chapter3b

Active Processes

• ATP is necessary to transport substances that are:– Too large– Non-soluble– Unable to move across its concentration gradient

• Active Transport– Primary & Secondary Active Transport

• Vesicular Transport– Endocytosis & Exocytosis

Primary Active Transport

•The energy to do work comes directly from the hydrolysis of ATP

•Example: Sodium-Potassium Pump

The Na+ - K+ Pump

• Na+-K+ ATPase maintains ↑ [K+] inside cells & ↑ [Na+] outside cells

• Binds 3 Na+ : 2 K+

• Creates cell membrane’s electrochemical gradient

• Crucial to cardiac & skeletal muscle, and nervous functions

• See video

Resting Membrane Potential

• Selective permeability allows for the generation of a membrane potential (voltage)

• At rest, the cell membrane has a (-) membrane potential

• Important to excitable tissue like nervous tissue

Secondary Active Transport

• Indirectly driven by primary active transport through the creation of ionic gradients

• Molecules or ions move from regions of lower concentration to regions of higher concentration

Vesicular Transport

• Large particles, macromolecules, and larger volumes of fluids do not fit through channels of protein pumps and must be transported in and out of the cell through vesicles

• Exocytosis: Out of the cell

• Endocytosis: Into the cell

Exocytosis

• Secretions within vesicles dock at the membrane and are released as the cell & vesicular membranes fuse

• Used during hormone secretion, neurotransmitter release, mucus secretion & waste elimination

Endocytosis: Clathrin-mediated

• The main process used for endocytosis

• Clathrin-mediated transport is used during phagocytosis, pinocytosis, and receptor-mediated endocytosis

• Clathrin is a lattice-like protein that cages in cargo for transport into the cell

Endocytosis: Phagocytosis

• Primarily used by defense cells like WBCs and macrophages

• Large, solid substances such as bacteria and dead cells are engulfed and subsequently destroyed

Endocytosis: Pinocytosis

• Droplets of extracellular fluid containing dissolved particles are folded into the plasma membrane

• Nutrients dissolved in extracellular fluid are taken into the cell

• Particularly important in the lining of the small intestine

Endocytosis: Receptor-mediated

• Selective mechanism for bringing specific macromolecules into the cell

• Receptors bind with their specific ligands (enzymes, insulin, hormones) and are endocytosed

The Cytoplasm

• Gel-like material, consistency of room temperature butter

• Contains the cytosol (viscous, clear liquid), organelles (“cell machinery”), the cytoskeleton (scaffolding & motor units), and inclusion bodies (stored nutrients, pigmentation)

• Most cellular activities take place in the cytoplasm

Mitochondria

• Organelle whose inner membrane is folded into shelf-like partitions called “cristae”

• Abundance depends on cell type

• Function: "Power plants of the cell”

Ribosomes

• Small granules dispersed throughout the cytoplasm and on the membranes of Rough ER, composed of ribosomal RNA and protein

• Function: protein synthesis

Endoplasmic Reticulum

• Network of interconnected parallel membranes that is continuous with the nuclear membrane

• Rough ER: Studded with ribosomes

• Smooth ER: No ribosomes

RER & Protein Synthesis

• Newly synthesized proteins are taken into the RER cisternae where they undergo modification before they are transported where needed

• RER is also a “membrane factory”. External face is site for phospholipid synthesis

Smooth Endoplasmic Reticulum• Contains enzymes that

catalyze the synthesis of lipids and steroids

• Plays a role in lipid metabolism and drug detoxification

• Catalyzes the decomposition of glycogen to release glucose

• In muscle, stores Ca+2

Golgi Apparatus

• Flattened membranous sacs ("cisternae") arranged in stacks associated with many vesicles

• Function: modification, packaging, and transport of proteins, the cell’s “traffic director”

Lysosomes

• Spherical membranous sacs containing digestive enzymes (acid hydrolase)

• Lysosome Function: “demolition crew of the cell“

Review: Endomembrane System

Peroxisomes

• Membrane sacs that look like lysosomes abundant in liver and kidney cells containing oxidases

• Oxidases detoxify harmful substances & neutralize free radicals by converting them into hydrogen peroxide and water