Oganelles and Membranes Handout

Below is a copy of a handout that may help in studying for Exam 1

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:

1. 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

2. 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.

 

3. 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.

4. 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.

5. 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.

 

1. 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.

 

2. 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.