Biology 304101 Dept. Of Biological Sciences University of Jordan Prof. Dr. Samih Tamimi
Chapter 6: A Tour of the Cell
CONCEPT 1: The Basic Unit of Life
CELLS ARE THE BASIC UNIT OF STRUCTURE AND FUNCTION FOR LIVING THINGS. ALL LIVING THINGS ARE
MADE OF CELLS. CELLS ARISE FROM PRE-EXISTING CELLS. Collectively, these are known as the Cell Theory!
 Prokaryotes:
 NO NUCLEUS, but do have nucleoid region with DNA present
 Small and Simple – few organelles
 Have cell membranes and cytoplasm
 Ex. Bacteria
 Eukaryotes:
 Contain nuclei
 Contains organelles that perform specialized functions
 Unicellular or multicellular
 Ex. Plant and animal cells
CONCEPT 2: The Cell Membrane
The cell membrane is a selectively permeable membrane that controls what enters and leaves the cell. It consists of a phospholipid
bilayer with proteins dispersed throughout the layers. A phospholipid is amphipathic (has both a hydrophobic (tails) and hydrophilic
(heads) region).
The fluid mosaic model describes the
membrane as a fluid structure with various
proteins embedded in or attached to a double
layer of phospholipids. Membranes are NOT
sheets of molecules locked rigidly in place –
most lipids and some proteins can drift about
laterally (in the plane of the membrane).
Integral proteins have nonpolar regions that
completely span the hydrophobic interior of the
membrane. Peripheral proteins are loosely
bound to the surface of the membrane.
Cholesterol molecules are embedded in the
interior of the bilayer to stabilize the membrane.
Phospholipids move along the plane of the
membrane rapidly – while some proteins are
kept in place by the cytoskeleton. Other proteins
drift slowly.
The external surface of the plasma membrane also has carbohydrates attached to it, forming the glycocalyx – which is very important
for cell-to-cell recognition. Proteins in the plasma membrane provide a wide range of functions: proteins transport molecules,
electrons, and ions through channels, pumps, carriers, and electron transport chains. Receptor proteins are critical to many systems.
1. Desmosomes serve as anchors for filaments and rivet cells together.
2. Channel proteins – act as passageways through the phospholipid bilayer for large things - these are integral proteins –
penetrate the hydrophobic core of the lipid bilayer.
4. Receptor proteins – receive info about the environment outside the cell and transmit it into the cell – no physical molecule
passes through this, just INFO - may be integral or peripheral proteins – not embedded in the bilayer, can be extensions of
integral proteins
5. Marker Proteins – identify the type of cell using carbohydrate chains (glycoproteins) - may also have glycolipids that are
not attached to proteins
Biology 304101 Dept. Of Biological Sciences University of Jordan Prof. Dr. Samih Tamimi
CONCEPT 3: Surface Area to Volume Ratio
Surface area acts as limiting factor in size of cell because is a two dimensional unit. Volume is three dimensional, so increases more
quickly than the surface area can accommodate. Larger organisms do not generally have LARGER CELLS, simply MORE CELLS!
CONCEPT 4: Cytoplasm & Cell Wall
CYTOPLASM includes the entire region between the nucleus and the cell membrane! The semi-fluid substance that fills this area is
called CYTOSOL, and this is the liquid in which the organelles are suspended.
Cell walls are found in plant cells (another barrier in ADDITION to the cell membrane). The cell wall protects the cell, gives support
to cell, and is made of polysaccharide called cellulose. It is very porous and allows molecules to pass through, but is NOT
SELECTIVELY PERMEABLE!!!
CONCEPT 5: Organelles
Review the following biological animations as you study this section of the notes:
http://bcs.whfreeman.com/thelifewire/content/chp04/0402001.html
and
www.cellsalive.com
Control:
*Nucleus (plant and animal)
*Centrosome (plant and animal)
Assembly, Transport, and Storage:
*Endoplasmic reticulum (plant and animal)
*Ribosomes (plant and animal)
*Golgi apparatus (plant and animal)
*Vacuoles (plant -1 large, and animal - many)
*Lysosomes (animal)
*Leucoplasts (plant only)
Energy Transformations:
*Chloroplasts and Chromoplasts (plant only)
*Mitochondria (plant and animal)
CONCEPT 6: The Endomembrane System
Many of the different membranes of the eukaryotic cell are part of an ENDOMEMBRANE SYSTEM. Membranes in cell are not
identical in structure or function (modifications are present according to job). The endomembrane system includes:
• nuclear envelope
• endoplasmic riticulum
• Golgi apparatus,
• Lysosomes
• Vacuoles
• plasma membrane
RED arrows show some of the pathways of the
membrane migration.
Nuclear envelope is connected to rough ER…which
is confluent with smooth ER.
Membrane produced by the ER flows in the form of
transport vesicles to the Golgi, which in turn pinches
off vesicles that give rise to lysosomes & vacuoles.
