Lecture 2 Flashcards
Plasma membrane
Fluid matrix
Phospholipid bilayer
Cholesterol and proteins scattered throughout
Carbohydrates which are attached to some phospholipids and proteins, extends form the surface to form the glycocalyx
Regulates the movement of most substances both into and out of the cell
Lipid components of plasma membrane
Insoluble in water, which ensures that the plasma membrane will not simply dissolve when it comes into contact with water
Nonpolar physical barrier to most substances
Only small and nonppolar substances can readily penetrate this barrier without assistance
Components:
-Phospholipids: polar and hydrophilic (water-loving) head, nonpolar and hydrophobic (water-hating) tails (x2); phospholipid bilayer ensures that cytosol remains inside the cell, and interstitial fluid remains outside
-Cholesterol: four-ring lipid molecule; scattered within the inner hydrophobic regions of the phospholipid bilayer; stops tails from sticking together
-Glycolipids: lipids with attached carbohydrate head; each carbohydrate part is attached to a phospholipid molecule located on the outer phospholipid bilayer; extend like antennae from the cell’s external phospholipid surface, where they are exposed to the interstitial fluid; contribute to glycocalyx
Integral membrane proteins
Embedded within, and extend completely across, the phospholipid bilayer
Hydrophobic regions within the integral proteins interact with the hydrophobic interior of the membrane
Hydrophilic regions of the integral proteins are exposed to the aqueous environments on either side of the membrane
Many are glycoproteins
Peripheral membrane proteins
Not embedded within the lipid bilayer
They are attached loosely to either the external or the internal surfaces of the membrane
Often “anchored” to the exposed parts of an integral protein
Roles of membrane proteins
-Transport proteins: provide a means of regulating the movement of substances across the plasma membrane (ex. channels, carrier proteins, pumps, symporters, and antiporters)
-Cell surface receptors: bind specific molecules called ligands which will initiate muscle contraction
-Identity markers: communicate to other cells that they belong to the body. cells of the immune system identity markers to distinguish normal, healthy cells from foreign, damaged, or infected cells that are to be destroyed
-Enzymes: may be attached to either the internal or the external surface of a cell for catalyzing chemical reactions
-Anchoring sites: secure the cytoskeleton to the plasma membrane
-Cell-adhesion proteins: for cell-to-cell attachments. proteins that form membrane junctions perform a number of functions, including binding cells to one another
Glycocalyx
“Coating of sugar” at the cell’s external surface
Composed of the carbohydrates of glycolipids and glycoproteins that extend outward from plasma membrane
Membrane transport
Passive processes:
1. Diffusion
-Simple diffusion
-Facilitated diffusion (channel-mediated, carrier-mediated)
2. Osmosis
Active processes:
1. Active transport
-Primary active transport
-Secondary active transport (symport, antiport)
2. Vesicular transport
-Exocytosis
-Endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis)
Passive processes (membrane transport)
Does not require expenditure of cellular energy
Depend upon the kinetic energy (or random movements) of ions and molecules as each moves down its concentration gradient (from where there is more to where there is less)
Types:
-Diffusion
-Osmosis
Active processes (membrane transport)
Differ from passive processes in that they require expenditure of cellular energy
Types:
-Active transport
-Vesicular transport
Diffusion (passive process)
Movement of either ions or molecules down their concentration gradient
Depends upon a concentration gradient (more in one area than another) (note: the greater the difference in concentration of the substance in one area than the other, the steeper the concentration gradient)
Involves the spreading out of ions and molecules (their random movement allows the substance to spread out or diffuse)
If unopposed, diffusion results in reaching equilibrium
Diffusion involving a cell (2 types)
Solutes always diffuse from an area where they are more concentrated to an area where they are less concentrated
The chemical characteristics of the diffusing solute dictate whether it moves across the plasma membrane unassisted between the phospholipid molecules or if it is assisted with a protein embedded in the plasma protein
2 types:
-Simple diffusion
-Facilitated diffusion
Simple diffusion
Pass without aid of membrane protein (unassisted)
Small and nonpolar molecules move into or out of a cell down their concentration gradient
