Cell structure and function There are three fundamental theory: 1. All living things are made up of cells 2. Cells are the smallest units (or most basic building blocks) of life 3. All cells come from preexisting cells through the process of cell division. Prokaryotic vs Eukaryotic cells Prokaryotic cells are organism of the domains bacteria and archaea that consist of prokaryotic cells Eukaryotic cells are organism of the domains protists, fungi, animals and plants that consist of eukaryotic cells Basic features of all cells - Semi-permeable plasma membrane Cytosol which is a semifluid, jellylike substance Chromosomes Ribosomes Prokaryotic cell Eukaryotic cell No nucleus Contain nucleus Smaller cell Larger cell DNA concentrated in a region called DNA in nucleus the nucleoid No membrane bound organelles Presence of organelles membrane-bound Cytoplasm bound by the plasma Cytoplasm in the region between the plasma membrane and nucleus Nucleus - Contains most of the cell’s genes and is usually the most visible organelle - Note that mitochondria and chloroplasts do contain some genes - The nuclear envelope encloses the nucleus to separate its content from the cytoplasm - The nuclear membrane is a double membrane meaning that each membrane consists of a lipid bilayer and embedded proteins - The membranes are perforated by openings which are called pores - A pore complex (protein complex) lines each pore and regulates entry and exit of proteins, RNAs and large macromoelcules - The nuclear lamina which is the nuclear inner side of the envelope is lined and it composed of proteins + maintains the shape of the nucleus by mechanically supporting the nuclear envelope - Nuclear matrix is a framework of protein fibers extending throughout the nucleus interior - Nuclear lamina and nuclear matrix work together to help organize the genetic material resulting in functioning efficiently - Main function is to control the gene expression Chromosomes, Chromatin and Nucleolus - Chromosomes are located inside in the nucleus where DNA is organized into individual units and it carry genes - Chromatin is where the DNA and proteins of chromosomes are together and it condenses to form individual chromosomes as cell starts to prepare to divide - NOTE: ribosomes (not organelles since not membrane bounded) help to produce proteins - The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis - Proteins imported from the cytoplasm are assembled with rRNA to form the large and small subunits of ribosomes thus, the subunits then exit the nucleus through the pores and go to the cytoplasm where they assemble (the subunits) to form ribosomes - Examples of cells active in protein synthesis: 1. Prominent (important) nucleoli 2. Large numbers of ribosomes - Ribosomes carry out protein synthesis in 2 locations: 1. In cytosol (free ribosomes) it synthesize proteins needed in the cytosol 2. On the outside of endoplasmic reticulum or nuclear envelope (bound ribosomes) it synthesize proteins inserted in the membranes for packaging within certain organelles or for export from certain cells EX: pancreatic cells secrete proteins so that it would need more bound ribosomes to help in packaging within certain organelles - The nucleus can directs protein synthesis by synthesizing mRNA then exit the nucleus through the pores and go to the cytoplasm where they (the subunits) assemble to form ribosomes Endomembrane system The endomembrane system (endo- = “within”) is a group of membranes and organelles in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes a variety of organelles, such as the nuclear envelope, lysosomes, the endoplasmic reticulum and Golgi apparatus 1. 2. 3. 4. 5. 6. Nuclear envelope ER Golgi apparatus Lysosomes Vacuoles Plasma membrane These components that are part of endomembrane system are either continuous or connected through or via transfer by vesicles Functions of endomembrane system: - Synthesis of proteins Transport of proteins into membrane and organelles or out of the cell Metabolism Movement of lipids Detoxification of poisons ER - The endoplasmic reticulum (ER) plays a key role in the modification of proteins and the synthesis of lipids. It consists of a network of membranous tubules and flattened sacs called cisternae. They are hollow and the space inside is called the lumen] There are 2 types of ER: - Smooth ER which lacks ribosomes but helps in synthesizing lipids. For example, steroid hormones released by cells in the testes and ovaries Metabolizes carbohydrates, detoxifies drugs and poisons especially in liver cells and stores calcium ions in muscle cells - Rough ER which contain ribosomes attach to it contain 2 parts of functions 1. Secretary protein synthesis: has a bound ribosomes which helps in secreting polypeptide chains into ER lumen through pores in the ER membrane thus, the enzymes in the ER membrane covalently bonded carbohydrates to the proteins to form glycoproteins 2. Intracellular transport: distributes the transport vesicles and secretary proteins that are surrounded by membrane. These vesicles bud off of the ER membrane from a specialized region called transitional ER Compartmentmentalizing the cell is part of rough ER - It is basically a membrane factory for the cell - It grows in place by adding membrane proteins and phospholipids to its own membrane - The ER membrane expands and portions of it are transferred in the form of transport vesicles to other components of the endomembrane system Golgi apparatus When vesicles bud off from the ER, where do they go? Before reaching their final destination, the lipids and proteins in the transport vesicles need to be sorted, packaged, and tagged so that they wind up in the right place. This sorting, tagging, packaging, and distribution takes place in the Golgi apparatus Functions: - Modifies products of the ER such as glycoproteins and phospholipids - Manufactures certain macromolecules such as polysaccharides needed in the cell wall of plants - Sorts and packages materials into transport vesicles The receiving side of the Golgi apparatus is called the cis face and the opposite side is called the trans face. Transport vesicles from the ER travel to the cisface, fuse with it, and empty their contents into the lumen of the Golgi apparatus. Cis face: receiving department which is located near the ER. It transport vesicles from the ER travel to the cisface, fuse with it, and empty their contents into the lumen of the Golgi apparatus Trans face: shipping department which help give rises to vesicles that pinch off and travel to other sites Lysosomes (Digestive compartments) - The lysosome is an organelle that contains digestive enzymes and acts as the organelle-recycling facility of an animal cell. It breaks down old and unnecessary structures so their molecules can be reused. - Lysosomes are part of the endomembrane system, and some vesicles that leave the Golgi are bound for the lysosome - Lysosomal enzymes work best only in acidic environment inside the lysosomes. In addition to that, if lysosome leaks its contents to the cytosol, then the released enzymes would not be vey effective since the cytosol is almost neutral or normal - Hydrolytic enzymes and lysosomal membranes are made by rough ER and then transferred to the Golgi apparatus for further processing and some lysosomes arise by budding from the trans face of the golgi apparatus - Lysosomes can also digest foreign particles that are brought into the cell from outside. For example, the lysosomes fuses with the food vacuole and digest the molecules. Then the digested molecules pass into cytosol and become nutrients for the cell. NOTE that some white cells also do engulf bacteria by phagocytosis which is the ingestion of bacteria by phagocytosis and then destroy them using lysosomes How are proteins of the inner surface of the lysosomal membrane and the digestive enzymes themselves spared from destruction (damaged)? The 3D shape of these proteins help to protect vulnerable bonds from enzymatic attack Role of lysosomes in apoptosis Apoptosis is a programmed cell death - If there is a large number of lysosomes leak their enzymes into the cytosol then as a result, it could be cell death or apoptosis - If lysosomes ere dysfunctional, then the hydrolytic enzymes won’t be able to function well and the lysosomes become filled up with indigestible materials starting to interfere with other cellular activities. Not only that, a disease can occur such as Tay-Sachs disease which is a lipid digestive enzyme that is missing. As a result, the brain becomes impaired by an accumulation (gradual gathering of something) of lipid in the cell How lysosomes can be used for reuse? Lysosomes use enzymes to recycle the cell’s own organelles and macromolecules using the process called autophagy. During autophagy, a damaged organelle or small amount of cytosol becomes surrounded by a double membrane then, the lysosome fuses with the outer membrane of this vesicle. Then, the lysosomal enzymes then dismantle (pull apart or take to pieces) the enclosed material and release the resulting small organic compounds into the cytosol for reuse Vacuoles - Plants cells are unique because they have a lysosome-like organelle called the vacuole - The large central vacuole stores water and wastes, isolates hazardous materials, and has enzymes that can break down macromolecules and cellular components, like those of a lysosome - Plant vacuoles also function in water balance and may be used to store compounds such as toxins and pigments (colored particles) It can perform a variety of functions in different kinds of cells - Food vacuoles are formed by phagocytosis - Contractile vacuoles which are found in many freshwater protists, pump excess water out of cells - Central vacuoles which are found in many mature plant cells + hold organic compounds and water - Plants and fungi lack lysozymes but have hydrolytic vacuoles with a similar function - Only in plants, they have small vacuoles that can hold reserves of important organic compounds - Some plant vacuoles contain pigments such as red or blue pigments that provide color to petals - Some plants protect themselves from herbivore by storing poisonous compounds in vacuole IMPORTANCE OF THE LARGE CENTRAL VACUOLE - The large central vacuole plays a major role in the growth of plant cells, which enlarges as the vacuole absorbs water and