Chapter 5: Membrane Structure and Function

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What are the three main receptors in reference to opioids and what do they control? What receptor does morphine act on?

Mu, delta, and kappa are responsible for enabling opioids to do their job.

Mu: molecular switches that trigger the brain reward system

Delta: play a role in determining the person’s emotional state

Kappa: associated with pain relief

Morphine acts on the mu receptor and has the potential for abuse and addiction.

What are membranes?

They are made of a phospholipid bilayer and they create a division between the cell and the external environment and the compartments within the cell. 

What is the purpose of having separate compartments? What is the name for this?

It allows multiple and incompatible chemical processes to occur at the same time. 

• “Division of labor” allows cells to operate more efficiently and respond to changes in the environment

What else do membrane components include?

• They have protein molecules, either wholly or partially embedded in the bilayer.

• Cholesterol, found in animal plasma membranes. Helps with modifying the fluidity of the membrane over a range of temperatures.

• Steroids found in plants.

What are phospholipids considered and why? What does this explain?

Amphipathic molecules because they have a hydrophobic tail and a hydrophilic head. This explains why they form a bilayer in water.

What can be used to study the nature of membrane proteins? What is the research method associated with this?

Electron micrographs. 

• Freeze-fracture. Freezes first and then splits the membrane so up and low separate. Proteins are still there and go with one of the layers.

What are integral proteins and peripheral proteins? What is a characteristic about integral proteins?

Integral Proteins: Embedded in the plasma membrane

• Some are on only one surface of the bilayer

• Hydrophilic ends are on both surfaces, interacting with polar 

Peripheral proteins: proteins that are only on the cytoplasmic side

What are integral proteins held in place by?

By attachments to protein fibers of the cytoskeleton (inside)

Fibers of the extracellular matrix (ECM) (outside)

Which organism(s) cells only has ECM? What does ECM have?

Animal cells. It has various protein fibers and complex and big carbohydrate molecules.

What is the fluid-mosaic model? What is a characteristic of membrane structures?

Fluid-mosaic model: Model for the plasma membrane based on the changing location and pattern of protein molecules in a fluid phospholipid bilayer.

Membranes are not rigid but rather flexible. They have molecules, phospholipids, cholesterol, and proteins.

What are the functions of the lipid, cholesterol, and plasma membrane contents?

Lipid: responsible for fluidity. 

Cholesterol: prevents the plasma membrane from becoming too fluid at high temps and too solid at low temps. 

Plasma Membrane: “mosaic” because it has many proteins.

Why are cells flexible?

A phospholipid bilayer is fluid.

At body temperature, the phospholipid bilayer’s consistency is…

Like olive oil.

What makes the bilayer more fluid?

The greater concentration of unsaturated fatty acid residues.

Membrane proteins are associated with…

ECM or cytoskeleton, or both.

They hold the protein in place and the fluid phospholipid bilayer.

Are membranes identical?

No. Carb chains are only attached to molecules on the outside and peripheral proteins are on one surface on the other. Membranes are asymmetrical. 

What are glycolipids and glycoproteins?

Glycolipids: phospholipids that have attached carbohydrate (sugar) chains.

Glycoproteins: proteins that have attached carbohydrate (sugar) chains. 

What is the basis for blood types A, B, and O?

Carbohydrates

What is glycocalyx?

In animal cells, carb chains are attached to proteins and give the cell a sugar coat. The glycocalyx protects the cell and has cell-to-cell adhesion, reception of signaling molecules, and cell-to-cell recognition. 

Functions of Proteins?

Channel: Passing molecules through the membrane. 

Carrier: passing molecules through the membrane. Transports sodium and potassium ions across the plasma membrane of a nerve cell. Important for nerve impulse conduction. 

Cell recognition proteins: glycoproteins. Help the body recognize when it is being swarmed by pathogens so an immune response can happen. 

Receptor: a shape that allows only specific molecules to bind to it. It causes it to change shape and bring a cellular response. 

