Download Ch. 6: A Tour of the Cell

Chapter 6
Cells: The Working Units of Life
PART I
AP Biology
Cells: The Working Units of Life
•
Key Concepts
•
Cells Provide Compartments for Biochemical
Reactions
•
Prokaryotic Cells Do Not Have a Nucleus
•
Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
•
The Cytoskeleton Provides Strength and Movement
•
Extracellular Structures Provide Support and
Protection For Cells and Tissues
Overview: The Fundamental Units of Life
•
•
•
•
All organisms are made of cells
Cell: simplest collection of matter that can live
Cell structure is DIRECTLY related cellular function
All cells related by their descent from earlier cells
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Cell theory was the first unifying theory of
biology:
1. Cells are the fundamental units of life.
2. All organisms are composed of cells.
3. All cells come from preexisting cells.
Cells Provide Compartments for
Biochemical Reactions
• Most cells are tiny, in order to maintain a good
surface area-to-volume ratio.
• The volume of a cell determines its metabolic
activity per unit of time.
• The surface area of a cell determines the
amount of substances that can enter or leave
the cell.
Why Cells Are Small
volume increases more rapidly
than its surface area!
Cells must maintain a large
surface area-to-volume
ratio in order to function!
Some large cells increase surface area by
folds in the cell membrane.
Concept 6.1: To study cells, biologists use
microscopes and the tools of biochemistry
• Cells are small but can be complex
o
Scientists use microscopes to see cells too small to see with the
naked eye
• Light microscope (LM): visible light passes
through specimen, then through glass lenses that
magnifies image
o
can magnify effectively to about 1,000 times the size of the actual
specimen
o
Most subcellular structures, including organelles (membraneenclosed compartments), are too small to be resolved by an LM
• Quality of an image depends on:
o Magnification: ratio of an object’s image size to its real
size
o Resolution: measure of the clarity of the image the
minimum distance of two distinguishable points
o Contrast: visible differences in parts of the sample
• Various techniques enhance contrast and enable
cell components to be stained or labeled
The Scale of Life
Light
Microscope
Electron
Microscope
Fig. 6-3ab
TECHNIQUE
RESULTS
(a) Brightfield (unstained
specimen)
50 µm
(b) Brightfield (stained
specimen)
Fig. 6-3cd
TECHNIQUE
(c) Phase-contrast
(d) Differential-interferencecontrast (Nomarski)
RESULTS
Fig. 6-3e
TECHNIQUE
RESULTS
(e) Fluorescence
50 µm
Fig. 6-3f
TECHNIQUE
RESULTS
(f) Confocal
50 µm
Electron Microscopes
• Electron microscopes (EMs) used to study subcellular
structures
• Types:
o Scanning electron
microscopes (SEMs):
focus beam of electrons
onto the surface of a
specimen
• Images look 3-D
o Transmission electron
microscopes (TEMs):
focus beam of electrons
through a specimen
• Used mainly to study internal structure of cells
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Cell Fractionation/Centrifugation
• Cell fractionation: takes cells apart
and separates major organelles from
one another
o enables scientists to determine the functions of
organelles
• Ultracentrifuges fractionate cells into
their component parts
• Biochemistry and cytology help
correlate cell function with structure
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Cell Fractionation/Centrifugation
Cells can be
separated from
one another on
the basis of•size & density
and the isolated
structures can
then be analyzed
chemically.
The Two Types of Cells
1. Prokaryotes have no membrane-enclosed
compartments.
2. Eukaryotes have membrane-enclosed
compartments called organelles, such as the
nucleus.
Prokaryotic Cells Do Not Have a Nucleus
• Prokaryotic cells:
o Are enclosed by a cell membrane
o Have DNA located in the nucleoid region
• The rest of the cytoplasm consists of:
o Cytosol (water and dissolved material) and
suspended particles
o Ribosomes—sites of protein synthesis
Fig. 6-6
Fimbriae
Nucleoid
Ribosomes
Plasma membrane
Bacterial
chromosome
Cell wall
Capsule
0.5 µm
(a) A typical
rod-shaped
bacterium
Flagella
(b) A thin section
through the
bacterium
Bacillus
coagulans (TEM)
A Prokaryotic Cell
Prokaryotic Cells Do Not Have a Nucleus
• Most prokaryotes have a rigid cell wall outside
the cell membrane.
