Cell Structures and Function
Structure determines form.
Form determines function.
Cells
Chapter 6
Overview: The Fundamental Units of Life
• All organisms are made of cells
• The cell is the simplest collection of matter
that can live
• Structure is correlated function
• All cells are related by their descent from
earlier cells
Fig. 6-1
Eukaryotic cells have internal membranes that
compartmentalize their functions
• The basic structural and functional unit of every
organism is one of two types of cells:
prokaryotic or eukaryotic
• Only organisms of the domains Bacteria and
Archaea consist of prokaryotic cells
• Protists, fungi, animals, and plants all
consist of eukaryotic cells
Comparing Prokaryotic and Eukaryotic Cells
• Basic features of all cells:
– Plasma membrane
– Semifluid substance called cytosol
(cytoplasm)
– DNA (chromosomes with genes)
– Ribosomes (make proteins)
• Prokaryotic cells are characterized by having
– No nucleus
– No membrane-bound organelles
– A single, circular chromosome
Fig. 6-6
Fimbriae
Nucleoid
Ribosomes
Plasma membrane
Bacterial
chromosome
Cell wall
Capsule
0.5 µm
(a) A typical
rod-shaped
bacterium
Flagella
(b) A thin section
through the
bacterium
Bacillus
coagulans (TEM)
• Eukaryotic cells are characterized by having
– DNA in a nucleus that is bounded by a
membranous nuclear envelope
– Membrane-bound organelles
• Eukaryotic cells are generally much larger than
prokaryotic cells
• The logistics of carrying out cellular metabolism
sets limits on the size of cells
• The surface area to volume ratio of a cell is
critical
• As the surface area increases by a factor of n2,
the volume increases by a factor of n3
• Small cells have a greater surface area relative
to volume
Surface area increases while
total volume remains constant
5
1
1
Total surface area
[Sum of the surface areas
(height  width) of all boxes
sides  number of boxes]
Total volume
[height  width  length 
number of boxes]
Surface-to-volume
(S-to-V) ratio
[surface area ÷ volume]
6
150
750
1
125
125
6
1.2
6
A Panoramic View of the Eukaryotic Cell
• A eukaryotic cell has internal membranes that
partition the cell into organelles
• Plant and animal cells have most of the same
organelles
Nuclear
envelope
ENDOPLASMIC RETICULUM (ER)
Flagellum
Rough ER
NUCLEUS
Nucleolus
Smooth ER
Chromatin
Centrosome
Plasma
membrane
CYTOSKELETON:
Microfilaments
Intermediate
filaments
Microtubules
Ribosomes
Microvilli
Golgi
apparatus
Peroxisome
Mitochondrion
Lysosome
NUCLEUS
Nuclear envelope
Nucleolus
Chromatin
Rough endoplasmic
reticulum
Smooth endoplasmic
reticulum
Ribosomes
Central vacuole
Golgi
apparatus
Microfilaments
Intermediate
filaments
Microtubules
Mitochondrion
Peroxisome
Chloroplast
Plasma
membrane
Cell wall
Plasmodesmata
Wall of adjacent cell
CYTOSKELETON
The endomembrane system regulates protein
traffic and performs metabolic functions in the
eukaryotic cell
• Components of the endomembrane system:
– Nuclear envelope
– Endoplasmic reticulum
– Golgi apparatus
– Lysosomes
– Vacuoles
– Plasma membrane
• These components are either continuous or
connected via transfer by vesicles
The cytoskeleton is a network of fibers that
organizes structures and activities in the eukaryote
• The cytoskeleton is a network of fibers
extending throughout the cytoplasm
• It organizes the cell’s structures and activities,
anchoring many organelles
• It is composed of three types of molecular
structures:
– Microtubules
– Microfilaments
– Intermediate filaments
The Extracellular Matrix (ECM) of Animal Cells
• Animal cells lack cell walls but are covered by an elaborate
extracellular matrix (ECM)
• The ECM is made up of glycoproteins such as collagen,
proteoglycans, and fibronectin
• ECM proteins bind to receptor proteins in the plasma
membrane called integrins
• Functions of the ECM:
