Introduction
Functional Anatomy of Prokaryotic and Eukaryotic Cells
- All living cells can be classified into two groups: Prokaryotes and Eukaryotes.
- We have covered the complete anatomy of prokaryotes, which include Bacteria and Archaea.
- Now we will cover the anatomy of Eukaryotes, which include all complex life forms, like humans and plants.
- But eukaryotes also include microorganisms such as algae, protozoa, fungi, and helminths.
- Though many of the general features between prokaryotes and eukaryotes are similar, eukaryotic cells are larger and more complex than prokaryotic cells.
Section 3-1
Eukaryotic Cells
- There are two major types of eukaryotic cells: animal and plant.
- Though both are important for the ecosystem of the world, we will be focusing primarily on animal and animal-like cell anatomy.
Animal versus Plant Cells
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Animal cells have lysosomes, secretory vesicles, cilia, whereas plant cells do not.
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Plant cells have chloroplasts, vacuoles, the cell wall (although different in structure from bacterial cell walls) and plasmodesmata (little pores through which substances can pass from cell to cell)
Eukaryotic External Structures
Motility
- Motility = Eukaryotes (both animal and microorganism) are capable of motility, and they utilize flagella and cilia to achieve this.
- The flagella in eukaryotes is very similar to that in prokaryotes, except that instead of flagellin in the filament, eukaryotic flagella have microtubules in the filament.
- Microtubules are long hollow tubes made of the protein tubulin.
- The movement of the eukaryotic flagellum is also different from the movement of the prokaryotic flagellum; instead of moving in a rotary- whip like fashion, the flagella of eukaryotes moves back and forth in a wave like pattern.
- This feature is important for the second motility tool, cilia.
- Cilia are very similar to flagella, except that they are shorter and cover the cell membrane densely. The wave motion of cilia is what sweeps unwanted items out of the lungs.
Cell Wall and Glycocalyx
- Eukaryotic cells do not contain peptidoglycan.
- This is significant medically because antibiotics, such as penicillin and cephalosporins act against peptidoglycan and therefore do not affect human eukaryotic cells.
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Organism Outer coating Construction material Major molecule Algae cell wall polysaccharide cellulose Fungi cell wall polysaccharide chitin Yeasts (nonfilamentous) cell wall polysaccharide glucan, mannan Protozoa just an outer coating; no cell wall pellicle protein animal cells glycocalyx sticky carbohydrates glycoproteins, glycolipids
- The eukaryotic cells with cell walls are plants, algae, and fungi.
- The cell wall of algae and plants contain cellulose, and the cell wall of fungi contain chitin.
- Both of these compounds are much simpler than the prokaryotic peptidoglycan, but all of these compounds add a layer of protection stronger than the cell membrane.
- Though chitin comprises the surface of fungi, which appear to be soft, chitin is actually much stronger than cellulose. Insect and crustacean exoskeletons are made of chitin.
- The thickness of the cellulose or chitin wall is what dictates the variations in strength (ex. blueberries vs carrots).
- Cell Surface = Like prokaryotes, some eukaryotic cells are covered in glycocalyx, a sugar coat, which allows the cells to adhere to surfaces and each other better.
- For example, the “slime” of algae or fish is glycocalyx.
- For example, the “slime” of algae or fish is glycocalyx.
Plasma Membrane
- Plasma Membrane = The plasma membrane of all organisms is a lipid bilayer of similar composition.
- The major differences between the plasma membrane of eukaryotes and prokaryotes, is that eukaryotic membranes have different peripheral and integral proteins embedded and animal membranes contain sterols.
- Sterols are a subgroup of steroids that are important for stabilizing the cell membrane.
- Because animal cells lack a cell wall, they need sterols to create a level of resistance to the environment; otherwise eukaryotic membranes would break at the first sign of a non-isotonic solution. (Cholesterol is an example of an important sterol.)
- Transport Across Plasma Membrane = All of the same transport options that exist for prokaryotes are available for eukaryotic cells (passive, active, facilitated, osmosis, etc).
