Leeza-Marie Williams

Endomembrane system:

The endomembrane system consists of a group of membrane bound organelles that are found in eukaryotes. These organelles work together to modify, package, and transport lipids and proteins. It is important to know that mitochondria, chloroplasts, and peroxisomes are not included in the endomembrane system.

The endoplasmic reticulum (ER) consists of a rough ER and a smooth ER which are important in the modification process of proteins and the synthesis of lipids. Structurally, the ER consists of membranous tubules that have a hollow space called lumen, and flattened sacks.

Rough ER have ribosomes attached to its cytoplasmic surface which makes protein chains that are sent into the lumen. The protein that is not transferred fully are anchored in the membrane. Proteins inside of the membrane fold and are modified with the addition of carbohydrate side chains. Modified proteins are incorporated into the membrane of the ER, other organelles, or secreted from the cell. Proteins that are sent out of the cell are first sent to the Golgi apparatus. Phospholipids that are manufactured in the rough ER are also transported out via vesicles. Smooth ER have several functions including synthesizing carbohydrates, lipids, and steroids, detoxification, and storing calcium.

As mentioned previously, proteins that bud off of the ER are sent to the Golgi apparatus where there are sorted, tagged, packaged, and distributed to the right location. The Golgi apparatus has a cis face where it receives protein and a trans face where protein is sent away from the apparatus. A fusion occurs between transport vesicles of the ER and the cis face of the Golgi apparatus where the contents within the transport vesicles are emptied into the lumen of the Golgi apparatus. Further modification may occur to proteins and lipids within the Golgi apparatus. For example, the addition or removal of short chained molecules of sugar or phosphate groups may occur. Modified proteins are sorted based on their amino acid sequences and chemical tags and are packaged into vesicles that bud away from the Golgi apparatus at its trans face. Some of the vesicles that budded off of the Golgi apparatus are sent to the lysosome or vacuole or fuse with the plasma membrane to secrete proteins outside of the cell.

The lysosome contains digestive enzymes that breaks down structures to reuse their molecules. Lysosomes also have the ability to digest foreign particles. For example, macrophages are a class of white blood cells that folds inward to engulf pathogens. Once the pathogen is contained, it forms a phagosome when it is pinched off from the plasma membrane. The phagosome and the lysosome fuse to form a compartment consisting of digestive enzymes that destroy the pathogen. Vacuoles are an alternative to lysosomes in plants which stores water, wastes, isolates pathogens, and breaks down macromolecules.

Mitochondria:

Mitochondrion are different from most other organelles because it has its own circular DNA, similar to the DNA of prokaryotes, and reproduces independently of the cell. Therefore, mitochondrion are endosymbiotic. Mitochondria convert oxygen and nutrients into adenosine triphosphate (ATP). ATP is the chemical energy “currency” of the cell that powers the cell’s metabolic activities. This process is called aerobic respiration and is the reason animals breathe oxygen. Without mitochondria, more complex animals, like humans, would likely not exist because their cells would only be able to obtain energy with the absence of oxygen or anaerobic respiration which is less efficient than aerobic respiration.

Most mitochondria are oblong organelles between 1 and 10 micrometers in length. Both the inner and outer mitochondrial membranes resemble the plasma membrane in molecular structure. Each are 60 to 70 Ǻ thick and are composed of two layers of phospholipid molecules in between two layers of protein molecules. The outer and the inner membranes contain specific channels for molecules to transport through them. The two membranes may be connected at adhesion sites. The outer and inner mitochondrial membranes are separated from each other by a narrow space called the intermembrane space.

The outer membrane is freely permeable to most small molecules. such as ions, ATP, ADP, and nutrient molecules because it consists of transmembrane channels formed by the protein, porin. The inner membrane is freely permeable only to oxygen, carbon dioxide, and water. Its structure is highly complex, including all of the complexes of the electron transport system, the ATP synthetase complex, and transport proteins. The wrinkles, or folds within the innermembrane are organized into layers called the cristae which divides the mitochondrion into compartments running perpendicular to the long axis of the rod shaped mitochondrion. The cristae greatly increase the total surface area of the inner membrane which provides enough space for the previously mentioned structures to fit inside of the mitochondria. The matrix contains enzymes that are responsible for the citric acid cycle reactions. The matrix also contains dissolved oxygen, water, carbon dioxide, the recyclable intermediates that serve as energy shuttles. An electron carrier is a molecule that accepts an electron from one molecule and transports it into another, this movement of electrons by carriers is known as the electron transport chain. Carriers can exist in either an oxidized or reduced form, in an oxidized form the carrier is available to receive additional electrons, in a reduced form the carrier is actively carrying electrons.

