Anatomy of Cell

Question 1

Describe the composition of each of the following components based on average, %range, and comment: water, protein, lipid, carbohydrate, and inorganic.

Answer 1

Water: 80%, 75-85%, 90% free/ 5% bound.
Protein: 15%, 10-20%
Lipid: 2.5%, 2-3%
Carbohydrate: 1.5%, ~1%
Inorganic: 1.0%, 1%

Question 2

Describe the features common to all eukaryotic cells.

Answer 2

Outer membrane, inner cytosol (solution of proteins, carbohydrates, and electrolytes. It has both fluid and gel-like properties), cytoskeleton - determines the shape and fluidity of cell (made from thin and intermediate filaments and microtubules), membrane bound organelles within cytosol, other structures which may or may not be bound by a membrane - inclusions.

Question 3

What is a plasma membrane?

Answer 3

Bimolecular layer of amphipathic phospholipid molecules with their hydrophilic heads at the outer and inner surfaces and their hydrophobic fatty acid chains facing towards the middle of the 2 layers.

Question 4

What does the plasma membrane contain?

Answer 4

Integral proteins which the cell inserts into the membrane. These include receptors, channels, transporters, enzymes, and cell attachment proteins.

Question 5

What ability does the cell have?

Answer 5

Exocytose and endocytose material through the cell membrane.

Question 6

In what way do membrane proteins diffuse?

Answer 6

Laterally, but many are anchored, so it’s important to realise that many proteins are not distributed equally within the cell membrane.

Question 7

What is the cell membrane permeable and impermeable to?

Answer 7

Highly permeable to: water, oxygen, and small hydrophobic molecules.
Virtually impermeable to charged ions (eg Na+).

Question 8

On what basis can structures in the cytoplasm be identified under the microscope?

Answer 8

Size, shape, and staining reactions.

Question 9

What are organelles?

Answer 9

Small, intracellular ‘organs’ with a specific function and structural organisation.

Question 10

List examples of organelles.

Answer 10

Mitochondria, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, lysosomes, and the nucleus.

Question 11

What are inclusions?

Answer 11

They are dispensable and may be present only as transients.

Question 12

What do inclusions represent?

Answer 12

Components that have been synthesised by the cell itself (pigment, glycogen stores, lipid droplets, presecretion product) or taken up from the extracellular environment (endocytotic vesicle).

Question 13

What are the 3 main classes of filaments?

Answer 13

Microfilaments (7nm in diameter) - composed of the protein actin, intermediate filaments (>10nm in diameter) - composed of 6 main proteins, which vary in different cell types, microtubules (25nm in diameter) - composed of 2 tubulin proteins.

Question 14

What can actin molecules do?

Answer 14

Assemble into filaments and later dissociate, making them very dynamic cytoskeletal elements.

Question 15

What is the function of intermediate filaments?

Answer 15

Bind intracellular elements together and to plasma membrane.

Question 16

List the type of intermediate filament and its location/cell type.

Answer 16

Neurofilaments - nerve cells, glial fibrillary acidic protein - glial cells of nervous system, desmin, muscle cells, cytokeratins - epithelial cells, vimentin - mesenchymal cells, filesin - lens of the eye, lamin - nuclei of all cells.

Question 17

What do intermediate filaments form?

Answer 17

A network throughout the cytoplasm.

Question 18

What are microtubules?

Answer 18

Hollow tubule composed of 2 types of tubulin subunits, α & β in an alternating array. It can be assembled and disassembled.

Question 19

Where do microtubules originate?

Answer 19

From a special organising centre called the centrosome. Include stabilizing proteins: microtubule-associated proteins (MAPS).

Question 20

What is the microtubules important in?

Answer 20

Cilia, flagella, and the mitotic spindle.

Question 21

What do microtubules serve as?

Answer 21

The ‘motorway’ network of the cell.

Question 22

What are the two proteins that attach to the microtubules and move along them?

Answer 22

Dynein and kinesin.

Question 23

What is the function of dynein and kinesin?

Answer 23

Kinesin is an ATPase that moves toward the cell periphery and dynein is an ATPase that moves toward the cell centre. These proteins are very important in the movement of components in cells with long processes, eg the very long axonal process of many neurons.

Question 24

Describe the structure of the nucleus.

Answer 24

It is enclosed by a nuclear envelope, composed of an inner and an outer nuclear membrane with nuclear pores providing continuity with the cytoplasm. The outer nuclear membrane is studded with ribosomes and is continuous with the cytoplasmic rough endoplasmic reticulum.

