The Discovery of the Cell

1
The Discovery of the Cell

• What is the cell theory?
• The cell theory states
• - All living things are made up of cells.
• - Cells are the basic units of
  structure and function in living things.
• - New cells are produced from existing
  cells.

2
Early Microscopes

• It was not until the mid-1600s that scientists
  began to use microscopes to observe living
  things.
• In 1665, Englishman Robert Hooke used an early
  compound microscope to look at a nonliving thin
  slice of cork, a plant material.
• Under the microscope, cork seemed to be made of
  thousands of tiny, empty chambers that Hooke
  called cells. The term cell is used in biology
  to this day.
• Today we know that living cells are not empty
  chambers, but contain a huge array of working
  parts, each with its own function.

3
Early Microscopes

• In Holland, Anton van Leeuwenhoek examined pond
  water and other things, including a sample taken
  from a human mouth. He drew the organisms he saw
  in the mouthwhich today we call bacteria.

4
The Cell Theory

• Soon after Leeuwenhoek, observations made by
  other scientists made it clear that cells were
  the basic units of life.
• In 1838, German botanist Matthias Schleiden
  concluded that all plants are made of cells.
• The next year, German biologist Theodor Schwann
  stated that all animals were made of cells.
• In 1855, German physician Rudolf Virchow
  concluded that new cells could be produced only
  from the division of existing cells, confirming a
  suggestion made by German Lorenz Oken 50 years
  earlier.

5
Electron Microscopes

• Light microscopes can be used to see cells and
  cell structures as small as 1 millionth of a
  meter. To study something smaller than that,
  scientists need to use electron microscopes.
• Electron microscopes use beams of electrons, not
  light, that are focused by magnetic fields.
• Electron microscopes offer much higher
  resolution than light microscopes.
• There are two major types of electron
  microscopes transmission and scanning.

6
Electron Microscopes

• Transmission electron microscopes make it
  possible to explore cell structures and large
  protein molecules.
• Because beams of electrons can only pass through
  thin samples, cells and tissues must be cut first
  into ultra thin slices before they can be
  examined under a transmission electron
  microscope.
• Transmission electron microscopes produce flat,
  two-dimensional images.

7
Electron Microscopes

• In scanning electron microscopes, a pencil-like
  beam of electrons is scanned over the surface of
  a specimen.
• Because the image is of the surface, specimens
  viewed under a scanning electron microscope do
  not have to be cut into thin slices to be seen.
• Scanning electron microscopes produce
  three-dimensional images of the specimens
  surface.

8
Electron Microscopes

• Because electrons are easily scattered by
  molecules in the air, samples examined in both
  types of electron microscopes must be placed in a
  vacuum in order to be studied.
• Researchers chemically preserve their samples
  first and then carefully remove all of the water
  before placing them in the microscope.
• This means that electron microscopy can be used
  to examine only nonliving cells and tissues.

9
Prokaryotes and Eukaryotes

• Eukaryotes are cells that enclose their DNA in
  nuclei.
• Prokaryotes are cells that do not enclose DNA in
  nuclei.

10
Prokaryotes

• Prokaryotic cells are generally smaller and
  simpler than eukaryotic cells.
• Despite their simplicity, prokaryotes grow,
  reproduce, and respond to the environment, and
  some can even move by gliding along surfaces or
  swimming through liquids.
• The organisms we call bacteria are prokaryotes.

11
Eukaryotes

• Eukaryotic cells are generally larger and more
  complex than prokaryotic cells.
• Most eukaryotic cells contain dozens of
  structures and internal membranes. Many
  eukaryotes are highly specialized.
• There are many types of eukaryotes plants,
  animals, fungi, and organisms commonly called
  protists.

12
Cell Organization

• What is the role of the cell nucleus?
• The nucleus contains nearly all the cells DNA
  and, with it, the coded
• instructions for making proteins and other
  important molecules.

13
Cell Organization

• The eukaryotic cell can be divided into two
  major parts the nucleus and the cytoplasm.
• The cytoplasm is the fluid portion of the cell
  outside the nucleus.
• Prokaryotic cells have cytoplasm as well, even
  though they do not have a nucleus.

14
Cell Organization

• Many cellular structures act as if they are
  specialized organs. These structures are known as
  organelles, literally little organs.
• Understanding what each organelle does helps us
  to understand the cell as a whole.

