Cell Structure for AS Level. Student and teacher can use this for their study

Cell Structure
• All organisms are made of cells
 Unicellular
Prokaryote or Eukaryote
 Multicellular
• The cell is the simplest collection of matter
that can live
 Cell is basic unit of life
 (contains: nucleus, cytoplasm, cell membrane)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cell Size
10 m
1 m
0.1 m
1 cm
1 mm
100 µm
10 µm
1 µm
100 nm
10 nm
1 nm
0.1 nm Atoms
Small molecules
Lipids
Proteins
Ribosomes
Viruses
Smallest bacteria
Mitochondrion
Nucleus
Most bacteria
Most plant and
animal cells
Frog egg
Chicken egg
Length of some
nerve and
muscle cells
Human height
Unaided
eye
Light
microscope
Electron
microscope
1 mm = …….. µm
1 µm = …….. nm

Cell Magnification
• Magnification –
how many types
larger the image is
than the actual
size of the
specimen
• Resolution – the
ability to
distinguish
between two close
together objects

Light microscopes
Light (or optical) microscopes use lenses to project a magnified
image of an object onto the eye.
Light microscopes are limited to a magnification of 1500× by their
resolving power (resolution). This is a measure of their ability to
distinguish between two separate points. A light microscope
cannot resolve two points that are closer than half a wavelength
of visible light (250 nm).
Magnification is a measure of how many times bigger the image is
than the object:
size of image
actual size of the object
magnification =

Calculations of actual size from magnification given
© Pearson Education Ltd 2008
This document may have been altered from the original
Week 1
Please note that magnification
sizes are subject to variation on
different screens

An amoeba seen under a light microscope; (a) with a x10 objective
(b) with a x40 objective
Week 1
different screens

Eyepiece graticule and stage micrometer at (a) x40 magnification and (b) x100
magnification
Week 1
different screens

Magnification
4.55μm
Photomicrographs often have magnification bars to allow
calculation of the actual size of specimens.

Question 1 Paramecium caudatum
x600

x600
Measured length = 142mm
142 ÷ 600 = 0.237mm
0.237mm = 237μm
Question 1 Paramecium caudatum

Mean measured length of the four
largest chloroplasts = 39.25mm
39.25 ÷ 9000 = 0.0044mm
0.0044mm = 4.4μm
x9000
Question 2 chloroplasts

Question 3 a bacterium
Measured length = 128mm
128 ÷ 0.002mm = magnification
Magnification = x64000

Question 4 seven week human embryo

Measure the actual length of the
scale bar and divide by the
length it represents
Magnification = 25 ÷ 10 = x2.5
Question 4 seven week human embryo

Question 5 head of a fruit fly

Measure the actual length of the
scale bar and divide by the
length it represents
Magnification = 12.5 ÷ 0.2 = x62.5
Question 5 head of a fruit fly

(a) Measure the actual length of
the scale bar and divide by
the length it represents
Magnification = 25 ÷ 0.02 = x1250
(b) 47mm
(c) 47 ÷ 1250 = 0.0376mm
0.0376mm = 37.6μm
Question 6 pollen grain

Question 7 red blood cells in an arteriole

Measured length of scale bar = 30mm
Diameter = 25mm [approx]
Actual diameter = 25 ÷ 3000 = 0.0083mm
0.0083mm = 8.3μm
Question 7 red blood cells in an arteriole

Measured width = 34mm
Actual width = 34 ÷ 15000 = 0.0023mm
0.0023mm = 2.3μm
Question 8 a mitochondrion

Question 9 bacteriophage [a type of virus]

Measured length of phage = 29mm
Magnification = 29 ÷ 0.0002 = 145000
Magnification = 1.45 x 105
Question 9 bacteriophage [a type of virus]

Question 10 potato cells
starch grains

Mean diameter of the cells = 38mm [approx]
Diameter of the cells = 38 ÷ 240 = 0.158mm
0.158mm = 158μm
Question 10 potato cells

Microscopy
• In a light microscope (LM), visible light
passes through a specimen and then through
glass lenses, which magnify the image
• LMs can magnify effectively to about 1,000
times the size of the actual specimen
• Most subcellular structures, including
organelles (membrane-enclosed
compartments), are too small to be resolved by
an LM

