2.1- Other features of euksryotic cells

Question 1

What is the structure of ribosomes?

Answer 1

• Small spherical organelles, about 20nm in diameter.
• Made of ribosomal RNA
• Made in the nucleolus, as two separate subunits, which pass through the nuclear envelope into the cell cytoplasm and then combine.
• Some remain free in the cytoplasm and some attach to the endoplasmic reticulum.

Question 2

What is the function of ribosomes?

Answer 2

• Ribosomes bound to the exterior of RER are mainly for synthesising proteins that will be exported outside the cell.
• Ribosomes that are free in the cytoplasm, either singly or in clusters, are primarily the site of assembly of proteins that will be sued inside the cell.

Question 3

What is the structure of centrioles?

Answer 3

The centrioles consists of two bundles of microtubules at right angles to each other.
-The microtubules are made of tubulin protein subunits and are arranged to form a cylinder.

Question 4

What is the function of centrioles?

Answer 4

-Before a cell divides, the spindle, made of threads of tubulin, forms from the centrioles.
-Chromosomes attach to the middle part of the spindle and motor protein walk along the tubulin threads, pulling the chromosomes to opposite ends of the cell
Centrioles are involved in the formation of cilia and undulipodia:
-Before the cilia form, the centrioles multiply and line up beneath the cell surface membrane.
-Microtubules then sprout outwards from each centriole, forming a cilium or undulipodium.
Centrioles are usually absent from cells of (higher) plants but may be present in some unicellular green algae, such as Chlamydomonas.

Question 5

What is the structure of the cytoskeleton?

Answer 5

A network of protein structures within the cytoplasm. It consists of :

• rod- like microfilaments made of subunits of the protein actin; they are polymers of actin and each microfilament is about 7nm in diameter
• Intermediate filaments about 10 nm in diameter.
• Straight cylindrical micro tubules made of protein subunits called tubulin about 18-30 nm in diameter.
• The cytoskeleton motor protein myosins, kinesins and dyneins are molecular motors. They are also enzymes and have a site that binds to and allows hydrolysis of ATP as their energy source.

Question 6

What is the function of cytoskeleton?

Answer 6

The protein microfilaments, within the cytoplasm give support and mechanical strength, keep the cell’s shape stable and allow cell movement.
Microtubules also provide shape and support to the cells and help substances and organelles to move through the cytoplasm within a cell.
-They form the track along which motor proteins (dynein and kinesin) walk and drag organelles from one part of the cell to another
-They form the spindle before a cell divides. These spindle threads enable chromosomes to be moved within the cell.
-Microtubules also make up the cilia , undulipodia and centrioles. Intermediate filaments are made up of a variety of proteins. They:
-Anchor the nucleus within the cytoplasm
-Extend between cells in some tissues, between special junctions, enabling cell to cell signalling and allowing cells to adhere to a basement membrane , therefore stabilising tissues

Question 7

What is the structure of cellulose cell wall?

Answer 7

The cell wall of plants is on the outside of the plasma membrane. It Is made from bundles of cellulose fibres

Question 8

What is the function of cellulose cell wall?

Answer 8

Absent from animal cells, the cell wall is strong and can prevent plant cells from bursting when turgid (swollen).
The cell walls of plant cells:
-provide strength and support
-Maintain the cells shape
-Contribute to the strength and support of the whole plant
-Are permeable and allow solutions (solute and solvent) to pass through.
Fungi have cell walls that contain chitin, not cellulose