15
Comparing the Cell to a Factory

• The eukaryotic cell is much like a living
  version of a modern factory.
• The specialized machines and assembly lines of
  the factory can be compared to the different
  organelles of the cell.
• Cells, like factories, follow instructions and
  produce products.

16
The Nucleus

• In the same way that the main office controls a
  large factory, the nucleus is the control center
  of the cell.
• The nucleus contains nearly all the cells DNA
  and, with it, the coded instructions for making
  proteins and other important molecules.

17
The Nucleus

• The nucleus is surrounded by a nuclear envelope
  composed of two membranes.

18
The Nucleus

• The nuclear envelope is dotted with thousands of
  nuclear pores, which allow material to move into
  and out of the nucleus.

19
The Nucleus

• Like messages, instructions, and blueprints
  moving in and out of a main office, a steady
  stream of proteins, RNA, and other molecules move
  through the nuclear pores to and from the rest of
  the cell.

20
The Nucleus

• Chromosomes contain the genetic information that
  is passed from one generation of cells to the
  next.
• Most of the time, the threadlike chromosomes are
  spread throughout the nucleus in the form of
  chromatina complex of DNA bound to proteins.

21
The Nucleus

• When a cell divides, its chromosomes condense
  and can be seen under a microscope.

22
The Nucleus

• Most nuclei also contain a small, dense region
  known as the nucleolus.
• The nucleolus is where the assembly of ribosomes
  begins.

23
Organelles That Store, Clean Up, and Support

• What are the functions of vacuoles, lysosomes,
  and the cytoskeleton?
• Vacuoles store materials like water, salts,
  proteins, and carbohydrates.
• Lysosomes break down lipids, carbohydrates, and
  proteins into small
• molecules that can be used by the rest of the
  cell. They are also involved
• in breaking down organelles that have outlived
  their usefulness.
• The cytoskeleton helps the cell maintain its
  shape and is also involved in
• movement.

24
Vacuoles and Vesicles

• Many cells contain large, saclike,
  membrane-enclosed structures called
• vacuoles that store materials such as water,
  salts, proteins, and
• carbohydrates.

25
Vacuoles and Vesicles

• In many plant cells, there is a single, large
  central vacuole filled with liquid. The pressure
  of the central vacuole in these cells increases
  their rigidity, making it possible for plants to
  support heavy structures such as leaves and
  flowers.

26
Vacuoles and Vesicles

• Vacuoles are also found in some unicellular
  organisms and in some animals.
• The paramecium contains an organelle called a
  contractile vacuole. By contracting rhythmically,
  this specialized vacuole pumps excess water out
  of the cell.

27
Vacuoles and Vesicles

• Nearly all eukaryotic cells contain smaller
  membrane-enclosed structures called vesicles.
  Vesicles are used to store and move materials
  between cell organelles, as well as to and from
  the cell surface.

28
Lysosomes

• Lysosomes are small organelles filled with
  enzymes that function as the cells cleanup crew.
  Lysosomes perform the vital function of removing
  junk that might otherwise accumulate and
  clutter up the cell.

29
Lysosomes

• One function of lysosomes is the breakdown of
  lipids, carbohydrates, and proteins into small
  molecules that can be used by the rest of the
  cell.

30
Lysosomes

• Lysosomes are also involved in breaking down
  organelles that have outlived their usefulness.
• Biologists once thought that lysosomes were only
  found in animal cells, but it is now clear that
  lysosomes are also found in a few specialized
  types of plant cells as well.

31
The Cytoskeleton

• Eukaryotic cells are given their shape and
  internal organization by a network of protein
  filaments known as the cytoskeleton.
• Certain parts of the cytoskeleton also help to
  transport materials between different parts of
  the cell, much like conveyer belts that carry
  materials from one part of a factory to another.
• Microfilaments and microtubules are two of the
  principal protein filaments that make up the
  cytoskeleton.

32
Microfilaments

• Microfilaments are threadlike structures made up
  of a protein called actin.
• They form extensive networks in some cells and
  produce a tough, flexible framework that supports
  the cell.
• Microfilaments also help cells move.
• Microfilament assembly and disassembly is
  responsible for the cytoplasmic movements that
  allow cells, such as amoebas, to crawl along
  surfaces.

33
Microtubules

• Microtubules are hollow structures made up of
  proteins known as tubulins.
• They play critical roles in maintaining cell
  shape.
• Microtubules are also important in cell
  division, where they form a structure known as
  the mitotic spindle, which helps to separate
  chromosomes.

