Learning Objectives 2
Describe what a cell is and the two general types of cells.
Describe the structure and functions of cell membranes.
Describe several ways in which molecules move across membranes.
Describe how cells are connected and how they communicate with each other.
Describe nine important landmarks in eukaryotic cells.
What is a Cell?
cell: smallest unit of life that
can function independently
and perform all the
necessary functions of life,
including reproducing itself
4
Cell
A cell is a three-dimensional structure, like a fluid-
filled balloon, in which many of the essential chemical
reactions of life take place.
Nearly all cells contain DNA (deoxyribonucleic acid).
5
Cell 6
Although most cells are too small to see with the
naked eye, there are a few exceptions, including hens’
eggs from the supermarket, each of which is an
individual cell.
The ostrich egg, weighing about three pounds, is
the largest of all cells.
Anton Van Leeuwenhoek
first to microscopically
observe microorganisms
in water
first to see bacteria
8
Robert Hooke
discovered cell in 1655
examined (under a coarse, compound microscope) very thin slices of cork and saw a multitude of tiny pores that he remarked looked like the walled compartments a monk would live in
did not know the real structure or function of a cell
9
What Hooke saw… 10
Illustration from Robert Hooke's
Micrographia, in which he
proposed that living things were
composed of minute structures
called "cells."
Cell Theory
Matthias Schleiden
German botanist
Theodor Schwann
German physiologist
Rudolf Virchow
German pathologist
11
Cell Theory
All living things or organisms are made of cells and their products.
Cells are the basic building units of life.
New cells are created by old cells dividing into two.
Schleiden & Schwann
1839
Virchow
1858
12
Landmarks in Cell Biology
1595: Jansen (Holland) - 1st compound microscope (>1
lens)
1626: Redi - argue against spontaneous generation
1655: Hooke - “cells” in cork
1674: Leeuwenhoek - protozoans
1833: Brown - cell nucleus
1839: Cell Theory (S & S)
1840: von Roelliker - sperm/egg are also cells
1857: Kolliker - mitochondria
13
Landmarks in Cell Biology
1858: Virchow – cells from pre-existing cells
1869: Miescher – DNA
1879: Flemming – chromosome behavior during
mitosis
1898: Golgi – golgi apparatus
1939: 1st TEM
1953: Watson, Crick, Wilkins – DNA double helix
1965: 1st commercial SEM
14
Every cell on earth is either…
a eukaryotic cell (from the Greek for
“true nucleus”)
central control structure called a nucleus
(contains cell’s DNA)
organisms composed of eukaryotic cells
are called eukaryotes
16
…OR a prokaryote
no nucleus; its DNA simply resides in the middle of the cell
smaller, evolutionarily older, structurally more simple, unicelllular
metabolically diverse some fuel activities in presence/absence of
oxygen using almost any energy source on earth sulfur in deep sea hydrothermal vents hydrogen sun
17
Eukaryotes…
1.5 billion years after prokaryotes
not all eukaryotes are multicellular Protista: nearly all are single-celled
organisms visible only with a microscope
22
Which feature below is only found in eukaryotic cells?
1. Ribosomes
2. DNA
3. Cell membrane
4. Mitochondria
25
Endosymbiosis Theory
explains presence of two organelles in eukaryotes
chloroplasts (plants/algae)
mitochondria (plants/animals)
2 different types of prokaryotes close
partnerships w/each other
small prokaryotes capable of performing photosynthesis
may have come to live inside a larger “host” prokaryote
photosynthetic “boarder” may have made some of
energy from photosynthesis available to host
26
Endosymbiosis Theory
2 cells become more and more dependent on each other until neither cell could live without the other, and they became a single, more complex organism
eventually, photosynthetic prokaryote chloroplast
similarly, prokaryote unusually efficient @ converting food and oxygen easily usable energy might take up residence in another prokaryote mitochondrion
27
Which evidence below supports the theory that
mitochondria and chloroplasts were originally
bacteria?
1. Circular DNA is present in both organelles.
2. Both organelles are larger than other
organelles in the cell.
3. Both organelles are surrounded by a single
lipid bilayer.
