Chapter 4 CELL MEMBRANE, CYTOSKELETON & CELL-CELL INTERACTIONS

A. Cell Membrane Structure Cell membrane is a phospholipid bilayer embedded with mobile proteins. Phosphate head of phospholipid is hydrophilic. Fatty acid tails are hydrophobic. Proteins

Types of membrane proteins: F Transport proteins - move substances across membrane F Cell surface proteins - establish “self” F Cellular adhesion molecules (CAMs) - enable cells to stick to each other F Receptor proteins - receive & transmit messages into a cell

Additional molecules may be associated with proteins & phospholipids: ] cholesterol ] sugar molecules F glycoproteins F glycolipids

B. Movement Across Membranes Cell membranes are selectively permeable. 1. Simple diffusion (passive) Substance moves across phospholipids from an area of high to an area of low concentration, without using energy. Substance moves down its concentration gradient Ex. O 2, CO 2

Simple Diffusion Diffusion continues until dynamic equilibrium is reached. Cell membrane Transport protein

Movement of water across membranes by simple diffusion is called osmosis. Water is driven to move because the membrane is impermeable to solute(s).

Tonicity Refers to differences in solute concentration on either side of a semipermeable membrane. ] Isotonic - both solutions have the same solute concentrations. ] Hypotonic - solution with the lower solute concentration. ] Hypertonic - solution with the higher solute concentration.

What is effect of immersing an animal cell in a hypertonic or hypotonic solution?

What is effect of immersing a plant cell in a hypertonic or hypotonic solution? Cell immersed in hypotonic solution Cell immersed in hypertonic solution

2. Facilitated Diffusion (passive) Substance moves through a transport protein from an area of high to an area of low concentration, without using energy. Substance moves down its concentration gradient Ex. glucose

Facilitated Diffusion Cell membrane Transport protein

Cell: Environment: 1% sucrose 3% sucrose 1% glucose 2% glucose 1% fructose 1% fructose 97% water 94% water Assume cell membrane is permeable to water, glucose & fructose, but impermeable to sucrose. In which direction will sucrose, glucose, fructose & water move?

3. Active Transport (active) Substance moves through a transport protein from an area of low to an area of high concentration; requires energy. Substance moves against its concentration gradient Ex. ions (Na +, K +, Cl - )

Active transport Cell membrane Transport protein ATP

Active transport of Na + & K + through the sodium-potassium pump (transport protein).

4. Cotransport The active transport of one substance is coupled to the passive transport of another. Ex. sucrose (plant cells)

5. Exocytosis, Endocytosis & Transcytosis Movement of large particles across membranes with the help of vesicles. Exocytosis - vesicles move particles out of the cell. Ex. release of enzymes from head of sperm; neurotransmitter release

Endocytosis - vesicles move particles into the cell. Three types of endocytosis: PinocytosisPhagocytosis Receptor- mediated endocytosis

Transcytosis - combines endocytosis & exocytosis. Vesicles rapidly transport particles through cells. Ex. transport of nutrient monomers through cells lining digestive tract & into the bloodstream

C. Cytoskeleton The structural framework of a cell. 1. Microtubules - hollow, thick elements made of the protein tubulin. Functions: ] move chromosomes apart during cell division ] form cilia & flagella

2. Microfilaments - long, thin elements made of the protein actin. Functions: ] connect cells to each other ] move vesicles & organelles within cytoplasm ] help cells move

3. Intermediate filaments - elements with diameters in between that of microtubules & microfilaments. Made of various proteins (ie. keratin) Functions: ] maintain cell shape ] connect cells to each other & to underlying tissue (skin cells)

D. Intercellular Junctions Structures that connect cells of multicellular organisms to form tissues. 1. Animal cell Connections Tight Junctions - cell membranes of adjacent cells are fused, creating a tight seal. Ex. cells lining small intestine; cells lining capillaries in brain

Desmosomes - intermediate filaments weld cell membranes of adjacent cells together in isolated spots. Ex. skin cells Gap Junctions - channels that link the cytoplasm of adjacent cells, allowing exchange of materials. Ex. heart muscle cells

2. Plant Cell Connections Plasmodesmata - channels that link the cytoplasm of adjacent plant cells, allowing the exchange of cytoplasm & organelles. Ex. cells conducting water & nutrients

E. Cell-Cell Interactions 1. Cell Adhesion Process that uses membrane proteins called cellular adhesion molecules (CAMs) to direct the migration of cells. Various CAMs function in sequence to: ] guide WBCs to injury sites ] guide embryonic cells to help form placenta ] establish nerve connections involved in learning & memory

CAMs directing WBCs to injury sites.

2. Signal Transduction Process by which cells receive, amplify, & respond to outside stimuli.