CELL ORGANELLES
Major Types of
Organelles
Organelles are small, specialized structures within
eukaryotic cells. Each has a particular
function or set of functions that it carries out within the cell. The name literally means “little organ” to
indicate that organelles are to cells what organs are to whole organisms. Different cell types can differ in their
proportions of certain types of organelles depending on what the function of
that cell type is. The size, shape, and
internal structure of organelles are strongly related to what they do within
the cell. Organelles demonstrate the
unifying theme that states, “the
relationship between structure an function underlies living systems.”
1. Plasma membrane: All cells, even those of
prokaryotes, are surrounded by a membrane. This structure serves the
“gatekeeper” function of the cell. Its
chemical composition is that of the lipid bilayer, which is two molecules
thick. Each phospholipid molecule has an
inward pointing hydrophobic (water fleeing) tail and an outward pointing
hydrophilic (water attracting) head. One
head is oriented toward that outside of the cell and the other toward the cell
interior. The two tails are sandwiched
between the two heads. The text describes this in the context of the fluid mosaic model of cell membranes.
The chemistry of life occurs in water and the structure of
the plasma membrane reflects the interaction of these phospholipids molecules
with water. A good demonstration of this
is the behavior of oil when placed in water. The oil molecules are hydrophobic and
individual molecules are organized into droplets surrounded by the water
medium.
Other structural features of the membrane include their many
transmembrane proteins, interior proteins, and cell surface markers that are
related to the many specialized processes that happen on or across the
membrane. Animal cells also incorporate cholesterol to provide more structure.
Plant cells, for example, do not contain cholesterol, which
is the reason that food labels for peanut butter, plant oils, crackers, bread,
etc. do not show cholesterol as an ingredient.
The comparable molecules in plant membranes are phytosterols, which we metabolized differently than cholesterol and
which do not add to cholesterol levels in our diet.
Membrane transport
processes:
- Passive
transport processes: These do not require energy expenditure on the
part of the cell.
- Diffusion: The movement of nonpolar (uncharged) organic molecules (such as steroid hormones) and of some common, small molecules like CO2, O2, and CO occurs directly by diffusion. The direction of diffusion is determined by the concentration gradient across the membrane. The net movement of diffusing molecules is from the higher to the lower concentration
- Ion diffusion: Cation (+) and anion (-) diffuse through specific ion channels that are made across the membrane by transport processes. As with 1. above, the net movement is determined by differences in concentration.
- Osmosis: This process is the diffusion of free water across the selectively permeable plasma membrane. Because water moves more freely than ions, its movement is the primary passive process equalizing the concentration of ions on either side of the membrane. The net movement of water is from where ions are least concentrated to where ions are most concentrated. The terms hypotonic, hypertonic, and isotonic solution are relevant here. We will consider these terms in the first lab exercise. It has recently become known that the movement of water during osmosis occurs through specialized protein channels called aquaporins
- Active Transport Processes: These are transport processes that do require the cell to expend energy in the form of ATP. The energy in ATP is stored in the covalent bonds of this molecule and is “spent” when the energy is needed to power a cell processes. ATP is produced by the oxidative respiration process associated with the mitochondria, where the chemical bond energy from food in the form of glucose is transferred to ATP molecules.
- Protein carriers: These molecules allow for the transmembrane movement of materials against the concentration gradient. The sodium (Na+)-potassium (K+) pump is such a carrier. It is estimated that 1/3 of the energy in an animal cell that is not currently dividing is used by this process.
- Coupled transport systems: This form of active transport involves moving a molecule against its concentration gradient by using the energy stored in the accumulated gradient of another molecule.
Processes 3. – 5. all involve the
bulk transport of molecules across the plasma membrane.
- Endocytosis: In this process, particles are engulfed by the membrane and transported into the cell. An example of where this type of process would occur is in immune system cells that engulf foreign particles like bacterial cells.
- Pinocytosis: This is like 3. above except that liquid
is engulfed instead of a solid particles.
Egg cells that are “nursed” by other cells take in needed liquid
nutrients by this process.
- Exocytosis:
This is the reverse of endocytosis/pinocytosis. Hormones, neurotransmitters, and
digestive enzymes that are produced by specialized cells are released into
the blood stream or other location by this process.
The details above have mostly to
do with the structure of plasma membranes with respect to the function of
moving materials across the membrane.
Other key features of membranes include cell surface markers that may be glycoproteins and glycolipids that
are involved in the body’s “self-recognition” and in the recognition of
particular tissue types, e.g., the surface antigens that determine the A, B, O
blood types. Also, the interactions
between cells, such as those that occur in the same tissue, are determined by
membrane proteins that facilitate cell-to-cell
adhesion.
2. Cytoplasm: This is not an organelle per se but a
region of the cell that is commonly referred to. The cytoplasm is simply the cell material
inside of the plasma membrane, but outside of the nucleus.
3. Nucleus:
This is generally the largest organelle in the cell and holds an
organism’s genetic instructions in the form of DNA. The DNA is contained in structures called
chromosomes, which we’ll look at in detail during the genetics section of the
course. Mitosis and meiosis are
each nuclear (of the nucleus) division processes that we’ll cover later in this
section of the course. The nucleus is
surrounded by a double membrane and has small opening, nuclear pores, through
which certain materials may pass. The
process of transcription that makes
a copy of gene that is then transported to the cytoplasm.
