Cells

Cell Theory

• Cells are the basic unit of life.

• All living organisms are made of cells.

• Two main types:

  • Prokaryotic cells → simple, no nucleus (e.g., bacteria)

  • Eukaryotic cells → complex, with nucleus (plants & animals)

• Cell Theory explained:

  • All living things are made of cells.

  • Cells are the basic unit of structure and function.

  • All cells come from pre‑existing cells.

What Are Organelles?

• Organelles = specialised parts of the cells which perform a specific function

  • “little organs” inside the cell

• Each organelle has a specialized structure and function, just like organs in the body.

• Together, they keep the cell alive and functioning.

• Found in the cytoplasm of eukaryotic cells.

Animal Cell (12 organelles)

• Cell Membrane

  • Thin, flexible boundary

  • Controls entry/exit of substances, maintains homeostasis

• Nucleus

  • Surrounded by nuclear envelope, contains DNA

  • Directs protein synthesis & cell division

• Nucleolus

  • Dense region inside nucleus

  • Produces ribosomes (protein factories)

• Nucleus membrane

  • Double membrane around nucleus

  • Protects DNA, controls movement in/out nucleus

• Cytoplasm

  • Jelly-like fluid

  • Site of chemical reactions; suspends organelles

• Mitochondria

  • Double membrane, inner folds (cristae)

  • Releases ATP via respiration

• Ribosomes

  • Tiny particles (in cytoplasm or in ER)

  • Build proteins from amino acids

• Endoplasmic Reticulum (ER)

  • Rough ER (with ribosomes)

    • Rough ER: helps protein transport

  • Smooth ER (no ribosomes)

    • Smooth ER: helps lipid (fat) synthesis

• Golgi Apparatus

  • Stacks of flattened sacs

  • Packages and modifies proteins to export out cell

• Lysosomes

  • Small sacs with enzymes

  • Breakdown wastes, pathogens, old organelles

• Centrosome

  • Region near nucleus with centrioles (spindle fibres)

  • Organise spindle fibres during cell division

• Vacuole

  • Small fluid-filled scas

  • Store water nutrients, wase

Comparing Animal to Plant Cells

• Similarities: Both have nucleus, cytoplasm, mitochondria, ER, Golgi, ribosomes.

• Differences:

  • Plant

    • Have cell membrane, cell wall, chloroplasts, large central vacuole

    • Do NOT have lysosomes or centrioles

  • Animal

    • Have cell membrane, lysosomes, centrioles, many smaller vacuoles

    • Do NOT have cell wall, no chloroplasts, small vacuoles

• Plant cells often more regular/rectangular, animal cells more irregular/round.

Plant cell (8 Organelles)

• Cell Wall

  • Rigid, made of cellulose

  • Provides strength & shape

• Chloroplasts

  • Contain chlorophyll, double membrane

  • Photosynthesis, to make oxygen and glucose

• Large Vacuole

  • Fluid-filled sac

  • Stores water, maintains turgor pressure → helps keep shape/structure, physiological functions and growth

• Nucleus

  • Surrounded by nuclear envelope, contains DNA

  • Directs protein synthesis & cell division

• Mitochondria

  • Double membrane, inner folds (cristae)

  • Releases ATP via respiration

• Endoplasmic Reticulum (ER)

  • Rough ER (with ribosomes)

    • Rough ER: helps protein transport

  • Smooth ER (no ribosomes)

    • Smooth ER: helps lipid (fat) synthesis

• Golgi Apparatus

  • Stacks of flattened sacs

  • Packages and modifies proteins to export out cell

• Ribosomes

  • Tiny particles (in cytoplasm or in ER)

  • Build proteins from amino acids

Microscopy

• Microscopy = using instruments to magnify and study cells and structures too small for the naked eye.

• Two main types:

  • Light Microscopes → use visible light and lenses.

  • Electron Microscopes → use beams of electrons for much higher magnification.

Light microscope

• Magnify up to ×1000.

• Use glass lenses and light source.

• Can view living cells.

• Often used in schools and labs.

• Stains (e.g., iodine, methylene blue) make structures visible.

Electron Microscopes

• Magnify up to ×1,000,000

• Use electrons instead of light

• Two types:

  • Transmission Electron Microscope (TEM): shows internal ultrastructure

  • Scanning Electron Microscope (SEM): shows 3D surface detail

  • Cannot view living cells (samples must be fixed)

  • Images to right are of the smooth endoplasmic reticulum in each microscope

  • Electron microscopes have higher resolution than light microscopes