Chapter 8. Movement across the Membrane

1

2003-2004 AP Biology

Chapter 8.

Movement across the Membrane

2004-2005 AP Biology

Cell membrane

 Cells have an inside & an outside

 Cell membrane is the boundary

Can it be an impenetrable boundary?

NO!

Why not?

The cell needs materials in

&

products or waste out!

3

2004-2005 AP Biology

Semi-permeable membrane

 Need to allow passage through the membrane

 But need to control what gets in or out

membrane needs to be semi-permeable

So how do you build a

semi-permeable membrane?

2004-2005 AP Biology

The Cell’s needs

 What kinds of molecules need to get through?



sugars & carbohydrates



peptides & proteins



lipids



nucleic acids



water

5

2004-2005 AP Biology

Cell membrane structure

 Basic structure of cell membrane is phospholipid bilayer

What molecules can go directly through?

2004-2005 AP Biology

Cell membrane permeability













So what gets through?

• Lipid-soluble molecules (non-polar, hydrophobic)

7

2004-2005 AP Biology

Cell membrane permeability



Put holes in it!

But the cell needs control…

Put selective holes in it!

How?

PROTEINS!

2004-2005 AP Biology

Transport proteins



open channel

conformational changes

Cellular Donuts!

Each transport protein is specific as to the substances that it will translocate (move).

For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.

Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.These channel proteins allow fast transport.

For example, water channel proteins, aquaprorins, facilitate massive amounts of osmosis.

Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport.

9

2004-2005 AP Biology

Getting through cell membrane



 diffusion of hydrophobic molecules

 high → low concentration gradient



 diffusion of hydrophilic molecules

 through a protein channel

 high → low concentration gradient



 diffusion against concentration gradient

 low → high

 uses a protein pump

 requires ATP

2004-2005 AP Biology

Passive transport is diffusion



biological systems

Universe tends towards disorder

 Diffusion

movement from high → low concentration of that item

Movement from high concentration of that substance to low concentration of that substance.

11

2004-2005 AP Biology

Diffusion of 2 solutes

 Each substance diffuses down its own concentration gradient, independent of concentration gradients of other

substances

Things tend to get mixed up evenly.

2004-2005 AP Biology

Facilitated diffusion

 Movement of molecules down

concentration gradient via transport protein

high → low concentration

protein channel

13

2004-2005 AP Biology

Gated channels

 Some channel proteins open only in presence of stimulus (signal)

stimulus usually different from transported molecule

 ex: ion-gated channels

when neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na⁺ ions to enter nerve cell

 ex: voltage-gated channels

change in electrical charge across nerve cell membrane opens Na⁺ & K⁺ channels

When the neurotransmitters are not present, the channels are closed.

2004-2005 AP Biology

Gated

channels

15

2004-2005 AP Biology

Active transport

 Cells may need molecules to move

need to pump against gradient

protein pump

requires energy (usually ATP)

conformational change in transport protein

2004-2005 AP Biology

Active transport

 examples

Na⁺ – K⁺ pump in neurons

H⁺ pumps in mitochondria & chloroplasts

nitrate & phosphate pumps in plant roots

Plants: nitrate & phosphate pumps in roots.

 Why?

 Nitrate for amino acids

 Phosphate for DNA & membranes

Not coincidentally these are the main constituents of fertilizer

 Supplying these nutrients to plants

 Replenishing the soil since plants are depleting it

17

2004-2005 AP Biology

 The sodium-potassium pump actively maintains the

gradient of sodium (Na⁺) & potassium ions (K⁺) across the membrane

 an animal cell has higher concentrations of K⁺ and lower concentrations of Na⁺ inside the cell

 sodium-potassium pump uses the energy of one ATP to pump 3 Na⁺ ions out and 2 K⁺ ions in

2004-2005 AP Biology

Transport summary

19

2004-2005 AP Biology

How about large molecules?

 Moving large molecules into & out of cell

through vesicles & vacuoles

endocytosis

 phagocytosis = “cellular eating”

 pinocytosis = “cellular drinking”

 receptor-mediated endocytosis

exocytosis

exocytosis

2004-2005 AP Biology

Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

fuse with lysosome for digestion

non-specific process

triggered by ligand signal

21

2003-2004 AP Biology

Movement of water across

the cell membrane

2004-2005 AP Biology

The special case of water

 Water is critical to biological systems, so we talk about water separately

 Osmosis

diffusion of water down its concentration gradient

23

2004-2005 AP Biology

Concentration of water

 Direction of osmosis is determined by comparing total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water

hypotonic hypertonic water

net movement of water

2004-2005 AP Biology

Managing water balance

 Cell survival depends on balancing

water uptake & loss

25

2004-2005 AP Biology

Managing water balance

 Isotonic

animal cell immersed in isotonic solution

blood cells in blood

no net movement of water across plasma membrane

water flows across

membrane, at same rate in both directions

volume of cell is stable

2004-2005 AP Biology

Managing water balance

 Hypotonic

animal cell in hypotonic solution will gain water, swell & burst

 Paramecium vs. pond water

 Paramecium is hypertonic

 H₂O continually enters cell

 to solve problem, specialized organelle, contractile vacuole

 pumps H₂O out of cell = ATP

plant cell

 turgid

27

2004-2005 AP Biology

Water regulation

 Contractile vacuole in Paramecium

2004-2005 AP Biology

Managing water balance

 Hypertonic

animal cell in hypertonic

solution will loose water, shrivel

& probably die

 salt water organisms are hypotonic compared to their environment

 they have to take up water &

pump out salt

plant cells

 plasmolysis = wilt

29

2004-2005 AP Biology

Any Questions??