Chapter 3: The Cellular Level of Organization
Chapter Objectives
THE PLASMA MEMBRANE
1. Describe the “fluid mosaic model” concept
2. Describe the components of the lipid bilayer.
3. Distinguish between integral and peripheral proteins in cell membranes.
TRANSPORT ACROSS THE PLASMA MEMBRANE
4. Distinguish between passive and active processes, including direction of particle
movement and energy requirements.
5. Explain the concept of simple diffusion and list substances that pass through the bilayer
by simple diffusion.
6. Explain the process of osmosis as the passive movement of water through a selectively
permeable membrane
7. Explain the differences between simple and facilitated diffusion.
8. Describe the process of channel-mediated facilitated diffusion and identify substances
transported by the process.
9. Describe the process of carrier-mediated facilitated diffusion and identify substances
transported by the process.
10. Define active process and describe the energy source used to drive the process.
11. Describe the process of primary active transport, relating the process to the
sodium/potassium pump.
12. Describe the process of secondary active transport and the role of symporters and
antiporters.
13. Describe the process of exocytosis and explain its importance.
14. Describe the process of phagocytosis and explain its importance.
15. Describe the process of pinocytosis and explain its importance.
16. Describe the process of receptor-mediated endocytosis and explain its importance.
17. Show the relation of osmotic pressure to tonicity by the effect on red blood cells of
different concentrations of solute in a surrounding solution.
ORGANELLES
18. Describe the basic structural features and functions of organelles.
19. Discuss the function of the nucleus, nuclear envelope, nucleoli, and chromatin.
20. Indicate the origin and components of ribosomes that allow them to produce proteins, and
their association with other organelles in this process.
21. Compare and contrast the two forms of Endoplasmic reticulum and describe their
functions.
22. Describe the structure and function of the Golgi apparatus.
23. Describe the structure and function of the lysosomes.
24. Describe the role of mitochondria in producing the energy-storage molecule ATP.
25. Distinguish the characteristics and functions of microfilaments, microtubules, and
intermediate filaments.
26. Show the relationship of cytoskeletal elements to centrioles and the centrosome and note
their purpose in dividing and nondividing cells
27. Describe the arrangement of microtubules in flagella and cilia that allow these projections
to perform the different types of transport movements.
PROTEIN SYNTHESIS
28. List the steps of protein synthesis and describe what occurs in each.
Chapter Lecture Notes
Plasma Membrane
Plasma membrane – flexible yet sturdy barrier that surrounds and contains the cytoplasm of a
cell (Fig 3.2)
Fluid mosaic model
Lipid bilayer
Phospholipids
Cholesterol
Glycolipids
Membrane proteins (Fig 3.3)
Integral proteins – extend into or through the lipid bilayer
Peripheral proteins – associated with the polar heads or integral proteins
Glycoproteins – integral or peripheral proteins that have carbohydrate molecules
attached
Transport Across Membranes (Table 3.1)
Process
Energy Source
Description
Examples
Simple Diffusion
(Fig 3.4 & 3.5)
Kinetic energy
Movement of fats, oxygen,
carbon dioxide through the
lipid portion of the membrane
Osmosis (Fig 3.8)
Kinetic energy
Net movement of particles
from an area of their
higher concentration to an
area of their lower
concentration, that is
along their concentration
gradient
Simple diffusion of water
through a selectively
permeable membrane
Channel-Mediated
Facilitated Diffusion
(Figs 3.5 & 3.6)
Kinetic energy
Same as simple diffusion,
but the diffusing
substance goes through a
membrane protein channel
Carrier-Mediated
Facilitated Diffusion
(Figs 3.5 & 3.7)
Kinetic energy
The diffusing substance
must attach to its
membrane protein carrier
Movement of glucose into
most body cells
Filtration
Hydrostatic
pressure
Movement of water and
solutes through a
semipermeable membrane
from a region of higher
hydrostatic pressure to a
region of lower
hydrostatic pressure, that
is along a pressure
gradient
Movement of water, nutrients,
and gasses through a capillary
