Chapter 5 – The Plasma
Membrane and Transport
State Standard
• Standard 1.a. – Cells are enclosed within semipermeable membranes that regulate their
interaction with their surroundings
Membrane Function
1. Forms a boundary between living cells and
their surroundings.
2. Controls the movement of molecules into
and out of the cell
- Selective permeability – allows some
substances to cross more easily than
others and blocks passage of some
substances altogether
Membrane Function
3. Takes up substances the cell needs and
disposes of the cell’s wastes.
4. Membranes can contain enzymes that
function in various chemical reactions.
Membrane Structure
• The plasma membrane
is 8 nm thick.
• It is mainly made up of
phospholipids and
proteins.
Membrane Structure
carbohydrate
cholesterolc
protein
Phospholipid bilayer
Membrane phospholipids
• Phospholipids have a
hydrophilic head and
two hydrophilic tails.
Membrane phospholipids
• The phospholipids form a bilayer
- The hydrophilic heads face outward and
the hydrophobic tails face inward
Hydrophobic
tails
Membrane Proteins
Phospholipid
bilayer
Integral Protein
Peripheral protein
• Membrane proteins are
found within the
phospholipid bilayer
• They can extend all the
way through the bilayer
– integral proteins
• They can be found
embedded on either
surface – peripheral
proteins
Functions of Membrane Proteins
• Some proteins transport substances
across the membrane by forming
channels or by physically moving
them across the membrane.
Membrane Proteins
Functions of membrane proteins include:
• Transport of substances across the membrane
• Join cells to one another
• Act as enzymes
• Bind to chemical messengers from other cells
• Act as identification tags so cells can recognize
one another
• Attach to cytoskeleton and the extracellular
matrix
Membrane cholesterol and carbohydrates
Cholesterol
Carbohydrates
• Cholesterol is found within
the phospholipid bilayer
• It helps stabilize the
phospholipids
• Located on outer layer of
membrane only
• Bound to proteins
(glycoproteins) or
phopholipids (glycolipids)
• Act as identification tags for
cell cell recognition
Fluid Mosaic Model
Fluid Mosaic Model
• The membrane is a fluid mosaic
• Fluid – the phospholipids and the proteins
drift within the membrane
• Mosaic – the diversity of proteins found within
the membrane and the arrangement
of these proteins in the membrane
Transport Across the Membrane
Diffusion
• The tendency of particles of any kind to
spread from regions of high concentration to
regions of low concentration
• Requires no energy, results from the random
motion of atoms and molecules.
Transport Across the Membrane
Passive Transport
• Substances diffuse through
the membrane without
work by the cell
- They spread from areas of
high concentration to areas
of lower concentration
- At equilibrium the
molecules continue to move
but there is no net change
in the concentration on
either side of the
membrane
Molecule
of dye
Membrane
EQUILIBRIUM
Transport Across the Membrane
• Osmosis is the passive
transport of water
• Water travels from an
area of lower solute
concentration to an
area of higher solute
concentration
Transport Across the Membrane
• Hypertonic
- the solution with the higher solute
concentration
• Hypotonic
- the solution with the lower solute
concentration
• Water moves from the hypotonic solution to
the hypertonic solution.
Transport Across the Membrane
• Water continues to cross the membrane until
the solute concentrations are equal on both
sides of the membrane
• Isotonic
- solutions with equal solute
concentrations
• Water molecules continue to move across the
membrane but at the same rate in both
directions
Osmosis Problems – Draw a diagram illustrating the
problem, identify the solutions as hyper, hypo or
isotonic, and show the direction osmosis will occur
1. A cell containing 2%
solute is placed in a solution
containing 10% solute.
4. A candy containing 10%
water is placed in a solution
containing 90% water.
2. A candy containing 15%
solute is placed in a solution
containing 12% solute.
5. A fish contains 95% water
lives in freshwater that
contains 68% water.
3. A cell containing 7%
solute is placed in a solution
containing 9% solute.
How Does Osmosis Affect Cells?
Distilled Water
Induces turgor in cells
Induces turgor in animal
cells
Makes plant cell pull away
from cell wall
Makes plant cell lyse
Makes animal cell lyse
Makes animal cell shrivel
3x Saline
How Does Osmosis Affect Cells?
1. Explain the different outcomes of adding
distilled water to the plant and animal cells.
2. Which conditions best model the effects of
drought on plant cells? Explain your answer.
3. Hypothesize why plants grow better when
the water surrounding their cells is slightly
hypotonic to the cell.
Water balance between cells and their
surroundings is crucial to organisms
• Osmosis causes cells to shrink in a hypertonic
solution and swell in a hypotonic solution.
Transport Across the Membrane
• Small non-polar
molecules diffuse freely
through the
phospholipid bilayer
• Many other kinds of
molecules diffuse
through pores created
by membrane proteins
• This type of transport is
called facilitated
diffusion
Solute molecule
Transport
protein
Transport Across the Membrane
Active Transport
Transport Across the Membrane
• Transport proteins can move solutes across
the membrane against a concentration
gradient.
- This is called active transport
• Active transport requires ATP – cells use
energy to move the molecule across the
membrane.
• The solute is moved from low concentration to
high concentration.
Transport Across the Membrane
• To move large molecules or particles across
the membrane a vesicle may fuse with the
membrane and expel its contents – exocytosis
Transport Across the Membrane
• Or the membrane may fold inward, trapping
material from the outside, and form a vesicle
that carries the material into the cell –
endocytosis
Transport Across the Membrane
• There are 3 kinds of endocytosis
Pseudopod of
the amoeba
Food being
ingested
Plasma membrane
Material bound to receptor
proteins
Transport Across the Membrane
There are 3 kinds of endocytosis:
1. Phagocytosis – a cell brings in
macromolecules, other cells, or particles
2. Pinocytosis – the cell takes in fluid and
dissolved solutes
3. Receptor-mediated endocytosis – specific
molecules bind to receptors on the
membrane that signal to the cell to take the
molecule in
• Harmful levels of cholesterol can accumulate
in the blood if membranes lack cholesterol
receptors