Cell Membrane Structure & Function

Amphipathic

Contains polar (hydrophilic) and non polar (hydrophobic) regions. ex: phospholipids

Phospholipid bilayer

Consists of two layers of phospholipids with the hydrophobic tails on the inside. Allow non-polar particles to enter, but often bounces out polar particles.

Structure of Membrane

Contains phospholipids, channel/transport proteins, steroids (carbs), and proteins

Factors affecting membrane fluidity

Unsaturated lipids prevent packing and keep it fluid. Saturated lipids allow it to pack together, making it viscous. Cholesterol buffers fluidity and prevents it from being too fluid/viscous

Peripheral proteins

Proteins bound to membrane surface

Integral proteins

Hydrophobic core of the membrane

Transmembrane proteins

Span the whole membrane

Functions of membrane proteins

Transport, enzymatic activity, signal transduction, cell-cell recognition, intracellular joining, attatching to ECM and cytoskeleton

Glycolipid

Carb bounded to lipid

Glycoprotein

Carb bounded to protein

Selective permeability

Some substances can cross more easily than others. exhibited by membrane. depends on both the lipid bilayer and the specific
transport proteins it contains

Transport proteins

Allow different substances to cross the membrane (embedded in membrane) involved in facilitated diffusion and active transport.

Channel proteins

Embedded in membrane and has a hydrophilic channel that certain molecules/ions can use to move through

Aquaporins

Special types of channel protein that helps water move through

Ion channel

Special types of channel protein that helps ions move through

Diffusion

Diffusion is the movement of particles of any
substance so that they spread out evenly into the
available space (dynamic equalibrium). move from high to low conc. (down a conc. grad) through a membrane

concentration gradient

the region along which the density of a
chemical substance increases or decreases

osmosis

specific type of diffusion involving free water (water not clustered around another molecule)

Passive transport

transport of a substance across a cell membrane by diffusion (facilitated or not); expenditure of energy is not required

tonicity

is the ability of a surrounding solution to
cause a cell to gain or lose water (depends on conc. of water and other solutes that can move through membrane)

Isotonic

solution conc. is same for both, water diffuses at same rate back and forth, no net movement.
normal for animal cells. makes plant cells flaccid

hypotonic

less solute and more water. water moves/diffuses away from this area,.
causes animal cells to lyse. makes plant cells turgid (ideal condition, means firm)

hypertonic

more solute and less water. water moves/diffuses towards this area.
causes animal cells to shrivel and plant cells to plasmolyse (membrane shrivels away from walls)

osmoregulation

control of solute concentration and water balance
ex: freshwater protists using contractile vacuole to pump out excess water

facilitated diffusion

transport proteins assist in moving molecules down a conc. gradient (high to low)

gated channels

special type of ion channel that opens/closes in response to a stimulus

active transport

transport of a substance across a cell membrane against the concentration gradient by hydrolyzing ATP into ADP for energy. all proteins involved are carrier proteins

enables cells to maintain solute
concentrations that differ from the environment

sodium pottasium pump

uses a phosphate from ATP to energize the transport of
K+ into the cell and Na+ out of the cell. allows our neurons to refire

membrane potential

Voltage across a membrane, created by differences in the distribution of positive and negative ions

inside is neg and attracts cations inside, outside is pos and wants anions outside

electrochemical gradient

a chemical force (the ion’s concentration gradient) and an electrical force (the effect of the membrane potential on the ion’s movement) combined to drive ion diffusion across a membrane

electrogenic pump

a transport protein that generates voltage across a membrane, storing energy that can be used for cellular work (USES ATP IN THIS PROCESS)

proton pump

transports H+ ions out of the cell. assists in cotransport of sucrose back into the cell

cotransport

passive transport helps provide energy to drive the active transport of another solute

exocytosis

transport vesicles migrate to the
membrane, fuse with it, and release their contents
outside the cell

endocytosis

In endocytosis, macromolecules are taken into the
cell in vesicles & the membrane surrounds the
material for transport into a vesicle.
3 types: phago, pino, and receptor mediated

phagocytosis

a cell engulfs a particle by extending pseudopodia around it and packing it in a membranous sac called a food vacuole, and then fuses with a lysosome to digest the particle

pinocytosis

molecules are taken up when extracellular fluid is “gulped” into tiny vesicles. takes in any and all substances, nonspecific

receptor mediated endocytosis

Receptor proteins bound to specific solutes from the extracellular fluid are clustered in coated pits that form coated vesicles
Emptied receptors are recycled to the plasma
membrane by the same vesicle