Cell Structure

(a) The ultrastructure of a cell is its detailed internal structure — the organelles and membranes too small to be seen with a light microscope.

(b) It is studied using the electron microscope, which has a much higher resolution than the light microscope.

(a) The nucleolus makes ribosomal RNA and assembles ribosomes.

(b) The smooth endoplasmic reticulum synthesises lipids (and steroids); it has no ribosomes.

(c) The Golgi apparatus modifies, packages and labels proteins into vesicles for secretion or transport.

(a) Eukaryotic cells contain the larger 80S ribosomes in their cytoplasm.

(b) The mitochondria and the chloroplasts inside eukaryotic cells contain the smaller 70S ribosomes.

Proteins for export are synthesised by ribosomes attached to the rough endoplasmic reticulum (rER) during translation. Inside the rER the polypeptide is folded and processed, then a section of the rER membrane pinches off as a transport vesicle containing the protein.

The transport vesicle moves to and fuses with the Golgi apparatus, emptying the protein into it. In the Golgi the protein is further modified (for example by adding carbohydrate), and is then packaged and labelled into a secretory vesicle.

The secretory vesicle moves to the cell surface membrane, fuses with it, and the protein is released from the cell by exocytosis.

A mitochondrion is surrounded by a double membrane. The inner membrane is folded into projections called cristae, which provide a large surface area for the proteins (electron carriers and ATP synthase) involved in the later stages of aerobic respiration.

The space enclosed by the inner membrane is the fluid matrix, which contains the enzymes for the Krebs cycle. This arrangement makes the mitochondrion the site of aerobic respiration, in which large amounts of ATP are produced.

Cells with a high energy demand, such as muscle or secretory cells, therefore contain many mitochondria.

The process begins in the nucleus, where the gene coding for amylase is transcribed into messenger RNA (mRNA). The mRNA then passes out of the nucleus through a nuclear pore.

In the cytoplasm the mRNA attaches to ribosomes on the rough endoplasmic reticulum (rER), where it is translated into the amylase polypeptide. The polypeptide enters the rER, where it is folded and processed. A transport vesicle then buds off the rER, carrying the protein.

The transport vesicle moves to and fuses with the Golgi apparatus. Inside the Golgi the amylase is modified, packaged and labelled, and is then enclosed in a secretory vesicle that buds off the Golgi.

The secretory vesicle moves towards the cell surface membrane (using ATP supplied by mitochondria), fuses with it, and the amylase is released out of the cell by exocytosis.

(a) The nucleus can be identified because it is usually the largest organelle, it is surrounded by a double membrane (nuclear envelope) with pores, and it often contains a dense nucleolus. (Any two of these.)

(b) A mitochondrion has a double membrane with the inner membrane folded into cristae, whereas a lysosome is a simple, small, round membrane-bound sac with no internal folds and is generally much smaller.

(c) The rough ER has ribosomes (small dark dots) attached to its membranes, giving it a studded appearance, while the smooth ER has no ribosomes attached, so its membranes look smooth.

(a) Magnification. The scale bar represents 2 µm but measures 8 mm on the image. Convert the image measurement to the same units: 8 mm = 8000 µm.

$M = \frac{\text{image size}}{\text{actual size}} = \frac{8000\ \mu\text{m}}{2\ \mu\text{m}} = 4000$

The magnification is ×4000.

(b) Actual length of the chloroplast. The chloroplast measures 56 mm = 56 000 µm on the image. Using $A = I \div M$:

$A = \frac{56\,000\ \mu\text{m}}{4000} = 14\ \mu\text{m}$

The actual length of the chloroplast is 14 µm.

Both the bacterial (prokaryotic) cell and the animal (eukaryotic) cell are surrounded by a cell surface membrane, and both contain cytoplasm, ribosomes and DNA — so they share the basic machinery of life.

However, they differ in several important ways. The animal cell has a true, membrane-bound nucleus containing its DNA, whereas the bacterial cell has no nucleus; its DNA is a single circular molecule lying free in the cytoplasm, and it may also carry small rings of DNA called plasmids. Eukaryotic DNA is linear and wound around histone proteins, which prokaryotic DNA is not.

The animal cell contains many membrane-bound organelles — mitochondria, rough and smooth endoplasmic reticulum, Golgi apparatus and lysosomes — while the bacterial cell has none of these. The ribosomes also differ: the animal cell has larger 80S ribosomes, the bacterial cell smaller 70S ribosomes.

Finally, the bacterial cell is smaller overall and is surrounded by a cell wall made of murein (peptidoglycan), sometimes with a capsule and flagellum, whereas a typical animal cell has no cell wall.

A protein-secreting cell is adapted by having large numbers of the organelles involved at each stage of the secretory pathway.

First, it contains many ribosomes and an extensive rough endoplasmic reticulum. Because the secreted product (such as an antibody) is a protein, it must be made by translation on ribosomes; the more ribosomes and rER, the higher the rate of protein synthesis and folding. The proteins are then carried away from the rER in transport vesicles.

Second, the cell has many Golgi bodies. The large quantity of protein arriving from the rER must be modified, packaged and labelled; numerous Golgi stacks allow this processing and the production of many secretory vesicles to occur rapidly.

Third, the cell contains numerous mitochondria. Both protein synthesis and the movement and fusion of vesicles during exocytosis are active processes that require ATP; abundant mitochondria provide this energy through aerobic respiration.

Finally, the secretory vesicles fuse with the cell surface membrane and release the protein by exocytosis. The whole cell is therefore organised so that protein is made on the rER, processed in the Golgi, and exported across the membrane, with mitochondria powering the process at every energy-requiring step.