Chapter 3 - Plasma Membrane & Cells: The Living Units

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Describe the Plasma Membrane and its function..

-Biomolecular layer of lipids and proteins in a constantly changing fluid mosaic
-Separates ICF from ECF
-Plays a dynamic role in cellular activity
-Selectively permeable

Phospholipids

-Membrane lipid: 95%
-Phosphate head thats polar and hydrophilic
-Fatty acid tails that are non-polar and hydrophobic

Glycolipids

-Membrane lipid: 5%
-Lipids with polar sugar groups on outer membrane surface

Cholesterol

-Membrane Lipid: 20%
-Increases membrane stability and fluidity

Integral Proteins

-Membrane Protein
-Firmly inserted into the membrane (most are transmembrane)

Function:
-Transport proteins (channels & carriers), enzymes or receptors

Peripheral Proteins

-Membrane Protein
-Loosely attached to integral proteins
-Include filaments on intracellular surface and glycoproteins on extracellular surface

Functions:
-Enzymes, motor proteins, cell-to-cell links, provide support on intracellular surface, and form part of glycocalyx

What are the functions of Membrane Proteins?

1. Transport
2. Receptors for signal transduction
3. Attachment to cytoskeleton and extracellular matrix
4. Enzymatic activity
5. Intercellular joining
6. Cell-cell recognition

MEMBRANE PROTEINS - Transport:

-A protein that spends the membrane may provide a hydrophilic channel that is selective for a particular solute.
-Some transport proteins hydrolyze ATP as an energy source to actively pump a substance across the membrane

MEMBRANE PROTEINS- Receptors for signal transduction:

-The membrane protein exposed to the outside of the cells may have a binding site with a specific snap that fits the snap of a chemical messenger, such as a hormone.
-The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell.

MEMBRANE PROTEINS- Attachment to the cytoskeleton and extracellular matrix (ECM):

-Elements of the cytoskeleton and the extracellular matrix may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins.
-Others play a role in cell movement or bind adjacent cells together.

MEMBRANE PROTEINS- Enzymatic activity:

-A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution.
-In some cases, several enzymes in a membrane act as a team that catalyze sequential steps of a metabolic pathway.

MEMBRANE PROTEINS- Intercellular joining:

-Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions.
-Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions.

MEMBRANE PROTEINS- Cell-cell recognition:

-Some glycoproteins serve as identification tags that are specifically recognized by other cells.

What are Tight Junctions and what do they do?

-Impermeable junction that encircles the cell
-Prevents fluid and most molecules from moving between cells, useful in the stomach.

What are Desmosomes and what do they do?

-Anchoring junctions scattered along the sides of cells
-Rivets or spot-welds that anchor cells together, useful in the heart.

What are Gap Junctions and what do they do?

-A small junctions the allows chemical substances to pass between cells
-Transmembrane proteins form pores that allow small molecules to pass from cell to cell (electrically), for spread of ions between cardiac or smooth muscle cells.

Passive Processes

1. Simple diffusion
2. Carrier-mediated facilitated diffusion
3. Channel-mediated facilitated diffusion
4. Osmosis

Simple Diffusion

-Non polar and lipid-soluble substances; one exception, WATER.
-Diffuse directly through the lipid bilayer
-Diffuse through channel proteins

Facilitated Diffusion

-Certain lipophobic molecules (e.g glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:
-exhibit specificity (selectivity)
-are saturable; rate is determined by number of carriers of channels
-can be regulated in terms of activity and quantity

Facilitated Diffusion using Carrier Proteins

-Transmembrane integral proteins transport specific polar molecules (e.g. sugars and amino acids)
-Binding of substrate causes shape change in carrier

Facilitated Diffusion using Channel Proteins

-Aqueous channels formed by transmembrane proteins selectively transport ions of water
-2 Types:
*Leakage channels - always open
*Gated channels - controlled by chemical or electrical signals

Osmosis

-Movement of solvent (water) across a selectively permeable membrane
-Occurs when the concentration of a solvent is different on opposing sides of a membrane
-Water diffuses through the lipid bilayer and through channels called aquaporins constructed by membrane proteins in the RBCs & kidney tubule cells
-Water concentration is determined by solute concentration because solute particles displace water molecules.

Osmolarity

-The measure of total concentration of solute particles
-When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached.

What is the importance of Osmosis?

-When osmosis occurs, water enters or leaves a cell
-Change in cell volume disrupts cell function

Tonicity

The ability of a solution to cause a cell to shrink or swell.

Isotonic

A solution with the same solute concentration as that of the cytosol.

Hypotonic

A solution having lesser solute concentration than that of the cytosol.

Hypertonic

A solution having greater solute concentration than that of the cytosol.

PLASMA MEMBRANE TRANSPORT - Filtration:

-The passage of water and solutes through a membrane by hydrostatic pressure.
-Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area.

What are the 2 types of Active Transport and what do they require?

1. Active Transport
*Primary active transport
*Secondary active transport
2. Vesicular Transport

-Both require carrier proteins and use ATP to move solutes across a living plasma membrane.
-Moves solutes against a concentration gradient.

Primary Active Transport

-Energy from hydrolysis of ATP causes shape change in transport protein so that bound solutes (ions) are "pumped" across the membrane.
-Best seen in: Na+/K+ Pump

Na+/K+ Pump

-Located in all plasma membranes
-Involved n primary and secondary active transport of nutrients and ions
-Maintains electrochemical gradients essential for functions of muscle and nerve tissues.

