Chapter 3
The Cellular Level of Organization
Basic, living, structural and functional unit of the body
compartmentalization of chemical reactions within
specialized structures
regulate inflow & outflow of materials
use genetic material to direct cell activities
Cytology = study of cellular structure
Cell physiology = study of cellular function
Generalized Cell Structures
Plasma membrane =
cell membrane
Nucleus = genetic material of cell
Cytoplasm = everything between the membrane and the nucleus
cytosol = intracellular fluid
organelles = subcellular structures with specific functions
The Typical Cell
Not all cells contain all of these organelles.
Plasma Membrane
Flexible but sturdy barrier that surround cytoplasm of cell
Fluid mosaic model describes its structure
“sea of lipids in which proteins float like icebergs”
membrane is 50 % lipid & 50 % protein
held together by hydrogen bonds
lipid is barrier to entry or exit of polar substances
proteins are “gatekeepers” -- regulate traffic
50 lipid molecules for each protein molecule
Lipid Bilayer of the Cell Membrane
Two back-to-back layers of 3 types of lipid molecules
Cholesterol and glycolipids scattered among a double row of
phospholipid molecules
Phospholipids
Comprises 75% of lipids
Phospholipid bilayer = 2 parallel layers of molecules
Each molecule is amphipathic (has both a polar & nonpolar region)
polar parts (heads) are hydophilic and face on both surfaces
a watery environment
nonpolar parts (tails) are hydrophobic and line up next to
each other in the interior
Glycolipids within the Cell Membrane
Comprises 5% of the lipids of the cell membrane
Carbohydrate groups form a polar head only on the side of
the membrane facing the extracellular fluid
Types of Membrane Proteins
Integral proteins
extend into or completely across cell membrane
if extend completely across = transmembrane proteins
glycoproteins have the sugar portion facing the
extracellular fluid to form a glycocalyx
gives cell “uniqueness”, protects it from being digested,
creates a stickiness to hold it to other cells or so it can hold a fluid layer
creating a slippery surface
Peripheral proteins
attached to either inner or outer surface of cell membrane
and are easily removed from it
Membrane Proteins
Functions of Membrane Proteins
Formation of Channel
passageway to allow specific substance to pass through
Transporter Proteins
bind a specific substance, change their shape & move it
across membrane
Receptor Proteins
cellular recognition site -- bind to substance
Functions of Membrane Proteins
Cell Identity Marker
allow cell to recognize other similar cells
Linker
anchor proteins in cell membrane or to other cells
allow cell movement
cell shape & structure
Act as Enzyme
speed up reactions
Membrane Fluidity
Membranes are fluid structures (oil layer)
self-sealing if punctured with needle
Selective Permeability of Membrane
Lipid bilayer
permeable to nonpolar, uncharged molecules -- oxygen, CO2,
steroids
permeable to water which flows through gaps that form in
hydrophobic core of membrane as phospholipids move about
Transmembrane proteins act as specific channels
small and medium polar & charged particles
Macromolecules unable to pass through the membrane
vesicular transport
Gradients Across the Plasma Membrane
Membrane can maintain difference in concentration of a
substance inside versus outside of the membrane (concentration gradient)
more O2 & Na+ outside of cell membrane
more CO2 and K+ inside of cell membrane
Membrane can maintain a difference in charged ions between
inside & outside of membrane (electrical gradient or membrane potential)
Thus, substances move down their concentration gradient and
towards the oppositely charged area
Gradients Across
Membrane
Concentration gradient
Electrical gradient
Transport Across the Plasma Membrane
Substances cross membranes by a variety of processes:
mediated transport moves
materials with the help of a
transporter protein
nonmediated transport does
not use a transporter protein
active transport uses ATP to
drive substances against their
concentration gradients
passive transport moves substances down their concentration
gradient with only their kinetic energy