Molecule ex. respiratory gases, small fatty acids, ethanol, and urea
Cannot be regulated by plasma membrane
Facilitated diffusion
Requires carrier protein or channel
Small charged ions or polar molecules are effectively blocked from passing through the membrane by the nonpolar bilayer
2 types:
-Channel-mediated diffusion
-Carrier-mediated diffusion
Channel-mediated diffusion (facilitated diffusion)
Movement of small ions (ex. Na+, K+) across plasma membrane through water-filled protein channels
Each channel is specific for one type of ion
Down concentration gradient
2 types:
-Leak channel: continuously open
-Gated channel: usually closed, opens only in response to a stimulus and then stays open for just a fraction of a second before it closes
Carrier-mediated diffusion (facilitated diffusion)
Movement of polar molecules (ex. glucose, amino acids) across the plasma membrane
Molecules larger size requires that their movement be assisted by a carrier protein
Down concentration gradient
Involves conformational change in carrier protein for the transport of the molecule
Uniporter = carrier that transports only 1 type of substance
3 primary events take place:
1. The carrier protein within the plasma membrane binds the polar molecules
2. This induces the carrier protein to change shape and move or carry the polar molecules to the other side of plasma membrane
3. There, it is released
Osmosis (passive process)
Diffusion of water
Solutes can be permeable or nonpermeable (when talking about osmosis, we refer to things as solutes when they are nonpermeable)
A solution with a greater concentration of solutes contains a lower concentration of water
Water moves toward the solution with the greater solute concentration
Water can pass through membrane in 2 ways:
1. Simple diffusion (slower) = limited amounts “slip between” the molecules of the bilayer
2. Facilitated diffusion (faster) = through aquaporin channels
Osmotic pressure
The pressure exerted by the movement of water across a semipermeable membrane due to a difference in water concentration
The steeper the gradient, the greater the amount of water moved by osmosis and the higher the osmotic pressure
Tonicity
The ability of a solution to change the volume or pressure of the cell by osmosis
When water crosses the plasma membrane of a cell by osmosis, the cell either gains or loses water with an accompanying change in the cell’s volume
We describe the relative water concentration and the net movement of water by osmosis when cells are immersed in each of these categories of solutions
3 types:
1. Isotonic solution
2. Hypotonic solution
3. Hypertonic solution
Isotonic solution
Has the same relative concentration of water as the cell’s cytosol and there is no net movement of water between the solution and the cell
Hypotonic solution
Has more water than the cell’s cytosol and there is a net movement of water by osmosis into the cell (can cause rupture = lysis)
Hypertonic solution
Has less water than a cell’s cytosol and there is a net movement of water by osmosis out of the cell (can cause shrinking = crenation)
Active transport (active process)
The movement of a solute against its concentration gradient (low area to high area)
2 types (distinguished by their specific energy source):
-Primary active transport
-Secondary active transport
Primary active transport (active transport)
Uses ATP
Types of ion pumps:
-Ca2+ pumps
-H+ pumps
-Na+/K+ pumps
Ca2+ pump (primary active transport)
Embedded in plasma membranes of erythrocytes
Move calcium out of erythrocyte to prevent it from becoming rigid due to accumulation of calcium
Therefore, erythrocyte remains flexible enough to move through capillaries
H+ pump (primary active transport)
Functions in maintaining cellular pH
Required in establishing an H+ gradient between outer and inner compartment of mitochondria as a part of the electron transport system
The diffusion of H+ down its concentration gradient allows ATP synthase to harness the energy to form ATP from ADP and Pi
Required to establish the low pH in lysosomes
Na+/K+ pump (primary active transport)
Exchange pump because it moves Na+ out of the cell (against gradient) and K+ into the cell (against gradient)
There is a 1:2:3 ratio for this pump: 1 ATP is required to pump 2 K+ ions into the cell and 3 Na+ ions out of the cell
Steps:
1. 3 sodium ions and ATP bind to sites on the cytoplasmic surface of the Na+/K+ pump
2. ATP is split into ADP and Pi, resulting in both the binding of the free phosphate (Pi) to the pump and release of energy that causes the Na+/K+ pump to change conformation (shape) and release the Na+ ions into the interstitial fluid
