Chapter 5

• All cells use energy, whether it is a leaf growing or a human running • The metabolic pathways inside cells use the chemical energy of ATP to synthesize molecules, cause muscle contraction, etc

• A metabolic pathway consists of a series of individual chemical reactions, each with its own enzyme • Enzymes are proteins that speed a reaction in cells due to its particular shape

• ATP is called the universal energy “currency” of life • An analogy: before we can spend currency (money), first we must make currency (money)….Similarly, before a cell can spend ATP, it first must make ATP

• Cellular respiration (occurs in mitochondria) transforms the chemical energy in carbohydrates in to that of ATP molecules • All cells are continuously making and breaking down ATP

• Simplest definition: The ability to do work or bring about change (synthesizing molecules, moving objects, generating heat and light) • Solar energy: this is energy from the Sun

Kinetic Energy: energy of movement (light heat, electricity and large objects – Mechanical energy is when chemical energy is converted to kinetic energy • Potential Energy: stored energy - in chemical bonds, electrical in battery – Chemical energy, the kind of energy in food, is potential energy

• Thermodynamics defines the basic properties of energy • First Law of Thermodynamics: Assuming no input of energy to a system, the amount of energy remains the same. Energy can not be created or destroyed • Second Law of Thermodynamics: When energy is converted from one form to another, the amount of useful energy decreases

• When leaf cells photosynthesize, they use solar energy to form CO 2 and water; other energy is lost as heat

• A moose uses the energy derived from carbohydrates to power its muscles • As its cells use this energy, none is destroyed, but some becomes heat, which dissipates into the environment

• According to the Second Law: – No energy conversion process is 100% efficient; i.e., cells are about 40% efficient. THERE IS A LOSS OF USABLE ENERGY • Therefore the second law can be paraphrased: every energy transformation makes the Universe less organized and more disordered – Entropy: this is the term used to indicate the relative amt of disorganization • For this reason, living things are dependent on an outside source of energy that ultimately comes from the Sun

• ATP is the common energy currency of cells • When cells require energy, they “spend” ATP • ATP is constantly being generated from ADP (adenosine phosphate) and a molecule of inorganic phosphate

• The food molecules we eat do not drive the work in our cells directly • Instead, chemical energy is released by the breakdown of organic molecules during cellular respiration within the mitochondria

• From these organic molecules, the mitochondria produces ATP • These molecules of ATP then power cellular work

• ATP is a nucleotide containing a nitrogen base (adenine), sugar (ribose) and 3 phosphate groups • When energy is required ATP is broken down to form ADP and phosphate

• When ATP drives work in cells, phosphate groups don’t just disappear; ATP energizes molecules in cells by transferring phosphate groups to those molecules

• This helps cells perform 3 types of work: • 1) chemical work: ATP supplies the energy needed to synthesize macromolecules that make up the cell, and therefore the organism • 2) Transport work: ATP supplies the energy needed to pump substances across the plasma membrane • 3) Mechanical work: ATP supplies the energy needed to permit muscles to contract, cilia to beat, chromosomes to move, etc.

• Your cells spend ATP continuously • It is a renewable energy source; ATP can be restored by adding a phosphate group back to ADP • This is the cycling of ATP, or the ATP cycle • This is known as a coupling reaction

• The chemical reactions that occur in organisms are collectively called METABOLISM

Few metabolic reactions would occur, however, without the assistance of ENZYMES Enzymes are specialized proteins that speed up chemical reactions Most enzymes are named for their substrate with –ase at the end (i.e., sucrase is the enzyme for sucrose)

• To increase a chemical reaction in the laboratory, you can heat it up —but this is not an option for our cells!

• Enzymes enable metabolism (chemical reactions) to occur at cooler by reducing the amount of activation energy required to break the bonds of reactant molecules • recall a chemical reaction: • A + B  C and D; A & B are the reactants, C & D are the products

• An enzyme does this by binding to reactant molecules and putting them under some kind of stress, making it easier to break their bonds and start a reaction

• Each enzyme is very selective in the reaction it catalyzes • This specificity is based on the ability of the enzyme to recognize the shape of a certain reactant molecule, which is termed the enzyme’s SUBSTRATE

• A special region of the enzyme called the ACTIVE SITE has a shape and chemistry that fits it into the substrate molecule

• When a substrate molecule slips into the docking station, the active site changes shape slightly to embrace the substrate and catalyze the reaction • This is called INDUCED FIT

• Enzymes are used over and over again; after the products are released from the active site, the enzyme is once again available to accept another molecule

• Enzymes are biological catalysts and are usually made from protein with complex 3-D structure • They possess the all the attributes of catalysts and they are also: – Very specific – e.g. starch vs. cellulose – Activity is regulated by the molecules whole reactions they catalyze • Enzymes have a pocket called active site into which substrates fit – Active site has complimentary shape and electric charge for substrate • Only certain molecules can enter active site

