7.3 Cell Boundaries

Cell Membrane

Cell Membrane (plasma

membrane): thin flexible layer that

surrounds all cells.

◦ main function: Regulate movement

Protection/ shape

◦ Selectively permeable

Fluid Mosaic made of: Phospholipids,

Cholesterol, Proteins, Carbohydrates

Phospholipid Bilayer

Phospholipid Bilayer: (two layers) each

layer is made of individual phospholipids

◦ Head: phosphate group: Hydrophilic

◦ Tail: Fatty Acid Tail: two strings of

hydrogen and carbon atoms called fatty

acid tails: Hydrophobic

Cholesterol

Cholesterol: This helps keep the cell

membrane flexible and fluid.

◦ Makes the membrane stronger making it

harder for small molecules to pass through

Proteins

Proteins help to determine the function

of the membrane.

Some include:

◦ Pumps

◦ Gates

◦ Receptors

◦ Enzymes

Proteins continued….

◦ Integral proteins: inside the bilayer (at

least one)

◦ Peripheral proteins: outside the lipid

bilayer (act as receptors)

Channel proteins

◦ Allows for certain molecules to pass through

the cell membrane

Carbohydrates

Carbohydrates: have the function of cell

recognition and adhesion

◦ Glycoprotein: proteins with carbohydrates

attached

◦ Glycolipid: phospholipids with carbohydrates

attached

Quick Review

Solution: mixture of two or more

substances

Solutes: Substances dissolved in solutions

Concentration of a solution: mass of

solute in a given volume of solution

Ex: 12 grams of salt in 3 liters of water=

12g/3L or 4g/L

How are materials transported

across the membrane?

Diffusion: movement of particles from an

area where it is more concentrated to an

area where it is less concentrated

◦ Does not require energy

Equilibrium: when the concentration of

the solute is the same throughout a

system

Q: When equilibrium is reached will

particles of the solution continue to

move across the cell membrane in

both directions?

Answer: YES but since the particles

are moving in each direction there is

no change in concentration

Osmosis: is the diffusion of water

through a selectively permeable

membrane.

◦ Isotonic: means same strength (same

strength on both sides)

◦ Hypertonic: above strength: more

concentrated solution

◦ Hypotonic: below strength

Q: Why can water pass through

the cell membrane when it is a

polar molecule?

Answer: Because it is small

enough to pass through the

membrane BUT it does so very

slowly

Facilitated Diffusion: cell membrane

channels that help to facilitate diffusion

◦ High to low concentration

◦ No energy required

It is going to take some ATP

Active Transport

Active Transport: When cells move materials in the opposite direction of diffusion ◦ Require energy

Smaller molecules are transported with pumps which are found in the membrane ◦ Move molecules and ions such as calcium,

potassium, and sodium ions

Larger molecules are transported by endocytosis and exocytosis which involve changes in the shape of a cell membrane

Endocytosis: is the process of taking material into

the cell by means of infoldings or pockets of the cell

membrane.

◦ Two examples:

Phagocytosis: “cell eating” amoebas use this

method to eat. Takes a lot of energy.

Pinocytosis: “cell drinking” fill with fluid

Exocytosis: when cells release large amounts of

material from the cell. The membrane of the vacuole

surrounding the material fuses with the cell

membrane