes Used For?

The light microscope allows us to observe the world far beyond the limitations of the unaided eye.

With the naked eye, we cannot differentiate two dots separated by less than 0.2 mm. With the light microscope, we can distinguish two dots sepa-rated by a distance up to a thousand times smaller.

To be able to observe a specimen under the microscope, light must pass through it. The samples we examine under the microscope must be very thin. The slides are often stained to increase contrast and better visualize the specimen.

The light microscope is a system of magnifying lenses adjusted to a se-ries of mechanical devices. These devices make it possible for light to pass through the specimen, reach our eyes, and produce a precise, mag-nified image of it.

Interactivity

1) The eyepieces are the lenses placed in front of our eyes. They magnify the images that reach us through the objective lens by between 5 and 15 times.

2) The objective is a lens. It produces an initial enlarged image of the specimen and projects it onto the eyepiece. Most microscopes have a few objectives which magnify the image between 4 and 40 times, and are held by a revolving piece called revolver.

3) The stage is the platform which supports the specimen to be examined. There is a hole through the center to let the light pass through and illuminate the sample.

4) The condenser is an optical system that focuses the light from the light source onto the sample. It includes a diaphragm which is used to regulate sample illumination.

5) The focus wheels move the stage up and down to obtain a focused image of the sample. The coarse focus is for coarse adjustments, whereas the fine focus adjusts using more precise movements.

6) Most microscopes use a lightbulb as the light source. Light travels from the lightbulb to our eyes, passing through the sample and the lenses.

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Parts of a Light Microscope

Video

Simulator

Protozoa magnified 400 times. Onion bulb magnified 1,000 times.

Human ovary magnified 1,000 times. Paper magnified 400 times.

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Microscope simulator to observe the samples in this activity and learn about the function of each of their parts.

Step-by-step explanation of how to prepare a vegetable sample to be viewed under the light microscope.


What Tells Living Organisms Apart

When observing microscopic organisms or samples of animals and plants, we keep noticing very small elements. These elements are called cells.

Some living organisms are extremely tiny and are made up of a single cell. Larger organisms are formed by groupings of many cells.

Nonliving matter is not formed by cells.

a. Observe a high-power magnifi-cation of the protozoan sample (1). Pay close attention to the shape of these living organisms and what their inside is like.

Make a sketch of one of these or-ganisms.

b. Observe the inner layer of an onion bulb under high power (2). One of the many “pieces” that make up the sample is highlighted. Pay attention to the shape and inside part of each one of these elements.

Draw and label the main character-istics you observed.

c. Now observe the sample of the mammal ovary (3). Notice the highlighted structures.

Make a drawing showing the charac-teristics of these elements.

d. What characteristics do this sam-ple and the onion bulb sample have in common? Do these characteristics resemble those of protozoa?

e. Observe the paper specimen (4). Describe what you see.

f. Can you identify structures that resemble what you saw in the sam-ples taken from living organisms?

Explain

Image Gallery

Gallery of light microscopy images showing animal cells, plant cells, fungi, and microorganisms. The im-age above depicts cells in the diges-tive tract of a mammal.

Image Gallery


The Cell: Structural Unit of Life

Each and every living organism, be it an animal or a plant, a large-sized organism or a microscopic being, is formed by tiny parts called cells.

All living organisms are formed by cells.

The chemical reactions that make life possible take place in a controlled way inside the cell.

The cell is the basic unit of life.

How Many Cells?

How many cells are required to form a living organism?

Many microscopic living organisms are formed by a single cell. We refer to them as one-celled or unicellular organisms.

Gallery depicting different organisms. In this image: one-celled algae.

Side Note

The average adult human body is made up of approximately 100 tril-lion cells.

Protozoa, bacteria, or some algae are ex-amples of one-celled organisms.

Other organisms are larger in size and more complex: they are formed by many cells, so we call them many-celled organisms.

Plants, fungi, and animals may contain hundreds, thousands, millions, and even billions of cells.

Interactivity

Elephants are bigger than mice because they have more cells, but not because their cells are bigger.

Animation

This animation will allow you to compare the size of different cells.

Notice that all microscopy images include a scale. What is the ap-proximate size of the cells in these images?

A protozoon of the species Paramecium caudatum. Cells of a moss leaf. Bacteria of two different species.


Cell Sizes

Living organisms are formed by different-sized cells.

Cells are very small, too small to be measured in millimeters.

We use an appropriate unit of length to refer to cell dimensions: the micrometer or micron.

A micrometer is defined as the thousandth part of the millimeter and is expressed in µm.

