Cells - Structure and Function

Important Events in the Discovery of Cells

• 1665 - Robert Hooke looks at cork under a microscope. Calls the chambers he see "cells"
• 1665 - 75 Anton van Leeuwenhoek, the person incorrectly given credit for the invention of the microscope (actually, he was just damn good at making and using them, and his scopes soon became the standard, and history has just given him credit as the inventor of the microscope), studies organisms living in pond water (like you did in lab). He calls them "Animalcules."
• 1830 - German scientists Schleiden and Schawann summarize the findings of many scientists and conclude that all living organisms are made of cells. This forms the basis of the Cell Theory of Biology

The Cell Theory of Biology

• All organisms are composed of cells
• The cell is the structural unit of life - units smaller than cells are not alive
• Cells arise by division of preexisting cells - spontaneous generation does not exist
• Cells can be cultured to produce more cells
  • in vitro = outside organism or cell
  • in vivo = inside organism or cell

Properties of Cells

Cells are complex and highly organized

• They contain numerous internal structures
• Some are membrane bound (organelles) while others do not

Cells contain a genetic blueprint and machinery to use it

• Genes are instructions for cells to create specific proteins
• All cells use the same types of information
  • The genetic code is universal
  • The machinery used for synthesis is interchangeable
• However, for this to function properly, information transfer must be error free
  • Errors are called mutations

Cells arise from the division of other cells

• Daughter cells inherit the genes from the mother cells
• Binary fission - cell division in bacteria
• Mitosis - the genetic complement of each daughter cell is identical to the other and to the mother cell. This is asexual reproduction
• Meiosis - the genetic complement of each daughter cell is reduced by half and each daughter cell is genetically unique. This is used in sexual reproduction
• Daughter cells inherit cytoplasm and organelles from the mother cells
  • Asexual - organelles from mother cell
  • Sexual - organelles predominately from one parent
    • In eukaryotes, the chloroplasts and mitochondria come from the egg cell
    • This can be used to trace the evolutionary origin of the organism

Cells acquire and utilize energy

• Plant cells undergo photosynthesis
  • convert light energy and CO₂ to chemical energy (ATP and glucose)
• Most cells respire
  • release energy found in organic compounds
  • convert organic compounds to CO₂ and O₂
  • make ATP

Cells can perform a variety of chemical reactions

• Transform simple organic molecules into complex molecules (anabolism)
• Breakdown complex molecules to release energy (catabolism)
• Metabolism = all reactions performed by cells

Cells can engage in mechanical activities

• Cells can move
• Organelles can move
• Cells can respond to stimuli
  • chemotaxis - movement towards chemicals
  • phototaxis - movement towards light
  • hormone responses
  • touch responses

Cells can regulate activities

• Cells control DNA synthesis and cell division
• Gene regulation - cells make specific proteins only when needed
• Turn on and off metabolic pathways

Cells all contain the following structures:

• Plasma membrane - separates the cell from the external environment
• Cytoplasm - fluid-filled cell interior
• Nuclear material - genetic information stored as DNA
• Ribosomes

Types of Cells

Prokaryotes Pro = before; karyon = nucleus relatively small - 5 to 10 um lack membrane-bound organelles earliest cell type Archaea Originally thought to be prokaryotes relatively small - 5 to 10 um lack membrane-bound organelles Usually live in extreme environments (thermophiles, halophiles, etc) Eukaryotes Eu = true; karyon = nucleus contain membrane-bound organelles Evolved from prokaryotes by endosymbiotic association of two or more prokaryotes Include Protists, Fungi, Animals, and Plants

Features of Prokaryotic Cells

Capsule - outer sticky protective layer Cell Wall - rigid structure which helps the bacterium maintain its shape this is in NO way the same as the cell wall of a plant cell Plasma membrane - separates the cell from the environment Mesosome - infolding of plasma membrane to aid in compartmentalization Nucleoid - region where nakedDNA is found Cytoplasm semi-fluid cell interior no membrane-bound organelles location for metabolic enzymes location of ribosomes for protein synthesis

Properties of Eukaryotic Cells

• Cytoskeleton - flexible tubular scaffold of microfilaments
  • maintains cell shape and provides support
  • anchors organelles & enzymes to specific regions of the cell
  • contractility and movement (amoeboid movement)
  • intracellular transport - tracks for vesicle and organelle movement by motor proteins
• Cytoskeleton components
  • Microfilaments
    • solid protein (actin) which is assembled at one end and disassembled at the other end
    • actin filaments can change lenght - a process known as treadmilling
    • actin frequently interacts with myosin, a second protein capable of movement - like in your muscles
    • excellent animated youtube video (don't worry about the calcium binding)
  • Intermediate filaments
    • rope-like fibrous proteins - defined by size, not by composition like microfilaments and microtubules
    • provide structural reinforcement
    • anchor organelles
    • keep nucleus in place
  • Microtubules
    • hollow tubes of tubulin (a globular protein)
    • maintains cell shape
    • anchor organelles
    • movement of organelles
    • track for motor proteins such as kinesin
    • excellent animated youtube video (skip to about a minute in)
• Summary of Cytoskeleton Components

• Cilia and Flagella - involved in cellular movement
  • composed of microtubules
  • cilia - short, numerous, complex
  • flagella - longer, fewer, less complex
  • both arranged in a 9+2 pattern with dynein arms projecting outward
  • Movement is through associations with dynein, a motor protein similar to myosin and kinesin

