The Cell

Cell Theory

  • Cells are fundamental units of life
  • All organisms are composed of cells
  • All cells come from preexisting cells
  • same as studying life
  • Life is continuous- we can trace back to cells

Cells like to keep a good surface area to volume ratio

  • Volume determines metabolic activity per unit of time
  • Surface area determines how many substances enter and leave
  • Basically need a high ratio so that enough nutrients can enter and waste can exit for the processes to happen
  • Cells increase surface area by folding

Cell Membrane

  • made of lipids
  • Selectively permeable barrier that helps maintain homeostasis
  • Important in communication and receiving signals
  • Has proteins for binding cells
      * Plasmodesmata are membrane-lined channels found in plants that transport substances and connect them
      * Extracellular matrices are found in animals
        * Like a string of webs connected to the membrane that hangs outside
          * Integrin connects the extracellular matrix to the membrane
          * Connected to microfilaments in the cell membrane and the collagen outside
          * for cell movement, the protein changes shape and detach
        * Fibrous component is collagen, gel-like component is proteoglycans
          * collagen is a protein, proteoglycans are proteins and polysaccharides
          * contribute to physical properties of bone, cartilage, skin, bone, and other tissues
          * help filter materials like in kidneys
          * orient cell movement and repair
      * Cell junctions protrude through and between cells to bind them together
        * Tight junction
          * prevents substances moving in the space between the cells
        * Desmosomes
          * allows for substances to move in the matrix between cells
        * Gap junction
          * run between membrane pores, allows for substances to pass through them
  • Cell Wall
      * Bacteria
        * have cell wall containing peptidoglycan (amino sugars and saccharides)
        * some bacteria have a additional outer cell wall, very permeable
        * some have slimy layer of polysaccharides called the capsule
        * some bacteria, including cyanobacteria, contain molecules in membrane that can photosynthesize
      * plants
        * semi-rigid, provides support and limits its volume
        * cellulose
        * gel-like matrix contains proteins and polysaccharides
        * act as barrier to infection
        * contributes to form during growth