1 µm = 0.001 mm

1,000 µm = 1 mm

Most cells range between 1 and 100 µm in length.

A Matter of Quantity

The size of a many-celled organism bears no relation to the size of its cells.

The cells of an ant, a mouse, and an elephant are similar in size.

The size and weight of a many-celled organism depend largely on the number of cells it has.

A mouse has fewer cells than an elephant, but more than an ant.

When an organism is in the process of growing, the number of cells that form it increases, but not their size.

A baby elephant has far fewer cells than its parents.

The name prokaryote is derived from Greek. The prefix pro means “befo-re” and karyon means “nucleus.” On the other hand, eukaryote means “true nucleus.”


Images

See the characteristics of prokary-otic cells.

Cell Structure

Cells have a wide range of shapes and sizes. However, all cells share these three main features:

Side Note

Interactive animation about the common characteristics of cells.

Interactivity

1) All cells are surrounded by a plasma membrane. This membrane is not rigid and works as a filter to regulate the passing of substances between the inside of the cell and the outside environment.

2) The cytoplasm is the internal environment of the cell and is mostly made up of water. Inside it there are dissolved substances, and in some cells there are some membrane-bound structures called organelles.

3) The genetic material regulates cell functions. In some cells, this material is contained inside a membrane-bound compartment called nucleus, whereas in some others it floats freely in the cytoplasm.

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Cell Basic Structure

Cells are classified into two large groups according to whether they have genetic material inside a membrane-bound compartment. This compartment is called nucleus.

Prokaryotic cells do not have a nucleus. The genetic material in this cells floats freely in the cytoplasm.

Eukaryotic cells do have a nucleus. In addition, there are mem-brane-bound structures called organelles in the cytoplasm. Or-ganelles carry out different functions.

Cell Types

Prokaryotes

Prokaryotic cells do not form complex many-celled organisms. Bacteria is the largest and most diverse group of organisms with prokaryotic cells.

The exclusive characteristics of this type of cells are:

Lack of a nucleus and organelles: the genetic material of prokaryotes is immersed in the cytoplasm, which does not contain organelles either.

Bacterial wall: a wall surrounding the cell membrane confers rigidity and resistance to the cells.

Also, they normally have the following characteristics:

Small dimensions: most prokaryotic cells are around 1 µm.

Characteristic shapes: these cells present simple shapes: spherical, rod-shaped, corkscrew-shaped, or bean-shaped.

Appendages: some cells can have filaments they use to move around.

nucleus

cytoplasm

plasma membrane


Image Gallery

Questions

1. Why do plant cells tend to be more regularly shaped than animal cells?

2. Do all plant cells contain chloropolasts?

3. Why are the cells in the green parts of plants this color?

Image gallery depicting different types of eukaryotic cells.

Eukaryotes

The exclusive characteristics of eukaryotic cells are:

Presence of nucleus: the genetic material is contained inside a membrane-bound compartment: the nucleus.

Presence of organelles: they have other compartments, called organelles, inside the cytoplasm.

Eucaryotic cells range between 10 and 150 µm in length.

All many-celled living organisms are formed by the organized bonding of many eukaryotic cells.

Animals, plants, and fungi are examples of many-celled eukaryotes.

Some one-celled organisms are made up of a single eukaryotic cell.

Protozoa, one-celled algae, and yeasts are single-celled eukaryotes.

Both animals and plants are made up of eukaryotic cells.

Although they share most of their traits, plant cells have some specific characteristics:

A rigid plant cell wall surrounds the outside of the membrane. That’s why their shape is usually polygonal.

The cells of leaves and other green parts in plants contain chloro-plasts. Photosynthesis takes place inside these organelles. Their color is due to the high concentration of chlorophyll.

They have vacuoles, large organelles that store water and dis-solved substances.

Animal and Plant Cells

Animal Cell

cell wallnucleus

plasma membrane

vacuole

chloroplasts

cytoplasm

Plant Cell

Outlines

Cell Diversity

Animals are made up of lots of animal cells. Likewise, plants are made up of millions of plant cells.

However, not all the cells in an animal or in a plant are the same, nor are they all exactly like the cell models shown previously.

Many-celled organisms are formed by different types of cells.

Both the shape and structure of each type of cell are related to their role in the organism.

Photo and image gallery of different types of cells in the human body.

Image Gallery

Organization Into Tissues

Cells of the same type are organized into tissues and together carry out a specific function in the organism.

Then different tissues are also organized into functional structures that are known as organs.

Leaves are a plant organ made up of different tissues.