Chloroplasts:

The process of photosynthesis is conducted using chloroplasts which are used to convert light energy into chemical energy. Since chloroplasts are involved with storing energy and synthesizing metabolic materials, they are categorized as a type of plastid. Various types of plastids are required depending upon the amount of light surrounds the leaf as it grows. Leucoplasts are used to synthesize starch, oil, and protein. Chromoplasts that are yellow to red synthesize carotenoids. Green chloroplasts contain chlorophyll a and chlorophyll b which are involved with absorbing light energy required for the process of photosynthesis.

To describe the structure of the chloroplast, it is enclosed in a double membrane consisting of two layers within the membrane called the intermembrane space. In terms of permeability, the outer portion of the double membrane is more permeable than the inner portion of the membrane. The stroma comprises of most of the chloroplast volume by containing a semi fluid material consisting of enzymes. Like mitochondria, chloroplast have their own DNA, ribosomes, and RNA which are found in the stroma. More complex plants have internal membranes (lamellae) with stacks (granum) of closed hollow disks (thylakoids). Lumens or internal spaces allow for the connection of thylakoids. Within the thylakoids are embedded chlorophyll molecules. Light travels in photons and are absorbed by molecules of chlorophyll. Once absorbed, electrons are emitted and hydrogen ions are propelled around the thylakoid stack. When hydrogen ions surround the thylakoid stack, formation of an electrochemical gradient occurs and the stroma produces ATP. Light independent reactions that involve carbon fixation occur in the stroma whereby low energy carbon dioxide molecules transforms into glucose, a high energy compound.

Cytoskeleton:

Eukaryotes possess three types of protein fibers in their cytoskeleton: microfilaments, intermediate filaments, and microtubules. Microfilaments are the narrowest type of protein fibers with a diameter of 7nm consisting of actin filaments which are linked monomers of protein that resembles the structure of a DNA’s double helix. Within microfilaments are actin subunits. Actin filaments have two structurally different ends that allow them to have directionality. They provide direction when myosin or motor proteins are needed to move around in the cell. Actin filaments also function as a means to enable the cell to transport protein containing vesicles and organelles. Actin and myosin work together in muscle cells to form sarcomeres which are structures of overlapping filaments. When sarcomeres slide past each other, the muscles within our body contracts. Since actin filaments have the ability to assemble and disassemble quickly, it plays a vital role in cell movement.

Intermediate filaments consist of multiple wound strands of fibrous proteins and are the in between the size of microfilaments and microtubules with a diameter of 8 -10 nm. Intermediate filaments differ than actin filaments in that intermediate filaments are more permanent and are specialized to handle tension in order to maintain the shape of the cell. They also serve to anchor the nucleus and other organelles in a specific location.

Contrary to their name, microtubules have the largest diameter at 25nm of all three types of cytoskeleton fibers. Microtubules consist of an arrangement of straw like α-tubulin and β-tubulin subunits. Microtubules are similar to actin filaments in that they have the ability to grow and shrink whenever tubulin proteins are added or removed. Since both actin filaments and microtubules have directionality, they both resist compression forces due to possessing two ends that are structurally different from one another. Without microtubules, motor proteins like kinesins and dyneins would not be able to transport vesicles within the cell. Microtubules are also important in cell division whereby microtubules assemble into spindles that pulls the chromosomes apart.

Protein trafficking:

Membrane bounded transport vesicles transport proteins embedded in the rough ER to the Golgi apparatus. The Golgi apparatus consists of cisternae which are flattened membrane sacs. Translation and modifications of proteins occurs in the rough ER. Modifications are completed in the Golgi apparatus as proteins pass through the complex to ensure that the proteins reach the correct final destination. Each region of the Golgi apparatus contains different enzymes depending on how it plans to modify proteins. As mentioned previously under the topic about the endomembrane system, proteins travel from the cis to the trans regions of the Golgi apparatus where they are then released to the lysosomes or vacuoles or outside of the cell.

