Chapter 7: Membrane structure, osmosis, transport across membranes

Lecture 9

What are the functions of the cell membrane?

1. Cell-cell recognition and communication involve specific molecules on cell surfaces

2. Regulate transport into and out of the cell

How do membranes regulate trasnport into and out of the cell?

1. Channels (Na⁺, K⁺, etc.), endocytosis

2. Cell-cell connections

What are some functions of membrane proteins?

1. Transport

2. Enzymatic activity

3. Signal transduction

4. Intercellular joining

5. Cell-cell recognition

6. Attachment to the cytoskeleton and extracellular matrix (ECM)

What does the extracellular matrix (ECM) do?

Helps the cell adhere and communicate

What are the general components of the ECM?

1. Proteins

2. Filaments of cytoskeleton

3. Carbohydrates

4. Glycolipids

5. Collagen

6. Fibronectin

What do integrins do on the ECM?

Bind outside ECM and transmit info from ECM to cytoskeleton inside

How are charged and larger/polar molecules transported across membranes?

Through hydrophilic membrane proteins that act as channels or as shuttles

Is transport selective/exclusive and unidirectional/bidirectional

Transport is selective and often unidirectional

How do small molecules transport across the membrane?

They easily flow through

What are two kinds of transport across membranes?

1. Passive

2. Active

What is passive transport?

Cell does not spend energy, down concentration gradient

What kind of molecules usually move passively across membranes?

Hydrophobic and small uncharged molecules

What is diffusion?

Transport of a solute down a concentration gradient

What is osmosis?

Transport of water down its concentration gradient

What are the two types of passive transport?

1. Diffusion

2. Osmosis

If diffusion stops at equilibrium, do molecules also stop moving?

No, at equilibrium, individual molecules do not stop moving, but their concentrations remain constant

How do solutes that are too large and can’t diffuse move?

Water molecules will move through the membrane to dilute more concentrated solute (OSMOSIS)

What are the relative measures of the ability for osmosis?

1. Hyperosmotic (hypertonic)

2. Isoosmotic (isotonic)

3. Hypoosmotic (hypotonic)

What is a hypotonic solution?

Concentration of solute is lower than the solution its being compared to (water moves in)

What is a hypertonic solution?

Concentration of solute is higher than the solution its being compared to (water moves out)

What is an isotonic solution?

The concentration of solute is same/equal to the concentration of the solution its being compared to (water flows equally)

Which way does water move in the selectively permeable membrane?

Water moves from lower to higher solution concentration (to dilute it)

How is osmosis characterized?

1. Osmotic pressure

2. Cells have many solutes that can’t diffuse through membranes and are hypertonic to pure water

What is osmotic pressure?

The tendency for a solution to take up water when separated from pure water by a selectively permeable membrane

How is osmotic pressure measured?

Measured in mOsm = sum of concentrations of all ions

What are the 3 types of cellular osmotic environment?

Animal cell

1. Hypotonic = lysed

2. Isotonic = normal

3. Hypertonic = shriveled

Plant cell

1. Hypotonic = turgid (normal)

2. Isotonic = flaccid

3. Hypertonic = plasmolyzed

What is facilitated diffusion?

An intermediate step between diffusion and active transport

Which way does facilitated diffusion move in a concentration gradient?

Moves down a concentration gradient (usually hydrophilic substances)

What facilitates facilitated diffusion?

Transport proteins

How does transport proteins function?

They function like enzymes

1. Bind specifically to transport substrate (cargo)

2. Exhibit saturation by transport substrate

What is active transport?

Works against a concentration gradient and requires energy

What is the sodium-potassium pump?

Type of active transport where cells have high concentrations of potassium ions and low concentrations of sodium ions

How does the sodium-potassium pump maintain its gradient?

By active export of sodium and import of potassium

What is the composition of the sodium-potassium pump?

1. Four-protein complex that spans the membrane

2. Binds 3 sodium ions inside the cell and receives energy by phosphorylation

3. Expels sodium ions to outside

4. Binds 2 potassium ions outside the cell → dephosphorylated to return to original conformation → release potassium ions inside cell → binds 3 ions of sodium

5. Causes hydrolysis of ATP → called sodium-potassium ATPase

What is the chlorine channel?

Ion transport channel regulated by phosphorlyation (cAMP dependent)

What do defects in the chlorine channel cause? What is the channel called?

Cystic fibrosis → CFTR = CF transmembrane conductance regulator

What is co-transport?

H⁺ gradient can drive another active transport, occurs through an ATP-dependent pump

How does co-transport increase membrane potential?

By removing protons

How are large molecules transported across membranes?

1. Phagocytosis

2. Pinocytosis

3. Receptor-mediated endocytosis

What is phagocytosis?

Non-specific engulfing and internalizing a particle

What is pinocytosis?

Non-specific extracellular fluid and dissolved substances are taken in

What is receptor-mediated endocytosis?

Specific internalization (the major type of transport of macromolecules across membranes)

How does receptor-mediated endocytosis work?

1. Macromolecules bind to a specific receptor on cell surface (coated pits)

2. Coated pits are coated with clathrin on the inner side of the membrane

3. Binding results in internalization and formation of transport vesicles (endosomes)

What are some examples of receptor-mediated endocystosis?

1. Uptake of cholesterol

2. Virus infenction of endocytosis (influenza)

What is the structure of a phospholipid?

Hydrophilic head, hydrophobic tail (one tail has a “kink” caused by double bond in unsaturated carbon)

What is a simple membrane?

Phospholipid bilayer

What is the fluid mosaic model of the phospolipid bilayer?

hydrophilic region of protein is outside, hydrophobic region is within

What does fluid mosaic mean?

Membrane lipids drift laterally and even “flip flop”

How are lateral movements of lipids visualized?

Fluoresence photobleaching

When do membranes solidify?

At low temperatures

What is the dual effect of cholesterol?

Cholesterol reduces fluidity at higher temperature and increases it at lower temperatures

What are membrane proteins made up of?

Integral and peripheral

What are integral proteins?

Proteins that are at least partly inserted into membranes; most completely span it (several times)

What are peripheral proteins?

Proteins that are attached to the membrane surface, but not inserted

What are transmembrane proteins?

Hydrophobic

Are biological membrane asymmetric or symmetric?

They’re asymmetric, their composition is different for inner and outer layers