Cells and Their Functions

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Cells and Their Functions
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Cells

• Think back to biology, what is a cell?
• Cell - the functional unit of life in all living
  things.
• This is the area where all chemical reactions and
  reproduction takes place.
• They are the smallest independent unit of life.

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Cells

• There are three classifications of cells, one of
  which we are really concerned with in zoology.
• The first two lack nuclei and other organelles.
• These two types of cells are usually referred to
  as prokaryotes.
• They are in the domains Archaea and Eubacteria.

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Cells

• The type of cell we are most concerned with is
  the Eukaryotic cell.
• These cells have a membrane bound nucleus that
  contains the DNA.
• They also contain organelles that are responsible
  for specific tasks in the cell.

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Eukaryotic Cells

• Eukaryotic cells have specialized structures
  called microfilaments and microtubules.
• These structures are arranged in the
  cytoskeleton, a structure which give shape to the
  cell and allows for intracellular movement.
• All eukaryotic cells have three basic parts
• Plasma membrane
• Cytoplasm
• Nucleus

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Eukaryotic Cells

• The plasma membrane is the outer boundary of the
  cell.
• Its purpose is to allow for the passage of
  substances in and out of the cell.
• It is semi-permeable and is composed of protein
  molecules dispersed throughout a phospolipid
  bilayer.
• There are also special receptors for larger
  molecules.

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Eukaryotic Cell

• The cytoplasm is the semi-fluid (think jello)
  portion of the cell.
• It encases, protects, and cushions the internal
  organelles.
• It is essentially the space that is not an
  organelle.

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Eukaryotic Cell

• The final structure is the nucleus.
• The nucleus is where everything is controlled.
• The nucleus has its own boundary from the rest of
  the cell.
• It contains the chromosomes which are essential
  to reproduction of the cell.

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Cell Size

• The majority of cells are to small to see without
  the aid of a microscope.
• There are a few reasons for this small size.
• First, the nucleus must be able to maintain
  control of the cytoplasm.
• Second, the surface area of the cell membrane
  cannot keep up with increases in volume.
• Finally, the amount of DNA is huge. To package
  it into the cell requires a significant decrease
  in size.

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Cell Membranes

• The membrane is a double layer of lipids and
  proteins.
• It is also fluid rather than solid.
• The lipid layer contains a phosporus unit which
  gives it the name of a phospholipid.
• This phospholipid layer acts like a sea where the
  proteins float like icebergs.

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Cell Membranes

• The model of the cell membrane is referred to as
  the fluid-mosaic model.
• The fluid refers to the fluid make-up of the
  membrane, while the mosaic refers to the many
  different proteins in the bilayer.
• The fact that the membrane is fluid means that it
  is constantly in a state of flux and therefore
  always changing while retaining its uniform
  structure.

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Cell Membranes

• The fluid-mosaic model has a few important
  principles
• The phospholipids have one polar end and one
  non-polar end.
• Cholesterol is present in the plasma and
  organelle membranes to make them less permeable
  to water soluble substances.
• The proteins may be located on the inner or outer
  surface of the membrane or embedded within it.
• Carbohydrates (glycoproteins) and lipids
  (glycolipids) can unite with proteins on the cell
  surface

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Cell Membranes

• Cell membranes serve six functions
• Regulate material movement.
• Separate the inside from the outside.
• Separate various organelles.
• Provide surface area for certain reactions.
• Separate cells from one another.
• Serve as a site for receptors.

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Cell Membranes

• The cell membrane has a particular quality called
  selective permeability.
• This means that the membrane can allow certain
  substances in and keep some substance out.
• This allows the cell to maintain what is known as
  homeostasis.
• The maintenance of a relatively stable constant
  environment despite changes in the cells external
  surroundings.

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Cell Membranes

• There are a number of ways in which cells move
  materials across the membrane.
• Some may involve the use of the cells energy
  while others are require no energy consumption by
  the cell.

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Simple Diffusion

• Cells are in constant, random motion as long as
  the temperature is above absolute zero.
• This movement occurs from areas where there are a
  lot of cells to areas where there are few cells.
• This will continue until they reach equal
  proportions.
• This process is what we call simple diffusion.
• This accounts for the majority of short distance
  transport of movement in and out of the cell.

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Facilitated Diffusion

• Polar molecules are not soluble in liquids.
• Therefore, they must have a means of moving
  across the cell membrane.
• The molecules may diffuse through protein
  channels in the cell membrane.
• These channels offer direct passage through the
  membrane without coming into contact with the
  hydrophobic layer of the membrane.
• Some materials may need assistance passing across
  the membrane.
• This help in movement is referred to as
  facilitated diffusion.

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Osmosis

• When water moves across the membrane, is carries
  a special name - osmosis.
• Osmosis, like the previous means of movement is
  from high to low.

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Osmosis

• With osmosis, there are certain terms we are
  concerned with.
• Tonicity refers to the amount of solutes in water
  in and out of the cell.
• With tonicity, there are varying levels of
  concentration to describe the tonicity.
• Isotonic the concentration is the same in and
  out.
• Hypertonic the concentration is higher outside
  than in.
• Hypotonic the concentratin is lower outside the
  cell than inside.

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Filtration

• Small molecules can be forced through the
  membrane by hydrostatic pressure.
• Larger molecules (proteins) will remain in place
  when filtration occurs.
• Filtration is common in humans and some animals
  in the kidney.
• This is the first step in the formation of urine
  or other waste products.
• Increased blood pressure forces the water out of
  the nephron in the kidney causing increased urine
  output.