Cell signaling:

There are four general categories of chemical signaling: paracrine signaling, autocrine signaling, endocrine signaling, and signaling by direct contact. The main difference between these types is the distance that the signal travels through the organism to reach the target cell.

Paracrine signaling occurs between cells when they are relatively close to one another. In paracrine signaling the cells communicate through releasing ligands that can diffuse through the space between the cells. Paracrine is critical to the process of development and to synaptic signaling of nerve cells transmitting signals.

Autocrine signaling involves a cell releasing a ligand that binds to the receptors on its own surface or inside of the cell. It is beneficial for enabling cells to reinforce their correct identities during development. Autocrine signaling is also beneficial to the spread of cancer from its original site to other parts of the body, metastasis.

The process of signals that are produced by specialized cells and released into the bloodstream where they are carried to target cells is called endocrine signaling. Endocrine signaling transmit signals over long distances. Hormones are signals that are produced in one part of the body and are carried to other targets via circulation.

Signaling by direct contact occurs when two cells bind to each other because they have complementary proteins on their surfaces. Once bound, the interaction causes the shape of one or both to change which transmits a signal. The immune system is heavily dependent upon this type of signaling whereby the immune cells use protein markers to recognize the body’s own cells from pathogens.

Endosymbiotic theory:

With regards to mitochondria, a hypothesis proposed among the scientific community, states that millions of years ago small, free-living prokaryotes were engulfed by larger prokaryotes because they were able to resist the digestive enzymes of the host organism. The two organisms developed a symbiotic relationship over time. The larger organism providing the smaller with ample nutrients and the smaller organism providing ATP molecules to the larger one. Through further evolution, the larger organism developed into the eukaryotic cell and the smaller organism developed into the mitochondrion.

Works Cited:

Caprette, David. “Structure of Mitochondria.” Rice University, 26 May    2005. Web. 9 Feb. 2017.

<http://www.ruf.rice.edu/~bioslabs/studies/mitochondria/mitotheory.html>.

Davidson, M. (2000, October 1). Chloroplasts. Retrieved April 22, 2017, from

https://micro.magnet.fsu.edu/cells/chloroplasts/chloroplasts.html

Davidson, Michael. “Mitochondria.” Molecular Expressions Cell Biology and Microscopy Structure and Function of Cells and Viruses. Florida State University, 1 Oct. 2000. Web. 9 Feb. 2017. <https://micro.magnet.fsu.edu/cells/mitochondria/mitochondria.html>.

Introduction to cell signaling. (n.d.). Retrieved April 22, 2017, from

https://www.khanacademy.org/science/biology/cell-signaling/mechanisms-of-cell-signaling/a/introduction-to-cell-signaling

McClean, P., Ph.D., & Johnson, C. (n.d.). Retrieved April 22, 2017, from

http://vcell.ndsu.nodak.edu/animations/proteintrafficking/transport_02.htm

“Structure Mitochondria.” Tutor Vista, n.d. Web. 9 Feb. 2017.           <http://www.tutorvista.com/biology/structure-mitochondria>.

The cytoskeleton. (n.d.). Retrieved April 22, 2017, from

https://www.khanacademy.org/science/biology/structure-of-a-cell/tour-of-organelles/a/the-cytoskeleton

The endomembrane system. (n.d.). Retrieved April 22, 2017, from

https://www.khanacademy.org/science/biology/structure-of-a-cell/tour-of-organelles/a/the-endomembrane-system

Leeza-Marie Williams

Membrane as a dynamic structure:

The general structure of all biological membranes consists of noncovalent bonds that hold lipid and protein molecules together. The dynamic structure of cell membranes enables molecules to move within the membrane. A cluster of lipid molecules packed together forms a lipid bilayer which functions as an impermeable barrier to water soluble or hydrophilic molecules. Protein molecules are responsible for most of the functions within the membrane. Because of protein molecules, specific molecules can travel across and catalyzing membrane associated reactions like ATP synthesis are mediated. Protein molecules also serve as a means of providing structure towards connecting the cytoskeleton through the lipid bilayer to the extracellular matrix or to an adjacent cell. Some protein molecules act as receptors in detecting and transducing chemical signals in the cell’s environment.

