Lecture #4

Cell Biology
 Cell Size
• Most cells cannot be seen with the naked eye:
• Smallest cell is a bacterium called Mycoplasma (~0.2µm)
• Largest cell is a bird egg
• Plant and animal cells are ~10 times larger than bacterial
cells

• Cell size has a number of restrictions:

• Cell must have enough space to house all of the
necessary DNA, protein, and internal structures that are
needed for survival and reproduction

• Cell size must also be able to facilitate the uptake of

needed nutrients and removal of wastes

• The surface area to volume ratio is important:

• Large cells have a smaller ratio than do small cells
• This means that larger cells have a smaller surface area
relative to their volume than do small cells
Prokaryotic Cells vs Eukaryotic Cells
• There are two primary groups of cells separated
based on size difference and complexity:
1. Prokaryotes: bacterial and archaea
2. Eukaryotes: protists, fungi, plants and animals

• Prokaryotic cells and eukaryotic cells have a number

of common features including: Prokaryotic Cell
• Both are bound by a plasma membrane
• Both contain chromosomes which are composed of DNA
• Both contain ribosomes which function to synthesize
proteins

• Prokaryotes and eukaryotes are different because

eukaryotes:
• Contain a membrane bound nucleus
• Contain membrane bound organelles that are
not found in prokaryotes Eukaryotic Cell
 Prokaryotic Cells
• Prokaryotic cells are very, very small
• 1-10µm in diameter
• This is one tenth (1/10) the diameter of a eukaryotic cell

• Prokaryotic cells do not have a nucleus

• The DNA of a prokaryotic cell is coiled into a specific region of the cell called
the nucleoid
• The nucleoid is not surrounded by a membrane
• Usually only one chromosome is present in prokaryotic cells and it is circular
not linear

• Ribosomes of prokaryotes are smaller and slightly

different than the ribosomes of eukaryotes
• The differences in ribosomal structure, make
bacterial ribosomes an attractive antibiotic target
• This allows the antibiotic to be given safely without
harming ribosomes of human cells (eukaryotic)
 Prokaryotic Cells
• Outside of the bacterial plasma membrane there is a cell wall
• The cell wall is a tough structure that is chemically complex
• The cell wall functions to protect the interior of the cell in addition to
maintaining the cell’s shape

• Some bacteria have an additional layer outside of the cell wall called a
capsule
• The capsule also functions in protection but also allows the bacterium to
adhere to surfaces such as catheters and human tissues (capsule is very sticky)

• Surface like projections called pili are also

present in some prokaryotes:
• Aid in bacterial attachment to surfaces and other bacterium

• Motile bacteria have flagella:

• May be one or more
• Longer than a pilus
• Facilitates motility
 Eukaryotic Cells
• Plants cells, animals cells, fungal cells and protists are all eukaryotic
• Eukaryotic cells are very different from prokaryotic cells

• Eukaryotic cells have a nucleus which is bound by a double membrane

• The nucleus contains a nucleolus which is absent in prokaryotes
• The nucleus has multiple chromosomes which are linear in shape

• Eukaryotic cells also have membrane bound

organelles present within the cytoplasm
• These organelles have specific cellular functions
 Eukaryotic Cells
• Organelles of a eukaryotic cells can be placed into four groups
according to function
• Group 1: function in manufacturing
• Nucleus, ribosomes, golgi apparatus and the endoplasmic reticulum

• Group 2: function to breakdown (hydrolyze) molecules

• Lysosomes, vacuoles, peroxisomes

• Group 3: function to process energy

• Mitochondria (found in all eukaryotic cells) and
chloroplasts (found only in plant cells)

• Group 4: function in structural support,

movement and communication among cells
• Cytoskeleton, plasma membrane and the cell wall
 Eukaryotic Cells
• Plant cells are very similar to animal cells with a few key differences:
• Plant cells do not have lysosomes or centrioles

• Plant cells have a rigid cell wall (animal cells do not): for protection and the
maintenance of cell shape

• Plasmodesmata are channels within plant cell wall to allow for the exchange
of materials

• Chloroplasts are only found in plant cells and they are the site of
photosynthesis

• A large central vacuole is also unique to plant

cells and functions in storage of water and
chemicals
 Plasma Membrane Structure
• The plasma membrane separates the interior of the cell
from the surrounding environment
• It also controls the passage of materials into and out of the cell

• The plasma membrane is incredibly thin

• Both the plasma membrane and the membranes of the

eukaryotic organelles are composed of phospholipids
• Consist of hydrophilic phosphate head and hydrophobic fatty
acid tails
• The phospholipid structure allows the formation of the
phospholipid bilayer with hydrophilic heads facing the outside
and hydrophobic tails facing the inside of the bilayer (shielded
from water)
 Plasma Membrane Structure
• Within the lipid bilayer there are membrane proteins
• The portion of the membrane proteins exposed to the exterior of the cell are
hydrophilic and the portions embedded within the middle of the membrane
are hydrophobic

• Any proteins that are not embedded by simply attached to the exterior of the
membrane are hydrophilic

