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The Cell Chapter 6
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• The cell is the basic unit of Life.
• All living organisms are composed of cells.
• Each Cell can reproduce by making a copy of its self.
Study The Figures
Cells come in many shapes and sizes.
Chicken egg
1 cm
100 !m
10 ! m
1!m
100 nm
Most plant
and Animal
cells
Nucleus
Most bacteria
Mitochondrion
Smallest bacteria
Ribosomes
Proteins
1 nm
Lipids
Small molecules
Figure 6.2
0.1 nm
•The ratio of Surface to Volume is proportional to r2/r3=1/r.
Frog egg
Viruses
10 nm
•Surface is (4" r2) proportional to r2
Atoms
Electron microscope
1 mm
•Volume of a sphere is (4/3" r3) proportional to r3
Surface area increases while
total volume remains constant
5
1
1
Electron microscope
0.1 m
The range is from about
0.1 !m to 1 m, but most
cells are between 1-100
!m in diameter.
Human height
Length of some
nerve and
muscle cells
Light microscope
1m
Unaided eye
10 m
Cell size is limited by the rate of diffusion of molecules across the
membrane vs. the volume. The logistics of carrying out cellular
metabolism sets limits on the size of cells
Measurements
1 centimeter (cm) = 10!2 meter (m) = 0.4 inch
1 millimeter (mm) = 10–3 m
1 micrometer (!m) = 10–3 mm = 10–6 m
1 nanometer (nm) = 10–3 mm = 10–9 m
Figure 6.8
Total surface area
(height " width "
number of sides "
number of boxes)
6
150
750
Total volume
(height " width " length
" number of boxes)
1
125
125
Surface-to-volume
ratio
(surface area ÷ volume)
6
1.2
6
All cells have several basic features in common:
• A smaller cell has a higher surface to volume ratio, which
facilitates the exchange of materials into and out of the cell
• When r becomes large the ratio becomes to small to provide
enough nutrients for the cell to function.
• Recall
•The ratio of Surface to Volume is proportional to r2/r3=1/r.
• They are bounded by a membrane called the plasma
membrane.
• Inside is a semifluid substance called the Cytosol
• They contain chromosomes
• They all have ribosomes
How do the few large cells (Nerve cells, fertilized Egg)
deal with this problem?
• There are two types of cells that are structurally
different,
– Prokaryotic and Eukaryotic cells.
Prokaryotic cells
• Have no nuclear membrane.
• The DNA in grouped together in a region called the Nucleoid,
• Lack membrane bound organelles and are usually smaller.
• Bacteria and Archaea (extremophiles, extreme thermophiles) are
Prokaryotic.
Eukaryotic cells
• Contain a true nucleus, bounded by a membranous nuclear envelope
• Are generally quite a bit bigger than prokaryotic cells
• Plants, Animal, Protists and Fungi are Eukaryotic.
• Have extensive and elaborately arranged internal membranes, which
form organelles
Concept : Eukaryotic cells have internal membranes that
compartmentalize their functions
• Plant and animal cells
– Have most of the same organelles
An animal cell
Nuclear envelope
ENDOPLASMIC RETICULUM (ER)
Rough ER
NUCLEUS
Nucleolus
Smooth ER
Chromatin
Flagelium
Plasma membrane
Centrosome
CYTOSKELETON
Microfilaments
Intermediate filaments
Ribosomes
Microtubules
Microvilli
Golgi apparatus
Peroxisome
Figure 6.9
A plant cell
Mitochondrion
Lysosome
In animal cells but not plant cells:
Lysosomes
Centrioles
Flagella (in some plant sperm)
Organelles.
•
The Nucleus contains most of the genes in an Eukaryotic
cell
•
It is surrounded by a double membrane nuclear envelope
full of nuclear pores that let large molecules flow between the
nucleus and the cytoplasm.
•
The Nucleolus is inside the Nucleus and here the Ribosomal
RNA’s are synthesized and assembled and then transported
to the cytoplasm where they combine to form ribosomes.
