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