• Bacterial cell walls contain peptidoglycans.
• Some bacteria have an additional outer
membrane that is very permeable.
• Other bacteria have a slimy layer of
polysaccharides, called the capsule.
Prokaryotic Cells Do Not Have a Nucleus
• Some bacteria, including cyanobacteria, have
an internal membrane system that contains
molecules needed for photosynthesis.
Prokaryotic Cells Do Not Have a Nucleus
• Some prokaryotes swim by means of flagella,
made of the protein flagellin.
• A motor protein anchored to the cell
membrane or outer membrane spins each
flagellum and drives the cell.
Prokaryotic Flagella
Prokaryotic Flagella
Prokaryotic Cells
• Cytoskeleton: Some rod-shaped bacteria have
a network of helical actin-like protein structures
to help maintain their shape.
Concept 6.2: Eukaryotic cells have internal
membranes that compartmentalize their functions
•
ALL organisms composed of one of two o Eukaryotic
• Protists, fungi, animals, plants
types of cells:
• Plasma membrane
o Prokaryotic
• Semifluid substance called cytosol
• Only organisms of Domain
Bacteria and Archaea
• Chromosomes (carry genes)
• No nucleus
• Ribosomes (make proteins)
• DNA in a nucleus that is bounded by a
• DNA in an unbound region called
membranous nuclear envelope
the nucleoid
• Membrane-bound organelles
• No membrane-bound
• Cytoplasm in the region between the plasma
organelles
membrane and nucleus
• Cytoplasm bound by the plasma
• Eukaryotic usually much larger than
membrane
prokaryotic cells
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Quick reminder of classification!
A FEW
Plasma Membrane
• Plasma
membrane:
selective barrier
that allows sufficient
passage of oxygen,
nutrients, and waste
to service the
volume of every cell
• General structure of
biological membrane
= double layer of
phospholipids
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 6-7
Outside of cell
(a) TEM of a plasma
membrane
The cell membrane
allows cells to maintain
homeostasis!
Inside of
cell
0.1 µm
Carbohydrate side chain
Hydrophilic
region
Hydrophobic
region
Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
Cell Size
• The logistics of carrying out cellular metabolism sets limits on
the size of cells
• Surface Area :Volume
o As the surface area increases
by a factor of n2, the volume
increases by a factor of n3
• Small cells = greater
surface area relative
to volume = MORE
diffusion/cellular
processes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
• Eukaryotic cells have a nucleus, cell
membrane, cytoplasm, and
ribosomes, as well as membraneenclosed compartments called
organelles.
• Each organelle plays a specific role in
the cell.
Eukaryotic Cell
• Has internal membranes that partition the cell into organelles
• Plant and animal cells have most of the same organelles
Concept 6.3: The eukaryotic cell’s genetic instructions are
housed in the nucleus and carried out by the ribosomes
• Nucleus: contains most of the DNA in a eukaryotic cell
o contains most of the cell’s genes; usually the most conspicuous organelle
• Nuclear envelope: encloses the nucleus, separating it from the
cytoplasm
• Nuclear membrane: double membrane; each membrane
consists of a lipid bilayer
• Ribosomes: use the information from the DNA to make
proteins
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 6-9a
Nuclear
envelope
ENDOPLASMIC RETICULUM (ER)
Flagellum
Rough ER
NUCLEUS
Nucleolus
Smooth ER
Chromatin
Centrosome
Plasma
membrane
CYTOSKELETON:
Microfilaments
Intermediate
filaments
Microtubules
Ribosomes
Microvilli
Golgi
apparatus
Peroxisome
Mitochondrion
Lysosome
Eukaryotic Cells
Not found in
plant cells
Fig. 6-9b
NUCLEUS
Nuclear envelope
Nucleolus
Chromatin
Rough endoplasmic
reticulum
Smooth endoplasmic
reticulum
Ribosomes
Large central vacuoles are
unique to plant cells!
