AP Biology - Unit 4: Membranes & Transport

Fluid Mosaic Membrane

composed of several elements (integral membrane protein, cholesterol, carrier protein, peripheral membrane protein, channel protein, lipid bilayer)

Phospholipid bilayer

Phosphate (polar), glycerol, fatty acids, polar heads, non-polar tails

Fluidity of phospholipid bilayer depends on…

• Degree of saturation of the fatty acids in phospholipid  

• Steroids like cholesterol in membrane alter fluidity depending on temperature

Fluid Mosaic elements

Glycoprotein, polysaccharide, cholesterol, extracellular fluid, glycolipid, cytoplasm, filaments of cytoskeleton (actin + microtubules)

Bilayer

barrier to passage of water and water soluble molecules 

Glycolipids & glycoproteins

cell & tissue identity markers;  indicate self vs. other 

Glycoprotein

protein with attached carbohydrate chains, present in cell membranes, extracellular matrix, and secreted proteins; they play roles in cell signaling, immune response, and cell adhesion.

ex: antibodies, hormones, and enzymes

Glycolipid

lipid molecule that contains a carbohydrate group attached to it; plays not only a structural role to maintain membrane stability but also facilitates cell-cell communication acting as receptors, anchors for proteins and regulators of signal transduction

Cytoskeleton

holds membrane in shape, position proteins (microtubules) & move membrane & organelles (actin)

Microtubules

position proteins

Actin

move membrane & organelles

Cell Identity markers

Glycoprotein/glycolipid (ex: blood type, tissue type)

Cell to Cell Adhesion 

Intercellular junction; forces things thru membrane not around

Catalyze reactions

enzyme, product, substrate; in series on the membrane

ex: smooth ER, mitochondria

Gates, Channels, Pumps 

Transport proteins & ATP; membranes are selectively permeable by regulating movement of ions or molecules by shape

Receptor

Receptor proteins

Ex: hormone, neurotransmitter, causes internal activity change

Attach to cytoskeleton

Proteins attach to cytoskeleton and extracellular matrix; gives cell structure

changes shape of cell

Signaling Between Cells steps

• Initiating cell releases signal molecule (hormone, NTS)

• To intercellular fluid or blood stream

• Signal molecule binds to protein receptor of target cell

• Leads to change in internal activity of target cell

Paracrine signaling

allows cells to communicate with each other by releasing signaling molecules that bind to and activate surrounding cells; injured cell, local mediator, mast cell releases histamine to target cells

• Nearby cell – signal thru intercellular fluid

• Walls of blood vessels expand

Contact-dependent signaling

requires cells to be in direct membrane-membrane contact, membrane-bound signal molecule connects signaling and target cells

• Infected cell presenting piece of pathogen

• Helper T cell will destroy

Neuronal signaling

synapses permit information transfer by interconnecting neurons to form the circuitry on which neural processing depends; neuron (sending cell) sends signaling molecule (neurotransmitter) to target cell through synapse

• target cell can be neuron, muscle, gland

Endocrine signaling

the signaling molecules (hormones) are secreted by specialized endocrine cells and carried through the circulation to act on target cells at distant body sites; endocrine cell secretes hormone through blood to receptor on target cells, or other glands/muscle

Passive transport

Ions & molecules move down the gradient. High to Low (no energy required from cell), spread b/c of their molecular motion 

Diffusion

ions move down gradient w/o energy from cell depending on permeability via protein channels specific to each ion

Facilitated Diffusion

larger molecules use a shape specific carrier protein BUT can become saturated b/c it only carries one at a time

Osmosis

movement of water from high to low water conc but depends on solute conc.; moves through aquaporin channels

Hyperosmotic solution

High solute concentration, low water concentration, water moves out of the cell → cells shrink

Isosmotic solution

Equal solute concentration, equal water concentration, water moves both in and out of the cell → cell normal, suitable

Hyposmotic solution

Low solute concentration, high water concentration, water moves into the cell → cells swell/expand

Cytolysis

animal cell bursting

Turgid

plant cell swelling

Plasmolysis

plant cell shrinking

Why does solute matter? 

• More solute, less room for water so it lowers water potential

• More solute, less water available because it is busy with Hydrogen bonding & keeping non-polar molecules excluded

Hydrostatic pressure

the pressure that any fluid in a confined space exerts

Osmotic equilibrium

occurs when the fraction of water molecules in solution matches the fraction of pure water molecules that have enough energy to overcome the pressure difference

Why does pressure matter?

More pressure more water potential 

BUT Plant cells can fight back against osmosis because of cell wall

Water potential purpose

To quantify the effect of  solute concentration and internal pressure on the direction of water movement

Ψ

(water potential)

Ψ_P      +         Ψ_s

_{(Pressure potential + solute potential)}

Water will always move from 

high water potential to low water potential

Water potential calculation

Ψ         =         Ψ_P      +       Ψ_s    which is –i x C x R x T

(Water potential = Pressure potential given + -i  x  C  x  R  x  T

i

# ions produced by dissociation 

C

concentration of solute 

R

.0831

T

273 + ^oC = K

Bulk passage

Movement of large particles/quantities by engulfing (active transport)

Endocytosis

into cell via a vesicle 

Phagocytosis

solid (cell “eating”) 

Pinocytosis

liquid (cell “drinking”) 

Exocytosis

out of cell via vesicle (ex: wastes, product) 

Receptor Mediated Endocytosis

uses protein receptors along membrane to pick up only appropriate molecules for endocytosis

Sodium-Potassium Pump

pumps Na out of cell and K in – both move up their gradients; requires energy form cells (ATP) → active transport

Ex: neurons

B/c phosphate groups are large and charged

they change the shape of the protein to open/close gate

Coupled transport

Uses Na K pump to set up gradient that provides potential & kinetic energy to drive a coupled pump that will transport another molecule (sugar) → secondary active transport

Charge changes the shape of the protein so…

it drags another molecule with it

Proton pump

1^st establishing proton gradient provides potential energy to drive ATP synthase to store energy in ATP (active transport)

Passive transport

Osmosis, diffusion, and facilitated diffusion

STUDY DIAGRAMS

YES MA’AM