– Support
– Adhesion
– Movement
– Regulation
Collagen
Proteoglycan
complex
EXTRACELLULAR FLUID
Fibronectin
Integrins
Plasma
membrane
Microfilaments
CYTOPLASM
MEMBRANE STRUCTURE AND
FUNCTION
Chapter 7
Plasma Membrane
• Bilayer of phospholipids
• Phospholipid
– The 2 tails are hydrophobic fatty acids
– The head is a hydrophilic phosphate
group
– The tails and head are connected by
glycerol
• Cell membranes also contain proteins and
carbohydrates
Figure 8.1 Artificial membranes (cross sections)
Figure 8.4 The fluidity of membranes
• Held together by weak hydrophobic
interactions
• Lipids and proteins can drift laterally within
membrane
• Cholesterol regulates membrane fluidity
(makes less fluid when it is warmer, more
fluid when it is colder) and is only found in
animal cells
• Unsaturated fatty acids have “kinks” at the
double bonds causing them to not pack
together closely – makes membrane more
fluid
• Membrane proteins
–Integral proteins - generally
transmembrane with hydrophobic
regions
–Peripheral proteins - generally
attached to membrane’s surface
• Cell-to-cell recognition
–“Markers”- glycoproteins,
glycolipids, & oligosaccharides
(short polysaccharides)
Figure 8.9 Some functions of membrane proteins
Figure 8.7 The structure of a transmembrane protein
Figure 8.6 The detailed structure of an animal cell’s plasma membrane, in cross
section
TRAFFIC ACROSS MEMBRANE
• Selective Permeability - regulates
the type and rate of molecular traffic
into and out of cell
–Nonpolar (hydrophobic) molecules
• dissolve in membrane
• cross easily
• ex. CO2 and O2
–Polar (hydrophilic) molecules
• Small, uncharged pass through
easily (ex. water)
• Larger, uncharged will not easily
pass (ex. glucose)
• All ions, even small ones, have
difficulty penetrating hydrophobic
region (ex. Na+, H+)
• Passive Transport
–Requires no energy
–Diffusion - the net movement of a
substance down a concentration
gradient (from high to low
concentration)
• Concentration gradient - regular,
graded concentration change over a
distance
• Net movement - the overall
movement away from center of
concentration
• Results from random molecular
motion, although net movement may
be directional
• Increases entropy (increases
disorder)
• Decreases free energy (-ΔG) so it is
a spontaneous process
• Rate of diffusion depends on
permeability of membrane
• Water diffuses freely across most
membranes
• Net movement stops at equilibrium
Figure 8.10 The diffusion of solutes across membranes
–Osmosis -diffusion of water
• Water diffuses down its
concentration gradient
• Direction is determined by total
solute concentration, regardless
of type or diversity of solutes in
solutions
Figure 8.11 Osmosis
• Hypertonic - greater solute
concentration
• Hypotonic - lower solute
concentration
• Isotonic - same solute concentration
• Water moves from hypotonic to
hypertonic areas.
Figure 8.12 The water balance of living cells
• Effects of osmosis
–In hypertonic solution: animal
cells shrivel, plant cell are
plasmolyzed (cell membrane
pulls away from cell wall)
–In a hypotonic solution: animal
cells are lysed (pop), plant cells
are turgid (firm)
–In isotonic: animal cells normal,
plant cells are flaccid (limp)
• Water potential (Ψ) is the
measure of the tendency for a
solution to take up water when
separated from pure water by a
selectively permeable membrane
• Water moves from high to low
water potential
• Water potential depends on
solute potential and pressure
potential (in cells)
–Ψ = Ψp + Ψs
»Ψp is pressure potential
»Ψs is solute potenial
–Water potential of pure water
is zero so Ψs of any solution
will always be negative
–Increasing solute, makes Ψs
more negative
–Increasing Ψs decreases
water potential.
–Pressure potential is influenced by water
movement into and out of plant cells.
