Membrane Structure and Function

Passive Transport

Membranes organize the chemical activities of cells. Separate

Regulate

Surround organelles in eukaryotes

Are selectively permeable

Membrane phospholipids form a bilayer. Phospholipids have hydrophilic and

hydrophobic zones

In water

The hydrophobic interior provides a barrier that regulates passage into and out of the cell

The membrane is a fluid mosaic of phospholipids and proteins.

It is a mosaic because of the proteins

It is fluid because the individual molecules move about.

Carbohydrates are found on the outside and provide recognition between cells

Cholesterol helps stabilize the fluidity at different temperatures.

Carbohydrates act as identification tags for cells

Passage Through The Phospholipid Bilayer What molecules can pass through the

phsopholipid bilayer? Small non polar Large non polar Small polar What molecules cannot pass freely through

the bilayer? Large polar Ions

Passive transport is diffusion across a membrane.

What is diffusion? Diffusion is the tendency for particles to spread

out spontaneously from an area of high concentration to an area of low concentration. It requires no work. Remember the picture…

Where is there more potential energy?

What is a concentration gradient?

Molecules move from a region of higher to a region of lower concentration.

This provides a net direction to their movement

It is called going down the concentration gradient At equilibrium, molecules continue to move back and

forth, but there is no net directional flow

Example – There are 100 people stuffed into this room; there are only 10 people in the room next door.

Where would you move?

Does the diffusion of more than one kind of particle work together or separately?

Do the particles stop moving once equilibrium

is reached?

Does the diffusion of more than one kind of particle work together or separately?

NO!

If a molecule can move freely through the phospholipid bilayer what always controls the direction of its movement? Concentration gradient.

Remember that the concentration gradient represents potential energy.

Water, Water Everywhere, But Not a Drop to Drink….

Osmosis

What is osmosis? The diffusion of water across a semi-

permeable membrane.

The cell can concentrate solutes that are not permeable to the phospholipid bilayer on one side of the membrane.

Which way will water move?

Water will follow the solutes.

What do the terms hypotonic, hypertonic and isotonic mean?

Hypotonic = lower solute concentration and therefore higher water concentration

Hypertonic = greater solute concentration and therefore lower water concentration

Isotonic = equal solute concentration Using the terms hypotonic and hypertonic,

which way will water flow? From hypotonic to hypertonic.

??

Which side is hypotonic and which side is hypertonic?

A B

Which way will the water

move?

WHY?

Water balance between cells and their surroundings is crucial to organisms. Cell membranes act as selectively permeable

membranes between the cell contents and its surroundings.

If a plant or an animal cell is isotonic with its surroundings, no osmosis occurs, and the cells do not change.

Cells in a hypotonic environment An animal cell will gain water and lyse (pop). A plant cell will swell and become turgid (stiff).

Cells in a hypertonic environment

An animal cell in a hypertonic solution will lose water and shrivel. This process is called crenation.

A plant cell in a hypertonic solution will lose water and lose turgor. This process is called plasmolysis.

Is this cell in a hypotonic, hypertonic, or isotonic environment?

Isotonic

Is this cell in a(n) hypotonic, hypertonic, or isotonic environment?

Hypotonic

Do Water Molecules Stop Moving in Isotonic Conditions? No. They continue to diffuse, however there is no

net movement!

Two solutions equal in solute concentrations so that osmosis does not occur between them are isotonic to each other.

However, even in an isotonic solution, water molecules are moving; they move at equal rates in both directions. There is no net movement.

Specific proteins facilitate diffusion across membranes. Facilitated diffusion occurs when a transport

protein allows a solute to diffuse. What solutes need facilitated diffusion? Ions The cell does not expend energy, therefore,

this process is passive.

Cells expend energy for active transport. Active transport involves the aid of a transport protein Particles move up a concentration gradient

(from an area of low concentration to an area of high concentration).

Energy is required to help the protein to move the particles.

What molecular form does this energy usually have?

ATP

Active Transport

Active Transport Example

Facilitated Diffusion OR Active Transport?

Review of passive and active transport:

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Review of passive and active transport:

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Review of passive and active transport:

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Review of passive and active transport:

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Review of passive and active transport:

Exocytosis and endocytosis transport large molecules. In exocytosis, membrane-bound vesicles (tiny

membrane-bounded sacs) containing large molecules fuse with the plasma membrane and release their contents outside the cell.

In endocytosis, the plasma membrane surrounds materials outside the cell, closes around the materials, and forms membrane-bound vesicles to contain the materials. Two important types of endocytosis are

phagocytosis (“cell eating”) and pinocytosis (“cell drinking”).

How Do Large Molecules Move Across a Membrane? Endocytosis or Exocytosis Energy is needed Molecules move using a “pocket” created by the cell “Endo” means “inside”, “exo” means “outside”

What Are The Different Methods In Which Large Molecules May Enter a Cell? Pinocytosis & Phagocytosis Pinocytosis – “cellular drinking”

Large molecules in solution are ingested by cell Phagocytosis – “cellular eating”

Cell sends out extensions of cytoplasm that surround and engulf large particles

Pinocytosis

Phagocytosis