3. 2 K+ ions from the interstitial fluid then bind to sites on the outer cellular surface of the Na+/K+ pump. At the same time, the Pi produced earlier by ATP splitting is released into the cytosol
4. This transport protein reverts back to its original shape, resulting in the release of the K+ ions into the cytosol. The Na+/K+ pump is now ready to begin the process again
Secondary active transport (active transport)
Does not use ATP
Involves the movement of a substance down its concentration gradient to provide the energy to move a different substance up its concentration gradient
The Na+ gradient is often the source of energy because its concentration gradient across the plasma membrane is extremely steep
Secondary active transport mechanisms are ultimately dependent upon the primary active transport mechanisms of Na+/K+ pumps. These pumps produce and sustain a distinct concentration gradient difference between Na+ on opposite sides of the plasma membrane, with substantially more Na+ in the interstitial fluid and less Na+ in the cytosol
2 types:
-Symport: uses a symporter protein; glucose binds symporter in plasma membrane and then both glucose and Na+ are transported into the cell
-Antiport: uses an antiport protein; an antiporter moves the two substances in the opposite direction
Vesicular transport (active process)
Allows for movement of large substances (or large amounts of a substance)
2 types:
-Exocytosis
-Endocytosis
Exocytosis (vesicular transport)
The means by which large substances or large amounts of a substance are secreted from the cell
Steps:
1. Vesicle nears plasma membrane
2. Vesicle membrane fuses with plasma membrane
3. Plasma membrane opens to outside of cell
4. Release of vesicle contents into the interstitial fluid and integration of vesicle membrane components into the plasma membrane
Endocytosis
The cellular uptake of large substances or large amounts of a substance from the external environment into the cell
Steps = similar to exocytosis but in reverse
3 types (differentiated based upon the specific material being transported and the mechanism involved):
-Phagocytosis
-Pinocytosis
-Receptor-mediated endocytosis
Phagocytosis (endocytosis)
Means cellular eating
Very few cells perform phagocytosis
Steps:
1. Occurs when a cell engulfs or captures a large particle external to the cell by forming pseudopodia (false feet) to surround the particle
2. Once the particle is engulfed, it is enclosed with what was previously part of the plasma membrane
3. This newly formed vesicle typically fuses with a lysosome
4. The molecules composing the ingested material are broken down or digested by the enzymes within the lysosome
Pinocytosis (endocytosis)
Also known as cellular drinking
Performed by most cells
Steps:
1. Occurs when multiple small regions of the plasma membrane invaginate and multiple vesicles are formed as the cell internalizes interstitial fluid that contains dissolved solutes
2. Nonspecific process because all solutes dissolved within the interstitial fluid are taken into the cell
Receptor-mediated endocytosis (endocytosis)
Uses receptors on the plasma membrane to bind specific molecules and bring them into the cell
Enables the cell to obtain bulk quantities
Steps:
1. Ligands within interstitial fluid attach to their distinct integral membrane protein receptors in the plasma membrane to form a ligand-receptor complex
2. Those complexes move laterally in the plane and accumulate at special membrane regions that contain clathrin protein on the internal surface of the membrane
3. Clathrin-coated regions holding complexes fold inward to form an invagination called a clathrin-coated pit
4. Invagination deepens and pinches off, and the lipid bilayer fuses to form a clathrin-coated vesicle, which then moves into the cytosol
5. Clathrin coat must be enzymatically removed before the vesicle may proceed to its intracellular destination
6. Following entry, receptors and ligands are uncoupled
7. Ligands may be stored, modified, or destroyed, and receptors (unless damaged) are returned to the plasma membrane
Connective vs epithelial tissue
Connective = loosely packed with lots of extracellular matrix whereas epithelial is tightly packed
Connective = lots of blood vessels whereas epithelial has no blood vessels
Connective = covered by other tissues whereas epithelial tissue forms the surface layers
Epithelium/epithelial tissue
Composed of 1+ layers of closely packed cells, and it contains little to no extracellular matrix between these cells
Covers the body surfaces, lines the body cavities and organ cavities, and forms glands
Epithelia characteristics (exhibited by all)