wall; formation of kidney
filtrate
Primary Active
Transport (solute
pumping) (Fig 3.10)
ATP (cellular
energy)
Movement of a substance
through a membrane
against a concentration (or
electrochemical gradient;
requires a membrane
carrier protein)
Movement of Na+ and K+
across the membrane against
their concentration gradient
Secondary Active
Transport (Fig 3.11)
ATP (cellular
energy)
Energy stored in a Na+ or
H+ concentration gradient
is used to drive other
substances across the
membrane against their
concentration gradients
Movement of calcium ions out
of the cell while Na+ is
moving in (antiporters);
dietary glucose and amino
acids moving into cells at the
same time Na+ is also moving
in (symporters)
Passive processes
Movement of water into and
out of cells through the lipid
bilayer and/or membrane
pores (aquaporins)
Movement of ions into and out
of cells
Active processes
Bulk transport
Exocytosis
ATP
Secretion or ejection of
substances from a cell; the
substance is enclosed in a
membranous vesicle,
which fuses with the
plasma membrane and
ruptures, releasing the
substance to the exterior
Secretion of neurotransmitters,
hormones, mucus, etc.;
ejection of cell wastes
Phagocytosis
(endocytosis)
(Fig 3.13)
ATP
In the human body, occurs
primarily in protective
phagocytes (some white blood
cells, macrophages)
Pinocytosis
(endocytosis)
(Fig 3.14)
ATP
Receptormediated
endocytosis (Fig
3.12)
ATP
“Cell eating”; A large
external particle (proteins,
bacteria, dead cell debris)
is surrounded by a
“seizing foot” and
becomes enclosed in a
plasma membrane
“Cell drinking”; Plasma
membrane sinks beneath
an external fluid droplet
containing small solutes;
membrane edges fuse,
forming a fluid-filled
vesicle
Selective endocytosis
process; external
substance binds to
membrane receptors, and
coated pits are formed
Occurs in most cells;
important for taking in solutes
by absorptive cells of the
kidney and intestine
Means of intake of some
hormones, cholesterol, iron,
and other molecules
Tonicity (Fig 3.9)
Isotonic – concentrations of solutes are the same on both sides of the membrane
0.9% NaCl solution (normal physiological saline) is isotonic for red blood cells (RBC)
Hypotonic – a solution that has lower concentration of solutes than the cytosol
Lysis or hemolysis in RBC
Hypertonic – a solution that has greater concentration of solutes than the cytosol
Crenation – cell shrinkage
Nucleus
Nucleus – control center (Fig 3.1 & 3.24)
contains chromosomes - heredity material - called chromatin when cell is not dividing
(Fig 3.25)
before cell division, chromosomes are copied by a process called replication (Fig 3.31)
Nucleolus - assembly plant for ribosomes
surrounded by nuclear membrane which has pores in it through which substances enter and
exit
Ribosomes
Ribosomes - contain both rRNA and ribosomal proteins (Fig 3.18)
named ribosomes because of the high content of RNA (literally RNA bodies)
one ribosome consists of one larger and one smaller subunit
functions as the workbench for protein synthesis
some ribosomes are free ribosomes - no attachment to organelles - concerned primarily with
synthesizing proteins for use inside cell
some ribosomes are attached to ER, hence rough ER - involved in the synthesis of proteins
for insertion in the cell membrane or for export from the cell
Endoplasmic Reticulum
Endoplasmic reticulum (ER) - system of membrane-enclosed channels continuous with nuclear
membrane and Golgi complex (Fig 3.19)
rough ER - has attached ribosomes - proteins synthesized are stored by the ER and sugar
groups may be added to form glycoproteins - then transported from ER to Golgi
smooth ER - no ribosomes attached - provides a surface area for chemical reactions
site of steroid, fatty acid, phospholipid synthesis (ex: in testis provides surface for
enzymes involved in testosterone synthesis)
site of carbohydrate synthesis, detoxification of alcohol, pesticides, carcinogens (ex: liver
synthesis of glycogen)
stores Ca2+ in muscle (but called sarcoplasmic reticulum (SR) in muscle)
Golgi Complex
Golgi complex - consists of stacks of flattened sacs (like pancakes) that can form vesicles for
exocytosis, lysosomes, or for storage (vesicles are membrane bound sacs that are smaller
than vacuoles) (Fig 3.20)
Golgi receives proteins, carbohydrates, lipids from vesicles made from ER and collects, sorts,
