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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 Figure 2.12 Cilia are extensions of the plasma movement. 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.

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.

Inner membrane Outer membrane

Cristae

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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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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.

CELL WALL

PLASTID

CENTRAL VACUOLE

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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 cytoplasm

Present- in plants

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

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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.

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.

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.

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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 http://img.mit.edu/newsoffice/images/article_images/original/20120803125952-0.jpg. 25 Jul. 2013. "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. <http://www.learnerstv.com/animation/animation.php?ani=162&amp> "Cytoskeleton Microtubules | Cell Biology." YouTube. YouTube, 21 Oct. 2009. Web. Summer 2013. <http://www.youtube.com/watch?v=5rqbmLiSkpk&> "Eucaryotic Cell Interactive Animation." Cell Models: An Interactive Animation. N.p., n.d. Web. Summer 2013. <http://www.cellsalive.com/cells/cell_model.htm> "Golgi Apparatus." Golgi Apparatus. N.p., n.d. Web. Summer 2013. <http://www.johnkyrk.com/golgiAlone.html> Postlethwait, John H. "Cell Structure and Function." Modern Biology. Student Ed. [S.l.]: Holt, Rinehart and Winston, 2009. 72-90. Print.

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"The Basics of Biology DVD Series." The Basics of Biology DVD Series. 21 Oct. 2009. Web. Summer 2013. Retrieved through youtube.com. < http://www.youtube.com/watch?v=-aSfoB8Cmic > Tortora, Gerald J., and Sandra R. Grabowski. "Table 1.1 Components and Functions of the Eleven Principle Systems of the Human Body." Introduction to the Human Body: Essentials of Anatomy and Physiology. 6th ed. N.p.: John Wiley & Sons, 2004. 4-5. Print.

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