• Nucleus
  • Nucleus surrounded by a double membrane with pores
    • Outer nuclear membrane continuous with ER (therefore part of the endomembrane system)
    • Proteins destined for import into the nucleus have a molecular tag (the "zip code") called Nuclear Localization Signal (NLS)
    • Importins bind to proteins with NLS and import them into the nucleus
    • Exportins do the same thing to export proteins out of nucleus
  • Nuclear matrix - protein-containing fibrilar network
  • Nucleoplasm - the fluid substance in which the solutes of the nucleus are dissolved
  • Chromosomes - histone protein and DNA complexes
    • heterochromatin - highly compact, supercoiled chromatin
    • euchromatin - long, filamentous strands of chromatin (gene transcription?)
  • Nucleolus - involved in the synthesis and assembly of ribosomes (plural = nucleoli)

• Ribosomes  
  • Technically not an organelle, since there is no membrane, but they are prominent cellular structures and usually lumped in with the organelles
  • The "factories" of the cell - involved in protein synthesis
  • Facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis
  • May either be free or bound to ER
    • Proteins which are fated to be secreted or transported to an endomembrane organelle are typically produced by the ribosomes bound to the ER (see signal hypothesis below)
    • Proteins which are fated to remain in the cytoplasm are typically produced by ribosomes floating free in the cytoplasm
  • Made up of two subunits, the large and the small subunit
  • Both subunits are constructed out of protein and RNA (called rRNA)
  • The ribosomes of prokaryotes and eukaryotes vary slightly with regard to size and shape

• Endomembrane System
  • Endoplasmic Reticulum - an extensive membranous network continuous with the outer nuclear membrane.
    • Rough ER - has ribosomes associated with it and is involved in secreted protein synthesis
    • Smooth ER - lacks ribosomes and is involved in membrane lipid synthesis
  • Golgi Apparatus
    • Flattened vesicles in stacks which receive protein from ER
    • Form secretory vesicles to transport proteins to different parts of the cell (vacuole, lysosome, etc) or for secretion
    • cis face - "receiving" side of Golgi apparatus
    • trans face - "shipping" side of Golgi apparatus
    • Sweet Home Apparatus
  • Lysosome
    • lysosomes are special types of vacuoles
    • contain enzymes for use in the hydrolytic breakdown of macromolecules
    • Three ways to enter a lysosome - phagocytosis, autophagy, and receptor-mediated endocytosis
• Peroxisome
  • Eukaryotic organelle that degrades fatty acids and amino acids
  • Also degrades the resulting hydrogen peroxide
  • There is a variety of different types of peroxisomes, each which breaks down different type of molecules
• Plant Central Vacuole - major storage space in center of plant cell with many functions
  • Digestive - break down of macromolecules
  • Storage - ions, sugars, amino acids, toxic waste
  • Maintain cell rigidity - high ionic concentration generates high water potential

• Transport of Proteins - the ER, Golgi, and transport vesicles
  • Ribosomes (see below) synthesize proteins
    • Those destines for secretion contain a short sequence of amino acids that interact with a receptor protein on the surface of rough ER
    • The ribosome interacts with the receptor protein, causing the synthesized protein to enter the ER
    • The receptor protein removes the signal sequence
  • Proteins that enter the ER are usually glycolated
    • The carbohydrate groups serve as chemical markers for protein distribution
    • The carbohydrate groups can be modified within the ER
  • In the Golgi Apparatus, proteins are segregated for transport to other regions in the cell
    • The sugars attached to proteins bind to receptors in the trans walls of the Golgi
    • These walls eventually bud off into the cytoplasm
    • The vesicles then bind to the appropriate endomembrane system (ER, plasma membrane, etc) and release the contents.

Images of Vesicle Transport Between Endomembrane Organelles  

Endo- and exocytosis video on YouTube

• Mitochondria
  • Found in ALL eukaryotic cells (yes, even in plant cells)
  • Site of aerobic respiration
    • sugars + O₂ - - > ATP + CO₂ + H₂O
  • Contain DNA which codes for mitochondrial proteins, ribosomes, etc.
  • Divide by a process similar to binary fission when cell divides
  • Enclosed in a double membrane system
    • Inner Membrane forms the Cristae (invaginations into interior region)
      • Site of energy generation
    • Matrix is the soluble portion of the mitochondria
      • Site of carbon metabolism
      • Location of mDNA
      • Site of mitochondrial protein synthesis
• Chloroplasts
  • Found only in plant cells
  • Site of photosynthesis
    • conversion of solar energy to chemical energy in the form of ATP and sugars
  • Contain DNA which codes for chloroplast proteins, ribosomes, etc.
  • Divide when plant cell divides
  • Enclosed in a double membrane envelope that does not invaginate into the chloroplast
  • Thylakoid is a third internal membrane system
    • contains membrane-bound photosynthetic pigments
    • site of photochemistry (the conversion of light energy to ATP)
    • site of O₂ generation
  • Stroma is soluble portion of chloroplast
    • site of CO₂ fixation
    • site of sugar synthesis (carbon metabolism)
    • location of cpDNA
    • site of chloroplast protein synthesis

Summary of Cellular Components - note that the list of "Endomembrane System" organelles is not complete!

Endosymbiotic Origin of Chloroplasts and Mitochondria

• Free-living prokaryote eaten by host
• Genes transferred to host nucleus
• Some genes retained but most lost - can no longer survive outside of host
• Symbiotic relationship
  • photosynthetic symbiont provides sugar - degenerates to form chloroplast
  • aerobic symbiont provides a more efficient energy generation system - degenerates to form mitochondria
  • host provides stable environment, nutrients, energy, and most proteins
• Evidence for Endosymbiotic Theory
  • Chloroplasts and mitochondria have DNA
    • does not code for all proteins
    • some genes in nucleus
    • proteins imported rom cytoplasm
  • Organelle proteins similar to bacterial form
  • Ribosome structure and metabolic enzymes more similar to bacterial forms