Started: 9/10/13 LEAP Biology Chapter 5 Started: 9/10/13 LEAP Biology

Membrane structure and function

5.1 Membranes are fluid mosaics of lipids and proteins with many functions Membranes are composed of - a bilayer of phospholipids with - Embedded and attached proteins - In a structure called a fluid mosaic Many phospholipids are made from unsaturated fatty acids that have kinks in their tail These kinks prevent phospholipids from packing together, keeping them in liquid form

5.1 In animal cell membranes, cholesterol helps Stabilize membranes at warmer temp and Keep membrane fluid at lower temp

Membrane proteins perform many functions Some membrane proteins: maintain cell shape through attachment to cytoskeleton and extracellular matrix function as receptors for chem messengers from other cells function as enzymes Membrane glycoproteins are involved on the intercellular junctions that attach adjacent cells to each other Membranes may exhibit selective permeability allowing some substances to cross more easily than others

5.2 Evolution Connection: Membranes form spontaneously which is a critical step in origin of life Phospholipids, the key ingredient of biological membranes, spontaneously self- assemble into simple membranes The formation of membrane-enclosed collections of molecules was a critical step in the evolution of the first cells

5.3 Passive Transport is diffusion across a membrane with no energy investment Diffusion – tendency of particles to spread out evenly in an available space Particles move from a more concentrated particle area to a less concentrated area This means that particles diffuse down their concentration gradient Eventually the particles reach equilibrium where the concentration is the same throughout Diffusion across a cell membrane does not require energy so it is called passive transport The concentration gradient itself represents potential energy for diffusion

5.4 osmosis- diffusion of water across membrane One of the most important substances that crosses membranes is Water osmosis- diffusion of water across a selectively permeable membrane The top of figure 5.4 shows what happens If a membrane permeable to water but not to a solute separates 2 solutions with different concentrations of solute Water will cross the membrane Moving down its own concentration gradient Until the solute concentration on both sides is equal

5.5 water balance btwn cells and their surroundings is crucial to organisms Tonicity – term that describes the ability of a solution to cause a cell to gain or lose water Tonicity mostly depends on concentration of a solute on both sides of the membrane

5.5 Types of Solutions How will animal cells be affected when placed in solutions of various tonicities? When an animal cell is placed into…. Isotonic Solution – the concentration of solute is the same on both sides of the membrane and cell volume won’t change Hypotonic Solution – solute concentration is lower outside cell, water molecules move into cell, and the cell will expand and may burst Hypertonic Solution – solute concentration is higher outside of cell, so water molecules move out of cell and the cell will shrink For an animal cell to survive in a hypotonic or hypertonic solution environment, it must engage in Osmoregulation, the control of water balance

5.6 transport proteins can facilitate diffusion across membranes Hydrophobic substances easily diffuse across membrane Polar or charged molecules do not easily cross cell membranes, and instead move across membranes with the help of specific transport proteins in process called Facilitated Diffusion which: Does not require energy And relies on the concentration gradient

5.6 transport proteins can facilitate diffusion across membranes Channel Proteins: proteins that function by becoming a hydrophilic tunnel for passage of ions or other molecules Carrier proteins: proteins that bind passenger, change shape, and release their passenger on the other side In both of these situations that protein is specific for the substrate which can be sugars, amino acids, ions, and even water Because water is polar, its diffusion through a membrane’s hydrophobic interior is relatively slow The very rapid diffusion of water into and out of certain cells is made possible by a protein channel called an aquaporin

Cells expend energy in the active transport of a solute In active transport, a cell: Must expend energy to Move a solute against its concentration gradient

5.9 Exocytosis and endocytosis transport large molecules across membranes A cell uses 2 mechanisms to move large molecules across membranes Exocytosis – used to export bulky molecules such as proteins or polysaccharides Endocytosis – used to import substances useful to the livelihood of the cell In both cases, material to be transported is packaged within a vesicle that fuses with the membrane

5.9 3 kinds of endocytosis (Endocytosis – used to import substances useful to the livelihood of the cell) Phagocytosis – engulfment of a particle by wrapping cell membrane around it, forming a vacuole Pinocytosis – is the same thing except that fluids are taken into small vesicles Receptor –Mediated Endocytosis: uses receptors in a receptor-coated pit to interact with a specific protein, initiating the formation of a vesicle