vesicular transport move materials across membranes in small
vesicles -- either by exocytosis or endocytosis
Principles of Diffusion
Random mixing of particles in a solution as a result of the
particle’s kinetic energy
more molecules move away from an area of high concentration
to an area of low concentration
the greater the difference in concentration between the 2
sides of the membrane, the faster the rate of diffusion
the higher the temperature, the faster the rate of diffusion
the larger the size of the diffusing substance, the slower
the rate of diffusion
increasing diffusion distance, slows rate of diffusion
When the molecules are evenly distributed, equilibrium has
been reached
Diffusion
Net diffusion from the higher dye concentration to the
region of lower dye
Equilibrium has been reached in the far right cylinder
Osmosis
Net movement of water through a selectively permeable
membrane from an area of high water concentration to an area of lower water
concentration
diffusion through lipid bilayer
aquaporins (transmembrane proteins) that function as water
channels
Only occurs if membrane is permeable to water but not to
certain solutes
Pure water on the left side & a membrane impermeable to
the solute found on the right side
Net movement of water is from left to right, until
hydrostatic pressure (osmotic pressure ) starts to push water back to the left
Affects of Tonicity on RBCs in Lab
Normally the osmotic pressure of the inside of the cell is
equal to the fluid outside the cell
cell volume remains constant (solution is isotonic)
Effects of fluids on RBCs in lab
water enters the cell faster than it leaves
water enters & leaves the cell in equal amounts
water leaves the cell
Effects of Tonicity on Cell Membranes
Isotonic solution
water concentration the same inside & outside of cell
results in no net movement of water across cell membrane
Hypotonic solution
higher concentration of water outside of cell results in
hemolysis
Hypertonic solution
lower concentration of water outside of cell causes
crenation
Diffusion Through the Lipid Bilayer
Important for absorption of nutrients -- excretion of wastes
Nonpolar, hydrophobic molecules
oxygen, carbon dioxide, nitrogen, fatty acids, steroids,
small alcohols, ammonia and fat-soluble vitamins (A, E, D and K)
Diffusion Through Membrane Channels
Each membrane channel specific for particular ion (K+, Cl-,
Na+ or Ca+2)
Slower than diffusion through membrane but still 1million K+
through a channel in one second
Channels may be open all the time or gated (closed randomly
or as ordered)
Facilitated Diffusion
Substance binds to specific transporter protein
Transporter protein conformational change moves substance
across cell membrane
Facilitated diffusion occurs down concentration gradient
only
if no concentration difference exists, no net movement
across membrane occurs
Rate of movement depends upon
steepness of concentration gradient
number of transporter proteins (transport maximum)
Facilitated Diffusion of Glucose
Glucose binds to transport
protein
Transport protein changes
shape
Glucose moves across cell
membrane (but only down
the concentration gradient)
Kinase enzyme reduces
glucose concentration inside
the cell by transforming
glucose into glucose-6-phosphate
Transporter proteins always bring glucose into cell
Active Transport
Movement of polar or charged substances against their
concentration gradient
energy-requiring process
energy from hydrolysis of ATP (primary active transport)
Na+, K+, H+, Ca+2, I- and Cl-, amino acids and
monosaccharides
Primary Active Transport
Transporter protein called a pump
works against concentration gradient
requires 40% of cellular ATP
Na+/K+ ATPase pump
most common example
all cells have 1000s of them
maintains low concentration of Na+
and a high concentration of K+ in the cytosol
operates continually
Maintenance of osmotic pressure across membrane
cells neither shrink nor swell due to osmosis & osmotic
pressure
sodium continually pumped out as if sodium could not enter
the cell (factor in osmotic pressure of extracellular fluid)
K+ inside the cell contributes to osmotic pressure of
cytosol
Digitalis
Slows the sodium pump, which lets more Na+ accumulate heart
muscle cells.