• LMs can magnify effectively to about 1,000
times the size of the actual specimen
• Various techniques enhance contrast and
enable cell components to be stained or
labeled
• Most subcellular structures, including
organelles (membrane-enclosed
compartments), are too small to be resolved by
an LM

• Two basic types of electron microscopes
(EMs) are used to study subcellular structures
• Scanning electron microscopes (SEMs)
focus a beam of electrons onto the surface of a
specimen, providing images that look 3-D
• Transmission electron microscopes (TEMs)
focus a beam of electrons through a specimen
• TEMs are used mainly to study the internal
structure of cells

Fig. 6-4
(a) Scanning electron
microscopy (SEM)
TECHNIQUE RESULTS
(b) Transmission electron
microscopy (TEM)
Cilia
Longitudinal
section of
cilium
Cross section
of cilium
1 µm
1 µm

Cell Fractionation
• Cell fractionation takes cells apart and
separates the major organelles from one
another
• Ultracentrifuges fractionate cells into their
component parts
• Cell fractionation enables scientists to
determine the functions of organelles
• Biochemistry and cytology help correlate cell
function with structure

Fig. 6-5
Homogenization
TECHNIQUE
Homogenate
Tissue
cells
1,000 g
(1,000 times the
force of gravity)
10 min Differential centrifugation
Supernatant poured
into next tube
20,000 g
20 min
80,000 g
60 min
Pellet rich in
nuclei and
cellular debris
Pellet rich in
mitochondria
(and chloro-
plasts if cells
are from a plant)
Pellet rich in
“microsomes”
(pieces of plasma
membranes and
cells’ internal
membranes)
150,000 g
3 hr
Pellet rich in
ribosomes

Comparing Prokaryotic and Eukaryotic Cells
• Basic features of all cells:
– Plasma membrane
– Semifluid substance called cytosol
– Chromosomes (carry genes)
– Ribosomes (make proteins)

• Prokaryotic cells are characterized by having
– No nucleus
– DNA in an unbound region called the nucleoid
– No membrane-bound organelles
– Cytoplasm bound by the plasma membrane

Fig. 6-6
Fimbriae
Nucleoid
Ribosomes
Plasma membrane
Cell wall
Capsule
Flagella
Bacterial
chromosome
(a) A typical
rod-shaped
bacterium
(b) A thin section
through the
bacterium
Bacillus
coagulans (TEM)
0.5 µm

• Eukaryotic cells are characterized by having
– DNA in a nucleus that is bounded by a
membranous nuclear envelope
– Membrane-bound organelles
– Cytoplasm in the region between the plasma
membrane and nucleus
• Eukaryotic cells are generally much larger than
prokaryotic cells

• The logistics of carrying out cellular metabolism
sets limits on the size of cells
• The surface area to volume ratio of a cell is
critical
• As the surface area increases by a factor of n2,
the volume increases by a factor of n3
• Small cells have a greater surface area relative
to volume

Fig. 6-8
Surface area increases while
total volume remains constant
5
1
1
6 150 750
125 125
1
6 6
1.2
Total surface area
[Sum of the surface areas
(height  width) of all boxes
sides  number of boxes]
Total volume
[height  width  length 
number of boxes]
Surface-to-volume
(S-to-V) ratio
[surface area ÷ volume]

A Panoramic View of the Eukaryotic Cell
• A eukaryotic cell has internal membranes that
partition the cell into organelles
• Plant and animal cells have most of the same
organelles
BioFlix: Tour Of An Animal Cell
BioFlix: Tour Of A Plant Cell

Fig. 6-9a
ENDOPLASMIC RETICULUM (ER)
Smooth ER
Rough ER
Flagellum
Centrosome
CYTOSKELETON:
Microfilaments
Intermediate
filaments
Microtubules
Microvilli
Peroxisome
Mitochondrion
Lysosome
Golgi
apparatus
Ribosomes
Plasma
membrane
Nuclear
envelope
Nucleolus
Chromatin
NUCLEUS

