Chapter 7-3 – Cell Boundaries

• Regulates what enters and leaves the cell.

• Provides protection and support.

• Highly selective barrier!!!!

The Plasma Membrane: Cell Membrane

Diffusion Through Cell Boundaries

• The plasma membrane is semi-permeable, which means the membrane regulates what can go into and out of the cell.

What the plasma membrane is made out of.

phospholipid bilayer of membrane – semi permeable

Outside of cell

Hydrophilichead

Hydrophobictail

PhospholipidCytoplasm (inside of cell)

• Most membranes have specific proteins and cholesterol embedded in the phospholipid bilayer.

• Two main types of proteins help regulate traffic across the membrane and perform other functions.

• Integral and peripheral.

The Plasma Membrane: A Fluid Mosaic of Lipids and Proteins

• Surface Carbohydrates function in cell recognition, cell signaling and cell adhesion.

• Cholesterol helps keep the phospholipids spaced apart.

The Plasma Membrane: A Fluid Mosaic of Lipids and Proteins

Figure 4.UN12

Outside of cell

Cytoplasm (inside of cell)

Protein

Phospholipid

Hydrophilic

Hydrophilic

Hydrophobic

Figure 4.6b

Fluid mosaic model of membrane

Outside of cell

Hydrophilicregion of

protein

Hydrophilichead

Hydrophobictail

Hydrophobicregions of

protein

Proteins

Cytoplasm (inside of cell)

• The plasma membrane is a fluid mosaic.

Fluid because molecules can move freely past one another.

A mosaic because of the diversity of proteins in the membrane.

The Plasma Membrane: A Fluid Mosaic of Lipids and Proteins

Cell Walls – ONLY IN PLANTS!!

• Plant cells have rigid cell walls surrounding the membrane.

• Plant cell walls

– made of cellulose

– protect the cells

– porous enough to allow water, oxygen and carbon dioxide to pass through easily.

– maintain cell shape

– keep cells from absorbing too much water.

• In a solution particles are constantly moving!

• The particles collide with one another and spread out randomly.

• As a result of this movement, particles tend to move from areas where they are more concentrated to areas where they are less concentrated.

Diffusion

Diffusion

Movement of molecules from areas of high concentration to areas of low concentration

Molecules of dye Membrane

Diffusion – no energy required!!!!

Equilibrium

• Some substances do not cross membranes without the help from channel proteins embedded in the plasma membrane.

– These substances cross the plasma membrane by a process called facilitated diffusion through protein channels.

– No energy input is needed, the molecules diffuse from areas of higher concentration to areas of lower concentration through the channel protein.

Diffusion across Membranes

high concentration

low concentration

Osmosis and Water Balance

• The diffusion of water across a selectively permeable membrane is osmosis.

• Water is moving from where it is more concentrated to where it is less concentrated.

• When we compare two solutions we say that…

– a hypertonic solution has a greater concentration of solutes

– a hypotonic solution has a lesser concentration of solutes

– an isotonic solution has an equal concentration of solutes

– Remember that when we make a solution the solvent is the liquid and the solute is the solid or liquid we place in the solvent.

Osmosis and Water Balance

Osmosis and Water Balance

Hypotonic solution

Hypertonic solution

Sugarmolecule

Selectivelypermeablemembrane Osmosis

Hypotonic solution

Hypertonic solution

Sugarmolecule

Selectivelypermeablemembrane Osmosis

Isotonic solutions

Osmosis

Osmosis and Water Balance

• Osmoregulation is the control of water balance within a cell or organism.

• Organisms must have a way to balance the water that exits and enters their cells.

• The cells in our body are not in danger of swelling because our cells are bathed in fluids like blood that are isotonic.

Water Balance in Cells

Animal cell

Plant cell

Normal

Flaccid (wilts)

Lysing

Turgid (normal)

Shriveled

Shriveled

Plasmamembrane

H2OH2O H2O H2O

H2OH2OH2O H2O

(a) Isotonic

solution

(b) Hypotonic

solution(c) Hypertonic

solution

Animal cell

Plant cell

Normal

Flaccid (wilts)

H2OH2O

H2O H2O

Isotonic

solution

A cell in an isotonic solution in which the concentration of solutes is the same inside and outside the cell. The cell is balanced and behaves normally.

A cell in a hypotonic solution such as fresh water. Because the cell has a higher concentration of solutes than the fresh water, water rushes into the cell and the cell swells.

Bursting

Turgid (normal)

H2O

H2O

Hypotonic

solution

The cell wall of the plant cell prevents the

plant cell from bursting!

Animal cells don’t have cell walls and will

eventually burst!!

A cell in a hypertonic solution in which the concentration of solutes in the solution is higher than the concentration of solutes inside the cell, which results in the movement of water out of the cell and the cell shrinking.

Shriveled

Shriveled

Plasmamembrane

H2O

H2O

Hypertonic

solution

The plasma membrane pulls

away from the cell wall in the process

of plasmolysis, which usually kills

the cell.

When plant cells lose water the plasma membrane pulls away from the cell wall and the plant cell shrivels

resulting in the plant wilting.

Active Transport: The Pumping of Molecules across Membranes

• Active transport requires that a cell expend energy to move molecules across a membrane against a concentration difference, meaning, from areas of low solute concentration to areas of high solute concentration.

• This is done by proteins embedded in the plasma membrane called protein pumps.

Figure 5.16-2

Lower solute concentration

Higher solute concentration

ATP

Solute

Active transport

Passive Transport(requires no energy)

Active Transport(requires energy)

Diffusion Facilitated diffusion Osmosis

Higher solute concentration

Lower solute concentration

Higher water concentration(lower solute concentration)

Lower water concentration(higher solute concentration)

Solute

Higher soluteconcentration

Lower soluteconcentration

ATP

So

lute

So

lute

Wate

r

So

lute

MEMBRANE TRANSPORT

Membrane Transport Summary