CHAPTER 8MEMBRANE STUCTURE AND

FUNCTION

• Plasma membrane is selectively permeable, (allowing some substances to cross more easily than others)

• PM is flexible – bends and changes shape

Plasma Membrane

Mostly lipids and proteins; some carbohydrates

• Lipids are in the form of phospholipids (mostly)

• Phospholipids are amphipathic molecules.– have both hydrophobic regions and hydrophilic regions.

Macromolecules in PM

Phospholipids …Phospholipids …

… are amphipathic… are amphipathicn

on

pol

ar

non

pola

r

hydrophilic

hydrophilic

hydrophobic

amphi-

“on both sides”

1895 – PM is made of lipids (water interaction??) 1925 – Phospholipid bilayer (flexibility; adhesion??)1935 – Davson & Danielli – proteins are outside

phospholipid bilayer (hydrophobic proteins, variability???)

Membrane models- history

1950 - EM

Membrane modelsFreeze-fracture studies (SEM) – proteins in the

middle of PM Fluid Mosaic Model – Singer and Nicholson (1972)

Phopholipid makes a bilayer; Integral and peripheral proteins are found ‘within’ or on ‘one side’ of the phospholipid bilayer as the name indicates

Hydrophilic regions of proteins (polar side chains) and phospholipids (polar head)– in contact with water

Hydrophobic tails/nonpolar protein regions are in a nonaqueous environment - tucked away in the middle of the membrane

Weak hydrophobic interactions maintain membrane structure; cholesterol - maintains fluidity;

Glycolipids (antigens) found on outside side of the membrane - provide cell-cell recognition (immune function)

Ion channels are formed by integral proteins to transport polar substances

Some integral/peripheral proteins are enzymes or receptors or junctions - function in cell communication

Fluid Mosaic model

Fig. 8.2b

Fluid Mosaic model

Why ‘Fluid’ and Why ‘Mosaic’?Fluid Mosaic

Proteins and lipids moveLaterally and can also Flip

Cholesterol (restricts it; except at low temp)

Structure is not the same on the inside and outside and from region to region (relate to lipids, integral and peripheral proteins, carbohydrates)

Function is related to structures(AP Theme!); therefore function is a mosaic - elaborate…

• Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other.

• Fluidity depends on unsaturation of fatty acids and presence of cholesterol. More unsaturation, more fluidity (why?).

• At normal and high temp. cholesterol limits fluidity; at low temp. cholesterol actually prevents freezing of membrane (increases fluidity)

Membranes are fluid

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.

Membranes are mosaics of structure and function

– Peripheral proteins are loosely bonded to the surface (inside or outside) of the PM, often connected to the other membrane proteins.

– Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (a transmembrane protein).

-Glycolipid (antigens) only on Extracellular side

Proteins in MembranesIntegral Proteins form alpha helix coils in the membrane

Proteins are connected to cytoskeleton (intracellular) and Extra Cellular Matrix (outside)

Integral Transmembrane Protein: Asymmetry in amino acids

gives the two membrane faces (extracellular and intracellular) their different characteristics

Integral Transmembrane Protein: Asymmetry in amino acids

gives the two membrane faces (extracellular and intracellular) their different characteristics

• Membranes have distinctive inside and outside faces.– The two layers may differ

in lipid composition, and proteins in the membrane have a clear direction.

– The outer surface also has carbohydrates.

Mosaic Structure Cont’d

• The proteins in the plasma membrane may provide a variety of major cell functions.

Fig. 8.9

Mosaic Function

Provides a hydrophilicchannel that is selective

for a particular solute

Provides a hydrophilicchannel that is selective

for a particular solute

Campbell; Fig 8.9

“active site”“active site”

“conformational change”

“conformational change”

– important in tissue and organ development.– rejection of foreign cells by the immune system.– Carbohydrates are covalently bonded either to lipids,

forming glycolipids, or, more commonly, to proteins, forming glycoproteins

– Example: The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells

Membrane carbohydrates are important for cell-cell recognition

• Small molecules and ions moves across the plasma membrane in both directions.

Transport Across the Membrane

Sugars, amino acids, oxygen, ions

Wastes, ions, CO2

Easy Access Need Assistance

• Hydrophobic molecules (lipids): hydrocarbons, CO2, and O2

• Small polar molecules - water

• Can get through because of small size or hydrophobic nature

• Ions and polar molecules like glucose, amino acids

• Size + charged nature prevents easy acess

• When do these substances need to get in?

Proteins can assist and regulate the transport of ions and polar molecules

Diffusion

Simple Facilitated

Energy Requirements

Requires no energy other than that of molecular motion

Channel

Proteins

Molecules go through lipid bilayer

Requires TRANSPORT

proteins

PassiveTransport

PassiveTransport

Carrier

Proteins

Diffusion is the tendency of molecules of any substance to spread out in the available space

driven by the intrinsic kinetic energy of molecules (so temp increase will…….).

a substance will diffuse from where it is more concentrated to where it is less concentrated, i.e. down its concentration gradient.

Passive transport- 1) Simple Diffusion

O2, and CO2; small polar molecules

include ethanol, H2O, and urea.

1) Channel proteins -provide corridors allowing a specific molecule or ion to cross the membrane.

