Chapter 3 – Cells: The Living Units
Overview of the Cellular Basis of life –
Cells are the structural unit of all living things
50 – 60 trillion cells in an adult human body
Approximately 200 different types (with different sizes, shapes, and functions)
Cells range from 2µm (1/12,000 inch) to over 3 feet
Cells have the same basic parts (generalized/composite cell)
Nucleus—controls cell activity, centrally located
Cytoplasm—area containing necessary organelles
Plasma membrane—outer cell boundary
The Plasma Membrane: Structure - The Fluid Mosaic Model:
double-layer (bilayer) of phospholipids with interspersed proteins that are constantly changing
polar, phosphate-containing head contacts HOH
non-polar, fatty acid tail avoids HOH
bilayer has tail in contact with heads outward
allows membrane to self-assemble into a closed structure when forming/repairing
10% of externally-facing phospholipids have sugar groups attached (glycolipids)
branching sugar groups are on some external proteins that stick far out in the extracellular space
(glycocalyx) and function as highly specific biological markers
cholesterol is present for stability
integral (transmembrane) proteins span the entire width of the membrane and help in transporting
materials thru the membrane (either as channels or carriers)
peripheral proteins are not embedded in the membrane but attached to the ends of the integral
proteins (usually enzymes or perform other mechanical functions)
(KNOW FIGURE 3.3 – THE FLUID MOSAIC MODEL – PAGE 58)
Specialization of the Plasma Membrane:
microvilli –
minute, finger-like extensions of the plasma membrane extending from an exposed cell surface
increase the surface area of the membrane
often found on absorption cells (intestinal cells)
membrane junctions –
with the exception of blood cells (red and white) and reproductive cells (sperm and egg) most
other cells are bound closely together
2 factors help in binding:
– adhesive glycoprotein in the glycocalyx
– plasma membranes of adjacent cells fit together in tongue-and-groove fashion
The Plasma Membrane: Functions – Membrane Transport:
extracellular (interstitial) fluid is composed of HOH, aa, sugars, fatty acids, vitamins, hormones,
neurotransmitters, salts, and waste.
membrane is selectively (differentially) permeable
(allows only some things to pass thru-keeps beneficial things inside & harmful things out)
passive processes
does not require cellular energy
move from [high] to [low] (along/down their [gradient])
smaller molecules move faster
warmer temperatures make faster reactions
eventually the molecules will be evenly distributed thru the whole environment and diffusion
stops (equilibrium)
types:
simple diffusion –
diffusion thru a plasma membrane will only occur if the molecule is
lipid soluble and small enough to go thru the membrane pores by itself
nonpolar, lipid soluble molecules only
O2, CO2, fats and alcohol
small polar and charged particles can pass through HOH-filled channels made by
channel proteins
facilitated diffusion –
molecules too big to go through on their own can get help from carrier proteins
not exactly sure how this happens
very selective (a carrier will only bind with a specific substrate)
limited by the number of carriers present in the membrane
osmosis –
the movement of HOH through a selectively permeable membrane
cannot move thru lipids bilayer but they can move thru pores
if the [solute] is different on both sides, the [HOH] will be different as well
(more [solute] – less [HOH])
(less [solute] – more [HOH])
(both move toward equilibrium)
the [total] of all solutes in a solution is the solution’s osmolarity
osmotic imbalances cause cells to shrink (loss of HOH) or swell (gain of HOH)
until [solute] reaches equilibrium or the cells breaks
ability of a cell to alter internal HOH volume is tonicity
isotonic solutions have equal [solute] as cells –
no net movement of HOH
hypertonic solutions have higher [solute] than cells –
HOH moves out of the cell causing shrinkage (crenation)
hypotonic solutions have lower [solute] than cells –
HOH moves into the cell causing swelling (lysis)
(distilled HOH is the ultimate hypotonic solution)
filtration –
HOH and solutes are forced through a membrane or capillary wall by fluid
(hydrostatic) pressure
depends on a pressure gradient that pushes solute-containing fluid (filtrate)
from high pressure to low pressure
not selective – only molecules too large to go thru are held back
occurs in kidney – first step of urine formation
active processes –
requires ATP to transport substances
molecules may be too large, unable to dissolve in bilayer, or moving against a [gradient]
types:
solute pumping –
moves aa and ions (Na+, K+, Ca++) against a [gradient]
carrier protein uses ATP to change shape to carry solutes
vesicular (bulk) transport –
moving large particles/macromolecules thru plasma membranes
exocytosis
moving substances out of the cell
hormone/mucus secretion, neurotransmitter release, waste ejection
cell product is enclosed in a membranous sac (vesicle) that migrates to
the cell membrane, fuses with it, and ruptures into the
interstitial fluid
endocytosis
moving substances into a cell
plasma membranes infolds, a vesicle forms, pinches off the cell
membrane, moves into the cytoplasm, and contents are used by
the cell
3 types:
phagocytosis (“eating cell”) – intake of solid material that fuses
with a lysosome for digestion
pinocytosis (“drinking cell”) – intake of liquid material
(important in nutrient absorption)
receptor – mediated endocytosis is very selective – plasma
membrane proteins (receptors) bind only with certain
substances
The Cytoplasm –
cellular material between plasma membrane and nucleus
major functional area
most cellular activities take place here
3 parts:
cytosol is the viscous, semitransparent fluid where the other elements are found including a
variety of solutes (proteins, salts, sugars, etc.)