–Pressure potential is the physical
pressure exerted on either side of a
membrane.
–Increasing Ψp increases Ψ
–A positive pressure potential means a
plant cell is turgid and a negative
pressure means it is flaccid.
–Osmotic pressure is equal to, but
opposite pressure potential.
–Ψp = 0 for things that are not cells (ex.
lakes)
• Osmoregulation - controlling
water balance
–Contractile vacuoles pump out
water (ex. Paramecium)
–Pumping out salts (ex. Bony fish)
–Facilitated Diffusion - diffusion
across membrane with the help of
transport proteins
• Passive because solutes move
down their concentration gradient
and no energy is required
• Facilitated Diffusion Animation
Figure 8.13 The contractile vacuole of Paramecium: an evolutionary adaptation for
osmoregulation
Figure 8.14 Two models for facilitated diffusion
• Transport proteins
–Specific for the solutes that they
transport
–Conformational change in protein allows
solute to be transferred to other side
–Gated proteins - channel opens in
response to electrical or chemical signal
–Ex. Aquaporins – channel proteins that
transport water via facilitated diffusion
»Problems with aquaporins associated
with glaucoma, cataracts, and kidney
diseases
• Active Transport - an energy
requiring process during which a
transport protein pumps a molecule
across the membrane, against its
concentration gradient (from low to
high concentration)
–Requires +ΔG
–Helps cell maintain steep ionic
gradients across cell membrane
–Uses ATP (energy)
–
Sodium Potassium Pump animation
Figure 8.15 The sodium-potassium pump: a specific case of active transport
–Ex. Sodium-potassium pump
• Transport protein has binding sites
for Na+ on interior side and sites for
K+ on exterior side
• Na+ binds to protein and stimulates
ATP to phosphorylize the protein
thereby changing its shape
• This changed shape allows Na+ to
be expelled outside of cell and
allows K+ to bind on outside of
protein
• K+ binding triggers release of
phosphate from protein
• Loss of phosphate restores
proteins original shape and expels
K+ into cell
• Na+ K+ pump translocates 3 Na+
ions out of cell for every 2 K+ ions
pumped into cell.
• Ion pumps generate voltage across
membrane
–Because anions and cations are
distributed unequally across cell
membranes, all cells have voltages
across their membranes (batteries)
–Membrane potential - voltage
across membrane
• ranges from -50 to -200 mv (the
inside of the cell is more negative
than the outside)
–That negative inside favors the
passive transport of cations into the
cell and anions out of cell.
–Electrochemical gradient - diffusion
gradient resulting from combined
effects of membrane potential and
concentration gradient
• Ions may not always diffuse down
their concentration gradient, but
always diffuse down their
electrochemical gradient
–Electrogenic pump - a transport
protein that generates voltage
across membrane
• Ex. Sodium potassium pump (in
animals): 3 Na+ move out and only
2 K+ move in (net charge of +1 on
outside of cell)
• Ex. Proton pump (in bacteria,
fungi, and plants): actively
transports H+ outside of cell
Figure 8.17 An electrogenic pump
Ion pump animation
• Cotransport - a process where a
single ATP-powered pump actively
transports one solute and indirectly
drives the transport of other solutes
against their concentration gradient
•
Cotransport animation
Figure 8.18 Cotransport
–One example in plants:
• An ATP driven proton pump sends
H+ outside of the cell
• Then H+ diffuses back into cell via
a specific transport protein
• As H+ diffuses, sucrose can ride
the proton’s “coattails” and move
into the cell (against its own
concentration gradient).
– One example in humans:
• If someone has severe diarrhea or
is badly dehydrated from running…
–Give person solution high in
glucose and salt
–Solutes transported to blood
causes water to move into blood
from colon (rehydration)
–Cotransport involves Na+ and
glucose so both needed
Figure 8.19 The three types of endocytosis in animal cells
• Exocytosis - process of exporting
macromolecules (ex. proteins and
polysaccharides) from a cell by fusion
of vesicles with a cell membrane
• Endocytosis - process of importing
macromolecules (ex. proteins and
polysaccharides) into a cell by forming
vesicles derived from the cell
membrane
•
•
Endo animation
More…
–Phagocytosis - endocytosis of
solid particles
–Pinocytosis - endocytosis of fluid
droplets
–Receptor-mediated endocytosis a ligand binds to a receptor site in a
coated pit and causes a vesicle to
form and ingest material. This is
more discriminating than
pinocytosis.
