Question
Many biological processes are cyclical. Examples of cycles can be found at the level of the cell, the organism and the ecosystem.
(a) Explain how changes to the cell cycle can result in tumour formation.
(b) Outline the role of the right atrium in the cardiac cycle.
(c) Describe processes in the carbon cycle that produce or use carbon dioxide.
Answer/Explanation
(a) The cell cycle is the process by which cells grow, copy their DNA, and divide. It is tightly controlled by proteins (like cyclins) that make sure each step happens at the right time. If this control is lost for example, due to damage in the DNA or mutations in genes that regulate the cycle the cell may keep dividing when it shouldn’t. This uncontrolled cell division forms a tumour. Tumours can be benign (not spreading) or malignant (cancerous, spreading to other tissues).
(b) The right atrium is one of the heart’s four chambers. It receives deoxygenated blood from the body through two large veins: the superior vena cava and inferior vena cava. During the cardiac cycle, the right atrium contracts and pushes blood into the right ventricle. From there, the blood goes to the lungs to pick up oxygen. So, the right atrium plays a key role in starting the flow of blood through the heart and to the lungs.
(c) In the carbon cycle, carbon dioxide (CO₂) moves between the atmosphere, organisms, and the environment. Here are key processes:
- Respiration: All living organisms (plants, animals, and microbes) release CO₂ when they break down food for energy.
- Combustion: Burning fossil fuels or wood releases stored carbon as CO₂ into the air.
- Photosynthesis: Plants use CO₂ from the air to make glucose. This removes CO₂ from the atmosphere.
- Decomposition: When organisms die, decomposers break them down, releasing CO₂ back into the air.
These processes keep carbon cycling through the Earth’s systems.
Question
Fluids inside unicellular and multicellular organisms allow materials to be moved.
(a) Explain how vesicles are used by cells to move materials.
(b) Describe the transport of carbon compounds such as sucrose and amino acids in phloem.
(c) Outline how food is moved from the stomach to the large intestine.
Answer/Explanation
(a) Vesicles in cells:
Vesicles are small sacs with membranes that carry materials inside the cell. They form by budding from organelles like the Golgi apparatus or endoplasmic reticulum, transport substances through the cytoplasm, and then fuse with target membranes to deliver their contents either inside or outside the cell.
(b) Transport of carbon compounds in phloem:
Sucrose and amino acids are actively loaded into phloem sieve tubes at the source (like leaves), lowering water potential inside the tubes. Water then moves in by osmosis, increasing pressure and causing sap to flow toward sink tissues (e.g., roots, fruits) by bulk flow.
(c) Food movement from stomach to large intestine:
Food moves through the digestive system by peristalsis, which are coordinated wave-like contractions of circular and longitudinal muscles in the gut wall. These contractions push food forward from the stomach through the small intestine and into the large intestine.