Brent Cornell

    
Organelles are specialised sub-structures within a cell that serve a specific function

Prokaryotic cells do not typically possess any membrane-bound organelles, whereas eukaryotic cells possess several

Universal Organelles  (prokaryote and eukaryote):

Ribosomes

Structure:  Two subunits made of RNA and protein; larger in eukaryotes (80S) than prokaryotes (70S)

Function:  Site of polypeptide synthesis (this process is called translation)

Cytoskeleton

Structure:  A filamentous scaffolding within the cytoplasm (fluid portion of the cytoplasm is the cytosol)

Function:  Provides internal structure and mediates intracellular transport (less developed in prokaryotes)

Plasma membrane

Structure:  Phospholipid bilayer embedded with proteins (not an organelle per se, but a vital structure)

Function:  Semi-permeable and selective barrier surrounding the cell

Eukaryotic Organelles  (animal cell and plant cell):

Nucleus

Structure:  Double membrane structure with pores; contains an inner region called a nucleolus

Function:  Stores genetic material (DNA) as chromatin; nucleolus is site of ribosome assembly

Endoplasmic Reticulum

Structure:  A membrane network that may be bare (smooth ER) or studded with ribosomes (rough ER)

Function:  Transports materials between organelles (smooth ER = lipids ; rough ER = proteins)

Golgi Apparatus

Structure:  An assembly of vesicles and folded membranes located near the cell membrane

Function:  Involved in the sorting, storing, modification and export of secretory products

Mitochondrion

Structure:  Double membrane structure, inner membrane highly folded into internal cristae

Function:  Site of aerobic respiration (ATP production)

Peroxisome

Structure:  Membranous sac containing a variety of catabolic enzymes

Function:  Catalyses breakdown of toxic substances (e.g. H₂O₂) and other metabolites

Centrosome

Structure:  Microtubule organising centre (contains paired centrioles in animal cells but not plant cells)

Function:  Radiating microtubules form spindle fibres and contribute to cell division (mitosis / meiosis)



Plant Cells Only

Chloroplast

Structure:  Double membrane structure with internal stacks of membranous discs (thylakoids)

Function:  Site of photosynthesis – manufactured organic molecules are stored in various plastids

Vacuole  (large and central)

Structure:  Fluid-filled internal cavity surrounded by a membrane (tonoplast)

Function:  Maintains hydrostatic pressure (animal cells may have small, temporary vacuoles)

Cell Wall

Structure:  External outer covering made of cellulose (not an organelle per se, but a vital structure)

Function:  Provides support and mechanical strength; prevents excess water uptake

Animal Cells Only

Lysosome

Structure:  Membranous sacs filled with hydrolytic enzymes

Function:  Breakdown / hydrolysis of macromolecules (presence in plant cells is subject to debate)