Eukaryotic Cell Structure and Function

Question 1

Describe cell theory

Answer 1

The cell theory states the cell is a fundamental unit of structures, function and organisation in all living organisms, and new cells are formed from other existing cells.

Question 2

State the three tents of cell theory

Answer 2

All living organisms are composed of one or more cells

The cell is the most basic unit structure in all organisms

All cells arise from pre-existing cells

Question 3

Name the advantages of have membranous organelles

Answer 3

The presence of membranes surrounding the organelles allows the maintenance of characteristic differences between the contents of each organelle and the cytosol. The compartmentalisation of specific reactions provide different local environments for which incompatible processes can occur simultaneously.

The presence of membranes also helps to increase membrane surface area, thus increasing efficiency of many reactions by providing optimal enzyme concentration for reactions to occur

Question 4

Describe how transport vesicles from the ER are processed to transport substances to other organelles in the cell

Answer 4

New cisternae are constantly being formed at the cis face by receiving transport vesicles from the ER. The membranes of transport vesicles from the ER fuses with the cis face membrane and deposit their contents into Golgi cisternal space.
At the trans face, membranes bud off to form secretory vesicles, which contain materials to be transported to the extracellular matrix.
Membranes can also bud off from the Golgi trans face to form lysosomes.
Between the Golgi sacs, Golgi vesicles are responsible for transferring materials between the parts of the Golgi.
Some Golgi vesicles also bud off from the trans face to transport substances to other organelles in the cell

• cis face: “receiving” side of Golgi apparatus
• trans face: “shipping” side of Golgi apparatus

Question 5

Outline the functions of the nucleus

Answer 5

The nucleus encloses genetic material and protects DNA from metabolically active cytoplasm. The double membrane (nuclear envelope) is perforated with pores to enable exchange of substances between the nucleus and cytoplasm.

Question 6

Outline the functions of the nuclear envelope

Answer 6

The nuclear envelope is a double membrane and it separates the contents of the nucleus from the cytoplasm

Question 7

Outline the functions of the nucleoplasm

Answer 7

The nucleoplasm is an aqueous matrix within the nucleus containing proteins, metabolites, ions, RNA and chromatin

Question 8

Outline the functions of the nucleolus

Answer 8

It is composed of DNA carrying rRNA genes, RNA and protein, which functions to synthesize a specific type of RNA known as ribosomal RNA (rRNA) that forms a component of ribosomes

Question 9

Outline the functions of the rough endoplasmic reticulum and relate it to its significance

Answer 9

These rough ER-bound ribosomes are sites of protein synthesis where a polypeptide chain is synthesised at the bound ribosome
The polypeptide chain then enters the ER lumen, which is the site of protein folding, through a protein channel in the rER membrane where the polypeptide chain folds into its native conformation
These proteins are either destined for export, or are targeted to various cellular organelles

Cells that are active in protein secretion usually have abundant rough ER. Some proteins synthesised in the rough ER can also directly enter the membrane of the ER to form ER membrane proteins. Proteins that leave the rER are enclosed in vesicles known as transport vesicles

Question 10

Outline the functions of the smooth endoplasmic reticulum

Answer 10

The smooth ER is a network of tubules which lack ribosomes, resulting in its smooth appearance. It functions in diverse metabolic processes which require ATP, such as:

• Synthesis of lipids (including oils, phospholipids and steroids like sex hormones); cells that are active in hormone secretion usually have abundant sER.
• Metabolism of carbohydrates,
• Detoxification of drugs and poisons
• Storage of calcium ions for use in muscle contraction, as well as cell signalling

Question 11

Outline the functions of the Golgi body and relate it to its significance

Answer 11

Some modifications include glycosylation (addition of sugar groups) and trimming (removal of excess monomers). Different Golgi cisternae contain different enzymes for modification, and hence ER products are progressively modified as they move through the stack of the Golgi complex from the cis face to the trans face

Therefore, cells that are active in any form of secretion usually have abundant GA, as the abundant flattened cisternae provide increased surface area for vesicle reception and budding. Multiple cisternae also allow for different modification processes to occur simultaneously.

Question 12

Explain the adaptation of the stroma and thylakoid in chloroplasts

Answer 12

Stroma
The inner membrane encloses a semi-fluid compartment known as the stroma, which contains circular DNA that enables the synthesis of chloroplast proteins.
Stroma also contains enzymes required for the light-independent reactions (Calvin cycle) of photosynthesis

Thylakoids
Thylakoids are the site for light-dependent reactions of photosynthesis. The thylakoids allow for increased surface area for attachment of chlorophyll and other photosynthetic pigments.
Compartmentalisation of thylakoid lumen is
also necessary for the setting up of proton
gradient across the thylakoid membranes.

Question 13

Describe the contents of and outline the functions of the lysosomes

Answer 13

It contains hydrolytic enzymes (proteases, nucleases, lipases and acid phosphatases) that can digest most biological macromolecules. Due to the acidic (pH 5) nature of lysosome contents and hydrolytic activity of enclosed enzymes, lysosomal contents must be prevented from spilling into the cytoplasm under normal cell conditions. Therefore, segregation of contents within the membrane provides optimal pH for hydrolytic reactions and protects cellular contents from hydrolysis.

