Cell Biology 1010- Chapter 10

Plasmodesmata Structure

Open channels where the cell membrane of one cell is continuous with adjacent cell membrane. Have a desmotubule connecting the smooth ER membrane of adjacent cells. Plasmodesmatae can change the size of their opening between closed, open and dilated cells.

What is pectins role in the middle lamella?

Pectins are negatively charged so they attract water and make a hydrated gel. Ca2+ and Mg2+ interact with the negative charges in the pectins and cement the cell walls of the adjacent cells together.

Cell Wall

Protective ECM outside the plasma membrane. Usually stronger, thicker and more rigid that animal ECM. Provide rigidity for mechanical support and also play a role in the maintenance of cell shape and direction of cell growth. Also prevents osmotic lysis. Composed of a primary and secondary cell wall.

What are the two macromolecules that mainly form the ECM?

Proteins and polysaccharides (carbohydrates) NEVER FAT Proteins form large fibers Polysaccharides give the ECM a gel-like character.

Elastin

Provides elasticity Needed in ares that expand and return to their original shape such as the lungs or the blood vessels. A protein in the ECM that is composed of elastic fibers. Form many covalent crosslinks to make a fiber with remarkable elastic properties. Covalent crosslinks stretch, but don't break apart and hold proteins together. Behaves like a rubber band, stretching under tension and snapping back when the tension is released.

Collagen

Provides high tensile strength to many parts of an animal's body. Main protein found in bone, cartilage, tendons, skin and the lining of blood vessels and internal organs.

Middle Lamella and Fruit Ripening

Ripening of fruit secret pectinases such that ripe fruit is less firm than unripe fruit. Pectin gives firm nature in unripe plants. Ethylene induces the secretion of pectinases, which breakdon pectin in the middle lamella and the primary cell wall, and the fruit becomes tender.

Multicellular

Single organism composed of more than one cell. Can attain amazing sizes and most likely came into being approximately 1 billion years ago. Plants and animals are always multicellular.

Gap Junctions

Small gap between plasma membranes of cells at junctions. Abundant in tissues and organs where the cells need to communicate with each other. Example: cardiac muscle cells are interconnected by many gap junctions.

What is one important component to being multicellular?

Surface area to cell size ratio. The smaller the cell, the better ratio it will have. This ratio is important especially in cell communication.

Secondary Cell Wall

Synthesized only once the cell stops growing. Deposited between plasma membrane and primary cell wall. Not flexible.

Tight Junctions and Rodenticides

Target the tight junctions, meaning that blood can ooze between the cells. Dried up blood is visible in the nose or under the tail of the dead rodent.

Functions of the ECM Strength

The ECM is the tough stuff that prevents tearing in the skin of animals. Can protect the soft parts of the body, such as the internal organs. Carbohydrate part of the ECM is responsible for strength.

What is the function of gap junctions?

The allow adjacent cells to share metabolites and directly signal each other. They form gaps, the only other protein that does this is cadherin.

Primary Cell Wall Composition

1. Cellulose: Beta-glucose polymers that associate with each other via hydrogen bonding to form microfibrils that provide great tensile strength. 2. Hemicellulose: NO glucose, made of sugars other than glucose. 3. Glycans: branched polysaccharides with a structural and organizational component. 4. Pectins: charged (most of the time negatively) that attract water and have a gel-like character that provides the cell wall with the ability to resist compression.

Assembly of Collagen

1. Procollagen peptides (called α fibrils) are synthesized into the rough ER lumen via cotranslation protein sorting. 2. Three α fibrils assemble into a triple helix called procollagen. 3. Extension sequences are on either end of the procollagen, these prevent further extension of procollagen. 4. Procollagen with extension sequences is secreted from the cell into the ECM. 5. The extension sequences are removed and the protein is now a collagen. 6. The collagen can connect with other collagens to form a collagen fibril. 7. Multiple collagen fibrils come together to form a large collagen fibril.

Tensile Strength

A measure of how much stretching force a material can bear without tearing apart.

Plasmodesmata

A membrane-lined, ER-containing channel that connects the cytoplasm of adjacent plant cells. Allows the passage of ions, water and signaling molecules between the cytosol of adjacent plant cells. Functionally similar to gap junctions.

What is the main benefit of being multicellular?