Enzymatic: carry out metabolic reactions directly. Without these, a cell would never be able to perform the chemical reactions needed to maintain its metabolism.

Junction: junction between animal cells. Cilia of cells that line the respiratory tract to beat in unison. 

What is the purpose of the plasma membrane?

Regulates what comes in and out of the cell. 

What does "selective permeable" mean?

Allows only certain substances to come into the cell.

Molecules require how much energy to cross the membrane?

None

What is diffusion?

Movement of molecules or ions from a region of higher to lower concentration; it requires no energy and tends to lead to an equal distribution (equilibrium)

Energy needed for hydrophobic/nonpolar and hydrophilic/polar molecules cross/fuse to membranes?

Hydrophobic/nonpolar: no energy 

Hydrophilic/polar: Energy. Chemically incompatible

What kind of molecules can freely cross the membrane? Are they polar or nonpolar? What do they follow?

Carbon dioxide, oxygen, glycerol, and alcohol. They are nonpolar.

• They follow their concentration gradient, from high to low. 

What is a concentration gradient?

Model for the plasma membrane based on the changing location and pattern of protein molecules in a fluid phospholipid bilayer.

Concentration of oxygen vs carbon dioxide?

Oxygen: higher outside than inside.

Carbon dioxide: higher inside than outside. - moves from inside to outside.

What are aquaporins?

Allow water to cross a membrane more quickly than expected. Allows cells to equalize their water pressure differences between their inside and outside so the membranes do not burst. 

Ions and polar molecules and their passage through the membranes?

Examples: glucose and amino acids.

They move slowly 

To move quickly, they are assisted by carrier proteins. 

What does each carrier protein need?

They recognize the shapes of molecules and must combine with an ion (e.g., sodium Na⁺) or a molecule (glucose) before changing its shape and moving the molecule. They are specific.

What is bulk transport?

How large particles can exit or enter the cell

What is the difference between exocytosis and endocytosis? What is vesicle formation reserved for?

Exocytosis: fusion of a vesicle with the plasma membrane moves to a particle to outside the membrane. Golgi body often produces the vesicles that carry these cells 

Endocytosis: vesicle formation moves a particle inside the plasma membrane.

*Reserved for the movement of macromolecules or larger (ex: virus)

What are the three ways that endocytosis can occur?

Phagocytosis: when the material taken in is large. Common in single-celled organisms (amoeba). Can happen in humans, too.

Pinocytosis: when vesicles form around a liquid or around tiny particles. (Blood cells, cells that line the kidney tubules or the intestinal wall, and plant root cells use this to ingest.)

Involve a significant amount of the plasma membrane, happening all the time.

Cells do not shrink in size; loss of plasma membrane is balanced by exocytosis.

Receptor-Mediated Endocytosis: A receptor protein recognizes compatible molecules and take them into the cell.

What is diffusion? What is a net movement?

Molecules moving from a high to low concentration until equilibrium is achieved and the molecules are distributed equally. A physical process.

Sum of the motion.

What is a solution, solute, and solvent?

Solution:  Fluid (the solvent) that contains a dissolved solid (the solute). 

Solute: Substance that is dissolved in a solvent, forming a solution. SOLID. 

Solvent: Liquid portion of a solution that dissolves a solute.

What is osmosis? Osmotic pressure?

Diffusion of water across a selectively permeable membrane from high to low concentration. 

• Pressure that develops in a system because of osmosis

What is a isotonic solution? What is tonic?

Isotonic Solution: A solution that is equal in solute concentration to that of the cytoplasm of a cell; causes a cell to neither lose nor gain water by osmosis.

Tonic: The solute concentration (osmolarity) of a solution compared to that of a cell. If the solution is isotonic to the cell, there is no net movement of water; if the solution is hypotonic, the cell gains water; and if the solution is hypertonic, the cell loses water. (SIMPLE TERM: Strength of the solution)

• 0.9% solution of the salt sodium chloride (NaCl) is isotonic to red blood cells

What is a hyptonic solution? What is the percentage of a hyptonic solution? What is cytolysis?