•
Ribosomes are organelles made from ribosomal RNA and
proteins and they carry out the protein synthesis by
translating messenger RNA to amino acids.
Nuclear envelope
Nucleolus
Chromatin
NUCLEUS
Centrosome
Rough
endoplasmic
reticulum Smooth
endoplasmic
reticulum
Ribosomes (small brwon dots)
Central vacuole
Tonoplast
Golgi apparatus
Microfilaments
Intermediate
filaments
CYTOSKELETON
Microtubules
Mitochondrion
Peroxisome
Plasma membrane
Chloroplast
Cell wall
Wall of adjacent cell
Figure 6.9
Plasmodesmata
In plant cells but not animal cells:
Chloroplasts
Central vacuole and tonoplast
Cell wall
Plasmodesmata
Endoplasmic reticulum ER
The endomembrane system
•Regulates protein traffic and performs metabolic
functions in the cell
•Includes many different structures
–
Consists of a network of continuous tubules and sacks called
cisternae.
–
The ER membrane is continuous with the outer nuclear membrane.
–
There are two distinct regions of ER
•The Smooth ER functions in divers metabolic processes such as lipid
synthesis, metabolism of carbohydrates and detoxification.
•Liver cells have much smooth ER. Storage of Ca+2 in muscle.
•The Rough ER is studded with ribosomes
•These produce proteins that are secreted in transport
vesicles, Most of these proteins are glycoproteins.
•The Rough ER also adds to the membrane by synthesizing
proteins and phospholipids.
Other Organelles
The Golgi apparatus
•
Receives many of the transport vesicles produced in the
rough ER
The Lysosomes are membrane bounded sacs that are full of
digestive enzymes.
– These enzymes can break down proteins, fats, polysaccharides
and nucleic acids.
– Inside Lysosomes PH is around 5.
Functions of the Golgi apparatus include
• Modification and sorting of the products of the rough ER
• Manufacture of certain macromolecules
(polysaccarides)
•
Peroxisomes generate H2O2 and also break it down. Inside
fatty acids are broken down and other compounds are
detoxified such as alcohol.
•
Vacuoles are also membrane (tonoplast) bounded sacs but
they are larger than Vesicles and are used too store material.
Water, dye, waste products. Only found in plants.
Mitochondria
Chloroplast
•Chloroplast
•
Mitocondria are the energy factory of the cells and produce ATP.
•
They have their own DNA and ribosomes.
–Is a specialized member of a family of closely related plant organelles called
plastids
•
They are enclosed by two membranes.
–Contains chlorophyll
•
The inner membrane is convoluted with infoldings called Cristae
–Is the photosynthesizing center
•
The space inside is called the mitochondrial Matrix.
–Is found in leaves and other green organs of plants and in algae
•Chloroplast structure includes
–Thylakoids, flattened membranous sacs
Mitochondrion
Intermembrane space
–Stroma, the internal fluid
Outer
membrane
Chloroplast
Free
ribosomes
in the
mitochondrial
matrix
Ribosomes
Chloroplast
DNA
Inner
membrane
Cristae
Matrix
Figure 6.17
Mitochondrial
DNA
1 !m
Thylakoid
100 !m
The Cell is not a sack of fluid. It has a definite structure:
Maintained by the Cytoskeleton.
•
There are three main types of fibers that make up the cytoskeleton
The 3 main structural proteins in the cytoskeleton (Table 6.1).
•
Microtubules function in structure and movement of particles along cells
– D=25 nm, L=200nm – 25 !m. Tubulin
– Also found in Cilia Flagella
•
Actin (Microfilaments) used in muscle with myosin for movement.
– D=7 nm,
•
Intermediate filaments reinforce cell shape and fix the positions of organelles.
– D=8-12 nm.
– They are more permanent than the other filaments and are the main component of
the cell cortex.
•
Stroma
Inner and outer
membranes
Granum
The Cell wall provides rigidity to the plant cell.
