Biology: Energy Pathways

First law of thermodynamics

Energy cannot be created or destroyed, only transformed or transferred.

Second law of thermodynamics

Energy transformations increase entropy (disorder) in the universe.

Free energy

Energy available to do work in a system.

Metabolism

All chemical reactions within an organism.

ATP

Adenosine triphosphate; the main energy carrier in cells.

ADP

Adenosine diphosphate; ATP after losing one phosphate group.

Autotroph

Organism that produces its own food, usually through photosynthesis.

Heterotroph

Organism that obtains food by consuming other organisms.

Linked reaction

A reaction whose products feed into another; often couples energy usage and release.

Potential energy

Stored energy due to position or structure.

Chemical reaction

Process where substances are changed into new substances.

Reactants

Starting substances in a chemical reaction.

Products

Substances formed from a chemical reaction.

Endergonic

Reaction that absorbs energy; not spontaneous.

Exergonic

Reaction that releases energy; spontaneous.

Cellular respiration

Process by which cells release energy from glucose using oxygen.

Glycolysis

First step of cellular respiration; breaks glucose into pyruvate.

Pyruvate

Product of glycolysis; enters mitochondria for further processing.

Pyruvate oxidation

Conversion of pyruvate into acetyl-CoA before the Krebs cycle.

Krebs cycle

Cycle that generates energy carriers by oxidizing acetyl-CoA.

Citric acid cycle

Another name for the Krebs cycle.

Electron transport chain

Final stage of cellular respiration; uses electrons to create a proton gradient for ATP production.

ATP synthase

Enzyme that produces ATP using a proton gradient.

NADH

High-energy electron carrier formed during cellular respiration.

NAD+

Electron carrier; oxidized form of NADH.

FADH2

Electron carrier formed during the Krebs cycle.

FAD+

Oxidized form of FADH2.

ATP

Main energy molecule in cells (repeated for context).

ADP

Low-energy molecule that can be converted to ATP (repeated for context).

Energy level

The amount of energy an electron has in an atom or molecule.

Outer membrane (mitochondria)

The outer boundary of the mitochondrion.

Inner membrane (mitochondria)

Location of the electron transport chain in mitochondria.

Intermembrane space

Area between inner and outer membranes in mitochondria.

Mitochondrial matrix

Internal space in mitochondria where the Krebs cycle takes place.

Fermentation

Anaerobic process that allows glycolysis to continue producing ATP.

Lactic acid fermentation

Anaerobic process producing lactic acid; occurs in muscles.

Ethanol fermentation

Anaerobic process producing ethanol and CO₂; occurs in yeast.

Aerobic

Requires oxygen.

Anaerobic

Does not require oxygen.

Endosymbiotic theory

Theory that mitochondria and chloroplasts originated from engulfed prokaryotic cells.

Photosynthesis

Process by which plants use sunlight to make glucose from CO₂ and water.

Wavelength

Distance between peaks of waves; determines light's energy and color.

Visible light

Portion of electromagnetic spectrum visible to humans.

Absorbance

The amount of light absorbed by a substance.

Transpiration

Loss of water vapor from plant leaves through stomata.

Guard cells

Cells that open and close the stomata.

Cuticle

Waxy layer covering the surface of plant leaves.

Stoma/Stomata

Pores in leaves that regulate gas exchange and water loss.

Chloroplast

Organelle where photosynthesis occurs.

Outer membrane (chloroplast)

The outermost layer of the chloroplast.

Inner membrane (chloroplast)

Inner layer surrounding the stroma in the chloroplast.

Thylakoid membrane

Membrane where light-dependent reactions take place.

Granum/Grana

Stack(s) of thylakoids in the chloroplast.

Lumen

Interior space of a thylakoid.

Stroma

Fluid surrounding the grana where the Calvin cycle occurs.

Photosynthetic pigment

Molecules like chlorophyll that absorb light for photosynthesis.

Accessory pigment

Pigments that broaden the spectrum of light absorbed (e.g., carotenoids).

Reaction center

Part of the photosystem where light energy is transferred to electrons.

Ground state

Electron's lowest energy level.

Excited state

High-energy state of an electron after absorbing light.

NADPH

Electron carrier used in the Calvin cycle.

NADP+

Electron carrier; oxidized form of NADPH.

Calvin cycle

Set of light-independent reactions that produce G3P from CO₂.

G3P

A 3-carbon sugar produced in the Calvin cycle; can become glucose.

Tri phosphates

Group of 3 phosphphates