Chapter 3: Cells: The Living Units

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

Cell Diversity

The Composite Cell

The Plasma Membrane: Structure

• Very thin, selectively permeable phospholipid bilayer

• Membrane proteins: surface receptors, pumps, channels, cytoskeletal anchors

• Membrane carbohydrates- Cell recognition, “ID tags”

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

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

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

Plasma Membrane Special Structures

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”

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

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

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

Membrane Transport

• Passive Processes– Diffusion

• Simple Diffusion• Facilitated Diffusion (channel & carrier-mediated)• Osmosis

– Filtration

• Active Processes– Active Transport– Vesicular Transport

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]

DiffusionMolecules diffuse through the membrane if:1)Lipid-soluble2)Small3)Assisted by a carrier molecule

Types of Diffusion:1)Simple Diffusion: Unassisted diffusion2)Facilitated Diffusion: Assisted diffusion

a)Channel-mediatedb)Carrier-mediated

3) Osmosis: Diffusion of solvents (water)

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

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

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

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

Diffusion –vs- Osmosis

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

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