4.1 Cells as Life’s Fundamental Unit

A Guide to the Natural World

David Krogh

© 2011 Pearson Education, Inc.

Chapter 4 • Lecture OutlineLife’s Home: The Cell

Biology

Fifth Edition


4.1 Cells as Life’s Fundamental Unit


Cells as Life’s Fundamental Unit

• With the possible exception of viruses, every form of life on Earth either is a cell or

is composed of cells.


Cells as Life’s Fundamental Unit

• Cells come into existence only through the activity of other cells.


4.2 Prokaryotic and Eukaryotic Cells


Prokaryotic and Eukaryotic Cells

• All cells can be classified as prokaryotic or eukaryotic.



• Prokaryotic cells either are bacteria or another single-celled life-form called archaea.



• Setting bacteria and archaea aside, all other cells are eukaryotic.



• Eukaryotic cells have most of their DNA contained in a membrane-lined compartment, called the cell nucleus, whereas prokaryotic cells do not have a nucleus.



• Eukaryotic cells tend to be much larger than prokaryotic cells. They have more of the specialized internal structures called organelles than do prokaryotic cells.



• Many eukaryotes are multicelled organisms, whereas all prokaryotes are single-celled.


Prokaryotic cells Eukaryotic cells

within membrane-bound nucleusspread through much of cell

DNA

much smaller

Size

always single-celled

Organization

Organelles

only one type of organelle

much larger

often multicellular

many types of organelles

Figure 4.2


4.3 The Eukaryotic Cell


The Eukaryotic Cell

• There are five principal components to the eukaryotic cell: the nucleus, other organelles, the cytosol, the cytoskeleton, and the plasma membrane.


The Eukaryotic Cell

• Organelles are “tiny organs” within the cell that carry out specialized functions, such as energy transfer and materials recycling.

© 2011 Pearson Education, Inc. Figure 4.4

The Animal CellThe nucleus contains the cell’s primary complement of DNA.

nuclear poresDNA

nuclear envelopenucleolus

Mitochondria are the powerplantorganelles that extract energyfrom food and put it into a formcells can use.

plasma membrane

The folds of the rough endoplasmicreticulum form a set of chambers within which proteins are processed.

All the cell’s structures outside the nucleus are immersed in a jelly-like fluid called the cytosol. Composed mostly of water, the cytosol is a location for countless chemical reactions carried out within the cell.

The smooth endoplasmic reticulumis the site of the production of lipidmolecules such as estrogen and testosterone.

free ribosomes

cytoskeleton

lysosome

transport vesicle

How are cell proteins sorted and shipped, so that they end up at the right location? Partly through the work of the Golgi complex.


The Eukaryotic Cell

• The cytosol is the jelly-like fluid outside the nucleus in which these organelles are immersed.

• The cytosol should not be confused with the cytoplasm, which is the region of the cell inside the plasma membrane but outside the nucleus.


The Eukaryotic Cell

• The cytoskeleton is a network of protein filaments.

• It functions in cell structure, cell movement, and the transport of materials within the cell.


The Eukaryotic Cell

• The plasma membrane is the outer lining of the cell.

• A membrane can be defined as the flexible, chemically active outer lining of a cell or of its compartments.


The Eukaryotic Cell

Components of eukaryotic cells

nucleus other organelles cytosol cytoskeleton plasma membrane


4.4 A Tour of the Animal Cell’s Protein Production Path


Tour of the Animal Cell’s Protein Production Path

• Information for the construction of proteins is contained in the DNA located in the cell nucleus.


The Protein Production Path

• This information is copied onto a length of messenger RNA (mRNA) that departs the cell nucleus through its nuclear pores and goes to the sites of protein synthesis, structures called ribosomes, which lie in the cytoplasm.



• Many ribosomes that receive mRNA chains process only a short stretch of them before migrating to, and then embedding in, one of a series of sacs in a membrane network called the rough endoplasmic reticulum (RER).



• The polypeptide chains produced by the ribosomal “reading” of the mRNA sequences are dropped from ribosomes into the internal spaces of the RER.