Question 25

What is the function of the nucleus?

Answer 25

Contains chromosomes and is the location of RNA synthesis. Both mRNA and tRNA are transcribed in the nucleus, and rRNA is transcribed in the nucleolus, a 1-3µm diameter dense area within the nucleus.

Question 26

What does the nucleus contain?

Answer 26

Euchromatin (DNA that is more dispersed and is actively undergoing transcription), and heterochromatin (DNA that is highly condensed and not undergoing transcription).

Question 27

Where are the ribosomes formed and what are they instrumental in?

Answer 27

The nucleolus and are instrumental in protein synthesis.

Question 28

What are the ribosomes made from?

Answer 28

Each ribosome is made up of a small subunit - which binds RNA, and a large subunit which catalyses the formation of peptide bonds.

Question 29

What does the export of ribosomes depend on?

Answer 29

The nuclear pore complex.

Question 30

What is the endoplasmic reticulum?

Answer 30

The endoplasmic reticulum forms a network of interconnecting membrane-bound compartments in the cell. In micrographs, it appears as flattened membrane sheets or elongated tubular profiles.

Question 31

What is the rough endoplasmic reticulum?

Answer 31

It is studded with ribosomes and plays a vital role in the synthesis of proteins destined for insertion into membranes or for secretion. It is also associated with glycoprotein formation.

Question 32

What is the amount of ER present in the cell?

Answer 32

It varies with how active the cell is. Cells that are relatively metabolically inactive have relatively little ER.

Question 33

What is the function of the smooth endoplasmic reticulum?

Answer 33

Continues the processing of proteins produced in the RER. The SER plays a vital role in the site of synthesis of lipids.

Question 34

What is the amount of SER present in the cell?

Answer 34

Most cells contain relatively little SER, but in some (eg cells synthesizing steroid hormones) it is extensive.

Question 35

What is the golgi apparatus?

Answer 35

Composed of a group of flattened, membrane bound cisternae. These are arranged in sub-compartments.

Question 36

Describe the function of the golgi apparatus.

Answer 36

Transport vesicles arrive at the golgi from the SER/RER. Golgi cisterns function in the modification and packaging of macromolecules that were synthesised in the ER. It adds sugars, cleaves some proteins, and sorts macromolecules into vesicles.

Question 37

What is the mitochondria?

Answer 37

Oblong, cylindrical organelles, typically 0.5-2µm in length. They are composed of an outer and an inner membrane. The inner membrane is extensively folded to form cristae, which act to increase the available surface area. They contain their own DNA and system for protein production.

Question 38

What is the function of the mitochondria?

Answer 38

Power generators of the cell. They function in the generation of ATP, via oxidative phosphorylation, and in the synthesis of certain lipids and proteins.

Question 39

What are intercellular junctions?

Answer 39

Specialized membrane structures which link individual cells together into a functional unit. They are particularly prominent in epithelia.

Question 40

List the 3 types of junctions.

Answer 40

Occluding junctions - link cells to form a diffusion barrier, anchoring junctions - provide mechanical strength, communication junctions - allow movement of molecules between cells.

Question 41

What are occluding junctions?

Answer 41

Appear as a focal region of close apposition between adjacent cell membranes. Are also known as tight junctions or zonula occludens. They prevent diffusion.

Question 42

What are anchoring junctions (zona/zonula adherens)?

Answer 42

Adherent junctions: link submembrane actin bundles of adjacent cells.

Question 43

What are transmembrane cadherin molecules?

Answer 43

They bind to each other in the extracellular space, and, through link molecules, to actin of the cytoskeleton.

Question 44

What is the function of desmosomes?

Answer 44

Link submembrane intermediate filaments of adjacent cells. Also called macula adherens. Desmosomes are very common in the skin where they provide mechanical stability.

Question 45

What is the function of communicating junctions?

Answer 45

Allow selective diffusion of molecules between adjacent cells. Often termed gap junctions.

Question 46

What is each circular patch of communicating junctions patched with?

Answer 46

Several hundred pores.

Question 47

What are pores of gap junctions produced by?

Answer 47

Connexon proteins.

Question 48

Where are gap junctions found?

Answer 48

In epithelia, but also in some smooth muscle and in cardiac muscle, where it is critical for the spread of excitation.

Question 49

What is a junctional complex?

Answer 49

Close association of several types of junctions found in certain epithelial tissues.