Table 6.1
ECM
Cells attach to each other via cell junctions.
•
The ECM (Extra Cellular Matrix is made up of glycoproteins and other
macromolecules secreted by the cells.
•
Fibronectin binds to the membrane bound Integrin proteins.
•
Collagen fibers and Proteoglycans give strength and structure to the ECM.
EXTRACELLULAR FLUID
Collagen
A proteoglycan
complex
Polysaccharide
molecule
Core
protein
Integrins
Microfilaments
Integrin
Figure 6.30
Plant Cells have Plasmodesmata through which movement of
large molecules is facilitated.
•
Tight junctions fuse cells together and prevent leaking of fluid
between cells.
•
Desmosomes are like rivets that fasten cells together and are
reinforced by intermediate filaments inside the cytoplasm.
•
Gap junctions allow small molecules to pass between cells.
Carbohydrates
Fibronectin
Plasma
membrane
•
Proteoglycan
molecule
CYTOPLASM
Summary
•
Types of intercellular junctions in animals
•The Cell has a definite structure and separate organelles.
TIGHT JUNCTIONS
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 !m
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
Figure 6.31
1 !m
Extracellular
matrix
Gap junction
0.1 !m
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.
•The Nucleus houses the cells genetic material (DNA)
•Organelles perform many different functions
•Mitochondria and Chloroplasts
–Are the ATP producing factories
•Cells are joined together by different junctions
–Tight junctions, Desmosomes and Gap junctions
•The ECM: made up of glycoproteins and other
macromolecules
•
Chapter 7 Membrane Structure and Function.
•
The Plasma Membrane (PM) functions to separate the
cell environment from the outside environment.
•
It consists of a phospholipids bilayer, with many
embedded or peripheral proteins.
–
Unsaturated hydrocarbon
tails with kinks
WATER
Saturated hydroCarbon tails
Cholesterol
(c) Cholesterol within the animal cell membrane
Figure 7.5
•
The Fluid mosaic model suggests that the membrane
proteins are like icebergs floating in water.
•
At very low temperatures the membrane is no longer fluid.
•
Cells deal with this by inserting cholesterol or other lipids into
the membrane this lowers the freezing point.
Membrane proteins have several functions.
•
Transport
– Channels allow larger molecules to diffuse across the membrane.
– Transporters transport molecules across using ATP.
Fibers of
extracellular
matrix (ECM)
EXTRACELLULAR
SIDE OF
MEMBRANE
Carbohydrate
Glycolipid
Cholesterol
Figure 7.7 p128
Viscous
(b) Membrane fluidity
Figure 7.2 p126
Figure 5.13 p71 6th ed
Microfilaments
of cytoskeleton
Affects the fluidity of the plasma membrane
Fluid
WATER
Glycoprotein
The type of hydrocarbon tails in phospholipids
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
•
Enzymatic Activity
•
Signal Transduction
•
Recognition, usually glycoproteins
•
Intercellular connections, adhesion molecules.
•
Anchoring Proteins that bind to the ECM and/ or the Cytoskeleton
and hold the cell or proteins in place.
Membrane proteins have several functions.
(d) Cell-cell recognition. Some glycoproteins serve as identification tags that
are specifically recognized by other
cells.
(a) Transport. (left) Channels selectively
allow larger molecules to diffuse across the
membrane.
(right) Transport proteins many use ATP
as an energy source to actively pump
substances across the membrane against
gradients.
Glycoprotein
ATP
(e)
Enzymes
(b) Enzymatic activity. A protein built into the
membrane may be an enzyme with its active
site exposed to substances in the adjacent
solution.
Signal
(f) Attachment to the cytoskeleton and
extracellular matrix (ECM).
Microfilaments or other elements of the
cytoskeleton may be bonded to
membrane proteins.
(c) Signal transduction. A membrane protein
may have a binding site with a specific
shape that fits the shape of a chemical
messenger.
