Chapter 4: Tour of the Cell

Unified Cell Theory

Unified Cell Theory

Unified Cell Theory:

• Cells are the most basic unit of life
• All living things are composed of cells
• Cells only develop from other cells

Types of Cells

Prokaryotes vs Eukaryotes

There are two types of cells: Prokaryotes and Eukaryotes

Both have:

• DNA
• Ribosome
• Cell Membrane
• Cytoplasm

Prokaryotes:

• DNA is not in a nucleus, rather in the nucleoid region
• No membrane bound organelles
• Includes bacteria and archaea

Eukaryotes:

• DNA is in a nucleus
• Has membrane bound organelles
• Includes plants, animals, fungi and protists

Plants vs Zombies

• Animals have:
  • Lysosome
  • Centriole
• Plants have:
  • Chloroplast
  • Central Vacuole
  • Cell wall

DNA, Ribosomes

Central Dogma of Life

The central dogma of life describes the process of cellular information:

• DNA stores the genetic information
• Transcription: DNA information is transcribed to RNA
• Translation: RNA information is translated as a specific protein formation
• The specific proteins perform a specific function, carrying out the DNA’s information

Nucleus

Nucleus:

• Only in eukaryotes
• Stores the DNA
• Referred to as the “control center of the cell”
• Components:
  • DNA
  • Nucleolus: Dark area where rRNA and ribosome are constructed
  • Nuclear Envelope: Double membrane that surrounds the nucleus
    • Contains pores for molecules to enter and exit
    • Filled with nucleoplasm

DNA

• Prokaryotes have a single circular DNA chromosome, located in the nucleoid region
  • They may also have plasmids: smaller circular DNA separate from the main strand
• Eukaryotes have multiple linear DNA chromosome, located in the nucleus
  • Eukaryotic DNA is packed into a DNA-protein complex named chromatin
• DNA information is transcribed to mRNA (messenger RNA). In eukaryotes, this happens in the nucleus. The mRNA is then translated to protein in the ribosome.

Ribosomes

Ribosome:

• Synthesize proteins from amino acids
• Composed of rRNA and protein

Protein synthesis:

• tRNA (transfer RNA) transfers amino acids to the ribosome
• The ribosome synthesizes the amino acids to protein based on the mRNA instructions

Structure:

• Unlike other organelle, ribosome are not membrane bound
• Contains two parts; a large and a small subunit
  • In protein synthesis, the mRNA is threaded between the large and small subunits
  • The small subunit translates the mRNA
  • The amino acids are arranged and bonded in the large subunit

Location:

• Can be free in the cytoplasm, attached to the cell membrane, nuclear envelope or RER
• Free ribosome generally synthesize protein that will remain in the cytoplasm; RER ribosome generally synthesize protein that will leave the cell or be embedded in the cell membrane.

Polyribosome: Cluster of multiple ribosome

Mitochondria, Chloroplasts

Mitochondria

Mitochondria:

• Only in eukaryotes
• Performs cellular respiration; manufactures ATP
• Referred to as the “powerhouse of the cell”

Structure:

• Contains an outer membrane
• Contains an inner membrane with cristae - folds
• The area in between the two membranes is the intermembrane space.
• The area inside the inner membrane is the mitochondrial matrix.

Chloroplasts

Chloroplasts:

• Only in plant cells
  • Only in green parts of plants; not in, for example, the edible part on an onion
• Perform photosynthesis

Structure:

• Contains a double outer membrane
• Thylakoids: Disc-like structures within the chloroplast
  • Chlorophyll: Pigments within the thylakoid membrane; absorb sunlight
    • Does not absorb green wavelength sunlight, instead reflecting it, resulting in a green appearance
  • Thylakoid lumen: Within the thylakoid membrane
• Granum (plural: grana): Stack of thylakoids
• Stroma: Space inside chloroplast not occupied by thylakoids, filled with fluid
• Some bacteria can perform photosynthesis. They have thylakoids with chlorophyll, but do not have chloroplasts.

Endosymbiosis Theory

Endosymbiosis Theory:

• Theorizes that mitochondria and chloroplasts were originally independent prokaryotic cells that later became integrated into eukaryotes in a symbiotic relationship
• Indicated by the following features found in both mitochondria and chloroplasts:
  • Double membrane
  • Contain their own DNA and ribosomes
  • Can reproduce independently

Endomembrane System

Overview

• Only in eukaryotes
• System of organelles that modify, package and transport proteins, carbohydrates and lipids
• Generally, the endomembrane system specifically handles molecules that are bound to leave the cell or be embedded in the cell membrane
• Endomembrane organelle are membrane-bound and have interior lumen with separate conditions from the cytoplasm, specific for their functions.
• Materials are transported between endomembrane organelle in transport vesicles

Endoplasmic Reticulum

Endoplasmic Reticulum:

• Interconnected membrane that extends from the nuclear envelope’s outer membrane
• Has two parts: Rough ER and Smooth ER
• Rough ER:
  • Studded with ribosomes
  • Modifies protein and phospholipids
• Smooth ER:
  • Does not have ribosome
  • Modifies lipids and cholesterol; performs select detoxification; stores calcium

Golgi Apparatus

Golgi Apparatus:

• Set of non-connected membrane discs
• Packages, labels and ships proteins
• Has two sides:
  • Cis side: faces the ER
  • Trans side: faces away from the ER

Vesicles

Vesicles:

• Dynamic membrane sac, used to transfer material
• Formed from the ER and Golgi membrane; sacs bud off a departure point with material inside, then fuse to the arrival point and release their material