TRANSPORT IN PLANTS
CHAPTER 36
Figure 36.1 An overview of transport in whole plants
OSMOSIS AND WATER
POTENTIAL
• From high to low concentration =
from hypotonic to hypertonic
• Water potential Ψ – from high to
low water potential
–Solute concentration and
pressure
–Measured in megapascals (MPa)
• Plasmolyze – when a plant’s cell
membrane pulls away from cell wall
• Turgor pressure – when a plant cell
swells against its cell wall
• Turgid – plant cells that are “swelled”
due to turgor pressure (healthy)
• Plants have aquaporins (as do
animals).
Figure 36.4 Water relations of plant cells
Figure 36.5 A watered tomato plant regains its turgor
BULK FLOW
• Movement of fluid driven by
PRESSURE
• Occurs in both xylem (water carrying
cells/tube) and phloem (food carrying
cells/tube), but how the pressure
made is different
–Xylem pressure due to
transpiration
–Phloem pressure due to active
transport of sucrose
TRANSPORT OF XYLEM SAP
• Root pressure – accumulation of
minerals lowers water potential so
water moves into cortex (“center”
tissue) from high to low water potential
• Can only move water upward a few
meters
• Transpiration allows for xylem sap to
be pulled upward very far!
Figure 36.9 Guttation
• Water evaporates from mesophyll
cells in leaves through stomata
(holes in leaves)
• Remaining water film on cells forms
menisci
• The more concave the menisci
become the more transpiration
increases
• More evaporation = more
transpiration
Figure 36.10 The generation of transpirational pull in a leaf
• This increases the water tension (negative
pressure)
• Draws water upward and out of xylem through
mesophyll like water in a syringe or straw
• Cohesion of water due to H bonding enables
water to be pulled upward in narrow tracheids
and vessels without water breaking apart
• Adhesion of water to hydrophilic walls of xylem
help it to fight gravity
• This bulk-flow is solar-powered because sun is
responsible for transpiration
Figure 36.11 Ascent of water in a tree
TRANSPIRATION CONTROL
• Must leave stomata open at some
point to get CO2
• Guard cells open and close stomata
–Flaccid guard cells close stomata
–Turgid guard cells open stomata
Figure 36.12 An open (left) and closed (right) stoma of a spider plant (Chlorophytum
colosum) leaf
Figure 36.12x Stomata on the underside of a leaf
Figure 36.13a The mechanism of stomatal opening and closing
• Guard cells become turgid by pumping
out H+ (proton pump) which makes cell
negative inside
• Then K+ comes in (attracted to
negative inside)
• This decreases water potential inside
so water moves into cell via osmosis
• Guard cells become flaccid when
stomata close and K+ leaves causing
water to leave cell
Figure 36.13b The mechanism of stomatal opening and closing
• In general, stomata open during day
and close at night
–Blue-light receptor in guard cell
signals proton pumps to start
–Guard cells do photosynthesis and
some of that ATP runs proton
pumps
–Low CO2 can signal opening
–Internal clocks
ADAPTIONS TO REDUCE
TRANSPIRATION
• C4 and CAM plants are better
able to assimilate and use CO2
• Stomata in depressions on
underside of leaf
• Some dessert plants shed
leaves in driest months
TRANSLOCATION OF
PHLOEM SAP
• Most prevalent solute is sucrose,
but sap also includes amino acids,
hormones, and minerals
• Moves both from root to shoot and
from shoot to root
• General movement from sucrose
source to sucrose sink
• Proton pumps (via cotransport) accumulate sucrose in
phloem cells
• This reduces water potential and causes the uptake of
water. This hydrostatic pressure forces sap to flow
along tube.
