Cell Membranes and Osmosis

- Cell membrane

- Controls what enters and leaves cell

What separates a cell from its surroundings?

Cell membrane is made out of two layers of fats. These fats are called

“phospholipids”Phospholipid

Since your cell membrane is made out of two layers of fats, the cell membrane is often called a “phospholipid bilayer”

LipidBilayer

The outside of the cell membrane likes to be near water

Hydrophilic – loves water

The inside of the cell membrane hates to be near water

Hydrophobic – hates or fears water

In addition to phospholipids, the cell membrane also has proteins in it.

These proteins help a cell to control what comes in and out of the cell.

Fluid mosaic model

These proteins can float around on the cell membrane like icebergs in the ocean and decorate the cell like a mosaic.

Cell membranes are “selectively permeable”

Permeable – EVERYTHING can pass through membrane

Non-Permeable – NOTHING can pass through membrane

Selectively-Permeable – SOME THINGS can pass through membrane

As long as it is small enough, it will DIFFUSE across cell membrane from high to low concentration

Not all molecules can diffuse through a cell membrane. However, one molecule can always

diffuse -

WATER!

Osmosis (diffusion of water) – water flows from an area of high concentration to an area of low concentration

Try osmosis with an animal cell. Assume starch – a really BIG sugar molecule – cannot move across an animal cell membrane.

Start by comparing the percent water inside and outside the cell.

Then, draw an arrow pointing in the direction that water will flow (high to low).

Finally, draw the result. (NOTE: animal cells can swell and shrink like a balloon – to a certain extent. Your own red blood cells can explode if they swell too much).

100% water

80% water

20% starch

Cell will lose water and shrivel

Try osmosis with an animal cell. Assume starch – a really BIG sugar molecule – cannot move across an animal cell membrane.

Start by comparing the percent water inside and outside the cell.

Then, draw an arrow pointing in the direction that water will flow (high to low).

Finally, draw the result. (NOTE: animal cells can swell and shrink like a balloon – to a certain extent. Your own red blood cells can explode if they swell too much).

80% water

20% starch

100% water

Cell will expand or even burst!

Use mouse to click here for an Osmosis Video

Turn on speakers

PLASMOLYSIS

Plasmolysis – a special case involving osmosis and plantsPlasmolysis – water moving from high to low concentrations in plant cells

Why would osmosis be different in plant cells? (Think: what makes plant cells different from animal cells?)

Plant cells have a cell membrane AND a cell wall.

100% water 80% water

20% starch

WHAT DO YOU THINK THIS CELL WILL LOOK LIKE?

PLASMOLYSIS

80% water

20% starch

100% water

Use mouse to click here for plasmolysis Video

Which one is in salt water?

Red blood cells in pure water

Red blood cells in salt water

PASSIVE AND ACTIVE TRANSPORT

How can cells transport nutrients and molecules that are too big to pass through their membranes?

Proteins – 1. Channels open up and allow certain

molecules into the cell.

2. Receptors “grab” molecules and bring them into the cell.

Proteins can work by “passive” or “active” transport

Passive transport (also known as “facilitated diffusion”) – proteins help molecules move across cell membrane from high to low; NO ENERGY is required

PASSIVE TRANSPORT

Animation Link

Animation Link 2

Active transport – proteins help move molecules across cell membrane from LOW to HIGH; ENERGY IS required

ACTIVE TRANSPORT

Animation Link

Not all molecules can move through cell membrane. Some are just too big! Different methods are needed.

ENDOCYTOSIS AND EXOCYTOSIS

Endocytosis – cell wraps membrane around object and pulls it in

There are three specific types of endocytosis

ENDOCYTOSIS

1. Phagocytosis – white blood cells “eat” bacteria and digest them

Animation Link

There are three specific types of endocytosis

ENDOCYTOSIS

2. Pinocytosis – cell is “drinking” in liquids

There are three specific types of endocytosis

ENDOCYTOSIS

3. Receptor-aided endocytosis– protein receptors work to pull molecules into cell

There are three specific types of endocytosis

EXOCYTOSIS

Exocytosis – molecules are released from cell