Chapter 4
Cells: Organization and Communication
The cell theory
 A cell is the basic unit of life
 All living things are made up of cells
 New cells arise from preexisting cells
There are two major types of cells in all living organisms
 Prokaryotic cells
o Thought to be the first cells to evolve
o Lack a nucleus
o Represented by bacteria and archaea
 Eukaryotic cells
o Have a nucleus that houses DNA
o Contain many membrane-bound organelles
Basic parts of all eukaryotic cells
 A plasma membrane that surrounds and delineates the cell
 Cytosol that is the semi-fluid portion inside the cell that contains organelles
 A nucleus containing DNA
Plant cells
 Plant cells have specialized organelles not found in animal cells
 Central vacuole plays a role in cell processes and also in maintaining proper
cell rigidity (turgor)
 Chloroplasts are organelles important for photosynthesis
Animal cells
Cell (or plasma) membrane
 The basic structure is a phospholipid bilayer
o It contains cholesterol for support
 There are embedded proteins that can move easily within the horizontal plane
 Many of the proteins and lipids have carbohydrates attached - glycoproteins
and glycolipids
o These are important for cell communication
Movement across the membrane
 The cell membrane is selectively permeable
 Movement across the membrane can occur passively or actively (requiring
energy)
Passive transport
 Simple diffusion
o Net movement is down the concentration gradient
o From an area of higher concentration to an area of lower
concentration
Passive transport
 Osmosis - special case of diffusion of water across the membrane
 Water will tend to move to equalize the solute concentration on either side of
the membrane
 Water requires specialized proteins (aquaporins) to cross the membrane
o It is actually a case of facilitated diffusion
o But if there are aquaporins in place, movement is virtually unhindered
Tonicity is a comparative term
 Isotonic solutions have equal amounts of solute inside and outside the cell and
thus do not affect the cell
 Hypotonic solutions have a lower solute concentration than the inside of the
cell and can lead to lysis (bursting)
 Hypertonic solutions have a higher solute concentration than the inside of the
cell and can lead to crenation (shriveling)
Passive transport
 Facilitated diffusion
o Some of the membrane proteins function as receptors and channels to
allow certain solutes to cross the membrane
 This is the main means by which glucose enters cells
Active transport
 To move something up its concentration gradient requires energy (ATP ➜
ADP)
o Usually it will move molecules or ions from an area of lower
concentration (outside the cell) to an area of higher concentration
(inside the cell)
o This is especially important for nutrients and metabolites
Active transport
 Some transport proteins act as pumps
 These often have reciprocal functions
o The Na+/K+ pump moves Na+ out of and K+ into the cell
Endocytosis and exocytosis
 Some molecules move into the cell via endocytosis (engulfing)
 Some molecules move out of the cell via exocytosis
o Both methods use vesicles
The organelles
 The organelles are enclosed in biological membranes
 The microenvironments inside the organelles can be different from the cytosol
providing specific functions
Cytoskeleton
 Provides the infrastructure of the cell
 Shape but flexibility, support for organelles, roadways for movement of
material within the cell
o Composed of various protein fibers
Structures involved in movement
 Flagella propel some cells via whiplike movements
 Cilia beat in rhythm to move other things past the cells
o Upper respiratory tract, oviduct
Nucleus
 Bound by a porous nuclear envelope that controls what enters and exits the
nucleus (through nuclear pores)
 Houses DNA and associated proteins making up chromatin
Endomembrane system
 A series of membranes (some part of organelles) allowing molecules to be
transported in the cell
 It consists of the nuclear envelope, endoplasmic reticulum (ER), Golgi
apparatus, lysosomes, and vesicles
o Rough endoplasmic reticulum – studded with ribosomes used to make
proteins
o Smooth endoplasmic reticulum – lacks ribosomes but aids in making
carbohydrates and lipids
o Golgi apparatus – flattened stacks that process, package, and route
proteins and lipids from the ER
o Vesicles – small membranous sacs used for transport
o Lysosomes – membranous vesicles made by the Golgi that contain
digestive enzymes involved in intracellular digestion and recycling of
old organelles
Mitochondria
 An organelle with a highly folded inner membrane
 Involved in the production of ATP (energy for the cell) through cellular
respiration requiring oxygen
o They are thought to have originally derived from an engulfed
prokaryotic cell