Structure of the Cell Membrane The composition of nearly all cell membranes is a double-layered sheet called a lipid bilayer. Because the various components of the bilayer (phospholipids, proteins, carbohydrate chains & cholesterol) are able to move around, it is often referred to as a fluid-mosaic model. The cell membrane regulates what enters and leaves the cell.

Functions of the Plasma Membrane Embedded enzyme proteins help carry out chemical reactions of the cell Receptor proteins allow to receive chemical messages Surface carbohydrate molecules allow recognition & communication between cells Transport proteins serve as channels or pumps to move materials in and out of cells Proteins from adjacent cells allow intracellular joining Embedded cholesterol molecules provide reinforcement

Diffusion Through Cell Boundaries Particles in a solution tend to move from an area where they are more concentrated to an area where they are less concentrated. (visualize this as a downhill movement, [high] to [low]) This process is called diffusion When the concentration of the solute is the same throughout a system, the system has reached equilibrium.

Osmosis The diffusion of water through a selectively permeable membrane. Osmosis is the diffusion of water through a selectively permeable membrane. In the first beaker, water is more concentrated on the right side of the membrane. As a result, the water diffuses (as shown in the second beaker) to the area of lower concentration.

Osmotic Pressure & Tonicity If you compare two solutions, the more concentrated solution (less water) is hypertonic. The more dilute (more water) solution is hypotonic. When the concentration is equal in both solutions, the solution is described as isotonic.

Effect of Tonicity on Cells

Facilitated Diffusion Cell membranes have protein channels that act as channels (pores) or carrier molecules, making it easy for certain molecules to cross. The movement of specific molecules across cell membranes through protein channels is known as facilitated diffusion. This process usually involves solute molecules (rather than solvent molecules) and is a form of passive transport because it requires no energy, ([high] to [low])

Aquaporins Membrane proteins channels; Critical role in water transport into and out of cells (facilitated diffusion) At least 11 variants in humans; understanding of their specific physiological roles is currently a major area of research Important role in kidneys (maintain body fluid balance)

Active Transport Active transport involves the use of energy (ATP) to move substances across a cell membrane Ex) Movement of materials in the opposite direction from which the materials would normally move - against a concentration difference This is achieved by membrane proteins that act as pumps Visualize this as UPHILL, [low] to [high]) Large materials also move by active transport regardless of concentration gradient

Active Transport of Large Materials (“Bulk Transport”) Endocytosis is the process of taking large materials into the cell using vesicles, or pockets, of the cell membrane. (endo=enter) Phagocytosis – cell “eating”; actively moving solids into the cell Pinocytosis – cell “drinking”; actively moving liquids into the cell Exocytosis, the membrane of the vesicle surrounding the material fuses with the cell membrane, forcing the contents out of the cell (exo=exit)

Life depends on the fact that energy can be converted from one form to another.

Energy Energy: the capacity to rearrange matter/ “do work” There are two basic forms of energy: Kinetic energy- energy of motion; moving objects do work by transferring motion to other matter Examples: think of an example where this is true Includes: heat (random motion of molecules) & light Potential energy- stored energy as a result of location or structure Includes: Chemical energy- energy available to cell to do work

Energy Transformations Thermodynamics- study of energy transformation that occur in a system (collection of matter) What is the difference between an open and closed system? 1st Law: Law of Energy Conservation 2nd Law: Energy conversions increase entropy of the universe Efficiency of car vs cell energy conversions

Which do you think is more efficient? Fuel Energy conversion Waste products Heat energy Carbon dioxide Gasoline Which do you think is more efficient? Combustion Kinetic energy of movement Oxygen Water Energy conversion in a car Heat Figure 5.11 Energy transformations (with an increase in entropy) in a car and a cell. Cellular respiration Glucose Carbon dioxide Oxygen Water Energy for cellular work Energy conversion in a cell

Explain the following statement: “In a thermodynamic sense, a cell or an organism is an island of low entropy in an increasingly random universe.” Concept: Chemical reactions either release or store energy. How could one detect if energy was released or stored?

In an exergonic reaction, energy is released. Bonds of reactants contain more energy than those in the products. Examples of exergonic reactions? Reactants Amount of energy released Potential energy of molecules Figure 5.12A Exergonic reaction, energy released. Energy released Products

Endergonic reaction, energy is absorbed from surroundings. Products contain more energy in bonds than reactants. Examples of endergonic reactions? Products Amount of energy required Potential energy of molecules Energy required Figure 5.12B Endergonic reaction, energy required. Reactants

Potential Energy Diagrams

Identify the following as exergonic or endergonic. Use of gasoline in a lawn mower Photosynthesis Synthesis of a protein in a cell Synthesis of glycogen in a cell Cell Respiration Hydrolysis of ATP All the chemical reactions that take place in an organism is referred to as metabolism. A Metabolic pathway is a series of rxns that builds or breaks down a complex molecule (controlled steps) Energy released from exergonic rxns drives endergonic rxnsin a cell – energy coupling

c. 3 negatively charged phosphate groups Phosphate group Adenine Adenosine ATP: a. Ribose b. Adenine c. 3 negatively charged phosphate groups Phosphate group Adenine ATP analogy: “compressed spring”” Triphosphate (ATP) Ribose Is the hydrolysis of ATP exergonic or endergonic? Hydrolysis Figure 5.13A The structure and hydrolysis of ATP. The reaction of ATP and water yields ADP, a phosphate group, and energy. + Adenosine Diphosphate (ADP)

ATP drives cellular work Transport Chemical Mechanical * Phosphorylating certain molecules results in work to be completed. Explain this based on what you learned about energy.

Key Point: ATP is a renewable resource the cell can regenerate. Phosphorylation Hydrolysis Energy from exergonic reactions Energy for endergonic reactions Figure 5.13C The ATP cycle.

Why don’t macromolecules such as DNA, carbohydrates, lipids spontaneously break down into simpler less energetic molecules? Energy barrier must be overcome, bonds between atoms must be weakened Referred to as energy of activation (EA) How can this be accomplished? 1. add heat- problem? 2. enzyme- proteins that function as biological catalyst, lowers EA and are not consumed in reaction

Figure 5.14 The effect of an enzyme is to lower EA Reaction without enzyme EA without enzyme EA with enzyme Reactants Energy Net change in energy (the same) Reaction with enzyme Figure 5.14 The effect of an enzyme is to lower EA. Products Progress of the reaction

Catalytic cycle of an enzyme: Substrate(sucrose) c. Active site b. Enzyme (sucrase) a. Induced Fit d. Products released f. Substrate hydrolyzed e.

Enzyme Functioning If enzyme shape changes (denatured) then it can no longer function. Denaturation can be caused by: temperature, pH, pressure. Enzyme non-protein helpers: cofactor(inorganic- Zn, Fe, Cu ions) or coenzyme (organic- vitamins) Enzyme function can be blocked by an inhibitor

Competitive vs. Non-Competitive Inhibition

Enzyme Inhibitors Important in regulating cell metabolism Feedback inhibition- product of metabolic reaction blocks the reaction from occurring Important to medicine and agriculture: Component to medicines and pesticides-blocks enzyme functioning Some antibiotics work by inhibiting enzymes of pathogenic bacteria Some HIV drugs (proteases) target viral enzymes Many toxins and poisons interrupt metabolism in this way- cause irreversible effects Nerve gases bind in the active site of an enzyme vital to the transmission of nerve impulses Some pesticides work by targeting the corresponding enzyme in insects