Its three major functions are:
Digestion of materials taken into cells - Food particles are engulfed by endocytosis to form food vacuoles, which fuse with lysosomes to form endosomes
Autophagy of worn-out organelles - Unwanted structures within the cell are enclosed by a membrane of unknown origin, forming a vesicle. This vesicle then fuses with the lysosome to form an autophagic vacuole
Autolysis - For autolysis to occur, there must be a mass release of lysosomal contents in the whole cell.

Question 14

Outline the functions of the mitochondria

Answer 14

Mitochondria are the sites of cellular respiration, the catabolic process that generates ATP by extracting energy from sugars, fats and other metabolic fuels in the presence of oxygen.

Question 15

Describe the inner membrane of the mitochondria and explain the purpose for its adaptation

Answer 15

The outer membrane is smooth but the inner
membrane is highly convolute with infoldings known as cristae (singular: crista) to increase the surface area for the attachment of various enzyme systems and involved in cellular respiration.

The compartment enclosed by the inner
membrane is known as the mitochondrial
matrix, which houses enzymes, circular DNA,
RNA and ribosomes so the mitochondria can
synthesise its own proteins

The compartment enclosed by the inner membrane is known as the mitochondrial matrix, which houses enzymes, circular DNA, RNA and ribosomes so the mitochondria can synthesise its own proteins. This is also the site of Krebs cycle.

The compartmentalisation of the mitochondrial matrix is also necessary for a proton gradient across the inner mitochondrial membrane to be set up. The generation of the proton gradient is due to the inner mitochodrial membrane being impermeable to protons.

Question 16

Outline the functions of chloroplasts

Answer 16

Chloroplasts are the sites of photosynthesis, and are only present in photosynthetic cells. They convert solar energy to chemical energy by absorbing sunlight, and use it to drive the synthesis of organic compounds from carbon dioxide and water

Question 17

Outline the functions of ribosomes

Answer 17

The ribosome serves as the site of protein synthesis in both eukaryotes and prokaryotes.

Ribosomes attached to the rER (bound ribosomes) synthesize proteins that are destined for export, insertion into membrane, or targeted to various membrane-bound organelles (e.g. lysosomes).

Free cytosolic ribosomes on the other hand, synthesise proteins that remain in the cell and function within the cytosol.

Question 18

Name the locations where ribosomes are found

Answer 18

The ribosome can be found in four cellular locations:

• attached to the rough endoplasmic reticulum,
• free in the cytosol,
• in the mitochondrial matrix and
• in the chloroplast stroma

Question 19

Outline the function of the cytoskeleton

Answer 19

The general functions of the cytoskeleton can be summarised as follows:

• Giving mechanical support to the cell and maintaining cell shape
• Allowing for anchorage and directs the movements of organelles and molecules within the cell
• Providing cell motility (cilia & flagella)

The cytoskeleton is constructed from at least three types of protein fibres: microtubules, microfilaments and intermediate filaments

Microtubules are used in structural support. Specialised arrangements of microtubules are used for varying functions. These arrangements of microtubules arise from microtubule organizing centres (MTOCs), which give anchorage and orientation to the microtubule assembly. Microtubules are also involved in transportation of molecules from one site in the cell to another.
Microfilaments are involved in the movement of cells, e.g. amoeboid crawling, and changes in cell shape.
Intermediate filaments may stabilise organelles, like the nucleus, or they may be involved in specialised cell junctions.

Question 20

Outline the functions of the centrioles

Answer 20

Before cell division, each centriole replicates itself and move to opposite poles of the cell. The centrosome act as MTOC (microtubule organising centre) for the formation of spindle fibres that play a role in nuclear division.

**Centrioles are found in animal cells, but are absent in plant cells. They are located near the nucleus, in a region known as the centrosome that contains specialised proteins required for microtubule assembly. Centrioles are found in pairs at right angles to each other, each member of the pair consisting of nine triplets of microtubules arranged in a ring.

Question 21

Describe the features and thus outline the functions of the plant cell wall

Answer 21

The plant cell wall is a relatively rigid and inflexible structure consisting mainly of the structural polysaccharide cellulose. It is secreted by the plant cell from within, and supports and defines the shape of plant tissues.

It:

• Protects the cells from both mechanical injury and invasion
• Cellulose’s high tensile strength enable the cell to withstand the hydrostatic pressure exerted by uptake of water by the cell and enables plant cells to prevent excessive uptake of water

The plant cell wall is freely permeable to all but very large molecules. The region between the cell walls of adjacent cells is separated by the middle lamella. This region is rich in polysaccharides such as pectin that helps the cells adhere together.

Question 22

Outline the functions of the cell surface membrane

Answer 22

A partially permeable barrier controlling exchange between the cell and its environment.

Question 23

Outline the functions of the vacuole (plant) [SDPPG]

Answer 23

STORAGE of organic compounds (e.g. proteins) and inorganic ions (e.g. K+ and en
DISPOSAL site for toxic metabolic by-products
Contains PIGMENTS (e.g. red and blue pigments that colour the petals)
Plant PROTECTION by accumulating compounds that are toxic or unpalatable to consumers
Cell GROWTH and elongation as water accumulates in the vacuole; plant cells can therefore increase in size with minimal investment in cytoplasm synthesis and without sacrificing surface area to volume ratio, as cytoplasmic contents are pushed to the periphery of the cell.

Question 24

Describe the structure of a typical bacterial cell (Not found in notes?)

Answer 24

Small and unicellular
Peptidoglycan cell wall
Circular DNA
70S ribosomes
Lack of membrane-bound organelles)