Cell Structures and Their Functions

# Cell Structures and Their Functions ## Introduction The cell is the basic living unit of all organisms. The simplest organisms consist of single living cells, whereas humans are composed of about 100 trillion cells. It was discovered by Robert Hooke through the microscope. Light microscopes allo...

# Cell Structures and Their Functions ## Introduction The cell is the basic living unit of all organisms. The simplest organisms consist of single living cells, whereas humans are composed of about 100 trillion cells. It was discovered by Robert Hooke through the microscope. Light microscopes allow us to visualize general features of cells. Electron microscopes are used to study the fine structure of cells. All the cells of an individual originate from a single fertilized cell. During development, cell division and specialization give rise to a variety of cell types (nerve, muscle, bone, fat, and blood cells). About 200 different types of cells compose the human body. Each cell type has important characteristics that are critical to the human function of the body. The cells have unique roles that support homeostasis and contribute to the many functional capabilities of the human body. ## Cell * Basic structural and functional unit of all organisms. * Smallest structure capable of performing all the activities vital to life. * Highly organized unit containing organelles, which perform specific functions. * The nucleus contains genetic material, and cytoplasm is a living material between the nucleus and the cell membrane. ## Functions of the Cell 1. **Cell Metabolism and energy use** - chemical reactions occurring within cells are referred to collectively as cell metabolism. Energy released during metabolism is used for cell activities, such as synthesis of new molecules and muscle contraction. Heat produced during cell metabolism helps maintain body temperature. 2. **Synthesis of new molecules** - The structural and functional characteristics of cells is determined by the types of molecules they produce. Cells differ from each other because they synthesize different kinds of molecules, including lipids, proteins, and nucleic acids. 3. **Communication** - Cells produce and receive chemical and electrical signals that allow them to communicate with one another. Ex. Nerve cells communicate with one another and with muscle cells, causing muscle cells to contract. 4. **Reproduction and inheritance** - Most cells contain a copy of all the genetic information of the individual. This genetic information ultimately determines the structural and functional characteristics of the cell. During the growth of an individual, cells divide to produce new cells, each containing the same genetic information. Specialized cells of the body, the gametes are responsible for transmitting genetic information to the next generation. 5. **Protection and support** - produce and secrete various molecules that provide protection and support of the body. 6. **Movement** - movement of the body occur because of molecules located within specific cells such as muscle cells. ## Major Parts of the Cell 1. **Cell Membrane / Plasma Membrane** * outermost component of the cell. * extremely thin and delicate. * major components: * **phospholipids** (lipids that contain phosphorus) * form a lipid bilayer, which is a double layer of phospholipid molecules * w/ hydrophilic/polar regions - water loving or soluble in water. * w/ hydrophobic regions (water fearing or insoluble in water) -will not associate with water, and this characteristic enables the cell membrane to act as a selective physical barrier. * **Proteins** - CHON molecules float on the phospholipid molecules and some extend from the inner to the outer surface of the plasma membrane. * **Membrane proteins** can function as: * **Marker molecules** – cell surface molecules that allow cells to identify one another or other molecules. They are mostly glycoproteins. Allow immune cells to distinguish between self-cells and foreign cells, such as bacteria or donor cells in an organ transplant. * **Attachment proteins** – allow cells to attach to other cells or to extracellular molecules. * **Transport proteins** – extend from one surface of the plasma membrane to the other and move ions or molecules across the plasma membrane. It includes channel proteins, carrier proteins, and ATP-powered pumps. * **Channel proteins** form membrane channels, which are like small pores extending from one surface of the plasma membranes to the other. Permits water, ions, and other solutes to bypass lipid portion of the cell membrane. * **Carrier proteins** – bind and transport solutes across the cell membrane. This process may or may not require energy. Bring