Chapter 4

Cell Structure

Molecules make up cells

• 30% of cell mass = macromolecules (carbs, lipids, proteins, nucleic acids)

• 70% of cell mass = Water >> important for cell structure

Discovery of cells

Father of Microbiology

• Anton van Leeuwenhoek

  • Made a microscope to see microscopic organisms

  • Saw cells in corks, named after monks’ chambers

Why Are Cells so Small?

Surface Area: Volume ratio

Surface Area: amt of area exposed on surface (membrane)

V: amt of space inside

As organisms get larger the SA:V gets smaller

SA must be greater than V to adequately pass materials thru cell membrane

Cell Theory

Early 1800s

Matthias Schleiden (botanist) and Theodor Schwann (zoologist) hypothesized cells have a life of their own

1. All living things are made of cells

2. Cells = basic unit of life

3. Cells arise from pre-existing cells (not spontaneous generation as previously thought)

4. Hereditary info is passed from cell (DNA)

5. All cells have the basic chemical composition (carbs, proteins, lipids, etc.)

6. Energy flow occurs within cells (metabolism)

3 Main Parts of a Cell

Cell Membrane

• Separates inside of cell from outside environment (lipid bilayer)

• Proteins in membrane dictates what can pass through

• Inside the membrane is cytosol

• Cytosol: mix of water, sugars, ions, protons. Organelles suspended in cytosol

Nucleus

• DNA is in there

• Defining characteristic of eukaryotes

Cytoplasm

• Cytosol, organelles, all other cellular components collectively called the cytoplasm

• This is where DNA is suspended in prokaryotes

Prokaryotes don’t have a nucleus, Eukaryotes have a nucleus

Bacteria and Archaea

Prokaryotes

• Unicellular, form colonies

• Lack a nucleus

• Archaea is more closely related to eukaryotes than to bacteria

Structural Differences

Prokaryotes are less elaborate inside than eukaryotes

• Ribosomes

  • Found in pro- and eukaryotes

  • Smaller and simpler in pro-

  • Synthesize proteins

• Plasmids (DNA)

  • Freely floating DNA inside prokaryotes since they have no nucleus

• Nucleoids

  • Nucleus-like structure in some prokaryotes

  • Cluster of DNA

• Cell Membrane

  • Found in pro- and euk-

  • Separates cell insides from external environment

  • Regulates what passes through cell

• Cell Wall

  • Encloses cell membrane

  • Rigid

  • Provides protection

• Capsule

  • Sticky layer of proteins

  • Helps bacteria stick to surfaces

  • Provides protection

• Pili

  • Protein filaments sticking out

  • Helps to move and stick to surfaces

  • Used for reproduction (called conjugation - sexual-like reproduction in bacteria)

• Flagella

  • Long, slender cellular projections

  • Used for motion like a propeller

Functions of the Nucleus

• Store DNA

• Controls molecules that pass thru nuclear membrane

Components of Nucleus

1. DNA

2. Chromatin

   1. Tightly packed DNA w/ specific proteins bound together

3. Nucleoplasm

   1. Fluid inside nucleus

   2. Chromatin suspended here

4. Nucleolus

   1. Dense region of protein and nucleic acids

   2. Ribosomes made here

5. Nuclear Envelope

   1. Membrane that surrounds nucleus; made of lipid bilayer

Endomembrane system

• Function: modifies, transports, and packages proteins and lipids in the cell

• Made of different organelles

  • Vesicles: sacs formed by budding. Cell taxi

  • Endoplasmic reticulum (ER): made from nuclear envelope. Ribosomes give it a rough appearance. Protein synthesis

  • Golgi Body: has enzymes to put “finishing touches” on proteins and lipids made on ER.

    • May attach sugars, oligosaccharides, or phosphate groups. Finished product: membrane proteins/lipids, proteins for secretion, or enzymes

Steps of the Endomembrane system

1. Protein made on rough ER

2. Wrap in vesicle

3. Drifts to golgi

4. Attaches to one side of golgi

5. Moves through the golgi, adding finishing touches

6. Exits other side of golgi

7. Drifts to cell membrane

8. Exocytosis: moving something inside the cell the the outside

Mitochondria

Breaks the sugar bonds (glucose) to make ATP

Mitochondria History

• Resemble bacteria (endosymbiotic theory)

• Have own DNA (DNA is circular like bacterial DNA)

• Divide independently of cell

• Has own ribosomes

Chloroplasts

Chloroplasts are a specific type of  plastid that perform photosynthesis

Plastids: organelles in plants and algae that help w/ photosynthesis, storage, and pigmentation

Cytoskeleton

• System of interconnected protein filaments

• Reinforce, organize, and move cell structures

• Some permanent, some form when needed

Parts of cytoskeleton

• Microtubules

  • Hollow cylinders

  • Function: movement and shape

  • Assemble and disassemble as needed

  • Ex. mitosis

• Microfilaments

  • Forms a mesh of microfilaments

  • Function: supports plasma membrane and movement

• Intermediate filaments

  • Stable framework

  • Function: structure and strength

  • Made of various proteins

  • Ex. keratin

Cell Movement

Motor Proteins

Move materials throughout the cell, activated by ATP

Cilia

Short, hairlike structures. Projects from cell membrane

Pseudopods

“False feet” temporary stretched lobe, move and grab prey

Cell Interactions

Cell Junctions

Structure that connects cell to other cells and the environment

• Cells send and receive signals

• Some junctions form tissues

Tight Junctions

• Found in animals

• Make up tissues

• Fasten cell membranes of adjacent cells together

• Keep bodily fluid contained

Adhering Junctions

• In animals

• Make up cardiac muscles and skin

Gap Junctions

• In animals

• Closable channels between cells

• Open to let water, ions, and molecules from cell’s cytoplasm to another

• Let substances flow quickly between cells

Plasmodesmata

• In plants

• Open channels connecting cytoplasm of two cells

• Let substances flow quickly between cells

• Similar to gap junctions in animal cells