Chapter 7-3 – Cell Boundaries

• Regulates what enters and leaves the cell.

• Provides protection and support.

• Highly selective barrier!!!!

The Plasma Membrane: Cell Membrane

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 cellHydrophilic

region ofprotein

Hydrophilichead

Hydrophobictail

Hydrophobicregions of

protein

Proteins

Cytoplasm (inside of cell)

Membrane Protein Functions

• Cells must control the flow of materials in and out of the cell.

• Membrane proteins perform many functions.

– Cell signaling = relays messages to inside of cell

– Cytoskeletal attachment = cell shape

– Transport = a tunnel that substances pass through

– Intercellular joining = linking adjacent cells

– Cell-cell recognition = sugar (carbohydrate) ID tags

– Enzymatic activity = assembly line of chemical pathway

Figure 5.11Cell signaling

Attachment to the cytoskeletonand extracellular matrix

Enzymatic activity

Cytoskeleton

Cytoplasm

Cytoplasm

Transport

Fibers ofextracellularmatrix

Intercellularjoining

Cell-cellrecognition

• 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.

Diffusion Through Cell Boundaries

• Living cells exists in a liquid environment in order to survive.

• The plasma membrane plays a crucial role in regulating the movement of dissolved molecules from one side of the membrane to the liquid of the other side.

Diffusion Through Cell Boundaries – Measuring Concentration

• Cytoplasm of a cell contains a solution of many different substances in water.

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

• 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!!!!

Net diffusion Net diffusion Equilibrium

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

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) Isotonicsolution

(b) Hypotonicsolution

(c) Hypertonicsolution

Animal cell

Plant cell

Normal

Flaccid (wilts)

H2OH2O

H2O H2O

Isotonicsolution

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

Hypotonicsolution

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

Hypertonicsolution

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.

• 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

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 OsmosisHigher solute concentration

Lower solute concentration

Higher water concentration(lower solute concentration)

Lower water concentration(higher solute concentration)

Solute

Higher solute concentration

Lower solute concentration

ATP

So

lute

So

lute

Wat

er

So

lute

MEMBRANE TRANSPORT

Membrane Transport Summary

• Endocytosis – process of taking materials into the cell by means of infoldings, or pockets of the plasma membrane that form into vesicles once inside the cell.

• Large molecules, clumps of food or even whole cells can be taken into the cell this way.

– Phagocytosis – “cell eating” example = ameobas

– Pinocytosis – “cell drinking”

Exocytosis and Endocytosis: Traffic of Large Molecules That Requires Energy

Endocytosis

vesiclecytoplasm

outside of the cell

plasma membrane

Exocytosis and Endocytosis: Traffic of Large Molecules That Requires Energy

• Exocytosis is the secretion of large molecules within transport vesicles to the outside of the cell.

Outside of cell

Cytoplasm

Plasmamembrane

Exocytosis

vesicle