Lysosome

Lysosome:

• Only in animals, not plants
• Vesicle containing digestive enzymes
• Fuses with other vesicles and digests their contents
• Breaks down old organelle, foreign agents; involved in cell suicide

Vacuoles, Peroxisomes

Vacuoles

Vacuole:

• Only in eukaryotes
• Permanent membrane bound sac; usually used for storage

Central Vacuole

Central Vacuole:

• Only in plant cells
• Large, fluid filled central vacuole
• Function: structure, storage, digestion, maintaining chemical balance
• Supports cell structure by swelling with fluid and providing turgor pressure against the cell wall

Peroxisomes

Peroxisome:

• Only in eukaryotes
• Small membrane bound organelle; detoxifies and breaks down molecules through oxidation
• Glyoxysomes: Special peroxisomes in plants that convert fat to sugars.

Cytoplasm

Cytoplasm

Cytoplasm:

• Area within the cell membrane (excluding the nucleus in eukaryotes)
• Filled with cytosol, a jelly-like substance
• Contains the cytoskeleton
• Organelle are suspended in the cytosol, anchored by the cytoskeleton

Cytoskeleton

Cytoskeleton: Intracellular skeletal system

• Functions:
  • Provides cellular structure
  • Anchors organelle
  • Creates pathways for organelle and materials
  • Facilitates cell motility
  • Aids in cell division

Components: (mostly applicable specifically for eukaryotes)

• Microtubules
  • Largest cytoskeleton component.
  • Hollow tubes made of tubulin monomers
  • Maintain cell structure; resist compression
  • Form tracks for organelle movement
  • Play a role in cell division
  • Aid cell motility
• Intermediate Filaments
  • Smaller than microtubules
  • Made of protein
  • Provide structural support; holds organelles in place
• Microfilaments
  • Smallest component
  • Made of actin fibers
  • Can rapidly assemble and disassemble
  • Maintain structure; provide tension around inner side of cell membrane
  • Play a role in cell division
  • Aid in cell motility

Centrosome:

• Central microtubular hub
• In animals, contains centrioles

Centrioles:

• Two perpendicular cylinders; each made of 9 sets of three microtubules
• Plays a role in cell division

Cell Membrane, Cell Wall, Extracellular Activity

Cell Membrane

Cell Membrane: Surrounds the cell

• Composed of a phospholipid bilayer
• Fluid Mosaic Model
• Selectively permeable

Cell Wall

Cell Wall

• Plants, fungi and prokaryotes have an exterior cell wall beyond the cell membrane
• Cellulose: Composes plants’ cell wall
  • Plants’ cell wall and central vacuole conjunctively maintain rigidity, support, strength and structure.
• Chitin: Composes fungi’s cell wall
• Peptidoglycan: Composes bacterias’ cell wall
• Capsule: Some bacteria are further surrounded by a capsule
• Flagella: Present in some cells: tail-like propeller by which the cell moves
• Cilia: Present in some eukaryotes: short, hairish protrusions which can move the cell or move substances along the cell surface.
  • In eukaryotes, cilia and flagella are made of microtubules arranged in a 9+2 structure: 2 central microtubules surrounded by 9 outer microtubule doublets
• Pili: Present in some bacteria: thin cell-membrane protrusions
  • Aid in adhesion to surfaces and in transferring genetic material.

Extracellular Activity

Plants:

• Plant cell membranes are surrounded by a rigid cellulose wall
• Neighboring cell walls are connected
• Neighboring cells communicate and transfer material through plasmodesmata connections
• Plasmodesmata:
  • “Tunnel” between two plant cells
  • Goes through the wall between the two cells and connects their cytoplasm
• Animals:
  • Extracellular Matrix (ECM): Area between cells
    • Filled with various proteins, mostly collagen
    • Holds cells in place; facilitates communication between cells
    • Binds with integrin proteins on the cell membrane surface
    • Facilitates intercellular communication; substances in the matrix can send signals to the cell through the integrin
  • Animal cells have three types of junctions:
    • Gap junction:
      • Similar to plasmodesmata - protein tunnel from one cell’s cytoplasm to its neighbor’s
    • Tight junction:
      • Fuses the surface of neighboring cells together
      • Proteins in the two cell membranes bond
      • Prevents things from passing between the cells; creates a waterproof barrier
    • Desmosomes:
      • Protrusions from the cell that latch to desmosomes of the adjacent cell
      • Attaches the two cells, but not tightly

Schematics

Types of Cells

• Prokaryotic

  

• Eukaryotic

  • Animal

    

  • Plant

    

Cell Components

• Nucleus

  

• Nucleus, ER

  

• DNA
  • See chapter 3
  • Nucleosome
    • DNA wraps around histone proteins

    

  • Chain of nucleosomes

    

  • Nucleosome chains form chromatin

    

  • Loose chromatin

    

  • Chromatid
    • Prior to DNA duplication, chromatin condenses and is visible as individual chromatid

    

• Ribosome

  • Ribosome

    

  • Translation

    

• Mitochondria

  

• Chloroplast

  

• Endomembrane System

  • Endomembrane system

    

  • Endoplasmic Reticulum

    

  • Golgi Apparatus

    

  • Vesicle

    

• Cytoskeleton

  • Cytoskeleton

    

    

  • Tubulin strand

    

  • Microtubule

    

• Phospholipid bilayer

  

• Cilia / Flagella cross section

  

• Plasmodesmata

  

• ECM

  

• Animal cell junctions

  • Gap junction

    

  • Tight junction

    

  • Desmosome