Biology 304101 Dept. Of Biological Sciences University of Jordan Prof. Dr. Samih Tamimi
CONCEPT 7: The Formation & Function of Lysosomes
A lysosome is a membrane-bounded sac of hydrolytic enzymes that the cell uses to digest macromolecules. Hydrolytic enzymes and
lysosomal membranes are made by rough ER and then transferred to the Golgi apparatus for further processing. At least some
lysosomes probably arise by budding from the trans face of the Golgi apparatus (see figure below).
Lysosomes carry out intracellular digestion in a variety of circumstances. Amoebas and many other protists eat by engulfing smaller
organisms or other food particles, a process called phagocytosis. The food vacuole formed in this way then fuses with a lysosome,
whose enzymes digest the food. Digested products then pass into the cytosol and become nutrients for the cell. Some human cells
also carry out phagocytosis. Among them are macrophages, cells that help defend the body by destroying bacteria and other invaders.
Lysosomes also use their hydrolytic enzymes to recycle the cell’s own organic material, a process called autophagy. This occurs
when a lysosome engulfs another organelle or a small amount of cytosol. With the help of the lysosomes, the cell continually renews
itself. Programmed destruction of cells (apoptosis) by their own lysosomal enzymes is important in the development of many
multicellular organisms (such as tadpoles into frogs). This even occurs in the hands of human embryos (which are webbed until
lysosomes digest the tissue between the fingers).
A variety of inherited disorders called lysosomal storage diseases affect lysosomal metabolism. In Pompe’s disease, the liver is
damaged by an accumulation of glycogen due to the absence of a lysosomal enzyme needed to break down that polysaccharide. In
Tay-sacs disease, a lipid-digesting enzyme is missing or inactive, and the brain becomes impaired by an accumulation of lipids in the
cells.
CONCEPT 8: Mitochondria, Chloroplasts, and the Endosymbiotic Theory
Review the following biological animations as you study this section of the notes:
http://www.sumanasinc.com/webcontent/animations/content/organelles.html
Mitochondria: site of cell respiration in animals AND plants. Chloroplasts: site of photosynthesis in plants. How did these specialized
compartments arise during the evolution of Eukaryotic cells? Biologists theorize that mitochondria and chloroplasts are probably
descendants of primitive prokaryotes that were engulfed by the ancestors of eukaryotic cells.
Biology 304101 Dept. Of Biological Sciences University of Jordan Prof. Dr. Samih Tamimi
CONCEPT 9: Other Cellular Structures
Peroxisomes job is to generate and degrade hydrogen peroxide—contain enzymes that transfer hydrogen from various substrates and
make H2O2 as a by-product They aAlso detoxify alcohol in liver cells. H2O2 is toxic, but peroxisomes contain enzymes that convert
it to water.
Cytoskeleton is a network of fibers extending into cytoplasm of cell that provides structural support, and aids in cell motility and cell
regulation. It is made up of microtubules (thickest), microfilaments (thinnest), and intermediate filaments.
Centrosomes and Centrioles
• Microtubules often grow out of centrosome (central area of cell)
• Within the centrosome are centrioles, each composed of nine sets of triplet microtubules arranged in a ring; CENTRIOLES
aid in chromosome separation
Cilia and Flagella
• The movement of these two locomotor appendages is controlled by microtubules
• Cilia are short projections, flagella are much longer
• Movement may not be for entire organism; may be part of a larger unit – ex. Cilia lining windpipe propel foreign substances
out.
Biology 304101 Dept. Of Biological Sciences University of Jordan Prof. Dr. Samih Tamimi
Cell surfaces and Junctions:
Cell wall – plant cells (much thicker than plasma membrane, contains microfibrils made of cellulose)
o Protects plant cell
o Maintains shape
o Prevents excessive water uptake
 primary cell wall -- in young plant cells, thin and flexible (when mature, hardening materials are added for
strength)
 middle lamella – between cell walls of adjacent cells, contains pectins (thick polysaccaride)
 secondary cell wall – may be added when plant is older, found between plasma membrane and primary
wall, consists of several laminated layers (Ex. Wood)
Plasmodesmata:
Plasmodesmata in plants; channels that allow cytosol to pass through and connect the living contents of adjacent cells (see page 134)
1) Plant cells first construct thin primary walls, often adding stronger secondary walls to the inside of the primary wall when the
cell’s growth ceases.
2) A sticky lamella cements adjacent cells together – thus, multilayered partition between these cells consists of adjoining walls
individually secreted by cells.
3) THESE WALLS DO NO ISOLATE THE CELLS – the cytoplasm of one cell is continuous with the cytoplasm of its
neighbors via plasmodesmata (channels through the walls).
Animal Cell Junctions:
tight junctions – animals – membranes of neighboring cells
are actually fused; prevent leakage of extracellular fluid
across a layer of epithelial cells
desmosomes – animals; “anchoring junctions” – function
like rivets, fastening cells together in strong sheets (are
reinforced by intermediate filaments made of keratin)
gap junctions – animals; “communicating junctions”
provide cytoplasmic channels between adjacent animal cells
Neighboring cells often adhere, interact, and
communicate through special patches of direct physical
contact…intracellular junctions help integrate cells!