4 A Tour of the Cell
Jan 11, 2016
4A Tour of the Cell
Overview: The Fundamental Units of Life
• All organisms are made of cells• The cell is the simplest collection of matter
that can be alive• All cells are related by their descent from
earlier cells• Though cells can differ substantially from one
another, they share common features
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Figure 4.1
Concept 4.1: Biologists use microscopes and the tools of biochemistry to study cells
• Most cells are between 1 and 100 m in diameter, too small to be seen by the unaided eye
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Microscopy
• Scientists use microscopes to visualize cells too small to see with the naked eye
• In a light microscope (LM), visible light is passed through a specimen and then through glass lenses
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• Three important parameters of microscopy– Magnification– Resolution– Contrast
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Figure 4.2
Most plant andanimal cells
Length of somenerve andmuscle cells
VirusesSmallest bacteria
Human height
Chicken egg
Frog egg
Human egg
NucleusMost bacteriaMitochondrion
Super-resolution
microscopy
Atoms
Small molecules
Ribosomes
ProteinsLipids
Un
aid
ed e
ye
LM
10 m
EM
1 m
0.1 m
1 cm
1 mm
100 m
10 nm
1 nm
0.1 nm
100 nm
10 m
1 m
Figure 4.2b
Most plant andanimal cells
VirusesSmallest bacteria
NucleusMost bacteriaMitochondrion
Super-resolution
microscopy
Atoms
Small molecules
Ribosomes
Proteins
Lipids
EM
100 m
10 nm
1 nm
0.1 nm
100 nm
10 m
1 m
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 light microscopy
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• 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 three-dimensional
• Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen
• TEM is used mainly to study the internal structure of cells
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Figure 4.3a
50
m
Brightfield(unstained specimen)
Brightfield(stained specimen)
Differential-interferencecontrast (Nomarski)
Phase-contrast
Light Microscopy (LM)
Figure 4.3b
50
m
10
m
Fluorescence Confocal
Light Microscopy (LM)
Figure 4.3c
Scanning electronmicroscopy (SEM)
Transmission electronmicroscopy (TEM)
Longitudinal sectionof cilium
Cross sectionof cilium
Cilia
2 m
Electron Microscopy (EM)
Cell Fractionation
• Cell fractionation breaks up cells and separates the components, using centrifugation
• Cell components separate based on theirrelative size
• Cell fractionation enables scientists to determine the functions of organelles
• Biochemistry and cytology help correlate cell function with structure
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Concept 4.2: Eukaryotic cells have internal membranes that compartmentalize their functions
• The basic structural and functional unit of every organism is one of two types of cells: prokaryotic or eukaryotic
• Organisms of the domains Bacteria and Archaea consist of prokaryotic cells
• Protists, fungi, animals, and plants all consist of eukaryotic cells
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Comparing Prokaryotic and Eukaryotic Cells
• Basic features of all cells – Plasma membrane– Semifluid substance called cytosol– Chromosomes (carry genes)– Ribosomes (make proteins)
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• 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
• Typically much smaller than eukaryotic cells (1-5um)
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Figure 4.4
(a) A typical rod-shapedbacterium
0.5 m(b) A thin section through
the bacterium Bacilluscoagulans (TEM)
Bacterialchromosome
Fimbriae
Nucleoid
Ribosomes
Cell wall
Plasma membrane
Capsule
Flagella
• 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 (10-100um)
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• The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell
• The general structure of a biological membrane is a double layer of phospholipids
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Figure 4.5
0.1 m
(a) TEM of a plasmamembraneOutside of cell
(b) Structure of the plasma membrane
Insideof cell
Hydrophilicregion
Hydrophilicregion
Hydrophobicregion
Carbohydrate side chains
Phospholipid Proteins
Figure 4.5a
0.1 m
(a) TEM of a plasmamembrane
Outside of cell
Insideof cell
• Metabolic requirements set upper limits on the size of cells
• The ratio of surface area to volume 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
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Figure 4.6
750
Surface area increases whiletotal volume remains constant
125
150
125
6
1
6
1
61.2
5
1
Total surface area[sum of the surface areas(height width) of all boxsides number of boxes]
Total volume[height width length number of boxes]
Surface-to-volumeratio[surface area volume]
A Panoramic View of the Eukaryotic Cell
• A eukaryotic cell has internal membranes that divide the cell into compartments—organelles
• The plasma membrane and organelle membranes participate directly in the cell’s metabolism
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Animation: Tour of a Plant Cell
Animation: Tour of an Animal Cell
Figure 4.7a
CYTOSKELETON:
NUCLEUS
ENDOPLASMIC RETICULUM (ER)
Smooth ER
Rough ERFlagellum
Centrosome
Microfilaments
Intermediatefilaments
Microvilli
Microtubules
Mitochondrion
Peroxisome Golgi apparatus
Lysosome
Plasmamembrane
Ribosomes
Nucleolus
Nuclearenvelope
Chromatin
Figure 4.7b
CYTO-SKELETON
NUCLEUS
Smooth endoplasmicreticulum
Chloroplast
Central vacuole
MicrofilamentsIntermediatefilaments
Cell wall
Microtubules
Mitochondrion
Peroxisome
Golgiapparatus
Plasmodesmata
Plasma membrane
Ribosomes
NucleolusNuclear envelope
Chromatin
Wall of adjacent cell
Rough endoplasmicreticulum