Figure 7.9
Intercellular joining. Membrane
proteins of adjacent cells may hook
together in various kinds of junctions,
such as gap junctions or tight junctions.
Receptor
Figure 7.9
The Cell membrane separates the cell
environment from the outside
Diffusion.
•
Diffusion is the tendency for molecules of any substance to spread out
evenly into the available space
Molecules of dye
Membrane (cross section)
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
• However, the cell must be able to exchange material
with the outside environment, to be able to grow and
function.
• The Plasma Membrane is selectively permeable.
•
Small non-polar molecules can diffuse across
•
Ions can diffuse across through specific ion
channels
Figure 7.11 p131
Net diffusion
Net diffusion
Equilibrium
Osmosis
•
Diffusion of water, from higher concentration of water to lower.
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same
concentration
of sugar
• Gasses and not to large hydrophobic molecules can
diffuse through the phospholipids bilayer down a
concentration gradient. Small polar molecules can’t.
•
Facilitated diffusion lets larger polar molecules move across
the membrane.
•
Carrier proteins Undergo a subtle change in shape that translocate the
solute-binding site across the membrane
•
Channel protein usually only allow specific molecules across. No
external energy is required since diffusion is driving the transport.
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
Figure 7.12
(a) A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
Figure 7.15 p135
[Na+] high
[K+] low
Active Transport
Na+
Na+
Na+
Na+
Na+
[Na+] low
[K+] high
Na+
•
CYTOPLASM
Active Transport moves molecules against a
concentration gradient, and requires energy usually in the
form of ATP.
ATP
P
ADP
Na+
–
–
Uniports only move one type of molecule across.
Galactose in E.Coli
Na+
Na+
Co-transporters move two types of molecules across.
Either in the same direction glucose and Na transporter
or in the opposite direction the Na+/K+ pump. 3 Na+ out
and 2 K+ in.
K+
P
K+
K+
K+
K+
Fig 7.16 p136
K+
The proton pump
Review: Passive and active transport compared
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
•
Plant cells use a proton pump which pumps H+ ions out of
cells.
•
This gradient of H+ then drives the transport of sucrose into
the cell using the Sucrose-H+ co-transporter.
–
+
H+
ATP
H+
+
–
H+
Proton pump
H+
–
+
H+
–
+
ATP
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse
through the lipid bilayer.
H+ Diffusion
of H+
Sucrose-H+
cotransporter
Facilitated diffusion. Many
hydrophilic substances diffuse
through membranes with the
assistance of transport proteins,
either channel or carrier proteins.
–
Figure 7.19 p137
Figure 7.17
H+
+
–
Sucrose
+
Bulk Transport
•
Exocytosis occurs when vesicles inside the cell fuse with the PM thus
releasing the contents of that vesicle in to the extracellular space.
•
Endocytosis is the reverse of exocytosis.
–
PHAGOCYTOSIS
EXTRACELLULAR
CYTOPLASM
FLUID
Pseudopodium
Phagocytosis, Pinocytosis and Receptor Mediated Endocytosis
1 !m
RECEPTOR-MEDIATED ENDOCYTOSIS
Pseudopodium
of amoeba
Coat protein
Receptor
Coated
vesicle
“Food” or
other particle
Bacterium
Food
vacuole
Ligand
Coated
pit
PINOCYTOSIS
0.5 !m
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
Coat
protein
Vesicle
Figure 7.20 p139
Plasma
membrane
0.25 !m
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).
•
Figure 6.2 p95
•
Figure 6.9 p100
•
Figure 6.17 p110
•
Figure 6.18 p111
•
Figure 6.29 p119
•
Figure 6.31 p121
•
Fig 5.13 p71 6th ed
•
Figure 7.2 p125
•
Figure 7.7 p127
•
Fig 7.9 p128
•
Figure 7.11 p131
•
Figure 7.12 p132
•
Fig 7.15 p134
•
Fig 7.16 p135
•
Fig 7.18 and
•
Fig 7.19 p136
•
Fig 7.20 p138