Topic 1: Cell Biology → Big Idea: The evolution of multicellular organisms allowed cell specialization and cell replacement. 1.1 Introduction To Cells ● According to the cell theory, all living organisms are composed of cells. The c ell theory is composed of 3 principles: ■ All organisms are composed of one or more cells ■ Cells are the smallest unit of life ■ All cells come from preexisting cells // Cells do not generate spontaneously. (prokaryotes divide by binary fission, while eukaryotes divide by fission, mitosis or meiosis. ● Evidence for the c ell theory: ○ Subcellular components have not been observed to maintain all functions of life on their own, unlike cells. All organisms are either unicellular or multicellular, e.g: humans are multicellular and bacteria are unicellular. (Robert Hooke, Antonie van Leeuwenhoek, Matthias Schleiden) ○ So far, scientists have not found any living entity that is not composed of at least one cell. Louis Pasteur’s famous experiment proved that cells do not generate spontaneously. After sterilizing chicken broth by boiling it, only after exposure to preexisting cells was life able to establish itself in the sterilized chicken broth, proving that cells would not spontaneously reappear. ● A cell in itself is alive, but its subcellular components are not. Life is an emergent property that arises at the level of the cell Subcellular components have not been observed to maintain all functions of life on their own ● Organisms composed of only one cell (unicellular) carry out all functions of life in that one cell ○ The functions of life are: ■ Metabolism → The sum of all chemical reactions in a living organism (e.g: cell respiration, digestion, photosynthesis) ■ Reproduction→ To produce offspring through either sexual (2 parents) or asexual (1 parent) reproduction ■ Homeostasis → The maintenance of a stable internal environment (e.g: temperature, water, glucose concentration, acid-base balance) ■ Excretion → The removal of metabolic waste (e.g: urine in animals, CO2 in plants) ■ Growth→ Living things can grow or change in size/shape ■ Response → Living things can recognize and respond to changes in the external environment ■ Nutrition → The ability to feed either by synthesizing organic molecules (photosynthesis) or by absorbing them (eating) to provide energy for metabolism. ● Two organisms can be used to demonstrate the functions of life: Paramecium a nd C hlorella. Figure 1: P aramecium Figure 2: Chlorella ● A cell’s surface area to volume ratio (SA:V) is an important limitation for cell size ○ A cell’s surface area is determined by its cell membrane, which regulates transport of molecules in and out of the cell. Oxygen (and other small molecules) can be absorbed and waste can also be excreted through the cell membrane. ■ A cell’s internal region constitutes its volume. Many metabolic processes occur within the cell’s internal region, which require gases and chemical nutrients to produce waste. ■ As a cell grows bigger, its internal volume increases and the surface area expands. However, the volume increases at a much faster rate than the surface area, and so the SA:V ratio decreases. ■ A smaller cell requires less nutrients and waste to be transported, and so the cell would have more surface area to transport the nutrients and waste products. ■ A larger cell requires more nutrients and waste products to be transported, and would have less cell membrane surface area to transport them. ■ As a cell grows, eventually the surface area can no longer serve the requirements of the cell (nutrients in and waste out through the cell membrane). Decreasing surface area to volume ratio will stimulate cell division through mitosis or binary fission. By dividing, the size of the cell is reduced and kept within SA:V limits ● Some cells have certain adaptations to maximize SA:V ratio, which include: ○ Long extensions of the cell membrane ○ Thin, flattened shape ○ Bristle-like extensions (microvilli) ● Multicellular organisms have properties that emerge due to the interactions of their cellular components ○ Emergence occurs when an entity is observed to have properties its parts do not have on their own. These properties or behaviors emerge only when the parts interact in a wider whole. ● At each level of biological organization, new properties emerge ○ Multicellular organisms are able to complete functions that individual cells could not undertake, this is due to the interactions between cells producing new functions. ● Specialized tissues can develop by cell differentiation in multicellular organisms ○ Every cell in a multicellular organism contains all genes of that organism. However, not all of those genes are activated in every cell or at the same time. When the gene is activated, the gene will encode for specific proteins. These proteins will affect the structure and function of cells. ○ By activating certain genes and not others, the cells are able to differentiate and form specialized tissues. Differentiation depends on gene expression which is regulated mostly during transcription. It is an advantage for multicellular organisms as cells can differentiate to be more efficient unlike unicellular organisms who have to carry out all of the functions within one cell. ○ In development after the zygote divides to form the blastocyst ( around 120-130 cells), and then the gastrula, which is differentiated into several dermal layers of cells (mesoderm, endoderm, ectoderm, and germ cells) that form into specific specialized cells. ○ ● Differentiation involves the expression of some genes and not others in a cell’s genome ○ Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature. All cells of an organism share an identical genome – each cell contains the entire set of genetic instructions for that organism. The activation of