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