The Cell
AP Biology
1. All organisms are
made up of cells
2. The cell is the
basic unit of
structure and
function in all
organisms
3. All cells come
from pre-existing
cells
Underlying the diversity of life
is a striking unity
Activities of life
Most everything you think of a whole organism needing
to do, must be done at the cellular level…
reproduction
growth & development
energy utilization
response to the environment
homeostasis
Types of Cells
Eukaryote
animal cells
Prokaryote
bacteria cells
Eukaryote
plant cells
Organelles
• Specialized structures
mitochondria
• specialized functions
• cilia or flagella for locomotion
• Containers
• partition cell into compartments
• create different local environments
chloroplast
• separate pH, or concentration of materials
• distinct & incompatible functions
• lysosome & its digestive enzymes
• Membranes as sites for chemical reactions
Golgi
• unique combinations of lipids & proteins
• embedded enzymes & reaction centers
• chloroplasts & mitochondria
ER
• What jobs do cells have to do?
• building proteins
• proteins control
every cell function
• make energy
• for daily life
• for growth
• build more cells
• growth
• reproduction
• repair
Why study protein production?
proteins
cells
DNA
organism
Building Proteins
• Organelles involved
• nucleus
• ribosomes
• endoplasmic reticulum
(ER)
• Golgi apparatus
• vesicles
The Protein Assembly Line
nucleus
ribosome
ER
Golgi
apparatus
vesicles
TO:
endoplasmic
reticulum
nucleus
protein
on its way!
DNA
RNA
vesicle
TO:
TO:
TO:
vesicle
ribosomes
TO:
finished
protein
protein
Making Proteins
Golgi
apparatus
Putting it together…
nucleus
nuclear pore
cell
membrane
Making proteins
protein secreted
rough ER
ribosome
vesicle
proteins
smooth ER
transport
vesicle
cytoplasm
Golgi
apparatus
Cells gotta live!
• What jobs do cells have to do?
• make proteins
• proteins control
every cell function
• make energy
• for daily life
• for growth
• build more cells
• growth
• reproduction
• repair
ATP
Cells need power!
• Making energy
• take in food & digest it
• take in oxygen (O2)
• make ATP
• remove waste
ATP
1960 | 1974
Lysosomes
• Function
• little “stomach” of the cell
• digests macromolecules
• “clean up crew” of the cell
• cleans up broken down
organelles
• Structure
• vesicles of digestive enzymes
Where
old organelles
go to die!
Christian de Duve
only in animal cells
Lysosomal enzymes
• Lysosomal enzymes work best at pH 5
• organelle creates custom pH
• how?
• proteins in lysosomal membrane
pump H+ ions from the cytosol
into lysosome
• why?
• enzymes are very sensitive
to pH
• why?
• enzymes are proteins —
pH affects structure
• why is this an adaptation: digestive enzymes which function at pH different
from cytosol?
• digestive enzymes won’t function well if some leak into cytosol = don’t
want to digest yourself!
But sometimes cells need to die…
• Lysosomes can be used to kill cells when they are
supposed to be destroyed
• some cells have to die for proper
development in an organism
• apoptosis
• “auto-destruct” process
• lysosomes break open & kill cell
• ex: tadpole tail gets re-absorbed
when it turns into a frog
• ex: loss of webbing between your
fingers during fetal development
• ex: self-destruct of cancerous cell
syndactyly
Fetal development
6 weeks
15 weeks
When things go wrong…
• Diseases of lysosomes are often fatal
• digestive enzyme not working in lysosome
• picks up biomolecules, but can’t digest one
• lysosomes fill up with undigested material
• grow larger & larger until disrupts cell & organ function
• lysosomal storage diseases
• more than 40 known diseases
• example:
Tay-Sachs disease
build up undigested fat
in brain cells
From food to making Energy
• Cells must convert incoming energy to forms that they can use for
work
• mitochondria:
from glucose to ATP
• chloroplasts:
from sunlight to ATP & carbohydrates
• ATP = immediate energy
• carbohydrates = stored energy
ATP
ATP
+
Mitochondria & Chloroplasts
• Important to see the similarities
• transform energy
• generate ATP
• double membranes = 2 membranes
• semi-autonomous organelles
• move, change shape, divide
• internal ribosomes, DNA & enzymes
Lynn Margulis
U of M, Amherst
Membrane-bound Enzymes
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6 +
6O2
 6CO2 + 6H2O + ATP
Membrane-bound Enzymes
carbon + water + energy  glucose + oxygen
dioxide
6CO2 + 6H2O +light

