Unit 2: Chapter 5

Biological Membranes

Structure of the Cell Membrane

6 main components

1. Phospholipids

2. Proteins

3. Cholesterol

4. Carbohydrates

5. Glycoproteins

6. glycolipids

Phospholipids (review)

• Membrane is a bilayer– One layer of polar heads – they are

hydrophilic because they love water– One layer of nonpolar tails – they are

hydrophobic because they hate water

Membranes have proteins!• 2 main types

1. Integral proteins• Inserted into the membrane• May be unilateral – reach only partway across

the membrane• Or they may be transmembrane – completely

span the membrane• Removal disrupts cell membrane

2. Peripheral proteins• Not embedded in membrane, but attached to the

membrane surface by either integral proteins or filaments from the cytoskeleton

• Removal has little effect on cell membrane

7 functions of membrane proteins

1. Anchoring cell2. Passive transport3. Active transport4. Enzyme activity5. Signal transduction – transmitting info

into cell6. Cell recognition – like ID tags7. Junction between cells – cell adhesion

Cholesterol

• Controls the fluidity of the cell membrane

History of the Cell Membrane

• 2 models1. Sandwich Model

2. Fluid Mosaic Model

Sandwich Model

• Davson and Danielli in 1935

• Perceived the plasma membrane as a sandwich where the proteins were the bread and the phospholipid bilayer was the meat

• Singer and Nicholson in 1972

• Said the membrane is fluid and must be

• If it solidifies, its permeability will change and enzymes will denature

• Said the membrane is mosaic – there are proteins embedded in it

Fluid Mosaic Model

Nature of protein and lipid mobility

• Lateral movement of lipids/proteins is quick• Lipids and proteins rarely flip across the lipid

bilayer

Basic Terms to Understand

• Selectively permeable – prevents passage of most materials through the membrane

• Solute – what is dissolving (salt, sugar, etc.)

• Solvent – what it is dissolving in (water etc.)

• Solution – mixture of solvent and solute

7 ways substances can get into a cell

1. Diffusion

2. Bulk flow

3. Osmosis

4. Facilitated diffusion

5. Active transport

6. Vesicle mediated transport

7. Cell to cell junction

Diffusion

• Moves materials from a high concentration to a low concentration

• Requires no energy – type of passive transport• Easy passage through – oxygen, carbon

dioxide, nitrogen, and small polar molecules• Slow passage through – large polar molecules

like glucose and charged ions– Proteins allow movement of charged/polar molecules

• Particles move until equilibrium

Bulk Flow

• Molecules move all together in same direction due to hydrostatic pressure

Osmosis

• ALL things undergo diffusion. • Water also diffuses, however, the water diffusion

is not evident unless it crosses a membrane.• Osmosis is the diffusion of water across a

membrane from high to a low concentration• No energy is required – type of passive transport• Since cells have membranes, osmosis is

important to cells

Membranes and Osmosis

• Tank w/ semipermeable membrane: water may pass, solute can’t

• At first the concentration of solute is very high on the left.• But over time, the water moves across the

semipermeable membrane, and dilutes the solute.

Water Moves Because It is Polar• Because water is polar, it binds to the solute by

hydrogen bonds • The concentration of water is higher on the right• Water will then flow across the membrane, down its

concentration gradient, to the left

Osmotic pressure

Water flow

}Solution presses

Three osmotic environments

• Hypertonic

• Hypotonic

• Isotonic– Same solute concentration inside and outside

the cell– Water flows in and out of the cell equally in

both directions– Most cells in our body are isotonic

Hypertonic

• High concentration of solute outside the cell

• Therefore there is more water inside the cell

• Water will move out of the cell

• If this process continues, the cell collapses and dies – this is called plasmolysis

Hypotonic

• Low concentration of solute outside the cell

• Therefore there is more water outside the cell

• Water will move inside the cell

• This causes the cell to expand, causing turgor pressure in plant cells Animal cells could burst – this is called cytolysis