Image

Human male sex cell (sperm). Human hearing cell.Human nerve cell (neuron).


Examples of living organisms which consist of only one eukaryotic cell can be found in nature.

Protozoa, single-celled algae, and yeasts are one-celled eukaryotic organisms.

Protozoa cells and single-celled algae cells may have the following ex-ternal structures:

Cell walls which give them rigidness.

Mobile appendages which enable them to move in liquid media.

Like plant cells, single-celled algae have chloroplasts in their cytoplasm. Protozoa and yeast, on the other hand, do not have these organelles.

One-Celled Eukaryotic Organisms

The skin is an animal organ made up of different tissues organized into layers.

Image

membrane nucleus

cell wall

chloroplastscytoplasmmobile appendage

Yeasts are one-celled fungi. Like all fungi, they are not equipped with chloroplasts.

Protozoa are one-celled eukaryotic organisms that have no chloro-plasts.

In contrast, one-celled al-gae contain chloroplasts.

Side Note



Elaborate

Classifying Cells

All living organisms are formed by cells. However, there are differences between the cells of different types of organisms.

In fact, if we know the par-ticular cell characteristics of each type of organism, we can learn about the type of organism these cells belong to.

We will determine the type of organism cells belong to through the analysis of microscopic images. To this end, a dichotomous key will be used.

A dichotomous key is is a method for determining the identity of something. It is like a a road map with many forks in it; each branch or choice is a question with a yes or no answer. Answer-ing “yes” makes you follow one path, whereas “no” means choosing the other option. After a series of choices, the user is led to the correct name of the item.

· Observe this dichotomous key and indicate the figure on the left hand side of the page it corresponds to:

Figure 1

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Can you observe cells?

Do these cells have nuclei?

3 Are these cells grouped into tissues?

4 Do these cells have a wall shaping them like a polygon?

Figure 2

Figure 3

Figure 4

Start

Living Organism

Nonliving Organism /

Eukaryote

Prokaryote /

Many-Celled Eukaryote

One-Celled Eukaryote /

Plant /

Animal /


Observe these microscopic images carefully and classify them using a dichotomous key.

Task

a. Image 1 b. Image 2

c. Image 3 d. Image 4

e. Image 5

EXERCISES


1. Where to Find CellsIndicate which of these objects are formed by cells and which are not.

The Cell: Structural Unit of Life

2. One, a Few, Many...Place these living organisms in descending order according to the number of cells they contain.

How Many Cells?

a. q Yes q No b. q Yes q No c. q Yes q No d. q Yes q No

e. q Yes q No f. q Yes q No g. q Yes q No h. q Yes q No

i. q Yes q No j. q Yes q No k. q Yes q No l. q Yes q No

a. Dog b. Locust c. Whale d. Lemon tree

e. Paramecium f. Mouse g. Mosquito h. Gorilla

EXERCISES


3. Microscopic SizesAs you know, the dimensions of what we see under the microscope are expressed in micrometers.

a. How many micrometers are there in a millimeter?

b. Express the length of the microorganisms below in micrometers.

Paramecium caudatum: 0.101 mm = ........... µm Giardia lamblia: 0.019 mm = ........... µm Phacus acuminatum: 0.055 mm = ........... µm Escherichia coli: 0.002 mm = ........... µm

4. Cell Lengthsa. Observe this image obtained under the microscope and, using the reference scale, in-dicate the maximum length of the highlighted protozoon.

Cell Sizes

5. Aligned ProtozoaA sample of water from a puddle is examined un-der the microscope, and a protozoon is detected. Its length is 125 µm.

a. If 1,320 protozoa were laid out in a straight line, what length would be obtained?

b. How many protozoa would have to be laid out in a straight line to cover a length of 250 m?

q 60 µm q 490 µm q 200 µm q 110 µm q 10 µm

b. Observe this image obtained under the micro-scope and, using the reference scale provided, indi-cate the maximum length of the highlighted cell.

q 50 µm q 0.24 mm q 400 µm q 10 µm q 0.12 mm

c. Observe this one-celled alga. Indicate, using the scale provided, the maximum length of this cell.

q 100 µm q 200 µm q 20 µm q 50 µm q 250 µm

d. Finally, observe these bacteria. Indicate, using the scale, the length of these cells.

q 10 µm q 1 µm q 0.1 µm q 5 µm q 5 mm

EXERCISES


6. Approximately How Many?An estimate reveals that there are approximately 10 trillion cells in 5 kg of animal mass.

Assuming this estimate is correct, complete the sentences below with the correct number.