- However, eukaryotic cells have a few additional options.
- The most important being endocytosis.
- Endocytosis is the process by which eukaryotic cells can transport external resources into the cell by engulfing the resource with the plasma membrane.
- Eukaryotic endocytosis is capable of transporting very large objects into the cell, including prokaryotes.
- When a eukaryotic cell consumes a prokaryote, it is called phagocytosis.
- The most important being endocytosis.
- However, eukaryotic cells have a few additional options.
Section 3-2
Eukaryotic Cells - Internal structure
- Cytoplasm = Eukaryotes and prokaryotes have similar cytoplasms (80% water and 20% solutes, including proteins, carbohydrates, lipids, inorganic ions, and small compounds).
- One thing to note, is that cytoplasm is not found in the nucleus of eukaryotic cells.
- The nucleus contains nucleoplasm, which has a different composition than cytoplasm. Specifically, nucleoplasm contains nucleotides for making DNA and RNA, which the cytoplasm does not contain.
- The nucleus contains nucleoplasm, which has a different composition than cytoplasm. Specifically, nucleoplasm contains nucleotides for making DNA and RNA, which the cytoplasm does not contain.
- One thing to note, is that cytoplasm is not found in the nucleus of eukaryotic cells.
- Ribosomes = are similar in structure and function in both eukaryotes and prokaryotes.
- However, the ribosomes in eukaryotic cells are larger than those of prokaryotes.
- The exception is that ribosomes contained in the mitochondrion are identical to prokaryotic ribosomes.
- This is the basis of the theory that mitochondria are a symbiotic evolution of bacteria in eukaryotic cells.
- This is the basis of the theory that mitochondria are a symbiotic evolution of bacteria in eukaryotic cells.
- The exception is that ribosomes contained in the mitochondrion are identical to prokaryotic ribosomes.
- However, the ribosomes in eukaryotic cells are larger than those of prokaryotes.
- Organelles = membrane bound structures with specific shapes and specialized functions in the cytoplasm of eukaryotic cells.
- Prokaryotes do not contain membrane-bound organelles.
- Prokaryotes do not contain membrane-bound organelles.
- Nucleus = the largest organelle in the cell.
- It is spherical and contains a bulk of the cell’s DNA.
- The nucleus is encompassed by a double membrane called the nuclear envelope, which contains a completely different set of proteins than the plasma membrane.
- The nucleus is perforated with small holes called nuclear pores, which allow small substances to go in and out of the nucleus.
- Nuclear pores are important for coordinating cellular processes, including RNA export and protein synthesis.
- Within the nucleus is another condensed region of DNA called the nucleolus; which is responsible for creating the subunits of ribosomes (rRNA).
- Structure of DNA in the nucleus
- The DNA in the nucleus is organized into threadlike masses called chromatin, which is tightly knit DNA wrapped around (see figure below)
- NUCLEOSOME – fundamental unit of chromatin; DNA wrapped less than two turns around 8 histone proteins
- The association with histones is crucial for two reasons:
- 1) keeps the incredibly large, and unwieldy DNA organized;
- 2) helps to regulate gene expression, so that not all genes are being transcribed all the time.
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- During cell replication, the chromatin condenses together to form chromosomes, which makes sure that DNA is accurately duplicated and reliable split between the two sister cells.
- During cell replication, the chromatin condenses together to form chromosomes, which makes sure that DNA is accurately duplicated and reliable split between the two sister cells.
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- Endoplasmic Reticulum (ER) = The ER is an extensive network of membranous sacs called cisternae.
- The ER extends off of the nuclear envelope and has two parts.
- 1) The rough endoplasmic reticulum gets its name because of the many ribosomes that stud the surface (making it look bumpy or “rough”).
- These ribosomes on the rough ER specialize in producing proteins involved in the secretory pathways, or proteins that associate with membranes.