genes within a given cell by chemical signals will cause it to differentiate. ○ Within the nucleus of a eukaryotic cell, DNA is packaged with proteins to form chromatin. Active genes are usually packaged in an expanded form called euchromatin that is accessible to transcriptional machinery. Inactive genes are typically packaged in a more condensed form called heterochromatin (saves space, not transcribed). Differentiated cells will have different regions of DNA packaged as euchromatin and heterochromatin according to their specific function ○ ● The capacity of stem cells to divide and differentiate along different pathways is necessary in embryonic development and also makes stem cells suitable for therapeutic uses ○ Stem cells are cells that are not yet fully differentiated but have the ability to divide and differentiate into different types of cells (e.g: one stem cell can differentiate into a blood cell, a liver cell or a kidney cell). Stem cells are necessary in embryonic development as all the cells in adult organisms stem from embryonic stem cells. ○ When a cell differentiates and becomes specialized, it loses its capacity to form alternative cell types. ● There are four main types of stem cells present at various stages of human development: ○ Totipotent→ Can form any cell type, including placental tissue (e.g: zygote) ○ Pluripotent→ Can form any cell type (e.g: embryonic stem cells) ○ Multipotent → Can form a number of closely related cell types (e.g: haematopoeitic adult stem cells) ○ Unipotent→ Cannot differentiate, but are capable of self renewal ● Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae ○ Striated Muscle Fibres ■ Muscles cells fuse to form fibres that may be very long (>300mm). Consequently, they may have multiple nuclei despite being surrounded by a single, continuous plasma membrane. This challenges that idea that cells always function as autonomous units. ○ Aseptate Fungal Hyphae ■ Fungi may have filamentous structures called “hyphae” which are separated into cells by internal walls called “septa”. Some fungi are not partitioned by septa and hence have a continuous cytoplasm along the length of the hyphae. This challenges the idea that living structures are composed of discrete cells. ○ Giant Algae ■ Certain species of unicellular algae may grow to very large sizes. This challenges the idea that larger organisms are always made of many microscopic cells. IB EXAM-STYLE QUESTION: D escribe the characteristics of P aramecium that enable it that enable it to perform the functions of life. ○ Parameica are surrounded by small hairs called cilia which allow it to move (Response) ○ Engulf food via a specialized membranous feeding groove called a cytostome (Nutrition) ○ Food particles are enclosed within small vacuoles that contain enzymes for digestion (Metabolism) ○ Solid wastes are removed via an anal pore, while liquid wastes are pumped out via contractile vacuoles (Excretion) ○ Essential gasses (O2) enter and exit (CO2) the cell via diffusion (Homeostasis) ○ Paramecia divide asexually (fission) although horizontal gene transfer can occur via conjugation (Reproduction) ● The use of stem cell therapy to treat Stargardt’s disease and other conditions ○ Stem cells can be used to replace damaged or diseased cells with healthy, functioning ones. ○ This process requires: ■ The use of biochemical solutions to trigger the differentiation of stem cells into the desired cell type ■ Surgical implantation of cells into the patient’s own tissue ■ Suppression of host immune system to prevent rejection of cells ■ Careful monitoring of new cells to ensure they do not become cancerous ● Examples of Stem Cell Therapy ○ Stargardt’s Disease ■ An inherited form of juvenile macular degeneration that causes progressive vision loss to the point of blindness. This is caused by a gene mutation that impairs energy transport in retinal photoreceptor cells, causing them to degenerate. The disease is treated by replacing dead cells in the retina with functioning ones derived from stem cells. ○ Parkinson’s Disease ■ A degenerative disorder of the central nervous system caused by the death of dopamine-secreting cells in the mid-brain. Dopamine is a neurotransmitter responsible for transmitting signals involved with the production of smooth, purposeful movements. Individuals with Parkinson’s disease typically exhibit tremors, rigidity, slowness of movement and postural instability. Treatment is by replacing dead nerve cells with living, dopamine-producing ones. ○ Other therapeutic examples ■ Leukemia → Bone marrow transplants for cancer patients who are immunocompromised as a result of chemotherapy ■ Paraplegia → Repair damage caused by spinal injuries to enable paralyzed victims to regain movement ■ D iabetes→ Replace non-functioning islet cells with those capable of producing insulin in type I diabetes ■ Burn victims→ Graft new skin cells to replace damaged tissue ● Calculating magnification 1mm = 1000μm 1μm = 0,001mm Section Review Questions 1. Metabolic waste requires sufficient surface are for it to be excreted. If the cell’s SA:V ratio is low, the cell’s excretory need would not be met and waste would build up inside the cell. 2. Paramecium a nd C hlorella get their nutrients in different ways; a P aramecium has an oral groove from which it can absorb organic molecules to provide energy for itself, while Chlorella uses chloroplasts to synthesize its own organic compounds. 3. Some cells, like nerve cells and and muscle cells, have a greatly reduced ability to reproduce once they become specialized. 4. One problem discovered early on in stem cell research was that stem cells cannot be distinguished by their appearance. They can only be isolated from other cells on their basis of their behaviour. 