34
Microtubules

• In animal cells, structures known as centrioles
  are also formed from tubulins.
• Centrioles are located near the nucleus and help
  to organize cell division.
• Centrioles are not found in plant cells.

35
Microtubules

• Microtubules help to build projections from the
  cell surface, which are known as cilia and
  flagella, that enable cells to swim rapidly
  through liquids.
• Microtubules are arranged in a 9 2 pattern.
• Small cross-bridges between the microtubules in
  these organelles use chemical energy to pull on,
  or slide along, the microtubules, allowing cells
  to produce controlled movements.

36
Organelles That Build Proteins

• What organelles help make and transport
  proteins?
• Proteins are assembled on ribosomes.
• Proteins made on the rough endoplasmic reticulum
  include those that will be released, or secreted,
  from the cell as well as many membrane proteins
  and proteins destined for lysosomes and other
  specialized locations within the cell.
• The Golgi apparatus modifies, sorts, and
  packages proteins and other materials from the
  endoplasmic reticulum for storage in the cell or
  release outside the cell.

37
Organelles That Build Proteins

• Cells need to build new molecules all the time,
  especially proteins, which catalyze chemical
  reactions and make up important structures in the
  cell.
• Because proteins carry out so many of the
  essential functions of living things, a big part
  of the cell is devoted to their production and
  distribution.
• Proteins are synthesized on ribosomes, sometimes
  in association with the rough endoplasmic
  reticulum in eukaryotes.

38
Ribosomes

• Ribosomes are small particles of RNA and protein
  found throughout the cytoplasm in all cells.
• Ribosomes produce proteins by following coded
  instructions that come from DNA.
• Each ribosome is like a small machine in a
  factory, turning out proteins on orders that come
  from its DNA boss.

39
Endoplasmic Reticulum

• Eukaryotic cells contain an internal membrane
  system known as the endoplasmic reticulum, or ER.
  
• The endoplasmic reticulum is where lipid
  components of the cell membrane are assembled,
  along with proteins and other materials that are
  exported from the cell.

40
Endoplasmic Reticulum

• The portion of the ER involved in the synthesis
  of proteins is called rough endoplasmic
  reticulum, or rough ER. It is given this name
  because of the ribosomes found on its surface.
• Newly made proteins leave these ribosomes and
  are inserted into the rough ER, where they may be
  chemically modified.

41
Endoplasmic Reticulum

• The other portion of the ER is known as smooth
  endoplasmic reticulum (smooth ER) because
  ribosomes are not found on its surface.
• In many cells, the smooth ER contains
  collections of enzymes that perform specialized
  tasks, including the synthesis of membrane lipids
  and the detoxification of drugs.

42
Golgi Apparatus

• Proteins produced in the rough ER move next into
  the Golgi apparatus, which appears as a stack of
  flattened membranes.
• The proteins are bundled into tiny vesicles that
  bud from the ER and carry them to the Golgi
  apparatus.

43
Golgi Apparatus

• The Golgi apparatus modifies, sorts, and
  packages proteins and other materials from the ER
  for storage in the cell or release outside the
  cell. It is somewhat like a customization shop,
  where the finishing touches are put on proteins
  before they are ready to leave the factory.

44
Golgi Apparatus

• From the Golgi apparatus, proteins are shipped
  to their final destination inside or outside the
  cell.

45
Organelles That Capture and Release Energy

• What are the functions of chloroplasts and
  mitochondria?
• Chloroplasts capture the energy from sunlight and
  convert it into food that
• contains chemical energy in a process called
  photosynthesis.
• Mitochondria convert the chemical energy stored
  in food into compounds
• that are more convenient for the cells to use.

46
Organelles That Capture and Release Energy

• All living things require a source of energy.
  Most cells are powered by food molecules that are
  built using energy from the sun.
• Chloroplasts and mitochondria are both involved
  in energy conversion processes within the cell.

47
Chloroplasts

• Plants and some other organisms contain
  chloroplasts.
• Chloroplasts are the biological equivalents of
  solar power plants. They capture the energy from
  sunlight and convert it into food that contains
  chemical energy in a process called
  photosynthesis.

48
Chloroplasts

• Two membranes surround chloroplasts.
• Inside the organelle are large stacks of other
  membranes, which contain the green pigment
  chlorophyll.