Function of membrane proteins:

              Each membrane contains a set of membrane proteins that enables the membrane to carry out specific activities. Proteins are bound to different locations such as the membrane surface or the domains of one or both sides of the membrane. Proteins that are bound to the extracellular membrane are involved in cell to cell signaling and interactions. Proteins that are bound within the membrane form channels and pores to allow for the movement of molecules across the membrane.

              There are two general categories of membrane proteins based on the behavior of membrane-protein interactions: integral (intrinsic) and peripheral (extrinsic). Through embedding in the phospholipid bilayer, integral proteins contain hydrophobic residues that interact with fatty acyl groups located within membrane phospholipids which allows the protein to bind to the membrane. Since most integral proteins take up most of the space within the phospholipid bilayer, they are called transmembrane proteins. These transmembrane proteins consist of membrane spanning domains called α helices or multiple β strands. Some integral proteins have covalently bonded fatty acids that are anchored to a portion of the membrane however the polypeptide chain does not enter the phospholipid bilayer.

              Peripheral membrane proteins which are also known as extrinsic proteins are indirectly bound to the membrane via interactions with lipid polar head groups or with integral membrane proteins. Therefore, extrinsic proteins do not interact with the hydrophobic core of the phospholipid bilayer. Peripheral membrane proteins located in the region where the cytosol and the plasma membrane meet contain the protein kinase C which plays a role in signal transduction by moving to and from the cytosol and the cytosolic face of the plasma membrane. Other proteins are found on the outer surface of the plasma membrane.

Phospholipid bilayers:

              Phospholipids are the most abundant type of lipids which consists of a hydrophilic, polar head group and two hydrophobic, fatty acid tails that vary in length from between 14 to 24 carbon atoms. One tail contains unsaturated or cis- bonds while the other tail is saturated. Double bonds create kinks in the tail which alters the length of the tail. Since phospholipids contain a polar and a nonpolar end, there are amphipathic. Because of their amphipathic structure, when placed into water, phospholipids spontaneously arrange themselves where the hydrophilic end faces the water and the hydrophobic end is tucked into the lipid bilayer in the shape of micelles or bilayers. Micelles occur when all of the fatty acid tails face one another and form a spherical shape while bilayers occur when all of the fatty acid tails face one another and form a raft-like shape.

The amphipathic and shape of the phospholipid as well as temperature determines the fluidity of a lipid bilayer. A shorter fatty acid chain length reduces the tendency of hydrocarbon tails which means the hydrocarbon to interact whereby the formation of kinks occurs due to cis- double bonds. Therefore, at lower temperatures the membrane remains fluid. Some organisms like bacteria and yeasts maintain their fatty acid structures when the temperature fluctuates. In reference to a decrease in temperature, these organisms increase the synthesis of cis-double bonds to avoid a decrease in bilayer fluidity. Since the lipid bilayer is not 100% fat, cholesterol also impacts the fluidity of the membrane by immobilizing regions of hydrocarbon chains that are closer to the hydrophilic end of the phospholipid. Therefore, an increase in cholesterol results in a decrease in fluidity of the phospholipid.

Passive transport:

              Passive transport is the most basic form of transporting molecules in and out of the cell by using a process called diffusion. Diffusion is the random movement of molecules across a membrane. Since passive transport does not require the expenditure of energy by the cell, substances diffuse from an area of high concentration to an area of low concentration across the membrane. Diffusion results in the net movement of molecules from one region to the other in presence of a concentration gradient. Small molecules like oxygen and carbon dioxide move easily across the plasma membrane through the process of diffusion. Hydrophobic substances such as triacylglycerols do not encounter problems when trying to diffuse in and out of a membrane. However, some hydrophilic molecules cannot move passively from regions of higher concentration to regions of lower concentration. Therefore, these molecules must protein transporters through the process of facilitated diffusion. The difference between facilitated diffusion and simple diffusion is that diffusion only uses the lipid bilayer to move molecules while facilitated diffusion uses a membrane transporter and bypasses the lipid bilayer.