4. Ribosomes:
Made of an RNA-protein complex these organelles are the site of protein
synthesis. In eukaryotes, they are
associated with the rough endoplasmic reticulum (see description below). Ribosomes have been called “universal
machines” because they also occur in the prokaryotic cells, where they are also
involved in protein synthesis.
5. Endoplasmic reticulum (ER): this is a sheet-like
organelle composed of a phospholipids bilayer (like the plasma membrane) and
studded with proteins.
- Rough
ER: This type of ER is
associated with the presence of ribosomes and is the site of protein
synthesis, a topic covered in the second part of the course.
- Smooth
ER: This type of ER is the
site of carbohydrate and lipid synthesis.
The ratio of smooth ER/rough ER in a cell depends on the function
of that cell. In the liver, smooth
ER is involved in the detoxification processes.
6. Golgi apparatus: The is the collective name of the
individual units that comprise it, the Golgi bodies. It is named for the Italian physiologist who
first described the organelle. Within
the cell, the Golgi apparatus is the site of collection, packaging, and
distribution of the molecules that are synthesized in the ER and used elsewhere
in the cell or outside of the cell.
The nucleus, ER, and Golgi bodies interact closely in the
production of biomolecules: Genetic instructions in the nucleus code directly
for the production of a protein, or indirectly for the production of a
carbohydrate or lipid. The molecules are
produced on the smooth or rough ER and then moved to the Golgi apparatus. After being packaged into small vesicles, these molecules are
transported to other cell locations or moved across the plasma membrane and
into the bloodstream by exocytosis.
Vesicles themselves are small, membrane-bound sacs.
7.Lysosomes:
These are membrane-bound organelles that have a digestive function
within the cell; lysosomes arise from the Golgi apparatus. Their primary role is the enzymatic
degradation of biomolecules—carbohydrates, lipids, proteins, and nucleic
acids—and phagocytized cells. The
degradation of phagocytized cells would take place, for example, in the white
blood cells of the immune system.
8. Mitochondria:
Mitochondria are the site of oxidative respiration in cells. During this process, which requires O2
and produces CO2 as a by-product, the energy in the chemical bonds
of glucose molecules are (partially) converted into the chemical bond energy of
ATP (adenosine triphosphate), which is the universal currency of energy that
cells use to power cellular processes, such as active transport and biomolecule
synthesis. This process is only about
30% efficient with the remainder of the energy contained in the glucose molecule
bonds lost as heat, as dictated by the second law of thermodynamics. Cellular
respiration is closely tied to what we normally refer to as “respiration”
in that the need for acquiring O2 and getting rid of CO2
when breathing stems from this process associated with the mitochondrion.
Metabolically active cells, like those in the liver or
muscle, can have hundreds of individual mitochondria. On average, individual mitochondria are
replaced once every 10 days or so. There
are a number of well-understood medical conditions that result from faulty
mitochondrial metablolism. For example,
a lethal condition found in infants, cardioencephalomyopathy, results from a
genetic change that disrupts normal mitochondrial function. A free-living bacterium (Paracoccus denitrificans) that resembles mitochondria now serves as
a research model for understanding the underlying cause of this disease.
Mitochondrial structure provides important insights into the
evolution of eukaryotic cells from prokaryotic (bacterial) cells. They bear a strong resemblance to bacterial
cells in that they have their own circular DNA, are capable of division, and
are of comparable size. We will continue
with this idea in considering the endosymbiont
theory as part of the next cell topic.
9.Cytoskeleton: This network of protein
microfilaments and microtubules serves as a type of cell
“scaffolding” within the cytoplasm. It contributes to cell shape, the anchoring
of organelles within the cytoplasm, and the movement of cells (cell motility). Microtubules called spindle fibers are also
critical in the process of cell division, the final topic in this section on
cell biology.
10. Centrioles:
These are found in animal (but not plant) and in certain other
cells. Located in the cytoplasm, this
organelle divides and organizes spindle fibers during mitosis and meiosis.
11.Chloroplasts: Though not present in animal cells,
these organelles of plants and algae are critical to animals because of their
role in fixing carbon during the process of photosynthesis.
All of the world’s major food chains are dependent upon the
transformation of solar energy into chemical bond energy. Chloroplasts are responsible for producing
this chemical bond energy, in the form of carbohydrates, from water, CO2,
and sunlight.
Like mitochondria, chloroplasts are the size of bacteria,
surrounded by a double membrane, capable of reproducing by fission (splitting)
and have their own circular DNA.
12. Cell wall:
Like chloroplasts, this cellular structure is not present in animal
cells but has great importance to animals.
The chemical composition of cell walls includes cellulose, a structural
carbohydrate. Plant material containing
cellulose is generally not digestible to animals because they lack the ability
to produce cellulase, the digestive enzyme that degrades cellulose. Animals that eat a diet high in cellulose
must rely on bacterial or protist symbionts to aid in the digestion of
cellulose. The “roughage” present in
human diets is partly cellulose.
13. Cilia and flagella: These are extracellular (outside the cell)
structures attached to the cell. Their
fine structure indicates that they contain cytoplasm and are surrounded by
plasma membrane, despite extending beyond the general boundary of the plasma
membrane. In vertebrates, ciliated cells
(e.g., in the respiratory tract or the inner ear) are involved in many
important physiological and sensory processes.
Animal sperm cells, once ejaculated, are propelled by a flagellum.
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