Eucaryotic Cell Structure and Cell Components
Diagram of a Eucaryotic cell -- see text for identification
Eucaryotic cells are organized into different
- Compartments bounded by membranes (to be
discussed in Ch. 8)
- Cytoplasm = central
metabolic compartment, bounded by cell membrane. Other compartments inside
cytoplasm are called organelles
allows specialized functions to be carried out in different
The cytoplasm: site of protein synthesis and many metabolic
- Contains many ribosomes = particles on which
- View figures of ribosomes
- Ribosome size measured
in Svedberg (S) units; derived from sedimentation in ultracentrifuge (used
before electron microscopes were available)
- Prokaryotes: ribosomes made of 30S and 50S
subunits, assemble into 70S ribosome
ribosomes made of 40S and 60S subunits, assemble into 80S ribosome
- In bacteria, ribosomes occupy 25% of cell
volume, use 90% of cell energy. Less in many specialized eukaryotic cells, but
still the dominant activity of almost all
- Contains many enzymes for general
- Compartment in which foodstuffs enter
and from which wastes leave cell
- Contains fiber
of the cytoskeletal system, which organize cytoplasmic
- Contains many different
- View cartoon of the interior of a eukaryotic cell
The Nucleus: locus of DNA & RNA synthesis and protein
The Endomembrane system: moving materials into different
- Contains chromatin = DNA-protein
complexes. Chromatin can condense into chromosomes during cell
- Site of RNA synthesis. 80% of RNA =
ribosomal RNA. Remaining 20% leaves nucleus as t-RNA & m-RNA, directs
protein synthesis (to be discussed Ch.
- Review the role of the nucleus and ribosomes in protein synthesis (Campbell website activity)
- View diagram of cell nucleus
- Contains nucleolus = assembly plant
for ribosomes. Ribosomal proteins are made in cytoplasm, must be transported
back into nucleus. Ribosomal RNA is made in nucleus. These two elements are
integrated inside nucleolus to create ribosomal subunits. These are then
exported out of nucleus through nuclear
View diagram of Nucleolus
- Bounded by nuclear membrane =
double layered structure. Contains many nuclear pores, allow
material to move in and out of nucleus
Pores have octagonal "doors" made of protein; open and close on either side
depending on specific signals. Pore has diameter of about 10 nanometers (10 x
10-9 m), smaller than diameter of a complete ribosome. Pore can open
up to as much as 26 nm in response to certain signals. Some signals allow motion
in but not out, other signals control reverse
diagram of nuclear pore structure
Endomembrane system = set of interconnected
compartments: endoplasmic reticulum (ER), Golgi body, lysosomes, cell
- Endoplasmic Reticulum
- Rough ER: synthesizes proteins for export or
movement to different cell compartments (but not to
cytoplasm). View quicktime movie of rER [353 KB, 00:59 min, from Cells.De Online service for cell biology] .
- Signal hypothesis: certain
mRNAs encode proteins designated for export. These carry a peptide signal at
growing end, causes growing protein to move to ER ("docking"), insert peptide
into membrane, translocate growing polypeptide chain across ER membrane. When
protein synthesis is complete, polypeptide folds up inside ER, not in cytoplasm.
- View diagram showing ribosomes attaching to ER
- Smooth ER (sER): synthesizes lipids,
detoxifies drugs and poisons (in liver).
View smooth ER -- structure labeled "1".
- Golgi body
- functions as intracellular
"post office" for sorting new proteins made on rER.
- Vesicles containing protein
pinch off from ER, fuse with cis face of Golgi. Inside Golgi, oligosaccharide
chains on proteins are modified. Vesicles pinch off from trans face of Golgi,
carry proteins to several possible destinations: export (out of cell),
lysosomes, peroxisomes, cell membrane, etc.
- View diagrams showing how substance move through Golgi body
- View animation of secretion from rER to Golgi to cell exterior (Campbell website activity)
- compartments to break down old
proteins, foreign materials, many wastes.
- Contain ~40 hydrolytic enzymes:
lipases, proteases, nucleases, etc. Break down organic polymers of all types.
- "Suicide bags" if opened up on cell itself = apoptosis.
- Lysosomes are used in phagocytosis, a process in which foreign materials are brought into the cell and "chewed up".
- View animation showing phagocytosis. (Flash animation by Tom Terry, 2001)
- View diagrams of lysosomes
- animation of secretion from rER to Golgi to lysosome (Campbell website activity)
- Cell membrane (aka plasma membrane) - see Ch. 8
- large membrane compartments
(contrasted with small membrane bags called vesicles).
- Plant cells have
especially large vacuole called the central vacuole, can occupy most of the
volume of a plant cell. Stores pigments, wastes, water, poisons, and
Organelles involved in energy transformations are separate
from the endomembrane system
- "Energy organelles" have unique
- are enclosed by double membrane system
- contain DNA and ribosomes (70S, not 80S like cytoplasmic ribosomes)
- make some of their own proteins
- from their own genes
- divide by binary fission (but not autonomous, cannot
- grow or sustain life outside of cell)
- Mitochondria = centers for respiratory
catabolism. Oxygen combined with chemicals to break down foods, generate
cell energy. Contain outer and inner compartments, with many membranous cristae
that "criss-cross" the internal space. Found in virtually every eukaryotic cell.
