Bio Ch.5: The Plasma Membrane

plasma membrane functions

defining the outer border of all cells and organelles, managing what enters and exits the cell, receiving external signals and initiating cellular responses—>receptors, and adhering the neighboring cells

phospholipid

• the main plasma membrane of an amphiphilic lipid molecule

• hydrophilic head: polar phosphate group and glycerol molecule

• hydrophilic head: 2 nonpolar fatty acid chains—>can be saturated (carbons are saturated with H; all single C-C bonds) or unsaturated (at least 1 double C=C bond)

phospholipids arrange themselves….

in a bilayer with polar heads facing outward and hydrophobic tails facing inward

components of the plasma membrane

cholesterol, proteins, glycoproteins, and cytoskeleton proteins

plasma membrane cholesterol

lipid that helps regulate the fluid nature of the membrane

plasma membrane proteins

act as transporters, enzymes, or in binding and adhesion

plasma membrane glycoproteins

proteins with carbohydrate molecules attached that serve to identify the function of the cell to other cells and the immune system

plasma membrane cytoskeleton proteins

interior network of proteins that support the plasma membrane and provide shape to the cell

fluid mosaic model

• a mosaic of the components that give the membrane a fluid character

• allows the cell to maintain structure and have some flexibility

membrane fluidity is due to…

• temperature (lipids move more with high temp)

• lipid packing (shorter tails=less stiff)

• saturation of fatty acids (more C-C bonds=high fluidity)

• cholesterol (high fluidity at lower temps, low fluidity at higher temps—>maintains proper rigidity)

the plasma membrane is…

selectively permeable (allows some molecules to pass through, but not others)

What molecules can and can’t pass through the lipid bilayer?

small hydrophobic molecules (O2, CO2) and small uncharged polar molecules (H2O, ethanol) can, but large uncharged polar molecules (amino acids, glucose, and nucleotides) and ions (H+, Na+, K+, Ca2+, CL-, Mg2+) can’t without transport proteins

integral proteins

• integrated completely into the bilayer

• 1 or more regions that are hydrophobic (composed of hydrophobic amino acids) and others that are hydrophilic

• locations and the number of regions determine how they arrange within the bilayer

peripheral proteins

occur only on the surface

types of membrane proteins

anchoring, recognition, enzymes, receptors, carriers, and channels

anchoring proteins

stabilizers; attach to inside or outside of structures

recognition proteins

identifiers; label cells as normal or abnormal

enzymes

catalyze reactions

receptor proteins

bind and respond to ligands (ions, hormones)

carrier proteins

transport specific solutes through membrane

channels

• regulate water flow and solutes passing through membrane

• gated channels open or close to regulate passages of substances

faulty membranes can…

cause disease

oligosaccharide carbohydrates

• “few sugars”

• located on the exterior surface of the plasma membrane, bound to either proteins (forming glycoproteins) or to lipids (forming glycolipids)

• function in cell-to-cell recognition and attachment

plasma membranes are asymmetric

• the inner surface differs from the outer surface

• interior proteins anchor fibers of the cytoskeleton to the membrane

• exterior proteins bind to the extracellular fluid

• glycoproteins bind to substances the cell needs to import

concentration gradient

• chemical gradient

• may exist across a biological membrane, where the concentration is higher on 1 side of the membrane compared to the other side

• form of potential energy that converts to kinetic energy when there’s a flow of the chemical across the membrane

electrical gradient

• the cytoplasm contains more negatively charged molecules than the extracellular fluid

• critical for proper cell function—>determines movement of the flow

electrochemical gradients

arise from the combined effects of concentration gradients and electrical gradients

transport across the membrane can either be…

passive (requiring no energy) or active (requiring energy—>ATP)

3 mechanisms of membrane transport

1. diffusion (passive transport)

2. facilitated diffusion (passive transport)

