Anatomy & Physiology Lecture: Active Transport

Active Membrane Transport

• Occurs when cells uses energy, directly or indirectly, to move solutes across the membrane AGAINST their concentration gradient (low to high concentration) 

  • Requires energy (ATP) 

• Two major active transport processes

  • Active transport 

  • Vesicular transport 

• Both require ATP to move solutes across a plasma membrane for any of these reasons: 

  • Too large for channels 

  • Not lipid soluble

  • Unable to move down concentration gradient

Primary active transport

• Energy comes directly from hydrolysis of ATP by transport protein called pumps 

• Energy from hydrolysis of ATP changes the shape of transport protein, causing solutes (ions) bound to protein to be pumped across membrane 

Secondary Active transport

• Energy stores in concentration gradient of ions created by primary active transport pumps 

• Always move more than one substance at a time using a cotransport protein

• Also called cotransport 

  • Relies on ion gradient created by primary active transport system 

  • Energy stored in gradients indirectly drives transport of other solutes 

    • Low Na concentrations inside the cell increases Na”s drive to enter the cell

    • Na can drag other molecule with it as it enters via carrier proteins 

    • Some sugars, amino acids, and ions are usually transported into cells via secondary active transport

  • Symporters: transport two different substances in the same direction

  • Antiporters: transport one substance into cell while transporting a different substance out of cell

Primary Active transport: Sodium Potassium Pump

• Most studied pump

• The pump protein is an enzymes that pumps 3 NA^+ out of the cell and 2 K^+ into cell for each ATP molecule used 

• Located in the plasma membrane, but especially active in excitable cells (nerves and muscles) 

• Leakage channels located in the plasma membrane result in leaking of NA^+ into the cell and leaking of K^+ out of cell 

  • Both travel down their concentration gradient

• Maintains electrochemical gradients, which involve both concentration and electrical charge of ions 

  • Essential for functions of muscle and nerve tissues

Vesicular Transport

• Moves large substances or large amounts across cellular membranes inside bubble-like, membranous sacs called vesicles 

  • Requires energy (usually ATP) 

Phagocytosis

• Types of endocytosis, referred to as “cell eating” 

  • Membrane projections form and engulf solids

  • They formed vesicle is called a phagosome

  • Exhibited by macrophages and certain other white blood cells 

    • Move by amoeboid motion cytoplasm flows into temporary entensions that allow cell to creep 

Pinocytosis

• Referred to as “cell drinking” or fluid-phase endocytosis

• Plasma membrane infolds, bringing extracellular fluid and dissolved solutes inside cell

• Allows certain cells to “sample” environment 

• Nutrient absorption in the small intestine 

• Membrane components are recycled back to membrane 

• No receptors are used, process is nonspecific 

Receptor - Mediated Endocytosis

• Involves the endocytosis and transcytosis of specific molecules

• Allows cells to selectively concentration material against gradient 

• Receptors are high specific to certain substances

  • Enzymes 

  • Insulun

  • Low density lipoproteins 

  • Iron 

  • Flu viruses 

  • Diphtheria, and cholera toxins

Exocytosis

• Process of ejecting material from cell 

• Activated by cell-surface signals or changes in membrane voltage

• Ejected substances enclosed in secretory vesicle

• Proteins on vesicle called v-SNARE finds and hooks up to target t-SNARE proteins on membrane

  • Docking process triggers exocytosis 

• Exocytosed substances:

  • Hormones 

  • neurotransmitters

  • Mucus

  • Cellular Wastes 

Transcytosis

moves substances into, across, and then out of the cell

Vesicular Trafficking

moves substances from area to another within the cell 

Endocytosis

• transport into cell 

  • Phagocytosis

  • Pinocystosis 

  • Receptor-mediated endocytosis

exocytosis

• transport out of cell 

  • Involves formation of protein coated vesicles 

  • Usually involve receptors, selective process 

    • Substance entering cell binds to its unique receptor 

  • Some pathogens can hijack receptors allowing them transport into cell 

  • After entry, vesicles may: 

    • Fuse with lysosome

    • Undergo trancytosis