              There are two types of membrane transporters: a channel and a carrier. Channel membrane transporters allow molecules of a specific shape and charge to pass through an opening similar to a tunnel. Gated membrane channels require a chemical or an electrical signal in order to respond and allow molecules in and out of the membrane. Carrier membrane transporters bind to specific molecules and then transports them across the membrane. When molecules bind to the carrier membrane, a conformational change occurs in the membrane protein whereby it is determined if a molecule can be transported across the lipid bilayer. One membrane carrier is open to movement of molecules on one side of the cell and the other membrane carrier located on the opposite side performs a similar function.

              Water molecules also are allowed to pass through by using simple diffusion the lipid bilayer because of their small size and because of aquaporins which are protein channels specifically geared towards transporting water molecules through facilitated diffusion. Solvents like water that move across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration do so through the process of osmosis. Movement of water occurs due to a force such as gravity or pressure on a cell wall.

Active transport:

              In instances where molecules cannot diffuse through the membrane from regions of low concentration to regions of high concentration, the cell must expend energy in the form of ATP in order to move these molecules through the process called active transport. Naturally, glucose moves from inside of the cell to outside of the cell, therefore the cell must use active transport in order to bring in more glucose molecules from outside of the membrane. Substances are transported in and out of the cell using proteins that are embedded in the cell membrane act as pumps in moving substances against the concentration gradient. For example, as in the case of the sodium-potassium pump, the sodium concentration inside of the cell is lower than outside of the cell, however, the potassium concentration inside of the cell is higher than outside of the cell. Both sodium and potassium must use primary active transport in order to move against the concentration gradient. Primary active transport directly uses a source of chemical energy to move molecules against the concentration gradient. Sodium is pumped out of the cell while potassium is pumped into the cell. Since the protein that is used to move sodium and potassium in opposite directions, the protein transporter is referred to as an antiporter. Likewise, protein transporters that move two molecules in the same direction are called symporters or co transporters.

              Through the process of secondary active transport, an electrochemical gradient is used to move molecules against the concentration gradient without the use of ATP, a chemical source of energy. The electrochemical gradient is formed when there is an increase in the concentration of small ions on one side of the membrane. The build up of potential energy can be used to move molecules across the membrane against the concentration gradient. When protons are moved across the membrane, a difference in charge occurs which causes the formation of an electrical gradient. The protons move from regions of common positive charge to regions of opposite or negative charge. An electrochemical gradient, as its name suggests, forms when a gradient has both charged and chemical aspects.

              To summarize, the difference between primary active transport and secondary active transport is that primary active transport uses chemical energy of ATP to move molecules while secondary active transport uses the build up of potential energy in order to power an electrochemical gradient that enables for the movement of molecules.

Bulk transport:

              When macromolecules like proteins or polysaccharides are moved across the plasma membrane in a process called bulk transport occurs. Bulk transportation has two forms, that both require the expenditure of ATP: exocytosis and endocytosis. During exocytosis, secretory vesicles formed by the Golgi apparatus are used to excrete molecules out of the cell by fusing to the unneeded molecules and transporting them to the plasma membrane. Once at the plasma membrane, the molecules are released into the extracellular space. Endocytosis is the process in which molecules outside of the cell are encapsulated by vesicle made out of a plasma membrane that folds inwards. Using specialized proteins, the pocket then pulls apart from the membrane and forms a vesicle in the cell.

The three types of endocytosis are as followed: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Nicknamed “cellular eating,” phagocytosis involves having the cell membrane pocket the macromolecule to form a phagosome, a food vacuole. Pinocytosis is nicknamed “cellular drinking” because the cell plasma membrane engulfs small amounts of extracellular fluid. Receptor-mediated endocytosis occurs when receptors proteins located on the cell’s surface cluster in coated pits which are regions of the plasma membrane capture specific target molecules that otherwise would not be captured via phagocytosis nor pinocytosis.

