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2.2 Prokaryotic Cells & 2.3 Eukaryotic cells 2.2.1 Draw and label a diagram of the ultrastructure of E. coli as an example of a prokaryote Include: Label and Name function of each structure. cell wall Plasma membrane Cytoplasm pili Flagella Ribosomes Nucleoid region Cell Wall: Made of a murein (not cellulose), which is a glycoprotein or peptidoglycan (i.e. a protein/carbohydrate complex). There are two kinds of bacterial cell wall, which are identified by the Gram Stain technique when observed under the microscope. Gram positive bacteria stain purple, while Gram negative bacteria stain pink. The technique is still used today to identify and classify bacteria. We now know that the different staining is due to two types of cell wall Plasma membrane: Controls the entry and exit of substances, pumping some of them in by active transport. Cytoplasm: Contains all the enzymes needed for all metabolic reactions, since there are no organelles. Ribosome: The smaller (70 S) type are all free in the cytoplasm, not attached to membranes (like RER). They are used in protein synthesis which is part of gene expression. Nucleoid: Is the region of the cytoplasm that contains DNA. It is not surrounded by a nuclear membrane. DNA is always a closed loop (i.e. a circular), and not associated with any proteins to form chromatin. Flagella: These long thread like attachments are generally considered to be for movement. They have an internal protein structure that allows the flagella to be actively moved as a form of propulsion. The presence of flagella tends to be associated with the pathogenicity of the bacterium. The flagella is about 20nm in diameter. This structure should not be confused with the eukaryotic flagella seen in protoctista. Pilli: These thread like projections are usually more numerous than the flagella. They are associated with different types of attachment. In some cases they are involved in the transfer of DNA in a process called conjugation or alternatively as a means of preventing phagocytosis. Slime Capsule: A thick polysaccharide layer outside of the cell wall, like the glycocalyx of eukaryotes. Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against phagocytosis. In some species the capsules of many cells in a colony fuse together forming a mass of sticky cells called a biofilm. Dental plaque is an example of a biofilm. Plasmids: Extra-nucleoid DNA of up to 400 kilobase pairs. Plasmids can self-replicate particularly before binary fission. They are associated with conjunction which is horizontal gene transfer. It is normal to find at least one anti-biotic resistance gene within a plasmid. This should not be confused with medical phenomena but rather is an ecological response to other antibacterial compounds produced by other microbes. Commonly fungi will produce anti-bacterial compounds which will prevent the bacteria replicating and competing with the bacteria for a resource. conjugation Direct contact between bacterial cells in which plasmid DNA is transferred between a donor cell and a recipient cell. There is no equal contribution to this process, no fertilization and no zygote formation. It cannot therefore be regarded as sexual reproduction 2.2.3 Identify structures in electron micrographs of E.coli Test Your Knowledge 2.2.4 State that prokaryotic cells divide by binary fission Prokaryotic cells divide by binary fission. This is an asexual method of reproduction in which a 'parental' cell divides into two smaller but equally sized cells. The cells are genetically identical and form the basis of a reproductive clone. a little extra information for the interested reader. The process of binary fission takes place in four stage: (a). Reproduction signal: The cell receives a signal, of internal or external origin that initiates the cell division. E.coli replicates about once every 40 minutes when incubated at 37o C. If however we increase the concentration of carbohydrate nutrients that the cell is supplied with then the division time can be reduced to 20 minutes. There is a suggestion here that an external signal (nutrient concentration) is acting as the reproductive signal. (b). Replication of DNA: bacterial cells have a single condensed loop of DNA. This is copied by a process known as semi-conservative replication to produce two copies of the DNA molecule one for each of the daughter cells The replication begins at a single point (ori)on the loop of DNA. The process proceeds around the loop until two loop have been produced, each