Cellular Level of Organization

1. Cellular Level of Organization Lesson 1

2. Main Parts of the Cell • Plasma membrane • Forms the cell’s flexible outer surface • A selective barrier that regulates the flow of material into and out of the cell • Maintains homeostasis • Acts in communication among cells and between cells and their external environment

3. Cytoplasm • Contains all of the cellular components between the plasma membrane and the nucleus • Composed of two parts • Cytosol • Fluid portion made up of water, dissolved solutes, and suspended particles • Organelles • Cellular organs that have a specific shape and function within the cell

4. Nucleus • Large organelle that contains the cell’s nuclear DNA • Regulates the overall function of the cell • Responsible for cell reproduction

5. Structure of the Plasma Membrane • Consists of a lipid bilayer made up of three types of lipid molecules: phospholipids, cholesterol, and glycolipids • Lipids have both polar and nonpolar parts and are terms amphipathic • Head of the lipid is polar and the hydrocarbon tail is nonpolar • In the bilayer, the nonpolar tails form the inside of the layer and the polar heads are oriented outward • The nonpolar inside is hydrophobic and the polar outside is hydrophylic

6. Arrangement of Membrane Proteins • Integral proteins • Extend into or through the lipid bilayer and are firmly embedded in it • Transmembrane proteins extend from the cytosol to the extracellular fluid • Are amphipathic • Act as transporter proteins allowing certain materials into and out of the cell

7. Peripheral proteins • Found either inside or outside of the lipid bilayer • Act as cell identifiers • Allow cells to recognize each other as belonging to the organism

8. Functions of Membrane Proteins • Integral membrane proteins • Ion channels: pores that allow specific ions through the plasma membrane • Transporters: selectively move a polar substance or ion from one side of the membrane to the other • Receptors: serve as cellular recognition sites

9. Enzymes: catalyze specific chemical reactions at the inside or outside surface of the cell • Linkers: anchor proteins in the plasma membranes of neighboring cells to one another or to protein filaments • Cell-identity markers: enable a cell to recognize other cells of the same kind during tissue formation and/or to respond to potentially dangerous foreign cells.

10. Peripheral proteins • Enzymes • Linkers • Support the plasma membrane • Anchor integral proteins • Move materials and organelles within cells • Change cell shape in dividing and muscle cells • Attachment of cells to each other

11. Selective Permeability • Plasma membranes allow some materials to freely pass through it by diffusion while keeping other materials from entering the cell • Transport channels allow specific materials into the cell that cannot diffuse across the plasma membrane • The ability to regulate the entrance of materials into the cells is called selective permeability • Materials will pass back and forth across the plasma membrane in response to concentration and electrical gradients that form from the differences in solute concentration within and outside the cell

12. Transport Across the Plasma Membrane • Occurs through two distinct processes • Active or facilitated processes • Require energy • These substances are normally being moved from areas of lower concentration to areas of higher concentration • Passive processes • Do not require energy • These substances move from areas of higher concentration to areas of lower concentration

13. Types of Passive Transport • Diffusion • Kinetic motion of the solutes and solvent randomly move the particles across the plasma membrane until equilibrium is reached • Normally the solutes and/or solvent move from areas of higher concentration to areas of lower concentration • Some ions and molecules can diffuse directly through the plasma membrane • Other ions and molecules will diffuse through specific ion channels and can only pass through the plasma membrane at specific sites

14. Osmosis • Diffusion of a solvent through a selectively permeable membrane • The solvent is usually water • The solvent moves from areas of higher concentration to areas of lower concentration • Besides directly diffusing through the lipid bilayer, water can also move through aquaporins, integral proteins that act as water channels

15. Osmosis and Cells • As long as the osmotic pressure of the solution outside of the cell is the same as the osmotic pressure inside the cell, the cell volume remains relatively constant. • A solution’s tonicity is a measure of the solution’s ability to change the volume of cells by altering their water content. • Isotonic solution: any solution in which a cell maintains its normal shape and volume • Hypotonic solution: water molecules enter the cell faster than they leave causing the cell to increase in size until it bursts • Hypertonic solution: water molecules leave the cell faster than they enter causing the cell to shrink called crenation

16. Facilitated Diffusion • A solute binds to a specific transporter on one side of the membrane and is released on the other side after the transporter undergoes a change in shape. • Solutes move down a concentration gradient until equilibrium is reached; then a solute moves into the cell at the same rate as it moves out of the cell. • The number of transporter proteins is limited and once all the transporters are in use, saturation is reached and increasing the concentration will not increase the rate of transport.

17. Two Types of Active Transport • Primary Active Transport • Energy from ATP is used to change the shape of transporter proteins, which “pump” a solute across a plasma membrane against its concentration gradient. • Cells typically use about 40% of their ATP on primary active transport.

18. Secondary Active Transport • Energy stored in a sodium ion or a hydrogen ion concentration gradient is used to drive other substances across the membrane against their own concentration gradients. • Transport proteins that carry two different molecules in the same direction are called symporters while those that move them in opposite directions are called antiporters.

19. Transport in Vesicles • A vesicle is a small, spherical sac that can transport materials into or out of a cell. • Endocytosis: materials move into a cell in a vesicle formed from the plasma membrane. • Exocytosis: materials move out of a cell by the fusion of the vesicle with the plasma membrane and release of the material to the external environment. • Transcytosis: vesicles undergo endocytosis on one side of the cell, move across the cell, and then undergo exocytosis on the opposite side.