1.2 The Ultrastructure Of Cells ● Prokaryotes have a simple cell structure without compartmentalization ○ Prokaryotes are organisms whose cells lack a nucleus ○ Prokaryotes can be classified into two different domains: ■ Archaebacteria→ found in extreme environments like high temperatures, salt concentrations or pH (e.g: Extremophiles) ■ Eubacteria→ Traditional bacteria including most known pathogenic forms (e.g: Escherichia coli) ● All prokaryotes have certain characteristics in common: ○ Prokaryotes are single-celled organisms and are much smaller than eukaryotes. ○ The size of most prokaryotes is between 1μm and 10μm, but can vary from 0.2μm to 750μm . ○ Prokaryotes are divided into two domains: the bacteria, unicellular organisms that have a wide range of shapes and ubiquitous in all habitats, and archaea, unicellular prokaryotic microorganisms similar to bacteria but possess some genes and metabolic pathways that are closely related to those of eukaryotes. ○ Prokaryotes exist in different shapes, such as: coccus, b acillus, spirillum, cocobacillus, and s pirochete. Some of the prokaryotes are pleomorphic (do not possess constant shape), while some exist as aggregate communities. ● Features of prokaryotes: ○ Cell Wall → Rigid outer covering made of peptidoglycan maintains shape and prevents bursting (lysis) ○ Slime Capsule → A thick polysaccharide layer used for protection against desiccation (drying out) and phagocytosis (engulfing large molecules using the plasma membrane) ○ Flagella → Long, slender projections containing a motor protein that enables movement. ○ Pili→ Hair-like extensions that enable adherence to surfaces (attachment pili) or mediate bacterial conjugations (sec pili) ○ Cell Membrane → Semi-permeable and selective barrier surrounding the cell ○ Cytoplasm→ Internal component of the cell i nside the cell membrane (includes organelles and semi-solid gel) ○ Cytosol→ Fluid component inside the cell membrane ○ Nucleoid→ Region of the cytoplasm where the DNA is located (DNA strand is circular and called a genophore) ○ Plasmids → Autonomous circular DNA molecules that may be transferred between bacteria during horizontal gene transfer ○ Ribosomes → Complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryotic ribosomes are 70S) ● Eukaryotes have a compartmentalized cellular structure ○ Eukaryotes are organisms whose cells do contain a nucleus. ○ They have a more complex structure and are believed to have evolved from prokaryotic cells (via endosymbiosis) ● Eukaryotes can be divided into four different kingdoms: ○ Protista→ Unicellular or multicellular organisms without specialized tissue ○ Fungi→ Have a cell wall made of chitin and obtain nutrition via heterotrophic absorption (absorbing nutrients from other organisms) ○ Plantae → Have a cell wall made of cellulose and obtain nutrition autotrophically via photosynthesis ○ Animalia→ No cell wall and obtain nutrition via heterotrophic ingestion (eating) ● Prokaryotes divide by binary fission ○ This involves replication of their DNA and elongation of the cell such to the point that it will partition to divide into 2 cells. ○ Binary fission is a form of asexual reproduction, meaning that the 2 cells produced are genetically identical to the original cell. ● Summary of all organelles (for both prokaryotes and eukaryotes): ○ Cell Wall → The cell wall protects and maintains the shape of the cell. In prokaryotes it is made up of a carbohydrate-protein complex called peptidoglycan. In eukaryotes, animal cells do not contain cell walls but plant and fungi it is composed of cellulose and chitin (or both in some fungi), respectively. ○ Plasma Membrane→ The plasma membrane is just under the cell wall and is similar in composition in both prokaryotes and eukaryotes. To a large extent, the plasma membrane controls the movement of materials in and out of the cell and ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ plays a role in binary fission. Flagella (sing. flagellum) → Some bacteria have flagella that enable locomotion. Pili→ Pili are hair-like growths on the outside of the cell wall. These structures can be used for attachment, but their main function is to join bacterial cells in preparation for the transfer of DNA from from one cell to another (sexual reproduction). Ribosomes → Organelles that occur in both eukaryotic and prokaryotic cells and function as sites of protein synthesis. These small structures occur in very large numbers that produce a lot of proteins. Prokaryote ribosomes = 70S, eukarytoe ribsomes = 80S. Nucleoid Region → The nucleoid region occurs only in prokaryotic cells and is a non-compartmentalized region that contains a single, long, continuous, circular thread of DNA, the bacterial chromosome. It is the region involved in cell control and reproduction. Golgi Apparatus→ The Golgi apparatus consists of flattened sacs, called cisternae, stacked on top of each other. This organelle functions in the collection, packaging, modification and distribution of materials synthesized in the cell. One side of the apparatus is near the rough ER called the c is side, which receives products from the ER. These products then move into the cisternae of the Golgi apparatus. They continue to move to the discharging or opposite side, called the trans side. The trans side has sacs called vesicles to transport modified materials to wherever they are needed, inside or outside the cell. Nuclear Pore → Allows communication between the nucleus and the rest of the cell. Rough Endoplasmic Reticulum → An extensive network of tubules or channels that extend almost everywhere in the cell. Its structure enables its function, which s the transportation of materials throughout the internal regions of the cell. The rough ER has ribosomes attached to its exterior. Smooth Endoplasmic Reticulum → An extensive network of tubules or channels that extend