• There, the polypeptide chains fold up, thus becoming proteins, and undergo editing.


Tour of an Animal Cell

Suggested Media Enhancement:

Tour of an Animal Cell

• To access this animation go to folder C_Animations_and_Video_Filesand open the BioFlix folder.



• Some ribosomes are not embedded in the RER but instead remain free-standing in the cytosol.



• Materials move from one structure to another in the cell via the endomembrane system.

• Here a piece of membrane, with proteins or other materials inside, can bud off from one organelle, move through the cell, and then fuse with another membrane-lined structure.



• Membrane-lined structures that carry cellular materials are called transport vesicles.



• Once protein processing is finished in the rough ER, proteins undergoing processing move, via transport vesicles, to the Golgi complex.

• They are processed further and marked for shipment to appropriate cellular locations.


The Golgi Complex

Figure 4.9

Golgi complex

1. Transport vesicle from RER fuses with Golgi

2. Protein undergoes more processing in Golgi

cisternae

cisternalspace

vesicle

to cytosol

to plasma membrane

for exportout of cell

3. Proteins are sorted and shipped…

Side chains are edited (sugars may be trimmed, phosphate groups added).


4.5 Cell Structures Outside the Protein Production Path


Cell Structures Outside the Protein Production Path

• The smooth endoplasmic reticulum is a network of membranes that functions to synthesize lipids and to detoxify potentially harmful substances.


Lysosomes and Cellular Recycling

• Lysosomes are organelles that break down worn-out cellular structures or foreign materials that come into the cell.

• Once this digestion is completed, the lysosomes return the molecular components of these materials to the cytoplasm for further use.


lysosome

worn-outorganelle

digestiveenzymes

1. Lysosome fuses with worn-out organelle.

2. Organelle broken down.

3. Small molecules returned to cytosol.

4. Waste molecules expelled from cell.

5. Usable molecules recycled to make new organelles.


Mitochondria and Energy

• Mitochondria are organelles that function to extract energy from food and to transform this energy into a chemical form the cell can use, the molecule ATP.


outermembrane

innermembrane

watercarbon dioxideATP

foodoxygen

Mitochondrion

Figure 4.11

Mitochondria and Energy


4.6 The Cytoskeleton: Internal Scaffolding


The Cytoskeleton: Internal Scaffolding

• Cells have within them a web of protein strands, called a cytoskeleton.



• The cytoskeleton provides the cell with structure, facilitates the movement of materials inside the cell, and facilitates cell movement.



• There are three principal types of cytoskeleton elements.

• Ordered by size, going from smallest to largest in diameter, they are microfilaments, intermediate filaments, and microtubules.


(a) Microfilaments (in red) (b) Intermediate filaments (c) Microtubules

7 nmMain function: changes in cell shape

10 nm 25 nmMain function: maintenance of cell shape Main functions: maintenance

of cell shape, movement of organelles, cell mobility (cilia and flagella)

Figure 4.12


Microfilaments

• Microfilaments are made of the protein actin.


Microfilaments

• They help the cell move and capture prey by forming rapidly in the direction of movement and decomposing rapidly at their other end.


Microfilaments


Intermediate Filaments

• Intermediate filaments provide support and structure to the cell.


Microtubules

• Microtubules play a structural role in cells and facilitate the movement of materials inside the cell by serving as transport “rails.”


Microtubules

• Cilia and flagella are extensions of cells composed of microtubules.


Cilia

• Cilia extend from cells in great numbers, serving to move the cell or to move material around the cell.


Flagella

• By contrast, one—or at most a few—flagella extend from cells that have them.


Flagella

• The function of flagella is cell movement.


(a) Transport monorails

transportvesicle

motorproteins

microtubule

(c) Flagellum(b) Cilia

Figure 4.14


4.7 The Plant Cell


Tour of a Plant Cell

Suggested Media Enhancement:

Tour of a Plant Cell

• To access this animation go to folder C_Animations_and_Video_Filesand open the BioFlix folder.