glucose into the cytoplasm and transport Na, K, and Ca into and out of the cell. * **Receptor proteins** – can attach to specific chemical signals and sensitive to specific extracellular materials that bind to them and trigger a change in a cell's activity. Part of an intercellular communication system that enables cell recognition and coordination of cell activities. * **Enzymes** catalyze chemical reactions on either the inner or the outer surface of the plasma membrane. * Also contains cholesterol, CHO, water and ions. * **Selective permeability** - allows some substances to move into and out of the cell but restricts the passage of other substances. The lipid bilayer of the cell membrane is permeable to water and to lipid-soluble molecules such as fatty acids, fat-soluble vitamins, steroids, O2, and CO2. The lipid bilayer is not permeable to ions, water-soluble compounds, and large molecules such as glucose and proteins. * **Fluid-mosaic model** an arrangement of molecules in the cell membrane and suggest that is highly flexible and can change its shape and composition through time. * **Functions:** * **Physical isolation** – The cell membrane is a physical barrier that separates the inside of the cell from the surrounding extracellular fluid. Conditions inside and outside the cell are very different, and those differences must be maintained to preserve homeostasis. * **Regulation of exchange with the environment** – controls the entry of ions, and nutrients, the elimination of wastes, and the release of secretions. * **Sensitivity** - The cell membrane is the first part of the cell affected by changes in the extracellular fluid. It also contains a variety of receptors that enable the cell to recognize and respond to specific molecules in its environment. * **Structural support** - specialized connections between cell membranes, or between membranes and extracellular materials, give tissue a stable structure. * **Protection** 2. **Cytoplasm** * Cellular contents between the plasma membrane and the nucleus * Site of all cellular activities except from those occurring in the nucleus. * Has two components: * **Cytosol** - fluid portion of the cytoplasm that surrounds organelles and constitutes about 55% of total cell volume. Varies in composition and consistency from one part of the cell to another, typically 75-90% water plus various dissolved solutes and suspended particles. Among these are various ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, enzymes and waste products. It also contains insoluble materials known as cytoplasmic inclusions which are aggregates of chemicals either produced by the cell or taken in by the cell. Examples: stored nutrients such as glycogen granules in muscle and liver cells and lipid droplets in fat cells. * **Organelles** (or little organs) - internal structures that perform specific functions essential to normal cell structure, maintenance, and metabolism. The number and type of organelles within each cell determine the cell's specific structure and functions. ## Cytoplasmic Organelles **Ribosomes** * Sites of protein synthesis, using information provided by the DNA of the nucleus. * Each ribosome consists of small and large subunits of ribosomal RNA and protein. Large and small subunits are made in the nucleolus of the nucleus. * Types: * **free ribosomes** – scattered throughout the cytoplasm, and synthesize proteins used in the cytosol. * **fixed ribosomes** attached to the endoplasmic reticulum (ER), and the proteins they manufacture enter the ER, where they are modified and packaged for export. * Ribosomes are also located within mitochondria, where they synthesize mitochondrial proteins. * Ribosomes are found in all cells, but their number varies depending on the type of cell and its activities. Liver cells, which manufacture blood proteins, have more ribosomes than do fat cells, which synthesize triglycerides. **Endoplasmic Reticulum (ER)** * Network of folded membranes connected to the membranous nuclear envelope surrounding the nucleus. * Types: * **Rough ER** – w/ ribosomes attached to the surface, and synthesize large amounts of protein for export from the cell. * **Smooth ER** no ribosomes attached, site for lipid and carbohydrate synthesis, and participates in the detoxification of chemicals within cells. In liver cells enzymes of the smooth ER also help release glucose into the bloodstream and inactivate or detoxify a variety of drugs and potentially harmful substances, including alcohol, pesticides, and carcinogens (cancer-causing agents). **Golgi Apparatus** * Named for Camillo Golgi, Italian histologist. * Stacks of flattened sacks, formed by membranes. * **Functions:** collect, modify, package and distribute proteins and lipids. * Proteins