Less Na+ concentration gradient across the membrane
Na+/Ca+2 antiporters slow down so more Ca+2 remains inside
the cardiac cells
Strengthening the force of contraction
Balance between concentration of Na+ and Ca+2 in cytosol
& extracellular fluid is important
Vesicular Transport of Particles
Endocytosis = bringing something into cell
phagocytosis = cell eating by macrophages & WBCs
particle binds to receptor protein
whole bacteria or viruses are engulfed & later digested
pinocytosis = cell drinking
no receptor proteins
receptor-mediated endocytosis = selective input
mechanism by which HIV virus enters cells
Exocytosis = release something from cell
Vesicles form inside cell, fuse to cell membrane
Release their contents
digestive enzymes, hormones, neurotransmitters or waste
products
replace cell membrane lost by endocytosis
Cytosol = Intracellular fluid
55% of cell volume
75-90% water with other components
large organic
molecules (proteins, carbos & lipids)
suspended by electrical charges
small organic molecules (simple sugars) & ions
dissolved
Site of many important chemical reactions
production of ATP, synthesis of building blocks
Cell Organelles
Nonmembranous organelles lack membranes & are indirect
contact with cytoplasm
Membranous organelles surrounded by one or two lipid bilayer
membranes
Cytoskeleton
Network of protein filaments throughout the cytosol
Functions
cell support and shape
organization of chemical reactions
cell & organelle movement
Centrosome
Found near nucleus
Pericentriolar area
formation site for mitotic spindle and microtubules
Centrosome
2 centrioles(90 degrees to each other)
9 clusters of 3 microtubules (9+0 array)
role in formation of cilia & flagella
Cilia and Flagella
Structure
pairs of microtubules
(9+2 array)
covered by cell
membrane
basal body is
centriole
responsible for initiating
its assembly
Differences
cilia
short and multiple
flagella
longer and single
Ribosomes
Packages of Ribosomal RNA & protein
Free ribosomes are loose in cytosol
synthesize proteins found inside the cell
Membrane-bound ribosomes
attached to endoplasmic reticulum or nuclear membrane
synthesize proteins needed for plasma membrane or for export
Endoplasmic
Reticulum
Network of membranes forming flattened sacs or tubules
called cisterns
half of membranous surfaces within cytoplasm
Rough ER
continuous with nuclear envelope & covered with attached
ribosomes
synthesizes, processes & packages proteins for export
Smooth ER -- no attached ribosomes
synthesizes phospholipids, steroids and fats
detoxifies harmful substances (alcohol)
Smooth & Rough Endoplasmic Reticulum
Golgi Complex
3-20 flattened, curved
membranous sacs called cisterns
Convex side faces ER & concave side faces cell membrane
Processes & packages proteins produced by rough ER
Cystic Fibrosis
Deadly inherited disorder
Chloride ion pump protein is not properly secreted from the
golgi or rough ER
Result is an imbalance in the transport of fluid and ions
across the plasma membrane
buildup of thick mucus outside of certain cells
respiratory and digestive problems
Lysosomes
Membranous vesicles
formed in Golgi complex
filled with digestive enzymes
Functions
digest foreign substances
autophagy(autophagosome forms)
recycles own organelles
autolysis
lysosomal damage after death
Tay-Sachs Disorder
Affects children of eastern European-Ashkenazi descent
seizures, muscle rigidity, blind, demented and dead before
the age of 5
Genetic disorder caused by absence of single lysosomal
enzyme
enzyme normally breaks down glycolipid commonly found in
nerve cells
as glycolipid accumulates, nerve cells lose functionality
chromosome testing now available
Peroxisomes
Membranous vesicles
smaller than lysosomes
form by division of preexisting peroxisomes
contain enzymes that oxidize organic material
Function
part of normal metabolic breakdown of amino acids and fatty
acids
oxidizes toxic substances such as alcohol and formaldehyde
contains catalase which decomposes H2O2
Mitochondria
Double membrane organelle
central cavity known as matrix
inner membrane folds known as crista
