g) the ultrastructure of eukaryotic cells and the functions of the different cellular components

What are cells?

• All living organisms are made up of cells

• Understanding cell structure and function is a key concept in biology

• The term cell was first coined by Robert Hooke in the 17th century

• Advances in light microscopy and electron microscopy have improved understanding of cell structure and function

  • Improvements in microscopy have been essential to understanding cell ultrastructure

Prokaryotic and eukaryotic cells

• Two main types of organism: prokaryotes and eukaryotes

  • Prokaryotic organisms consist of prokaryotic cells (single-celled organisms, e.g. bacteria)

  • Eukaryotic organisms are made of eukaryotic cells

    • Both types contain organelles

    • Eukaryotic cells are larger and more complex than prokaryotic cells
      

Organelles

• Structures found within cells

• Each organelle has a specific function

• Can be seen using an electron microscope

  • The internal structure of organelles is known as cell ultrastructure

Animal cells organelles:

Plant cells organelles:

Are eukaryotic cells
Contain all organelles found in animal cells plus:

• a cell wall with plasmodesmata (‘channels’ for exchanging substances

• vacuole (compartment that contains cell sap),

• and chloroplasts (the organelles involved in photosynthesis).

Plasma membrane (cell surface membrane)

• Found on the surface of animal cells

• Found just inside the cell wall of plant and prokaryotic cells

• Made mainly of lipids and proteins

  • Function:

    • Regulates movement of substances into and out of the cell

    • Contains receptor molecules that respond to chemicals (e.g. hormones)
      

Cell wall.

• Rigid structure surrounding plant cells

• Made mainly of cellulose (a carbohydrate)

  • Function:

    • Supports plant cells

    • Fungal and bacterial cell walls are not made of cellulose

Nucleus

• Large organelle surrounded by a double membrane (nuclear envelope)

• Nuclear envelope contains pores

  • Contains:

    • Chromatin (DNA + proteins)

    • Nucleolus

• Function:

  • Controls cell activities by controlling DNA transcription

  • DNA provides instructions to make proteins

  • Nuclear pores allow substances (e.g. RNA) to move between nucleus and cytoplasm

  • Nucleolus makes ribosomes

Lysosome

• Round, membrane-bound organelle

• No clear internal structure

  • Function:

    • Contains digestive enzymes

    • Digests invading cells

    • Breaks down worn-out cell components

• Enzymes are kept separate from cytoplasm by the membrane

Ribosome

• Very small organelle

• Found free in cytoplasm or attached to rough ER

• Made of proteins and RNA

• Not surrounded by a membrane

  • Function:

    • Site of protein synthesis

Rough endoplasmic reticulum (RER)

• System of membranes enclosing fluid-filled spaces

• Surface covered with ribosomes

  • Function:

    • Folds and processes proteins made at ribosomes

Smooth endoplasmic reticulum (SER)

• Similar structure to RER

• Does not have ribosomes

  • Function:

    • Synthesises and processes lipids

Vesicle

• Small, fluid-filled, membrane-bound sac in cytoplasm

  • Function:

    • Transports substances:

    • In and out of the cell (via plasma membrane)

    • Between organelles

    • Formed by Golgi apparatus, ER, or plasma membrane

Golgi apparatus

• Group of flattened, membrane-bound sacs

• Vesicles often seen at edges

  • Function:

    • Processes and packages proteins and lipids

    • Forms lysosomes

Mitochondrion

• Oval-shaped organelle

• Double membrane:

• Outer membrane

• Inner membrane folded into cristae

• Inside is the matrix containing respiration enzyme

  • Function:

  • Site of aerobic respiration

  • Produces ATP

  • Found in large numbers in energy-demanding cells
    Exam Tip: Never say mitochondria “produce energy” – they produce ATP

Chloroplast

• Found in plant cells

• Small, flattened organelle

• Surrounded by a double membrane

  • Contains:

    • Thylakoid membranes

    • Grana (stacks of thylakoids)

    • Lamellae (link grana)

    • Stroma

• Function:

  • Site of photosynthesis

    • Light-dependent reactions occur in grana

    • Light-independent reactions occur in stroma

Centriole

• Small, hollow cylinders

• Made of microtubules

• Found in animal cells and some plant cells

  • Function:

    • Involved in chromosome separation during cell division

Cilia

• Small, hair-like structures on surface of some animal cells

  • Structure:

  • Outer membrane

    • 9 pairs of microtubules surrounding 2 central microtubules (9 + 2)

• Function:

  • Movement of substances along cell surface

  • Tip: In trachea, cilia move mucus and dirt out of lungs

Flagellum

• Longer than cilia

• Surrounded by plasma membrane

• Same 9 + 2 microtubule arrangement as cilia

  • Function: microtubles contract to make flagellum move

    • Propels cells forward (e.g. sperm cells)

    • Tip: In humans, only sperm cells have flagella

    • Tip: Singular = flagellum / plural = flagell