synthesized by rough ER enter the Golgi apparatus and are modified to form glycoproteins and lipoproteins. The proteins are then packaged into vesicles that pinch off from the margins of the Golgi apparatus and are distributed to various locations. Some vesicles contain enzymes that are used within the cell; some vesicles carry proteins to the plasma membrane, where the proteins are secreted from the cell by exocytosis; other vesicles contain proteins and lipids that become part of the cell membrane. **Secretory Vesicles** * small, membrane bound sacs * **Functions:** * transport materials produced within cells to the exterior of the cells. * store materials produced within cells. * secretory vesicles pinch off from the Golgi apparatus and move to the surface of the cell and accumulate in the cytoplasm and are released to the exterior when the cell received signal. * formed from the Golgi apparatus. **Lysosomes** * membrane-bound vesicles containing intracellular digestive enzymes. * formed from the Golgi apparatus * **Functions:** * remove damaged organelles or pathogens within cells. * Their enzymes are activated when they fuse w/ the membranes of damaged organelles. The enzymes within the lysosomes break down the materials in the endocytic vesicle. White blood cells phagocytize bacteria. Enzymes within lysosomes destroy the bacteria. Also when tissues are damaged, ruptured lysosomes within the damaged cells release their enzymes and digest both healthy and damaged cells. * help recycle worn-out structures - lysosome can engulf another organelle, digest it, and return the digested components to the cytosol. * **Human liver cell recycles about half of its contents every week.** * **Lysosomal enzymes may also destroy the entire cell (autolysis).** **Peroxisomes** * Small, membrane-bound vesicles similar to a lysosome. * Contain enzymes oxidases that can oxidize (remove hydrogen atoms) various organic substances and catalase that decomposes hydrogen peroxide. * **Functions:** break down of fatty acids, amino acids, and hydrogen peroxide (by-product of fatty acid and amino acid break down that can be toxic to the cell); oxidize toxic substances. * Enzymes break down hydrogen peroxide into oxygen and water thus protecting the cell from its damaging effects. * **Hydrogen peroxide is a potentially dangerous free radical (ions or molecules that contain unpaired electrons and can be destructive to vital compounds such as proteins).** * **Cells active in detoxification, such as liver and kidney cells, have many peroxisomes.** **Proteasomes** * tunnel-like structures, similar to channel protein, and not bounded by membranes. * contain enzymes (proteases) that cut proteins into small peptides. * a typical body cell contains thousands of proteasomes. * **Functions:** * continuous destruction of un-needed, damaged, or faulty proteins. * break-down of proteins into amino acids, which can be recycled into new proteins. **Mitochondria** * powerhouse of the cell. * small, spherical, rod shaped or thin filamentous structure in the cytoplasm. * double membrane, with inner folds (cristae) enclosing important metabolic enzymes. * **Function:** major site of ATP synthesis (produce 95% of ATP required by the cell). * The process of mitochondrial energy production is known as aerobic metabolism or cellular respiration. * The number of mitochondria in a particular cell varies with the cell's energy demands. * Active cells such as muscle, liver, and kidneys use ATP at a high rate and have more numbers of mitochondria than cells that require less energy. * Red blood cells have no mitochondria. **Cytoskeleton** * internal protein framework of various threadlike filaments and hollow tubules that gives the cytoplasm strength and flexibility. * extending throughout the cytosol. * also involved in cell movement. * components: * **Microtubules** * Largest and hollow structures formed from protein subunits. * form the primary components of the cytoskeleton. * maintain cell shape. * movement of organelles within a cell. * provide support to the cytoplasm. * assist in cell division (migration of chromosomes). * forming essential components of centrioles, spindle fibers, cilia and flagella. * **Microfilaments** * Small fibrils formed from protein subunits that structurally support the cytoplasm. * Contribute to cell's strength and shape. * Provide mechanical support. * Involve in cell movement (muscle microfilament). * Anchor the cytoskeleton to integral proteins in the plasma membrane. * **Intermediate Filaments** * Fibrils formed from protein subunits that are smaller in diameter than microtubules but larger in diameter than microfilaments. * Provide mechanical support to the cell. **Centrioles** * Located in the centrosome, a specialized zone of cytoplasm close to the nucleus that is the center of microtubule formation. * Made of microtubules and facilitate chromosome movement during cell division. * Each centrosome contains two centrioles. * Each centriole is a small, cylindrical organelle composed of nine triplets, each consisting of three parallel microtubules joined together. * **All cells that are capable of dividing contain a pair of centrioles.