The Cellular Level of Organization

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A cell is the basic, living, structural and functional unit of the body.

Cell Theory:the building blocks of all plants and

animalsall cells come from the division of

preexisting cellscells are the smallest units that perform

all vital physiological functionseach 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

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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

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Cytoplasm refers to cytosol plus organelles and inclusions. cytosol - contains proteins, enzymes, nutrients, ions, and other small moleculesorganelles - highly organized structures with

characteristic shapes that are specialized for specific cellular activities.

inclusions - are temporary structures in thecytoplasm that contain secretions and storage products of the cell.

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Plasma membrane

• Physical isolation• Regulation of exchange with the

environment• Sensitivity to the environment

– Signal transduction• Structural support

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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.

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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.

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Membrane carbohydrates

• 3-5 % of membrane• Proteoglycans, glycoproteins and

glycolipids• Gylcocalyx

– Lubrication and protection– Anchoring and locomotion– Specificity in binding– Recognition

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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.

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Integral Proteins can be channels or transporters.

Peripheral proteins can be receptors, enzymes or can be cell identity markers

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Membrane proteins

• Anchoring proteins• Recognition proteins• Enzymes• Receptor proteins• Carrier proteins• Channels

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Intercelluar junctionsTight 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

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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

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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

cholesterol22

Membrane Physiology• Cell membrane function:

– Cellular communication– Establish an electrochemical gradient– Are selectively permeable

• Lipids• Size• Electrical charge• Presence of channels and transporters

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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

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Diffusion

• Rate depends on:– Temperature– Gradient size– Distance – Molecule size– Electrical forces

• Reaches equilibrium or• Physiological steady state

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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

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Osmosis

• Movement of WATER through a selectively permeable membrane

• Moves according to the conc. of water• Osmotic pressure

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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

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Cell in a hypertonic solution

crenation

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Cell in a hypotonic solution

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osmosis

• Eliminates conc. differences faster than solute diffusion

• Aquaporins - water channels

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Facilitated diffusion

• Uses carrier molecules• Down a conc. gradient• Specificity• Saturation limits• regulation

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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).

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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++

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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

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Vesicular TransportExocytosis – moving substances outside the cell

Endocytosis – taking substances into the cell

clathrin proteins

Pinocytosis – “cell drinking”

Phagocytosis – “cell eating”

Receptor mediated endocytosis

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Receptor mediated endocytosis

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Exocytosis

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