Cell energy and division
Cells must capture energy and reproduce. This lesson covers photosynthesis, respiration, and how cells divide.
ATP: energy currency
ATP (adenosine triphosphate) stores energy in bonds between its phosphate groups. Breaking off one phosphate releases energy to power muscle contraction, active transport, and building molecules. A cell recycles its ATP constantly, making and using millions of molecules per second.
Photosynthesis
In chloroplasts, plants use light energy to turn carbon dioxide and water into glucose and oxygen: 6CO2 + 6H2O + light -> C6H12O6 + 6O2. The light reactions capture energy and split water, releasing oxygen; the Calvin cycle uses that energy to fix carbon dioxide into sugar.
Cellular respiration
Cells release energy from glucose by respiration: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy. Glycolysis in the cytoplasm splits glucose; the Krebs cycle and electron transport chain in mitochondria complete the job, yielding about 30-32 ATP per glucose. Respiration is roughly photosynthesis in reverse.
Fermentation
Without oxygen, cells can still make a little ATP by glycolysis, regenerating needed molecules by fermentation. Muscles produce lactate during intense exercise; yeast produce ethanol and carbon dioxide, which is how bread rises and beer is brewed. Fermentation yields only 2 ATP per glucose.
The cell cycle
A dividing cell goes through interphase (growth and DNA copying) and then mitosis and cytokinesis (division). Checkpoints stop the cycle if DNA is damaged or incomplete. When checkpoints fail and cells divide uncontrollably, the result can be cancer.
Mitosis
Mitosis divides the copied chromosomes equally into two nuclei, in stages: prophase (chromosomes condense), metaphase (they line up at the centre), anaphase (copies pull apart), and telophase (new nuclei form). Cytokinesis then splits the cytoplasm. The result is two genetically identical cells, used for growth and repair.
Meiosis
Meiosis makes sex cells (gametes) with half the chromosome number: in humans, 23 instead of 46. It involves two divisions and produces four genetically different cells. Crossing over, swapping segments between paired chromosomes, and the random sorting of chromosomes create enormous variety in offspring.
Worked example
A human skin cell has 46 chromosomes. After mitosis each daughter cell has 46. A cell in the testes undergoing meiosis produces four sperm, each with 23. At fertilisation, 23 from the sperm plus 23 from the egg restores 46.
Key takeaways
ATP powers cellular work. Photosynthesis stores light energy in glucose; respiration releases it as ATP, mostly in mitochondria. Fermentation gives a little ATP without oxygen. Mitosis makes identical cells for growth and repair; meiosis makes varied gametes with half the chromosomes. Failed cell-cycle control can cause cancer.
Flashcard
Cell's energy currency
Check your understanding
What is the main purpose of mitosis in a multicellular organism?