Cell Structure and Function
1
Microscopes
•
Anton Leeuwenhoek invented the microscope in
the late 1600’s, which first showed that all living
things are composed of cells. Also, he was the first
to see microorganisms.
2
Cell Structure
•
In 1655, the English scientist Robert Hooke coined
the term “cellulae” for the small box-like structures
he saw while examining a thin slice of cork under a
microscope.
3
Cell Theory
•
•
In 1838 – 1839, German scientists Schleiden and
Schwann, proposed the first 2 principles of the cell
theory:
• All organisms are composed of one or more
cells.
• Cells are the smallest living units of all living
organisms.
15 years later, the German physician Rudolf
Virchow proposed the third principle:
• Cells arise only by division of a previously
existing cell.
4
Basic Cell Structure
•
•
•
•
•
All cells have the following basic structure:
A thin, flexible plasma membrane surrounds the
entire cell that regulates the passage of materials
between the cell and its surrounding
The interior is filled with a semi-fluid material called
the cytoplasm.
At some point, all cells contain DNA, the heritable
material that directs the cell’s activities
Also inside some cells are specialized structures
called organelles.
5
Cell Theory
•
The cell is the lowest level of structure that is
capable of performing all the activities of life.
• Regulate its internal environment.
• Take in and use energy.
• Respond to its local environment.
• Develop and maintain its complex
organization.
• Divide to form new cells.
6
Cell Characteristics
•
Two major kinds of cells - prokaryotic cells and
eukaryotic cells - can be distinguished by their
structural organization.
•
•
•
•
Eukaryotic cells - Contain membrane-enclosed
organelles, including a DNA-containing nucleus
Prokaryotic cells - Lack such organelles
The cells of the microorganisms called bacteria
and archaea are prokaryotic.
All other forms of life have the more complex
eukaryotic cells
7
The Cell
Nucleus
(contains DNA)
Prokaryotic cell
Eukaryotic
cell
DNA
(no nucleus)
25,000 
Organelles
8
Multicellular Organisms
•
•
Some organisms consist of a single cells, others are
multicellular aggregates of specialized cells.
Multicellular Organisms exhibit three major structural
levels above the cell:
• Similar cells are grouped into tissues
• Several tissues coordinate to form organs
• Several organs form an organ system.
9
Generalized Eukaryotic Cell
10
Cell Size Limit
•
Most cells are relatively small because as size increases,
volume increases much more rapidly than surface area.
• longer diffusion time
• limit to the volume of cytoplasm that can be effectively
controlled by genes.
11
Cell Size Limit
•
A cell must exchange materials with its
environment. Cell volume determines the
amount of materials that must be
exchanged, while surface area limits how
fast exchange can occur. In other words, as
cells get larger the need for materials
increases faster than the ability to absorb
them.
12
Visualizing Cells
13
Prokaryotic Cells
•
Simplest organisms
• Cytoplasm is surrounded by plasma membrane and
encased in a rigid cell wall composed of peptidoglycan.
• No distinct interior compartments
• Some use flagellum for locomotion, threadlike structures
protruding from cell surface
14
Eukaryotic Cells
•
Characterized by compartmentalization by
an endomembrane system, and the
presence of membrane-bound organelles.
• central vacuole
• vesicles
• chromosomes
• cytoskeleton
• cell walls
15
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Infolding
of the
plasma
membrane
Plasma
membrane
DNA
Cell wall
Prokaryotic
cell
Prokaryotic
ancestor of
eukaryotic
cells
Endoplasmic
reticulum (ER)
Nuclear
envelope
Nucleus
Plasma
membrane
Eukaryotic
cell
16
Endosymbiosis
•
Endosymbiotic theory suggests engulfed
prokaryotes provided hosts with advantages
associated with specialized metabolic
activities.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Internal
membrane
system
Aerobic
bacterium
Mitochondrion
Ancestral eukaryotic cell
Eukaryotic
cell with
mitochondrion
17
Animal Cell
18
Plant Cell
19
Cell Membrane
Fluid Mosaic Model
Glycoprotein
Extracellular fluid
Carbohydrate
Cholesterol
Peripheral
protein
Cytoplasm
Extracellular
matrix protein
Glycolipid
Transmembrane
proteins
Filaments of
cytoskeleton
20
Nucleus
•
•
•
•
•
Repository for genetic material
Chromatin: DNA and proteins
Nucleolus: Chromatin and ribosomal
subunits - region of intensive ribosomal RNA
synthesis
Nuclear envelope: Surface of nucleus bound
by two phospholipid bilayer membranes Double membrane with pores
Nucleoplasm: semifluid medium inside the
nucleus
21
Nucleus
22
Chromosomes
•
DNA of eukaryotes is divided into linear
chromosomes.
