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What is energy?
• Energy: capacity to do work– Potential energy
(Example: chemical bonds)
– Kinetic energy (Energy of motion)
Two Laws of Thermodynamics
1. Energy cannot be created or destroyed– It can be converted from
one form to another and transferred between objects or systems
2. Entropy tends to increase– Energy tends to disperse
spontaneously – Some energy disperses at
each energy transfer, usually in the form of heat
Energy In, Energy Out• Chemical reactions
– Reactants (molecules in)
– Products (molecules out)
• Endergonic reactions (energy-requiring)– Photosynthesis
• Exergonic reactions (energy-releasing)– Aerobic respiration
Energy Flows in One Direction
• Energy is not cycled – In from the sun; into and
out of ecosystems• All living things harvest
external energy– Producers and consumers
use energy to assemble, rearrange, and dispose of substances
• Substances cycle among organisms over time
What is ATP?• Adenosine
triphosphate (ATP) – Main energy carrier
between cell reaction sites
– Currency of the cell• Phosphorylation
– Phosphate-groups transfer to and from ATP
– Couple metabolic reactions that release usable energy to metabolic reactions
What enzymes do?• Enzymes are
catalysts – Speed reaction rates
by lowering activation energy
– Proteins in nature• Activation energy
– Minimum energy needed to start a reaction
• Each enzyme functions best within a characteristic range of temperature, salt concentration, and pH
How enzymes work?• Active site
– Small cleft in enzyme’s surface where reactions occur
• How enzymes lower activation energy – concentrating substrate molecules – orienting substrates to favor reaction – inducing fit between substrate and active
site– excluding water from active site
• Activation energy allows enzyme to react with substrate.
What are cofactors?• Most enzymes
require assistance of cofactors– Inorganic metal ions– Organic coenzymes
(vitamins)• Example: Catalase
(Antioxidant)– Cofactor: Iron
Metabolic pathways• Cells concentrate, convert,
and dispose of most substances in orderly, enzyme-mediated reaction sequences
• Biosynthetic pathways – Construct large molecules
from smaller ones– Require energy
• Photosynthesis– Main biosynthetic pathway
in the biosphere– Autotrophs
Degradative pathways• Degradative
pathways – Break down
molecules to smaller products
– Release usable energy
• Aerobic respiration – Main degradative
pathway in the biosphere
– Heterotrophs• Oxidation–reduction
(redox) reactions – Electron transfers
used in metabolic pathways
How substances cross membranes?• Diffusion
– process by which molecules intermingle as a result of their energy of motion
• Passive– facilitated diffusion, solute moves down
its concentration gradient and no energy input is required.
• Active Transport– Active transport uses energy to move a solute "uphill" against its gradient,
What is diffusion?• Diffusion
– Net movement of molecules to a region where they are less concentrated • Number of atoms or
molecules in a given volume
• Diffusion rates are influenced by:– Temperature – Molecular size– Gradients of pressure,
charge, and concentration
– Selective Permeability
What is facilitated diffusion?
• Many solutes cross membranes through transport proteins (open or gated channels)
• Facilitated diffusion (passive transport) does not require energy input– Solute diffuses down its
concentration gradient through a transporter
– Example: Glucose transporters
What is active transport?• Active transporters require
ATP energy to move a solute against its concentration gradient – Maintain gradients across cell
membranes– Example: Calcium pumps
What are endocytosis and exocytosis?
• Exocytosis– Cytoplasmic vesicle
fuses with plasma membrane
– Contents are released outside
• Endocytosis– Part of plasma
membrane forms a vesicle that sinks into the cytoplasm
Three types of endocytosis• Receptor-mediated
endocytosis– Substance binds to
surface receptors– Pit forms endocytotic
vesicle• Phagocytosis (“cell
eating”)– Amoebas use
pseudopods to engulf prey
• Bulk-phase endocytosis– Vesicle forms around
extracellular fluid
Which way will water move?
• Osmosis – The diffusion of
water across a selectively permeable membrane
– Water molecules follow their concentration gradient, influenced by solute concentration
What is tonicity?• Relative
concentrations of two solutes separated by a semipermeable membrane– Hypertonic fluid
(higher solute concentration)
– Hypotonic fluid (lower solute concentration)
– Isotonic solutions (two solutions with the same tonicity)