BIOLOGYUNIT(1)

what is a “proteome”?

the entire set of proteins expressed by a genome

why is the proteome larger than the genome? (particularly in eukaryotes)

because more than 1 protein can be produced from a single gene as a result of alternative RNA splicing.

genome = recipe book with 10 base recipes

Each recipe has:

• optional ingredientswhat is a “proteome”?

proteome = From 10 recipes, you can make:

• 30 different dishes

Genome
→ the complete set of genes in a cell or organism

Proteome
→ the complete set of proteins produced from that genome

how can proteomes be different if each cell has the same genome? - (each organism has different cells, but those cells in one organism have the same genome)

not all genes are expressed as proteins, the proteome depends on the cell type - every cell has the same genome, but cells only use the genes they need : the proteome includes only the protein a cell currently makes.

what factors may change the proteome of a given cell?

the set of proteins a cell expresses can depend on:

• metabolic activity

• cellular stress

• health status

• signalling molecules (hormones)

explain how factors affect the proteome.

the proteome is the set of proteins CURRENTLY being expressed, due to a specific set of commands for a specific cell type - an affecting factor can trigger a new set of commands from the cell’s genome. - these ‘new’ proteins stay, but are stopped producing or slowly destroyed

why do eukaryotic cells have internal membranes?

because of their size, eukaryotes have a relatively small surface area (area of cell/plasma membrane) to volume ratio (space inside the cell). it is therefore too small an area to carry out all the vital functions carried out by membranes.

internal membranes (e.g. ER, golgi, mitochondria) increase TOTAL membrane area allowing more chemical reactions and transport.

what is the function of “endoplasmic reticulum”?

they form a network of membrane tubules, continuous (physically connected - no gap in between) with the nuclear membrane - materials can move directly between the two

describe the structure of a “golgi apparatus”.

a series of flattened membrane discs.

what is the role of “vesicles”?

to transfer materials between membrane compartments - nuclei, golgi, ER, and mitochondria

what are “lysosomes” and what is their role?

they are membrane-bound organelles containing hydrolases (enzymes that break down molecules) that digest proteins, lipids, nucleic acid and carbohydrates. - “the recycling and waste disposal system of the cell”

why is rough endoplasmic reticulum (RER) described as “rough”?

rough ER has ribosomes on its cytosolic face

where are proteins and lipids, specifically lipids, synthesised?

in the endoplasmic reticulum (ER).

• lipids are synthesised in the SER, where it’s then inserted into the ER’s membrane

the synthesis of all proteins begins where in the cell?

cytosolic ribosomes (floating in cytosol). the synthesis of proteins is completed here and they remain in the cytosol. - Cytosolic proteins stay in the cytosol because they have no signal telling the cell to send them anywhere else. Proteins stay where they are made unless they contain a signal that directs them elsewhere (transmembrane protein).

what is a “transmembrane protein”, and what is it’s role?

transmembrane proteins carry a signal sequence which halts translation and directs the ribosome synthesising the protein, to dock with the ER forming RER

what is a “signal sequence”?

a short stretch of amino acids at one end of the polypeptide that determines the eventual location of a protein in the cell.

what occurs after the ‘docking’ of the ribosome to the ER?

translation restarts and the transmembrane protein is inserted into the membrane of the ER

describe the stages of ‘synthesis membrane components by RER’

1. proteins are synthesised by ribosomes in the cytoplasm.

2. a signal protein from the membrane binds to the ribosome and stops translation

3. the signal protein binds to a receptor and directs the ribosome to attach to the ER, forming RER.

4. a) translation restarts and the protein is inserted into the membrane of the ER

4 .b) once translation is finished, the ribosome detaches.

what happens once the proteins are inserted into the RER?

they are transported by vesicles that bud off from the ER and fuse with the golgi apparatus.

why do proteins go through these modifications?

-Proteins destined for secretion or membranes enter the ER for folding and initial modification, are transported to the Golgi apparatus for further modification and sorting, and are then delivered to their final destination.

what happens to proteins as they move through the golgi apparatus?

they undergo post-translational modification