almost everywhere in the cell. Its structure enables its function, which s the transportation of materials throughout the internal regions of the cell. The smooth ER does not have ribosomes attached to its exterior. It is involved in the production of membrane phospholipids and cellular lipids, the production of sex hormones such as testosterone and oestrogen, the storage of calcium ions in muscle cells (needed for contraction of muscle cells) and transportation of lipid-based compounds. Nucleolus → A dense, solid structure that is involved in ribosome synthesis. Vacuoles → I n animal cells, they are small and typically transport materials into and out of the cell. In plant cells, vacuoles use osmosis to absorb water and swell until they create internal pressure against the cell wall. This provides cell stability and support. In prokaryotic cells, aside from storage, the main role of the central vacuole is to maintain turgor pressure against the cell wall. Proteins found in the tonoplast (aquaporins) control the flow of water into and out of the vacuole through active transport, pumping potassium (K+) ions into and out of the vacuolar interior. Lysosomes → Lysosomes are intracellular digestive centres that arise from the Golgi apparatus. It does not have any internal structures. They are sacs bound by a single membrane that contain as many as 40 digestive enzymes. These enzymes ○ ○ ○ ○ ○ ○ ○ are all hydrolytic and catalyze the breakdown of proteins, nucleic acids, lipids and carbohydrates. Lysosomes fuse with old or damaged organelles from within the cell to break them down so their components can be recycled. Nuclear Membrane → Membrane around the nucleus that breaks down during cell division. Centrioles → T he main function of the centriole is to help with cell division in animal cells. The centrioles help in the formation of the spindle fibers that separate the chromosomes during cell division (mitosis). Mitochondria → Rod-shaped organelles that appear throughout the cytoplasm of the cell. They have their own DNA, allowing them some independence within the cell. They have a double-membrane: the outer membrane is smooth, but the inner membrane is folded into cristae. Inside the inner membrane is a semi-fluid substance called the matrix. Most mitochondrial reactions involve the synthesis of Adenosine Triphosphate (ATP). Cytoplasm → A region inside the plasma membrane of the cell where the organelles are found. The fluid portion of this region is called cytosol. Starch Granules → T he stored starch granules can be converted by enzymes (amylases) to glucose, and the glucose is utilized to generate energy during germination or whenever energy is needed. Chloroplasts → c hloroplasts allow plants to capture the energy of the Sun in energy-rich molecules. Centrosomes→ t he centrosome is an organelle that serves as the main microtubule organizing center of the animal cell, as well as a regulator of cell-cycle progression. Section Review Questions 5. DNA is more likely to get damaged, causing a greater chance of mutation. 6. Pili are the structures most involved in sexual reproduction of prokaryotic cells as they allow for attachment and horizontal gene transfer via conjugation. 7. Pili are the structures that allow for adherence to other surfaces. Considering that there are so many pili covering the exterior of the bacterium, this allows the bacterium to cling itself to the surface more strongly and require more force to be removed. 8. Muscle cells have many mitochondria, which allows them to respond quickly to the need for doing work. 9. Mitochondria and chloroplasts. 10. Chloroplasts produce simple carbohydrates. These carbohydrates are sources of chemical energy when their chemical bonds are broken. The energy can be used to produce ATP, which is necessary for cellular activities. Mitochondria enable the breakdown of the chemical bonds to release the energy. 11. The presence of a scale bar means that the actual size of the object in the micrograph can be worked out. 1.3 Membrane Structure ● Davson-Danielli model vs. Singer-Nicolson model: ○ Proposed by Hugh Davson and James Danielli (1935), the model used a lipid bilayer model suggesting that it was covered on both sides by a thin layer of globular protein. ○ A second model was proposed by Seymour J. Singer and Garth L. Nicolson (1972) proposed that proteins are inserted into the phospholipid bilayer and do not form a layer on the surface of it. They suggested that the proteins formed a mosaic floating in a fluid layer of phospholipids. ○ Reasons why Singer and Nicolson proposed a different model: ■ Not all membranes are identical or symmetrical, as the first model implied ■ Membranes with different functions also have different compositions and different structures, as can be seen by an electron microscope ■ A protein layer is not likely because it is very non-polar and would not interface with water, as shown by cell studies ■ Since 1972, only a few minor changes have been made to the Singer-Nicholson model ● Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules. ○ Amphipathic = Having both hydrophilic and hydrophobic components, Hydrophilic = Water-loving, Hydrophobic = Water-fearing ○ The current agreed model for the cellular membrane is the fluid mosaic model ○ ○ Phospholipid → ■ ■ The “backbone” of the membrane is a bilayer composed of huge numbers of molecules called phospholipids ■ Each phospholipid is made up of a three-carbon compound called glycerol, two carbons of which are attached to fatty acid hydrocarbon chains. The third carbon is attached to the highly polar phosphate group ■ Fatty acids are not water-soluble because they are non-polar, but the alcohol and phosphate groups are water-soluble because they are highly polar. This means that that the head of a phospholipid is