4. All of the above.
28
Plasma Membranes
lipid bilayer - phospholipids head: glycerol linked to a molecule containing phosphorous polar - has an electrical charge (water is also a polar
molecule and for this reason, other polar molecules mix easily with water.)
hydrophilic (“water loving”) 2 legs are long chains of carbon and hydrogen atoms. no electrical charge non-polar molecules do not mix with water hydrophobic (“water fearing”)
hydrophilic head region mixes easily with water, whereas their hydrophobic tail region does not mix with water
31
Plasma Membranes
phospholipid bilayer: in a cell’s plasma membrane, 2 of these sheets of phospholipids are arranged so that the hydrophobic tails are all in contact with each other and the hydrophilic heads are in contact with the watery solution outside and inside the cell
32
What determines whether a protein resides on
the surface or extends through the bilayer?
all the amino acids that make up each
protein have side chains that differ from
one another chemically some side chains are hydrophilic
some are hydrophobic
as protein is assembled final shape, side
chains can cause parts of protein to be
attracted to hydrophobic or hydrophilic
regions
35
What determines whether a protein resides on
the surface or extends through the bilayer?
transmembrane protein both hydrophobic and hydrophilic regions
peripheral membrane proteins entirely hydrophilic structure
bind only to head regions of phospholipids
can be positioned on either outer or inner side
of membrane
36
Lipids that make up the cell membrane are
hydrophobic. Hydrophilic molecules, like glucose,
cannot cross this barrier. What major component of
the plasma membrane helps glucose get into and
out of the cell?
1. Phospholipids
2. Carbohydrates
3. Nucleic acids
4. Proteins
5. Fatty acids
38
The Plasma Membrane – “Fluid Mosaic”
In addition to proteins, two other molecules are found
in the plasma membrane:
Short, branched carbohydrate chains
serve as part of a membrane’s fingerprint along with
recognition proteins
Cholesterol
helps the membrane maintain its flexibility, even at low
temps
some cells ~25%, others (bacteria, plants) - none
39
Smallmouth Bass and Yellow Perch are considered warm
water fish, whereas certain species of char are found in the
arctic. Which fish do you think might have the most
cholesterol in the membranes of its cells?
1. Smallmouth Bass
2. Yellow Perch
3. Arctic char
4. Both 1 and 2 are correct.
40
Membrane surfaces have a
“fingerprint” that identifies the cell. 41
Cells with an
improper fingerprint
are recognized as
foreign and are
attacked by your
body’s defenses
(antibodies).
Which of the cell membrane functions below is
possible due to the presence of proteins in the plasma
membrane?
1. The cell membrane determines what molecules will
enter or leave the cell.
2. The cell membrane contains proteins that can bind to
messages sent from other cells and relay the message
to the interior of the cell.
3. The cell membrane contains proteins that act as a
fingerprint for distinguishing between self and foreign
cells.
4. Both 1 and 2 are correct.
5. All of the above. 42
Passive transport is the spontaneous
diffusion of molecules across a membrane. 44
Two kinds of passive transport
Diffusion
Osmosis
Diffusion and Concentration Gradients 45
Diffusion passive transport (no
energy needed) Solute particle is dissolved
in a gas or liquid (solvent) moves from an area of
high solute concentration to an area of lower concentration
molecules tend to move down their concentration gradient
Simple & Facilitated Diffusion 46
simple diffusion molecules s/a O2, CO2 (small, no charge) pass directly
through lipid bilayer of membrane w/o assistance
facilitated diffusion spontaneous diffusion across a plasma membrane requires a
transport protein
may be electrically charged repelled by hydrophobic
middle region of phospholipid bilayer
OR may be too big to squeeze through the membrane
In either case, there must be a concentration gradient
from one side of the membrane to the other
Direction of Osmosis 49
Direction is determined only by a difference in total
concentration of all molecules dissolved in water doesn’t matter what solutes they are
Which side of the membrane has more “dissolved
stuff?” Water will move in that direction.
For this reason, small increases in the salinity of
lakes can have disastrous consequences for
organisms living there, from fish to bacteria
An amoeba (a single-celled protist) is placed in a
solution. The volume of the cell increases (cell
swells). Which statement below is the correct
interpretation of this result?
1. The amoeba is in a hypertonic solution.
2. Water is moving into the amoeba.
3. The amoeba is in a isotonic solution.
4. Water is moving out of the amoeba.
50
In active transport, cells use energy to
move small molecules 51
Molecules can’t always move spontaneously
and effortlessly in/out of cells active transport: when their transport takes energy
necessary if molecules to be moved are very large
or if they are being moved against their
concentration gradient
In all active transport, membrane proteins act
like motorized revolving doors push molecules into cell regardless of concentration
of on either side of membrane
Active Transport 52
To help break down food into more digestible
bits, cells lining stomach create an acidic
environment by pumping large numbers of H+
ions into the stomach contents, against their
concentration gradient.
