Sub-cellular structures and identification
Cell biology • Cell structure
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Nucleus: genetic control centre
The nucleus contains the cell’s genetic material (DNA) and controls cellular activities by directing protein synthesis and cell division. The nucleus sits in the cytoplasm and is surrounded by a nuclear membrane that separates DNA from cytoplasmic reactions, allowing regulated gene expression and safe storage of genetic information .
Cytoplasm and cell membrane: site of reactions and selective barrier
The cytoplasm is a watery matrix that holds organelles and is the location for most cellular chemical reactions. The cell membrane surrounds the cytoplasm and controls movement of substances into and out of the cell by selective permeability; this regulation maintains internal conditions needed for metabolism and diffusion-based transport .
Mitochondria: energy release by respiration
Mitochondria are small organelles where most of the cell’s respiration occurs. Internal folded membranes (cristae) increase surface area for enzymes and electron carriers, so energy release from glucose accelerates; more active cells contain more mitochondria to meet higher energy demands .
Ribosomes: protein synthesis
Ribosomes are small particles in the cytoplasm that assemble amino acids into proteins by translating messenger RNA. Their small size and abundance allow many proteins to form simultaneously, supporting growth, repair and enzyme production in all cell types that possess them .
Chloroplasts: light capture and photosynthesis (plant cells)
Chloroplasts contain chlorophyll and internal membrane stacks that capture light energy and convert carbon dioxide and water into glucose and oxygen during photosynthesis. High chloroplast numbers in leaf cells increase light absorption and glucose production; chlorophyll gives green parts of plants their colour and limits photosynthesis to green tissues that receive light .
Permanent vacuole and cell wall (plant and algal cells)
Many plant cells contain a permanent vacuole filled with cell sap (water with dissolved sugars and ions). Vacuole pressure (turgor) pushes cytoplasm against the cell wall and keeps cells rigid. The cell wall, made mainly of cellulose in plants and algae, provides structural support and resists inward or outward pressure, allowing plants to stand upright and maintain shape .
Plasmids and bacterial cell structure (prokaryotes)
Bacterial cells are prokaryotic and lack a nucleus; DNA exists as a single loop free in the cytoplasm. Many bacteria contain plasmids-small circular DNA rings that provide extra genes (for example, antibiotic resistance) and so increase genetic variation and adaptability. Bacterial cell walls provide support but are not made of cellulose; the cell membrane lies immediately inside the wall and also contains enzymes for some respiration-related reactions .
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