The main benefit is the division of labor. Example: the intestinal cells of animals and root cells in plants have become specialized for nutrient uptake. Other types of cells perform different roles, such as reproduction.

Middle Lamella

Additional component of the ECM that is found between most adjacent plant cells. First layer to form when cells divide.

Adhesive Proteins

Adhere ECM components together and to the cell surface. Provide organization to the ECM and facilitate the attachment of cells to the ECM. Fibronectin and laminin

Cell Junctions

Adhere cells to each other and to the ECM. Animals cells have a more varied group of junctions because they lack the structural support provided by the cell wall.

What are the four main categories of anchoring junctions?

Adherins, desmosomes, local adhesions and hemidesmosomes.

Plasmodesmata Open

Allow the passage of ions and small molecules such as sugars and cAMP. Similar to gap junctions in this state.

CAMs and Cell Signaling

CAMs originally thought to be only mechanical, but recent evidence points to a role in cell signaling. When the connection between the linker protein and the intracellular domain is lost, the cell is either preparing to move, change shape or divide (migration, arrangement and division). The signal is lost, the inside of the cell then undergoes a conformation change and the connection on the outside of the cell is lost as well.

Cadherins

Ca2+ dependent adhering molecule. CAMS (cell adhesion molecules) that create cell-to-cell junctions. Never between cells of different origins, which establishes and tissue specific system.

Homophilic Dimerization

Cadherins of one cell of a particular tissue can only bind with cells of the same tissue. Example: one type of cadherin is called E-cadnerin, and another is N-cadherin. E-cadherin in one cell can only bind to E-cadherin of another cell. Plays a key role in the proper arrangement of the cell throughout the body, particularly during embryonic development. By expressing only certain types of cadherins, each cell will only bind to other expressing the same type.

Plasmodesmata Dilated

Can dilate to allow the passage of macromolecules and even viruses between adjacent cells. Important for the passage of proteins and mRNA during plant development.

Gap Junction Composition

Composed of an integral membrane protein called connexin. Six connexin proteins in one cell align with six connexin proteins in an adjacent cell to form a connexon.

Connexon

Composed of six connexin proteins that align with six connexins in another cell. Connexon allows passage of ions and small molecules (amino acids). However, they are too small to allow the passage of RNA, proteins or polysaccharides. This means that cells that communicate via gap junctions still maintain their own distinctive set of macromolecues.

Integrin Protein Composition

Composed of two nonidentical subunits. Extracellular domain binds to fibronectin, which then binds to collagen. Intracellular domain binds to linker protein and then to intermediate or actin filaments within the cell.

Fibronectin

Connects cells to the ECM and helps to organize components in the ECM. Protein that attaches to proteins embedded in the plasma membrane and also binds with the fibers in the extracellular matrix.

Laminin

Connects cells to the ECM and helps to organize components in the basal lamina, a specialized ECM found next to epithelial cells. Alpha subunit connects epithelial cells to basal lamina. Beta and gamma subunits connect epithelial to fibers in the ECM.

Secondary Cell Wall Composition

Depends on the type of organ and type of plant, but will ALWAYS contain 1. Cellulose 2. Lignins 3. Most of the time, there are other polysaccharides also composing the secondary cell wall.

How do cadherins bind to each other?

Extracellular domains of two cadherin proteins, each in adjacent cells, bind to each other to promote cell-to-cell adhesion. Binding requires the presence of calcium ions, which change the conformation of the cadherin protein such that cadherins in adjacent cells bind to each other.

Primary Cell Wall

First to be synthesized, synthesized right after a cell is newly made. Very flexible, increases in size with cell.

Structural Proteins

Form large fibers that give the ECM its strength and elasticity. Collagen and elastin.

Gametes

Function in sexual reproduction. Sexual reproduction came about from multicellularity (meiosis)

Tight Junctions

Function is to prevent leakage between cells. Wraps so tightly that air can't even pass through. Forms a tight seal between adjacent cells. Means that substances can only diffuse through channels, they can't even go between cells.

What are the junction functions that the middle lamella performs?

Functions of both anchoring junctions and tight junctions done by the middle lamella.

Glycosaminoglycans

GAGS Long, unbranched polysaccharides connected by negatively charged disaccharides. Negative charge of disaccharides attracts ions and water, which leads to the glycosaminoglycans being very rigid, help to form cartilage.