H+ pumping increases ability to digest food but
comes at a great energy cost (ATP) because
protons would not normally flow into a region
against concentration gradient.
A normal cell has a high concentration of potassium on the
inside and a low concentration of potassium on the outside.
Also, a normal cell has a low concentration of sodium on the
inside and a high concentration on the outside. Yet, the net
movement of potassium is into the cell and the net
movement of sodium is out of the cell. What type of
transport is involved?
1. Osmosis
2. Facilitated transport
3. Active transport
4. Simple diffusion
54
Endocytosis and exocytosis are used
for bulk transport of particles 55
Many molecules are just too big to get into a
cell by passive or active transport.
membrane protein is only so big
some cells in the immune system must ingest
(and destroy) entire bacterial cells that are
invading your body
Endocytosis: cells engulf large
molecules/bacteria with plasma membrane.
Exocytosis: opposite process; releases materials
from the cell
3 types of endocytosis 57
Phagocytosis – “cell eating”
Pinocytosis – “cell drinking”
Receptor-mediated endocytosis
Faulty cell membranes are a primary
cause of cardiovascular disease 60
Cholesterol often builds up on artery walls
reduces area available for blood flow
causes artery to harden
too much circulating LDL cholesterol can lead to
cardiovascular disease and death it’s good
to remove as much cholesterol from bloodstream
as quickly as possible
Individuals lucky enough to have large numbers of
LDL receptors on liver cell membranes have a
significantly lower risk of cardiovascular disease
Faulty cell membranes are a primary
cause of cardiovascular disease 61
Those who consume food laden with too much cholesterol
or who have the misfortune of inheriting genes that code
for faulty liver cell membranes that have only a few LDL
receptors, can have a heavy load of circulating blood
cholesterol.
This outcome can result in the early onset of
cardiovascular disease.
Familial hypercholesterolemia – no LDL receptors
cholesterol accumulates in the arteries so rapidly that
cardiovascular disease begins to develop even before
puberty and death from heart attacks can occur before
the age of 30
Connections between cells hold them in place
and enable them to communicate with each other 64
Involves numerous types of protein
and glycoprotein adhesion molecules
Tight Junctions 65
form continuous, water-tight seals around
cells and also anchor cells in place
particularly important in the small
intestine where digestion occurs
Desmosomes 66
are like spot welds or rivets that fasten cells
together into strong sheets
function like Velcro: they hold cells together but
are not water-tight
found in much of the tissue-lining cavities of
animal bodies
Gap Junctions 67
Pores surrounded by special proteins that form open channels
between two cells
large enough for salts, sugars, amino acids, and electrical
signals to pass through
too small for the passage of organelles or larger molecules
such as proteins and nucleic acids
Important mechanism for cell-to-cell communication
heart: electrical signal telling muscle cells to contract is
passed from cell to cell through gap junctions
also important in allowing a cell to recognize that it has
bumped up against another cell
chemicals flowing from one cell to the next can signal to the
body to stop producing cells of a particular type.
Your skin is water proof. What type of cell-cell
junction is most likely involved in creating a water-
tight barrier between the outside and inside of your
body?
1. Tight junctions
2. Desmosomes
3. Gap junctions
69
How can a lack of communication
between cells lead to cancer? 70
Contact inhibition – normal cells generally stop dividing
when they bump up against other cells
no contact inhibition with cancer cells
divide continuously, eventually forming a mass of cells
(tumor)
Cancer cells usually have reduced numbers of gap
junctions
harder for a cell to detect that it has bumped up
against other cells
cell is more likely to continue dividing tumorous mass
of dividing cells
Plasmodesmata 71
Plasmodesmata
most plants have anywhere from 1,000 to
100,000 microscopic tube-like channels
connect cells to each other, enabling
communication and transport between them
1. The Nucleus 73
The nucleus is the largest and most
prominent organelle in most eukaryotic
cells
The nucleus has two primary functions:
genetic control center
storehouse for hereditary information
1. The Nucleus 74
Nucleolus
area near the center of the nucleus where subunits of
the ribosomes are assembled
ribosomes are the site of protein assembly
ribosomes are found in both prokaryotes & eukaryotes
ribosomes are found on endoplasmic reticulum and/or
free-floating in cytoplasm
Chromatin
DNA & surrounding proteins
1. The Nucleus 75
Nuclear membrane (nuclear envelope)
surrounds nucleus, separates it from cytoplasm
2 bilayers on top of each other
covered with tiny pores
made from multiple proteins embedded
within the phospholipid membranes
spans both bilayers
enable large molecules to pass from nucleus
cytoplasm
2. Cytoplasm & Cytoskeleton 78
Cytosol:
“soup" within which all other cell organelles
reside
most cellular metabolism occurs here
mostly water but also full of proteins
control cell metabolism
Cytoplasm: collective term for cytosol plus
organelles suspended within cytosol
2. Cytoplasm & Cytoskeleton 79
Cilia
short projections that often occur in large numbers on
a single cell
move fluid along/past a cell
sweeps airways to our lungs clean of debris (such as
dust) from the air that we breathe.