Energy and the cell

5.10 cells transform energy as they perform work Cells are small units, a chemical factory, housing thousands of chem reactions Cells use these chem reactions for: -Cell maintenance -Manufacture of cellular parts -And cell replication

5.10 - Energy Energy – capacity to cause change or to perform work 2 kinds: Kinetic Energy: energy of motion Heat: type of Kinetic energy; associated with random movement of atoms or molecules Light: type of K energy; can be harnessed to power photosynthesis Potential Energy: energy that matter posses as result of its location or structure Thermodynamics: Study of energy transformations that occur in matter

Thermodynamics: Study of energy transformations that occur in matter - Law 1 of Thermodynamics: energy in universe is constant Law 2: energy conversions increase disorder of universe Entropy – measure of disorder or randomness

5.11 chem reactions either release or store energy Cells use oxygen in reactions that release energy from fuel molecules In cellular respiration, the chemical energy stored in organic molecules is converted to a form that the cells can use to perform work Chemical reactions either: Release energy (Exergonic Reactions) Require input of energy and store energy (Endergonic Reactions)

Burning wood releases energy in Glucose as heat and light Exergonic Reactions Release energy in covalent bonds of the reactants Burning wood releases energy in Glucose as heat and light Cellular Respiration Involves many steps Releases energy slowly Uses some of released energy to produce ATP

Endergonic reactions Require input of energy Yields products rich in potential energy Begin with reactant molecules that contain little potential energy End with products that contain more chemical energy

Photosynthesis Type of endergonic process Energy-poor reactants, CO2 and H2O are used Energy is absorbed from sunlight Energy-rich sugar molecules are produced

Energy Coupling Uses the: Energy released from exergonic reactions to drive Essential endergonic reactions Usually using energy stored in ATP molecules

5.12 ATP drives cellular work by coupling exergonic and endergonic reactions ATP (adenosine triphosphate) powers nearly all forms of cellular work Consists of: - nitrogenous base adenine - 5 Carbon sugar Ribose and - 3 phosphate groups Hydrolysis of ATP releases energy by transferring its 3rd phosphate from ATP to some other molecules in a process called Phosphorylation Most cellular work depends on ATP energizing molecules by phosphorylating them

3 main types of cellular work Chemical Mechanical Transport ATP drives all three of these types of work ATP is renewable

How enzymes function

5.13 enzymes speed up cell’s chem reactions by lowering energy barriers Although biological molecules posses much potential energy, it is not released spontaneously an energy barrier must be overcome before a chem reaction can begin this energy is called the activation energy (EA) Enzymes function as biological catalysts by lowering EA needed for reaction to begin increase the rate of a reaction without being consumed by the reaction and Are usually proteins, although some RNA molecules can function as enzymes

5.14 a specific enzyme catalyzes each cellular reaction An Enzyme Is very selective on the reaction it catalyzes Has a shape that determines enzyme’s specificity The specific reactant that an enzyme acts on is called the enzyme’s substrate A substrate fits into a region of the enzyme called the active site Enzymes are specific because their active site fits only specific substrate molecules

most human enzymes work best at 35 – 40 degrees Celsius 5.14 For every enzyme, there are optimal conditions under which it is most effective Temperature affects molecular motion an enzyme’ optimal temp produces the highest rate of contact btwn the reactants and enzyme’s active site most human enzymes work best at 35 – 40 degrees Celsius The optimal pH for most enzymes is near neutrality

5.14 Many enzymes require non-protein helpers called cofactors, which Bind to active site Function in catalysts Some cofactors are inorganic such as Zinc, Iron, or Copper If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme

5.15 enzyme inhibitors can regulate enzyme activity in cell A chemical that interferes with an enzyme’s activity is called an inhibitor Competitive Inhibitors -block substrates from entering active site -reduce enzyme’s productivity Non - Competitive Inhibitors Bind to enzyme somewhere other than active site Change shape of active site Prevent substrate from binding Look at figure 5.15 A in book

5.15 Enzyme inhibitors are important in regulating cell metabolism In some reactions the product may act as an inhibitor of one of the enzymes in the pathway that produced it. This is called Feedback Inhibition