Chapter 3 Part A

Chapter 3 – Part A Cells: The Living Units

Why This Matters  Understanding the structure of the body’s cells explains why the permeability of the plasma membrane can affect treatment

3.1 Cells: The Living Units  Cell theory – A cell is the structural and functional unit of life – How well the entire organism functions depends on individual and combined activities of all of its cells – Structure and function are complementary  Biochemical functions of cells are dictated by shape of cell and specific subcellular structures

3.1 Cells: The Living Units  Cell diversity – Over 200 different types of human cells – Types differ in size, shape, and subcellular components; these differences lead to differences in functions

 Generalized Cell – All cells have some common structures and functions – Human cells have three basic parts: 1. Plasma membrane: flexible outer boundary 2. Cytoplasm: intracellular fluid containing organelles 3. Nucleus: DNA containing control center

Extracellular Materials  Substances found outside cells  Classes of extracellular materials include: – Extracellular fluids (body fluids – Cellular secretions (e.g., saliva, mucus) – Extracellular matrix: jelly-like substance that acts as glue to hold cells together

Part 1 - Plasma Membrane  Acts as an active barrier separating intracellular fluid from extracellular fluid  Plays dynamic role in cellular activity by controlling what enters and what leaves cell  Also known as the “cell membrane” 3.2 Structure of Plasma Membrane

© 2016 Pearson Education, Inc. 1  Consists of membrane lipids that form a flexible lipid bilayer  Specialized membrane proteins float through this fluid membrane, resulting in constantly changing patterns Membrane Lipids  Lipid bilayer is made up of: – 75% phospholipids, which consist of two parts:  Phosphate heads: are polar (charged), so are hydrophilic (water-loving)  Fatty acid tails: are nonpolar (no charge), so are hydrophobic (water-hating) – 5% glycolipids  Lipids with sugar groups on outer membrane surface – 20% cholesterol  Increases membrane stability

Membrane Proteins  Allow cell communication with environment  Most have specialized membrane functions  Two types: – Integral proteins; peripheral proteins

 Integral proteins – Firmly inserted into membrane – Most are transmembrane proteins (span membrane) – Have both hydrophobic and hydrophilic regions  Hydrophobic areas interact with lipid tails  Hydrophilic areas interact with water – Function as transport proteins (channels and carriers), enzymes, or receptors

 Peripheral proteins – Loosely attached to integral proteins – Include filaments on intracellular surface used for plasma membrane support – Function as:  Enzymes  Cell-to-cell connections

Glycocalyx  Consists of sugars (carbohydrates) sticking out of cell surface  Every cell type has different patterns of this “sugar coating” – Functions as specific biological markers for cell- to-cell recognition – Allows immune system to recognize “self” vs. “nonself”

Clinical – Homeostatic Imbalance 3.1  Glycocalyx of some cancer cells can change so rapidly that the immune system cannot recognize cell as being damaged.  Mutated cell is not destroyed by immune system so is able to replicate

© 2016 Pearson Education, Inc. 2 Cell Junctions  Some cells are “free” (not bound to any other cells) – Examples: blood cells, sperm cells  Most cells are bound together to form tissues and organs  Three ways cells can be bound to each other – Tight junctions – Desmosomes – Gap junctions

 Tight junctions – Integral proteins on adjacent cells fuse to form an impermeable junction that encircles whole cell  Desmosomes – Rivet-like cell junction formed when linker proteins (cadherins) interlock linker proteins of neighboring cell like a zipper  Gap junctions – Transmembrane proteins (connexons) form tunnels that allow small molecules to pass from cell to cell – Used to spread ions, simple sugars, or other small molecules between cells – Allows electrical signals to be passed quickly from one cell to next cell  Used in cardiac and smooth muscle cells

How do substances move across the plasma membrane?  Plasma membranes are selectively permeable – Some molecules pass through easily; some do not  Two ways substances cross membrane – Passive processes: no energy required – Active processes: energy (ATP) required

3.3 Passive Membrane Transport  Passive transport requires no energy  Two types of passive transport – Diffusion  Simple diffusion  Carrier- and channel-mediated facilitated diffusion  Osmosis – Filtration  Type of transport that usually occurs across capillary walls Diffusion  Collisions between molecules in areas of high concentration cause them to be scattered into areas with less concentration – Difference is called concentration gradient

© 2016 Pearson Education, Inc. 3 – Diffusion is movement of molecules down their concentration gradients (from high to low)  Energy is not required Diffusion (cont.)  Speed of diffusion is influenced by size of molecule and temperature  Molecules have natural drive to diffuse down concentration gradients that exist between extracellular and intracellular areas  Plasma membranes stop diffusion and create concentration gradients by acting as selectively permeable barriers  Nonpolar, hydrophobic lipid core of plasma membrane blocks diffusion of most molecules  Molecules that are able to passively diffuse through membrane include: – Lipid-soluble and nonpolar substances – Very small molecules that can pass through membrane or membrane channels – Larger molecules assisted by carrier molecules

Diffusion (cont.)  Simple diffusion – Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through phospholipid bilayer – Examples: oxygen, carbon dioxide, fat-soluble vitamins

 Facilitated diffusion – Certain hydrophobic molecules (e.g., glucose, amino acids, and ions) are transported passively down their concentration gradient by:  Carrier-mediated facilitated diffusion – Substances bind to protein carriers  Channel-mediated facilitated diffusion – Substances move through water-filled channels

 Carrier-mediated facilitated diffusion – Carriers are transmembrane integral proteins – Carriers transport specific polar molecules, such as sugars and amino acids, that are too large for membrane channels  Channel-mediated facilitated diffusion – Channels with aqueous-filled cores are formed by transmembrane proteins – Channels transport molecules such as ions or water (osmosis) down their concentration gradient  Osmosis – Movement of solvent, such as water, across a selectively permeable membrane – Water diffuses through plasma membranes  Through lipid bilayer (even though water is polar, it is so small that some molecules can sneak past nonpolar phospholipid tails)  Through specific water channels called aquaporins (AQPs) – Flow occurs when water (or other solvent) concentration is different on the two

 Osmolarity: measure of total concentration of solute particles  Water concentration varies with number of solute particles because solute particles displace water molecules – When solute concentration goes up, water concentration goes down, and vice versa  Water moves by osmosis from areas of low solute (high water) concentration to high areas of solute (low water) concentration WATER FOLLOWS SALT Diffusion (cont.)  When solutions of different osmolarity are separated by a membrane permeable to all molecules, both solutes and water cross membrane until equilibrium is reached – Equilibrium: Same concentration of solutes and water molecules on both sides, with equal volume on both sides  When solutions of different osmolarity are separated by a membrane that is permeable only to water, not solutes, osmosis will occur until equilibrium is reached – Same concentration of solutes and water molecules on both sides, with unequal volumes on both sides  Movement of water causes pressures: – Hydrostatic pressure: pressure of water inside cell pushing on membrane – Osmotic pressure: pressure of water outside cell pushing to move into cell by osmosis  The more solutes inside a cell, the higher the osmotic pressure  A living cell has limits to how much water can enter it  Water can also leave a cell, causing cell to shrink  Change in cell volume can disrupt cell function, especially in neurons  Tonicity – Ability of a solution to change the shape or tone of cells by altering the cells’ internal water volume  Isotonic solution has same osmolarity as inside the cell, so volume remains unchanged  Hypertonic solution has higher osmolarity than inside cell, so water flows out of cell, resulting in cell shrinking – Shrinking is referred to as crenation  Hypotonic solution has lower osmolarity than inside cell, so water flows into cell, resulting in cell swelling – Can lead to cell bursting, referred to as lysing