hydrophilic and the tails are hydrophobic. ■ The hydrophilic and hydrophobic regions cause phospholipids to align as a bilayer if there is water present and there is a large number phospholipid molecules. Because the phospholipid fatty acid tails do not attract each other strongly, the membrane tends to be fluid or flexible. This allows animal cells to have a variable shape and also allows the process of endocytosis to take place. What maintains the overall structure of the membrane is the tendency water has to form hydrogen bonds. (p.s: this point is very important for answering exam style questions about membrane properties and structure) ○ Cholesterol → ■ Membranes must be fluid to function properly. At various locations in the hydrophobic region (fatty acid tails) are cholesterol molecules ■ Cholesterol molecules help maintain the fluidity of the membrane by packing fatty acid tails tighter in warmer temperatures and loosening fatty acid tails in colder temperatures ■ Fatty acid tails can be saturated or unsaturated. Saturated fatty acid tails can be packed more closely, unlike unsaturated fatty tails ■ Plant cells do not have cholesterol molecules; they depend on saturated and unsaturated fatty acids to maintain proper membrane fluidity ○ Proteins → ■ Proteins are the molecules that create diversity in membrane functions ■ Different types of proteins are embedded in the fluid matrix of the phospholipid bilayer. This creates This creates the mosaic effect referred to in the fluid mosaic model ■ There two main types of proteins embedded in the phospholipid bilayer: integral proteinsand peripheral proteins. Integral p roteins an amphipathic character, with both hydrophilic and hydrophobic regions within the same protein (hydrophobic mid-section and hydrophilic region exposed on either side of the membrane). P eripheral proteins do not protrude into the middle hydrophobic region but remain bound to the surface of the membrane. They are often anchored by an integral protein. ● Membrane proteins are diverse in structure, function and position in the membrane. ○ Receptor Proteins(Hormone-binding sites) → Receptor proteins each bind to a specific molecule outside the cell which triggers a reaction in the cell or a cell action. These proteins can be either integral or peripheral. An example of this is the insulin receptor protein. Insulin is a hormone released by the pancreas when blood sugar levels are high. Insulin binds to the receptor protein which causes the cell to open the typically-closed glucose transport chain, which allows the glucose to enter the cell from the blood. ○ Enzyme Proteins(Immobilized proteins) → Enzyme proteins promote chemical ○ ○ ○ ○ reactions that synthesize or break apart biological molecules. These proteins can be either integral or peripheral. An example of an enzyme protein is ATP Synthase, which synthesizes adenosine triphosphate (ATP) . It works by adding a phosphate onto adenosine diphosphate (ADP) to create adenosine triphosphate (ATP). It is located in the membranes of chloroplasts and mitochondria (used both in photosynthesis and cellular respiration) and the cell membranes of some prokaryotes, meaning they are integral proteins. Adhesion Proteins(Cell Adhesion) → Adhesion proteins anchors the cell membrane to the inner cytoskeleton or proteins outside the cell wall as well as to other cells. These proteins can be either integral or peripheral. An example of adhesion proteins is Cadherins, a type of transmembrane proteins. In the presence of calcium, it binds binds cells within tissues together. Recognition Proteins(Cell-to-Cell Communication) → Recognition proteins serve as identification tags on the surface of a cell and are often glycoproteins(proteins with an attached with an attached sugar molecule). They can be either integral or peripheral. An example would Major Histocompatibility Complex (HMC) Proteins. These are proteins on the surface of cells that belong to a particular individual. HMC proteins interact with immune system cells to identify which cells belong to the body and which cells are foreign. Channel Proteins(Channels for passive proteins) → Channel proteins serve as pores or tunnels for large or hydrophilic molecules to be passively t ransported in and out of the cell. These proteins can only be integral proteins. An example is the glucose channel protein. Glucose is too large and to hydrophilic to diffuse naturally through the phospholipid bilayer. Through the channel proteins, glucose moves along a concentration gradient (from high to low concentration), so no energy is required. Pump Proteins (Pumps for Active Transport) → Pump proteins serve as pores or tunnels for large or hydrophilic molecules to be actively t ransported in and out of the cell. These proteins can only be integral proteins. An example of a pump protein is the sodium-potassium pump. Section Review Questions 12. The phospholipid bilayer is composed of large numbers of molecules called phospholipids. Phospholipids have a polar head and non-polar fatty acid tails. The head is made of a glycerol and a phosphate group, making it water-soluble and very hydrophilic. The tails are non-polar hydrocarbon tails, making them hydrophobic. Because of the hydrophilic and hydrophobic interactions, the phospholipids line up to form a bilayer with the hydrophilic heads on the outer side facing the water and hydrophobic tails tucked inwards. 13. Plant cells do not have cholesterol embedded in in their cell membranes and rely on saturated and unsaturated fatty acid tails to maintain fluidity, unlike animal cell membranes that rely on cholesterol to do that. So a plant-based diet would have significantly less cholesterol than one with high animal product intake. 