Organelles

  • found in the cytoplasm along cytosol, which is water and dissolved particles
  • Ribosomes
      * sites of protein synthesis
      * translates the nucleotide sequence of a messenger RNA into a polypeptide
      * both types of cells, has a large and a small subunit
      *
      * each subunit consists of ribosomal RNA and protein
      * not membrane bound organelles
      * they are free in the cytoplasm, in the rough endoplasmic reticulum, or in mitochondria or chlorplast
      * in prokaryotes they float freely in the cytoplasm
  • Nucleus
      * usually largest organelle
      * where DNA is stored and replicated
      * where DNA is transcribed to RNA
      * contains nucleolus
        * where ribosomes are made from RNA and proteins
      * two membranes called the nuclear envelope
        * has pores for substances to pass through
      * In the nucleus, DNA combines with proteins called chromatin to make chromosomes
        * strand of DNA encoded with genes
      * outer membrane is continuous with the endoplasmic reticulum
  • endomembrane system
      * nuclear envelope, endoplasmic reticulum, golgi apparatus, and lysosomes
  • vesicles
      * membrane-bound shuttles that transport substances between components and the membrane
  • Endoplasmic reticulum
      * network of interconnected membranes in the cytoplasm, large surface area
      * Rough endoplasmic reticulum
        * ribosomes attached to surface
        * newly made proteins enter the RER lumen, and are modified then tagged for delivery to specific locations
          * transported in vesicles that pinch off of the ER
        * all secreted proteins and most membrane proteins pass through RER
        * Polypeptides also transported to RER while synthesizing
          * folded into tertiary structure
          * many combine with carbohydrate groups, becoming glycoproteins
            * important in recognition and interaction between cells
      * Smooth Endoplasmic Reticulum
        * more tubular (like coral, RER is wall-y), no ribosomes
        * chemically modifies small molecules like drugs and pesticides
        * site of glycogen degradation
        * synthesis of lipids and steroids
        * stores calcium ions, important for cell responses
  • Golgi Apparatus
      * flattened sacs (cisternae) and small membrane enclosed vesicles
      * further modifies proteins from the RER
      * concentrates, packages, and sorts proteins
      * adds carbs to proteins
      * site of polysaccharide synthesis for plant cell walls
      * three regions
        * cis: recieves
        * trans: exports in vesicles to the membrane or lysosomes
        * medial: medial
  • Lysosomes
      * primary lysosomes originate from golgi apparatus
      * contain hydrolases (digestive enzymes), macromolecules are hydrolyzed into monomers
      * macromolecules enter the cell through phagocytosis
        * part of the membrane encloses the material and a phagosome is formed
        * phagosomes then fuse with primary lysosomes to make secondary lysosomes
        * secondary lysosomes hydrolyze food molecules
      * phagocytes are cells that specialize in taking in materials and breaking them down
      * autophagy is the programmed destruction of cell components (materials and organelles) using lysosomes
      * lysomal storage diseases happen when lysosomes fail to break cell components down
  • Mitochondria
      * energy-rich molecules begin breaking down in the cytosol
      * then it enters the mitochondria where it gets turned into ATP
      * need more energy=more mitochondria
      * 2 membranes
        * outer-very porous
        * extensive folds called cristae increase surface area
      * fluid filled matrix contains enzymes, DNA, and ribosomes
  • peroxisomes
      * collect and break down toxic by-products of metabolism (ex. H2O2) using special enzymes
  • plant organelles
      * plastids
        * some are used for storage
        * give plants their color
        * chromoplasts
        * leucoplasts (starch)
      * chloroplast
        * chlorophyll, photosynthesis, anabolic reaction
        * two membranes
        * internal membranes called thylakoids
          * granum- stack of thylakoids, converts light energy into chemical energy
          * stroma- aqueous matrix around grana
            * contains ribosomes and DNA, synthesizes carbs
      * glyoxysomes
        * convert lipids into carbs for growth
  • vacuoles
      * mainly in plants and fungi
      * storage of waste in toxins, deters herbavores
      * structure for plants, water enters vacuole through osmosis, creates turgor pressure
      * reproduction- colors attract pollinators
      * catabolism- enzyme in seed vacuoles hydrolyze stored food for early growth
      * contractile vacuoles can get rid of water by pumping it out and moving the cell

Cytoskeleton

  • some rod-shaped bacteria have network of actin-like, helical structures that help maintain shape
  • supports and maintains cell shape
  • holds organelles in position and can move them
  • involved in cytoplasmic streaming (things in cell move)
  • interacts with extracellular stuff to hold cell in place
  • dynamic instability
      * filaments can lengthen or shorten with more assembly or detachment
        * actin and myosin interact to contract muscles
  • motor protein
      * any protein that causes movement
  • 3 components
  • microfilaments
      * helps a cell or its parts move
      * determines shape
      * made from actin monomers that attach in the plus end and detach at the minus
        * actin is a protein, important in muscle contraction
  • intermediate filaments
      * 50 kinds, 6 classes
      * tough, ropelike, no dynamic instability
      * anchor cells in place
  • microtubules
      * thickest cytoskeleton component
      * rigid internal skeleton for some cells or regions
      * framework for motor proteins
      * made from dimers of protein tubulin, chains of dimers surround a hollow core
      * \
        * and - ends, dynamic instability
      * polymerization - rigidity, depolymerization- collapse
      * form internal skeleton for cellular appendages
      * cilia
        * short, usually many
        * stiffly moves cell
        * can move fluid over a stationary cell
      * flagella
        * some prokaryotes use flagella to move
        * one or two, longer, push or pull cell through water
      * cilia and flagella microtubules are arranged in 9+2 pattern
        * doublets- 9 fused pairs of tubules form a cylinder
        * one unfused pair in center
        * motion occurs when doublets slide past each other
        * dynein
          * motor protein that can change shape, causes sliding
        * nexin
          * crosslinks doublets and prevents sliding, so it bends
        * kinesin
          * combines with vesicles that walk along the microtubule