** * **Centrioles produce the spindle fibers that move DNA strands during cell division.** * **Mature red blood cells, skeletal muscle cells, cardiac muscle cells, and typical neurons lack centrioles; as a result, these cells cannot divide.** **Cilia** * Mobile extension of cell surface. * Varies from 1,000 per cell and contains specialized microtubules enclosed by the cell membrane. * Moves substances over the surface of the cell. * Numerous in the respiratory tract, helps lung clear of debris. **Flagella** * Whiplike locomotor organelle, usually occur one per cell. * Much longer than cilia. * propel sperm cells. * **Sperm cells are the only human cells that have flagella.** **Microvilli** * Minute finger-shaped projections of the cell membrane on the exposed surfaces of many cells. * supported by microfilaments but they do not actively move like cilia and flagella. * Increase the surface area of the membrane and aid in absorption. * Abundant in intestine, kidney and other areas in which absorption is an important function. ## 3. Nucleus * Largest organelle and located near the center of the cell. * Bounded by a nuclear envelope, consisting a double membrane surrounds the nucleus and separates its fluid contents, the nucleoplasm from the cytosol. * **Nucleoplasm** contains ions, enzymes, RNA and DNA nucleotides, proteins, small amounts of RNA, and DNA. * **Nuclear pores** point where the inner and outer membranes of the nuclear envelope come together to form a hole, and control the movement of substances between the nucleus and cytoplasm. * Most nuclei contain several nucleoli (plural nucleoli; singular nucleolus). Nucleoli are organelles that synthesize ribosomal RNA (rRNA) and assemble the ribosomal subunits into functional ribosomes. * **DNA** (Deoxyribonucleic acid) is mostly found within the nucleus, and small amounts are found in the mitochondria. DNA stores instructions for protein synthesis. * **Nuclear DNA** and associated proteins are organized into chromosomes. * Nucleus of the human body cell contains 23 pairs (46) of chromosomes (diploid cell), which carries the genetic material, composed of DNA and proteins (histones). One member of each pair is derived from the mother and one from the father. * **Functions:** * control center for cellular operations. * controls synthesis of different proteins. * determines both the structure of the cell and the functions it can perform by controlling which proteins are synthesized. * **All body cells have a single nucleus at some point in their life cycle, although some cells, such as red blood cells, lose their nuclei as they mature.** * **Skeletal muscle cells and osteoclasts have several nuclei.** * **Nucleoli are most prominent in cells that manufacture large amounts of proteins, such as muscle and liver cells.** * **Chromosomes in a dividing cell are tightly coiled, and they can be seen clearly as separate structures.** * **Chromosomes in a non-dividing cell are loosely coiled and dispersed throughout the nucleus as delicate filaments known as chromatin.** ## Cell Physiology ### Movement Through the Cell Membrane * Movement of substances across its cell membrane is essential to the life of a cell. Certain substances must move into the cell to support metabolic reactions. Other substances must be moved out because they have been produced by the cell for export or are cellular waste products. * Substances such as enzymes, glycogen and K ions are found at higher concentrations intracellularly; Na, Ca, and Cl extracellularly. * Nutrients must enter cells continually, and waste products must exit. * About two-thirds of the body fluid in the body is contained inside body cells known as **intracellular fluid** (ICF). Fluid outside body cells is called the **extracellular fluid** (ECF). The ECF in the spaces between the cells of tissues is **interstitial fluid**. The ECF in blood vessels is the **plasma**, and in lymphatic vessels is called the **lymph**. * Substances dissolved in body fluids include gases, nutrients, ions, and other substances needed to maintain life. Any substance dissolved in a fluid is called a **solute**, and the fluid in which it is dissolved is the **solvent**. * Body fluids are dilute solutions in which a variety of solutes are dissolved in water. The amount of a solute in a