These channel proteins allow fast transportWater channel proteins -aquaprorinsIons move through these channels

Passive transport- 2) Facilitated Diffusion

Channels

Open Closed

Channels that spend almost all of their time in theopen configuration are called

“leak” channels, or pores

Channels that spend almost all of their time in theopen configuration are called

“leak” channels, or pores

Channels that spend almost all of their time in the

closed configuration are called“gated”

Channels that spend almost all of their time in the

closed configuration are called“gated”

Ions, and H2O(SKIP details)

2) Carrier Proteins – Allows movement of

polar compounds

2) Carrier Proteins – Allows movement of

polar compounds

1. Molecules (substrates) actually bind with the carrier (amino acids, sugars, nucleosides, and other small molecules). Know this - ‘glucose’ and ‘aminoacid’ carrier proteins transport these substances into the blood from the small intestine!

2. Protein changes in shape (conformational change)

3. It allows molecules to pass through

1. Molecules (substrates) actually bind with the carrier (amino acids, sugars, nucleosides, and other small molecules). Know this - ‘glucose’ and ‘aminoacid’ carrier proteins transport these substances into the blood from the small intestine!

2. Protein changes in shape (conformational change)

3. It allows molecules to pass through

Passive transport- 2) Facilitated Diffusion

Transport proteins Enzymes

Specific binding sitesCan become saturated

Can be inhibited

Catalyze a process

Transport Proteins are like Enzymes

3. Osmosis is the passive transport of water

Tonicity describes how the size of a cell would change if it were placed in a solution

Tonicity describes how the size of a cell would change if it were placed in a solution

Fig. 8.11

Osmosis is the passive transport of water – it occurs until isotonicity is reached

Osmosis – passive diffusion of water across a semipermeable membrane

Cell survival depends on balancing water uptake and loss

Osmosis is the diffusion of water through a selectively permeable membrane.

Osmosis is the diffusion of water through a selectively permeable membrane.

Tonicity describes how the size of a cell would change if it were placed in the solution

Tonicity describes how the size of a cell would change if it were placed in the solution

Isotonic – same solute/water concentrations as inside cells

so cells retain their normalsize and shape

Isotonic – same solute/water concentrations as inside cells

so cells retain their normalsize and shape

H2O

H2O

Lab - hemodialysis animation

Be sure to know how these calculations were performed (on next test).A) Open system; osmosis

raises water in arm with more solute

B) Plunger pushes and opposes osmosis (back pressure)

C) Plunger keeps pushing, causing water to move against concentration gradient

D) Result: Water rises in arm on opposite side

Wilted(cells may be flaccid or plasmolysed)

Turgid

• Osmoregulation : maintanance of osmotic balance

• Paramecium – protist (contractile vacuole)

Cell survival depends on balancing water uptake and loss

• The cells of plants, prokaryotes, fungi, and some protists have cell walls that contribute to the cell’s water balance.

• Active transport requires the cell to use its own metabolic energy (ATP).

• Active transport is against the Concentration Gradient (solutes move from low concentration to high concentration)

4. Active transport is the pumping of solutes against their gradients

• The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membrane.

All cells maintain a voltage across their plasma membranes using pumps.

The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of cations (+) and anions (-) on opposite sides of the membrane.

This voltage, the membrane potential, ranges f50 to -200 millivolts.

Some ion pumps generate voltage across membranes

• The membrane potential acts like a battery.

• The membrane potential favors the passive transport of cations (+) into the cell and anions (-) out of the cell.

• Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane: – a chemical force based on an ion’s concentration

gradient – an electrical force based on the effect of the

membrane potential on the ion’s movement.

• Ions diffuse down their electrochemical gradient.• Special transport proteins, electrogenic pumps,

generate the voltage gradients across a membrane• Examples of electrogenic pumps – Na-K pump, proton

(H+) pump

Some ion pumps generate voltage across membranes

• Proton pump actively transports H+ out of the cell.

• Plants, fungi, mitochondria, chloroplast

• These electrogenic pumps store energy that can be used to make ATP.

oncentration gradient.

In cotransport, a membrane protein couples the transport of two solutes

• Small molecules and water Transport proteins

• Large macromolecules Vesicles

• Exocytosis: a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane.

Exocytosis and Endocytosis transport large molecules (need ATP)

• Endocytosis - a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane.

• Endocytosis is a reversal of exocytosis.

Fig. 8.19a

• One type of endocytosis is phagocytosis, “cellular eating”.

• In phagocytosis, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large vacuole.

• The contents of the vacuole are digested when the vacuole fuses with a lysosome.

• In pinocytosis, “cellular drinking”, a cell creates a vesicle around a droplet of extracellular fluid.

Fig. 8.19b

• Receptor-mediated endocytosis is very specific in what substances are being transported.

• Extracellular substances bind to special receptors, on the membrane surface, especially near coated pits.

• This triggers the formation of a vesicle

Fig. 8.19c

Transport Overview:• Simple Diffusion• Facilitated diffusion (carrier and channel proteins)• Osmosis – diffusion of wwater• Active Transport• Electrogenic pumps• Cotransport• Exocytosis• Endocytosis and Receptor- mediated endocytosis• Phagocytosis and Pinocytosis

• Why is the plasma membrane structure called the FLUID MOSAIC MODEL?

• Review your Lab