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Chapter 3: Cells:

The Living Units

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The Cellular Basis of Life: Cell

Theory

• A cell is the basic structural & functional unit

of living organisms

• The activity of an organism depends both the

individual & collective activity of its cells

• Biochemical activities of cells are dictated by

the relative number of their specific

subcellular structures

• Continuity of life has a cellular basis

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Cell Diversity

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The Composite Cell

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The Plasma Membrane: Structure

• Very thin, selectively

permeable phospholipid

bilayer

• Membrane proteins:

surface receptors,

pumps, channels,

cytoskeletal anchors

• Membrane

carbohydrates- Cell

recognition, “ID tags”

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The Plasma Membrane: Functions

• Defines the cell’s boundary

• Selectively permeable membrane that

transports substances in and out of the

cell

• Generates and maintains the resting

membrane potential

• Mediates cell-cell and cell-environment

interactions

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Transport: Protein channels and pumps transport specific

substances into the cell

Enzymatic Activity: Some enzymes are at the membrane, supplying

products where they are needed

Signal Transduction: Some membrane proteins are receptors that

receive external signals and initiate cascades of

chemical reactions

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Intercellular Joining: Cell adhesion molecules (CAMS) provide binding

sites for cell-cell interactions

Cell-Cell Recognition: Some glycoproteins serve as cellular “ID” tags that

allow cells to recognize each other

Attachment to the cytoskeleton and

ECM: Some membrane proteins act as anchors for the

cell, binding the cell to its environment and

supporting the cytoskeleton

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Plasma Membrane Special Structures

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Special Structures: Tight Junctions

• Interlocking junctional

proteins fused together to

form a relatively

impermeable barrier around

cells

• Prevents molecules from

passing between adjacent

cells

• Example: Digestive tract

lining to prevent “seeping”

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Special Structures: Desmosomes

• Plaque & linker proteins

interdigitate like a zipper

• Allow for cell sheets to

form

• Also prevents cell sheets

subjected to mechanical

stress from tearing

• Examples: Skin, heart

muscle

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Special Structures: Gap Junctions

• Communicating junction between cells

• Connexons are transmembrane proteins that form channels that span across adjacent cells

• Essential for excitable tissue that require rapid communication in order to synchronize activities

• Examples: Cardiac & smooth muscle

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Membrane Transport

• Cells are surrounded by extracellular fluid/

interstitial fluid containing nutrients, amino acids,

sugars, fatty acids, vitamins, etc.

• Cells need to extract essential substances from

interstitial fluid in order to survive

•Traffic across the plasma membrane is constant but

is also selective on what substances are introduced

into the cell

•Transport is either passive or active 14

Membrane Transport

• Passive Processes

– Diffusion

• Simple Diffusion

• Facilitated Diffusion (channel & carrier-mediated)

• Osmosis

– Filtration

• Active Processes

– Active Transport

– Vesicular Transport

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Diffusion

• The tendency of molecules or ions to scatter evenly

throughout the environment

• Molecules and ions move down their concentration

gradients until equilibrium is reached

[High] [Low]

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Diffusion

Molecules diffuse through the membrane if:

1)Lipid-soluble

2)Small

3)Assisted by a carrier molecule

Types of Diffusion:

1)Simple Diffusion: Unassisted diffusion

2)Facilitated Diffusion: Assisted diffusion

a)Channel-mediated

b)Carrier-mediated

3) Osmosis: Diffusion of solvents (water)

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Simple Diffusion

• Small, non-polar, lipid-soluble

substances diffuse directly

through the lipid bilayer from

regions of high concentration

to low concentration

• Examples: Oxygen, Carbon

dioxide, fat-soluble vitamins

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Channel-Mediated Facilitated Diffusion

• Protein channels form

“tunnels” that allow small, lipid-

insoluble molecules (water,

ions) into the cell

• Channels are selective, only

allowing molecules of certain

sizes and charges to pass

through 19

Carrier-Mediated Facilitated Diffusion

• Large, polar, lipid-

insoluble molecules such

as sugars and amino

acids bind to protein

carriers and are “shuttled”

across the membrane

• Carrier membrane

proteins are molecule-

specific 20

Osmosis

• Specific to the movement of

water (a solvent) through a

membrane from regions of high

water concentrations to regions

of low water concentrations

• Osmotic diffusion of water may

be directly through the lipid

bilayer or through aquaporins

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Diffusion –vs- Osmosis

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Tonicity

Hypertonic: Osmotic pressure

outside the cell is higher, water

leaves cell faster than it enters

Isotonic: Osmotic pressure in and

out of cell are equal, water enters

and leaves cell at same rate

Hypotonic: Osmotic pressure

inside cell the higher, water enters

the cell faster than it leaves 23

Filtration

• Movement of molecules

through membranes

from regions of high

hydrostatic pressure

to regions of lower

hydrostatic pressure

• Separation of solids

from fluids

• Generally applies only

to capillary walls

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