Fig. 6-9b
NUCLEUS
Nuclear envelope
Nucleolus
Chromatin
Rough endoplasmic
reticulum
Smooth endoplasmic
reticulum
Ribosomes
Central vacuole
Microfilaments
Intermediate
filaments
Microtubules
CYTO-
SKELETON
Chloroplast
Plasmodesmata
Wall of adjacent cell
Cell wall
Plasma
membrane
Peroxisome
Mitochondrion
Golgi
apparatus

Concept 6.3: The eukaryotic cell’s genetic
instructions are housed in the nucleus and carried
out by the ribosomes
• The nucleus contains most of the DNA in a
eukaryotic cell
• Ribosomes use the information from the DNA
to make proteins

The Nucleus: Information Central
• The nucleus contains most of the cell’s genes
and is usually the most conspicuous organelle
• The nuclear envelope encloses the nucleus,
separating it from the cytoplasm
• The nuclear membrane is a double membrane;
each membrane consists of a lipid bilayer

Fig. 6-10
Nucleolus
Nucleus
Rough ER
Nuclear lamina (TEM)
Close-up of nuclear
envelope
1 µm
1 µm
0.25 µm
Ribosome
Pore
complex
Nuclear pore
Outer membrane
Inner membrane
Nuclear envelope:
Chromatin
Surface of
nuclear envelope
Pore complexes (TEM)

• Pores regulate the entry and exit of molecules
from the nucleus
• The shape of the nucleus is maintained by the
nuclear lamina, which is composed of protein

• In the nucleus, DNA and proteins form genetic
material called chromatin
• Chromatin condenses to form discrete
chromosomes
• The nucleolus is located within the nucleus
and is the site of ribosomal RNA (rRNA)
synthesis

Ribosomes: Protein Factories
• Ribosomes are particles made of ribosomal
RNA and protein
• Ribosomes carry out protein synthesis in two
locations:
– In the cytosol (free ribosomes)
– On the outside of the endoplasmic reticulum or
the nuclear envelope (bound ribosomes)

Fig. 6-11
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large
subunit
Small
subunit
Diagram of a ribosome
TEM showing ER and ribosomes
0.5 µm

Concept 6.4: The endomembrane system regulates
protein traffic and performs metabolic functions in
the cell
• Components of the endomembrane system:
– Nuclear envelope
– Endoplasmic reticulum
– Golgi apparatus
– Lysosomes
– Vacuoles
– Plasma membrane
• These components are either continuous or
connected via transfer by vesicles

The Endoplasmic Reticulum: Biosynthetic Factory
• The endoplasmic reticulum (ER) accounts for
more than half of the total membrane in many
eukaryotic cells
• The ER membrane is continuous with the
nuclear envelope
• There are two distinct regions of ER:
– Smooth ER, which lacks ribosomes
– Rough ER, with ribosomes studding its
surface

Fig. 6-12
Smooth ER
Rough ER Nuclear
envelope
Transitional ER
Rough ER
Smooth ER
Transport vesicle
Ribosomes
Cisternae
ER lumen
200 nm

Functions of Smooth ER
• The smooth ER
– Synthesizes lipids
– Metabolizes carbohydrates
– Detoxifies poison
– Stores calcium

Functions of Rough ER
• The rough ER
– Has bound ribosomes, which secrete
glycoproteins (proteins covalently bonded to
carbohydrates)
– Distributes transport vesicles, proteins
surrounded by membranes
– Is a membrane factory for the cell

• The Golgi apparatus consists of flattened
membranous sacs called cisternae
• Functions of the Golgi apparatus:
– Modifies products of the ER
– Manufactures certain macromolecules
– Sorts and packages materials into transport
vesicles
The Golgi Apparatus: Shipping and
Receiving Center

Fig. 6-13
cis face
(“receiving” side of
Golgi apparatus) Cisternae
trans face
(“shipping” side of
Golgi apparatus)
TEM of Golgi apparatus
0.1 µm

