The Ultrastructure of the cell

Question 1

What is the cytokeleton ?

Answer 1

The cytoskeleton is a network of
fibers extending throughout the
cytoplasm
It organizes the cell’s structures and activities,
anchoring many organelles, provides cell shape, aids in motility, and responds to environmental signals

Question 2

Three main types of fibres

Answer 2

• Microtubules are the thickest of the three components of the cytoskeleton
• Microfilaments, also called actin filaments, are the thinnest components
• Intermediate filaments are fibers with diameters in a middle range

Question 3

Roles of the Cytoskeleton: Support,
Motility, and Regulation

Answer 3

Support: The cytoskeleton helps to support the cell and maintain its shape
Motility: It interacts with motor proteins to produce motility. inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton
Regulation: Recent evidence suggests that the cytoskeleton may help regulate biochemical activities

Question 4

Microtubules

Answer 4

Structure: Hollow tubes, wall cosists of 13 columns of tubulin molecules
Diameter: 25nm with 15nm lumen
Protein subunits: Tubulin
Main functions : Maintenance of cell shape
Cell Motility
Chromosome movements in cell division
Organelle movements

Question 5

Microtubules in centrosomes and centrioles

Answer 5

• In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring
• Before an animal cell divides, centrioles
  replicate
• Not all eukaryotes have centrioles – organize microtubules in other ways

Question 6

Microtubles beating in cilia and flagella

Answer 6

• Microtubules (specialised arrangement) control the beating of cilia and flagella, locomotor appendages of some cells
• Cilia and flagella differ in their beating patterns
• Flagella – undulating motion generates force in same direction as axis
• Cilia – more like oars – power and recovery strokes

Question 7

Cillia and flagella ultrastructure

Answer 7

A core of microtubules sheathed by the plasma
membrane (9+2)
A basal body that anchors the cilium or flagellum
(similar centriole)
A motor protein called dynein, which drives the bending movements of a cilium or flagellum

Question 8

Microfilaments

Answer 8

Structure : Two interwined strands of actin
Diameter: 7nm
Protein subunits: Actin
Main functions: Maintenance of cell shape
Changes in cell shape
Muscle contraction
Cytoplasmic streaming
Cell motility
Cell division

Question 9

Structure of microfilaments

Answer 9

They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape
Bundles of microfilaments make up the core of
microvilli of intestinal cells

Question 10

What do microfilaments that function in cellular motility contain?

Answer 10

They contain myosin and actin

Question 11

What is pseudopodia? And what is its function?

Answer 11

Cellular extensions - extend and
contract through the reversible assembly and
contraction of actin subunits into microfilaments

Question 12

What is a cytoplasmic streaming?
What is its purpose? Where does it occur ?

Answer 12

Directed flow of cytosol and organelles around plant cells through the mediation of actin.
- Aids in the delivery of nutrients, metabolites, genetic information, and other materials to all parts of the cell.
* Cytoplasmic streaming occurs along actin filaments in the cytoskeleton of the cell.
* Dependent on intracellular pH and temperature

a circular flow of cytoplasm within
cells

Question 13

How are microfilaments responsible for wound healing?

Answer 13

A contractile ring of actin and myosin is formed
around the wound

Question 14

Intermediate Filaments

Answer 14

Structure: Fibrous proteins supercoiled into thicker cables
Diameter: 8-12 nm
Protein subunits: One of several different proteins of the keratin family
Main functions: Maintenance of cell shape
Anchorage of nucleus ad certain other organelles
Formation of nuclear lamina

Question 15

What are plant cell walls and what are they made of ?

Answer 15

• The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water
• Plant cell walls are made of cellulose fibers which form bundles. Other polysaccharides form cross links between the cellulose fibers. Each layer of cellulose fibers runs in a different direction = increased mechanical strength

Question 16

What are the 3 layers of the cell wall?

Answer 16

• Primary cell wall: relatively thin and flexible
• Middle lamella: thin layer between primary walls of
  adjacent cells - glue
• Secondary cell wall (in some cells): added between the
  plasma membrane and the primary cell wall

* Plasmodesmata are channels between adjacent
plant cells

Question 18

Cell wall of young plant vs old cell

Answer 18

A young growing plant cell secretes a primary cell
wall which is flexible and can stretch and expand.
* When cell stops growing it strengthens its wall (new strong wall or multiple layers for secondary wall)

Question 19

What role do microtubules play in orienting deposition of cellulose in cell walls?

Answer 19

They influence cellulose deposition by either moving microtubules or directly changing the orientation of newly synthesized cellulose microfibrils.

Question 20

What is the ECM of animal cells and what is it made up of ?

Answer 20

Extracellular matrix (ECM)
The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin

Question 21

What attaches the cell to the ECM

Answer 21

protein fibronectin

Question 22

What are intergins ?

Answer 22

ECM proteins bind to receptor proteins in the
plasma membrane called integrins

Question 23

Glycocalyx

Answer 23

Coat formed of lipids and
polysaccharides that form part of the plasma
membrane
* Role in cell contact and cellular recognition and
increases mechanical strength of multicellular
tissues

Question 25

What are the functions of ECM

Answer 25

• Structural support
• Cell to cell adhesion
• Movement
• Regulation of biochemical processes

Question 26

What are intercellular junctions ?

Answer 26

• Intercellular junctions facilitate the contact between cells in tissues, organs, or organ
  systems.

Question 28

What are the different types of Intercellular junctions?

Answer 28

• Plasmodesmata
• Tight junctions
• Desmosomes
• Gap junctions

Question 29

Plasmodesmata

Answer 29

Are channels (20 – 40 nm in
Diameter) that perforate plant cell walls
* Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell
* The plasmodesmata allow relatively free passage of many molecules – no plasma membrane blocking them
either

Question 30

Tight junctions

Answer 30

At tight junctions, membranes of neighbouring
cells are pressed together, preventing leakage of
extracellular fluid

Question 31

Desmosomes

Answer 31

(anchoring junctions) fasten cells
together into strong sheets

Question 32

Gap junctions

Answer 32

(communicating junctions) provide
cytoplasmic channels between adjacent cells

Question 33

Cadherins

Answer 33

Protein filaments (transmembrane
glycoproteins) link two adjacent
cells
Also link actin filaments