The Cellular Level of Organization 1

A cell is the basic, living, structural and functional unit of the body. Cell Theory: the building blocks of all plants and animals all cells come from the division of preexisting cells cells are the smallest units that perform all vital physiological functions each cell maintains homeostasis at the cellular level. 2

Cells are measured in micrometers. Cells vary in size and shape. Shape is determined by function. Two types of cells: –Sex cells –Somatic (body) cells Cells are surrounded by extracellular fluid, which is called interstitial fluid in most tissues 3

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Every Eukaryotic cell has three main parts: Plasma (cell) membrane - separates inside of cell from external environment. Nucleus – organelle that contains the cell’s DNA and is surrounded by a double membrane. Cytoplasm – everything from the nuclear membrane to the plasma membrane 6

Cytoplasm refers to cytosol plus organelles and inclusions. cytosol - contains proteins, enzymes, nutrients, ions, and other small molecules organelles - highly organized structures with characteristic shapes that are specialized for specific cellular activities. inclusions - are temporary structures in the cytoplasm that contain secretions and storage products of the cell. 7

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Plasma membrane Physical isolation Regulation of exchange with the environment Sensitivity to the environment –Signal transduction Structural support 9

Most of the surface area of the cell membrane is made of phospholipid, but accounts for only 42% of the weight of the membrane. Proteins – important in many functions Also find glycolipids and cholesterol. Phosphoslipid is an amphipathic molecule – phosphate heads on the outside and inside, and fatty acid tails in the middle. 10

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Membrane is fluid - fatty acid tails are unsaturated The membrane is selectively permeable – it allows fat soluble substances to pass through (such as steroid hormones) and some other small, uncharged molecules. Cholesterol is a large molecule, and helps to stabilize the membrane. 12

Membrane carbohydrates 3-5 % of membrane Proteoglycans, glycoproteins and glycolipids Gylcocalyx –Lubrication and protection –Anchoring and locomotion –Specificity in binding –Recognition 13

Fluid mosaic model - proteins float like icebergs in a sea of phospholipids. Proteins can be integral proteins – go all the way through the membrane, or may be peripheral proteins -bound to the inside or outside membrane. 14

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Integral Proteins can be channels or transporters. Peripheral proteins can be receptors, enzymes or can be cell identity markers 16

Membrane proteins Anchoring proteins Recognition proteins Enzymes Receptor proteins Carrier proteins Channels 17

Intercelluar junctions Tight junctions – membranes of adjacent cells bound together by occludins and claudins formingAn impermeable junction. Desmosomes are protein “spot welds” in skin and cardiac muscle: –plaques, linker protein filaments, and thicker filaments across inside of cell 18

Intercellular junctions Gap junctions are tubular channels (connexons) that connect the cytoplasm of one cell with that of another. –Ions, simple sugars and other small molecules Cellular Adhesion Molecules help cells form temporary attachments to other cells. CAMs 19

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Membrane proteins Anchoring proteins Recognition proteins Enzymes Receptor proteins –Ligands bind Carrier proteins – allows establishment of electrochemical gradient Channels Rafts –lipid rafts – tails saturated; more cholesterol 22

Membrane Physiology Cell membrane function: –Cellular communication –Establish an electrochemical gradient –Are selectively permeable Lipids Size Electrical charge Presence of channels and transporters 23

Movement of materials Passive processes: –Depend on concentration and kinetic energy –Do not require energy –Move substances from an area of high concentration to an area of low concentration Down a concentration gradient 24

Diffusion Rate depends on: –Temperature –Gradient size –Distance –Molecule size –Electrical forces Reaches equilibrium or Physiological steady state 25

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Simple diffusion Channel mediated diffusion –0.8 nm –Size and charge –Interaction between ion and channel walls Rate limited by number of suitable channels - Na, K, Cl pass through membranes at a rate comparable to simple diffusion 28

Osmosis Movement of WATER through a selectively permeable membrane Moves according to the conc. of water Osmotic pressure 29

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Tonicity Concentration of one solution relative to another ( conc. in cytoplasm) Isotonic – equal concentrations –0.9 % NaCl or 5% glucose soln. Hypertonic – more concentrated Hypotonic – less concentrated 31

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Cell in a hypertonic solution crenation 34

Cell in a hypotonic solution 35

osmosis Eliminates conc. differences faster than solute diffusion Aquaporins - water channels 36

Facilitated diffusion Uses carrier molecules Down a conc. gradient Specificity Saturation limits regulation 37

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Filtration -a type of bulk flow where the movement of water and dissolved substances across a membrane is due to gravity or hydrostatic pressure (water pressure). 39

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Active Transport Depends on the use of energy (ATP) Moves substances up a concentration gradient (up hill) These systems are often called “pumps” –Na+ / K+ pump - Na/K ATPase –Others carry Ca ++, Mg ++, I -, Cl - and Fe ++ 42

Active transport Countertransport –Exchange pump Cotransport or symport –Move two different substances in same direction –One down a conc. Gradient –Use of energy to pump one substance back out 43

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Vesicular Transport Exocytosis – moving substances outside the cell Endocytosis – taking substances into the cell clathrin proteins Pinocytosis – “cell drinking” Phagocytosis – “cell eating” Receptor mediated endocytosis 45

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Receptor mediated endocytosis 48

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Exocytosis 50

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