Organelles. 1 (organelles)

Question 1

What are organelles?

Answer 1

A subcellular structure at the microscopic scale with one or more overall functions.

Question 2

What is the nucleus, and what are some of its characteristics?

Answer 2

This is the site of DNA replication and transcription, as well as mRNA and tRNA synthesis.
It has a double membrane, surrounded by a nuclear envelope, which controls what enters and exits the cell.
The nucleus contains over 99.5% of the cell’s genes.

Question 3

What is euchromatin?

Answer 3

The less condensed, transcribing regions that are transcriptionally active.
Have a high GC content.
Appears lighter when viewed, as the dye cannot bind to the transcriptionally active regions as well.
This is more abundant in cells that are more active and are producing proteins at a higher rate.

Question 4

What is heterochromatin?

Answer 4

This is more condensed material, usually depicting transcriptionally silent areas.
Often present in silenced genes, that are methylated, and have a higher AT content.
These appear darker when viewed as they take up stain more readily.

Question 5

What is the nucleolus, functions, locations, characteristics etc.

Answer 5

It is a region found within the nucleus, that has no membrane, and is responsible for ribosome assembly and rRNA processing.

Question 6

What are nuclear subdomains and what is their importance?

Answer 6

These are regions where each chromosome can map out mii organelles of some sort within the nucleu… i.e. each nucleus has organelles at different levels within it.
These chromosomes in the nucleus are not linked to one zone however, i.e. thy can loop out and be found in a different nuclear structure, i.e. they are not static to one area, they are moving around at some rate.

Question 7

How do ‘organelles’ / subdomains in the nucleus move around?

Answer 7

By random diffusion, as there is no cytoskeleton or motors in the nucleus.

Question 8

What is the function of the mitochondria and what are some of its characteristics?

Answer 8

It is the principal site of ATP synthesis, and the site of respiration (oxidative phosphorylation). It is involved in the Krebs cycle which is intermediary metabolism.
Also provides a calcium store, and can instigate apoptosis.
The mitochondria has a double membrane and its own genome, consisting of 13 oding genes and translational machinery.

Question 9

What is the function of peroxisomes?

Answer 9

They are involved in lipid metabolism, in the synthesis and breakdown of lipids.
Degradation of hydrogen peroxide to water … (H2O2 produced by catalase).

Question 10

How do peroxisomes vary between humans and plants?

Answer 10

In humans they appear very empty, however in plants, these proteins are so highly concentrated in the peroxisomes that there is an electron-dense paracrystalline matrix.

Question 11

What is the function of lipid droplets, and what are their characteristics?

Answer 11

They are involved in lipid synthesis, storage and breakdown… used as a signalling platform.
THey have no bilayer, only a monolayer. This is because their contents are almost entirely oil, so no bilayer is needed… hydrophilic heads are out to the water, and the hydrophobic tails are faced inwards towards the oil contents.

Question 12

What is the function of chloroplasts and what are their characteristics?

Answer 12

They are chlorophyll containing membranes involved in photosynthesis in plant cells (only found in plant cells).
They are chlorophyll containing membranes believed to be largely derived from one endosymbiotic event. Have thylakoids and the stroma.

Question 13

What are thylakoids and what is their function?

Answer 13

These are internal membranes found in chloroplasts. They stack up to form grana and play a role in light harvesting and the light dependent reactions of photosynthesis.

Question 14

What is the stroma and what occurs here?

Answer 14

The colorless fluid surrounding the grana in chloroplasts, within the stroma are grana (the stacks of thylakoids).
In the stroma, the final stages of photosynthesis occurs here.

Question 15

What is the function of vacuoles?

Answer 15

Acts as a storage organelle, by enclosing areas filled with both organic and inorganic molecules, along with water to support the organelle.
Plants have a very large vacuole, helping to make the cells easily expandable, allowing cells to change size and shape with availability of water.

Question 16

What is the tonoplast?

Answer 16

An active transport membrane surrounding the vacuole.

Question 17

What is the function of the SER (smooth endoplasmic reticulum).

Answer 17

It is involved in the synthesis of fatty acids and phospholipids as well as calcium storage in the cells.

Question 18

Which cells will contain a lot of SER?

Answer 18

Cells making a high level of lipids.