Solutions that cause cells to swell or even burst because of the intake of water. 

• Less than 0.9%

• Cytolysis is cells that have been disturbed

What is hemolysis? What is turgor pressure?

Hemolysis is cytolysis in red blood cells. 

• Swelling of plant cells in a hyptonic solution. A plant cell doesn’t burst. Important to keep plants upright.

What is a hypertonic solution? Percentage?

Solutions that cause cells to shrink or shrivel due to loss of water. Higher than 0.9%.

What is crenation and plasmolysis?

Crenation: Red blood cells in animals that have shrunk.

Plasmolysis: Contraction of the cytoplasm in the plant cell due to the loss of water.

What are facilitated and active transport? Which requires energy and which doesn’t?

Facilitated transport: how molecules (e.g., glucose and amino acids) are rapidly transported across the plasma membrane. They combine with carrier proteins. No energy.

Active Transport: Moves against the concentration gradient. Needs energy in the ATP form. A carrier protein is needed too. Also called “pumps” because they use proteins to move substances.

What does water use to travel through the plasma membrane?

A channel protein.

What is a sodium-potassium pump?

A carrier protein in the plasma membrane that moves sodium ions out of cells and potassium ions into cells. It is important in the function of nerve and muscle cells in animals.

What are the two types of animal cell surfaces? What is fibronectin?

Extracellular Matrix (ECM): meshwork of proteins and polysaccharides in close association with the cell that produced them.

Collagen (resist stretching) and elastin fibers (gives the ECM resilience)

Junctions Between the Cells—3 types:

Adhesion Junctions mechanically attach adjacent cells. Most common.

In desmosomes, cytoplasmic plaques attach to the intermediate filament cytoskeleton within each cell and are joined between cells by integral membrane proteins called cadherins. Sturdy and flexible. In some organs, heart, stomach, bladder, they hold the cells together.

In hemidesmosomes, the intermediate filaments of the cytoskeleton are attached to the ECM through integrin proteins.

Tight junctions bring cells even closer than desmosomes do.

Connect plasma membranes between adjacent cells together, think: zipper.

Gap Junction: allows cells to communicate. Forms when two identical plasma membrane channels join.

Important for heart and smooth muscle

Six plasma membrane.

Fibronectin is an adhesive protein that binds to a protein called integrin in the plasma membrane.

Allows the ECM to influence the activities cytoskeleton and shapes the cell’s activities and shape.

What are proteoglycans? 

Amino sugars in the ECM that form multiple polysaccharides that attach to a protein. They attach to a long centrally placed polysaccharide. 

• Resists compression of the extracellular matrix

What do all plant cells have? What is plasmodesmata?

A primary cell wall. It contains fibrils, where microfibrils are held together by noncellulose substances.

The cytoplasm of living cells is connected by plasmodesmata, narrow, membranelined channels that pass through the cell wall. It allows only small solutes and water to pass freely from cell to cell

What is signal transduction?

A process that occurs within a cell when a molecular signal (protein, hormone, etc.) initiates a response within the interior of the cell.

What is phosphorylated and dephosphorylated? What are the enzymes that conduct this?

Phosphorylated: phosphate that has been added to a molecule.

Dephosphorylated: phosphate group is removed

• Kinases and phosphatases

What is a G protein?

Assist in signal transduction. Interacts between the receptor and another membrane-bound protein, called an effector protein, then initiates the cellular response.

Signal transduction pathways often function using…What are the two most common ones?

Second messenger. 

• cAMP (cyclic AMP) 

• Calcium ions (Ca²⁺)

What are synaptic, paracrine, direct contact, and autocrine and their functions/examples?

Synaptic: transferring info. Within the nervous system. Ex: epinephrine, serotonin

Paracrine: molecules released over short distances Ex: blood clotting and tissue repair signaling.

Direct Contact: contact between receptors of neighboring cells or through channels like gap junctions. Ex: Contact in cells during embryonic development

Autocrine: release of chemicals by a cell to influence its own activity. Cytokines of the immune system