Chapter 4
Cell Structure and Function
Early Scientist
• Robert Hooke: first describe the cell
– Small compartments in cells from a cork
• Van Leeuwenhoek: developed the first
microscope
• Observed several living cells, including sperms
Cell Theory Scientist
• Schleiden- Botanist proposed that plants are
made up of cells
• Schwann- zoologist proposed that animals are
composed of cells.
• Virchow- concluded that cells come from
other cells
Cell Theory
1- All organisms are composed of one or more
cells
2- Smallest unit having the properties of life
3- The continuity of life arises directly from the
growth and division of single cells
Cell Size
• atoms --> DNA --> virus --> bacteria ->mitochondria--> Eukaryotic cells
• Cells must remain small in size due to the ratio of
surface area and volume
• As the cell increases in size, its surface area
becomes to small to support its internal structures.
Oxygen and other important substances cannot
diffuse fast enough. Cells that get too large, may
divide.
Structural Organization of Cells
• Plasma Membrane(“GATEKEEPER”) separates
the cell from the environment, permits the
flow of molecules across the membrane, and
contains receptors that can affect the cell’s
activities.
– Composed of lipid bilayer
– Very controlled
Cytoplasm
• It is everything between the plasma
membrane and nucleus
– This includes organelles, semifluid substance
(cytosol), and filaments (cytoskeleton)
Genetic Material
• 1. provides cellular "blueprint" that controls
the functions of the cell
2. In the form of DNA (Deoxyribonucleic acid)
3. DNA is universal for all cells, and all living
things - evidence of common ancestry
4. Chromatin is the complex of proteins and
DNA, it condenses into chromosomes before
cell division
Eukaryotic versus Prokaryotic
• Nucleus
• Membrane bound
organelles
• Examples: fungi, protist,
animal, and plants
• No Nucleus:
Chromosomes grouped
together known as a
nucleoid
• Smaller than eukaryotes
• Bacteria
The Nucleus
• --nuclear envelope contains pores for some things to
enter and exit
-- chromatin is DNA and proteins, when the cell begins
to divide, chromatin condenses and forms
chromosomes
--DNA remains in the nucleus, it sends instructions to
the cytoplasm via messenger RNA
--RNA directs the synthesis of proteins on ribosome in
the cytoplasm
--Nucleolus assembles ribosome within the nucleus,
ribosome contain the "tools" to construct proteins
Endoplasmic reticulum (ER)
• Transport System
(endoplasm means "within cytoplasm",
reticulum means "little net"
• --divides cell into compartments
--channels molecules through the cell's
interior, like a little highway
Smooth ER
• Smooth ER
• --mostly contains enzymes that unction in lipid
synthesis (such as hormones like estrogen and
testosterone)
Rough ER
• --has ribosomes which give it its "rough"
appearance
• --functions in protein synthesis
• --ER transports newly assembled proteins to
the Golgi Apparatus
Golgi Appratus
• Delivery System
• --flattened stacks of membranes
• --functions in collection, packaging and distribution of
molecules made in the cell and used elsewhere
• -- front end (cis) faces the ER, and the back end (trans)
faces the cell membrane
• --unprocessed proteins enter the Golgi apparatus, are
processed and exit near the cell membrane for export
• --the folded stacks are called cisternae
Jobs of Golgi Appratus
• 1. separates proteins according to their
destinations
2. modifies proteins (adds sugar and makes
glycoproteins)
3. packages materials into vesicles which are
exported outside the cell
Lysosome (Lip smacking good)
• --vesicles that are used to digest
• --contain high levels of degrading enzymes (to "lyse"
means to dissolve)
• --recycle old and worn out cell parts
• --"suicide sac“
• --digest other particles taken in by phagocytosis
• --this "food" is stored in food vacuoles, the
lysosomes fuse with the vacuoles and release
digestive enzymes
• --found in animal cells
Ribosomes
• --each is composed of two subunits, one large
and one small
• --mRNA is "read" by the ribosomes and amino
acids are assembled into proteins
• --ribosomes are manufactured by the
nuceolus inside the nucleus
•
•
•
•
Mitochondria
Powerhouse: cell furance
--contains its own DNA, support for Endosymbiosis Theory
--singular is "mitochondrion“
--2 membranes, one smooth outer membrane, and an
inner membrane folded into layers called cristae
• --Cristae has two compartments: the matrix and the
intermembrane space
• --mitochondria divide before cell division, they are not
synthesized like other cell parts
• --function to store energy for cell use. Energy is stored in
the form of ATP - adenosine triphosphate
Mitochondria
Chloroplast
•
•
•
•
--only found in plant cells
--has its own DNA, like mitochondrion
--functions to convert light energy to ATP
--consists of grana, closed compartments that
are stacked
• --thylakoids are the individual disk shaped
compartments that make up the grana
• --stroma is the fluid surrounded the thylakoids
Cytoskeleton
• Cell is not "just a bag in a bubble". Lots of
internal fibers = internal "skeleton". Not rigid
like bone; capable of being assembled, broken
down in minutes. Allows cell movement, cell
division, internal motion of compartments.
