Chapter 7

Chapter 7

Membrane Structure and Function

Plasma membrane is selectively permeable A. Membrane Structure

• Phospholipids and some proteins and carbohydrates have both hydrophobic and phillic regions

phospholipids are Amphipathic molecules. Both hydrophillic / phobic ends

• fluid mosaic model.

• http://www.johnkyrk.com/cellmembrane.html

Figure 7.7

Glycoprotein

Carbohydrate

Microfilaments

of cytoskeleton Cholesterol Peripheral protein

Integral protein

CYTOPLASMIC SIDE OF MEMBRANE

EXTRACELLULAR SIDE OF

MEMBRANE Glycolipid

Major Functions of Proteins Within the Cell Membrane

Figure 7.9

Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the

membrane that is selective for a particular solute.

(right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane.

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 are organized as a team that carries out sequential steps of a metabolic pathway.

Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a

conformational change in the protein (receptor) that relays the message to the inside of the cell.

(a)

(b)

(c)

ATP

Enzymes

Signal

Receptor

Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.

Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.31).

Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the

cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29).

(d)

(e)

(f)

Glyco- protein

Figure 7.9

Traffic Across Membranes

A membrane’s molecular organization results in selective permeability

• both directions.

• nutrients enter ,metabolic waste products leave.

• regulates [ions], Na

, K

, Ca

, and Cl

• Hydrophobic molecules, CxHx, CO

, and O

, dissolve in and cross easily.

• Ions ,polar and large molecules pass with difficulty.

 water, glucose

• transport proteins accommodate movement of

various solutes

Passive transport is diffusion across a membrane

• Diffusion

• Each substance diffuses, independent of the concentration gradients of other substances.

• passive transport diffusion, requires no energy

• The concentration gradient represents potential energy and drives diffusion.

Figure 7.11 B

Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both.

Each dye diffuses down its own concen- tration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side.

(b)

Net diffusion Net diffusion

Net diffusion

Net diffusion Equilibrium

Equilibrium

Osmosis is the passive transport of water

• Hypertonic Vs. hypotonic.

– Tap water hypertonic to distilled but hypotonic to seawater.

• isotonic = [solute]

• The diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis.

Figure 7.13

Hypotonic solution Isotonic solution Hypertonic solution Animal cell. An

animal cell fares best in an isotonic environ- ment unless it has special adaptations to offset the osmotic uptake or loss of water.

(a)

H₂O H₂O H₂O H₂O

Lysed Normal Shriveled

Cell survival depends on Osmoregulation

• maintain internal environment.

• Ex: freshwater protists, overcome “freshwater dilemma” with a

contractile vacuole

• plants, prokaryotes,

fungi, and some protists have walls

• turgid in hypotonic solution

• Flaccid / Plasmolysis -

in a hypertonic solution

Thermodynamics in Biology

• Entropy is always increasing

• Cells require energy to maintain order

– Useful energy is Free Energy (the energy available to do work)

• When entropy is low, FE is high

• When entropy is high, FE is low

• Energy transfers occur across membranes

– Movement of water across a membrane can be thought of as work

Water Potential

• Measures the force that causes water to move into a system

– The potential energy of water

– The ability of a plant to “ pull” water into root tissue

• Water always diffuses from high (less -)  to low (more -)



– Water moves from high to low conc.

  of distilled H2O water = 0

  = p

 osmotic (solute) potential

– Tendency to pull water in one direction

– Due to solutes, becomes more neg as [solute] increases

 p = pressure potential

Determining Water Potential

  = 



iCRT

• I = ionization constant

• C= molarity of solution (sucrose)

• R= pressure constant .0831

• T= Kelvin temp (Celcius + 273)

Water potential of a

solution equals the solute potential

• No pressure can build up in a beaker.

• We don’t know the pressure

potential of the potato cell, BUT we

do know that the potato gains NO

mass when the water potential of

the potato and the water potential

of the solution are equal

Specific proteins facilitate passive transport of water and selected solutes

• Many transport proteins simply provide corridors channel proteins

– Ex: water channel proteins, aquaporins

• The passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion.

• Transport proteins similar to enzymes.

• specific binding sites for the solute.

• Vmax

• Competitive inhibitors

• Gated channels, open or close depending on a physical or chemical stimulus *(different from the transported molecule.)

– Ex: neurotransmitters

Figure 7.15

Carrier protein Solute

A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes.

The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.

(b)

Figure 7.15

EXTRACELLULAR FLUID

Channel protein

Solute

CYTOPLASM A channel protein (purple) has a channel through which

water molecules or a specific solute can pass.

(a)

Facilitated Diffusion by Protein Channel or Carrier Protein

Active transport : pumping of solutes AGAINST their gradients

• active transport requires energy.

• maintain internal concentrations of small molecules (diffuse easily), but not LARGER or Polar molecules

• ATP supplies the energy for most active transport.

• shifts a phosphate group from ATP (forming ADP) to the transport protein.

• may induce a conformational change in the transport protein

 solute across the membrane.