• Certain molecules can inhibit a metabolic reaction by binding to an enzyme and disrupting its function • These are ENZYME INHIBITORS • They are “substrate imposters” that plug up the active site, somehow cause the active site to disfigure so can’t accept the enzyme

• Some enzymes are subject to allosteric regulation – Enzyme is enhanced or inhibited by small organic molecules – Regulator is neither substrate nor product, it binds to allosteric site and changes shape of active site • 2 or more molecules with similar structure can compete for active site → (Competitive inhibition)

• In some inhibitors, binding is reversible, enabling inhibitors to regulate metabolism • For example, if metabolism is producing more of a certain product than a cell needs, that product may inhibit an enzyme required for its production • This is called feedback regulation, keeping the cell from wasting resources that could be put to better use

Environmental influence on enzyme activity • Enzymes are sensitive to environmental conditions • Much of 3-D structure of proteins is cause by H-bonds and these bonds can be altered by chemical and physical surroundings • Each enzyme has optimum → – pH: 6-8, except pepsin – Salt concentration: used to preserve meat before refrigeration because salt inactivates bacterial enzymes – Temperature: temperature affects speed of molecule movement, so more movements increase likelihood of collision with active site. BUT when temperatures rise too high, then secondary and tertiary structure of enzyme become compromised • Some enzymes require coenzymes (derived from water soluble vitamins) – Bind to substrate and weaken bonds

• There are two main types of transport across the membrane: • 1. Passive transport (simple diffusion, facilitated diffusion, and osmosis) • 2. Energy-requiring transport (active transport, endocytosis, and exocytosis)

• The diffusion of WATER molecules across a cell membrane is called
osmosis
• Osmosis across the cell membrane is very important for living cells…we are 2/3 water!

• The diffusion of water across a cell membrane is due to concentration differences inside vs. outside a cell • All cells contain dissolved substances, so the flow of water across the membrane depends on the concentration of water molecules in the extra cellular fluid surrounding them

• Isotonic solutions → the solution outside the cell has the SAME concentration of water molecules as inside the cell • Hypertonic solutions → the solution outside the cell has LOWER concentration of water molecules than inside the cell – (water tends to leave cell → cell shrivels) • Hypotonic solutions→ the solution outside the cell has a HIGHER concentration of water molecules than inside the cell – (water enters cell → cell may burst)

• Diffusion always occurs from a higher to a lower concentration of water molecules

• If the concentration of water molecules (and therefore solutes) is equal on both sides, water molecules will move back in forth but it won't have any result on the overall amount of water on either side

• If the concentration of water molecules is higher (and therefore solute molecules lower) inside the cell, water molecules will leave the cell, causing the cell to shrink

• If the concentration of water molecules is less (and the solute concentration higher) inside the cell, water will enter the cell, causing the cell to swell • The cell may burst, which is referred to as “lysis” • In plant cells, the cell wall protects the cell from bursting. This turgor pressure keeps the plant erect

Movement across membranes

• Remember that the cell membrane is embedded with proteins • There are many different kinds of proteins with different functions • Channel, carrier, cell recognition, receptor, and enzymatic

• These are involved in the passage of molecules through a membrane • They have a channel that allows the substance to simply move through the membrane • Cystic Fibrosis is called by a faulty chloride channel, causing mucus to collect in the pancreas and liver ducts

• These are involved in the passage of molecules through a membrane • They combine with the substance and help it move across the membrane • Sodium and potassium ions move this way into nerve cells; this is a form of active transport and is referred to as the “sodium potassium pump”

• These are glycoproteins • These proteins help the body recognize when it is being invaded by pathogens so that an immune reaction can occur • The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve

• These are shaped in a way that a specific molecule can bind to it • This brings about a cellular response, for instance, insulin binds with liver cells —in response, it stores glucose

• These are proteins that carry out enzymatic reactions directly

• Instead, these are transported into and out of the cell by vesicle formation • Endocytosis is the term used when molecules are brought IN to the cell • Exocytosis is the term used when molecules are taken OUT of the cell

• During endocytosis, cells take in substances by vesicle formation • A portion of the cell membrane invaginates to envelope the substance, and then the membrane pinches off to form a intracellular vesicle

• There are three types of endocytosis: • 1. phagocytosis: this is when the molecule is very large • 2. pinocytosis: this is when the molecule is very small • 3. receptor-mediated endocytosis: this is when the molecule is very specific to the process

Movement across membranes: Endocytosis
• ‘cell drinking’ • A small patch of cytoplasm dimples around the fluid • Dimple buds into cytoplasm as a small vesicle • Cell acquires substances in the same concentration as in the extracellular fluid

Movement across membranes: Endocytosis
• Uptake of molecules or complexes of molecules • Can selectively concentrate molecules within cell • External receptor proteins bind with molecule • Coated pit: depression with concentration of receptors • When the molecules bond to receptor, the pit deepens • Receptor, bound molecule and some extracellular fluid move into cytoplasm in vesicle • This is the mechanism for cholesterol uptake

• During exocytosis, vesicles fuse with the plasma membrane and allows secretion to occur