a copy of the original. The process finishes at a single point on the loop of DNA called the ter position. (c). Segregation of DNA: One DNA loop will be provided for each of the daughter cells. As the new loops form the ori site becomes attached to some contractile proteins that pull the two ori sites, and therefore the loops, to opposite ends of the cell. This is an active process that requires the bacteria to use energy for the segregation. (d). Cytokinesis: Cell separation. This occurs once the DNA loop replication and segregation is complete. The DNA completes a process of condensing whilst the plasma membrane begins to form a 'waist' or constriction in the middle of the cell. As the plasma membrane begins to pinch and constrict the membrane fuses and seals with additional new membrane also being formed. Eukaryotic Cells 2.3.1 draw and label a diagram of ultrastructure of a liver cell as an example of an animal cell. •N:Nucleus •PM: plasma membrane •M: mitochondria •rER: Rough endoplasmic reticulum •GA: Golgi apparatus •L: Lysosome •MV: Microvilli •Bioflix: tour of the animal cell 2.3.2 Annotate the diagram from 2.3.1 with the functions of each named structure. Nucleus: This is the largest of the organelles. The nucleus contains the chromosomes which during interphase are to be found the nucleolus The nucleus has a double membrane with pores(NP). The nucleus controls the cells functions through the expression of genes. Some cells are multi nucleated such as the muscle fiber Plasma membrane: controls which substances can enter and exit a cell. It is a fluid structure that can radically The membrane is a double layer of water repellant molecules. Receptors in the outer surface detect signals to the cell and relay these to the interior. The membrane has pores that run through the water repellant layer called channel proteins. Plasma membrane: This image shows the junction between two liver cells. The image has been manipulated for clarity to see the two adjoining plasma membranes. Notice the mitochondria to the left and the rER to the right of the membranes. Mitochondria: location of aerobic respiration and a majot synthesis of ATP region.. Double membrane organelle. Inner membrane has folds called cristae. This is the site of oxidative phosphorylation. Centre of the structure is called the matrix and is the location of the Krebs cycle. Oxygen is consumed in the synthesis of ATP on the inner membrane The more active a cell the greater the number of mitochondria. Mitochondria: This micrograph of a mitochondria shows: Double outer membrane Folded inner membrane called the cristae. Matrix of the mitochondria These features are common to all mitochondria. Notice the rER above the mitochondria for scale and the dark granules of glycogen below the organelle. Rough endoplasmic reticulum (rER): protein synthesis and packaging into vesicles. rER form a network of tubules with a maze like structure. In general these run away from the nucleus The 'rough' on the reticulum is caused by the presence of ribosomes. Proteins made here are secreted out of the cell Endoplasmic reticulum (rER) The rER runs vertical in the image. Note the dark spots which are the ribosomes. A cell with a great deal of rER is producing proteins for secretion outside of the cell. The network of endoplasmic tubules allows proteins to be moved around within the cytoplasm before final packaging and secretion. Ribosomes: the free ribosome produces proteins for internal use within the cell Golgi apparatus: modification of proteins prior to secretion. proteins for secretion are modified possible addition of carbohydrate or lipid components to protein packaged into vesicles for secretion Golgi apparatus: The golgi apparatus in the diagram forms a stack of membrane envelopes on top of each other. Vesicles containing proteins fuse with the structure. The proteins are modified inside the apparatus usually with the addition of non-protein substances. Lysosomes: Digestive Compartments A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids Lysosomal enzymes work best in the acidic environment inside the lysosome © 2011 Pearson Education, Inc. Animation: ENDOMEMBRANE SYSTEM © 2011 Pearson Education, Inc. Some types of cell can engulf another cell by phagocytosis; this forms a food vacuole A lysosome fuses with the food vacuole and digests the molecules Lysosomes also use enzymes to recycle the cell’s own organelles and macromolecules, a process called autophagy © 2011 Pearson Education, Inc. Lysosomes simple membrane bound vesicle containing