Plant Cells and Osmotic Pressure• In plants, hypotonic solutions produce osmotic pressure

that produces turgor pressure– Turgor means “tight or stiff owing to being very full” – Keeps plant upright; in hypertonic conditions plants wilt

Hypotonic solution Hypertonic Hypertonic

Vacuole fills

Vacuole shrinks

Dialysis• Dialysis is the diffusion of solutes across a membrane• The selectively permeable membrane allows small

sugar molecules to move across the membrane, but large proteins cannot

Facilitated Diffusion

• Transport proteins move materials through membrane

• 3 kinds of transport proteins1. Uniport – carries a single molecule across

the membrane

2. Symport – moves 2 different molecules at the same time in the same direction

3. Antiport – exhanges 2 molecules in opposite directions

Vesicle Mediated Transport

• When vesicles or vacuoles fuse with the membrane to move substances in or out of the cell

• 2 main types1. Exocytosis – when vesicle expels contents

outside the cell

2. Endocytosis – when vesicles bring substances into the cell• 3 types

3 types of Endocytosis

1. Phagocytosis – solid being taken into the cell

2. Pinocytosis – liquid being taken into the cell

3. Receptor Mediated Endocytosis – substance binds to a specific receptor on the cell before it is brought in

Active Transport

• Energy is needed

• Moves materials against the concentration gradient

• Main example: sodium potassium pump

Sodium Potassium Pump

• Occurs in animal cells• Required ATP (active transport)• Exchanges 3 Na+ ions on inside for 2 K+

ions on outside• This exchange is uneven so an electric

potential is generated and so the membrane is now considered to be polarized

• Let’s see this in action

How do cells stick together and let materials?

1. Plasmodesmata

2. Gap junctions

3. Desmosomes

4. Tight junctions

Plasmodesmata

• They are channels

that allow movement

of certain molecules

and ions between plant cells

Gap Junctions

• Cytoplasmic channels between neighboring animal cells

• Let adjacent cells communicate

• Small dissolved molecules and electrical signals may pass from one cell to the other

• Very similar to plasmodesmata

Desmosomes• Function as rivets

and join animal cells together

• They are reinforced by intermediate filaments made of keratin

• Still permits materials to move around them in the intercellular space

Tight Junctions• Continuous belt

around animal cells that fuse membranes of neighboring cells

• It is leak proof

• Contains no intercellular space

Water Potential

• Chemical potential of water

• Measure of the energy available for reaction or movement

• Measures the ability of water to move and water always moves from areas of higher potential to areas of low potential

• Has the symbol Ψ (psi)

Water Potential Continued…

• Is measured in the unit bars

• The formula for calculated water potential is osmotic potential (solute) + pressure potential

So, the equation is

Ψ = Ψs + Ψp

Equation Components

Ψp = pressure on the system

= 0, if the system or container is open

** all of our problems will be open so

Ψp will always equal 0.

Ψs = change in water potential due to solute molecules

The more solute, the lower the water potential

Ψs = -iCRT

Wherei = ionization constant = # of ions in the solute = 1 when there are no ions present

C = Molar Concentrationusually given in problemequal to Molarity (M), or moles/volume

R = pressure constant = 0.0831 Liters X Bars/moles x Kelvin this number never changes

T = temperatureneeds to be in KelvinConversion of Celsius to Kelvin is

K = 273 + Celsius

Suppose we have a beaker of distilled water at room temperature. (0M) What is the water potential?

Ψ= Ψs + Ψp

Ψ = -iCRT + 0

Ψ = -(1)( 0) (0.0831)(23 + 273) + 0

Ψ = 0 + 0

Ψ = 0

Suppose we have an open beaker A that contains Sucrose (2.5M) at 25 C.

What is the water potential?Ψ = Ψ s + Ψ p

Ψ = -iCRT + 0

Ψ = -(1)(2.5)(0.0831)(25 + 273)

Ψ = -61.91 bars