surface area for chemical reactions of cellular respiration
Function
generation of ATP
powerhouse of cell
Mitochondria self-replicate
increases with need for ATP
circular DNA with 37 genes
only inherited from mother
Nucleus
Large organelle with double membrane nuclear envelope
outer membrane continuous with rough ER
perforated by water-filled nuclear pores (10X channel pore
size)
Nucleolus
spherical, dark bodies within the nucleus (no membrane)
site of ribosome assembly
Function of Nucleus
46 human DNA molecules or chromosomes
genes found on chromosomes
gene is directions for a specific protein
Non-dividing cells contain nuclear chromatin
loosely packed DNA
Dividing cells contain chromosomes
tightly packed DNA
it doubled (copied itself) before condensing
Normal Cell Division
Mitosis (somatic cell division)
one parent cell gives rise to 2 identical daughter cells
mitosis is nuclear division
cytokinesis is cytoplasmic division
occurs in billions of cells each day
needed for tissue repair and growth
Meiosis (reproductive cell division)
egg and sperm cell production
in testes and ovary only
The Cell Cycle in Somatic Cells
Process where cell duplicates its contents & divides in
two
23 homologous pairs of chromosomes must be duplicated
genes must be passed on correctly to the next generation of
cells
Nuclear division = mitosis
continuous process divided into 4 stages
prophase, metaphase, anaphase & telophase
Cytoplasmic division = cytokinesis
Interphase Stage of Cell Cycle
Doubling of DNA and centrosome
Phases of interphase stage -- G1, S, and G2
G1 = cytoplasmic increase (G0 if never divides again)
S = replication of
chromosomes
G2 = cytoplasmic growth
Replication of Chromosomes
Doubling of genetic material during interphase.
(S phase)
DNA molecules unzip
Mirror copy is formed along
each old strand.
Nitrogenous bases pick up complementary base
2 complete identical DNA molecules formed
Stages of Nuclear Division:Mitosis
Prophase
Metaphase
Anaphase
Telophase
Prophase
Chromatin condenses into visible chromosomes
pair of identical chromatids held together by a centromere
Nucleolus & nuclear envelope disappear
Each centrosome moves to opposite ends of cell
spindle is responsible for the separation of chromatids to
each new daughter cell
Metaphase
Chromatid pairs line up across the middle of cell at the
metaphase plate
Anaphase
Chromatids (daughter chromosomes) move toward opposite poles
of cell
movement is due to shortening of microtubules
Chromosomes appear V-shaped as they are dragged towards the
poles of the cell
pull is at centromere region
Telophase
Chromosomes stop moving & appear as dark, condensed
bundle
Chromosomes uncoil & revert to chromatin
Nucleoli and nuclear membrane reappear
Mitotic spindle breaks up
Cytokinesis
Division of cytoplasm and organelles
Begins in late anaphase with formation of cleavage furrow
Ends with 2 daughter cells in interphase
Control of Cell Destiny
Cell destiny is either to remain alive & functioning, to
grow & divide or to die
Homeostasis must maintain balance between cell
multiplication & cell death
Aging
Age alters the body’s ability to adapt to changes in the
environment
Theories to explain aging
cells have a limited number of divisions
glucose bonds irreversibly with proteins
free radical
theory---electrically charged molecules with an unpaired electron cause cell
damage
autoimmune responses due to changes in cell identity markers
Evidence of aging
damaged skin, hardened arteries, stiff joints
Cellular Diversity
100 trillion cells in the body -- 200 different types
Vary in size and shape related to their function
Cancer = out of control cell division
Hyperplasia =
increased number of cell divisions
benign tumor does not metatasize or spread
malignant---spreads due to cells that detach from tumor and
enter blood or lymph
Causes -- carcinogens, x-rays, viruses
every cell has genes that regulate growth & development
mutation in those genes due to radiation or chemical agents
causes excess production of growth factors
Carcinogenesis
multistep process that takes years and many different
mutations that need to occur