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