The Plant Cell

• Plant cells have most of the structures found in animal cells—ribosomes, a cell nucleus, a rough ER, and so forth—although plant cells do not have the lysosomes found in animal cells.


The Plant Cell

• Plant cells have three structures not found in animal cells: • a cell wall• a large central vacuole• the organelles called chloroplasts


cell wall

chloroplast

central vacuole

cytoskeleton

Plant cells have a cell wall, chloroplasts, and a central vacuole, while animal cells do not.

nuclear envelopenuclear poresDNAnucleolus

nucleus

rough endoplasmicreticulum

smooth endoplasmicreticulum

free ribosomes

Golgi complexcytosol

plasma membrane

mitochondrion

Figure 4.17

The Plant Cell


The Central Vacuole

• The central vacuole stores nutrients and degrades waste products.


The Cell Wall

• The cell wall gives the plant structural strength and helps regulate the intake and retention of water.


Chloroplasts

• Chloroplasts are the sites of photosynthesis.


Chloroplasts

watercarbon dioxide

minerals

outer membrane

inner membrane

sugar (food)oxygen


Animation 4.1: The Structure of Cells

The Structure of Cells


4.8 Cell-to-Cell Communication


Cell-to-Cell Communication

• Cells are able to communicate with each other through special structures.


Communication Among Plant Cells

• Plant cells have channels, called plasmodesmata, that are always open and hence have the effect of making the cytoplasm of one plant cell continuous with that of another.


Communication Among Animal Cells

• Adjacent animal cells have channels, called gap junctions, that are composed of protein assemblages that open only as necessary.


Communication Among Animal Cells

• These gap junctions allow the movement of small molecules and electrical signals between cells.


Plant tissues

Animal tissues gap junction

plasmamembranes

cytoplasm

cytoplasm

plasmamembranecell walls

plasmodesmata

(a) Plasmodesmata

In plants, a series of tiny pores between plant cells, the plasmodesmata, allow for the movement of materials amongcells. Thanks to the plasmodesmata channels, the cytoplasm of one cell iscontinuous with the cytoplasm of the next; the plant as a wholecan be thought of as having a single complement of continuous cytoplasm.

(b) Gap junctions

In animals, protein assemblies come into alignment with one another, forming communication channels between cells. A cluster of many suchassemblies—perhaps severalhundred—is called a gapjunction.

Figure 4.20

© 2011 Pearson Education, Inc. Table 4.1

Table 4.1

Structures in Plant and Animal Cells

Name Function and Location Name Function and Location

NucleusSite of most of the cell’s DNALocation: Inside nuclear envelope

Synthesis of ribosomal RNALocation: Nucleus

Sites of protein synthesisLocation: Rough ER, Free-standing incytoplasm

Maintains cell shape, facilitates cellmovement and movement of materialswithin cellLocation: Cytoplasm

Protein-rich fluid in which organelles andcytoskeleton are immersedLocation: Cytoplasm

Processing, sorting of proteinsLocation: Cytoplasm

Digestion of imported materials and cell’sown used materialsLocation: Cytoplasm

Mitochondria

Rough endoplasmic reticulum

Smooth endoplasmic reticulum

Vesicles

Central vacuole (in plant cells only)

Chloroplasts (in plant cells only)

Cell walls(in plant cells only)

Transform energy from foodLocation: Cytoplasm

Protein processingLocation: Cytoplasm

Lipid synthesis, storage; detoxification ofharmful substancesLocation: Cytoplasm

Transport of proteins and other cellularmaterialsLocation: Cytoplasm

Nutrient storage, cell pressuremaintenance, pH balanceLocation: Cytoplasm

PhotosynthesisLocation: Cytoplasm

Limit water uptake; maintain cellmembrane shape, protect from outsideinfluencesLocation: Outside plasma membrane

Nucleolus

Ribosomes

Cytoskeleton

Cytosol

Golgi complex

Lysosomes (in animal cells only)

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4.1 Cells as Life’s Fundamental Unit