Cell membrane characteristics and transport mechanisms

1. Cell membrane characteristics and transport mechanisms

2. Plasma Membrane • Physical barrier - separates intracellular fluids from extracellular fluids • Helps in maintaining homeostasis • Plays a dynamic role in cellular activity – selectively permeable

3. CH3 + N CH3 CH2 CH3 CH2 Phosphategroup O Hydrophilic head O– P O O CH2 CH2 CH O O O O C C CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 Hydrophobic tails CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH3 Fluid Mosaic Model • Double bilayer of phospholipids • Phospholipids have hydrophobic tails and hydrophilic heads

4. The plasma membrane includes proteins • The functions of the plasma membrane include: • Isolation • Regulation of exchange with the environment • Sensitivity to the environment • Structural support

5. The plasma membrane includes proteins • The proteins in the plasma membrane includes: • Anchoring proteins • Recognition proteins (immune system) • Receptors • Carrier proteins • channels

6. Structures on the plasma membrane surfaces Microvilli, Cilia, Stereocilia Apical surface Lateral surface Specialized junctions

7. Features of Apical Surface of Epithelium - Microvilli • Projections that increase surface area • Folding of the plasma membrane http://cellbio.utmb.edu/microanatomy/epithelia/epith_lec.htm

8. Features of Apical Surface of Epithelium - Cilia • These structures are designed for motility. • Epithelia that need to move substances across their surface (like mucous in the air passages) have cilia. • Each cilium or flagellum has a basal body located at its base. • Basal bodiesanchor the cilia or flagella and are thought to be responsible for their formation. • They look like centrioles and are believed to be derived from them

9. Flagella:(ex) spermatoza • Extra long cilia • Moves cell http://www.lbl.gov/Science-Articles/Archive/sabl/2006/Jul/02.html

10. Cell junctions – 3 groups • Tight junction • designed to restrict the movement of material between the cells they link • Gap junction • create cytoplasmatic communication bridges between cells • Anchoring junction • attach cells to one another or to extracellular matrix

11. Tight Junctions • An intercellular junction between cells in which the outer layers of the cell membranes fuse, • reducing the ability of larger molecules and water to pass between the cells. • Tight junctions prevent the free movement of molecules between cells in the intestine and allow the intestinal cell to control absorption

12. Gap junctions • Example – intercalated discs in the heart, electrical synapses

13. Cell transport mechanisms - How things enter and leave the cell

14. 2 groups of movement • Passive transport – no energy is needed • Diffusion • Carrier-mediated • Active transport – requires ATTP • Pumps • Vesicular transport

15. Passive transport • All molecules in the body are in constant motion regardless of the presence of a membrane (kinetic energy) • Motion stops only at absolute zero • By international agreement, it is defined as 0K on the Kelvin scale, −273.15°C on the Celsius scale and −459.67°F on the Fahrenheit scale • When a membrane is present the movement in a certain direction can be limited or changed • A molecule will move in a certain direction until collide with another molecule. When this happens, the direction of the movement will change

16. Diffusion • Depends on a concentration gradient. (What is a concentration? A concentration gradient?) • The driving force is kinetic energy and it is influenced by: • Molecule size – the smaller the faster • Temperature – the warmer the faster

17. Diffusion • The movement of molecules will happen in ALL directions • What is usually important is the net rate of diffusion in a certain direction • The net movement will be from high to low concentration until equilibrium is reached • At equilibrium, the net movementis equal in all directions

18. Membrane permeability • Membrane can be: • Freely permeable (this does not apply to plasma membrane) – allows passage of all substances • Selectively permeable – permits passage of some materials and prevents passage of others • Impermeable – cells can be impermeable to specific substances, but no living cell has a completely impermeable membrane

19. Diffusion through cell membrane • Diffusion is divided into 2 types: • 1. Simple diffusion – the movement of particles through the membrane with no assistance • Nonpolar / lipid-soluble substances that diffuse directly through the lipid bilayer • Gases readily diffuse through lipid bilayer. (Ex. movement of oxygen inside cells and CO2 outside) • Diffusion of water and other lipid-insoluble molecules happens via protein channels • The channels are highly selective as a result of the diameter, shape, charge and chemical bonds

20. Diffusion through cell membrane • 2. facilitated diffusion - Assisted by carrier protein • Materials are bound to specific proteins and move through water-filled protein channels (big polar molecules; ex. – glucose) • The facilitated diffusion rate depends on the rate in which the carrier protein molecule can undergo changes that allow passage • Carrier Proteins • Are integral transmembrane proteins • Show specificity for certain polar molecules • Their number will influence the amount that can be transferred through the membrane

21. Osmosis • Osmosis is a simple diffusion of water. • It occurs through a selectively permeable membrane • Occurs when the concentration of a water is different on opposite sides of a membrane

22. Osmosis – osmolality, osmolarity and osmotic pressure • Osmolality (molecular weight) - One osmole is 1 gram molecular weight • Osmolarity (concentration) - One osmole in one liter • Osmotic pressure – defined by the concentration of solute particles in a solution • Is defined by the number of particles, not their size or nature • Each particle in a solution, regardless of its mass, exerts the same pressure against the membrane

23. Effects of Solutions of Varying Tonicity • Tonicity – description of how the solution affects a cell • Isotonic – solutions with the same solute concentration as that of the cytosol • Hypertonic – solutions having greater solute concentration than that of the cytosol • Hypotonic – solutions having lesser solute concentration than that of the cytosol

24. Passive Membrane Transport: Filtration • The passage of water and solutes through a membrane by hydrostatic pressure • Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area • Depending on the size of the membrane pores • only solutes of a certain size may pass through it.

25. Transport that uses ATP • A movement that can be against concentration gradient • Uses ATP to move solutes across a membrane • Two types: • Active transport - use of carrier proteins • Vesicular transport

26. Types of Active Transport • 2 types according to the source of energy used for the transport • Primary active transport • The energy for the transport derived directly from a high energy molecule – ATP • The hydrolysis of ATP causes phosphorylation of a transport protein that in turn changes its shape. • That change “promotes” the passage of materials (ex. Sodium-potassium pump)

27. Types of Active Transport • Secondary active transports – one ATP-powered pump can drive secondary transport of other solutes. • The energy is derived from the energy stored in creating the concentration gradient • This concentration difference was created by the primary active transport that used ATP • Secondary transport, like the primary, depends on carrier proteins, but without the need of energy

28. Active transport • Symport system – two substances are moved across a membrane in the same direction • Antiport system – two substances are moved across a membrane in opposite directions (Na/K)

29. Vesicular Transport • Transport of large particles and macromolecules across plasma membrane using vesicles and ATP • Endocytosis – enables large particles and macromolecules to enter the cell. Few types: • Receptor-mediated endocytosis – selective process that depends on the binding of extracellular material to a specific receptor • This binding initiates the endocytosis • Phagocytosis – “cell eating”; endocytosis of solid objects • pseudopods engulf solids and bring them into the cell’s interior • Happens in specialized cells • Pinocytosis – “cell drinking”; endocytosis of liquids. • This is not a selective process and does not involve receptor

30. Vesicular Transport • Exocytosis – moves substance from the cell interior to the extracellular space • Transcytosis – moving substances into, across, and then out of a cell • Vesicular trafficking – moving substances from one area in the cell to another

31. Passive Membrane Transport – Review

32. Active Membrane Transport – Review