energy
C6H12O6
+ 6O2
Mitochondria are everywhere!!
animal cells
plant cells
Cells gotta live!
• What jobs do cells have to do?
• building proteins
• proteins control
every cell function
• make energy
• for daily life
• for growth
• build more cells
• growth
• reproduction
• repair
Cytoskeleton
• Function
• structural support
• maintains shape of cell
• provides anchorage for organelles
• protein fibers
• microfilaments, intermediate
filaments, microtubules
• motility
• cell locomotion
• cilia, flagella, etc.
• regulation
• organizes structures
& activities of cell
Cytoskeleton
actin
 microtubule
 nuclei

Centrioles
• Cell division
• in animal cells, pair of centrioles
organize microtubules
• guide chromosomes in mitosis
Cell Size
2007-2008
Limits to cell size
• Lower limit
• smallest bacteria
• mycoplasmas
• 0.1 to 1.0 micron (µm = micrometer)
• most bacteria
• 1-10 microns
• Upper limit
• eukaryotic cells
• 10-100 microns
 micron = micrometer = 1/1,000,000 meter
 diameter of human hair = ~20 microns
What limits cell size?
• Surface to volume ratio
• as cell gets bigger its
volume increases
faster than its
surface area
• smaller objects
have greater
ratio of surface
area to volume
s:v
6:1
~1:1
6:1
Limits to cell size
• Metabolic requirements set upper limit
• in large cell, cannot move material in &
out of cell fast enough to support life
What process is this?
CH
aa
aa
NH3
aa
O2
CO2
NH3
O2
CH
aa
CO2
CHO
CO2
CHO
O2
NH3
CHO
O2
CH
aa
aa
CO2
CHO
O2
NH3
CH
aa
What’s the solution?
How to get bigger?
• Become multicellular (cell divides)
But what challenges do you have to solve now?
CO2
CO2
aa
aa
NH3
CO2
O2
CH
aa
CHO
CO2
NH3
CHO
CH
O2
NH3
aa
O2
aa
CH
NH3
CO2
NH3
CO2
NH3
CO2
CO2
NH3
O2
NH3
CO2
CO2
NH3
CHO
CO2
aa
Cell membrane
• Exchange structure
• plasma membrane functions as selective barrier
• allows passage of O2 & nutrients IN
• allows passage of products & wastes OUT
Phospholipid
Membrane
proteins
Cholesterol
Overview
• Cell membrane separates living cell from
nonliving surroundings
• thin barrier = 8nm thick
• Controls traffic in & out of the cell
• selectively permeable
• allows some substances to cross more easily
than others
• hydrophobic vs hydrophilic
• Made of phospholipids, proteins & other
macromolecules
Phospholipids
Phosphate
• Fatty acid tails
• hydrophobic
• Phosphate group head
• hydrophilic
Fatty acid
• Arranged as a bilayer
Structure and function relationship
Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
More than lipids…
• In 1972, S.J. Singer
& G. Nicolson
proposed that
membrane proteins
are inserted into
the phospholipid
bilayer
Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
Cytoplasm
Transmembrane
proteins
Filaments of
cytoskeleton
Membrane fat composition varies
• Fat composition affects flexibility
• membrane must be fluid & flexible
• about as fluid as thick salad oil
• % unsaturated fatty acids in phospholipids
• keep membrane less viscous
• cold-adapted organisms, like winter wheat
• increase % in autumn
• cholesterol in membrane
Membrane Proteins
• Proteins determine membrane’s specific functions
• cell membrane & organelle membranes each have
unique collections of proteins
• Membrane proteins:
• peripheral proteins
• loosely bound to surface of membrane
• cell surface identity marker (antigens)
• integral proteins
• penetrate lipid bilayer, usually across whole membrane
• transmembrane protein
• transport proteins
• channels, permeases (pumps)
Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
Classes of amino acids
What do these amino acids have in common?
polar & hydrophilic
Protein’s domains anchor molecule
• Within membrane
• nonpolar amino acids
• hydrophobic
• anchors protein
into membrane
• On outer surfaces of
membrane
• polar amino acids
• hydrophilic
• extend into
extracellular fluid &
into cytosol
Polar areas
of protein
Nonpolar areas of protein
Examples
Retinal
chromophore
H+
NH2
water channel
in bacteria
Porin monomer
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
H+
Cytoplasm
proton pump channel
in photosynthetic bacteria
function through
conformational change =
shape change
Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
identity marker
Cell adhesion
Cell surface
receptor
Attachment to the
cytoskeleton
Membrane carbohydrates
• Play a key role in cell-cell recognition
• ability of a cell to distinguish one
cell from another
• antigens
• important in organ &
tissue development
• basis for rejection of
foreign cells by
immune system
Movement across
the Cell
Membrane
Diffusion
• 2nd Law of Thermodynamics
governs biological systems
• universe tends towards disorder (entropy)
 Diffusion

