Cell Communication

Key Points:

  • the membrane is selectively permeable

Transport Across Membranes

  • Factors that determine whether a molecule can cross the plasma membrane

    • Size—smaller molecules pass easier

    • Attraction to water: hydrophobic molecules cross easier than hydrophilic ones—-hydrophilic ones need help from proteins to cross the membrane

  • Overall, most molecules are too big to pass through without help of a protein (CO2 actually can, though)

    • Even tiny ions like K+, Na+, and Cl- are too attracted to water to pass by

  • If more of a molecule is on one side the membrane than another, the molecules will move across the membrane until they’re evenly distributed across the membrane

    • Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration—passive process with no energy required

      • Happens because random distribution of molecules is more energetically favorable than an organized distribution. If they are free to move, molecules will bounce off each other until they reach reach a random distribution called equilibrium

Crossing the Border

  • if a molecule can cross a plasma membrane, will diffuse across the membrane until it reaches equilibrium. Can happen with or without help of protein

    • Simple diffusion happens when molecules diffuse across membranes without any help from transport proteins

    • Facilitated diffusion occurs when molecules diffuse across membranes with the help of transport proteins

  • Transport proteins

    • Channel proteins are built like soda straws. The polypeptide chains of these proteins loop around to build the walls of the “straw”—creates tunnel. When channel proteins insert into a membrane, they form fluid-filled tunnels that pass through the membrane. Small hydrophilic molecules can pass through. Some channel proteins are open at all times, some open and close in response to signals

    • Carrier proteins transport specific molecules across membranes. Each carrier protein has a binding site that is the right shape for the molecule that it can transport. When the right molecule binds to the protein, the protein changes shape and moves the molecule to the other side of the membrane

Water Diffusion

  • Diffusion of water called osmosis —-water moves from more to less concentrated area. Passive process.

  • This process continues until there is an equal concentration of water on both sides of the membrane, resulting in osmotic equilibrium.

    • Water is more concentrated where water is most pure—least solutes

  • Relative solute concentrations that determine movement of water:

    • Hypertonic: solutions have a great concentration of solutes

    • Hypotonic: solutions have lesser amount of solutes

    • Isotonic: solutions have the same concentration of solutes

*why salt is used as a preservative—water left bacteria cells when surrounded by hypertonic solution

*Osmosis affects people who suffer with diabetes.

Active transport

  • Cells need molecules to move opposite to their concentration gradient. They need cells to travel from less concentrated areas to more concentrated areas, which is essential for maintaining cellular functions and homeostasis.

  • This is called active transport

    • During active transport, carrier proteins pick up the molecule to be transported on one side of the membrane and, with the help of ATP, change shape in order to move the molecule to the other side of the membrane. These proteins are called pumps.

Sodium Potassium Pump

  1. The sodium-potassium pump picks up three sodium ions (Na+) from the inside of the cell

  2. An ATP molecule is split, and one of its phosphate groups is attached to the sodium-potassium pump

  3. The sodium-potassium pump changes shape, releasing the three sodium ions (Na+) outside of the cell

  4. The sodium-potassium pump picks up two potassium ions from the outside of the cell

  5. The phosphate group from ATP is released from the sodium potassium pump

  6. The sodium potassium pump returns to its original shape, releasing two potassium ions (K+) to the inside of the cell

For every round of action, pump moves threes sodium ions out of the cell and two K ions in the cell. Higher concentration of sodium outside the cell, higher potassium ion inside the cell—-greater positive charge outside the cell. Differences in ion concentration and electrical charge are important in the functioning of the nerve and muscle cells in animals.

Cell Communication

Multicellular organisms are made of cells that have to communicate with each other for proper function

Two essential components

  • attachment of cells to form tissues. Cells are connected to each other through various types of cell-cell attachments

  • Communication between cells to coordinate responses to signals. Cells can communicate directly with neighboring cells or send signals over long distances to communicate with cells farther away in the body

Cell-cell attachments

cell connections change based on function

  • tight junctions: bring cells together so tightly that even water can’t pass between cells. Protein pass through the membranes of both cells, holding the cells as if they were sewn together. The proteins are arranged in lines so that continuous bands of attachment are formed like seams of stiches through the tissue. Tight junctions are important in surface tissues like skin and mucous membranes where they can create an effective barrier to molecules and foreign organisms that would otherwise slip between the cells.

  • anchoring junctions: hold cells together tightly, but allow materials to move through the intercellular space. They give structure and strength to tissues. Anchoring junctions are very important in tissues that do a lot of work—for example, heart muscle cells. Three types of anchoring junctions use different proteins to make connections between cells, the cytoskeleton, and the extracellular matrix

    • Desmosomes anchor cells to each other by attachments between proteins called cadherins in the membranes of both cells. The cadherins also connect with intermediate filaments in the cytoplasm of the cells

    • Hemidesmosome anchor cells to the extracellular matrix. Proteins called integrins in the membranes of cells attach to extracellular matrix proteins toward the outside of the cell and to intermediate filaments in the inside of the cell

      • fibronectins outside of the cell

      • actin microfilaments inside cell (part of cytoskeleton)