Lysosomes: Digestive Compartments
• A lysosome is a membranous sac of hydrolytic
enzymes that can digest macromolecules
• Lysosomal enzymes can hydrolyze proteins,
fats, polysaccharides, and nucleic acids
Animation: Lysosome Formation

• Some types of cell can engulf another cell by
phagocytosis; this forms a food vacuole
• A lysosome fuses with the food vacuole and
digests the molecules
• Lysosomes also use enzymes to recycle the
cell’s own organelles and macromolecules, a
process called autophagy

Fig. 6-14
Nucleus 1 µm
Lysosome
Digestive
enzymes
Lysosome
Plasma
membrane
Food vacuole
(a) Phagocytosis
Digestion
(b) Autophagy
Peroxisome
Vesicle
Lysosome
Mitochondrion
Peroxisome
fragment
Mitochondrion
fragment
Vesicle containing
two damaged organelles
1 µm
Digestion

Fig. 6-14a
Nucleus 1 µm
Lysosome
Lysosome
Digestive
enzymes
Plasma
membrane
Food vacuole
Digestion
(a) Phagocytosis

Fig. 6-14b
Vesicle containing
two damaged organelles
Mitochondrion
fragment
Peroxisome
fragment
Peroxisome
Lysosome
Digestion
Mitochondrion
Vesicle
(b) Autophagy
1 µm

Vacuoles: Diverse Maintenance Compartments
• A plant cell or fungal cell may have one or
several vacuoles

• Food vacuoles are formed by phagocytosis
• Contractile vacuoles, found in many
freshwater protists, pump excess water out of
cells
• Central vacuoles, found in many mature plant
cells, hold organic compounds and water
Video: Paramecium Vacuole

Fig. 6-15
Central vacuole
Cytosol
Central
vacuole
Nucleus
Cell wall
Chloroplast
5 µm

The Endomembrane System: A Review
• The endomembrane system is a complex and
dynamic player in the cell’s compartmental
organization

Fig. 6-16-1
Smooth ER
Nucleus
Rough ER
Plasma
membrane

Fig. 6-16-2
Smooth ER
Nucleus
Rough ER
Plasma
membrane
cis Golgi
trans Golgi

Fig. 6-16-3
Smooth ER
Nucleus
Rough ER
Plasma
membrane
cis Golgi
trans Golgi

Concept 6.5: Mitochondria and chloroplasts
change energy from one form to another
• Mitochondria are the sites of cellular
respiration, a metabolic process that generates
ATP
• Chloroplasts, found in plants and algae, are
the sites of photosynthesis
• Peroxisomes are oxidative organelles

• Mitochondria and chloroplasts
– Are not part of the endomembrane system
– Have a double membrane
– Have proteins made by free ribosomes
– Contain their own DNA

Mitochondria: Chemical Energy Conversion
• Mitochondria are in nearly all eukaryotic cells
• They have a smooth outer membrane and an
inner membrane folded into cristae
• The inner membrane creates two
compartments: intermembrane space and
mitochondrial matrix
• Some metabolic steps of cellular respiration
are catalyzed in the mitochondrial matrix
• Cristae present a large surface area for
enzymes that synthesize ATP

Fig. 6-17
Free
ribosomes
in the
mitochondrial
matrix
Intermembrane space
Outer
membrane
Inner
membrane
Cristae
Matrix
0.1 µm

Chloroplasts: Capture of Light Energy
• The chloroplast is a member of a family of
organelles called plastids
• Chloroplasts contain the green pigment
chlorophyll, as well as enzymes and other
molecules that function in photosynthesis
• Chloroplasts are found in leaves and other
green organs of plants and in algae

• Chloroplast structure includes:
– Thylakoids, membranous sacs, stacked to
form a granum
– Stroma, the internal fluid

Fig. 6-18
Ribosomes
Thylakoid
Stroma
Granum
Inner and outer
membranes
1 µm

Peroxisomes: Oxidation
• Peroxisomes are specialized metabolic
compartments bounded by a single membrane
• Peroxisomes produce hydrogen peroxide and
convert it to water
• Oxygen is used to break down different types
of molecules

