Cell Structure (Endomembrane System)

What is the structure of ribosomes?

• Roughly spherical, about 20nm in diameter

• Non-membrane bound organelle

• Made up of a small subunit and a large subunit

• Bound and free ribosomes are structurally identical

How are ribosomal subunits made?

• Each is made of rRNA and proteins

• Assembled in the nucleolus before being exported out of the nucleus into the cytoplasm

What kind of ribosomes to prokaryotes and eukaryotes have?

Prokaryotes: 70S ribosomes ← 30S (small) + 50S (large) subunits

Eukaryotes: 80S ribosomes ← 40S (small) + 60S (large) subunits

What is the function of ribosomes? + free vs bound

• Site of protein synthesis, including enzymes

• Free: Proteins that function within the cytosol

• Bound: Proteins for secretion, packaging and insertion

What is the function of the endomembrane system?

To make, move and break down cellular products

What does the endomembrane system consist of?

• Nuclear envelope

• Rough and smooth endoplasmic reticulum

• Golgi apparatus

• Plasma membrane

What is the structure of the endoplasmic reticulum in general?

• Extensive network of membranous tubules and sacs called cisternae

• Membrane separates the internal compartment of the ER from the cytosol

What is the structure of the rough endoplasmic reticulum (rER)?

• Single membrane-bound organelle usually continuous with the outer membrane of the nuclear envelope

• Bound ribosomes stud the outer surface of the membrane

• Cisternae appear more flattened than that of sER

• Fluid-filled lumen / cisternal space is interconnected

What is the function of the rER?

Synthesizing, folding, and modifying proteins

How do proteins fold into their 3D conformation in the endomembrane system?

As proteins are synthesized by bound ribosomes on the rER, they enter the cisternal space to fold into their native 3D conformation

What is glycosylation?

• Carbohydrates being added to proteins to form glycoproteins in the rER

• Catalysed by enzymes found within the membranes of the rER

How are proteins transported from the rER?

Transport vesicles bud off the rER, carrying proteins to their next destination (usually the GA)

What is the function of the endoplasmic reticulum in general?

Membrane factory of the cell

• Grows by adding proteins (synthesized by rER) and phospholipids (synthesized by sea) to its own membrane

• ER membrane is transferred in transport vesicles to other components of the endomembrane system including the cell surface membrane

What are proteins synthesized by ribosomes meant for?

• Packaging into organelles

• Secretion out of the cell

• Insertion into the plasma membrane

What is the structure of the smooth endoplasmic reticulum (sER)

• Single membrane bound organelle

• Lacks ribosomes on its surface

• More tubular cisternae than the rER

What is the function of the sER in general?

Lipid synthesis (phospholipids, cholesterol, steroid hormones)

What is the specialized function of the sER in liver cells?

• Carbohydrate metabolism as it is involved in the pathway where glycogen is broken down into glucose

• Some enzymes detoxify drugs and poison

What is the specialized function of the sER in muscle cells?

sER is known as the sarcoplasmic reticulum and stores calcium ions involved in muscle contraction

What kind of cells have an abundance of rER?

Secretory cells actively synthesizing proteins e.g. pancreatic cells

What determines the location of ribosomes (cytosol or rER

• Presence of signal peptide

• The signal peptide is recognized by the signal recognition peptide in the cytoplasm that binds to a receptor in the rER, bringing the ribosome to the rER and the peptide is translocated into the rER lumen

How does the signal peptide come about?

When some proteins are being synthesized by the ribosomes, a short sequence of amino acids, the signal peptide, makes up the first stretch of the polypeptide that is synthesized

What is the structure of the Golgi Apparatus?

• Single membrane bound organelle

• Consists of a stack of flattened, membrane-bound sacs called cisternae and associated vesicles called Golgi vesicles

What are the faces of the Golgi Apparatus?

• Convex (cis) face, where vesicles from the ER fuse to add new vesicles to the Golgi Apparatus

• Concave (trans) face, giving rise to vesicles that pinch off and travel to other sites

What is the function of the Golgi Apparatus (in summary)?

Packing, sorting, targeting and packaging macromolecules for use within the cell or for secretion out of the cell

What is the function of the Golgi Apparatus (in detail)? [glycoproteins and glycolipids]

• Glycolysation: Add short sugar chains to proteins and lipids to form glycoproteins / glycolipids

• Modify existing glycoprotein and glycolipids made in the ER by cleaving a sugar molecule from their sugar chain or modifying their sugars

• Lysosome formation

• Produces polysaccharides which are secreted from the concave face of the GA, in transport vesicles that eventually fuse with the plasma membrane

• Sorts and targets completed materials to different parts of the cell or for secretion out of the cell

What is the structure of the lysosome?

• Small spherical vesicles

• Enclosed by a single membrane

What are the contents of the lysosome?

• Hydrolytic enzymes (proteases, lipases, nucleases) used to digest macromolecules with pH in the acidic range

• Contents are acidic

How are lysosomes formed?

• Hydrolytic enzymes and lysosomal membrane proteins are made by bound ribosomes on the rER

• Contents above are transported via vesicles to the convex face of the GA for further processing before the processed enzymes bud off the concave face of the GA to form lysosomes

How are vulnerable bonds on lysosomes shielded from enzymatic attack

Proteins on the inner surface of the lysosomal membrane and digestive enzymes within them are not hydrolyzed as the 3D shape of these proteins shield vulnerable bonds from enzymatic attack

What are the functions of lysosomes?

• Digestion of material taken in by endocytosis

• Autophagy

Describe the digestion of material taken in by endocytosis in lysosomes

• Lysosomes fuse with vesicles/vacuoles formed by endocytosis to digest the contents including food materials, foreign particles etc.

• Useful products of hydrolysis are absorbed and assimilated into the cytoplasm

• Unwanted products are released into the external medium by exocytosis

Describe autophagy in lysosomes

• The breakdown of unwanted structures within the cell

• Organic products from the breakdown are returned to the cytoplasm for reuse