Cell Structure

Every animal cell has these organelles, each with its own job:

• Cell membrane — thin layer around the cell. Controls what enters and leaves (semi-permeable). Made of phospholipids.
• Cytoplasm — jelly-like fluid inside. Site of most chemical reactions. Contains organelles.
• Nucleus — controls everything. Contains the DNA (genetic instructions).
• Mitochondria — bean-shaped 'powerhouses'. Site of aerobic respiration, producing ATP (energy) from glucose and oxygen.
• Ribosomes — tiny dots in the cytoplasm. Make proteins by joining amino acids.

Plant cells have ALL of these PLUS three EXTRA features:

• Cell wall — rigid layer outside the cell membrane, made of CELLULOSE. Gives plant cells their box-like shape and strength.
• Chloroplasts — green oval organelles containing CHLOROPHYLL. Site of PHOTOSYNTHESIS (the reverse of respiration — uses light energy to make glucose).
• Permanent vacuole — large fluid-filled sac that takes up most of the volume. Stores water and dissolved minerals; pressure (turgor) helps keep the plant upright.

Animal and plant cells are EUKARYOTES — they have a TRUE NUCLEUS and other membrane-bound organelles.

Bacterial cells are PROKARYOTES — they have NO nucleus. The DNA floats freely in the cytoplasm as a single CIRCULAR loop. Most other organelles are absent too.

Key differences:

Feature	Animal	Plant	Bacterium
Nucleus	Yes	Yes	NO
Cell wall	No	Yes (cellulose)	Yes (different material — peptidoglycan)
Chloroplasts	No	Yes	No
Mitochondria	Yes	Yes	No
Size	~10-100 μm	~10-100 μm	~1-10 μm (much smaller)
DNA shape	Linear chromosomes	Linear chromosomes	Single circular loop

Bacteria can also have:

• A flagellum (whip-like tail for movement).
• Plasmids (small extra DNA loops; carry antibiotic resistance genes).
• A slimy capsule outside the cell wall (protection).

Despite being simpler, bacteria are extraordinarily successful — they exist almost everywhere on Earth (your gut, deep-sea vents, glaciers, soil) and outnumber every other type of organism.

Cells are typically 10-100 μm across — too small to see with the naked eye. A LIGHT MICROSCOPE magnifies up to about 400× (school) or 1000× (research). You can see cell shapes, nuclei and (in plants) chloroplasts at this magnification.

For more detail (smaller organelles like ribosomes), you need an ELECTRON MICROSCOPE (magnifies up to ~1 000 000×). These are large, expensive lab machines.

Basic skills you'll use in MYP lab work:

• Total magnification = eyepiece × objective. Eyepiece is usually 10×; objectives are 4×, 10×, 40× → totals of 40×, 100×, 400×.
• Always start on the LOWEST power, focus, then move up to higher power.
• Use the COARSE focus knob on low power; SWITCH to fine focus on higher powers (to avoid crashing the lens into the slide).
• Magnification formula: image size = actual size × magnification.

Worked example. A cell appears 4 cm across when magnified 400×. What is its actual size? Actual = 4 cm / 400 = 0.01 cm = 0.1 mm = 100 μm.

Classic MYP lab work: prepare a slide of onion epidermis (stained with iodine) to see plant cells, or cheek cells (stained with methylene blue) for animal cells. Both demonstrate the cell structures and let you practice slide preparation.