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 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
• 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 aquire 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

Types of Cells

Prokaryotes

• Pro = before; karyon = nucleus
• relatively small - 5 to 10 um
• lack membrane-bound organelles
• earliest cell type

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

• Features shared with Prokaryotic cells
  • Rigid cell wall
    • Plant cells, some Fungi, some Protists
    • Animal cells lack cell wall
  • Plasma membrane
  • Cytoplasm with ribosomes
• 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
  • Intermediate filaments - rope-like fibrous proteins
    • 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
• 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
• Nucleus
  • Double membrane with pores
  • Outer membrane continuous with ER
  • Nuclear matrix - protein-containing fibrilar network
  • Nucleoplasm - the fluid substance in which the solutes of the nucleus are dissolved
  • Chromosomes - protein and DNA complexes
  • Nucleolus - involved in the synthesis and assembly of ribosomes
• Endomembrane System
  • Endoplasmic Reticulum - an extinsive membranous network continuous with the outer nuclear membrane.
    • Rough ER - has ribosomes 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
  • Lysosome
    • found only in animal cells
    • contain enzymes for use in the hydrolytic breakdown of macromolecules
  • 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

  Images of Vesicle Transport Between Endomembrane Organelles  

   

• Ribosomes  
  • 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
  • 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
• 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 mitochondira
      • 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 soluable portion of chloroplast
    • site of CO₂ fixation
    • site of sugar synthesis (carbon metabolism)
    • location of cpDNA
    • site of chloroplast protein synthesis

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