Cells: The Living Units Part A 3

Structure of a Generalized Animal Cell Figure 3.2

Plasma Membrane Separates intracellular fluids from extracellular fluids Bilayer consists of phospholipids, cholesterol, glycolipids and imbedded proteins

Functions of Membrane Proteins

Passive Membrane Transport: Diffusion Simple diffusion – nonpolar and lipid-soluble substances like steroids Facilitated diffusion Specific carries for some molecules Protein channels

Diffusion 6 16 11 Membrane permeable to solute & H2O

Osmosis - Diffusion of water Diffusion of water across a semipermeable membrane Occurs when the concentration of a solvent is different on opposite sides of a membrane Osmolarity – concentration of all solute molecules in a solution Tonicity – term describing relative osmolarity of two solutions Hypo – below (lower osmolarity) Hyper – above (higher osmolarity) Iso – equal (same osmolarity)

Osmosis 6 M 16 M 11 M 11 M Membrane permeable only to H2O

Active Transport Uses ATP to move solutes across a membrane Requires carrier proteins – pumps Categories of Carrier Proteins Uniport – a single substance Symport system – two substances, simultaneously, same direction Antiport system – two substances, simultaneously, opposite directions

Active Transport: Sodium-Potassium Pump 6 K+ is released and Na+ sites are ready to bind Na+ again; the cycle repeats. Extracellular fluid Binding of cytoplasmic Na+ to the pump protein stimulates phosphorylation by ATP. 1 Cytoplasm low high 2 Phosphorylation causes the protein to change its shape. low high Concentration gradients of K+ and Na+ 5 Loss of phosphate restores the original conformation of the pump protein. 3 The shape change expels Na+ to the outside, and extracellular K+ binds. 4 K+ binding triggers release of the phosphate group.

Types of Active Transport Primary active transport –phosphorylation of the carrier protein by ATP causes conformational change Secondary active transport – use of an exchange pump (such as the Na+-K+ pump) to indirectly drive the transport of other solutes

Membrane Potential Voltage across a membrane Na+ and K+ concentration gradients active transport of ions by pumps & diffusion of K+ through channels Ranges from –20 to –100 mV

Roles of Membrane Receptors Receive stimulus from outside cell and transmit signal into cell Ligand Proteins/peptides Neurotransmitters Ligand activates receptor Receptor activates a signaling pathway inside the cell

Cytoplasm Cytoplasm – material between plasma membrane and the nucleus Cytosol – aqueous solution of proteins, salts, sugars, etc.. Cytoplasmic organelles – metabolic machinery of the cell

Cytoplasmic Organelles Membranous nucleus mitochondria vesicles endoplasmic reticulum Golgi apparatus Nonmembranous cytoskeleton centrioles ribosomes

Nucleus Figure 3.28a

Chromosomes Double stranded molecules of DNA DNA compacted with proteins

DNA Replication DNA polymerase replicates each strand of original double helix Two copies are separated into daughter cells during mitosis Figure 3.31

Cell Cycle Interphase Mitotic phase G1 DNA synthesis (S) G2 Prophase Metaphase Anaphase Telophase Figure 3.30

From DNA to Protein – Gene Expression Regions of DNA constitute genes Genes encode for RNA RNA is produced by the biochemical process of transcription RNA leaves the nucleus and is used to direct protein synthesis –translation - in the cytoplasm

RNA Synthesis - Transcription

Ribosomes Composed of rRNA and proteins Carry out protein synthesis Cytoplasmic ribosomes synthesize soluble proteins ER membrane-attached ribosomes synthesize proteins to be incorporated into membranes or secreted from cell

Protein Synthesis - Translation Figure 3.37

Protein Synthesis by ER-bound Ribosomes Figure 3.19

Endoplasmic Reticulum (ER) Interconnected membrane compartments Continuous with the outer nuclear membrane rough ER and smooth ER

Golgi Apparatus Modification, concentration, and packaging of proteins Vesicles from the ER fuse with the Golgi apparatus Proteins pass through the Golgi apparatus Secretory vesicles leave the Golgi and move to designated parts of the cell

The Endomembrane System – ER-Golgi-Vesicles Figure 3.21

Vesicular Transport Transport of large particles and macromolecules across plasma membranes Exocytosis – out of cell Endocytosis – into cell Phagocytosis – large particles into cell Vesicular trafficking – place to place within cell

Specialized Vesicles Lysosomes Peroxisomes Vesicles containing digestive enzymes Digest materials ingested by the cell during endocytosis & phagocytosis Peroxisomes Vesicles containing oxidases and catalases Carry out biochemical reactions requiring O free radicals

Mitochondria Generates ATP by aerobic cellular respiration Krebs’ Cycle Oxidative-Phosphorylation

Cytoskeleton Dynamic, rod-like proteins throughout the cytosol and underlying plasma membrane Three major types Microtubules Intermediate filaments Microfilaments

Microtubules Dynamic, hollow tubes made of tubulin proteins Determine overall shape of cell and distribution of organelles Organized by the centrioles

Centrioles Barrel-shaped organelles located near the nucleus Pinwheel array of short microtubules Organize mitotic spindle during mitosis

Motor Molecules & Microtubules Examples: dyein, kinesin Powered by ATP Attach organelles to microtubules & microtubules to each other

Cilia & Flagella Whiplike, motile cellular extensions Cilia Flagella Found in multiple arrays on apical surface of some epithelial cells Respiratory tract Fallopian tubes Move substances in one direction across cell surfaces Flagella Single structure Moves cell Sperm are only human cell with flagellum

Cilia

Microfilaments Dynamic strands of the protein -actin Attached to the inside of the plasma membrane Braces and strengthens the cell surface Function in endocytosis, exocytosis and cell movement

Intermediate Filaments Tough, protein fibers resist pulling forces on the cell Attaches to inside of membrane at desmosomes

Cell Adhesion Complexes Anchor cells to each other Desomsomes Adherins junctions Tight junctions Allow cell motility by anchoring to the extracellular matrix Focal adhesions Allow cell-cell movement of relatively large molecules Gap junctions Cell adhesion molecules (CAMs) desmin cadherin integrin

Tight Junction Figure 3.5a

Desmosomes & Adherens Junctions Figure 3.5b

Gap Junction Figure 3.5c