https://www.khanacademy.org/science/biology/ecology/biogeochemical-cycles/v/the-water-cycle 2024-05-06T19:22:43.24334365Z The water cycle The water cycle, powered by the sun, involves water moving in various forms between sources and sinks. It starts with evaporation from bodies of water, condensation into clouds, and precipitation back to the earth. This cycle also includes transpiration from plants and sublimation from ice to vapor. The majority of Earth's water is saltwater, with only a small percentage being freshwater in lakes, rivers, and groundwater. Sal Khan video https://cdn.kastatic.org/googleusercontent/okxRKXMHovcsPlENtDy06lBLXXn7tUYfX5eG7MOD5nZYSr_KyM9ufnRznemT6SbIP9tt1CNOA0D36g4_qHKE9eE The water cycle The water cycle, powered by the sun, involves water moving in various forms between sources and sinks. It starts with evaporation from bodies of water, condensation into clouds, and precipitation back to the earth. This cycle also includes transpiration from plants and sublimation from ice to vapor. The majority of Earth's water is saltwater, with only a small percentage being freshwater in lakes, rivers, and groundwater. https://cdn.kastatic.org/ka-youtube-converted/jFjI6y46QRk.mp4/jFjI6y46QRk.mp4 494 Photosynthesis and cellular respiration https://www.khanacademy.org/science/middle-school-earth-and-space-science/x87d03b443efbea0a:weather-and-climate/x87d03b443efbea0a:the-water-cycle/a/the-water-cycle-ms 2026-04-27T10:10:46.754795089Z Key points: The water cycle Review your understanding of the water cycle in this free article aligned to NGSS standards. article https://www.khanacademy.org/science/biology/ecology/biogeochemical-cycles/v/nitrogen-cycle 2024-05-06T19:22:43.24334365Z The nitrogen cycle The nitrogen cycle is a crucial process that converts atmospheric nitrogen into a form that plants and other organisms can use. This process, known as nitrogen fixation, is primarily carried out by bacteria. Nitrogen is essential for life, forming a key component in amino acids, ATP, and DNA. The cycle also includes the return of nitrogen to the atmosphere through decomposition. Sal Khan video https://cdn.kastatic.org/googleusercontent/gQTO9JBwADmey2m_RcYCR_nTXRtFSeKzD8OmiRJn46LlNov4Q7sqsF2imvMCMS8i7OccArqSUWSxktBugc1seihb0g The nitrogen cycle The nitrogen cycle is a crucial process that converts atmospheric nitrogen into a form that plants and other organisms can use. This process, known as nitrogen fixation, is primarily carried out by bacteria. Nitrogen is essential for life, forming a key component in amino acids, ATP, and DNA. The cycle also includes the return of nitrogen to the atmosphere through decomposition. https://cdn.kastatic.org/ka-youtube-converted/DsCMYyQ0NWU.mp4/DsCMYyQ0NWU.mp4 345 Photosynthesis and cellular respiration https://www.khanacademy.org/science/ap-college-environmental-science/x0b0e430a38ebd23f:the-living-world-ecosystems-and-biodiversity/x0b0e430a38ebd23f:biogeochemical-cycles/a/ap-es-the-nitrogen-cycle 2026-06-25T10:00:38.725739878Z Key points: The nitrogen cycle Review your understanding of the nitrogen cycle in this free article aligned to AP standards. article https://www.khanacademy.org/science/biology/ecology/biogeochemical-cycles/v/carbon-cycle 2024-05-06T19:22:43.24334365Z The carbon cycle Carbon is crucial for life, forming molecules like glucose, ATP, amino acids, and DNA. The carbon cycle involves plants taking in carbon dioxide, fixing carbon, and creating organic molecules. Animals eat plants, metabolize the molecules, and release carbon dioxide. This cycle keeps carbon circulating in the biosphere. Sal Khan video https://cdn.kastatic.org/googleusercontent/PANHGUsQ7xlmTUmBIQ5gOGmQph_QMC3IukyvEWtMqz7ynXac-nf0w-2gbg0-Lsah0_bhu8pJPMKf_GMxQJUfwO3B The carbon cycle Carbon is crucial for life, forming molecules like glucose, ATP, amino acids, and DNA. The carbon cycle involves plants taking in carbon dioxide, fixing carbon, and creating organic molecules. Animals eat plants, metabolize the molecules, and release carbon dioxide. This cycle keeps carbon circulating in the biosphere. https://cdn.kastatic.org/ka-youtube-converted/_dYkByQ9Kmg.mp4/_dYkByQ9Kmg.mp4 517 Photosynthesis and cellular respiration https://www.khanacademy.org/science/biology/ecology/biogeochemical-cycles/a/the-carbon-cycle 2025-05-22T20:38:55.753848076Z The carbon cycle Learn how carbon moves through Earth's ecosystems and how human activities are altering the carbon cycle. article https://www.khanacademy.org/science/ap-biology/cellular-energetics/photosynthesis/v/photosynthesis 2022-11-16T04:22:43.7414897Z Photosynthesis Photosynthesis is essential for life on Earth. Photosynthesis involves two stages: the light-dependent reactions, which require sunlight and water to produce oxygen, ATP, and NADPH, and the light-independent reactions (or "dark reactions"), which use the products of the light-dependent reactions along with carbon dioxide to produce carbohydrates. Sal Khan video https://cdn.kastatic.org/googleusercontent/j_VtfkzgOpNbsv7JemuDeDS97LZKI7ZC8RIkPt6cuP5EVn0mb9KRFvsEabbBaOgSXSvbliJ5ekyFEvERUcbWx5sl