Cell structure and organelles: study guide

Key ideas

Cells are built from organelles that each do a particular job. Comparing plant, animal and bacterial cells, and calculating real sizes from micrographs, are the two skills tested most.

• The nucleus holds DNA, mitochondria make ATP by aerobic respiration, the rough ER makes proteins, the smooth ER makes lipids, and the Golgi modifies and packages proteins.
• Plant cells also have a cell wall, a large vacuole and chloroplasts. Animal cells have none of these, but have centrioles and lysosomes.
• Bacteria are prokaryotes: no nucleus, no membrane-bound organelles, circular DNA, small (70S) ribosomes and a cell wall of peptidoglycan.
• Magnification = image size ÷ actual size. Keep units consistent, usually converting everything to micrometers.

Key equations

• magnification = image size ÷ actual size
• actual size = image size ÷ magnification
• 1 mm = 1000 µm
• 1 µm = 1000 nm

Worked example

A cell appears 45 mm long on a micrograph with a magnification of ×900. Actual length = 45 mm ÷ 900 = 0.050 mm = 50 µm. With a scale bar of 20 mm labeled 10 µm, the magnification is 20 mm ÷ 10 µm = 20 000 µm ÷ 10 µm = ×2000.

Common mistakes

• Mixing millimeters and micrometers when calculating the actual size.
• Saying bacteria have no ribosomes. They have smaller 70S ribosomes.
• Placing chloroplasts, a cell wall or a large central vacuole in animal cells.

Practice

Test yourself with practice questions on this topic: Cell structure and organelles