Inside the Cell

AP Biology - Summer 2013

WEEK 2

Biochem QUIZ

Write a quick sentence to describe the

difference between each pair of

concepts:

1. Organic molecules vs. inorganic

molecules

2. Hydrogen bonds vs. covalent bonds

3. Monomers vs. polymers

4. Carbohydrates vs. proteins

5. Substrate vs. catalyst

Biochem QUIZ

1. Organic molecules vs. inorganic moleculesOrganic molecules contain covalently bonded carbon. Inorganic molecules may not contain carbon, or the carbon is not covalently bonded together.

2. Hydrogen bonds vs. covalent bondsHydrogen bonds are weak connections between different molecules. Covalent bonds are strong bonds between atoms in the same molecule.

3. Monomers vs. polymersMonomers are single, small molecules. Polymers are long molecules formed by connecting monomers.

4. Carbohydrates vs. proteinsCarbohydrates are made of carbon, hydrogen, and oxygen. Proteins also include nitrogen, sulfur, and other atoms.

5. Substrate vs. catalystThe substrate is the reactant, which changes during a chemical reaction. The catalyst or enzyme helps the reaction happen, but it will not change during the reaction.

The First Cells

Last week, we discussed the primordial soup, a theory to describe the creation of the macromolecules we need to form cells. But why do we need macromolecules?

The first thing a cell needs is a membrane that separates the inside from the outside.

And this membrane needs to organize itself, because there are no cells around yet to help it get together.

INTRODUCING…THE PHOSPHOLIPID!(the early Earth’s first self-organizing membrane

molecule)

Featuring…◦ A phosphate ―head‖

◦ Two lipid ―tails‖

◦ A glycerol ―backbone‖

The fatty acid tails are non-polar: they share electrons equally and have no charge.

The phosphate head is polar; it shares electrons unequally like water.

The phosphate heads are hydrophilic—

because they are polar, they like to mix

with water.

The lipid tails are hydrophobic—because

they are nonpolar, they hate water.

So if you put phospholipids in water, they naturally form

protective bubbles called micelles with the tails

pointing in.

But these micelles aren’t big enough to hold anything!

The Phospholipid Bilayer

By instead having a double layer, or bilayer, cells can have a watery inside and outside, while the fatty acid tails never touch water.

This membrane is semipermeable—some things, like water and dissolved gases, can pass through easily.

Large molecules and charged ionscannot go through without help.

The Fluid Mosaic Model

It’s not all phospholipids … there are also

embedded membrane proteins

Proteins can help with transport or

signalling Cholesterol is an

important part of the membrane. It also has hydrophilic and hydrophobic parts. It stabilizes and seals the membrane.

All the molecules on the membrane are fluid and movable.

Why Are Cells Small?

The plasma membrane is where all the action happens in a cell—so it’s important to have a high surface to volume ratio.

The bigger the cell, the smaller this ratio, meaning there is less plasma membrane to go around.

DIFFUSION

the movement of any

particle from an area

of high concentration

to an area of low

concentration

Diffusion is caused

by ―Brownian motion‖

– the random motion

of particles in any

fluid (gas or liquid).

Concentration of red

is high on the left side

Everything is even—this is

dynamic equilibrium

(low on the

right side)

TIME

OSMOSIS

a special name for

diffusion when the

diffusing particle is

water, and it’s going

across a membrane

If the membrane is

selectively permeable

(like the cell

membrane!) then water

can get through when

other molecules can’t.

This can lead to uneven water distribution

on the different sides of a membrane.

Cells in Solution Isotonic solutions have the same concentration of

water inside and outside. Water passes in and out, but equally in either direction.

Hypertonic solutions have less water on the outside. Water rushes out of the cell into the salty or sugary outside.

Hypotonicsolutions have more water outside. Water rushes into the cell.

Plant cells and animal cells react differently to water concentrations. Animal cells in hypotonic solution will lyse or burst, and cannot survive in pure water.

Plant cells, protected by their cell walls, simply grow turgid and crisp—they are healthy.

Passive Transport Osmosis happens automatically in the cell—

the membrane cannot stop it. It takes no effort!

Transport across the membrane with no energy expended is called passive transport.

Larger molecules can also move passively, but they must be passed through transport proteins that form large channels in the cell membrane. This is called facilitated diffusion.

Different ions and molecule types have different specific channels.

Active Transport

For a molecule to move in the wrong

direction (up the concentration

gradient), the cell must apply energy

(ATP) in a process called active

transport. Each type of ion

(K+, H+, Cl-,

Na+) has its

own specific

protein ―pump‖

for transport.

The Phospholipid Bouncer

Passes through the membrane freely:

◦ Water (H2O)

◦ Dissolved gases (N2, O2, CO2 )

◦ Small uncharged polar molecules (urea,

alcohol)

Transported through proteins:

◦ Ions (K+, Na+, Cl-)

◦ Small molecules (neurotransmitters)

Endocytosis and exocytosis:

◦ Very large polymers (proteins)

◦ Bacteria and viruses

The Structure of ATP

As we saw, active transport requires the cell to spend some energy. The ―energy currency‖ in a living cell for interactions like these is ATP, adenosine tri-phosphate.

ATP is composed of (a) an adenosine nucleotide, (b) a ribose sugar, and (c) three phosphate groups bonded together.

The ATP Energy Cycle When the cell needs some

energy from ATP, it gets it by breaking a chemical bond—specifically, the bond between the second and third phosphate group.

Now there are only two phosphate groups attached, so it’s no longer tri-phosphate. Instead, this low energy molecule is called ADP, adenosine di-phosphate.

Luckily, ADP is reusable, and can go to the mitochondrion in the cell to be ―recharged‖ and have another phosphate

If red blood cells cultured in an isotonic

medium are placed in distilled water,

they will most likely

(A) Remain unchanged

(B) Shrivel

(C) Swell and lyse

(D) Divide

(E) Become dormant

If red blood cells cultured in an isotonic

medium are placed in distilled water,

they will most likely

(A) Remain unchanged

(B) Shrivel

(C) Swell and lyse

(D) Divide

(E) Become dormant

Some videos

Diffusion through a membrane:

http://www.youtube.com/watch?v=2Th

0PuORsWY&feature=related

Diffusion and osmosis discussion

http://www.youtube.com/watch?v=W0

Dm06BsYBI