Structure of cells and organelles

Structure of cells and organellesProkaryotic cells

Major differences between prokaryotic and eukaryotic cells

Prokaryotic cells (bacteria, cynobacteria)Eukaryotic cells (green plants, fungi, animals)

Usually extremely small, 0.5-10m in diameterUsually larger cells, 10-100 m in diameter

Mostly binary fission, no spindleMitosis, meiosis, or both; spindle formed.

DNA is circular and lies free in the cytoplasm (no true nucleus). DNA is naked (not associated histone proteins or RNA to form chromosomes)DNA is linear and contained in a nucleus. DNA is associated with histone proteins and RNA to form chromosomes

In some prokaryotes, sexual system involves transfer of some DNA from one cell to anotherSexual system involves complete nuclear fusion between special sex cells (gametes), with equal contribution from both nuclei

Cell walls present, but chemically different from those of plants; they contain polysaccharides and amino acids (peptidoglycans)Cell walls present in plants and fungi; they made of cellulose or chitin

Few organelles; membraneous structures absent or very simple and existing briefly in the cellMany organelles, bounded by double membranes (chloroplasts, mitochondria, nucleus) and single membrane (Golgi apparatus, lysosome, vacuole, endoplasmic reticulum)

Protein synthesised in small ribosomes (70S)Protein synthesised in large ribosomes (80S)

Some cells have simple flagella; these are without microtubules, 20 nm in diameterSome cells have complex cilia and flagella; these have 9+2 arrangement of microtubules, 200 nm in diameter

Some can fix nitrogen for use in amino acid synthesisNone have this facility

Differences between plant and animal cells

PLANT CELLSANIMAL CELLS

Tough, slightly elastic cellulose cell wall present (in addition to the cell membrane)Cell wall absent-only a membrane surrounds the cell

Pits and plasmodesmata present in the cell wallNo cell wall and therefore no pits or plasmodesmata

Middle lamella join cell walls of adjacent cellsMiddle lamella absent-cells are joined by intercellular cement

Plastids, e.g. chloroplast and leucoplasts, present in large numbersPlastids absent

Mature cells normally have a large single, central vacuole filled with cell sapVacuoles, e.g. contractile vacuoles, if present, are small and scattered throughout the cell

Tonoplast present around vacuole Tonoplast absent

Cytoplasm normally confined to a thin layer at the edge of th cellCytoplasm present throughout the cell

Nucleus at edge of the cellNucleus anywhere in the cell but often central

Lysosomes not normally presentLysosomes almost always present

Centrioles absent in higher plants Centrioles present

Cilia and flagella absent in higher plantsCilia or flagella often present

Starch grains used for storageGlycogen granules used for storage

Only meristematic cells are capable of divisionAlmost all cells are capable of division

Few secretions are produced A wide variety of secretions are produced

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