• Composed of microfilaments and
microtubules
Centrioles
• --usually occur in pairs arranged at right
angles
• --assemble microtubules which influence the
cell shape and movement - part of the
cytoskeleton
• --also function in cell division, mitosis
• --only found in animal cells
Vacuoles
• --in plants the vacuoles are large and
centralized, storage of water makes the cell
turgid
• --in animals, they store food, water and other
substances (much smaller)
How Do Cells Move?
• Motor Proteins:
– Through controlled assembly and disassemby of
their subunits, microtubules, and microfilaments
grow or diminish in length, thereby the structures
attached to them are thereby pushed or dragged
through the cytoplasm
– Parallel arrays of microfilaments or microtubules
actively slide past one another to bring about
contraction, as in muscle
– Microtubules or microfilaments shunt organelles
from one location to another as in cytoplasmic
streaming
Cilia, Flagella, and False Feet
• Flagella are quite long, not usually numerous,
and found on one-celled protistians, and animal
sperm cells
• Cilia are shorter and more numerous and can
provide locomotion for free-living cells may
move surrounding water and particles if the
cilated cells is anchored
• Pseudopods “false foot” temporary lobes that
project from the cell, used in locomotion and
food capture
Cell Surface Specialization
• Eukaryotic Cell Walls
– Cell walls are carbohydrate frameworks for
mechanical support in bacteria, protistans, fungi,
and plants
– In growing plant parts, bundles of cellulose strands
form the primary cell wall (pliable enough to allow
for enlargement)
– Secondary walls are formed inside the primary
walls
• 25% is composed of ligin
Matrixes between animal cells
• The matrix between animal cells includes cell
secretions and materials drawn from the
surroundings between cells.
• Example: Cartilage consists of scattered cells
– Collagen embedded in a “ground substance” of
modified polysaccharide
Cell Junctions
• In plants tiny channels called plasmodesmata
cross the adjacent primary walls and
connected the cytoplasm
Continue…
• Animal cells display three types of junction:
– Tight Junctions: occur between cells of epithelial
tissues in which cytoskeletal strands of one cells
fuse with strands of neighboring cells causing an
effective seals
– Adhering junctions are like spot welds at the
plasma membranes of two adjacent cells that need
to be held together during stretching as in the skin
and heart.
– Gap Junctions: are small, open channels that
directly link the cytoplasm of adjacent cells
Cell Communication
• Signals and receptors allow cells to change
their activities
– Example: cell division
– Hormones- stimulators of cell activity
Prokaryotic Cells
• “Before nucleus”
• Indicates existence of bacteria before
evolution of cells with a nucleus; bacterial
DNA is clustered in a distinct region of the
cytoplasm (nucleoid)
• Bacteria are some of the smallest and
simplest cell
– Bacterial flagella project from the membrane and
permit rapid movement
– Rigid cell wall supports cell and surrounds the
plasma membrane which regulates transport into
and out of the cell
– Ribosomes, protein assembly sites, are dispersed
throughout the cytoplasm