packages as new vesicles, and delivers vesicles for storage, membrane use, or
exocytosis, lysosomes
Protein Trip: ribosomes (site of protein synthesis) → rough ER → transport vesicle → Golgi
→ secretory vesicle → release to exterior by exocytosis (Fig 3.21)
proteins and lipids used in cell membrane or inside cell follow a similar route
Lysosomes (Fig 3.22)
Lysosome - formed by Golgi and contain powerful digestive (hydrolytic) enzymes that:
recycle monomers in a cell from polymers (proteins, carbohydrates, lipids, nucleic acids)
destroy bacteria engulfed by white blood cells (WBC) when the phagocytic vesicle fuses
with lysosome
programmed destruction of cells:
programmed destruction of tissue between fingers and toes during development so
webbed hands and feet are absent
metamorphosis of tadpole to frog (lysosomes destroy cells of tail)
release of lysosomal enzymes outside of cell:
bone remodeling by osteoclasts (knee remodeled every 3-4 months)
rheumatoid arthritis due to leaking of lysosomal enzymes into joint cavity and destruction
of joint cavity - cortisone and aspirin both help to maintain integrity of lysosomal
membrane
lysosome works best in an acid pH (pH = 5) even though cytoplasm is pH 7:
contains proton (H+) pumps that gather H+ from cytoplasm and pump into lysosomes
lysosomes retain the dangerous hydrolytic enzymes but permit final products of digestion
to escape
Mitochondria
Mitochondria - powerhouse of cell - double membraned - found in both animal and plant cells,
but not in bacteria (Fig 3.23)
center of cellular respiration (Krebs cycle and electron transport chain) - 95 % of oxygen
brought into body is used in mitochondria as the final hydrogen acceptor to make
metabolic water (See Chapter 25)
Krebs cycle occurs in matrix - series of enzymes remove H from molecules such as glucose
and fatty acids and are accepted by NAD to make NADH2
Electron transport chain (ETC) - occurs along cristae - accepts the NADH2 from Krebs cycle
- ETC has a series of enzymes that split the hydrogen into electrons and protons electrons are transported from high energy levels to low energy levels and ATP is
then produced - at the end of ETC the proton and electron come back together to form
hydrogen which is then accepted by O2 to make H2O
mitochondria contains its own DNA - can reproduce independently
Cytoskeleton
Cytoskeleton - elaborate network of protein structures = "bones and muscles" (Fig 3.15)
microfilaments - thin strands of actin - aid in cell movement (ex. amoeboid movement as in
WBC), aid in cytokinesis
most highly developed in muscles
intermediate filaments - tough, insoluble protein fibers that are intermediate in size between
microfilaments and microtubules - protein varies with cell types (ex. keratin filaments
in epidermal cells)
microtubules - hollow tubes formed of globular proteins called tubulins
provide monorail system to move organelles/vesicles
also in centrioles, cilia, spindle fibers
Centrioles
Centrioles - paired cylindrical bodies each composed of 9 triplets of microtubules (protein
straws) (Fig 3.16)
Functions:
organize spindle fibers and asters during mitosis in animal cells (may not be necessary for
this purpose because plants produce spindle fibers during mitosis but they lack
centrioles)
form the bases of cilia and flagella (having centrioles at each pole in mitosis provides a
vehicle for transmission of centrioles to all cells)
Cilia/Flagella
Cilia/Flagella - Membrane bound sets of microtubules that move by means of ATP (Fig 3.17)
cannot produce cilia/flagella without centrioles
centrioles = 9 triplets of microtubules
cilia/flagella = 9 sets of doublets with central pair
Protein Synthesis
DNA in nucleus is used as instruction manual to produce proteins
Protein synthesis is a two step process: transcription and translation (Fig 3.26)
Transcription
Transcription - the synthesis of RNA under the direction of DNA, occurs in the nucleus (Fig
3.27)
Messenger RNA (mRNA) - RNA that carries a genetic message from the DNA to the
protein-synthesizing machinery
Translation
Translation - the synthesis of a polypeptide, which occurs under the direction of mRNA (Fig
3.29)
Triplet code - instruction for a polypeptide chain written in three-nucleotide word, codons
Translation is done by the ribosomes in cytoplasm (free) and at endoplasmic reticulum
(bound)