organelles are the metabolic machinery that have specific functions
inclusions are chemical substances present in some cells (stored nutrients, glycogen granules,
lipid droplets, melanin, etc.)
Organelles –
specialized cellular compartments with specific functions
most organelles are surrounded by its own plasma membrane
some are non-membrane bound
nucleus –
control center of the cell
most cells have only one
skeletal muscle cells, some liver cells, and bone destruction cells are multinucleate
red blood cells are anucleate (cannot reproduce and only live 3-4 months)
largest organelle in the cell
3 distinct regions –
nuclear envelope:
double layer membrane surrounding nucleus
outer membrane is continuous with rough ER
at various points, the two membrane layers fuse and forms a nuclear pore to allow for
the import of proteins and export of messenger and ribosomal RNA
nucleoli (singular nucleolus):
spherical bodies within the nucleus
not membrane-bound
assembly site for ribosomal subunits (large/numerous in cells making tissue proteins)
chromatin
system of bumpy threads in the nucleoplasm
made of DNA and histone (a globular protein)
during non-dividing times, the chromatin is extended and not easily seen
during cell division, the chromatin condenses to form visible chromosomes
endoplasmic reticulum (ER) –
extensive system of interconnected tubes and parallel membranes that make fluid-filled cavities
(cisternae)
ER membranes are continuous with the nuclear membrane
2 types:
rough ER
surface is studded with ribosomes
proteins made are transported thru the cisternae to be secreted
abundant in secretory cells, antibody-producing cells, and liver cells
considered to be the cell’s “membrane factory” (makes integral proteins)
smooth ER
continuous of the rough ER
tubules in a branching network
no role in protein synthesis
responsible for reactions involving lipid metabolism and synthesis
cholesterol, lipoproteins, steroid-based sex hormones, fat absorption/transport
helps in the detoxification of drugs
important in storing/releasing Ca++ during muscle contractions (sarcoplasmic reticulum)
ribosomes –
small
made of proteins and ribosomal RNA
2 globular subunits that fit together
site of protein synthesis
free ribosomes
– float in the cytoplasm
– make soluble proteins
membrane-bound (attached) ribosomes
– attached to the rough endoplasmic reticulum
– makes proteins for export or use in plasma membrane
can switch between synthesis locations
Golgi apparatus –
flattened, stacked, membranous sacs
associated with numerous vesicles
transport vesicles budded off from the rough ER fuse with the Golgi
modifies, concentrates, and packages the proteins and membranes made in the rough ER
some proteins are “tagged” for specific delivery, sorted, packaged, and shipped
mitochondria –
sausage-shaped
movable and change shape continuously
provide ATP for the cell
abundant in liver and muscle cells
contains DNA and RNA for self-replication by fission (very similar to bacteria)
peroxisomes –
contain oxidase that uses O2 to detoxify toxic substances (alcohol and formaldehyde)
destroy free radicals
(highly reactive chemicals with unpaired electrons that can destroy protein, lipid, and nucleic
acid structures)
self-replicating by fission
lysosomes –
spherical membranous vesicles
contains digestive enzymes
abundant in phagocytes
digest a wide variety of biological molecules
works best in acidic conditions (pH 5)
other important functions:
digesting particles ingested by endocytosis (especially bacteria, viruses, toxins, etc.)
