7-3 Cell Boundaries

All cells are surrounded by a thin, flexible barrier ◦ cell membrane. Many cells also produce a strong supporting layer

around the membrane known as a ◦ cell wall.

Cell Membrane

Cell wall

• Cell Membrane – regulates what enters and leaves the cell – provides protection and support.

Outside of cell

Cell membrane

Inside of cell (cytoplasm)

Protein channel

Proteins

Lipid bilayer

Carbohydrate chains

• cell membranes is a double-layered sheet called a lipid bilayer. – Gives a flexible structure – forms a barrier between the cell and surroundings.

Lipid bilayer

• contain protein molecules embedded. – Some have carbohydrate molecules attached to them.

Protein channel

Proteins

Carbohydrate chains

• Cell Wall – provides support and protection for the cell. – found in plants, algae, fungi, and many

prokaryotes. – lies outside the cell membrane. – porous enough to allow water, oxygen, carbon

dioxide, and other substances to pass through

• Diffusion Through Cell Boundaries • Cell are in a liquid environment. • The cell membrane

– regulates movement of dissolved molecules from one side of the membrane to the other side.

– Measuring Concentration • Solution -mixture of two or more substances. • Solutes – The stuff dissolved. • Concentration - mass of solute in a given volume of

solution, (mass/volume).

– Diffusion • Diffusion - Particles in a solution move from an area of

high concentrated to an area of low concentration • Equilibrium - concentration is the same throughout

High concentration

Low concentration

Movement of solute

•equilibrium

•solute particles diffuse in both directions.

– Diffusion • depends upon random particle movements

– without requiring the cell to use energy.

• Osmosis – diffusion of water through a selectively permeable

membrane.

Movement of water

Dilute sugar solution (Water more concentrated)

Concentrated sugar solution (Water less concentrated)

Sugar molecules

Selectively permeable membrane

• Water tends to diffuse from a highly concentrated region to a less concentrated region. – compare two solutions, the more concentrated solution is hypertonic

(“above strength”). – more dilute solution is hypotonic (“below strength”). – concentrations of solutions are the same on both sides of a membrane, the

solutions are isotonic (”same strength”).

– Osmotic Pressure • pressure osmosis exerts on the hypertonic side of a

selectively permeable membrane.

• Facilitated Diffusion • Cell membranes have protein channels that act as

carriers – making it easy for certain molecules to cross.

• facilitated diffusion. – the movement of specific molecules across cell membranes

through protein channels

• Hundreds of different protein channels have been found that allow particular substances to cross different membranes.

Protein channel

Glucose molecules

• facilitated diffusion – Fast – Specific – still diffusion.

• Passive transport – does not require energy • only occur if there is differences in concentration on

one side of a cell membrane as compared to the other side.

• Active Transport • movement of materials in the opposite direction from which the

materials would normally move – against a concentration difference

• Active transport requires energy. – Molecular Transport

• small molecules and ions are carried across membranes by proteins in the membrane.

• Energy is used – helps cells to concentrate substances in a particular location

Molecule to be carried

Endocytosis and Exocytosis ◦ Endocytosis process of taking large material into the cell by means of

infoldings, or pockets, of the cell membrane. pocket breaks loose and forms a vacuole within the cytoplasm. Examples

Phagocytosis extensions of cytoplasm surround a particle and package it

within a food vacuole. The cell then engulfs it. requires a considerable amount of energy

Pinocytosis tiny pockets form along the cell membrane, fill with liquid, and

pinch off to form vacuoles within the cell

– Exocytosis • Release of large amounts of material from the cell • membrane of the vacuole surrounding the material

fuses with the cell membrane, forcing the contents out of the cell.

END OF SECTION