• The membrane permeability to different molecules exploits polarity

differences:
• Non-polar molecules such as molecular oxygen (O2) and carbon dioxide (CO2)
are easily able to pass the hydrophobic interior of the membrane

• Some transmembrane proteins form channels

that allow hydrophilic molecules and ions to
pass through the membrane
 Nucleus
• Contains most of the cells DNA (mitochondria also has some DNA)
• Controls all cellular activity by directing protein synthesis
• The nucleus makes RNA (specifically messenger RNA, mRNA) according to directions
encoded in DNA
• The mRNA then exits the nucleus via pores in the membrane where it will be made into
protein in the cytoplasm

• Eukaryotic chromosomes are called chromatin

• Chromatin is DNA complexed with proteins (histones)
• The chromatin condenses into visible chromosomes during cell division

• The nucleus is bound by the nuclear envelope

• The nuclear envelope is a double membrane that contains protein
pores to allow certain materials to enter and exit the nucleus
• The nuclear envelope is continuous with the endoplasmic reticulum

• The nucleolus is a found within the nucleus

• The site of ribosomal synthesis
• Ribosomes are then exported through nuclear pores
 Ribosomes
• Ribosomes function to synthesize proteins
• All are composed of a large subunit (60S) and a small subunit (40S) which is
referred together as an 80S ribosome

• Cells that naturally produce a lot of protein will have more ribosomes
than cells that produce fewer proteins
• Cells that actively produce protein will also have a prominent nucleolus

• Ribosomes are found in two locations within the cell

• Free ribosomes are suspended in the cytoplasm, most proteins made by these
ribosomes function in the cytoplasm

• Bound ribosomes are attached to the endoplasmic reticulum, most proteins

produced by bound ribosomes will be either
inserted into membranes or secreted from the cell

• Cells that specialize in protein secretion will have

a high proportion of bound ribosomes
 The Endomembrane System
• The endomembrane system consists of a number of membranes some
of which are physically connected and some that are vesicular (vesicles
are sacs composed of membrane)
• Includes the nuclear membrane, the endoplasmic reticulum, the golgi
apparatus, lysosomes, vacuoles and the plasma membrane

• The nuclear membrane is connected to the endoplasmic reticulum

• The bulk of these organelles work together to synthesize, store and export
molecules
 The Endoplasmic Reticulum
• The endoplasmic reticulum has two different forms:
• The smooth endoplasmic reticulum: does not have any bound ribosomes
• The rough endoplasmic reticulum: has ribosomes bound to its surface giving it a rough
appearance

• The Smooth Endoplasmic Reticulum (smooth ER):

• Functions in lipid production: oils, phospholipids and steroids
• Ex) ovarian cells and testicular cells synthesize sex hormones which are
steroids, therefore these cells are rich in smooth endoplasmic reticulum

• Also function in detoxification of drugs

• As a person takes greater amounts of drugs, the amount of smooth ER and
associated detox enzymes increase leading to an increased drug tolerance
• This means that higher and higher doses of the drug are
necessary to achieve the same effect (ex) sedation
• Because the smooth ER does not respond to the drug
specifically this will also increase tolerance to other drugs

• Smooth ER also functions to store calcium ions

 The Endoplasmic Reticulum
• The Rough ER:
• The rough ER functions to produce more membrane phospholipids which
are then inserted into the membrane

• Bound ribosomes produce proteins that will be:

• Inserted into the membrane of the rough ER

• Transported to other organelles

• Or secreted by the cell

• Ex) insulin is a protein produced by bound
ribosomes and secreted from the cells in
the pancreas
 The Endoplasmic Reticulum
• Synthesis, modification and packaging of a secretory protein:
1. As the polypeptide is synthesized by a bound ribosome it is inserted into the
cavity of the rough ER via a protein pore
• Once inside the ER the protein folds into its final 3D shape

2. Short sugar chains are often linked to the polypeptide, producing a

glycoprotein

3. Once ready for export the molecule is packaged into a transport vesicle.

4. The transport vesicle now buds from the

ER membrane and the protein travels to the
golgi apparatus where it will be further processed
 The Golgi Apparatus
• Transport vesicles exit the ER and travel to the golgi apparatus

• The golgi apparatus consists of flattened sacs stacked atop one another
• The flattened stacks are not interconnected as they are in the ER

• The number of golgi stacks correlates with the quantity of protein that
the particular cell secretes

• The golgi apparatus functions to receive and modify proteins that have
arrived from the ER
 The Golgi Apparatus
• One side of the golgi apparatus serves as the receiving end and the
other side serves as the shipping end
• The receiving end serves to receive transport vesicles from the ER
• The shipping end gives rise to vesicles which bud off and travel to other sites
• Modification of proteins occurs as the protein travels from the receiving end
through to the shipping end
• Carbohydrate groups on glycoproteins may be modified in the golgi,
phosphate groups may also be added to proteins serving as identification tags
later on in protein transport

• Products of the golgi may be:

• Secreted from the cell
• Added to the plasma membrane or to
organelle membranes
• Ex) lysosome membranes
 Lysosomes
• Lysosomes are membrane bound sacs containing digestive enzymes
• Both the lysosomal enzymes and the lysosomal membranes are made by the
rough ER and modified by the golgi apparatus

• The digestive enzymes of the lysosome must be kept within a membrane

because they are acidic and would damage the other cell components

• Lysosomes may function by fusing with vacuoles containing food that

has been engulfed by the cell
• The lysosomal enzymes break down the food and release the nutrients back
into the cell where they can be metabolized to generate energy
 Lysosomes
• Lysosomes can also fuse with vacuoles containing engulfed bacteria,
killing organism

• The lysosome can also fuse with damaged organelles

• Breaking them down to release their components for cell use
• Allows the cell to continually renew itself

• Tae-Sachs Disease: results when a lipid digesting

enzyme is missing from lysosomes
• The lysosomes become engorged with fat
which accumulate in the brain
• Impairs brain function
• A child with Tae-Sachs disease will
die within a few years
 Vacuoles
• A vacuole is a multi-functional membrane bound sac

• The central vacuole of a plant cell functions to:

• Breakdown ingested material with its enclosed enzymes
• It also stores water and enlarges
• It stores necessary nutrients, other chemicals and waste products
• Some vacuoles in flowering plants also contain pigments which work to attract
pollinators
• They can also contain poisons that protect the plant from predation
Review of the Endomembrane System
 Mitochondria
• Mitochondria function to carry out cellular respiration in all Eukaryotic
cells
• Cellular respiration is the conversion of chemical energy from foods and sugars
into ATP (a molecule that stores energy and can be used to power cellular
processes)

• Mitochondria are enclosed by a double membrane:

• Both are phospholipid membranes containing embedded proteins that are
unique to the mitochondria

• There are two regions of space in the mitochondria:

1. The intermembrane space is the narrow region
between the inner and the outer membranes

2. The mitochondrial matrix is enclosed by the

inner mitochondrial membrane and houses
DNA, ribosomes and many of the enzymes
needed for cellular respiration
 Mitochondria
• The inner membrane has many folds and contains protein molecules
which function to synthesize ATP
• These folds are called cristae and act to increase the surface area of the
membrane

• The greater surface area allows the mitochondria to be better able to

synthesize ATP
 Chloroplasts
• Photosynthesis is the conversion of light energy from the sun into
chemical energy of sugar molecules
• Eukaryotes that are capable of photosynthesis use an organelle called the
chloroplast

• The chloroplast contains many membranes which partition the

organelle into compartments
• The chloroplast contains an inner and outer membrane separated by a thin
intermembrane space

• The area bound by the inner membrane is contains fluid called the stroma and
also contains enzymes, DNA and ribosomes
 Chloroplasts
• Thylakoids are the network of interconnected sacs located inside of the
chloroplast
• The space inside of the thylakoids is called the thylakoid space
• When the thylakoids are stacked atop one another they are called granum

• The grana (pleural of granum) function to absorb the sun’s energy

 Mitochondria and Chloroplasts
• Mitochondria and chloroplasts evolved by an endosymbiosis
• Both mitochondria and chloroplasts have a single circular molecule of DNA
much like bacteria
• The ribosomes in mitochondria and chloroplasts are more structurally similar to
bacterial ribosomes than to eukaryotic ribosomes
• The mitochondria and the chloroplasts both reproduce by splitting in half, very
similar to prokaryotic reproduction
• Both organelles are enclosed by a double membrane and the membrane is
highly similar to the plasma membrane of eukaryotes

• The endosymbiotic theory proposes that mitochondria and chloroplasts

were both originally prokaryotes that began living inside of larger cells
• Endosymbiont refers to a small cell living
within another cell (called the host cell)

• All eukaryotes have mitochondria but not all

eukaryotes have chloroplasts
 Internal Skeleton
• The cytoskeleton is a network of protein fibers that extend throughout
the cell cytoplasm
• The protein fibers provide structural support and assist in the movement of
organelles within the cell
• The cytoskeleton may also help regulate cell activity by transmitting signals
from the surface of the cell to the interior

• The cytoskeleton is composed of three cell types:

• Microfilaments
• Microtubules
• Intermediate filaments
 Internal Skeleton
1. Microfilaments:
• Solid rods composed of globular protein
called actin

• Arranged in a twisted double chain

• Form a three-dimensional network that

helps support the cell shape

• Microfilaments are also very important in

muscle contraction
 Internal Skeleton
2. Intermediate Filaments:
• Composed of various fibrous
proteins

• Have a ropelike structure

• Used to reinforce cell shape and to

anchor organelles in place

• Usually permanent cell fixtures

 Internal Skeleton
3. Microtubules:
• Straight, hollow tubes
• Composed of protein called tubulin
• Centrioles are composed of microtubules
which grow out of an area of the cell called
the centrosome
• The microtubules also shape and support
the cell
• Organelles can also be moved along
microtubules
• Ex) lysosomes can ‘walk’ along a
microtubule in order to reach a food
vacuole
• Microtubules are also very important in cell
division