Question 19

What is the RER (rough endoplasmic reticulum).

Answer 19

A series of connected flattened sacs, forming a part of a continuous membrane organelle within cytoplasm of eukaryotic cells, that run form the plasma membrane to nuclear envelope of the cell.
It is coated with ribosomes and can form a network inside the cell intimately linked to the mitochondria and peroxisomes.

Question 20

When was the ER first noted, and by who?

Answer 20

1945… by Porter, Claude, Meskers and Fullman with an electron microscope.

Question 21

What is the function of the RER?

Answer 21

They play a role in the synthesis of proteins on the ribosomes found on their surface. Involved in post-translational modification of proteins too… the starting point of secretory processes.

Question 22

Which cells will contain a lot of ER?

Answer 22

Plasma cells, as they are making very high levels of proteins in the form of antibodies.

Question 23

What is the cytoplasm?

Answer 23

Everything except the nucleus i.e. the part of the cell which is contained within the entire cell membrane other than the contents of the nucleus of the cell.

Question 24

What is the cytosol?

Answer 24

The remainder when all organelles are removed i.e. the intracellular fluid present inside cells.

Question 25

What is the function of the cytosol?

Answer 25

Intermediary metabolism (e.g. glycolysis). This is the intracellular process by which nutritive material is converted into cellular components. 
Signal transduction from one point of the cell to another.

Question 26

How is organelle movement aided in the cytosol?

Answer 26

By cytoskeletal fibres that form networks in the cytosol, of microtubules, microfilaments, intermediate filaments which support membranes, organise organelles and are important for cellular movement.

Question 27

What is the function of the Golgi Apparatus?

Answer 27

Modification, sorting and packaging of proteins from the ER for secretion to the plasma membrane.
Involved in the transport of lipids around the cells, and the creation of lysosomes.
They are typically sacs or folds called cisternae.

Question 28

What is an autophagosome and what is its function?

Answer 28

A type of lysosome, that is a spherical structure with a double layer membrane.
It is involved in the intracellular degradation system for cytoplasmic contents and for invading microorganism.
They are formed from a portion of cytoplasm including organelles enclosed by a membrane.
The outer membrane of the autophagosome fuses with endosomes, then lysosomes, then internal material is degraded.

Question 29

What are processing bodies (P-body)?

Answer 29

Distinct foci within the cytoplasm of the eukaryotic cell, consisting of many enzymes involved in mRNA decay and storage.
It has no membrane.

Question 30

What are sec bodies?

Answer 30

A sequestration of ER exit machinery, that act as a protection mechanism of cells under stress.

Question 31

What are the advantages of dividing up a cell into organelles (for the cell and the biologist)?

Answer 31

For the cell:
Physical segregation of activities (proteins etc.) and environments (ions, small molecules etc.).
Allows for highly specialised varied functions.

For the biologist:
Link function to location.
Allows for simplification of the cell… to produce models and experiments for hypothesis testing.
Can simplify biology to study in isolation.

Question 32

What are the disadvantages of dividing up a cell into organelles (for the cell and the biologist)?

Answer 32

For the cell:
Has to communicate between the different areas of segregation.
Extra level of complications.

For the biologist:
Studying an isolated system/ cell is not a true depiction of the conditions within an organism, hence could draw a wrong conclusion from the over-simplification.
The isolated cell/ organelle may not behave the same way during/ after the experiment as it would in the organism.

Question 33

What does cell specialisation require?

Answer 33

Activities of a range of organelles.
Specific cell shapes.
Specialised organelles and cytoskeleton.

Question 34

What are photoreceptors, and what are their characteristics in humans and in flies?

Answer 34

These are photon-gathering systems that have an increased membrane for light capture.

HUMAN:
There is an inner and an outer segment with a connecting cilium.
THe rod outer segment has expanded stacked membranes, with high amounts of rhodopsin.

FLY:
Apical surface of the cell has been turned on its side, so the rhodopsin is stacked in the microvilli of these rhabdomeres.

Question 35

What are some specializations of cyanobacteria and green plants?

Answer 35

The have elaborated a huge number of proteins by stacking up sites of membrane to increase the surface area.
These are related to bacteria, but are capable of photosynthesis.
They are prokaryotic.