Secondary Active Transport

-Use of an exchange pump (such as the Na+-K+ pump) indirectly to drive the transport of other solutes.
-Depends on an ion gradient created by primary active transport.
-Energy stored in ionic gradients is used indirectly to drive transport of other solutes.

Cotransport

Always transports more than one substance at a time.

Symport System

Two substances transported in the same direction.

Antiport System

Two substances transported in opposite directions.

Vesicular Transport (e.g. Neurotransmitters)

-Requires cellular energy (ATP)
-Transport of large particles and macromolecules and fluids across plasma membrane generally via exocytosis and endocytosis.

Exocytosis

Moves substances from the cell interior to the exterior space.

Endocytosis

Enables large particles and macromolecules to enter the cell.

Transcytosis

Transport into, across, and then out of the cell.

Substance (vesicular) Trafficking

Transport from one area or organelle in the cell to another.

What does Endocytosis and Transcytosis involve?

-Formation of protein-coated vesicles
-Often receptor mediated, therefore very selective

Phagocytosis

-A form of Endocytosis.
-Macrophages and some white blood cells
-The cell engulfs a large particle by forming projecting pseudopods around it and enclosing it within a membrane sac called a phagosome.
-The phagosome is combined with a Lysosome.
-Undigested contents remain in the vesicle (now called a residual body) or are ejected by exocytosis.
-Vesicle may or may not be protein-coated but has receptors capable of binding to microorganisms or solid particles.

Fluid-phase Endocytosis (Pinocytosis)

-A form of Endocytosis
-Nutrient Absorption in the small intestine
-The cell "gulps" drops of extracellular fluid containing solutes into tiny vesicles.
-No receptors are used, so the process is non-specific.
-Most vesicles are protein coated.

Receptor-mediated Endocytosis

-A form of Endocytosis
-Uptake of enzymes low-density lipoproteins, iron, and insulin.
-Extracellular substances bind to specific receptor proteins in a region of coated pits, enabling the cell to ingest and concentrate specific substances (Ligands) in protein-coated vesicles.
-Ligands may simply be released inside the cell, or combined with lysosomes to digest contents.
-Receptors are recycled to the plasma membrane in vesicles.

Name 4 examples of Exocytosis..

1. Hormone secretion
2. Neurotransmitter release
3. Mucus secretion
4. Ejection of wastes

What is Membrane Potential?

-Separation of oppositely charged particles (ions) across a membrane created a membrane potential.

What is a Resting Membrane Potential (RMP)?

-Voltage measured in resting state of cells.
-The point where K+ potential is balanced by the membrane potential.
-Results from Na+ and K+ concentration gradients across the membrane.
-Ranges from -50 to -100mV in different cells.

Membrane at rest?

Much more permeable to K+ than to Na+.

What is Steady State?

Potential maintained by active transport of ions via Na/K pump & changes occur only on the membrane surface (inside and out).

What are the 6 main steps for generation and maintenance of RMP?

1. The Na+/K+ pump continually ejects Na+ from cells and carries K+ back in.
2. Some K+ continually diffuses down its concentration gradient out of cell through K+ leakage channels.
3. Membrane interior becomes negative (relative to exterior) because of large anions trapped inside cell.
4. Electrochemical gradient begins to attract K+ back into cell.
5. RMP is established at the point where the electrical gradient balances the K+ concentration gradient.
6. A steady state s maintained because of the rate of active transport is equal to and depend on the rate of Na+ diffusion into the cell.

Cell-Environment Interactions

-Involves glycoproteins and proteins of glycocalyx
*Cell adhesion molecules (CAMs)
*Membrane receptors

What are the 5 roles of CAMs?

1. Anchor cells to extracellular matrix or to each other.
2. Assist in movement of cells past one another.
3. CAMs of blood vessel lining attract white blood cells to injured or infected areas.
4. Stimulate synthesis or degradation of adhesive membrane junctions.
5. Transmit intracellular signals to direct cell migration, proliferation and specialization.

MEMBRANE RECEPTORS - Contact Signaling:

Touching and recognition of cells; e.g. in normal development and immunity.

MEMBRANE RECEPTORS - Chemical Signaling:

-Interactions between receptors and ligands (neurotransmitters, hormones and paracrines) to alter activity of cell proteins (e.g. enzymes or chemically gated ion channels)

MEMBRANE RECEPTORS - Electrical Signaling:

-Voltage-regulated "ion gates" in nerve and muscle tissue.

MEMBRANE RECEPTORS - G protein-linkied receptors:

Ligand binding activates a G protein, affecting an ion channel or enzyme or causing the release of an internal second messenger, such as cyclic AMP.

What is the Cytoskeleton?

-Elaborate series of rods throughout the cytosol
*Microtubules
*Microfilaments
*Intermediate filaments

Microfilaments

-Dynamic actin strands attached to cytoplasmic side of plasma membrane
-Involved in cell motility, change in shape, endocytosis, and exocytosis.

Intermediate Filaments

-Tough, insoluble rope-like protein fibers.
-Resist pulling forces on the cell and attach to desmosomes.

Microtubules

-Dynamic hollow tubes
-Most radiate from centrosome
-Determine overall shape of cell and distribution of organelles.

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