Photosynthesis Photosynthesis is essential for life on Earth. Photosynthesis involves two stages: the light-dependent reactions, which require sunlight and water to produce oxygen, ATP, and NADPH, and the light-independent reactions (or "dark reactions"), which use the products of the light-dependent reactions along with carbon dioxide to produce carbohydrates. https://cdn.kastatic.org/ka-youtube-converted/-rsYk4eCKnA.mp4/-rsYk4eCKnA.mp4 817 Photosynthesis and cellular respiration https://www.khanacademy.org/science/cambridge-o-level-biology-cie/x2a084a90105205d2:plant-nutrition/x2a084a90105205d2:photosynthesis/e/overview-of-photosynthesis 2022-11-18T12:00:57.983091387Z Overview of photosynthesis This exercise has general questions on photosynthesis. Moumita Sen exercise https://www.khanacademy.org/science/ap-biology/cellular-energetics/photosynthesis/a/intro-to-photosynthesis 2021-04-29T07:54:51Z Intro to photosynthesis Conversion of light energy to chemical energy. Reactions of photosynthesis, where they take place, and their ecological importance. article https://www.khanacademy.org/science/ap-biology/cellular-energetics/photosynthesis/a/hs-photosynthesis-review 2026-06-25T12:40:44.68874125Z Learn: Photosynthesis review article https://www.khanacademy.org/science/ap-biology/cellular-energetics/cellular-respiration-ap/v/introduction-to-cellular-respiration 2023-04-04T21:01:09.417661961Z Cellular respiration introduction Cellular respiration is the process by which cells derive energy from glucose. The chemical reaction for cellular respiration involves glucose and oxygen as inputs, and produces carbon dioxide, water, and energy (ATP) as outputs. There are three stages to cellular respiration: glycolysis, the Krebs cycle, and the electron transport chain. Sal Khan video https://cdn.kastatic.org/googleusercontent/GIjWu39wgAGTFemxT5wnVdLLVqej9giH1ILSlN7M-_K4LMB_TG80HYc064xBM27IrHW6RDipGHqOajla5Z-WXcM Cellular respiration introduction Cellular respiration is the process by which cells derive energy from glucose. The chemical reaction for cellular respiration involves glucose and oxygen as inputs, and produces carbon dioxide, water, and energy (ATP) as outputs. There are three stages to cellular respiration: glycolysis, the Krebs cycle, and the electron transport chain. https://cdn.kastatic.org/ka-youtube-converted/2f7YwCtHcgk.mp4/2f7YwCtHcgk.mp4 859 Photosynthesis and cellular respiration https://www.khanacademy.org/science/ap-biology/cellular-energetics/cellular-respiration-ap/v/overview-of-cellular-respiration 2023-06-06T10:35:22.92767095Z Overview of cellular respiration Cellular respiration is how cells get energy from glucose. The process has three main parts: Glycolysis happens in the cytosol and breaks glucose into two pyruvate, producing 2 ATPs and 2 NADHs. The Krebs cycle occurs in the mitochondrial matrix, where pyruvate is turned into acetyl-CoA, which then goes through a series of reactions, producing ATP, NADH, and FADH2. Finally, oxidative phosphorylation uses NADH and FADH2 to create a proton gradient, which helps make more ATP. video https://cdn.kastatic.org/googleusercontent/Ml9Go_y5yN9tR6xTkhsCx4bj-GyM_NqD3a9m474HusoNa1pvmlbWVQXfK4BxtzvRALulCNxWZPW2q3cYL4LUvFY Overview of cellular respiration Cellular respiration is how cells get energy from glucose. The process has three main parts: Glycolysis happens in the cytosol and breaks glucose into two pyruvate, producing 2 ATPs and 2 NADHs. The Krebs cycle occurs in the mitochondrial matrix, where pyruvate is turned into acetyl-CoA, which then goes through a series of reactions, producing ATP, NADH, and FADH2. Finally, oxidative phosphorylation uses NADH and FADH2 to create a proton gradient, which helps make more ATP. https://cdn.kastatic.org/ka-youtube-converted/9zoS5WGsmpc.mp4/9zoS5WGsmpc.mp4 791 Photosynthesis and cellular respiration https://www.khanacademy.org/science/cambridge-o-level-biology-cie/x2a084a90105205d2:respiration/x2a084a90105205d2:introduction-to-the-cellular-respiration/e/cellular-respiration-introduction 2026-04-07T22:59:30.107127435Z Introduction to Cellular Respiration This exercise covers the overall process of cellular respiration, especially in the context of oxidation and reduction. Moumita Sen exercise https://www.khanacademy.org/science/ap-biology/cellular-energetics/cellular-respiration-ap/a/intro-to-cellular-respiration-and-redox 2023-08-07T17:09:37.131449861Z Introduction to cellular respiration and redox Intro to redox in cellular respiration. Substrate-level vs. oxidative phosphorylation. Electron carriers. article https://www.khanacademy.org/science/ap-biology/cellular-energetics/cellular-respiration-ap/a/steps-of-cellular-respiration 2021-04-29T07:51:33Z Steps of cellular respiration Cellular respiration is a metabolic pathway that breaks down glucose and produces ATP. The stages of cellular respiration include glycolysis, pyruvate oxidation, the citric acid or Krebs cycle, and oxidative phosphorylation. article