solution is its **concentration**. A **concentration gradient** is a difference in concentration between two different areas, the ICF and ECF. Solutes are moving from a high concentration area to a low concentration area are said to move down or with the concentration gradient. Solutes moving from a low concentration area to a high concentration area are said to move up or against the concentration gradient. * The **permeability** of the cell membrane determines precisely which substances can enter or leave the cytoplasm. If any substance can cross without difficulty, the membrane is **freely permeable**. Cell membranes are **selectively permeable**, permitting the free passage of some materials and restricting the passage of others. Whether or not a substance can cross the cell membrane is based on the substance's size, electrical charge, molecular shape, lipid solubility, or some combination of these factors. #### A. Passive Processes - substances move down its concentration gradient through the membrane, using only its own energy of motion (kinetic energy). Move ions or molecules across the cell membrane without any energy expenditure by the cell. 1. **Diffusion** * lons and molecules are in constant motion, colliding and bouncing off one another. * Movement of solute molecules from an area of higher concentration to an area of lower concentration in a solution. * Proceeding "down a concentration gradient” or “downhill.” As a result of the process of diffusion, molecules eventually become uniformly distributed, and concentration gradients are eliminated. * Ex: smell of fresh flowers in a vase sweetens the air in a large room; a drop of ink spreads to color an entire glass of water. Both examples begin with an extremely high concentration of molecules in a localized area. * substances diffuse across a membrane in one of two ways: * **Lipid bilayer** - Lipid-soluble molecules, such as O2, CO2, steroid hormones, fat-soluble vitamins (A,D,E, and K), nitrogen gases, fatty acids, and alcohol diffuse through the lipid bilayer of the cell membranes. * **Membrane channels** – ions and most water-soluble compounds such as water molecules, Na, K, Ca, and Cl pass through membrane channels. * The normal intracellular concentrations of many substances depend on diffusion and some nutrients enter and some waste product leave cells by diffusion. * Important means of transporting substances through the extracellular and intracellular fluids in the body and it tends to eliminate local concentration gradients. Ex: every cell in the body generates CO2, and its intracellular concentration is relatively high. CO2 concentrations are lower in the surrounding ECF. Because cell membranes are freely permeable to CO2, it can diffuse down its concentration gradient-dispersing from the cell's interior into ECF, and from the ECF into the bloodstream, for delivery to and elimination from the lungs. * **Factors Influencing Diffusion rate of Substances across Plasma membrane:** * **Steepness of concentration gradient** - the greater the difference in concentration between two sides of the membrane, the higher the rate of diffusion. When charged particles are diffusing, it is the steepness of the electrochemical gradient that determines the diffusion rate across the membrane. * **Temperature** – the higher the temperature, the faster the rate of diffusion. In a person who has fever, all of the body's diffusion processes occur more rapidly than the diffusion rate. * **Mass of the diffusing substance** - the larger the mass of the diffusing particle, the slower its diffusion rate. Molecules with smaller molecular weight diffuse more rapidly than larger ones. * **Surface area** - the larger the membrane surface area available for diffusion, the faster the diffusion rate. Ex: air sacs of the lungs have a large surface area available for diffusion of O2 from the air into the blood. Some lung diseases, such as emphysema reduces the surface area. This slows the rate of diffusion and makes breathing more difficult. * **Diffusion distance** – the greater the distance over which diffusion must occur, the longer it takes. Diffusion across a plasma membrane takes only a fraction of a second because the membrane is so thin. In pneumonia, fluid collects in the lungs and increases diffusion distance, thus slowing diffusion of oxygen into the blood. 