14. Amphipathic phospholipids possess both hydrophilic and hydrophobic qualities within the same molecule, they have a hydrophilic head and hydrophobic fatty acid tails. 15. Most recognition proteins are glycoproteins, meaning they have a glucose molecule attached to them. 1.4 Membrane Transport ● There are two general types of transport, active transport and passive transport. ○ Passive transport does not require any energy (ATP), and occurs along a concentration gradient (from an area of high concentration to an area of low concentration) ○ Active transport d oes require energy (ATP) because it moves against a concentration gradient (from low concentration to high concentration) ○ Passive transport has three different types: simple diffusion, facilitated diffusion and osmosis ● Diffusion: ○ Particles of a certain type move from a region of higher concentration to a region of lower concentration ○ In a living system, this often involves a membrane ○ Example: Oxygen enters the cell through the membrane and used by its mitochondria for respiration, which creates a lower concentration of oxygen inside the cell compared to the outside of it. Oxygen then diffuses into the cell as a result of this ● Facilitated diffusion: ○ Facilitated diffusion is a particular type of diffusion involving a membrane with specific carrier proteins that are capable of combining with the substance to aid its movement. ○ The carrier protein changes shape to accomplish this task but does not require energy ● Osmosis: ○ A type of diffusion along a concentration gradient, but only involves the passive movement of water across a semi-permeable membrane ○ A semi-permeable membrane is one that only allows certain substances to pass through ○ Ah ypertonic(or hyperosmotic) solution has a high concentration of total solutes ○ Ah ypotonic(or hypo-osmotic) solution has has a lower concentration of total solutes ○ Water moves from a hypotonic to a hypertonic solution across a semi-permeable membrane, until it becomes isotonic(equal ratio of water to solutes). When a solution is isotonic, there is no net movement of water ● The size and polarity of of molecules determine the ease with which various substances can cross membranes. ○ These characteristics and the ability of molecules to cross membranes are arranged along a continuum like this: Small and non-polar molecules ←⸺⸺⸺⸺⸺→ Large and polar molecules cross cross membranes easily membranes with difficulty ○ Example: O2 , CO2 and N2 move across membranes easily and passively, but ions like Na+ and K+ have difficulty passing through (as do glucose and sucrose molecules) ● Active transport: ○ Active transport requires energy in the form of ATP for the molecules to be transported ○ Active transport moves substances against the concentration gradient ○ This allows a cell to maintain interior concentrations of molecules that are different from exterior concentrations ○ ● Endocytosis and exocytosis: ○ Endocytosis and exocytosis are processes through which larger molecules can move across the plasma membrane ○ Endocytosis allows macromolecules to enter the cell while exocytosis allows molecules to leave ○ Both processes depend on the fluidity of the plasma membrane ○ The phospholipid molecules are not closely packed but have rather loose connections between the fatty acid tails. The hydrophilic and hydrophobic properties of the different regions of the phospholipid molecules cause them to form a stable bilayer in an aqueous environment ○ Endocytosisoccurs when a portion of the plasma membrane is pinched off to enclose macromolecules. This pinching off involves a change in the shape of the membrane. The ends of the membrane reattach because of the hydrophobic and hydrophilic properties of the phospholipids and the presence of water. This could not be possible without the fluid nature of the plasma membrane ○ Exocytosis is essentially the opposite of endocytosis. An example of exocytosis involves proteins produced in the cytoplasm of a cell. Protein exocytosis begins in the ribosomes of the rough endoplasmic reticulum: ■ Protein produced by the ribosomes of the rough endoplasmic reticulum enters the lumen (the inner space of the endoplasmic reticulum) ■ Protein exits the endoplasmic reticulum and enters the cis side of the Golgi apparatus; a vesicle is used here ■ As the protein moves through the Golgi apparatus, it is modified and exits on the trans side inside a vesicle ■ The vesicle with the modified protein inside moves to and fuses with the plasma membrane, which results in the secretion of the contents from the cell. Section Review Questions 16. In passive transport, molecules move spontaneously from a region of high concentration to a region of low concentration along a concentration gradient. Once both sides are equal, no net movement can observed and they are said to be “in equilibrium”. However, in active transport, ATP is used to move molecules against the concentration gradient, meaning that it will not be in equilibrium with the concentration of the other side at the end of the process. 17. Non-polar amino acids would be present to attach the integral proteins, because the inner region of the phospholipid bilayer is also non-polar. 18. Both endocytosis and exocytosis are examples of active transport because they require energy in the form of ATP to take place. 