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Active Transport

• When a cell needs to move things against its
  concentration gradient, energy in the form of ATP
  is required.
• This type of movement of substances is referred
  to as active transport.
• Similar to facilitated diffusion except the
  carrier protein must use energy from the cell to
  move the molecules from lower concentrations to
  higher.

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Active Transport

• There are a number of different active transport
  mechanisms cells use.
• One of the most common is the sodium-potassium
  pump.
• The type of active transport helps to maintain
  the balance of sodium and potassium ions in the
  cells environment.

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Endocytosis and Exocytosis

• Endocytosis and Exocytosis involve the bulk
  movement of materials across the membrane.
• There are three types of endocytosis
• Pinocytosis nonspecific uptake of small
  droplets of external fluid.
• Phagocytosis cell takes in solid material
  rather than liquid.
• Receptor mediated endocytosis receptor protein
  recognizes an extracellular molecule and binds
  with it.
• Many times these reactions cause the membrane to
  form a dent and surround the material with a
  vessicle.

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Endocytosis and Exocytosis

• One of the organelles you studied in biology is
  the Golgi apparatus.
• Its functions is to package proteins and other
  molecules .
• These materials are then excreted by the cell
  through exocytosis.

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Endocytosis and Exocytosis
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Cell Components

• We have already discussed the function of the
  cytoplasm.
• However, cytoplasm has two parts
• The cytomembrane system
• The cytosol

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Cell Components

• The cytomembrane system consists of the well
  defined structures we will talk about shortly.
• The cytosol suspends these structures and also
  contains some dissolved materials.

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Cell Components

• The next structure we are going to discuss is the
  ribosomes.
• The site where protein synthesis occurs.
• They are composed mostly of proteins and a
  special kind of RNA (rRNA).
• Some are attached to another structure and some
  float freely in the cytoplasm.

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Cell Components

• Cells need to transport materials throughout the
  cytoplasm.
• They also need a place to store enzymes and
  proteins.
• This structure is the endoplasmic reticulum.

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Cell Components

• The endoplasmic reticulum (ER) is a series of
  tubules and sacs that reach throughout the
  cytoplasm.
• The ER originates at the nuclear envelope and
  travels to the plasma membrane.
• ER also serve as an attachment point for the
  ribosomes - rough ER.
• ER without ribosomes is called smooth ER.
• Smooth ER functions to produce lipids, detoxify
  the cell, and also ion storage for muscles.

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Cell Components

• We have already touched upon the packaging unit
  for the cell, the Golgi apparatus.
• A series of membranes associated with the ER.
• Materials are passed from the ER to Golgi.
• These substances are usually proteins
• The Golgi before packaging and sorting marks the
  proteins for identification.

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Cell Components

• Lysosomes contain enzymes that are used in
  digesting materials for the cell.
• The enzymes are made by the ER.
• They are then transported to the Golgi and then
  made into lysosomes or vessicles that fuse with
  lysosomes.

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Cell Components

• Cells need a means by which to produce energy.
• The mitochondrion are where this energy
  production takes place.
• Interestingly, this structure contains its own
  DNA.

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Cell Components

• Cells need a means by which to connect organelles
  to one another.
• This connecting occurs via the cells
  cytoskeleton.
• The cytoskeleton contains three major components
• Microtubules
• Intermediate filaments
• Microfilaments

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Cell Components

• Cells must move and also move materials to them.
• Cilia and Flagella account for these movements.
• Flagella are 5-20 times longer than cilia and
  move in a different fashion than cilia.

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The Nucleus

• The big daddy of the cell, the one thing it cant
  live without, the control center, this is the
  nucleus.
• The nucleus has two functions
• Direct chemical reactions
• Storage of genetic material

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The Nucleus

• Each nucleus has three main components
• The nuclear envelope which seperates the nucleus
  from the rest of the cell.
• Chromosomes which contain the information in the
  form of DNA needed for heredity.
• The Nucleolus is present in non-dividing cells
  and serves as the pre-assembly point for
  ribosomes.

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Levels of Organization

• Animals have five levels of organization
• Cells
• Tissues
• Organs
• Organ systems
• Animal
• As we move up each level, it becomes more complex.

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Tissue Types

• Four main types of tissues
• Epithelial covers or lines a structure
• Connective support and bind
• Muscle movement of body and appendages
• Nervous communication within the body
• All of these tissues have a distinct appearance
• Differences in color, texture, and location.

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Tissue Types

• Epithelial Tissue
• Specialized for covering the internal and
  external body structures
• Comprised of tightly packed cells
• Classified into two groups
• Simple epithelial
• Stratified epithelial

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Tissue Types

• Connective Tissue
• Two main types
• Loose connective tissue
• Adipose tissue (fat)
• Fascia
• Fibrous connective tissue
• Tendons
• Ligaments
• Can be liquid or solid
• Bone
• Fat
• Blood
• Used in support and compartmentalization

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Tissue Types

• Muscle Tissue
• Used in movement of the body and appendages
• Three types
• Skeletal
• Attached to bones
• Smooth
• Stomach, intestines
• Cardiac
• Heart

Smooth
Skeletal
Cardiac
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Tissue Types

• Nervous Tissue
• Composed of several different types of cells
• Neurons
• Impulse conduction
• Neuroglia
• Protection, support, and nourishment
• Peripheral glial cells
• Help support the periphereal nervous system