Anatomy of the cell

Question 1

What is the Plaslamella/plasma membrane?

Answer 1

It is a 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 2

what does The cell membrane is selectively permeable. mean?

Answer 2

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

Question 3

What does the phospholipid bilayer contain?

Answer 3

Integral and peripheral proteins and cholesterol

Question 4

What are organelles?

Answer 4

Organelles are small, intracellular ‘organs’ with a specific function and structural organization. They are essential to life

Question 5

What are inclusions?

Answer 5

Inclusions are dispensable and may be present only as transients. They represent 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 6

What are the three classes of filaments that make up the cytoskeleton?

Answer 6

Microfilaments, intermediate filaments and microtubules.

Question 7

Desribe microfilaments.

Answer 7

Microfilaments (MF): Composed of fine strands of the protein actin.
7nm diameter (or 0.007µm)
Actin molecules can assemble into filaments and later dissociate, making them very dynamic cytoskeletal elements

Question 8

Describe intermediate filaments.

Answer 8

Bind intracellular elements together and to the plasmalemma
10-15nm in diameter (0.01-0.015µm)
More than 50 types, but divided into classes. These classes are used in pathology to identify tumour origins

Question 9

Describe microtubules.

Answer 9

Microtubules: hollow tubule composed of two types of tubulin subunits, α & β in an alternating array.
Can be assembled and disassembled.
Originate from a special organising centre called the centrosome.
Include stabilizing proteins: microtubule-associated proteins (MAPS). Microtubules polymerize in the central portion of the cell and radiate outwards. Thus they are polar.
Important in cilia, flagella and the mitotic spindle.
Microtubules serve as the ‘motorway’ network of the cell. Two proteins, dynein and kinesin attach to the microtubules and move along them. They associate with the membranes of organelles and vesicles and ‘drag’ them along the microtubule.
Microtubules radiate from a special microtubule organising centre also called the centrosome which contains at its core a pair of organelles called centrioles which themselves are composed mainly of specialized microtubule segments.

Question 10

What are dynein35 and kinesin?

Answer 10

Kinesin is an ATPase that moves toward the cell periphery.
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, for example the very long axonal process of many neurons

Question 11

Describe the nuclear membrane.

Answer 11

The nucleus is enclosed by a nuclear envelope, composed of an inner and an outer nuclear membrane with nuclear pores providing continuity with the cytoplasm.
Between these two sheets is the perinuclear cistern. The perinuclear cistern is continuous with the cistern of the endoplasmic reticulum.
The outer nuclear membrane is studded with ribosomes and is continuous with the cytoplasmic rough endoplasmic reticulum.

Question 12

What is the function of the nucleus?

Answer 12

The nucleus 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 13

What is the size of a rbc?

Answer 13

Approximately 7nm

Question 14

Describe the two types of DNA.

Answer 14

The nucleus contains:
Euchromatin (DNA that is more dispersed and is actively undergoing transcription)
Heterochromatin (DNA that is highly condensed and not undergoing transcription)

Question 15

Describe ribosomes.

Answer 15

Ribosomes are formed in the nucleolus and are instrumental in protein synthesis.
Each ribosome is made up of a small subunit - which binds RNA, and a large subunit which catalyses the formation of peptide bonds.
The export of ribosomes depends on the nuclear pore complex.

Question 16

What is the endoplasmic reticulum?

Answer 16

Reticulum means ‘net-like structure’. The endoplasmic reticulum (ER) forms a network of interconnecting membrane-bound compartments in the cell.
In micrographs, it appears as flattened membrane sheets or elongated tubular profiles.
There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Question 17

Describe the rough endoplasmic reticulum.

Answer 17

RER is so called because it is studded with ribosomes. It plays a vital role in the synthesis of proteins destined for insertion into membranes or for secretion. Not surprisingly, the amount of ER present in the cell varies with how active the cell is. Cells that are relatively metabolically inactive have relatively little ER. Rough endoplasmic reticulum is associated with protein synthesis and the initiation of glycoprotein formation.

Question 18

Describe the smooth endoplasmic reticulum.

Answer 18

The SER continues the processing of proteins produced in the RER.
The SER plays a vital role as the site of the synthesis of lipids.
Most cells contain relatively little SER, but in some (e.g. cells synthesizing steroid hormones) it is extensive. SER are associated with lipid production eg testosterone (cholesterol). Leydig cell produces testosterone.

Question 19

Describe the Golgi Apparatus.

Answer 19

The Golgi apparatus (complex) is composed of a group of flattened, membrane bound cisternae. These are arranged in sub-compartments.
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.
Adds sugars
Cleaves some proteins
Sorts macromolecules into vesicles.

Question 20

Describe the mitochondria.

Answer 20

Mitochondria are 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.
Mitochondria are the power generators of the cell. They function in the generation of ATP, via oxidative phosphorylation, and in the synthesis of certain lipids and proteins.
Contain their own DNA and system for protein production

Question 21

What are intercellular junctions?

Answer 21

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

Question 22

What are the three types of intercellular junctions?

Answer 22

There are 3 types of junctions:
Occluding junctions: link cells to form a diffusion barrier.
Anchoring junctions: provide mechanical strength.
Communicating junctions: allow movement of molecules between cells.

Question 23

What is the purpose of occluding junctions?

Answer 23

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

Question 24

Describe the different types of anchoring junctions.

Answer 24

Adherent Junctions: Link submembrane actin bundles of adjacent cells. Also termed zonula adherens. Transmembrane cadherin molecules bind to each other in the extracellular space, and, through link molecules, to actin of the cytoskeleton.

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

Question 25

Describe communicating junctions.

Answer 25

Communicating junctions:
Allow selective diffusion of molecules between adjacent cells.
Often termed gap junctions.
Each junction is a circular patch studded with several hundred pores.
Pores are produced by connexon proteins.
Found in epithelia, but also in some smooth muscle and in cardiac muscle, where it is critical for the spread of excitation.

Question 26

Describe the ways material can move in and out of the cell.

Answer 26

Diffusion
Via transport proteins (pumps or channels)
By incorporation into vesicles (vesicular transport)

Question 27

Describe endocytosis.

Answer 27

Material from the extracellular space can be incorporated into the cell by endocytosis. The cell membrane invaginates, fuses and the newly made endocytotic vesicle (endosome) buds into the cell. This process is often receptor mediated.

Question 28

Describe phagocytosis.

Answer 28

Bacteria or larger particulate material from the extracellular space can be incorporated into the cell by phagocytosis. The bacterium binds to cell surface receptors triggering extensions of the cell to engulf it forming a phagosome. The phagosome binds with a lysosome carrying digestive enzymes producing a phagolysosome.