1.5 The Origin of Cells ● Cells can only formed by division of pre-existing cells ○ The cell theory stated that: ■ All organisms are composed of one or more cells ■ Cells are the smallest unit of life ■ All cells come from preexisting cells // Cells do not generate spontaneously. (prokaryotes divide by binary fission, while eukaryotes divide by fission, mitosis or meiosis. ○ Prokaryotic cells are formed during a process called binary fission. ○ Eukaryotic cells form new identical cells by the process called mitosis (genetically identical) and form sex cells through meiosis (haploid cells which not genetically identical to the parent cell and contain half the genetic material). Cell division forms the new cells from pre-existing cells replaced the concept of spontaneous generation, where cells were formed from inanimate matter. ○ The chemical processes that contributed to the initial formation of biological life required specific conditions to proceed. This included a reducing atmosphere and high temperatures (>100ºC) or electrical discharges to catalyse chemical reactions. These conditions do not commonly exist on modern Earth and hence living cells cannot arise independently by abiogenesis. Instead, cells can only be formed by the division of pre-existing cells (biogenesis) ● The first cells must have arisen from non-living material. ○ Abiogenesis is the natural process of life arising from non-living matter such as simple organic compounds. ○ The non-living synthesis of simple organic molecules has been demonstrated by the Miller-Urey experiment. Stanley Miller and Harold Urey recreated the postulated conditions of pre-biotic Earth using a closed system of flasks and tubes. Water was boiled to vapour to reflect the high temperatures common to Earth’s original conditions The vapour was mixed with a variety of gases (including H2, CH4, NH3) to create a reducing atmosphere (no oxygen) This mixture was then exposed to an electrical discharge (simulating the effects of lightning as an energy source for reactions). The mixture was then allowed to cool (concentrating components) and left for a period of ~1 week After this time, the condensed mixture was analysed and found to contain traces of simple organic molecules. ● The origin of eukaryotic cells can be explained by the endosymbiotic theory ○ There is compelling evidence that mitochondria and chloroplasts were once primitive free-living bacterial cells ○ Symbiosis occurs when two different species benefit from living and working together. When one organism actually lives inside the other it's called endosymbiosis. The endosymbiotic theory describes how a large host cell and the bacteria ingested through endocytosis, could easily become dependent on one another for survival, resulting in a permanent relationship. As long as the smaller mitochondria living inside the cytoplasm of the larger cell divided at the same rate, they could persist indefinitely inside those cells ○ The smaller cell was provided food and protection by the larger cell and the smaller mitochondria would supply energy through aerobic respiration for the larger cell. Over millions of years of evolution, mitochondria and chloroplasts have become more specialized and today they cannot live outside the cell ● Evidence for the symbiotic theory: ○ Mitochondria are about the size of most bacterial cells ○ Mitochondria and prokaryotes both divide by fission ○ Both the mitochondria and prokaryotes divide independently of the host cell ○ Mitochondria have their own ribosomes which allows them to synthesize their own proteins ○ Mitochondria have have their own DNA, which more closely resembles the DNA of prokaryotic cells ○ Mitochondria have a double membrane on their exterior, which is consistent with an engulfing process (check pages 39-40 [could be different in the SL book] for more evidence of the endosymbiotic theory) Section Review Questions 19. Bacteria in the air were able to enter the flask and contaminate the broth 20. There is no clear evidence of how the first nucleus developed. It may have developed as mitosis-like divisions resulted in compact genomes. It may have occurred with plasma membranes breaking into the interior, resulting in the nuclear membrane. This is presently being hypothesized and researched. 21. Emergent properties indicate greater outcomes for the whole than would be expected when simply adding up the individual abilities of each part. When Hatena and the alga combine, their abilities are pooled together, allowing greater abilities than anticipated. 22. The cells on Earth today are here because of changes to the first cells that appeared on Earth. Endosymbiosis and other changes resulted in the more complex cells of today compared with the very simple first cells 1.6 Cell Division ● Mitosis is the division of the nucleus into two identical daughter nuclei ○ Mitosis functions as part of the process by which cells are cloned to make genetically identical daughter cells ○ There 4 reasons for cells to divide mitotically: ■ Tissue repair or replacement – damaged or aged cells are replaced with identical, healthy ones ■ Organism growth – multicellular make new cells through mitosis ■ Asexual reproduction – vegetative propagation in plants occurs via mitotic division ■ Embryonic development – zygotes undergo mitosis in order to develop into embryos ○ Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase, metaphase, anaphase, and telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle. ● Interphase: ○ The largest phase of the cell cycle in most cells, the longest, and most variable ○ Includes 3 smaller phases – G1, S, and G2 ■ G1 Phase – First intermediate gap stage in which the cell grows and prepares for DNA replication ■ S Phase – Synthesis stage in which DNA is replicated ■ G2 Phase – Second intermediate gap stage in which the cell finishes growing and prepares for cell division. Organelles may increase in number, DNA begins to condense from chromatin to chromosomes, and microtubules may begin to form ● Cyclinsare a group of proteins that control the cell’s progression through the cell cycle ○ Cyclins bind to cyclin-dependent protein kinases (CDKs), enabling them to act as enzymes ○ The activated enzymes cause the cell to move from G1 to the S phase, and from G2 to M phase ○ These points where CDKs function are called c heckpoints ○ ○ There four