movement from high  low concentration
Diffusion
• Move from HIGH to LOW concentration
• “passive transport”
• no energy needed
movement of water
diffusion
osmosis
Diffusion across cell membrane
• Cell membrane is the boundary between inside &
outside…
• separates cell from its environment
Can it be an impenetrable boundary?
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
NO!
OUT
IN
cell needs materials in & products or waste out
waste
ammonia
salts
CO2
H2O
products
Diffusion through phospholipid bilayer
• What molecules can get through directly?
• fats & other lipids
 What molecules can
lipid
inside cell
salt
NH3
NOT get through
directly?

polar molecules
 H 2O

outside cell
sugar
aa
H2O
ions
 salts, ammonia

large molecules
 starches, proteins
Channels through cell membrane
• Membrane becomes semi-permeable with protein
channels
• specific channels allow specific material across
cell membrane
inside cell
NH3
H2O
salt
aa
sugar
outside cell
Facilitated Diffusion
• Diffusion through protein channels
• channels move specific molecules across
cell membrane
facilitated = with help
• no energy needed
open channel = fast transport
high
low
“The Doorman”
Active Transport
• Cells may need to move molecules against
concentration gradient
• shape change transports solute from
one side of membrane to other
• protein “pump”
conformational change
low
• “costs” energy = ATP
ATP
high
“The Bouncer”
Active transport
• Many models & mechanisms
ATP
ATP
video
antiport
symport
Getting through cell membrane
• Passive Transport
• Simple diffusion
• diffusion of nonpolar, hydrophobic molecules
• lipids
• high  low concentration gradient
• Facilitated transport
• diffusion of polar, hydrophilic molecules
• through a protein channel
• high  low concentration gradient
• Active transport
• diffusion against concentration gradient
• low  high
• uses a protein pump
• requires ATP
ATP
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP
How about large molecules?
• Moving large molecules into & out of cell
• through vesicles & vacuoles
• endocytosis
• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
• exocytosis
exocytosis
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular signal
The Special Case of
Water
Movement of water
across
the cell membrane
2007-2008
Osmosis is diffusion of water
• Water is very important to life,
so we talk about water separately
• Diffusion of water from
high concentration of water to
low concentration of water
• across a
semi-permeable
membrane
Concentration of water
• Direction of osmosis is determined by comparing total
solute concentrations
• Hypertonic - more solute, less water
• Hypotonic - less solute, more water
• Isotonic - equal solute, equal water
water
hypotonic
hypertonic
net movement of water
Managing water balance
• Cell survival depends on balancing water uptake &
loss
freshwater
balanced
saltwater
Managing water balance
• Isotonic
• animal cell immersed in
mild salt solution
• example:
blood cells in blood plasma
• problem: none
• no net movement of water
• flows across membrane
equally, in both directions
• volume of cell is stable
balanced
Managing water balance
• Hypotonic
• a cell in fresh water
• example: Paramecium
• problem: gains water,
swells & can burst
• water continually enters
Paramecium cell
• solution: contractile vacuole
• pumps water out of cell
ATP
• ATP
• plant cells
• turgid
freshwater
Water regulation
• Contractile vacuole in Paramecium
Managing water balance
• Hypertonic
• a cell in salt water
• example: shellfish
• problem: lose water & die
• solution: take up water or
pump out salt
• plant cells
• plasmolysis = wilt
video
saltwater
1991 | 2003
Aquaporins
• Water moves rapidly into & out of cells
• evidence that there were water channels
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Osmosis…
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
Gradients (ATP Synthase)
Cotransport (Symport and Antiport)
Bozeman, Cell Communication
Bozeman, Signal Transduction Pathways
Cell Signals (DNA Learning Center)