    • Adherens junctions anchor cells to each other and to the ECM. Adherens junctions connect cells together by attachment of cadherin proteins in the membranes. The cadherin proteins also attach actin microfilaments in the cytoplasm of the cells. The actin microfilaments may form bands around the cells that help with the movement of tissue. Also form between cells and the extracellular matrix—in this case, integrins in the plasma membranes of cells form connections to proteins in the extracellular matrix

    • Communicating junctions

      • Allow direct passage of chemical/electrical signals from cell to cell

      • In most animal cells, cells communicate rapidly with each other by gap junctions, a narrow gap between two cells connected by channel proteins called connexins

        • allow small ions and small molecules to move directly from the cytoplasm of one cell to the next

        • in response to signals, the protein rings of the connexins can pull together, closing the opening to the tunnel and blocking the movements of materials

        • critical role in rapid nerve cell communication

Plant cells

  • plant cells stick together because of cell walls. First part of plant cell to be made is the middle lamella, a layer of sticky molecules that holds adjacent plant cells together

    • after this made, long cellulose molecules are embedded into sticky molecules to add strength and create the primary wall

    • in woody plants, a secondary wall containing molecule lignin is also added

  • Plant cell tunnels

    • tunnels of cytoplasm called plasmodesmata function similarly to gap junctions in animal cells

    • have been observed to exchange small organelles

Sending and Receiving Signals

  • signals that travel over a distance to reach their target are called hormones.

    • ex: insulin, which tells cells to take glucose out of blood to use for energy

  • For a cell to receive signals, must have receptors to recognize and change in response to it.

    • receptors for signaling molecules that can cross the plasma membrane, such as steroid hormones, are located inside of cells

    • The receptors for signaling molecules that can’t cross the plasma membrane are located within the plasma membrane of the cell

Receptors

Signal Transduction

Receptor proteins in plasma membranes bind to ligands on the outside of the cell and then cause a change in behavior on the inside of the cell. Because passed across membrane, called signal transduction

  1. The signal is received when the ligand binds to the receptor. The ligand is the signaling molecule, such as a hormone. Because the ligand is the original signal to the cell, it is called a primary messenger

  2. The signal is transduced when the receptor changes shape and becomes ready to cause a change on the inside of the cell. The binding of the ligand to the receptor changes the shape of the receptor protein, causing new binding sites on the receptor to become available.

  3. The signal is amplified when the receptor causes a change inside the cell that activates molecules called second messengers. Second messenger are molecules that increase in concentration inside the cell and spread the original signal around the cell. Once receptor can trigger the production of many second messenger molecules, which increase strength of signal within the cell

  4. The cell responds when second messengers cause changes in cell behavior. Second messengers may change cellular responses by activating or inhibiting proteins, such as enzymes, or by causing new proteins to be made through gene expression.

Amplifying the signal

Primary messengers are made in very low quantities in the body. Amplified by signal transduction pathway.

Principals of signal transduction pathways

  • Proteins can be activated or inactivated when molecules bind to them. A protein’s shape is essential to its function. When something binds to a protein, its shape will change—function may change from active to inactive

  • Adenosine triphosphate and guanine triphosphate are often used as a source of phosphate groups. Energy molecules that have three phosphate groups as a part of their structure, can remove phosphate group from then by hydrolysis and transfer the phosphate to another molecule

  • Transfer of phosphate group to a molecule is called phosphorylation. Enzymes called kinases transfer phosphate from ATP/GTP to other molecules

Enzyme Linked Receptors

  • A type of signal transduction pathways—enzyme linked receptors—are receptor proteins that have the ability to catalyze reactions inside the cell

  • One well studied group of these is the receptor tyrosine kinases (RTKs). *these are receptors that can phosphorylate a molecule

    • Signal Transduction for RTK

    • Step one: primary messenger binds to receptor. The binding of the primary messenger causes the receptor to change shape

    • Step Two: the receptor (RTK) is phosphorylated and becomes an active enzyme: after the receptor’s shape changes, it has a binding site for a phosphate group. After phosphate group in bound, enzymatic ability of the receptor is activated

    • Step Three: The receptor (RTK) activates another membrane associated protein called Ras. When Ras is activated, it binds a molecule of GTP.

    • Step 4: Ras kicks off phosphorylation cascade: activates 1st protein in the cascade by transferring a phosphate from its GTP to the protein. Activated protein catalyzes the phosphorylation of second protein, and that activates the third, and so on. Each activated protein in the pathway activates many copies of the next protein in the pathway which amplifies signal from the primary messenger

      • one proteins eventually causes change in cell behavior

G Proteins

G proteins: bind GTP. They wait near the membrane right next to receptor proteins they work with.

  1. The primary messenger binds the receptor. The binding of the primary messenger causes the protein to change shape

  2. The receptor activates its G protein. The G protein binds to a GTP molecule and splits in half

  3. Part of the G protein travels along the membrane and binds to an enzyme. The enzyme becomes activated by the binding of the G protein

  4. The enzyme catalyzes the production of second messenger molecules. The second messengers travel through the cell and trigger change in cell behavior

Deactivating the Signals

  • phosphatases: enzymes that remove phosphates from molecules. When phosphate removed from molecules in signal transduction pathways, the molecules are inactivated

  • Activated G proteins and Ras proteins convert GTP to GDP and deactivate themselves. Once inactivated, no longer activate the enzyme that produces second messengers

  • Second messenger molecules are short lived in cell. Inactivated by number of cellular mechanics