hydrolytic enzymes produced in the golgi apparatus. used to digest engulfed bacteria or viruses or old organelles used to digest macromolecules hydrolytic enzymes are retained within the vesicle membrane to prevent autodigestion of the cell. Figure 6.13 Nucleus Vesicle containing two damaged organelles 1 m 1 m Mitochondrion fragment Peroxisome fragment Lysosome Digestive enzymes Lysosome Lysosome Plasma membrane Peroxisome Digestion Food vacuole Vesicle (a) Phagocytosis (b) Autophagy Mitochondrion Digestion Figure 6.13a Nucleus 1 m Lysosome Digestive enzymes Lysosome Plasma membrane Digestion Food vacuole (a) Phagocytosis Figure 6.13aa Nucleus Lysosome 1 m Figure 6.13b Vesicle containing two damaged organelles 1 m Mitochondrion fragment Peroxisome fragment Lysosome Peroxisome Vesicle (b) Autophagy Mitochondrion Digestion Figure 6.13bb Vesicle containing two damaged organelles Mitochondrion fragment Peroxisome fragment 1 m 2.3.3 Identify structures from 2.3.1 in electron micrographs of liver cells In an electron micrograph the nucleus will be the largest of the organelles. In this image there is a dark stained region called the nucleolus which is the location of the DNA. The membrane has pores which allow the entry of cell signal molecules, nucleotides and the exit of mRNA. Generally the nucleus appears spherical however there are cells in which the nucleus has more unusual shape such as the multi-lobbed white blood cells. Comparing Prokaryotic and Eukaryotic Cells Basic features of all cells Plasma membrane Semifluid substance called cytosol Chromosomes Ribosomes © 2011 Pearson Education, Inc. (carry genes) (make proteins) Prokaryotic cells are characterized by having No nucleus DNA No in an unbound region called the nucleoid membrane-bound organelles Cytoplasm © 2011 Pearson Education, Inc. bound by the plasma membrane Eukaryotic cells are characterized by having DNA in a nucleus that is bounded by a membranous nuclear envelope Membrane-bound organelles Cytoplasm in the region between the plasma membrane and nucleus Eukaryotic cells are generally much larger than prokaryotic cells © 2011 Pearson Education, Inc. 2.3.4 Comparison of prokaryotic and eukaryotic cells Nuclear envelope Plant Cells NUCLEUS Nucleolus Chromatin Rough endoplasmic reticulum Smooth endoplasmic reticulum Ribosomes Central vacuole Golgi apparatus Microfilaments Intermediate filaments Microtubules Mitochondrion Peroxisome Chloroplast Plasma membrane Cell wall Wall of adjacent cell Plasmodesmata CYTOSKELETON Chloroplast Note: • double membrane • internal thylakoid membranes which contain the chlorophyll. • Stroma where the calvin cycle fixes CO2 into carbohydrates, oils or starch. Vacuole • The vacuole is a storage area for organic solute such as sugars and amino acids. • The vacuole is surrounded by a membrane called the tonoplast which has essentially the same type of structure as the plasma membrane. Cell Wall: Plant cell walls are composed of cellulose (2.3.6) In the electron micrograph we can see cytoplasmic connections through adjacent cells. These are called plasmodesmata. 2.3.5 State three differences between plant and animal cells TEST YOUR KNOWLEDGE Test Your Knowledge – comparing prokaryotic and Eukaryotic cells Build an Animal Cell and a Plant Cell Practice – Review Animal Cell Structure and Function Review Plant Cell Structure and Function 2.3.6 Outline two roles of extracellular components Plant Cell Wall: Found around all plant cells Composed of cellulose. Maintains the shape of the cell. Provides structural support against the force of gravity. prevents excessive uptake of water by the cell Animal extracellular matrix Basement membrane: a secretion formed from collagen and glycoproteins joined together by a third 'linkage' protein. Their exact composition varies form tissue to tissue. Support: the membrane surrounds the tissues of lines ducts. It provides structural support for the integrity of the tissue or organ. Usually found as the basal lamina or basement membrane of epithelial cells. Filter : The basement membrane of the kidney glomerulus provides the effective barrier for ultrafiltration Vascular niche : Interestingly cells often require a base on which to organise before they will form proper tissue. There are implications here for developmental biology, tissue repair, stem cell therapies and cancer treatment. Interstitial matrix Bone has a matrix which includes collagen with a calcium phosphate. Other tissues are surrounded by a matrix composed of a kind of gel that provides support for the tissue.