49
Mitochondria

• Nearly all eukaryotic cells, including plants,
  contain mitochondria.
• Mitochondria are the power plants of the cell.
  They convert the chemical energy stored in food
  into compounds that are more convenient for the
  cell to use.

50
Mitochondria

• Two membranesan outer membrane and an inner
  membraneenclose mitochondria. The inner membrane
  is folded up inside the organelle.

51
Mitochondria

• One of the most interesting aspects of
  mitochondria is the way in which they are
  inherited.
• In humans, all or nearly all of our mitochondria
  come from the cytoplasm of the ovum, or egg cell.
  You get your mitochondria from Mom!

52
Mitochondria

• Chloroplasts and mitochondria contain their own
  genetic information in the form of small DNA
  molecules.
• The endosymbiotic theory suggests that
  chloroplasts and mitochondria may have descended
  from independent microorganisms.

53
Cellular Boundaries

• What is the function of the cell membrane?
• The cell membrane regulates what enters and
  leaves the cell and also protects and supports
  the cell.

54
Cellular Boundaries

• A working factory has walls and a roof to
  protect it from the environment outside, and also
  to serve as a barrier that keeps its products
  safe and secure until they are ready to be
  shipped out.

55
Cellular Boundaries

• Similarly, cells are surrounded by a barrier
  known as the cell membrane.
• Many cells, including most prokaryotes, also
  produce a strong supporting layer around the
  membrane known as a cell wall.

56
Cell Walls

• The main function of the cell wall is to provide
  support and protection for the cell.
• Prokaryotes, plants, algae, fungi, and many
  prokaryotes have cell walls. Animal cells do not
  have cell walls.
• Cell walls lie outside the cell membrane and
  most are porous enough to allow water, oxygen,
  carbon dioxide, and certain other substances to
  pass through easily.

57
Cell Membranes

• All cells contain a cell membrane that regulates
  what enters and leaves the cell and also protects
  and supports the cell.

58
Cell Membranes

• The composition of nearly all cell membranes is
  a double-layered sheet called a lipid bilayer,
  which gives cell membranes a flexible structure
  and forms a strong barrier between the cell and
  its surroundings.

59
The Properties of Lipids

• Many lipids have oily fatty acid chains attached
  to chemical groups that interact strongly with
  water.
• The fatty acid portions of such a lipid are
  hydrophobic, or water-hating, while the
  opposite end of the molecule is hydrophilic, or
  water-loving.

60
The Properties of Lipids

• When such lipids are mixed with water, their
  hydrophobic fatty acid tails cluster together
  while their hydrophilic heads are attracted to
  water. A lipid bilayer is the result.

61
The Properties of Lipids

• The head groups of lipids in a bilayer are
  exposed to water, while the fatty acid tails form
  an oily layer inside the membrane from which
  water is excluded.

62
The Fluid Mosaic Model

• Most cell membranes contain protein molecules
  that are embedded in the lipid bilayer.
  Carbohydrate molecules are attached to many of
  these proteins.

63
The Fluid Mosaic Model

• Because the proteins embedded in the lipid
  bilayer can move around and float among the
  lipids, and because so many different kinds of
  molecules make up the cell membrane, scientists
  describe the cell membrane as a fluid mosaic.

64
The Fluid Mosaic Model

• Some of the proteins form channels and pumps
  that help to move material across the cell
  membrane.
• Many of the carbohydrate molecules act like
  chemical identification cards, allowing
  individual cells to identify one another.

65
The Fluid Mosaic Model

• Although many substances can cross biological
  membranes, some are too large or too strongly
  charged to cross the lipid bilayer.
• If a substance is able to cross a membrane, the
  membrane is said to be permeable to it.
• A membrane is impermeable to substances that
  cannot pass across it.
• Most biological membranes are selectively
  permeable, meaning that some substances can pass
  across them and others cannot. Selectively
  permeable membranes are also called semipermeable
  membranes.

66
Passive Transport

• What is passive transport?
• The movement of materials across the cell
  membrane without using
• cellular energy is called passive transport.

67
Passive Transport

• Every living cell exists in a liquid
  environment.
• One of the most important functions of the cell
  membrane is to keep the cells internal
  conditions relatively constant. It does this by
  regulating the movement of molecules from one
  side of the membrane to the other side.