2. **Osmosis** * a special type of diffusion. * Diffusion of water (solvent) across a selectively permeable membrane, from an area of higher water concentration to an area of lower water concentration. * Water molecules pass through lipid bilayer and water channels. * Important to cells because large volume changes caused by water movement can disrupt normal cell functions. * Water molecules tend to flow across a membrane toward an area containing a higher solute concentration. Water movement continues until concentrations, and total solute concentrations are the same on either side of the membrane. * **Osmotic Pressure** - force required to prevent the movement of water across a selectively permeable membrane. * The greater the concentration of a solution, the greater is its osmotic pressure and the greater the tendency for water to move into the solution. * The greater the concentration of a solution, the greater the tendency for water to move into the solution and the greater the osmotic pressure must be to prevent that movement * **Types of solutions:** * **Isotonic Solution** * concentration of various solutes and water are the same on both sides of the cell membrane. * cells maintain their normal shape and volume. * does not cause a net movement of water into or out of the cell, an equilibrium exists, and as one water molecule moves out of the cell, another moves in to replace it. * cells neither shrink nor swell. * ex. 0.9% NaCl * **Hypotonic Solution** * has lower concentration of solutes and higher concentration of water than the cytoplasm of the cell. * water molecules enter the cells by osmosis faster than they leave. * causes cells to swell/ rupture (lysis), RBCs rupture (hemolysis). * **Hypertonic Solution** * has higher concentration of solutes and lower concentration of water than the cytoplasm of the cell. * water molecules move out of the cells by osmosis faster than they enter. * causes cells to shrink (crenation). * **RBCs and other body cells may be damaged or destroyed if exposed to hypertonic or hypotonic solutions. For this reason, most IV solutions are isotonic.** * **Hypertonic solution is useful to treat cerebral edema. Infusion relieves fluid overload by causing osmosis of water from interstitial fluid into the blood.** * **Hypotonic solutions can be used to treat people who are dehydrated.** * **Water and most sports drinks are hypotonic.** 3. **Filtration** * movement of fluid through a partition containing small holes. * movement due to the pressure difference of a liquid through a filter that prevents some or all of the substances in the liquid from passing through. * **hydrostatic pressure** (pressure exerted by a liquid) forces water across a membrane. * occurs across the walls of small blood vessels, pushing water and dissolved nutrients into the tissues of the body. * **Filtration across specialized blood vessels in the kidneys is an essential step in the production of urine.** #### B. Active Processes – cellular energy in the form of ATP is used to “push” substances across the cell membrane against its concentration gradient. **Note:** facilitated diffusion is not considered as an active process. 1. **Mediated Transport** * process by which transport proteins mediate, or assist in the movement of ions and molecules across the plasma membrane. * **carrier molecules** (membrane proteins) within the cell membrane are involved in carrier-mediated transport mechanisms which function to move large, water-soluble molecules or electrically charged ions across the cell membrane and exhibit specificity (only specific molecules are transported). * has 3 characteristics: * **specificity** – each transport protein moves particular molecules or ions, but not others. Ex. Transport protein that moves glucose does not move amino acids or ions. * **Competition** – occurs when similar molecules or ions can be moved by transport protein. Although transport proteins exhibit specificity, a transport protein may transport very similar substances. The substance in the greater concentration or the substance for which the transport protein is most specific is moved across the plasma membrane at the greater rate. * **Saturation** - rate of movement of molecules or ions across the membrane is limited by the number of available transport proteins. Once, the concentration of the substance is increased so that all the transport proteins are in use, the rate of movement remains constant, even though the concentration of the substance increases. * **Types:** * **Facilitated diffusion** * Moves substances into or out of the cells from a higher to a lower concentration of that substance. * Passively transported across the cell membrane by carrier proteins. * ATP is not required. * Molecule to be transported binds to a receptor site on the carrier protein. Then the shape of the protein changes, moving the molecule to the inside of the cell membrane, where it is released into the cytoplasm. * Glucose and amino acids are transported by this process. * **Active Transport** * Carrier-mediated process that moves substances across the cell membrane from regions of lower concentration