Central vacuole
Golgi
apparatus
Microfilaments
Intermediate
filaments
Microtubules
Mitochondrion
Peroxisome
Chloroplast
Plasma
membrane
Cell wall
Plasmodesmata
Wall of adjacent cell
CYTOSKELETON
Eukaryotic Cells
Nucleus
• The nucleus is usually the largest organelle:
• Location of DNA and DNA replication
• Site where DNA is transcribed to RNA
• Contains the nucleolus, where assembly
of ribosomes from RNA and proteins
begins
Nucleus
• Pores regulate the entry/exit of molecules from nucleus
• Nuclear lamina: maintains shape of nucleus; composed of protein
• Chromatin: genetic materials formed by DNA and proteins
o Can condense into
chromosomes
• Nucleolus: located
within the nucleus;
site of ribosomal
RNA (rRNA)
synthesis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Ribosomes: Protein Factories
• Ribosomes: translate the nucleotide sequence of a mRNA
molecule into a polypeptide.
• They occur in both prokaryotic and eukaryotic cells
• Consists of one large and one small subunit.
o 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)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 6-11
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
TEM showing ER and ribosomes
Small
subunit
Diagram of a ribosome
Concept 6.4: The endomembrane system
regulates protein traffic and performs
metabolic functions in the cell
• Components of the endomembrane system:
o Nuclear envelope
o Endoplasmic reticulum
o Golgi apparatus
o Lysosomes
o Vesicles
o Plasma membrane
• Components are either continuous
or connected via transfer by vesicles
• Endoplasmic reticulum (ER):
accounts for more than half of
o continuous with the nuclear envelope
o Two distinct regions of ER:
• Smooth ER, which lacks ribosomes
• Rough ER, with ribosomes studding its
Copyright © 2008
Pearson Education, Inc., publishing as Pearson Benjamin Cummings
surface
The Endomembrane System
vesicles shuttle
substances between
the various
components, as well
as to the cell
membrane.
Functions of the ER
• Smooth ER
o
o
o
o
Synthesizes lipids
Metabolizes carbohydrates
Detoxifies poison
Stores calcium
• Rough ER
o Has bound ribosomes, which
secrete glycoproteins
(proteins covalently bonded to
carbohydrates)
o Distributes transport
vesicles, proteins surrounded
by membranes
o Is a membrane factory for the
cell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Golgi Apparatus:
Shipping and Receiving Center
• Golgi apparatus: consists of flattened membranous sacs called
cisternae
• Functions of the Golgi apparatus:
o Modifies products of the ER
o Manufactures certain macromolecules
o Sorts and packages materials into transport vesicles
Fig. 6-16-1
Nucleus
Rough ER
Smooth ER
Plasma
membrane
Fig. 6-16-2
Nucleus
Rough ER
Smooth ER
cis Golgi
trans Golgi
Plasma
membrane
Fig. 6-16-3
Nucleus
Rough ER
Smooth ER
cis Golgi
trans Golgi
Plasma
membrane
Lysosomes: Digestive Compartments
• Lysosome: membranous sac of hydrolytic enzymes that can
digest macromolecules
• Lysosomal enzymes can hydrolyze:
o proteins
o fat
o
polysaccharides
o nucleic acids
• Phagocytosis: some
cells engulf another
cell by; forms food
vacuole
• Lysosome fuses with food vacuole  digests molecules
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 6-14a
Nucleus
1 µm
contain hydrolases
(digestive enzymes),
and are the site where
macromolecules are
hydrolyzed into
monomers!
Lysosome
Lysosome
Digestive
enzymes
Plasma
membrane
Digestion
Food vacuole
(a) Phagocytosis
Lysosomes: Digestive Compartments
• Autophagy: is the programmed destruction of cell
components.
• Cells break down their own materials, and even
entire organelles, within lysosomes.
Fig. 6-14b
Vesicle containing
two damaged organelles
1 µm
Mitochondrion
fragment
Peroxisome
fragment
Lysosome
Peroxisome
Vesicle
(b) Autophagy
Mitochondrion
Digestion