68
Diffusion

• The cytoplasm of a cell is a solution of many
  different substances dissolved in water.
• In any solution, solute particles tend to move
  from an area where they are more concentrated to
  an area where they are less concentrated.
• The process by which particles move from an area
  of high concentration to an area of lower
  concentration is known as diffusion.
• Diffusion is the driving force behind the
  movement of many substances across the cell
  membrane.

69
Diffusion

• Suppose a substance is present in unequal
  concentrations on either side of a cell membrane.

70
Diffusion

• If the substance can cross the cell membrane,
  its particles will tend to move toward the area
  where it is less concentrated until it is evenly
  distributed.

71
Diffusion

• At that point, the concentration of the
  substance on both sides of the
• cell membrane is the same, and equilibrium is
  reached.

72
Diffusion

• Even when equilibrium is reached, particles of a
  solution will continue to move across the
  membrane in both directions.
•  
• Because almost equal numbers of particles
  move in each direction, there is no net change in
  the concentration on either side.

73
Diffusion

• Diffusion depends upon random particle
  movements. Substances diffuse across membranes
  without requiring the cell to use additional
  energy.
• The movement of materials across the cell
  membrane without using cellular energy is called
  passive transport.

74
Facilitated Diffusion

• Cell membranes have proteins that act as
  carriers, or channels, making it easy for certain
  molecules to cross.
• Molecules that cannot directly diffuse across
  the membrane pass through special protein
  channels in a process known as facilitated
  diffusion.
• Hundreds of different proteins have been found
  that allow particular substances to cross cell
  membranes.
• The movement of molecules by facilitated
  diffusion does not require any additional use of
  the cells energy.

75
Osmosis An Example of Facilitated Diffusion

• The inside of a cells lipid bilayer is
  hydrophobicor water-hating. Because of this,
  water molecules have a tough time passing through
  the cell membrane.
• Many cells contain water channel proteins, known
  as aquaporins, that allow water to pass right
  through them. Without aquaporins, water would
  diffuse in and out of cells very slowly.
• The movement of water through cell membranes by
  facilitated diffusion is an extremely important
  biological processthe process of osmosis.

76
Osmosis An Example of Facilitated Diffusion

• Osmosis is the diffusion of water through a
  selectively permeable membrane.
• Osmosis involves the movement of water molecules
  from an area of higher concentration to an area
  of lower concentration.

77
How Osmosis Works

• In the experimental setup below, the barrier is
  permeable to water but not to sugar. This means
  that water molecules can pass through the
  barrier, but the solute, sugar, cannot.

78
How Osmosis Works

• There are more sugar molecules on the right side
  of the barrier than on the left side. Therefore,
  the concentration of water is lower on the right,
  where more of the solution is made of sugar.

79
How Osmosis Works

• There is a net movement of water into the
  compartment containing the concentrated sugar
  solution.
• Water will tend to move across the barrier until
  equilibrium is reached. At that point, the
  concentrations of water and sugar will be the
  same on both sides.

80
How Osmosis Works

• When the concentration is the same on both sides
  of the membrane, the two solutions will be
  isotonic, which means same strength.

81
How Osmosis Works

• The more concentrated sugar solution at the
  start of the experiment was hypertonic, or above
  strength, compared to the dilute sugar solution.
  
• The dilute sugar solution was hypotonic, or
  below strength.

82
Osmotic Pressure

• For organisms to survive, they must have a way
  to balance the intake and loss of water.
• The net movement of water out of or into a cell
  exerts a force known as osmotic pressure.

83
Osmotic Pressure

• Because the cell is filled with salts, sugars,
  proteins, and other molecules, it is almost
  always hypertonic to fresh water.
• As a result, water tends to move quickly into a
  cell surrounded by fresh water, causing it to
  swell. Eventually, the cell may burst.

84
Osmotic Pressure

• In plants, the movement of water into the cell
  causes the central vacuole to swell, pushing cell
  contents out against the cell wall.
• Since most cells in large organisms do not come
  in contact with fresh water, they are not in
  danger of bursting.

85
Osmotic Pressure

• Instead, the cells are bathed in fluids, such as
  blood, that are isotonic and have concentrations
  of dissolved materials roughly equal to those in
  the cells.
• Cells placed in an isotonic solution neither
  gain nor lose water.

86
Osmotic Pressure

• In a hypertonic solution, water rushes out of
  the cell, causing animal cells to shrink and
  plant cell vacuoles to collapse.