Fig. 6-19
1 µm
Chloroplast
Peroxisome
Mitochondrion

Concept 6.6: The cytoskeleton is a network of fibers
that organizes structures and activities in the cell
• The cytoskeleton is a network of fibers
extending throughout the cytoplasm
• It organizes the cell’s structures and activities,
anchoring many organelles
• It is composed of three types of molecular
structures:
– Microtubules
– Microfilaments
– Intermediate filaments

Fig. 6-20
Microtubule
Microfilaments
0.25 µm

Roles of the Cytoskeleton: Support, Motility, and
Regulation
• The cytoskeleton helps to support the cell and
maintain its shape
• It interacts with motor proteins to produce
motility
• Inside the cell, vesicles can travel along
“monorails” provided by the cytoskeleton
• Recent evidence suggests that the
cytoskeleton may help regulate biochemical
activities

Fig. 6-21
Vesicle
ATP
Receptor for
motor protein
Microtubule
of cytoskeleton
Motor protein
(ATP powered)
(a)
Microtubule Vesicles
(b)
0.25 µm

Components of the Cytoskeleton
• Three main types of fibers make up the
cytoskeleton:
– Microtubules are the thickest of the three
components of the cytoskeleton
– Microfilaments, also called actin filaments, are
the thinnest components
– Intermediate filaments are fibers with
diameters in a middle range

Table 6-1
10 µm 10 µm 10 µm
Column of tubulin dimers
Tubulin dimer
Actin subunit
 
25 nm
7 nm
Keratin proteins
Fibrous subunit (keratins
coiled together)
8–12 nm

Table 6-1a
10 µm
Column of tubulin dimers
Tubulin dimer
 
25 nm

Table 6-1b
Actin subunit
10 µm
7 nm

Table 6-1c
5 µm
Keratin proteins
Fibrous subunit (keratins
coiled together)
8–12 nm

Microtubules
• Microtubules are hollow rods about 25 nm in
diameter and about 200 nm to 25 microns long
• Functions of microtubules:
– Shaping the cell
– Guiding movement of organelles
– Separating chromosomes during cell division

Centrosomes and Centrioles
• In many cells, microtubules grow out from a
centrosome near the nucleus
• The centrosome is a “microtubule-organizing
center”
• In animal cells, the centrosome has a pair of
centrioles, each with nine triplets of
microtubules arranged in a ring

Fig. 6-22
Centrosome
Microtubule
Centrioles
0.25 µm
Longitudinal section
of one centriole
Microtubules Cross section
of the other centriole

Cilia and Flagella
• Microtubules control the beating of cilia and
flagella, locomotor appendages of some cells
• Cilia and flagella differ in their beating patterns
Video: Chlamydomonas Video: Paramecium Cilia

Fig. 6-23
5 µm
Direction of swimming
(a) Motion of flagella
Direction of organism’s movement
Power stroke Recovery stroke
(b) Motion of cilia
15 µm

• Cilia and flagella share a common
ultrastructure:
– A core of microtubules sheathed by the plasma
membrane
– A basal body that anchors the cilium or
flagellum
– A motor protein called dynein, which drives
the bending movements of a cilium or
flagellum
Animation: Cilia and Flagella

Fig. 6-24
0.1 µm
Triplet
(c) Cross section of basal body
(a) Longitudinal
section of cilium
0.5 µm
Plasma
membrane
Basal body
Microtubules
(b) Cross section of
cilium
Plasma
membrane
Outer microtubule
doublet
Dynein proteins
Central
microtubule
Radial
spoke
Protein cross-
linking outer
doublets
0.1 µm

• How dynein “walking” moves flagella and cilia:
− Dynein arms alternately grab, move, and
release the outer microtubules
– Protein cross-links limit sliding
– Forces exerted by dynein arms cause doublets
to curve, bending the cilium or flagellum

Fig. 6-25
Microtubule
doublets
Dynein
protein
ATP
ATP
(a) Effect of unrestrained dynein movement
Cross-linking proteins
inside outer doublets
Anchorage
in cell
(b) Effect of cross-linking proteins
1 3
2
(c) Wavelike motion

Fig. 6-25a
Microtubule
doublets
Dynein
protein
(a) Effect of unrestrained dynein movement
ATP

