2.1.1 cell ultrastructure

Question 1

Nucleus

Answer 1

This is the largest organelle inside the cell. It is surrounded by a nuclear envelope Which is a double membrane containing nuclear pores, large holes which control the exit of substances such as RNA and proteins from the nucleus. The interior is called the nucleoplasm and is full of chromatin a DNA/protein protein complex in the ratio of 1 to 2 containing genes. When cell division occurs the chromatin becomes condensed into discreet observable chromosomes. The dark region of the cell is called a nucleolus and is involved in making ribosomes

Question 2

Mitochondrion

Answer 2

where aerobic respiration takes place in all eukaryotic cells. 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 site of ATP synthesis.

Question 3

Chloroplast

Answer 3

chloroplasts are where photosynthesis takes place, so are only found in photosynthetic organisms (plants and algae). Like mitochondria they are enclosed by a double membrane, but chloroplasts also have a third membrane called the thylakoid membrane. The thylakoid membrane is folded into thylakoid disks, which are then stacked into piles called grana. The space between the inner membrane and the thylakoid is called the stroma. The thylakoid membrane contains chlorophyll and other photosynthetic pigments arranged in photosystems, together with stalked particles, and is the site of photosynthesis and ATP synthesis. Chloroplasts also contain starch grains, ribosomes and circular DNA.

Question 4

Ribosomes

Answer 4

These are the smallest and most numerous of the cell organelles, and are the sites of protein synthesis. They are composed of protein and RNA, and are manufactured in the nucleolus of the nucleus. 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. They are often found in groups called polysomes. All eukaryotic ribosomes are of the larger, “80S”, type.

Question 5

Smooth endoplasmic reticulum

Answer 5

Series of membrane channels involved in synthesising and transporting materials, mainly lipids, needed by the cell.

Question 6

Rough endoplasmic reticulum

Answer 6

Similar to the SER, but 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.

Question 7

Golgi apparatus

Answer 7

Another series of flattened 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, while at the other side small vesicles bud off and move towards the cell membrane, where they fuse, releasing their contents by exocytosis.

Question 8

Vacoules

Answer 8

These are membrane-bound sacs containing water or dilute solutions of salts and other solutes. Most cells can have small vacuoles that are formed as required, but plant cells usually have one very large permanent vacuole that fills most of the cell, so that the cytoplasm (and everything else) forms a thin layer round the outside. Plant cell vacuoles are filled with cell sap, and are very important in keeping the cell rigid, or turgid. Some unicellular protoctists have feeding vacuoles for digesting food, or contractile vacuoles for expelling water.

Question 9

Lysosomes

Answer 9

These are small membrane-bound vesicles formed from the RER containing a cocktail of digestive 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 10

Undulipodium

Answer 10

This is a long flexible tail present in some cells and used for motility. It is an extension of the cytoplasm, surrounded by the cell membrane, and is full of microtubules and motor proteins so is capable of complex swimming movements. There are two kinds: flagella (no relation of the bacterial flagellum) are longer than the cell, and there are usually only one or two of them, while cilia are identical in structure, but are much smaller and there are usually very many of them.

Question 11

Centrioles

Answer 11

This is a pair of short micro tables involved in cell division. Before cell division the centrioles replicate themselves and then the two centrioles pull apart to opposite ends of the cell, where they initiate the spindle that organises and separates the chromosomes

Question 12

Cytoplasm/cytosol

Answer 12

This is the solution within the cell membrane. It contains enzymes for glycolysis (part of respiration) and other metabolic reactions together with sugars, salts, amino acids, nucleotides and everything else needed for the cell to function

Question 13

Cytoskeleton

Answer 13

This is a network of protein fibres extending throughout all eukaryotic cells, used for support, transport and motility. The cytoskeleton is attached to the cell membrane and gives the cell its shape, as well as holding all the organelles in position. There are three types of protein fibres (microfilaments, intermediate filaments and microtubules), and each has a corresponding motor protein that can move along the fibre carrying a cargo such as organelles, chromosomes or other cytoskeleton fibres. These motor proteins are responsible for such actions as: chromosome movement in mitosis, cytoplasm cleavage in cell division, cytoplasmic streaming in plant cells, cilia and flagella movements, cell crawling and even muscle contraction in animals.

Question 14

Microvilli

Answer 14

These are small finger-like extensions of the cell membrane found in certain cells such as in the epithelial cells of the intestine and kidney, where they increase the surface area for absorption of materials. They are just visible under the light microscope as a brush border.

Question 15

Cell membrane/plasma membrane

Answer 15

This is a thin, flexible layer round the outside of all cells made of phospholipids and proteins. It separates the contents of the cell from the outside environment, and controls the entry and exit of materials. The membrane is examined in detail later.

Question 16

Cell wall

Answer 16

This is a thick layer outside the cell membrane used to give a cell strength and rigidity. Cell walls consist of a network of fibres, which give strength but are freely permeable to solutes (unlike membranes). A wickerwork basket is a good analogy. Plant cell walls are made mainly of cellulose, but can also contain hemicellulose, pectin, lignin and other polysaccharides. They are built up in three layers called the primary cell wall, the secondary cell wall and the middle lamella. There are often channels through plant cell walls called plasmodesmata, which link the cytoplasms of adjacent cells. Fungal cell walls are made of chitin (poly-glucosamine). Animal cells do not have a cell wall, though they do have a layer of carbohydrate outside the cell membrane called the cell coat, or glycocalyx.