Flagella
much longer than cilia
occur in prokaryotes, single-celled eukaryotes, many
algae/plants
animals – only sperm
3. Mitochondria 81
cell’s all-purpose energy converters present in virtually all eukaryotic cells mitochondria allow us to convert energy contained
within chemical bonds of dietary carbohydrates, fats, and proteins in food CO2, H2O, and ATP (energy source used by all cells to fuel all functions/activities; more in ch. 4)
energy conversion requires a significant amount of oxygen, organisms’ mitochondria consume most of the oxygen used by each cell
3. Mitochondria - Endosymbiosis 83
As learned earlier, mitochondria may very well have
existed as separate single-celled bacteria-like organisms
billions of years ago
similar to bacteria in size and shape
may have originated when symbiotic bacteria took up
permanent residence within other cells
strongest evidence for this is that mitochondria have
their own DNA
Mixed in among the approximately 3,000 proteins in
each mitochondrion, there are anywhere from two to ten
copies of its own little ring-shaped DNA. This DNA carries
the instructions for making 13 important mitochondrial
proteins necessary for metabolism and energy production.
4. Lysosomes 84
“Floating garbage disposal”
mitochondria wear out after ~10 days of intensive
activity
white blood cells constantly track down and consume
bacterial invaders, which they then have to dispose of
1000s of ongoing reactions of cellular metabolism
produce many waste macromolecules that cells must
digest and recycle
4. Lysosomes 86
Round, membrane-enclosed, acid-filled
~ 50 different digestive enzymes and a super-acidic,
corrosive fluid
if a lysosome were to burst, it would almost immediately
kill cell by rapidly digesting all component parts
Some enzymes break down lipids, others carbohydrates,
others proteins, and still others nucleic acids
Frequently a first step when a cell consumes and begins
digesting a particle of food or even an invading bacterium
Most component parts of molecules that are digested (s/a
amino acids) released back into the cell where they can be
re-used by cell as raw materials
4. Lysosomes – Tay-Sachs Disease 87
Result of malfunctioning lysosomes
individual inherits an inability to produce a critical
lipid-digesting enzyme
lipids continue to be sent to the lysosome where they
accumulate, undigested
lysosome swells until it bursts and digests the whole cell
or until it chokes the cell to death
process occurs in large numbers of cells and eventually
leads to death
Which cellular component would be responsible for the classic shape of a red blood cell?
1. Nucleus
2. Cytoskeleton
3. Lysosome
4. Mitochondria
88
Endomembrane System 89
production and modification of biological molecules
mass of interrelated membranes spreading out from
& surrounding nucleus
~20% of cell’s volume
forms chambers within cell that contain their own
mixture of chemicals
smooth endoplasmic reticulum
rough endoplasmic reticulum
golgi apparatus
5. Endoplasmic Reticulum (Rough) 91
Large series of interconnected, flattened sacs connected
directly to nuclear envelope
rough ER almost completely surrounds the nucleus (usually)
called “rough” because surface is studded w/ribosomes
cells w/high rates of protein production have large numbers of
ribosomes
more on ribosomes in Chapter 5
5. Endoplasmic Reticulum (Rough) 92
Primary function: fold and package proteins that will be
shipped elsewhere
example: poisonous frogs package their poison in rough ER
of the cells in which it is produced before transporting it to
the poison glands on their skin
Proteins that are used within cell are generally produced
on free-floating ribosomes in cytoplasm
5. Endoplasmic Reticulum (Smooth) 94
no ribosomes bound to it
connected to rough ER but farther from nucleus
w/slightly different appearance
looks like a collection of branched tubes
synthesizes lipids such as fatty acids, phospholipids,
and steroids
exactly which lipids are produced varies throughout the
organism and across plant and animal species
following their production these lipids are packaged in
transport vesicles and are then sent to either other
parts of the cell or to the plasma membrane for export
5. Endoplasmic Reticulum (Smooth) 95
ALSO – smooth ER helps protect us from the
following through the use of detoxifying enzymes alcohol, antibiotics, barbiturates, amphetamines
toxic metabolic waste products produced in our bodies
Smooth ER proliferates in cells that are exposed to
large amounts of particular drugs increases the capacity for detoxification, but as the
cells become more and more efficient at detoxification,
our tolerances to the very drugs the smooth ER is trying
to destroy can increase.
