• 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 cell
Hydrophilicregion of
protein
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
• The cytoplasm of a cell contains a solutionof 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
Molecules of dye Membrane
Diffusion – no energy required!!!!
Net diffusion Net diffusion 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.
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.
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
• 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
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.