Organelles

Question 1

Tell me about a nucleus (4)

Answer 1

This is the largest organelle. It is surrounded by a nuclear envelope, which is a double membrane with nuclear pores – large holes containing proteins that control the exit of substances from the nucleus. The interior is called the nucleoplasm, which is full of chromatin – the DNA/protein complex (see unit 2). During cell division the chromatin becomes condensed into discrete observable chromosomes. The nucleolus is a dark region of chromatin, involved in making ribosomes.

Question 2

Tell me about mitochondria (6)

Answer 2

This is a sausage-shaped organelle (8micrometres long), and is where aerobic respiration takes place in all eukaryotic cells (anaerobic respiration takes place in the cytoplasm). Mitochondria release energy (in the formATP) from carbohydrates, lipids and other energy rich molecules. Cells that use a lot of energy (like muscle cells) have many mitochondria.

Mitochondria are surrounded by a double membrane: the outer membrane is simple and quite permeable, while the inner membrane is highly folded into cristae, which give it a large surface area. The space enclosed by the inner membrane is called the mitochondrial matrix, and contains small circular strands of DNA. The inner membrane is studded with stalked particles, which are the enzymes that make ATP.

Question 3

Tell me about ribosomes? (4)

Answer 3

These are the smallest and most numerous of the cell organelles, and are the sites of protein synthesis. Ribosomes are either found free in the cytoplasm, where they make proteins for the cell’s own use, or they are found attached to the rough endoplasmic reticulum, where they make proteins for export from the cell. All eukaryotic ribosomes are of the larger, “80S”, type.

Question 4

What are lysosomes and what do they do? (4)

Answer 4

These are small membrane-bound vesicles formed from the RER containing a cocktail of hydrolytic enzymes. They are used to break down unwanted chemicals, toxins, organelles or even whole cells, so that the materials may be recycled. They can also fuse with a feeding vacuole to digest its contents.

Question 5

Tell me about rough and smooth ER (4)

Answer 5

Endoplasmic Reticulum (ER). This is a series of interconnected membrane channels involved in synthesising and transporting materials. Rough Endoplasmic Reticulum (RER) is studded with numerous ribosomes, which give it its rough appearance. The ribosomes synthesise proteins, which are processed in the RER (e.g. by enzymatically modifying the polypeptide chain, or adding carbohydrates), before being exported from the cell via the Golgi Body.
 Smooth Endoplasmic Reticulum (SER) does not have ribosomes and is used to process materials, mainly lipids, needed by the cell.

Question 6

What is the Golgi body/apparatus and what does it do?

Answer 6

It is series of flattened stacks of membrane vesicles, formed from the endoplasmic reticulum. Its job is to transport proteins from the RER to the cell membrane for export. Parts of the RER containing proteins fuse with one side of the Golgi body membranes, and are modified (carbohydrate is added to form glycoproteins), while at the other side small vesicles bud off and move towards the cell membrane, where they fuse, releasing their contents by exocytosis.

Question 7

Even though this is Unit 2, tell me about chloroplasts!!! (10)

Answer 7

Membrane arrangement and disc shape provides large surface for light absorption;
layering of membrane allows a lot of pigment;
Permeable membrane allows diffusion of gases /carbon dioxide;
membranes provide surface for attachment of electron / hydrogen acceptors;
Contains chlorophyll for light absorption;
Contains different pigments to absorb different wavelengths;
Stacking / arrangement of grana/thylakoids maximises light catchment;
Stroma contains enzymes for photosynthesis;
Outer membrane keeps enzymes in chloroplast;
Starch grains / lipid droplets store products of photosynthesis;
Ribosomes / DNA for enzyme/protein synthesis;
Shape of chloroplast gives large surface area for CO2, absorption.

Question 8

Describe how you would obtain a sample of undamaged chloroplasts from leaves. (6)

Answer 8

1. Chop up leaves in a….
2. Cold: reduces enzyme activity
3. Buffered solution: prevents pH changes affecting enzymes
4. Isotonic: prevents osmosis and possible lysis or shrinkage of organelles
5. Filter and centrifuge filtrate
6. Centrifuge supernatant
7. at higher speed;
8. Chloroplasts in (second) pellet;

Key to remember you must separate organelles in order, most dense first then least dense, if you try and jump to a particular stage the sample will be contaminated

Question 9

What do we call the solid and the liquid when we have centrifuged some cells in order to separate the organelles?

Answer 9

After spinning the pellet of organelle is at the bottom the remaining solution is called the supernatant and poured (decanted) for centrifugation.

Question 10

Which organelle reminds Mrs Dobson of Miranda’s BUN? (Not what you thought I said!)

Answer 10

The ribosome (hehe)