MOVING THROUGH MEMBRANESMOVING THROUGH MEMBRANES

Basic Ideas of Biology

Unit 1

Issue One: Membrane Structure

Membranes are important to cell structure and function.

The plasma membrane

– surrounds the cell’s cytoplasm

– separates the cell from its environment

– regulates what moves in and out of the cell

Internal membranes surround and form part of the structure of many of the organelles of eukaryotic cells

Nucleus

Endoplasmic reticulum

Mitochondria and chloroplasts

Golgi bodies

Lysosomes

Phospholipids form the framework of a cell’s membranes.

A phospholipid molecule consists of two parts:

– a hydrophilic head, which contains phosphorus.

– a hydrophobic tail, composed of two fatty acid molecules

H2O

H2O H2OH2O

H2O H2O H2O H2O

H2O

Water makes up a major part of the cell’s internal and external surroundings.

Membrane

The tails project inward--away from the water.

The hydrophilic heads are in contact with the water.

The framework of the membrane is a phospholipid bilayer.

Phospholipid molecules arrange themselves in two layers facing each other.

H2O

H2O H2OH2O

H2O H2O H2O H2O

H2O

Proteins are also an important part of membrane structure.

Some proteins penetrate the entire phospholipid framework; others do not.

Carbohydrate molecules attached to the outer surface help the membrane recognize various substances.

Issue Two:Methods of Transport

Concentration Gradient

Diffusion is the net movement of molecules from a region where they are highly concentrated to a region where they are less highly concentrated.

Diffusion stops when molecules are evenly distributed.

Diffusion can occur through membranes.

Osmosis is the diffusion of water through a semipermeable membrane.

Solute molecules

Solvent Molecules•Think of an example of a substance that could be a solvent.

•Think of an example of a substance that could be a solute.

What would happen to a cell placed in pure water?

90% water

10% solute 100% water

What would happen to a cell placed in a 20% salt solution?

90% water

10% solute

Cell’s are about 90% water.

80% water

Transport proteins are embedded in cell membranes

Facilitated diffusion is a passive method that uses transport proteins.

Molecules move along a concentration gradient.

No energy is required.

Active transport uses transport proteins to move some types of molecules against a concentration gradient.

Active transport requires the cell to use energy.

Exocytosis and endocytosis also require energy. These processes involve rearrangement of the membrane.

oooooo

oooooo oo

o oo o o

o o o o oo o ooo o o oo o

o oo

Review: Substances pass through membranes in several different ways:

DiffusionOsmosisFacilitated

Diffusion Active TransportExocytosis and

Endocytosis

}Forms of Passive Transport;do not require energy

}Require energy as ATP.