3. active transport

passive transport

• involves diffusion, facilitated diffusion, and osmosis

• moves materials down their concentration gradient

• does not require an input of energy

the simplest form of passive transport is…

diffusion

diffusion

• simplest form of passive transport

• occurs when a substance from an area of high concentration moves down its concentration gradient (in membranes, this occurs through the lipid bilayer)

• net movement ceases if equilibrium is achieved

• only small nonpolar molecules (O2, CO2, lipid hormones, etc) can diffuse through biological membranes

factors that affect diffusion rates

• concentration: greater difference—>faster diffusion

• mass of the molecules: smaller molecules—>quicker diffusion

• temperature: higher temperatures—>molecules move faster

• solubility: more non-polar (lipid soluble)—>faster diffusion

• surface area: increased surface area—>sped up diffusion rates

• distance traveled: greater distances—>slower rates (important factor affecting upper limit of cell size)

• pressure: in some cells, blood pressure forces solutions through membranes—>sped up diffusion rates)

• solvent density: dehydration increases cytoplasm density—>reduced diffusion rates

channel proteins

• the top, bottom, and inner core are composed of hydrophilic AA—>attract ions and/or polar molecules

• some are open all the time

• others are gated, only opening when a signal is received

carrier proteins

• specific to a single substance

• binds to that substance, change their shape, and “carry it” to the other side

• many allow movement in either direction, as concentration gradients change

osmosis

• the diffusion of water across a membrane

• water always moves from an area of higher water concentration to one of lower concentration

• differences in water concentration occur when a solute cannot pass through the selective permeable membrane

tonicity

• describes how an extracellular solution can change the volume of a cell by affecting osmosis

• often correlated to the osmoslarity of a solution

• osmolarity describes the total solute concentration of a solution (permeable and non-permeable solutes)

when solutions of different osmoslarities are separated by a membrane permeable to water but not the solute…

water moves from the solution with lower osmolarity through the membrane

hypertonic, isotonic, and hyotonic describe…

the osmolarity of the cell to that of its extracellular fluid

hypertonic

extracellular fluid has a higher osmolarity than the cytosol—>water leaves the cell

isotonic

extracellular fluid has the same osmolarity than the cytosol—>water does not move

hypotonic

extracellular fluid has lower osmolarity than the cytosol—>water enters the cell

osmoregulation

• organisms whose cells have cell walls (plants, fungi, bacteria, and some protists) prefer hypotonic extracellular solutions

• the pressure exerted by the plasma membrane against the cell wall (turgor pressure) is critical to organismal growth and functions

• hypertonic solutions cause plasmolysis—>plasma membrane detaches from the cell wall

osmoregulation by other organisms

• freshwater protists use contractile vacuoles to pump water out of their cells so they do not burst

• marine invertibrates have internal salt concentrations that match their environment

• fish excrete diluted urine to get rid of excess H2O or salts

• osmoreceptors of the brain cells monitor solute concentrations in our blood, releasing hormones that affect kidney function

active transport

• occurs through transmembrane, integral carrier proteins called pumps

• 3 types of pumps: uniporter (carries 1 molecule or ion), symporter (carries 2 different molecules or ions in the same direction), and antiporter (carries 2 different molecules or ions in different directions)

primary active transport

moves an ion or molecule against its concentration gradient using energy from ATP hydrolysis

secondary active transport

uses electrochemical gradient created by primary active transport to move a different substance against its concentration gradient (many amino acids and glucose enter the cell this way)

sometimes cells need to import (endycytosis) or export (exocytosis) molecules/particles that are…

• too large to pass through a transport protein

• bulk transport is a type of active transport; energy required

3 types of endocytosis

1. phagocytosis (cellular eating): the cell membrane surrounds a particle and engulfs it

2. pinocytosis (cellular drinking): the cell membrane invaginates, surrounds a small volume of fluid, and pinches off

3. receptor-mediated endocytosis: uptake of a specific substance is targeted by binding to receptors on the external surface of the membrane

exocytosis

vesicles containing substances fuse with the plasma membrane; the contents are then released to the exterior of the cell