B2.2 Organelles and compartmentalisation Flashcards
B2.2.1—Organelles as discrete subunits of cells that are adapted to perform specific functions
Students should understand that the cell wall, cytoskeleton and cytoplasm are not considered organelles,
and that nuclei, vesicles, ribosomes and the plasma membrane are.
NOS: Students should recognize that progress in science often follows development of new techniques.
For example, study of the function of individual organelles became possible when ultracentrifuges had
been invented and methods of using them for cell fractionation had been developed
Eukaryote cells contain a variety of organelles (described in Section A2.2.6). Each type of organelle is adapted, by its structure, to perform one or more specific functions.
The plasma membrane, nucleus, vesicles and ribosomes are all organelles because they have specific functions and their structure is discrete (they are individually distinct).
Some cell structures are not organelles:
* cell wall-outside the plasma membrane so outside the boundary of the cell (extracellular)
* cytoplasm-has diverse rather than specific functions
* cytoskeleton-very extensive structure that extends through the cytoplasm and is not discrete.
B2.2.2—Advantage of the separation of the nucleus and cytoplasm into separate compartments
Limit to separation of the activities of gene transcription and translation—post-transcriptional
modification of mRNA can happen before the mRNA meets ribosomes in the cytoplasm. In prokaryotes
this is not possible—mRNA may immediately meet ribosomes.
Eukaryotes modify mRNA before translating it into polypeptides. This happens inside the nucleus, where RNA is produced by transcription. The nuclear membrane ensures that this post-transcriptional modification is completed before mRNA meets the ribosomes that will translate it in the cytoplasm.
In prokaryotes there is no nuclear membrane, so ribosomes can translate mRNA as soon as it is produced and post-transcriptional modification is not possible.
B2.2.3—Advantages of compartmentalization in the cytoplasm of cells
Include concentration of metabolites and enzymes and the separation of incompatible biochemical
processes. Include lysosomes and phagocytic vacuoles as examples.
The outer membrane of organelles such as lysosomes encloses their contents and creates a compartment that is separated from surrounding cytoplasm.
Advantages:
* The small volume allows enzymes and their substrates to be concentrated, speeding up enzyme activity.
* pH can be kept at the ideal level for the organelle’s function.
* Incompatible biochemical processes can be kept separate. For example, lysosomes contain many hydrolytic enzymes that digest proteins and other macromolecules. If not confined within a membrane, they would digest much of the cell.
Phagocytic white blood cells digest pathogens and some unicellular organisms digest prey, but not themselves, inside vacuoles, as described in Section B2.7.73.
B2.2.4—Adaptations of the mitochondrion for production of ATP by aerobic cell respiration
Include these adaptations: a double membrane with a small volume of intermembrane space, large
surface area of cristae and compartmentalization of enzymes and substrates of the Krebs cycle in the
matrix.
Structure and function are closely related in mitochondria. This is an example of adaptation and is due to evolution by natural selection.
outer mitochondrial membrane separates the contents of the
mitochondrion from the rest of the cell, creating a compartment with ideal conditions for aerobic respiration
inner mitochondrial membrane— contains electron transport chains and
ATP synthase, which work together to produce ATP by chemiosmosis
1-2 micrometres
cristae
tubular or shelf-like projections of the inner membrane which increase the surface area available for ATP production
naked loop of DNA and 70S
ribosomes allow the mitochondrion to synthesize some of its own proteins
matrix—
fluid inside the mitochondrion containing high concentrations of enzymes and substrates for the Krebs cycle and link reaction
intermembrane space the space between the outer and inner membrane is very small, so when protons are pumped into it by electron transport chains, a high proton concentration rapidly develops
B2.2.5—Adaptations of the chloroplast for photosynthesis
Include these adaptations: the large surface area of thylakoid membranes with photosystems, small
volumes of fluid inside thylakoids, and compartmentalization of enzymes and substrates of the Calvin
cycle in the stroma.
thylakoid membranes a system of membranes inside the chloroplast containing the photosystems, electron carriers and ATP synthase needed for the light-dependent reactions of photosynthesis
grana-stacks of thylakoids that give a large total surface area of membrane for the light-dependent reactions
thylakoid spaces-with a very small volume, so a high proton concentration builds up after relatively few photons of light have been absorbed
1-2 micrometres
naked loop of DNA and
7OS ribosomes allow the chloroplast to synthesize some its own proteins
stroma
-with a high concentration of Rubisco and other enzymes and substrates for the light-independent reactions (Calvin cycle)
store of starch
inner membrane
outer membrane
chloroplast envelope creates a compartment with optimal conditions for photosynthesis
B2.2.6—Functional benefits of the double membrane of the nucleus
Include the need for pores in the nuclear membrane and for the nucleus membrane to break into vesicles
during mitosis and meiosis.
A double nuclear membrane has these advantages:
1. Pores can be formed by joining the outer membrane to the inner membrane. Nuclear pores are needed for ribosomes
and mRNA to move from the nucleus where they are produced to the cytoplasm where they are used.
2. The nuclear membrane can easily break up into vesicles during mitosis and meiosis, releasing the chromosomes so
they can move within the cell. The vesicles move to the poles of the cell where they are later used to construct nuclear
membranes around the new daughter nuclei.
B2.2.7—Structure and function of free ribosomes and of the rough endoplasmic reticulum
Contrast the synthesis by free ribosomes of proteins for retention in the cell with synthesis by membranebound ribosomes on the rough endoplasmic reticulum of proteins for transport within the cell and
secretion.
F r e e r i b o s o m e s
synthesize proteins
that are released into
free
r i b o s o m e s
the cytoplasm, where
they perform their
functions. Ribosomes
bound to the rough
ER make proteins for
transport to other
membrane-bound Golgi
apparatus
organelles in the
cell. The proteins
enter the lumen of
the rough ER and are plasma membrane
then transported in
vesicles that bud off. Many of the proteins pass to the
Golgi apparatus and then on to the plasma membrane
for secretion by exocytosis.
B2.2.8—Structure and function of the Golgi apparatus
Limit to the roles of the Golgi apparatus in processing and secretion of protein
The Golgi apparatus is a stack of cisternae (flattened membrane-bound sacs), which is described in Section A2.2.17.
Polypeptides from the rough ER arrive at the Golgi apparatus in vesicles which fuse with the cisterna on the “cis” side. The
polypeptides are trafficked from cisterna to cisterna until they reach the “trans” side. There are enzymes in each cisterna
which modify polypeptides by adding non-amino acid structures or cutting and crosslinking. Gradually the final form of a
protein is developed. For example, proinsulin is converted to insulin. Vesicles bud off from the cisterna on the trans side and
carry the mature proteins either to the plasma membrane for secretion by exocytosis, or to another organelle in the cell.
B2.2.9—Structure and function of vesicles in cells
Include the role of clathrin in the formation of vesicles.
Clathin is a three-legged protein that can be assembled to form a spherical cage.
Clathrin cages are used to pull a patch of membrane inwards from the plasma membrane until a vesicle is formed. Many vesicles
remain coated in a framework of clathrin molecules.
clathrin-coated vesicle ready to bud off from the plasma membrane
extracellular space
The functions of vesicles are described in Section B2.1.13.