- 2) The smooth endoplasmic reticulum extends from the rough ER. It does not have ribosomes on the surface, because it does not synthesize proteins.
- The Smooth ER creates phospholipids, sterols, and fatty acids.
- The Smooth ER creates phospholipids, sterols, and fatty acids.
- Golgi Complex / Apparatus = A stack of cisternae that is responsible for modifying and transporting newly synthesized proteins to the exterior of the cell.
- Though the Golgi looks similar to the ER, the Golgi is not associated with the nucleus.
- Though the Golgi looks similar to the ER, the Golgi is not associated with the nucleus.
- Mitochondria = the “powerhouse of the cell” because it produces ATP, which is the primary source of energy for eukaryotic cells.
- (Carbohydrates are eventually converted to ATP, which is why carbohydrates are considered a primary source of energy for cells.)
- Mitochondria can occur in abundance within the cytoplasm (up to 2000 mitochondria in cell-types that demands a lot of energy, like muscles).
- They are tubular in shape and contain a dense network of internal membrane folds called cristae, which is where all of the chemical reactions occur.
- ATP Synthesis in Mitochondria
- The mitochondrion contains its own circular DNA, which encodes proteins involved in ATP synthesis.
- Mitochondria also contain their own ribosomes to help create the proteins involved in ATP synthesis.
- This allows for mitochondria to produce energy faster than if the resources were coming all the way from the nucleus.
- This allows for mitochondria to produce energy faster than if the resources were coming all the way from the nucleus.
- Chloroplast = Unique to algae and green plants.
- It is an organelle that contains chlorophyll and the enzymes required for photosynthesis.
- The role of chloroplasts is very similar to that of the mitochondria.
- Endosymbiotic Theory
- An evolutionary theory that eukaryotic cells obtained their organelles from bacteria.
- Though the theory applies to all organelles, the mitochondria and chloroplasts are the organelles with the most supportive evidence.
- 1) The mitochondria/chloroplast have their own ribosomes that are more similar to bacteria ribosomes than eukaryotic ribosomes.
- 2) The DNA contained within the mitochondria/chloroplast are genetically similar to Rickettsial bacteria or cyanobacteria, respectively.
- 3) Mitochondria/chloroplasts do not replicate by mitosis; they replicate exclusively by binary fission.
- Vesicles = Both the ER and the Golgi are capable of generating membrane enclosed spheres, vesicles.
- The ER and Golgi use these spheres to transport proteins around the cell.
- However, the Golgi is also able to create entire organelles using these membrane-enclosed spheres as a foundation. For example:
- Lysosomes - Contain digestive enzymes that break down molecules and kill bacteria.
- Peroxisomes – Membrane-enclosed spheres that are much smaller than lysosomes.
- Peroxisomes are important in metabolism, and contain enzymes that oxidize various organic substances.
- Vacuoles – Membrane-enclosed spheres, formed by the Golgi, that serve a very similar function in eukaryotes as inclusions serve for prokaryotes. Thus, various materials are stored in vacuoles which can be used in the future.
Section 3-3
Comparing Prokaryotic and Eukaryotic Cells
- Both prokaryotes and eukaryotes contain: nucleic acids, proteins, lipids, carbohydrates, and perform similar chemical reactions to metabolize food, build proteins, and store energy.
- However, there are chief distinguishing characteristics:
Prokaryotes
Eukaryotes
DNA is circularly arranged and not enclosed within a membrane DNA is arranged into multiple chromosomes, enclosed within a nucleus DNA is not associated with histones DNA is almost completely bound by histone
Do not contain organelles = specialized cellular structures that have specific functions Contain multiple organelles, including mitochondria, nucleus, chloroplasts, endoplasmic reticulum, golgi complex, and lysosomes Have cell walls with peptidoglycan Most cell walls contain cellulose or chitin, and do not contain peptidoglycan Replicate by binary fission Usually replicate by mitosis
Flagella contain flagellin and move in a whip- like motion. Flagella contain microtubules and move in a wave-like motion.