Works Cited:

Active transport. (n.d.). Retrieved April 10, 2017, from

https://www.khanacademy.org/science/biology/membranes-and-transport/active-transport/a/active-transport

Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland

Science; 2002. Chapter 10, Membrane Structure. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21055/

Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland

Science; 2002. The Lipid Bilayer. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26871/

Bulk transport. (n.d.). Retrieved April 10, 2017, from

https://www.khanacademy.org/science/biology/membranes-and-transport/bulk-transport/a/bulk-transport

Diffusion and passive transport. (n.d.). Retrieved April 10, 2017, from

https://www.khanacademy.org/science/biology/membranes-and-transport/passive-transport/a/diffusion-and-passive-transport

Endocytosis and Exocytosis. (n.d.). Retrieved April 10, 2017, from

https://www.wyzant.com/resources/lessons/science/biology/endocytosis-and-exocytosis

Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

Section 3.4, Membrane Proteins. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21570/

Leeza-Marie Williams

Learning Summation

What is biology?

According to the Norwegian University of Science and Technology, the word biology is derived from the Greek words bios and logos which means life and study, respectively. Simply, biology can be defined as the science of life and living organisms. An organism is a living entity consisting of one cell like bacteria, or several cells such as animals, plants and fungi.

While the definition of biology appears straightforward, biological science can range from the study of molecular mechanisms in cells, to the classification and behavior of organisms, and to how species evolve and interact between ecosystems.

Furthermore, Biology often overlaps with other sciences, for example, biochemistry and toxicology with biology, chemistry, medicine, and even astronomy, to name a few. Biology also interacts with social sciences with regards to the administration of biological resources, developmental biology, biogeography, evolutionary psychology and ethics.

Properties of water

As stated by the website Owlcation, water consists of five properties which are as followed: an attraction to polar molecules, a high-specific heat, a high heat of vaporization, a lower density of ice, and a high polarity.

Firstly, waters ability to attract to polar molecules can be attributed to cohesion and adhesion. Cohesion refers to water’s attraction to other water molecules, whereby, the hydrogen bonds in water hold other water molecules together. Because of water’s cohesiveness, water in its liquid state has surface tension which allows for insects, such as Water Striders, to walk on water. Furthermore, water’s cohesiveness enables it to maintain its liquid state instead of a gas state at moderate temperatures. Adhesion is water’s attraction between molecules of a different substance in which it is able to form hydrogen bonds. Due to water’s adhesiveness, capillary action occurs.

Secondly, high-specific heat is the amount of energy that is absorbed or lost by one gram of a substance to change the temperature by 1 degree Celsius. Since water molecules form many hydrogen bonds between one another, plenty of energy is needed to break down those bonds. Breaking the bonds allows individual water molecules to move freely about and have a higher temperature. If there are many individual water molecules moving about, then, they will create more friction and more heat, which means a higher temperature. The hydrogen bonds between water molecules absorb the heat when they break and release heat when they form, which minimizes temperature changes. Water helps maintain a moderate temperature of organisms and environments.

Thirdly, water’s high heat of vaporization is the other property responsible for its ability to moderate temperature. It refers to the amount of heat energy needed to change a gram of liquid into gas. Just like the properties of having a high specific heat, water also needs an ample amount of energy in order to break down the hydrogen bonds which causes a cooling effect.

Fourthly, when observing water’s density at cooler temperatures, the hydrogen bonds of water molecules form ice crystals because they are more stable and will maintain its crystal-like shape. Ice is less dense than water because of the hydrogen bonds being spaced out and being relatively apart. The low density is what allows icebergs to float and is the reason why only the top part of lakes are frozen.

Fifthly, water is a polar molecule that has a high level of polarity and attraction to ions and other polar molecules. As we already know, water can form hydrogen bonds, which make it a powerful solvent. Water molecules are attracted to other molecules that contain a full charge, like an ion, a partial charge, or polar. Salt is a polar compound that dissolves in water. Water molecules surround the salt molecules and separate sodium from the chloride by forming hydration shells around those two individual ions.

Works Cited:

“What Is Biology at NTNU?” Norwegian University of Science and Technology, n.d. Web. 25 Feb. 2017.

<https://www.ntnu.edu/biology/about-us/what-is-biology>.

“5 Properties of Water.” Owlcation, 13 June 2016. Web. 25 Feb. 2017. <https://owlcation.com/stem/5-

Properties-of-Water>.