Why Do Cells Need Energy?
A way to separate the inside of the cell from the outside → the membrane
A way to encode/transmit information → nucleic acids & proteins
Energy to do the work of the cell which includes
Movement, growth, pump ions, and perform reactions needed for cellular function, etc.
How Do Organisms Harvest Energy?
Organisms can harvest energy from their environment in one of two ways
Obtain energy from the sun → Phototroph
Plants are the most common → obtain energy from the sun to convert CO2 & water into sugar and O2
Obtain energy from chemical compounds → Chemotroph
Animals are the most common chemotrophs → obtain energy by ingesting other organisms and breaking them down into CO2 & water
How are organisms further classified based on their source of carbon?
Organisms can further be classified in 2 ways based on their source of carbon
If the organism can convert CO2 into glucose since they make their own organic source of carbon → Autotroph
If the organism is unable to convert CO2, they must ingest other organisms/molecules for a source of carbon → Heterotroph
Metabolism
Metabolism → chemical processes that occur in a living organism to maintain life
Include building up and breakdown of chemical compounds → allows energy to be harnessed or released
These reactions are continuously happening in cells
Many of these reactions are linked in such a way that the products of one reaction are the reactants of the next reaction → forming pathways
There are 2 branches of metabolism → Anabolism & Catabolism
Types of Energy
Energy that is not associated w/ movement but rather stored → Potential Energy
Energy of Motion → Kinetic Energy
Note: Energy can be converted from one form to another
Chemical Energy
Chemical bonds between pairs of atoms in a molecule hold onto a form of potential energy → Chemical Energy
Bond strength & potential energy are linked
Strong bonds = less potential energy
Weak bonds = more potential energy
ATP
ATP - The Energy Currency
Cells do not use all the energy they have at once → best to keep it stored in a chemical form that is readily accessible to the cell
Most common chemical form used by the cell is ATP → the energy currency of the cell
The ATP molecule contains energy in its chemical bonds
ATP (Adenosine Triphosphate) is composed of:
Adenine
A 5-carbon sugar → Ribose
3 Phosphate Groups
Note: If less than 3 Phosphate
Only 2 phosphate groups → ADP
Only 1 phosphate group → AMP
What is the purpose of ATP?
ATP provides energy in a form that all cells can readily use → to perform the work of the cell
Examples of cellular work
DNA, RNA, and protein synthesis
Vesicle movement in a cell
Pumping substances across membranes
Breaking down fats to release energy
Breaking down proteins into amino aids
Oxidizing sugars to produce ATP
ATP has a high amount of energy → the bonds between phosphate groups are weak
Has High Potential Energy
When the bonds between the phosphate groups are broken → the potential energy is released for the cell to do work
Important: the chemical energy of ATP is held in the bonds connecting the phosphate groups
Laws of Thermodynamics (1)
1st Law
Thermodynamics is the study of changes in energy that accompany events in the universe
Concepts allow us to predict the direction that events will take and whether input of energy is required
1st Law → Energy cannot be created nor destroyed
Energy can be converted from one form to another → e.g. conversion of sunlight into chemical energy
Laws of Thermodynamics (2)
2nd Law
Events in the universe have direction → proceed from a higher energy state to a lower energy state
These events are thermodynamically favourable and are said to be spontaneous → no input of external energy
Results in a reduction in the amount of usable energy → energy transformations are not 100% efficient
Chemical Reactions
Atoms keep their identity but the bonds linking them change during a chemical reaction
The concentration of products and reactants can influence whether a reaction proceeds forward or backward
Free Energy (G)
The amount of energy in a system available to do work → Gibbs free energy
ΔG is the symbol for the difference in Gibbs free energy between the reactants and products of a chemical process
If the products of a reaction have more free energy than the reactants → ΔG is Positive
Thermodynamically unfavourable → Endergonic
Needs an input of energy
If the reactants have more free energy than the products → ΔG is negative
Thermodynamically favourable → Exergonic
Energy is available for use in other processes
endergonic reactions require energy, exergonic release energy
Energy Available
Total Energy (H) = energy available to do work (G) + energy lost to entropy (S)
This can be rearranged to illustrate the state of Gibbs free energy
ΔG = ΔH - TΔS
H = total energy (enthalpy)
G = energy available to do work
S = energy lost to entropy or disorder
T = absolute temperature in Kelvin
Temperature influences the movement of molecules & adds to the degree of disorder
Remember Δ = Change = Final State - Initial State
ATP Hydrolysis: What happens when ATP is hydrolyzed?
The reaction of ATP with water is exergonic (spontaneous) → Released Energy (-ΔG)
At physiological pH → the phosphate groups in ATP are negatively charged & resist one another
Compared to ATP, which has 3 phosphate groups, ADP contains 2 → meaning it has a lower potential energy
Activation Energy
Chemical transformations require the breakage of certain covalent bonds within the reactant
Reactants must contain sufficient kinetic energy to overcome the activation energy barrier → EA
Enzymes catalyze reactions by decreasing magnitude of EA barrier → reaction can proceed
Cells can carry out many reactions w/ a +ΔG (unfavourable)
Why can cells carry out many reactions w/ a +ΔG (unfavourable)
Occurs for 2 reasons
Relative concentrations of reactants to products are maintained above that defined by equilibrium constant
Coupling endergonic and exergonic reactions
E.g. hydrolysis of ATP → drives endergonic reactions
How are chemical reactions catalyzed?
Chemical reactions are catalyzed by proteins called → Enzymes
Active site of an enzyme is formed by the assembly of certain amino acids in a 3D Structure
Enzyme active site binds the substrate(s) and converts it to the product(s)
Active site binds the substrate(s) and helps to stabilize the transition state → lowers the activation energy
Anabolic and catabolic reactions are catalyzed by enzymes
It is possible to affect enzyme activity
Enzyme activity can be reduced by 2 different kinds of inhibitors → reversible & irreversible
Enzyme activity can be increased by activators
Why are many metabolic pathways regulated
b/c energy is needed for chemical reactions
An example of regulation is where the final product in a pathway inhibits the 1st step of the reaction known as → negative feedback
Important for maintaining balance in the cell and in helping the cell to conserve energy