this enables the cells to become large but without adding more cytoplasm so there will be less demand for the cell - The cytosol would occupy only a thin layer between central vacuole. And the plasma membrane so the ratio of the surface area to volume is sufficient even though the plant cell is large Mitochondria - powerhouses or energy factories of the cell. Their job is to make a steady supply of adenosine triphosphate (ATP) which is the cell’s main energy-carrying molecule. The process of making ATP using chemical energy from fuels such as sugars is called cellular respiration, and many of its steps happen inside the mitochondria. - They are oval-shaped and have two membranes: an outer one, surrounding the whole organelle, and an inner one, with many inward protrusions called cristae that increase surface area. Benefits of cristae include increasing the surface area for ATP production and it does contain enzymes important to ATP production The space between the membranes is called the intermembrane space, and the compartment enclosed by the inner membrane is called the mitochondrial matrix. The matrix contains mitochondrial DNA (their own DNA) and ribosomes - The number of mitochondria in the cell depends on the cell’s level of metabolic activity. For example, cells that move or contract need more mitochondria per volume than less active cells. Chloroplasts - Chloroplasts are found in plants and algae. They're responsible for capturing light energy to make sugars in photosynthesis. In photosynthesis, light energy is collected and used to build sugars from carbon dioxide. The sugars produced in photosynthesis may be used by the plant cell, or may be consumed by animals that eat the plant, such as humans. Then, the energy contained in these sugars is harvested through a process called cellular respiration, which happens in the mitochondria of both plant and animal cells. - The membrane of a thylakoid disc contains light-harvesting complexes that include chlorophyll, a pigment that gives plants their green color. Thylakoid discs are hollow, and the space inside a disc is called the thylakoid space or lumen, while the fluid surrounding the thylakoids is called the stroma. This compartmental organization enables the chloroplast to convert light energy to chemical energy Mitochondria and chloroplasts likely began as bacteria that were engulfed by larger cells (the endosymbiont theory) which states that some of the organelles in eukaryotic cells were once prokaryotic microbes. They suggest that an early ancestor of eukaryotes engulfed an oxygen using nonphotosynthetic prokaryotic cell. Then, the engulfed cell formed a relationship with the host cell becoming an endosymbiont. The endosybionts evolved into mitochondria. Lastly, at least one of these cells may have then taken up a photosynethetic prokaryote which envolved into a chloroplast Why mitochondria and chloroplast have their own DNA? Bacteria, mitochondria, and chloroplasts are similar in size. Bacteria also have DNA and ribosomes similar to those of mitochondria and chloroplasts. Based on this and other evidence, scientists think host cells and bacteria formed endosymbiotic relationships long ago, when individual host cells took in aerobic (oxygen-using) and photosynthetic bacteria but did not destroy them. Through million of years of evolution, the aerobic bacteria became mitochondria and the photosynthetic bacteria became chloroplasts. Peroxisomes Peroxisomes are specialized metabolic compartments bounded by a single membrane Functions: breaking down fatty acids into smaller molecules, detoxification of alcohol and other harmful substances in liver cells What produces hydrogen peroxide H2O2) Peroxisomes contain enzymes that remove hydrogen atoms from various substrates and transfer them to oxygen (O2) to produce hydrogen peroxide (H2O2) - Hydrogen peroxide is toxic and is thus converted to water by enzymes in the peroxisome - ER and Golgi help to produce and package the enzymatic content of peroxisomes - Compartmental structure of cells needed in this example since the enzymes that produce H2O2 and those that dispose of this toxic compound are isolated away from other cellular components that could be damaged Cytoskeleton - It is a network of fibers extending throughout the cytoplasm that help organizes the cell’s structures and activities, anchoring many organelles - It is composed of three types of molecular structures 1. Microtubules 2. Microfilaments 3. Intermediate filaments Cell wall - It is an extracellular structure that are only in plant cells - Functions include such as protecting the plant cell, maintaining its shape, and preventing excessive uptake of water - Plant cell walls are made of cellulose fibers that are embedded in other polysaccharides and protein - Example of plant cells that contain cell walls: Prokaryotes, protists, bacteria, fungi, some unicellular eukaryotes Centriole Functions: - help with cell division in animal cells - help in the formation of spindle fibers that separate chromosomes during (cell division) Function of cell membrane: is the protective barrier that surrounds the cell and prevents unwanted material from getting into it