87
Osmotic Pressure

• Some cells, such as the eggs laid by fish and
  frogs, must come into contact with fresh water.
  These types of cells tend to lack water channels.
  
• As a result, water moves into them so slowly
  that osmotic pressure does not become a problem.

88
Osmotic Pressure

• Other cells, including those of plants and
  bacteria, that come into contact with fresh water
  are surrounded by tough cell walls that prevent
  the cells from expanding, even under tremendous
  osmotic pressure.

89
Osmotic Pressure

• Notice how the plant cell holds its shape in
  hypotonic solution, while the animal red blood
  cell does not.
• However, the increased osmotic pressure makes
  such cells extremely vulnerable to injuries to
  their cell walls.

90
Active Transport

• What is active transport?
• The movement of materials against a concentration
  difference is known as
• active transport. Active transport requires
  energy.

91
Active Transport

• Cells sometimes must move materials against a
  concentration difference.
• The movement of material against a concentration
  difference is known as active transport. Active
  transport requires energy.

92
Active Transport

• The active transport of small molecules or ions
  across a cell membrane is generally carried out
  by transport proteins, or protein pumps, that
  are found in the membrane itself.

93
Active Transport

• Larger molecules and clumps of material can also
  be actively transported across the cell membrane
  by processes known as endocytosis and exocytosis.
  
• The transport of these larger materials
  sometimes involves changes in the shape of the
  cell membrane.

94
Molecular Transport

• Small molecules and ions are carried across
  membranes by proteins in the membrane that act
  like pumps.
• Many cells use such proteins to move calcium,
  potassium, and sodium ions across cell membranes.
  
• Changes in protein shape seem to play an
  important role in the pumping process.

95
Molecular Transport

• A considerable portion of the energy used by
  cells in their daily activities is devoted to
  providing the energy to keep this form of active
  transport working.
• The use of energy in these systems enables cells
  to concentrate substances in a particular
  location, even when the forces of diffusion might
  tend to move these substances in the opposite
  direction.

96
Bulk Transport

• Larger molecules and even solid clumps of
  material may be transported by movements of the
  cell membrane known as bulk transport.
• Bulk transport can take several forms, depending
  on the size and shape of the material moved into
  or out of the cell.

97
Endocytosis

• Endocytosis is the process of taking material
  into the cell by means of infoldings, or pockets,
  of the cell membrane.
• The pocket that results breaks loose from the
  outer portion of the cell membrane and forms a
  vesicle or vacuole within the cytoplasm.

98
Endocytosis

• Large molecules, clumps of food, and even whole
  cells can be taken up by endocytosis.
• Two examples of endocytosis are phagocytosis and
  pinocytosis.

99
Endocytosis

• In phagocytosis, extensions of cytoplasm
  surround a particle and package it within a food
  vacuole. The cell then engulfs it.
• Amoebas use this method for taking in food.
• Engulfing material in this way requires a
  considerable amount of energy and, therefore, is
  a form of active transport.

100
Endocytosis

• In pinocytosis, cells take up liquid from the
  surrounding environment by forming tiny pockets
  along the cell membrane.
• The pockets fill with liquid and pinch off to
  form vacuoles within the cell.

101
Exocytosis

• Many cells also release large amounts of
  material from the cell, a process known as
  exocytosis.
•  
• During exocytosis, the membrane of the vacuole
  surrounding the material fuses with the cell
  membrane, forcing the contents out of the cell.

102
The Cell as an Organism

• How do individual cells maintain homeostasis?
• To maintain homeostasis, unicellular organisms
  grow, respond to the environment, transform
  energy, and reproduce.

103
The Cell as an Organism

• A single-celled, or unicellular, organism does
  everything you would expect a living thing to do.
  
• Just like other living things, unicellular
  organisms must achieve homeostasis, relatively
  constant internal physical and chemical
  conditions.
• To maintain homeostasis, unicellular organisms
  grow, respond to the environment, transform
  energy, and reproduce.

104
Multicellular Life

• How do the cells of multicellular organisms work
  together to maintain homeostasis?
• The cells of multicellular organisms become
  specialized for particular tasks and communicate
  with one another to maintain homeostasis.