to those of higher concentration against a concentration gradient. * Can also move substances from higher to lower concentrations. **ATP** provides the energy needed to move ions or molecules across the membrane * if ATP is not available, active transport stops. * Not dependent on a concentration gradient. This means that the cell can import or export specific materials regardless of their intracellular or extracellular concentrations. * All cells contain carrier proteins called **ion pumps** that actively transport the cations Na, K, Ca, and Mg across cell membranes. **Sodium-Potassium Pump** – moves Na out of cells and K into cells 1. three sodium ions and one ATP bind to sodium-potassium pump. 2. the ATP breaks down to ADP and a phosphate (P) and releases energy (that energy is used to power the shape change in the Na-K pump); phosphate remains bound to the Na-K pump-ATP binding site. 3. the Na-K pump changes shape, and the Na are transported across the membrane. 4. Na diffused away from the Na-K pump. 5. two potassium ions bind to the Na-K pump. 6. phosphate is released from the Na-K pump binding site. 7. Na-K site changes shape, transporting K across the membrane, and the K diffuse away from the pump. The Na-K pump can again bind to Na and ATP. * **Na & K are the principal cations in the body fluids. Na ions concentrations are high in the ECF but low in ICF. Low K concentration in ECF and high in ICF.** * **The presence of channel proteins in the membrane that are always open, sodium ions slowly diffuse into the cell, and potassium ions diffuse out.** * **Homeostasis within the cell depends on maintaining Na & K concentration gradients w/ the ECF.** * **Secondary Active Transport** * Concentration gradient established by the active transport of one substance such as Na across the cell membrane, provides the energy to move a second substance, such as glucose into the cell membrane against its concentration gradient. * **Cotransport** - diffusing substance moves in the same direction as the transported substance. * **Counter-transport** - movement of the diffusing substance in the opposite direction as the transported substance 2. **Vesicular Transport** * A vesicle is a membrane bound sac that surrounds substances within the cytoplasm. * Transport substances from one structure to another within the cells, take in substances from the ECF, and release substances into ECF. * Movement of vesicles requires energy supplied by ATP. * Types: * **Endocytosis** * substances brought into the cell are surrounded by a piece of the plasma membrane which buds off inside the cell to form a vesicle containing ingested substances. * packaging of extracellular materials in a vesicle at the cell surface for import into the cell. * Types: * **Phagocytosis** (cell eating) - large solid particles, such as bacteria, viruses or aged or dead cells or foreign substances are ingested. It is important in the elimination of harmful substances from the body by macrophages. In phagocytosis, a part of the plasma membrane extends around a particle and fuses so that the particle is surrounded by the membrane to form a vesicle that enters the cytoplasm. The vesicle fuses with one or more lysosomes, and lysosomal enzymes break down the ingested material. * **Pinocytosis** (cell drinking) – uptake of small droplets of ECF and the materials contained in the fluid, by the formation of small endocytic vesicles. * **Receptor-mediated endocytosis** - when a specific substance binds to a receptor molecule, endocytosis is triggered and the substance is transported into the cell. Ex. Cholesterol, insulin, and iron. receptor molecules on the cell surface bind to molecules to be taken into the cell. the receptor and the bound molecules are taken into the cell as a vesicle is formed. * **Exocytosis** * elimination of a material from a cell through the formation of vesicles. * vesicle is created inside the cell fuses with the cell membrane and discharges its contents into the extracellular environment. * membrane-enclosed secretory vesicles form inside the cell, fuse with the plasma membrane, and release their contents into the ECF. * results in secretion, the liberation of materials from a cell. * all cells carry out exocytosis, but it is important in two types of cells: * secretory cells that liberate digestive enzymes, hormones, mucus, or other secretions. * nerve cells that release substances called neurotransmitters. ### Whole Cell Activity The interactions between organelles must be considered for cell function to be fully understood. That function is reflected in the quantity and distribution of organelles. Cell metabolism, protein synthesis, and cell division illustrate the interactions of organelles that result in a