-Cellularity: composed almost entirely of tightly packed cells; minimal amount of extracellular matrix between cells
-Polarity: apical surface which is exposed either to external environment or internal body space; outside surface may have microvilli or cilia; lateral surfaces may contain membrane junctions; each epithelium has a basal surface (fixed/deep surface) where the epithelium is attached to a basement membrane with underlying connective tissue
-Attachment to basal membrane: bound at its basal surface to basement membrane
-Avascularity: no blood vessels; nutrients obtained across apical surface or by diffusion across basal surface
-Extensive innervation: richly innervated (nerves)
-High regeneration capacity: undergo cell division frequently; continual replacement occurs through cell division of the deepest epithelial cells (stem cells)
Epithelia primary functions
-Physical protection: protect both external and internal surfaces from dehydration, abrasion, and destruction by physical, chemical, or biological agents
-Selective permeability: may be relatively nonpermeable to some substances, while promoting and assisting the passage of other ions and molecules. All substances that enter or leave the body must pass through an epithelium
-Secretions: some epithelial cells are specialized to produce and release secretions (these form glands)
-Sensations: innervated by sensory nerve endings to detect or respond to stimulus
Classification of epithelial tissue
Number of cell layers:
-Simple epithelium = 1 layer
-Pseudostratified epithelium = appears layered (stratified) because nuclei are distributed at different levels, but all are attached to basement membrane (considering it simple)
-Stratified epithelium = contains 2+ layers; only bottom layer is attached to basement membrane
Cell type:
-Squamous cells = flat, wide, and somewhat irregular; arranged like floor tiles; nucleus is somewhat flatted
-Cuboidal cells = about as tall as they are wide; cell nucleus is spherical and located within the center of cell
-Columnar cells = slender and taller than they are wide; cell nucleus is oval and usually oriented lengthwise and in basal region of cell; transition cells can readily change their shape from polyhedral to more flattened depending upon the degree to which the epithelium is stretched
Glands (glandular epithelium?)
Either individual cells or multicellular organs composed predominantly of epithelial tissue
Secrete substances either for use elsewhere in the body or for elimination
Glandular secretions = mucin, ions, hormones, enzymes, or urea
Endocrine glands
Lack ducts
Secrete their products (hormones) into blood to be transported around the body
Hormones act as chemical messengers to influence cell communication
Exocrine glands
Typically originate from an invagination of epithelium that burrows into the underlying connective tissue
Usually maintain their connection with the epithelial surface by means of a duct
Unicellular exocrine glands: typically do not contain a duct; located close to surface of epithelium; most common type = goblet cell (usually found in both simple columnar epithelium and pseudostratified ciliated columnar epithelium)
Multicellular exocrine glands: contain numerous cells that work together to produce a secretion; gland often consists of acini (clusters of cells that produce the secretion, and 1+ smaller ducts, which merge to form a larger duct that transports the secretion to the epithelial surface); typically surrounded by fibrous capsule
Exocrine gland classification by anatomic form
Simple glands = single, unbranched duct
Compound glands = branched ducts
Tubular glands = secretory portion and duct have the same diameter
Acinar glands = secretory portion forms an expanded sac
Tubuloacinar glands = has both tubules and acini
Exocrine gland classification by method of secretion
Merocrine glands = package their secretions into secretory vesicles and release secretions by exocytosis; glandular cells remain intact and are not damaged by producing the secretion; ex. lacrimal (tear) glands, salivary glands, sweat glands, exocrine glands of pancreas, gastric glands of stomach
Apocrine glands = produce their secretory material when the cell’s apical portion pinches off, releasing cytoplasmic content; thereafter, the cell repairs itself in order to repeat secretory activity; ex. mammary glands and ceruminous glands of ear
Holocrine glands = formed from cells that accumulate a product; the entire cell then disintegrates; viscous mixture of both cell fragments and the product the cell produced prior to disintegration; the ruptured, dead cells are continuously replaced; ex. oil-producing (sebaceous) glands in skin