Fig. 6-25b
Cross-linking proteins
inside outer doublets
Anchorage
in cell
ATP
(b) Effect of cross-linking proteins
(c) Wavelike motion
1 3
2

Microfilaments (Actin Filaments)
• Microfilaments are solid rods about 7 nm in
diameter, built as a twisted double chain of
actin subunits
• The structural role of microfilaments is to bear
tension, resisting pulling forces within the cell
• They form a 3-D network called the cortex just
inside the plasma membrane to help support
the cell’s shape
• Bundles of microfilaments make up the core of
microvilli of intestinal cells

Fig. 6-26
Microvillus
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
0.25 µm

• Microfilaments that function in cellular motility
contain the protein myosin in addition to actin
• In muscle cells, thousands of actin filaments
are arranged parallel to one another
• Thicker filaments composed of myosin
interdigitate with the thinner actin fibers

Fig. 6-27
Muscle cell
Actin filament
Myosin filament
Myosin arm
(a) Myosin motors in muscle cell contraction
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
(b) Amoeboid movement
Nonmoving cortical
cytoplasm (gel)
Chloroplast
Streaming
cytoplasm
(sol) Vacuole
Cell wall
Parallel actin
filaments
(c) Cytoplasmic streaming in plant cells

Fig, 6-27a
Muscle cell
Actin filament
Myosin filament
Myosin arm
(a) Myosin motors in muscle cell contraction

Fig. 6-27bc
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
(b) Amoeboid movement
Nonmoving cortical
cytoplasm (gel)
Chloroplast
Cell wall
Streaming
cytoplasm
(sol)
Parallel actin
filaments
(c) Cytoplasmic streaming in plant cells
Vacuole

• Localized contraction brought about by actin
and myosin also drives amoeboid movement
• Pseudopodia (cellular extensions) extend and
contract through the reversible assembly and
contraction of actin subunits into microfilaments

• Cytoplasmic streaming is a circular flow of
cytoplasm within cells
• This streaming speeds distribution of materials
within the cell
• In plant cells, actin-myosin interactions and sol-
gel transformations drive cytoplasmic
streaming
Video: Cytoplasmic Streaming

Intermediate Filaments
• Intermediate filaments range in diameter from
8–12 nanometers, larger than microfilaments
but smaller than microtubules
• They support cell shape and fix organelles in
place
• Intermediate filaments are more permanent
cytoskeleton fixtures than the other two classes

Concept 6.7: Extracellular components and
connections between cells help coordinate cellular
activities
• Most cells synthesize and secrete materials
that are external to the plasma membrane
• These extracellular structures include:
– Cell walls of plants
– The extracellular matrix (ECM) of animal cells
– Intercellular junctions

Cell Walls of Plants
• The cell wall is an extracellular structure that
distinguishes plant cells from animal cells
• Prokaryotes, fungi, and some protists also have
cell walls
• The cell wall protects the plant cell, maintains its
shape, and prevents excessive uptake of water
• Plant cell walls are made of cellulose fibers
embedded in other polysaccharides and protein

• Plant cell walls may have multiple layers:
– Primary cell wall: relatively thin and flexible
– Middle lamella: thin layer between primary
walls of adjacent cells
– Secondary cell wall (in some cells): added
between the plasma membrane and the
primary cell wall
• Plasmodesmata are channels between
adjacent plant cells

Fig. 6-28
Secondary
cell wall
Primary
cell wall
Middle
lamella
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Plasmodesmata
1 µm

Fig. 6-29
10 µm
Distribution of cellulose
synthase over time
Distribution of microtubules
over time
RESULTS

The Extracellular Matrix (ECM) of Animal Cells
• Animal cells lack cell walls but are covered by
an elaborate extracellular matrix (ECM)
• The ECM is made up of glycoproteins such as
collagen, proteoglycans, and fibronectin
• ECM proteins bind to receptor proteins in the
plasma membrane called integrins

Fig. 6-30
EXTRACELLULAR FLUID
Collagen
Fibronectin
Plasma
membrane
Micro-
filaments
CYTOPLASM
Integrins
Proteoglycan
complex
Polysaccharide
molecule
Carbo-
hydrates
Core
protein
Proteoglycan
molecule
Proteoglycan complex