• Exist as strands of chromatin, except
during cell division
• Histones associated packaging proteins
23
The Nucleus And The Nuclear Envelope
24
Nucleolus
•
•
•
Protein synthesis occurs at tiny
organelles called ribosomes.
Ribosomes are composed of a large
subunit and a small subunit.
Ribosomes can be found alone in the
cytoplasm, in groups called
polyribosomes, or attached to the
endoplasmic reticulum.
25
Ribosomes
•
Ribosomes are RNA-protein complexes
composed of two subunits that join and
attach to messenger RNA.
• site of protein synthesis
• assembled in nucleoli
26
Endomembrane System
•
Compartmentalizes cell, channeling passage
of molecules through cell’s interior.
• Endoplasmic reticulum
• Rough ER - studded with ribosomes
• Smooth ER - few ribosomes
27
Rough ER
•
•
Rough ER is especially abundant in cells that secrete proteins.
•
As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the
cisternal space through a pore formed by a protein complex in the ER membrane.
•
As it enters the cisternal space, the new protein folds into its native conformation.
•
Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is
attached.
•
Secretory proteins are packaged in transport vesicles that carry them to their next stage.
Rough ER is also a membrane factory.
•
Membrane-bound proteins are synthesized directly into the membrane.
•
Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.
•
As the ER membrane expands, membrane can be transferred as transport vesicles to other
components of the endomembrane system.
28
Smooth ER
•
•
•
•
•
The smooth ER is rich in enzymes and plays a role in a variety of metabolic processes.
Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.
These include the sex hormones of vertebrates and adrenal steroids.
In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as alcohol
and barbiturates.
Smooth ER stores calcium ions.
•
Muscle cells have a specialized smooth ER that pumps calcium ions from
the cytosol and stores them in its cisternal space.
•
When a nerve impulse stimulates a muscle cell, calcium ions rush from
the ER into the cytosol, triggering contraction.
29
The Golgi apparatus
•
The Golgi apparatus is the shipping and receiving center for cell
products.
• Many transport vesicles from the ER travel to the Golgi apparatus
for modification of their contents.
• The Golgi is a center of manufacturing, warehousing, sorting, and
shipping.
• The Golgi apparatus consists of flattened membranous sacs—
cisternae—looking like a stack of pita bread.
• The Golgi sorts and packages materials into transport vesicles.
30
Functions Of The Golgi Apparatus
Golgi
apparatus
cis face
(“receiving” side of
Golgi apparatus)
1 Vesicles move
2 Vesicles coalesce to
6 Vesicles also
form new cis Golgi cisternae
from ER to Golgi
transport certain
Cisternae
proteins back to ER
3 Cisternal
maturation:
Golgi cisternae
move in a cisto-trans
direction
5 Vesicles transport specific
proteins backward to newer
Golgi cisternae
0.1 0 µm
4 Vesicles form and
leave Golgi, carrying
specific proteins to
other locations or to
the plasma membrane for secretion
trans face
(“shipping” side of
Golgi apparatus)
TEM of Golgi apparatus
31
Membrane Bound Organelles
Nucleus
•
•
•
Lysosomes – vesicle
containing digestive
enzymes that break down
food/foreign particles
Vacuoles – food storage
and water regulation
Peroxisomes - contain
enzymes that catalyze the
removal of electrons and
associated hydrogen
atoms
1 µm
Lysosome
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
(a) Phagocytosis: lysosome digesting food
32
Energy Organelles
•
•
•
Mitochondria
• bounded by exterior and interior
membranes
• interior partitioned by cristae
Chloroplasts
• have enclosed internal compartments of
stacked grana, containing thylakoids
• found in photosynthetic organisms
Both organelles house energy in the form of
ATP
33
Mitochondria
•
•
•
•
•
Mitochondria are found in plant and animal cells.
Sites of cellular respiration, ATP synthesis
Bound by a double membrane surrounding fluid-filled matrix.
The inner membranes of mitochondria are cristae
The matrix contains enzymes that break down carbohydrates and
the cristae house protein complexes that produce ATP
34
Chloroplasts
•
•
•
A chloroplast is bounded by two membranes enclosing
a fluid-filled stroma that contains enzymes.
Membranes inside the stroma are organized into
thylakoids that house chlorophyll.
