Cells: The WorkingUnits of Life

4

History of the Cell

Hooke – coined the term “cell”

Leewenhoek – bacteria are made of cells & first good microscope!

Concept 4.1 Cells Provide Compartments for Biochemical Reactions

Cell theory was the first unifying theory of biology.

1. Cells are the basic units of life.

2. Cells are the basic units of structure and function.

3. All cells come from preexisting cells.

Animal Cells

Concept 4.1 Cells Provide Compartments for Biochemical Reactions

Most cells are tiny

• to maintain a good surface area-to-volume ratio.

Figure 4.2 Why Cells Are Small

Figure 4.1 The Scale of Life

Concept 4.1 Cells Provide Compartments for Biochemical Reactions

To visualize small cells, there are two types of microscopes:

Light microscopes—use glass lenses and light

Resolution = 0.2 μm (OK)

Electron microscopes—electromagnets focus an electron beam

Resolution = 2.0 nm (awesome)

Figure 4.3 Microscopy

Concept 4.1 Cells Provide Compartments for Biochemical Reactions

The plasma membrane:

• Is a selectively permeable barrier that allows cells to maintain a constant internal environment

• Is important in communication and receiving signals

• Often has proteins for binding and adhering to adjacent cells

4.1 CelConcept 4. Provide Compartments for Biochemical Reactions

Two types of cells: Prokaryotic and eukaryotic

Prokaryotes are without membrane-enclosed compartments. NO NUCLEUS!

Eukaryotes have membrane-enclosed compartments called organelles, such as the nucleus.

In-Text Art, Ch. 4, p. 59

Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus

Prokaryotic cells:

• Are enclosed by a plasma membrane

• Have DNA located in the nucleoid

The rest of the cytoplasm consists of:

• Cytosol (water and dissolved material) and suspended particles

• Ribosomes—sites of protein synthesis

Figure 4.5 A Prokaryotic Cell

Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus

Other Characteristics of a prokaryote:

• rigid cell wall (containing peptidoglycan) outside the plasma membrane.

• Some bacteria have an additional outer membrane that is very permeable.

Other bacteria have a slimy capsule made of polysaccharides.

Some prokaryotes swim by means of flagella, made of the protein flagellin

Figure 4.6 Prokaryotic Flagella (Part 1)

Figure 4.7 Eukaryotic Cells (Part 1)

Figure 4.7 Eukaryotic Cells (Part 8)

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

Ribosomes—sites of protein synthesis:

both prokaryotic and eukaryotic cells

similar structure—one larger and one smaller subunit.

Ribosomes are not membrane-bound organelles—in eukaryotes, they are free in the cytoplasm, attached to the endoplasmic reticulum, or inside mitochondria and chloroplasts.

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

The nucleus is usually the largest organelle.

It is the location of DNA and of DNA replication.

It contains the nucleolus, where ribosomes are made

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

The nucleus is surrounded by two membranes that form the nuclear envelope.

Nuclear pores in the envelope control movement of molecules between nucleus and cytoplasm.

In the nucleus, DNA combines with proteins to form chromatin in long, thin threads called chromosomes.

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes.

Tiny, membrane-surrounded vesicles shuttle substances between the various components, as well as to the plasma membrane.

Figure 4.8 The Endomembrane System

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

Endoplasmic reticulum (ER)—network of interconnected membranes in the cytoplasm, with a large surface area

Two types of ER:

• Rough endoplasmic reticulum (RER)

• Smooth endoplasmic reticulum (SER)

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

Rough endoplasmic reticulum (RER) has ribosomes attached to begin protein synthesis.

Newly made proteins enter the RER lumen.

Once inside, proteins are chemically modified and tagged for delivery.

The RER participates in the transport.

All secreted proteins and most membrane proteins, including glycoproteins, which is important for recognition, pass through the RER.

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

Smooth endoplasmic reticulum (SER)—more tubular, no ribosomes

It chemically modifies small molecules such as drugs and pesticides.

It is the site of glycogen degradation in animal cells.

It is the site of synthesis of lipids and steroids.

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

The Golgi apparatus is composed of flattened sacs (cisternae) and small membrane-enclosed vesicles.

Receives proteins from the RER—can further modify them

Concentrates, packages, and sorts proteins

Adds carbohydrates to proteins

Site of polysaccharide synthesis in plant cells

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

The Golgi apparatus has three regions:

The cis region receives vesicles containing protein from the ER.

At the trans region, vesicles bud off from the Golgi apparatus and travel to the plasma membrane or to lysosomes.

The medial region lies in between the trans and cis regions.

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

Primary lysosomes originate from the Golgi apparatus.

They contain digestive enzymes, and are the site where macromolecules are hydrolyzed into monomers.

Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 1)

Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 2)

Figure 4.7 Eukaryotic Cells

Outer Membrane (very porous)

Cristae – increases Surface Area-

Matrix – Enzymes, DNA, Ribosomes

Figure 4.7 Eukaryotic Cells

Thylakoids – chlorophyll

Grana – stack of thylakoids

Stroma - aqueous

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

Peroxisomes collect and break down toxic by-products of metabolism, such as H2O2, using specialized enzymes.

Glyoxysomes, found only in plants, are where lipids are converted to carbohydrates for growth.

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

Vacuoles :

• Storage of waste products, food, enzymes, water, and pigments

• creates turgor pressure in plant cells

Contractile vacuoles in freshwater protists get rid of excess water