5. Endoplasmic Reticulum (Smooth) 96
Just as cells with high rates of protein production
have large numbers of ribosomes, we find huge
amounts of smooth ER in liver cells because they are
the primary sites of molecular detoxification
Other cells that are packed with ER include plasma
cells in the blood that produce immune system
proteins for export and pancreas cells that secrete
large amounts of digestive enzymes
5. Endoplasmic Reticulum (Smooth) 97
Chronic exposure to many drugs (from antibiotics to heroin)
can induce a proliferation of smooth ER, particularly in the
liver, and the smooth ER’s associated detoxification
enzymes. This proliferation in turn increases tolerance to the
drugs, necessitating higher doses. Moreover, the increased
detoxification capacity of the cells often enables the cell to
better detoxify other compounds, even if the person has
never been exposed to them. This increased detoxification
capacity can lead to problems. An individual who has been
exposed to large amounts of drugs, for example, may end
up responding less well to antibiotics.
Which cells below would have the smoothest endoplasmic reticulum?
1. Kidney cells
2. Kidney cells of an alcoholic
3. Liver cells
4. Liver cells of an alcoholic
98
6. Golgi Apparatus 99
Flattened stack of membranes (each of which is
called a Golgi body) that are not interconnected
Processes molecules synthesized within a cell—
primarily proteins and lipids—and packages those
that are destined for use elsewhere in the body
Also a site of carbohydrate synthesis, including the
complex polysaccharides found in many plasma
membranes
6. Golgi Apparatus 100
Transport vesicles bud from the endoplasmic reticulum move through cytoplasm Golgi apparatus vesicles fuse w/Golgi apparatus, dump contents
Golgi body.
~ 4 successive chambers , molecules get passed from one to the next enzymes make slight modifications to molecule (ex:
phosphates or sugars)
Molecules then bud off from Golgi apparatus in a vesicle, move cytoplasm
7. Cell Wall 103
Provides additional protection and support for plant cells
Made from polysaccharides, including cellulose
Animal cells do not have cell walls, but some archaea, bacteria, protists, & fungi do, although cell wall chemical composition differs from those found in plants
7. Cell Wall 104
Cell wall ~100 times thicker than plasma membrane, thus giving the plant tremendous structural strength some plants can grow several hundred feet tall also helps increase plants’ water resistance provides protection from insects, etc.
Cell wall does not completely seal off plant cells from one another porous, allowing water, solutes to reach plasma
membrane plasmodesmata: channels connecting adjacent cells,
allowing passage of molecules between cells
8. Vacuoles 105
Stands out in plant cells but also found in protists, fungi, and animals surrounded by a membrane filled w/fluid occupies 50 - 90% of plant cell’s interior space
Plays important role in five different areas of plant life nutrient storage waste management predator deterrence sexual reproduction physical support
8. Vacuoles 106
Nutrient storage: amino acids, sugars, ions, etc.
Waste management: vacuole retains waste products and
degrades them with digestive enzymes (like the lysosome in
animal cells)
Predator deterrence: poisonous, nasty-tasting materials can
accumulate inside vacuoles of some plants
Sexual reproduction: vacuole may contain pigments that give
some flowers their color, enabling them to attract birds and
insects that help the plant reproduce
8. Vacuoles 107
Physical support: high concentrations of dissolved substances
within the vacuole can cause water to rush into the cells via
osmosis
water increases fluid pressure inside vacuole
turgor pressure allows flowers, other plant parts to stand
upright
plant wilting is result of loss of turgor pressure
9. Chloroplast 108
found in all plants and
eukaryotic algae
site of photosynthesis
conversion of light energy
into chemical energy of
food molecules; oxygen is
a by-product (more in
chapter 4)
9. Chloroplast 109
Circular DNA (contains many of the genes essential for photosynthesis)
Dual outer membrane of chloroplast is consistent w/idea that a cell engulfed a photosynthetic bacterium, enveloping it with its plasma membrane in endocytosis.
Endosymbiosis theory chloroplasts might have originally been bacteria that
were engulfed by a predatory cell remained alive, became cell’s meal ticket, providing
food for the cell in exchange for protection