Small structures similar to bacteria in some size.
View mitochondria (protected)
- Chloroplasts = centers for photosynthetic
anabolism. Belong to group of plant organelles called plastids. Include
chloroplasts (photosynthesis), amyloplasts (store starch), chromoplasts (store
pigments). Trap light, convert energy to sugars (+ CO2, water).
Contain stacks of thylakoids, where green pigmented chlorophyll is embedded in
membrane to trap light.
theory: All organelles seem to share many properties with bacteria: contain
70S ribosomes (whereas rest of eukaryote cells contain 80S ribosomes), divide by
binary fission, contain circular DNA without nucleus, etc. Lynn Margulis
proposed endosymbiont hypothesis: that organelles derived from ancient
colonization of large bacteria (became the eucaryotic cell) by smaller bacteria
(became the mitochondria, chloroplast, etc.) Symbiosis = "living together".
Eventually, organelles lost ability to exist as separate organisms, cannot be
separated from cell. Recent evolutionary taxonomy by comparing ribosomal RNA
shows that this idea has lots of merit. Mitochondrial and plastid ribosomes are
very similar to current bacteria, very different from
Build a cell (Campbell website activity)
Cytoskeletal system provides internal fibrous structure to
Cell is not "just a bag in a bubble". Lots of
internal fibers = internal "skeleton". Not rigid like bone; capable of being
assembled, broken down in minutes. Allows cell movement, cell division, internal
motion of compartments.
- Largest diameter fiber. Found in cytoplasm of all eukaryotes.
- Involved in many
structures: cilia, flagella (9+2 arrangement); spindle fibers that polymerize
from centrioles during mitosis/meiosis.
- Made of tubulin protein; polymerizes
into hollow tubules 25 nm diameter.
- View cell
treated with anti-tubulin fluourescent
- Cilia and flagella: organelles of
locomotion. Contain 9 double rings of microtubules, 2 central microtubules.
- View micrographs showing structure of cilia
- View SEM of cilia on surface of epithelial
- Two motor proteins allow motion along microtubules
- Motor protein 1 -- Dynein
- Motor protein 2 -- Kinesin
- Also powered by ATP, also allows protein to move along microtubule
- Causes motion from negative (-) end of the microtubule toward the positive (+) end of the microtubule (where new tubulin is added to the microtubule).
- pulls things toward outer reaches of cell
- Example: in nerve cells, kinesin pulls vesicles away from center towards nerve endings.
- View animation of kinesin pulling vesicle along a microtubule
- View more sophisticated animation of kinesin moving along a microtubule. (Select either the Quicktime or MPEG movie titled "Structural Analysis of the Kinesin Motor Protein" )
- Microfilaments (= actin)
- Intermediate filaments
- a third kind of
- Made from keratin subunits. Not so quickly assembled and disassembled as
microtubules or microfilaments.
- May be involved in resisting tension, reinforcing cell shape, fixing location of nucleus.
Cell walls provide rigid structure around
- Found in plants, fungi, bacteria -- not in
- Allow cells to survive in plain
water, rigid structure (tree towering 150 feet high!)
- Thicker than cell membrane. Made from cellulose
(plants and fungi), other polysaccharides (bacteria).
plant cell walls
- Cells maintain contact by plasmodesmata
-- thin cytoplasmic connections, lined by membrane, that pass across cell wall
Cells don't end at their outer membrane; they possess an
extracellular matrix (ECM)
- Animal cells don't have walls, but do have ECM =
meshwork of macromolecules outside plasma membrane. Consists mainly of
glycoproteins (proteins with oligosaccharide chains), especially collagen.
- Some cells attached directly to ECM by bonding
to collagen or fibronectin.
- View diagram of ECM (protected)
Cells are joined by a variety of intracellular
- In multicellular organisms, adajcent cells are held together by several types of specialized junctions.
(found in animals): specialized "belts" that bind two cells tightly to each other, prevent fluid from leaking into intracellular space.
View animation of tight junctions
(Campbell website activity)
(found in animals): intercellular "rivets" that create tight bonds between cells, but allow fluids to pass through intracellular spaces.
animation of desmosomes (aka anchoring junctions)
(Campbell website activity)
(found in animals): formed by two connecting protein rings embedded in cell membrane of adjacent cells. Allows passage of water, small solutes, but not macromolecules (proteins, nucleic acids).
View animation of gap junctions (aka communicating junctions)
(Campbell website activity)
(found in plants): channels connecting cells; allow free passage of water and small solutes, but not macromolecules (proteins, nucleic acids).
figure showing plasmodesmata
(Campbell website activity)
Other useful sources of information:
- Cell-Tissue-Body Explorer - An interactive animated atlas of cells and functions of the human body. Note: 3-D renderings are very slow to load unless you have a very fast computer and connection.