105
Multicellular Life

• The cells of multicellular organisms are
  interdependent, and like the members of a
  successful baseball team, they work together.
• In baseball, players take on a particular role,
  such as pitcher, catcher, infielder, or
  outfielder. Messages and signals are sent and
  understood by teammates and coaches to play the
  game effectively.
• Cells in a multicellular organism work the same
  way. The cells of multicellular organisms become
  specialized for particular tasks and communicate
  with one another in order to maintain homeostasis.

106
Cell Specialization

• The cells of multicellular organisms are
  specialized, with different cell types playing
  different roles.
• Some cells are specialized to move, others to
  react to the environment, and still others to
  produce substances that the organism needs.
• No matter what the role, each specialized cell
  contributes to the overall homeostasis of the
  organism.

107
Levels of Organization

• The specialized cells of multicellular organisms
  are organized into tissues, then into organs, and
  finally into organ systems.

108
Life is Cellular
109
Limits to Cell Size

• What are some of the difficulties a cell faces
  as it increases in size?
• The larger a cell becomes, the more demands the
  cell places on its DNA. In addition, a larger
  cell is less efficient in moving nutrients and
  waste materials across its cell membrane.

110
Information Overload

• Living cells store critical information in DNA.
• As a cell grows, that information is used to
  build the molecules needed for cell growth.
• As size increases, the demands on that
  information grow as well. If a cell were to grow
  without limit, an information crisis would
  occur.

111
Information Overload

• Compare a cell to a growing town. The town
  library has a limited number of books. As the
  town grows, these limited number of books are in
  greater demand, which limits access.
• A growing cell makes greater demands on its
  genetic library. If the cell gets too big, the
  DNA would not be able to serve the needs of the
  growing cell.

112
Exchanging Materials

• Food, oxygen, and water enter a cell through the
  cell membrane. Waste products leave in the same
  way.
• The rate at which this exchange takes place
  depends on the surface area of a cell.
• The rate at which food and oxygen are used up
  and waste products are produced depends on the
  cells volume.
• The ratio of surface area to volume is key to
  understanding why cells must divide as they grow.

113
Ratio of Surface Area to Volume

• Imagine a cell shaped like a cube. As the length
  of the sides of a cube increases, its volume
  increases faster than its surface area,
  decreasing the ratio of surface area to volume.
• If a cell gets too large, the surface area of
  the cell is not large enough to get enough oxygen
  and nutrients in and waste out.

114
Traffic Problems

• To use the town analogy again, as the town
  grows, more and more traffic clogs the main
  street. It becomes difficult to get information
  across town and goods in and out.
• Similarly, a cell that continues to grow would
  experience traffic problems. If the cell got
  too large, it would be more difficult to get
  oxygen and nutrients in and waste out.

115
Division of the Cell

• Before a cell grows too large, it divides into
  two new daughter cells in a process called cell
  division.
• Before cell division, the cell copies all of its
  DNA.
• It then divides into two daughter cells. Each
  daughter cell receives a complete set of DNA.
• Cell division reduces cell volume. It also
  results in an increased ratio of surface area to
  volume, for each daughter cell.

116
Cell Division and Reproduction

• How do asexual and sexual reproduction compare?
• The production of genetically identical
  offspring from a single parent is known as
  asexual reproduction.
• Offspring produced by sexual reproduction
  inherit some of their genetic information from
  each parent.

117
Asexual Reproduction

• In multicellular organisms, cell division leads
  to growth. It also enables an organism to repair
  and maintain its body.
• In single-celled organisms, cell division is a
  form of reproduction.

118
Asexual Reproduction

• Asexual reproduction is reproduction that
  involves a single parent producing an offspring.
  The offspring produced are, in most cases,
  genetically identical to the single cell that
  produced them.
• Asexual reproduction is a simple, efficient, and
  effective way for an organism to produce a large
  number of offspring.
• Both prokaryotic and eukaryotic single-celled
  organisms and many multicellular organisms can
  reproduce asexually.

119
Examples of Asexual Reproduction

• Bacteria reproduce by binary fission.
• Kalanchoe plants form plantlets.
• Hydras reproduce by budding.

120
Sexual Reproduction

• In sexual reproduction, offspring are produced
  by the fusion of two sex cells one from each of
  two parents. These fuse into a single cell
  before the offspring can grow.
• The offspring produced inherit some genetic
  information from both parents.
• Most animals and plants, and many single-celled
  organisms, reproduce sexually.

121
Comparing Sexual and Asexual Reproduction