Fig. 6-30a
Collagen
Fibronectin
Plasma
membrane
Proteoglycan
complex
Integrins
CYTOPLASM
Micro-
filaments
EXTRACELLULAR FLUID

Fig. 6-30b
Polysaccharide
molecule
Carbo-
hydrates
Core
protein
Proteoglycan
molecule
Proteoglycan complex

• Functions of the ECM:
– Support
– Adhesion
– Movement
– Regulation

Intercellular Junctions
• Neighboring cells in tissues, organs, or organ
systems often adhere, interact, and
communicate through direct physical contact
• Intercellular junctions facilitate this contact
• There are several types of intercellular junctions
– Plasmodesmata
– Tight junctions
– Desmosomes
– Gap junctions

Plasmodesmata in Plant Cells
• Plasmodesmata are channels that perforate
plant cell walls
• Through plasmodesmata, water and small
solutes (and sometimes proteins and RNA) can
pass from cell to cell

Fig. 6-31
Interior
of cell
Interior
of cell
0.5 µm Plasmodesmata Plasma membranes
Cell walls

Tight Junctions, Desmosomes, and Gap Junctions in
Animal Cells
• At tight junctions, membranes of neighboring
cells are pressed together, preventing leakage of
extracellular fluid
• Desmosomes (anchoring junctions) fasten cells
together into strong sheets
• Gap junctions (communicating junctions) provide
cytoplasmic channels between adjacent cells
Animation: Tight Junctions
Animation: Desmosomes
Animation: Gap Junctions

Fig. 6-32
Tight junction
0.5 µm
1 µm
Desmosome
Gap junction
Extracellular
matrix
0.1 µm
Plasma membranes
of adjacent cells
Space
between
cells
Gap
junctions
Desmosome
Intermediate
filaments
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells

Fig. 6-32a
Tight junctions prevent
fluid from moving
across a layer of cells
Tight junction
Intermediate
filaments
Desmosome
Gap
junctions
Extracellular
matrix
Space
between
cells
Plasma membranes
of adjacent cells

Fig. 6-32b
Tight junction
0.5 µm

Fig. 6-32d
Gap junction
0.1 µm

The Cell: A Living Unit Greater Than the Sum of
Its Parts
• Cells rely on the integration of structures and
organelles in order to function
• For example, a macrophage’s ability to destroy
bacteria involves the whole cell, coordinating
components such as the cytoskeleton,
lysosomes, and plasma membrane

Fig. 6-UN1
Cell Component Structure Function
Houses chromosomes, made of
chromatin (DNA, the genetic
material, and proteins); contains
nucleoli, where ribosomal
subunits are made. Pores
regulate entry and exit of
materials.
Nucleus
(ER)
Concept 6.3
The eukaryotic cell’s genetic
instructions are housed in
the nucleus and carried out
by the ribosomes
Ribosome
Concept 6.4 Endoplasmic reticulum
The endomembrane system
regulates protein traffic and
performs metabolic functions
in the cell
(Nuclear
envelope)
Concept 6.5
Mitochondria and chloro-
plasts change energy from
one form to another
Golgi apparatus
Lysosome
Vacuole
Mitochondrion
Chloroplast
Peroxisome
Two subunits made of ribo-
somal RNA and proteins; can be
free in cytosol or bound to ER
Extensive network of
membrane-bound tubules and
sacs; membrane separates
lumen from cytosol;
continuous with
the nuclear envelope.
Membranous sac of hydrolytic
enzymes (in animal cells)
Large membrane-bounded
vesicle in plants
Bounded by double
membrane;
inner membrane has
infoldings (cristae)
Typically two membranes
around fluid stroma, which
contains membranous thylakoids
stacked into grana (in plants)
Specialized metabolic
compartment bounded by a
single membrane
Protein synthesis
Smooth ER: synthesis of
lipids, metabolism of carbohy-
drates, Ca2+ storage, detoxifica-
tion of drugs and poisons
Rough ER: Aids in synthesis of
secretory and other proteins from
bound ribosomes; adds
carbohydrates to glycoproteins;
produces new membrane
Modification of proteins, carbo-
hydrates on proteins, and phos-
pholipids; synthesis of many
polysaccharides; sorting of Golgi
products, which are then
released in vesicles.
Breakdown of ingested substances,
cell macromolecules, and damaged
organelles for recycling
Digestion, storage, waste
disposal, water balance, cell
growth, and protection
Cellular respiration
Photosynthesis
Contains enzymes that transfer
hydrogen to water, producing
hydrogen peroxide (H2O2) as a
by-product, which is converted
to water by other enzymes
in the peroxisome
Stacks of flattened
membranous
sacs; has polarity
(cis and trans
faces)
Surrounded by nuclear
envelope (double membrane)
perforated by nuclear pores.
The nuclear envelope is
continuous with the
endoplasmic reticulum (ER).

