Cells: The Living Unit
Cells – general
• 4 aspects of cell theory –
– A cell is the basic structural & functional unit of life
• Cell properties directly relate to the properties of life
– The activity of an organism depends on the activities
of individual and collective cells
– Principle of complementarity –
• Activities of cells are dictated by specific structures of cells
– Continuity (the smooth working) of life is based on a
cellular level
• Cells come from pre-existing cells
Cells – general cont.
• Characteristics of cells –
– Cells vary greatly in their size, shape, and
function
– All cells are composed primarily of carbon,
hydrogen, nitrogen, and oxygen (CHON)
– All cells have the same basic parts & some
common functions
– All general cells contain a plasma membrane,
cytoplasm, and nucleus
Cell diversity
Plasma membrane structure
• Plasma membrane –
– Defines the extent of the cell
– Separates bodies major fluid components
• Intracellular fluid within cells
• Extracellular fluid outside cells
• The plasma membrane is composed of a double
layer of phosopholipids embedded with smaller
amounts of cholesterol and glycolipids and proteins
Plasma membrane structure cont.
• Fluid mosaic model =
– Membrane structure composed of a double layer
(bilayer) of lipid molecules with protein molecules
dispersed in it.
– Proteins which “float” within the bilayer form an ever
changing mosaic pattern
• Composed of…
– Double layer of phospholipids (lying tail to tail with
their polar heads exposed to the water inside and
outside the cell) embedded with cholesterol, proteins,
and glycolipids
Plasma membrane structure cont.
• Surfaces of the plasma membrane –
– Outside surfaces of the cell contain more lipids
• Contains glycolipids (sugar lipid)
• Helps to make the outer surface more polar
– Inside contains integral proteins, phospholipids, & cytoskeleton
• Phospholipids –
– Modified lipid containing phosphorus
– 2 components –
• Hydrophilic –
– Polar “head”
– Has a charge
– Water loving
• Hydrophobic –
– Nonpolar “tail”
– Does not have a charge
– Water hating
P.M.
Plasma membrane structure cont.
• Plasma membrane proteins –
– Integral proteins are firmly inserted into the plasma
membrane
• Have both hydrophobic and hydrophilic regions
• Some protrude from only one side of the plasma membrane
– Usually involved in receptors for hormones or chemical
messengers or act as chemical relay messengers
• Transmembrane proteins span the entire plasma membrane
– Mainly involved in transport
– Channel proteins –
» Have a pore in which water soluble molecules pass
– Carrier proteins –
» Bind to substances to move them through the membrane
P.M
Plasma membrane structure cont.
– Peripheral proteins are not embedded in the plasma
membrane, but attach to integral proteins or to
phospholipids
• Can be removed w/o disturbing the plasma membrane
• Contain filaments that helps support the membrane on the
cytoplasmic side
• Some involved in changing the cells shape (during division)
– The glycocalyx is the fuzzy, sticky, carbohydrate-rich
area surrounding the cell
• “sugar covering”
• Provides a highly specific biological markers
– Recognize self from non-self
P.M.
Specialization of the plasma membrane
• Microvilli –
– “shaggy hairs”
– Fingerlike extensions of the plasma membrane that
increase the surface area of the cell
– Most often found in cells with an absorptive function
– Kidneys and intestines
– Contains actin –
• Contractile protein that helps stiffen the microvilli to keep
them erect
Specialization of the plasma membrane cont.
• Membrane junctions –
– Many cells are close together and in tight
communities
• Glycoproteins act as the cells “adhesive”
• Wavy contours of cells edges allow cells to stick
together in a tongue and grove fashion
• Special membrane junctions are formed
– Tight Junctions
– Desmosomes
– Gap Junctions
Specialization of the plasma membrane cont.
– Tight junctions
• Integral proteins on
adjacent cells fuse
together to form
impermeable junctions
• Prevents molecules
from passing through
the extracellular space
between cells
Specialization of the plasma membrane cont.
• Membrane junctions cont. – Desmosomes –
• Anchoring junctions
• Scattered like rivets to prevent the cells separation
• Held together by linker proteins that extend from the plaque
on the cytoplasmic face
• Linker proteins span from one cell to the other & attach
themselves to the other cells plaque
• Reduces the chances of ripping the cell when tension is
applied
Desmosomes
Specialization of the plasma membrane cont.
• Membrane junctions cont. –
– Gap junctions –
• Communication junction
• Allows chemical substances to pass between
adjacent cells
• Connected by connexons – hollow tubules that
allow the cells to communicate because their
cytoplasm's are so close
• Allows small molecules to pass between two cells
• In electrically charged tissues
Gap junctions