Structure and function of eukaryotic cell organelles

STRUCTURE AND FUNCTION OF ORGANELLES

Nucleus:

• Structure

  • Bound by a double membrane known as the nuclear envelope

  • Outer membrane: continuous with endoplasmic reticulum

  • Inner membrane: in contact with contents of the nucleus

  • Perforated with holes known as nuclear pores

  • Contains DNA

  • Contains one or more nucleolus

• Function

  • Control cell activity by regulating protein and enzyme synthesis

  • Nucleolus responsible for the synthesis and partial assembly of ribosomes

Cell surface membrane:

• Structure 

  • Made up of phospholipid bilayer

  • Hydrophilic head facing environment and cytosol

  • Hydrophobic tails directed inwards to form a hydrophobic core

• Function

  • Control movement of substances in and out of the cell

  • Prevents most water soluble substances from entering and exiting the cell

Centrioles:

• Structure

  • Made up of 9 triplets of microtubules arranged in a cylindrical structure

• Function

  • Produce spindle fibres that bind to chromosomes and separates them during cell division

  • Formation of cilia and flagella

Ribosome:

• Structure

  • Has no membranes

  • Made up of proteins and ribonucleic acid (RNA)

  • Has a large subunit and a small subunit

  • Location: Attached to rER or exist as free ribosome in the cytosol

• Function

  • Synthesise proteins

Endoplasmic Reticulum

• Structure

  • Made up of membranous tubes or sacs called cisternae

• Function

  • rER: site of protein synthesis

  • sER: synthesises lipids such as membrane phospholipids and steroid hormones (progesteron, testosteron)

Golgi apparatus

• Structure

  • Made up of flattened membrane bound sacs called cisternae (not interconnected)

  • Constantly being reformed at cis face by vesicles from endoplasmic reticulum and budded off at trans face for transport across the membrane

• Function

  • Chemically modifies, sorts, packs and transports molecules within it for secretion across the cell membrane or for delivery to other parts of the cell

Lysosome

• Structure

  • Bound by single membrane

  • Contains hydrolytic digestive enzymes

• Function

  • Digestion of molecules taken in by cell in endocytosis or phagocytosis

  • Autophagy: Digestion of worn out or improperly functioning organelles

  • Autolysis: Self digestion of a cell by release of substances within lysosomes

Mitochondria

• Structure

  • Double membrane separated by inter-membrane space

  • Outer membrane: Smooth and continuous

  • Inner membrane: folded extensively to form cristae that extend into the matrix

  • Matrix contains hereditary material (circular DNA, RNA) and ribosomes

• Function

  • Involved in cellular respiration to release energy

Vacuole

• Structure

  • Fluid-filled bound by a single membrane

  • Animal - relatively small, temporary (called vesicles)

  • Plant - Large, central, permanent vacuole surrounded by a membrane tonoplast, contains cell sap

• Function

  • Hydrolytic enzymes present sometimes - act as a lysosome

  • Waste products may accumulate in vacuole

  • Some dissolved substances act as food reserve

Chloroplast

• Structure

  • Large organelle - Bound by double membrane

  • Outer membrane: Smooth and continuous 

  • Inner membrane: Give rise to thylakoids or lamellae

  • Inside the chloroplast - gel-like stroma

  • Within stroma - thylakoids stacked to form grana (plural granum) connected by inter-granum lamellae

  • Contains photosynthetic pigments

• Function

  • Site of photosynthesis

Cell wall

• Structure

  • Mainly made of cellulose

  • Strong yet fully permeable and allows free passage of substance in and out of cell

• Function

  • Provide mechanical support

  • Serve as a protective layer to prevent plant cell from bursting

  • Protects plant cell from pathogens