Proteoglycans

Glycosaminoglycans of equal length that are connected to a core protein.

Integrins

Group of cell-surface receptor proteins that are a second type of CAMs that don't require Ca2+ to function.

Somatic Cells

In animals, devoted to the growth, development and survival of the organism.

How are plant cells organized in terms of cell junctions?

In plants, cellular organization is different because of the rigid cell wall. Plant cells are connected to each other by a component called the middle lamella, which cements their cell walls together. Also have junctions termed plasmodesmata that allow adjacent cells to communicate with each other.

Example of Tight Junctions

Intestine cells form a sheet that is one cell thick. One side faces the intestinal lumen, the other faces the ECM and blood vessels. Tight junctions between these cells prevent the leakage of materials from the lumen of the intestine into the blood and vice versa.

Larger ________ = larger ____________

Larger genomes mean larger proteomes Multicellular organisms produce a larger array of proteins than unicellular proteins do. These additional proteins are used for cell communication, arrangement and attachment of cells and cell specialization.

Plasmodesmata Closed

Whenever there is a large pressure difference between adjacent cells. Happens because of cell damage (when a cell is damaged there is a loss in turgor pressure)

What are the three types of cell junctions found in animal cells?

Anchoring junctions Tight junctions Gap junctions/ communication junctions

Anchoring Junctions

Attach cells to each other and to the ECM. Particularly common in parts of the body where the cells are tightly connected and form linings. Example: layer of cells that line the small intestine. Anchoring junctions keep intestinal cells tightly adhered to one another, thereby forming a strong barrier between the lumen of the intestine and the blood.

Functions of the ECM Organization/ Arrangement

Attachment of cells to the ECM plays a key role in the proper arrangement of cells throughout the body. Can bind body parts together, such as tendons to bones. Example: Embryology, cells migrate to different places to form different things.

What part of animals is composed mainly of the ECM?

Bone and cartilage of animals is composed largely of the ECM.

Functions of the ECM Structural Support

Bones of many animals composed of ECM. Skeletons not only provide structural support, but also facilitate movement.

Functions of the ECM Cell Signaling

One way that cells in multicellular organisms sense their environment is changes in the ECM. Some proteins that belong to the ECM can respond to signals.

How do cadherins bind to the cytoskeleton proteins?

Linker proteins on the interior of the cell connect cadherins to actin or intermediate filaments of the cytoskeleton. Promotes a more stable interaction.

Tight Junctions Composition

Made by occludin and claudin, form ropelike structures that encircle two plasma membranes. Occludin and claudin are in adjacent cells and bind to each other, forming a tight seal between cells. Not mechanically strong because they aren't bound to the cytoskeleton. Because of this tight junctions always have anchorage junctions nearby in order to hold the cells together.

What is the order of the synthesis of a plant ECM?

Middle lamella, primary cell wall and secondary cell wall.

How do the additional proteins of the proteome in multicellular organisms affect cell specialization?

Most important function of the additional proteins in multicellular organisms. Each type of cell makes specific types of proteins, which determine the function of the cell.

How do the additional proteins of the proteome in multicellular organisms affect the arrangement and attachment of cells?

Multiple cells need to be arranged in a complicated manner in multicellular organisms. These proteins can help direct the cells to the correct location.

Extracellular Matrix

Network of material produced by the cell and secreted from cells forming a complex meshwork outside of the cells. Plants are surrounded by a cell wall, while animal cells rely on the ECM.

Chitin

Nitrogen-containing polysaccharide. Forms the hard protective outer covering (exoskeleton) of insects and crustaceans.

How do the additional proteins of the proteome in multicellular organisms affect cell communication?

The proteins are found in many of the different types of receptors, such as the G-Protein Coupled Receptor, Ligand-Gated receptors and enzyme-linked receptors.

What is the main function of GAGS and proteoglycans?

The provide resistance to compression. Once secreted from cells, the form a gel-like component in the ECM. Due to its high water content, the ECM is difficult to compress and serves to protect cells. GAGs and proteoglycans are found in regions of the body that are subjected to harsh mechanical forces such as joints.

What part of plants are composed mainly of the cell wall?

The woody parts of plants are composed mainly of cell walls. Class of phenolics called lignins. The cell wall remains long after the cell has died, allowing plants to maintain their structure.