• Pressure gradient reinforced by unloading of sucrose
at sink.
• So building pressure at sucrose source and reduced
pressure at sucrose sink causes sap to flow from the
source to the sink.
• Xylem recycles water from source to sink.
Figure 36.17 Pressure flow in a sieve tube
Figure 36.18 Tapping phloem sap with the help of an aphid
Cell to Cell Communication
Chapter 11
Local and Long Distance Signaling
• Local signaling – influences cells in the
nearby vicinity
– Paracrine signaling – secreting cell releases
a regulator in the extracellular matrix
– Synaptic signaling – nerve cell releases
neurotransmitter into a synapse (space
between nerves)
• Long Distance signaling – can influence cells
all over body
– Hormone signaling – endocrine cells
secrete hormones into blood where they can
reach any cell
Figure 11.3 Local and long-distance cell
communication in animals
Figure 11.4 Communication by direct contact
between cells
Cell junctions and cell-cell recognition
Three Stages of Cell Signaling
• Reception – target cell’s detection of a
chemical signal
– Signal is detected when it binds to a
receptor
– Ligand – a signal molecule that binds to a
receptor
• Transduction – binding of signal to receptor
stimulates a change in the receptor.
– The changed receptor triggers a step or
many steps that lead to the cell response.
• Response – end result – the cell response
Figure 11.5 Overview of cell signaling
Receptors
• Intracellular receptors – found in
cytoplasm or on nucleus so signal must
pass through cell membrane first
–Testosterone receptor only found in
certain cells
• An activated testosterone receptor
acts as a transcription factor
• Transcription factors - turn on or
turn off genes
Figure 11.10 Steroid hormone interacting with
an intracellular receptor
• Cell Membrane Receptors –
found in cell membrane
–Three major types
• G-linked receptor
• Receptor tyrosine kinase
• Ligand-gated ion channel
Figure 11.6 The structure of a G-protein-linked
receptor
Figure 11.7 The functioning of a G-proteinlinked receptor
G Protein Animation
Figure 11.8 The structure and function of a
tyrosine-kinase receptor
Figure 11.9 A ligand-gated ion-channel
receptor
Transduction
• Signal transduction pathways – a chain of
molecular interactions (like falling dominoes)
• Often involves protein phosphorylation and
dephosphorylation
– Protein kinases – enzymes that transfer
phosphate groups from ATP to a protein
– Phosphorylation of protein often changes
protein from inactive to active form
– Protein phosphatases – enzymes that
rapidly remove P (often turn off pathway)
Figure 11.11 A phosphorylation cascade
•
•
•
•
Second Messengers
Second messengers – small, nonprotein,
water-soluble molecules or ions that are part
of signaling pathways
Two most common second messengers
– Ca2+
– cAMP or cyclic AMP (Cyclic adenosine
monophosphate)
Adenylyl cyclase – an enzyme in cell
membranes that converts ATP into cAMP
Phosphodiesterase – an enzyme that
converts cAMP into AMP
Figure 11.12 Cyclic AMP
Figure 11.13 cAMP as a second messenger
Figure 11.16 Cytoplasmic response to a signal: the stimulation of
glycogen breakdown by epinephrine
G Protein Animation for Epinephrine Ligand
Fine Tuning of Response
• Signal Amplification – number of activated
products is much greater than in preceding
step
• Specificity – different cells have different
proteins so two cells can respond to same
signal in different manner
– Ex. epinephrine stimulates liver to break
down glycogen and heart cells to contract
faster
• Scaffolding Proteins – large relay proteins
to which several other relay proteins are
attached
Figure 11.18 The specificity of cell signaling
Figure 11.19 A scaffolding protein
Apoptosis
• Apoptosis - triggered by signals that
activate cell suicide
• In C. elegans (small worm), death genes
called ced. When activated they stimulate
death by activating proteases and
nucleases.
• Very similar genes found in other animals
including humans
• Apoptosis problems are associated with
cancer, Parkinson’s, and alzheimer’s