A human being weighing 60 kg has approxi-mately ............................ trillion cells.

An elephant weighing 7,500 kg has approxi-mately ............................ trillion cells.

A dog weighing 10 kg has approximately ............................ trillion cells.

A Matter of Quantity

7. Cell PartsComplete this text about the main characteristics of cells.

The cell is the structural unit of all living organisms.

There is a boundary surrounding the cell called plasma ...................., a kind of filter that controls the passing of substances between the interior and exterior of the cell.

The internal environment of cells is called .................... and consists of an aqueous solution that, in some cells, has .................... inside.

The activity of the cell is ultimately controlled by the .................... material. In certain cells, this material is encapsulated in a .................... .

8. Structural Components Which of the three typical elements of a cell do these statements refer to?

It functions as a filter which controls the sub-stances that enter and leave the cell.

It contains mainly water and dissolved sub-stances.

It is, in some cells, encapsulated in the nucleus.

It controls the processes performed in the cell.

Cell Structure

9. Truths and Lies about CellsRead the statements below carefully and deter-mine whether they are true or false.

Cells are very small organisms, normally mea-suring less than 1 mm.

All living organisms are formed by thousands or millions of cells.

Some living organisms are not made up of cells.

All cells are tiny things separated from their en-vironment by a membrane. These things contain an aqueous solution and genetic material.

EXERCISES


10. Talking about ProkaryotesSelect a suitable option to complete the state-ments below.

I. As a rule, prokaryotic cells are ........................ eukaryotic cells.

q smaller thanq equal in size toq larger than

II. Prokaryotic cells do not form ........................ organisms.

q one-celledq many-celledq one-celled nor many-celled

III. Prokaryotic cells have a bacterial wall ........................ the plasma membrane.

q aroundq insideq replacing

IV. Contrary to eukaryotic cells, prokaryotic cells have no ........................ .

q membraneq cytoplasmq nucleus

V. .................... are prokaryotic cells.q Protozoaq One-celled algaeq Bacteria

Cell Types

Animal and Plant Cells

11. Talking about EukaryotesSelect a suitable option to complete the state-ments below.

I. Eukaryotic cells form ........................ organisms.q many-celled and one-celledq many-celledq one-celled

II. In contrast with prokaryotes, eukaryotic cells have a ........................ .

q nucleusq bacterial wallq cell membrane

III. The cytoplasm of eukaryotic cells contains ........................ .

q substances dissolved in water and organellesq substances dissolved in waterq organelles

IV. Eukaryotic cells are ........................ prokaryotic cells.

q smaller thanq equal in size toq larger than

V. Animals and plants are ........................ organisms.q eukaryotic and many-celledq eukaryotic and one-celledq prokaryotic

12. A Plant or an Animal?Indicate the elements which are: exclusive to plant cells; exclusive to animal cells; shared by animal cells and plant cells.

a. Nucleus

b. Cell wall

c. Organelles

d. Cytoplasm

e. Genetic material

f. Vacuole

g. Chloroplasts

h. Cell membrane

EXERCISES


13. Layers of the SkinThis diagram represents the outer layer of human skin, called the epidermis.The epidermis is formed by different types of cells arranged in layers.

a. Observe the image carefully, deter-mine the different types of cells you can see, and describe them.

b. What is special about the cells on the outermost layer?

14. One-Celled Algae and Protozoa

a. Read the statements below about the one-celled organism in the picture and determine wether they are true or false.

The cell has some mobile appendages.

There are chloroplasts in its cytoplasm.

This cell is a protozoan.

The cell has an external wall.

This cell is a single-celled alga.

b. Read the statements below about the one-celled organisms in the picture and determine wether they are true or false.

One-Celled Eukaryotic Organisms

c. Put the different layers that conform the human epidermis in order, starting with the outermost layer: stratum corneum, stratum luci-dum, stratum granulosum, stratum spinosum and stratum basale.

(Look up information to solve this exercise.)

There are chloroplasts inside these cells.

These cells have fine, long appendages.

They are single-celled algae.

They are protozoa.

These cells contain chlorophyll.

15. The Size of WhalesThere are no points of reference in the immense vastness of the sea. We can compare whales and see that some are bigger than others, but the-re is no way for us to ascertain—only with this picture—whether they are bigger than dolphins or hakes… However, we can affirm that they are much larger than protozoa. We do not need to use a reference point.Why is this statement unquestionable?

Organization Into Tissues

ANNOTATIONS


Related units:


Formed by Cells

Nutrition

Responses to the EnvironmentReproduction

Life Processes

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