Chlorophyll absorbs solar energy and carbohydrates
are made in the stroma.
35
Cytoskeleton
•
•
The eukaryotic cytoskeleton is a network of filaments and
tubules that extends from the nucleus to the plasma
membrane that support cell shape and anchor organelles.
Protein fibers
• Actin filaments - cell movement
• Microtubules - centrioles
• Intermediate filaments
36
Centrioles
•
•
Centrioles are short
cylinders with a 9 + 0
pattern of microtubule
triplets.
Centrioles may be
involved in microtubule
formation and
disassembly during cell
division and in the
organization of cilia and
flagella.
37
Cilia and Flagella
•
•
•
Contain specialized arrangements of microtubules
Are locomotor appendages of some cells
Cilia and flagella share a common ultrastructure
Outer microtubule
doublet
Dynein arms
0.1 µm
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
Plasma
membrane
(b)
0.5 µm
(a)
0.1 µm
Triplet
(c)
Cross section of basal body
38
Cilia and Flagella
•
•
•
Cilia (small and numerous) and flagella (large and single) have a
9 + 2 pattern of microtubules and are involved in cell movement.
Cilia and flagella move when the microtubule doublets slide past
one another.
Each cilium and flagellum has a basal body at its base.
39
Cilia and Flagella
(a) Motion of flagella. A flagellum
usually undulates, its snakelike
motion driving a cell in the same
direction as the axis of the
flagellum. Propulsion of a human
sperm cell is an example of
flagellatelocomotion (LM).
Direction of swimming
1 µm
(b) Motion of cilia. Cilia have a backand-forth motion that moves the
cell in a direction perpendicular
to the axis of the cilium. A dense
nap of cilia, beating at a rate of
about 40 to 60 strokes a second,
covers this Colpidium, a
freshwater protozoan (SEM).
15 µm 40
Cell Junctions
•
Long-lasting or permanent connections between
adjacent cells, 3 types of cell junctions:
TIGHT JUNCTIONS
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continuous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
DESMOSOMES
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
Tight junctions
Intermediate
filaments
Desmosome
Gap
junctions
Space
between Plasma membranes
cells
of adjacent cells
1 µm
Extracellular
matrix
Gap junction
0.1 µm
GAP JUNCTIONS
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one cell to an adjacent cell. Gap junctions
consist of special membrane proteins that
surround a pore through which ions, sugars,
amino acids, and other small molecules may
pass. Gap junctions are necessary for communication between cells in many types of tissues,
including heart muscle and animal embryos.
41
Tight Junctions
•
Connect cells into sheets. Because these
junctions form a tight seal between cells, in
order to cross the sheet, substances must
pass through the cells, they cannot pass
between the cells.
Tight
junction
42
Anchoring Junctions
•
Attach the cytoskeleton of a cell to the matrix
surrounding the cell, or to the cytoskeleton of
an adjacent cell.
Plasma
membranes
Intracellular
attachment
proteins
Cell
1
Cell
2
Cytoskeletal
filament
Intercellular
space
Transmembrane
linking proteins
Extracellular
matrix
43
Communicating Junctions
•
Link the cytoplasms of 2 cells together,
permitting the controlled passage of small
molecules or ions between them.
Two adjacent connexons
form a gap junction
Connexon
Adjacent plasma
membranes
Intercellular space
44
Prokaryotes
Eukaryotes
Organisms
Monera (bacteria)
All other
organisms
Size
Very small
(1 – 5 μm)
Much larger
(10 – 100 μm)
Complexity
Relatively simple
Complex
Cell wall
Usually present
(contains
peptidoglycan)
Sometimes
present (lacks
peptidoglycan)
45
Prokaryotes
Eukaryotes
Plasma
membrane
Always present
Always present
Internal
membranes
May contain
infoldings of the
plasma membrane
but usually lack
internal
membranes
Complex system of
internal membranes
divides cell into
specialized
compartments
46
Prokaryotes
Eukaryotes
Membranebound
organelles
Absent
Present
Ribosomes
Smaller and free
in the cytoplasm
Larger and may
be bound to ER
Cytoskeleton
Absent
Present
Flagella
Solid flagellin;
rotate
Microtubules;
bend
47
Prokaryotes
Eukaryotes
Structure of
genetic
material
Single, naked,
circular DNA
molecule
Many linear chromosomes,
each made of 1 DNA
molecule joined with
protein
Location of
genetic
material
In an area of the
cytoplasm called the
nucleoid
Inside a membrane-bound
nucleus
48