Fig. 6-UN1a
Concept 6.3
The eukaryotic cell’s genetic
instructions are housed in
the nucleus and carried out
by the ribosomes
Nucleus Surrounded by nuclear
envelope (double membrane)
perforated by nuclear pores.
The nuclear envelope is
continuous with the
endoplasmic reticulum (ER).
(ER)
Houses chromosomes, made of
chromatin (DNA, the genetic
material, and proteins); contains
nucleoli, where ribosomal
subunits are made. Pores
regulate entry and exit os
materials.
Ribosome Two subunits made of ribo-
somal RNA and proteins; can be
free in cytosol or bound to ER
Protein synthesis

Fig. 6-UN1b
Concept 6.4
The endomembrane system
regulates protein traffic and
performs metabolic functions
in the cell
Endoplasmic reticulum
(Nuclear
envelope)
Golgi apparatus
Lysosome
Vacuole Large membrane-bounded
vesicle in plants
Membranous sac of hydrolytic
enzymes (in animal cells)
Stacks of flattened
membranous
sacs; has polarity
(cis and trans
faces)
Extensive network of
membrane-bound tubules and
sacs; membrane separates
lumen from cytosol;
continuous with
the nuclear envelope.
Smooth ER: synthesis of
lipids, metabolism of carbohy-
drates, Ca2+ storage, detoxifica-
tion of drugs and poisons
Rough ER: Aids in sythesis of
secretory and other proteins
from bound ribosomes; adds
carbohydrates to glycoproteins;
produces new membrane
Modification of proteins, carbo-
hydrates on proteins, and phos-
pholipids; synthesis of many
polysaccharides; sorting of
Golgi products, which are then
released in vesicles.
Breakdown of ingested sub-
stances cell macromolecules,
and damaged organelles for
recycling
Digestion, storage, waste
disposal, water balance, cell
growth, and protection

Fig. 6-UN1c
Cell Component
Concept 6.5
Mitochondria and chloro-
plasts change energy from
one form to another
Mitochondrion
Chloroplast
Peroxisome
Structure Function
Bounded by double
membrane;
inner membrane has
infoldings (cristae)
Typically two membranes
around fluid stroma, which
contains membranous thylakoids
stacked into grana (in plants)
Specialized metabolic
compartment bounded by a
single membrane
Cellular respiration
Photosynthesis
Contains enzymes that transfer
hydrogen to water, producing
hydrogen peroxide (H2O2) as a
by-product, which is converted
to water by other enzymes
in the peroxisome

You should now be able to:
1. Distinguish between the following pairs of
terms: magnification and resolution;
prokaryotic and eukaryotic cell; free and
bound ribosomes; smooth and rough ER
2. Describe the structure and function of the
components of the endomembrane system
3. Briefly explain the role of mitochondria,
chloroplasts, and peroxisomes
4. Describe the functions of the cytoskeleton

5. Compare the structure and functions of
microtubules, microfilaments, and
intermediate filaments
6. Explain how the ultrastructure of cilia and
flagella relate to their functions
7. Describe the structure of a plant cell wall
8. Describe the structure and roles of the
extracellular matrix in animal cells
9. Describe four different intercellular junctions

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