degrading worn-out or nonfunctional organelles
breakdown of stored glycogen
breakdown of non-useful tissue, embryonic digit web, uterine lining during menstruation, and
bone tissue to release Ca++
autolysis happens when the lysosomal membrane breaks down and digests the cell
(low O2, injury, high vitamin A)
cytoskeleton –
series of rods going thru the cytoplasm
supports cellular structures
helps generate cell movement
not covered by a membrane
3 types:
microtubules
hollow tubes
all originate near the nucleus in an area called the centrosome
determines the overall shape of the cell and distribution of organelles
some organelles are attached to microtubules and are pulled thru the cytoplasm
constantly grows out from the cell center, breaking up, & reassembling
microfilaments
thin strands of contractile proteins
dense cross-linked network in a unique arrangement for each cell
responsible for cell motility or changes in cell shape
intermediate filaments
tough, insoluble protein fibers that vary with cell type
diameter intermediate between microtubules and microfilament
most stable and permanent of cytoskeletal elements
high tensile strength to resist pulling forces on the cell
helps form chromosomes
cilia and flagella –
whiplike extensions from the exposed surface of certain cells
cilia moves substances in one direction across cells
flagella are usually singular and used for movement
Cell Growth and Reproduction –
The Cell Life Cycle:
series of changes a cell goes thru from the time it is formed until it reproduces
2 major periods – interphase and mitotic phase
INTERPHASE –
period from cell formation to cell division
cell is carrying out all its routine activities
also prepares for cell division
divided into 3 subphases:
– G1 (growth 1)
cells are metabolically active
time length is variable
cells that will not divide are in the G0 phase
centrioles begin replication
– S (synthetic/synthesis)
DNA replicates itself
must occur before cell can divide
replication must be exact to avoid mutations
trigger is unknown
once replication starts it continues until it’s finished
- G2 (growth 2)
cell makes more organelles and membrane to allow for division
MITOTIC PHASE –
some cells divide almost continuously (skin and intestinal lining)
some cells divide more slowly (liver cells)
some cells do not divide at all (nervous, skeletal, & cardiac muscle)
the amount of nutrients a cell requires is directly related to its volume
surface area does not increase proportionally with volume and a growing cell will eventually “outgrow”
its surface area
cell division creates daughter cells that have a favorable surface area:volume ratio
two events:
mitosis (nuclear division)
cytokinesis (cytoplasmic division)
mitosis:
4 phases –
Prophase
first and longest phase of mitosis
chromatin coils and condenses to form chromosomes
each chromosome is two identical chromatin threads held together by a centromere
microtubules from the centriole lengthen and push the 2 centrioles apart (toward the poles)
nuclear membrane disappears
when centrioles are at the poles, some microtubules anchor the centrioles to the plasma
membrane and other microtubules attach to the centromere of each chromosome
Metaphase
chromosomes cluster at the equator
Anaphase
shortest phase
microtubules begin to shorten
one chromatid of each chromosome is pulled to its pole (chromatid is now a chromosome)
Telophase
like prophase in reverse
identical set of chromosomes at each pole
chromosomes uncoil into chromatin
nuclear membrane & nucleoli reappear
until cytokinesis, the cell is binucleate
Cytokinesis:
plasma membrane over the center of the cell is drawn inward to form a cleavage furrow
microfilaments make the furrow deepen
cytoplasmic mass is pinched into 2 parts
each resulting daughter cell has their own plasma membrane, organelles, and nucleus
(KNOW THE STAGES OF MITOSIS/CYTOKINESIS – FIGURE 3.30 – PAGES 96-97)
Development Aspects of Cells –
cells become specialized very early in development
cells in various regions of the embryo are exposed to different chemical signals that trigger them into
specific pathways of development
as development continues cells release chemicals that influence the development of neighboring cells
by triggering processes that switch some of their genes off
secret of cell specialization is the kinds of proteins made
cell specialization therefore leads to structural variation
cell aging has three theories:
sometimes attributed to cumulative effects of chemicals (both natural and man-made)
progressive disorders in the immune system results in a weakening and eventual failure of cell
responses
cessation of mitosis and cell aging are pre-programmed into our genetic makeup
(genetic theory)