different types of cyclins, each activated at different time of the cell cycle ○ ● Cell cycle checkpoints are mechanisms within Interphase that ensure the fidelity and continued viability of mitotic division in cells ○ G1 Checkpoint: Determines appropriate growth conditions (nutrients, cell size, presence of growth factors, etc.). Assess the level of DNA damage from ionising radiation or UV. ○ G2 Checkpoint: Determines the state of pre-mitotic cell. Suitable size required for successful division. Identify replication faults which may have which may have occurred to changes in the DNA sequence distorting genetic fidelity of the daughter cells ● Mitosis: ○ During mitosis the replicated chromosomes separate and move to opposite poles of the cell, thus providing for the same genetic material at each of the locations ○ When the chromosomes are at the poles of the cell, the cell then divides into 2 cells distinct from the larger parent, called daughter cells ● Mitosis involves four phases. In order, they are: ○ Prophase ○ Metaphase ○ Anaphase ○ Telophase ● During G2, the chromatin begins to condense in a process called s upercoiling: ○ DNA wraps around histones (DNA packaging proteins) to form nucleosomes ○ Nucleosomes are further wrapped into a solenoid (a coil) ○ Solenoids group together in looped domains to induce a final coiling that produces the chromosome ○ ○ In eukaryotic cells, before the S phase and replication, chromosomes are composed of one molecule of DNA. After replication, the chromosome includes two molecules. ○ These two identical molecules are held together by the c entromere, and each molecule is referred to as a c hromatid (together called sister chromatids). When the sister chromatids separate during mitosis, they each have their own centromere and are each called a chromosome ● Prophase: ○ ○ ○ ○ ○ The chromatin fibres become more tightly coiled to form chromosomes Nuclear envelope disintegrates Mitotic spindle begins to form The centromere of each chromosome has a region called the k inetochore thats attaches to the spindle ○ Centrosomes move to opposite poles, due to the elongating microtubules, thus elongating the cell ● Metaphase: ○ ○ ○ ○ ○ Chromosomes move to the equator of the cell, referred to as the metaphase plate The centromere of each chromosome lies on the plate Movement of chromosomes arises as a result of the action of the spindle fibers The centrosomes are now at opposite ends of the cell ● Anaphase: ○ ○ Usually the shortest phase of mitosis ○ Sister chromatids of each chromosome split ○ Chromatid movement arises due to shortening of the microtubules of the spindle ○ At the end of the phase, each pole of the cell has a complete, identical set of chromosomes ● Telophase: ○ ○ ○ ○ ○ Chromosomes start to elongate to form chromatin Nuclear envelope reappears Spindle apparatus disappears The cell is elongated and ready for c ytokinesis ● Cytokinesis: ○ Cell division does not occur in discrete stages but is rather a continuum. The stages used are only to clarify and help understand the cell cycle ○ Once nuclear division has occurred, the cell undergoes cytokinesis ○ In animal cells, it involves the inward pinching of the fluid plasma membrane to form cleavage furrows ○ Plant cells have firm cell walls, so they form cell plates instead. ○ ● Cancer: ○ Cancer occurs when a cell’s cycle becomes out of control ○ The result is a mass of abnormal cells called a tumour ○ The m itotic index is a tool for predicting the response of cancer cells to chemotherapy. It is the ratio of the number cells undergoing mitosis compared to the number of cells not undergoing mitosis ○ Example of mitotic index: ○ A primary tumouris one that occurs in the original site of cancer, and a secondaryt umouris one that has spread from the original location to another part of the organism Section Review Questions: 23. The microtubules are the main components of mitotic spindle fibres that separate the sister chromatids. If their production is disrupted, the daughter cells resulting from mitosis would not be identical, because they wouldn’t have an equal number of chromosomes. Further implications would be that the daughter cells would have improper DNA that will not be able to fulfill cellular needs and processes 24. The 24 chromosomes will separate in metaphase to become 48 sister chromatids 25. Cytokinesis is the last part of the cell cycle, that occurs right after the M phase and right before the G1 phase 26. Animal cells form a contractile ring of filaments that pinch inwards to form a cleavage furrow, which then splits into the two daughter cells. However, plants have a rigid cell wall, and so they form a cell plate from vesicles that develops between the cell wall and the plasma membrane and builds outwards until the cell splits 27. The mitotic index is equal to the the ratio of the number of cells undergoing mitosis to the number of cells not undergoing mitosis, and it can be used to predict the response of cancer cells to chemotherapy Chapter Review Practice Questions: 1. a. i. No chromosomes are visible, therefore it is in interphase ii. Cell growth and DNA replication b. Stem cells are not yet differentiated and retain the ability to divide c. Stem cells can be used to treat Stargardt’s disease, which causes continuous vision loss and eventually blindness. This disease is treated by injecting stem cells to replace dead cells in the retina. Once the stem cells differentiate and multiply, vision is eventually restored 2. A 3. C 4. D 5. a. i. 100 mm ÷ 0.0002 = 50,000 ii. 0.1 µ b. Mitosis replaces dead or damaged cells and helps in embryonic growth (zygotes must undergo mitosis to develop into embryos) c. Because stem cells are not yet differentiated, they can develop into a lot of different types of cells to replace damaged tissues and heal wounds