functioning cell. **Cell Metabolism** * The sum of all chemical reactions in the cell. * Allows organisms to grow and reproduce, maintain their structures, and respond to their environments. * In metabolic processes, O2 and nutrients broken down in the digestive system provide the chemical energy to power cellular activities * Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy. * **Categories of metabolism:** * **Catabolism** - a type of metabolic process occurring in living cells by which complex molecules are broken down to produce energy. It is known as decomposition reactions. Catabolic reactions produce more energy than they consume and often occur in the mitochondria. Ex. Digestive process split CHON in food into amino acids; glycolysis (glucose molecule is split into two molecules of pyruvic acid w/ net production of two ATPs). * **Anabolism** - a constructive metabolic process whereby energy is consumed to synthesize or combine simpler substances. Anabolic/synthesis reactions consume more energy than they produce and occur in the endoplasmic reticulum. Ex. Amino acids, into more complex organic compounds, such as enzymes and nucleic acid; Amino acids are used to build new proteins that make up the body structures such as muscles and bones. * **Aerobic respiration-** requires oxygen and produces carbon dioxide, water and up to 38 ATP molecules from a molecule of glucose. * **Anaerobic respiration** – does not require oxygen and produces lactic acid and two ATP molecules from a molecule of glucose. **Protein Synthesis** * Cells synthesize many chemicals to maintain homeostasis, much of the cellular machinery is devoted to protein production. Cells constantly synthesize large numbers of diverse proteins. The proteins determine the physical and chemical characteristics of cells and, on a larger scale, of organisms. * DNA contains thousands of genes that directs protein synthesis or provides the instructions for making proteins. * **Gene** a sequence of nucleotides providing a chemical set of instructions for making a specific protein. Each DNA molecule contains many different genes. * **Phases:** * **Transcription** * Making a copy of part of the information in DNA and occurs in nucleus. * Genetic information in DNA base triplets is copied into a complementary sequence of codons in a strand of RNA known as the messenger RNA (mRNA). * Transcription of DNA is catalyzed by the enzyme RNA polymerase (promotes the synthesis of mRNA). * Newly formed mRNA is a transcript (a copy) of the information contained in the gene. * mRNA travels from the nucleus to the ribosomes where the information in the copy is used to synthesize a protein by means of translation. * **Protein synthesis needs amino acids and specialized transport molecules, called transfer RNA (tRNA) which carries the amino acids to the ribosomes.** * **Transcription involves the synthesis of mRNA, tRNA, and rRNA (RNA component of ribosome) based on the nucleotide sequence in DNA** * **Translation** * Synthesis of protein using the information provided by the sequence of codons along the mRNA strand. * During translation, the sequence of codons determines the sequence of amino acids in the protein. * In addition to mRNA, translation requires ribosomes (consist of rRNA and proteins) and tRNA. * Begins when the newly synthesized mRNA leaves the nucleus and binds with ribosome in the cytoplasm. Molecules of tRNA deliver amino acids that will be used by the ribosome to assemble protein. * There are more than 20 different types of tRNA, at least one for each amino acid used in protein synthesis. Each tRNA molecule contains a complementary triplet of nitrogenous bases called anticodon, that will bind to a specific codon on the mRNA. #### Protein Synthesis Overview: 1. DNA contains the information necessary to produce proteins. 2. Transcription of one DNA strand results in mRNA, which is a copy of the information in the DNA strand needed to make a protein. 3. The mRNA leaves the nucleus and goes to ribosome. 4. Amino acids, the building blocks of proteins, are carried to the ribosome by tRNAs. 5. In the process of translation, the information contained in mRNA is used to determine the number, kinds and arrangement of amino acids in the protein. ## Cell Division * During the time between fertilization and physical maturity, the number of cells making up an individual increases from a single cell to roughly 75-100 trillion cells. This amazing increase in numbers occurs through a form of cellular reproduction called cell division. Even when development has been completed, cell division continues to be essential to survival because it replaces old and damaged cells. * Process whereby cells reproduce themselves. * The formation of two daughter