Chapter 5: Homeostasis and Transport

1. Chapter 5: Homeostasis and Transport Lesson 1: Cell Structures Involved in Cell Transport

2. Lesson Objectives • Describe the structure and function of the plasma membrane in relation to cell transport. • Identify the types of membrane proteins involved in cell transport. • Identify the roles of the cytoplasm and cytoskeleton in cell transport. • Identify the role of the endomembrane system in eukaryotic organisms in cell transport. • Discuss special transport structure of plant cells. • Outline the role of cell transport in homeostasis.

3. FROM OUTSIDE TO INSIDEThe Plasma Membrane • Barrier between cytoplasm inside and environment outside the cell • Protects, supports the cell • Controls what goes in and what goes out • Selective permeability allows only certain substances to pass through • Water most important substance; most essential molecules for life are soluble • Inorganic ions: sodium (Na+), potassium (K+), calcium (Ca2+), and chlorine (Cl-) • Gases: oxygen helps release energy that powers cellular reactions • Hormones: transmit messages

4. The Phospholipid Bilayer • Consists of two layers of phospholipids • hydrophobic, or water-hating, interior (tails) • hydrophilic, or water-loving, exterior (heads) • Proteins embedded within have a variety of functions • Hydrophobic molecules are nonpolar, easily cross; if small enough • Hydrophilic molecules are polar, need help to cross; usually through embedded proteins

5. Proteins Embedded in the Plasma Membrane • NOT INVOLVED IN TRANSPORT • Peripheral proteins loosely bound to membrane surface or to part of a integral protein • INVOLVED IN TRANSPORT • Integral proteins transmembrane protein; extend into and usually span interior of membrane

6. Integral proteins types… • Transport proteins ions, polar molecules • Types of transport proteins: • Channel proteins hydrophilic channel • Water passes through aquaporin channel proteins • Ions pass through ion channels • Other ions pass through gated channels; open and close in response to a stimulus • Carrier proteins hold molecules and change shape as they pass them through • Sodium-potassium pump

7. Cytoplasm’s Role in Intracellular Transport • Contains enzymes, salts, organelles, and a variety of organic molecules • Enzymes help dissolve cellular waste for export • Salts are conductors of electricity • Movement generated through churning of cytoplasmic streaming speeds up distribution of nutrients, proteins, and organelles within cell • http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter4/animation_-_cytoplasmic_streaming.html

8. Role of Cytoskeleton in the Cytoplasm • Consists of three different threadlike structures: microfilaments, intermediate filaments, microtubules • Microfilaments and microtubules both components of intracellular transport • Microfilaments composed of protein called actin; act like tracks within cells for myosin molecules • Microtubules tracks for vesicle intracellular transport

9. The Endomembrane System in Eukaryotic Cells • Separates and compartmentalizes interrelated functions of variety of organelles • Equates to a division of labor • Organelles involved: • Nuclear envelope, smooth endoplasmic reticulum, rough ER, Golgi, lysosomes, endosomes, vesicles, vacuoles, and plasma membrane • Some through direct connection; others through vesicular transport • Vesicles bud off of sending organelles; fuse with receiving organelles

10. The Endoplasmic Reticulum (ER) • Helps make and transport proteins and lipids • Connected to the pores of the nuclear envelope • Two types of endoplasmic reticulum: • Rough endoplasmic reticulum (RER) • Smooth endoplasmic reticulum (SER). • Rough Endoplasmic Reticulum (RER) • Studded with ribosomes; involved in protein synthesis, the production and transport of new membrane, and the modification and transport of newly formed proteins within the cell • Proteins synthesized on the RER are transported to other locations through vesicles formed in the SER. • Smooth Endoplasmic Reticulum (SER) • Not studded with ribosomes • Contains enzymes for lipid biosynthesis (change to phospholipids and steroids) • Forms transition vesicles that travel along microtubular tracks in the cytoplasm • Transport molecules made in the RER to the Golgi apparatus

11. The Golgi Apparatus • Processes proteins and prepares them for use both inside and outside the cell • Receives proteins from the ER that have been transported in vesicles, packages and labels them • Sends them on to their next destinations in another set of vesicles • Also involved in the transport of lipids around the cell http://www.johnkyrk.com/golgiAlone.html

12. Vesicles • Sac-like organelles • Store and transport large molecules • Pinch off ER and Golgi • Transport to plasma membrane; fuse with it • Going out = exocytosis • Pinch inward at plasma membrane • Transport to destinations inside the cell • Going in = endocytosis

13. Lysosomes and Endosomes • Lysosomes bud off of the Golgi apparatus; infused with hydrolytic enzymes • Digestive vesicles of the cell; contain enzymes called hydrolases • Digest proteins, nucleic acids, lipids, and complex sugars. • Vesicles enter the cell through endocytosis; sent to lysosomes contents processed • Break down and disarm many potentially pathogenic and foreign materials; expel them outside the cell through exocytosis • Endosomes formed during endocytosis allow materials from outside the cell to enter the cell • Formed when the cell’s plasma membrane folds inward to surround macromolecules, encircles them, brings them into the cell by pinching off the membrane at their point of entry • Vesicles larger than 100 nanometers in size are referred to as vacuoles.

14. Special Transport Structure in Plant Cells • Structure not found in animal cells, cell wall • Rigid layer that surrounds the plasma membrane • Supports and protects the cell • Tiny holes, or pores, in the cell wall called plasmodesmata (singular, plasmodesma) • Form open channels through which strands of cytosol connect between adjacent cells • Allow water, nutrients, and other substances to move into and out of the cells

15. Homeostasis and Cell Function • Cell to function normally=> stable state must be maintained inside the cell • Example: concentration of salts, nutrients, and other substances must be kept within a certain range • Process of maintaining stable conditions inside a cell (or an entire organism) is homeostasis • Homeostasis requires constant adjustments, because conditions are always changing both inside and outside the cell • Structures and processes described in this lesson and the next lesson play important roles in homeostasis and are consider homeostatic mechanisms • By moving substances into and out of cells, they keep conditions within normal ranges and maintain homeostatic regulation inside the cells and the organism as a whole • If homeostatic mechanisms fail to maintain homeostatic regulation disease or death of a cell or organism may follow.

16. Lesson Summary • A major role of the plasma membrane is transporting substances into and out of the cell. • The plasma membrane is selectively permeable, allowing only certain substances to pass through. • Proteins embedded within the plasma membrane help to move hydrophilic, polar molecules into the cell. • The cytoplasm is the internal medium for cell transport. • Vesicle aids in the import and export of macromolecules. • Plant cells have a special transport structures that are not found in animal cells, called plasmodesmata. • Cell transport helps cells maintain homeostasis by keeping conditions within normal ranges inside all of an organism’s cells. • If homeostatic mechanisms fail, homeostatic regulation may fluctuate and disease or death of a cell or organism can follow.