• The sodium-potassium pump maintains (Na⁺) and (K⁺) [ions]

across the membrane.

– Typically, cell has higher [K⁺] and lower [Na⁺]inside the cell.

– pump uses one ATP to pump three Na⁺ out and two K⁺ in.

– http://www.youtube.com/watch?v=1ZFqOvxXg9M&feature=related

Na+ binding stimulates phosphorylation by ATP.

2 Na⁺

Cytoplasmic Na⁺ binds to the sodium-potassium pump.

1

K⁺ is released and Na⁺ sites are receptive again;

the cycle repeats.

3 Phosphorylation causes the

protein to change its conformation, expelling Na⁺ to the outside.

4

Extracellular K⁺ binds to the protein, triggering release of the Phosphate group.

Loss of the phosphate 6 restores the protein’s original conformation.

5

CYTOPLASM

[Na⁺] low [K⁺] high Na⁺

Na⁺

Na⁺

Na⁺ Na⁺

P ATP

Na⁺ Na⁺ Na⁺

P ADP

K⁺

K⁺

K⁺

K⁺ K⁺

K⁺ [Na⁺] high

[K⁺] low

Active Transport - Na

& K

pump

Some ion pumps generate voltage across membranes

• All cells maintain a voltage across their plasma membranes.

– cytoplasm of a cell is negative

– This voltage, membrane potential, ranges from -50 to -200 millivolts.

• favors the passive transport of cations into the cell and anions out of the cell.

• Two combined forces, electrochemical gradient, drive the diffusion of ions across a membrane:

Ex: Nerve cell

• Special transport proteins, electrogenic pumps, generate the voltage gradients across a membrane

– sodium-potassium pump

– proton pump ex: cristae of mitochondria, bacteria

EXTRACELLULAR FLUID

+

H⁺

H⁺

H⁺ H⁺

H⁺

H⁺ Proton pump

ATP

CYTOPLASM

+ +

+ + –

– –

–

–

+

Cotransport: a membrane protein couples the transport of two solutes

• As the solute that has been actively transported (H+, Na+) diffuses back passively through a transport protein, its

movement can be coupled with the active transport of another substance (sucrose, glucose) against its

concentration gradient.

• Ex: Plants use H+ to move sucrose, Liver uses Na+ to move glucose *both molec. must be present

Figure 7.19

Proton pump

Sucrose-H⁺ cotransporter

Diffusion of H⁺

Sucrose

ATP H⁺ H⁺

H⁺

H⁺

H⁺

H⁺ H⁺

+ +

+ +

+ – +

– – – – –

Three General Classifications for Bind & Release Conformational Transport Proteins

• Uniport - one substance, one way

– Facilitated (passive) transport - glucose into liver

• Symport - Two substances, same way

– Cotransport - Na

& glucose, recovery of glucose into kidney

• Antiport - Two substances, opposite ways – Na

K

Pump

• Ch. 36, Cell Membranes and Transport and Water

Potential p. 767-771

Exocytosis and Endocytosis transport large molecules

• Large polysaccharides and proteins, cross membrane via vesicles.

– exocytosis Golgi transport vesicle  plasma

membrane  bilayers fuse and spill the contents to the outside.

– endocytosis a cell brings in macromolecules by forming vesicles from plasma membrane.

• Ex: phagocytosis (solid particle) Cellular eating.

• Ex: pinocytosis (liquid) Cellular drinking

 Large polysaccharides and proteins, cross membrane via vesicles.

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?

it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis http://www.youtube.com/watch?v=1w10R9lv7eQ

Receptor-Mediated Endocytosis

• very specific in what substances are being transported – Special acquisition

• special receptors bind ligands,(molec to be trans.) triggers formation of vesicle.

– Cholesterol travels in blood as (LDL) LD lipoproteins – These lipoproteins bind to LDL receptors and are

endocytosed.

• In familial hypercholesterolemia, LDL receptors are

defective accumulation of LDL and cholesterol in the blood.

– atherosclerosis.

http://www.sumanasinc.com/webcontent/animations/content/end ocytosis.html

0.25 µm

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coated pit

Coated vesicle

A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs).

Plasma membrane

Coat protein Receptor-mediated endocytosis enables the

cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the

extracellular fluid. Embedded in the membrane are proteins with

specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins.

Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules

(green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

• Relating cell type to function. Also see p. 824-6 in Campbell

• Epithelial Tissue layers – Cover outside of the body or line organs or cavities

• Squamous - flat cells found where cells are abraided off. Ex:

linings

– Passive transport – movement across cells without energy – Diffusion

• Cuboidal – square shape, specialized for secretion via transport proteins Ex: thyroid and salivary gland

– Active Transport – using energy to transport against a gradient.

• Columnar – rectangular shaped, oriented as upright columns.

Sophisticated endomembrane system, processing vessicles

» Ex: Lining of intestine or nasal cavity, active absorbtion and secretion

– Active transport

– Endo and Exocytosis