Human Biology Stage 3
Text: Chapter 2
Keywords
 Diffusion
 Carrier mediated transport
 Osmosis
 Facilitated diffusion
 Fluid mosaic model
 Active transport
 Phospholipid bilayer
 Hydrophobic
 Hydrophilic
 Channel proteins
 Carrier proteins
 Vesicular transport
 Endocytosis
 Pinocytosis
 Phagocytosis
 Exocytosis
 Receptor proteins
Review: diffusion & osmosis
 Diffusion is the movement of
molecules from an area of
high concentration to an area
of low concentration
 Osmosis is a special case of
diffusion. It is the diffusion in
water from a region of high
concentration through a
differentially permeable
membrane
 Both diffusion and osmosis
are passive process, they
require no energy input from
the cell
The cell membrane
 The cell membrane
surrounds the whole cell
and separates the
contents of the cell from
the extracellular fluid
Function of the cell membrane
 A physical barrier: separates
the cell from the extracellular
fluid
 The regulation of the passage
of materials: the membrane
controls the movement of
materials into/out of cells
 Sensitivity: the cell
membrane is the first part of
the cell affected by any
changes in the extracellular
fluid
 Support: cell membrane is
attached to microfilaments to
give cell support
Structure of cell membranes
 The main structure of
the membrane is
composed
phospholipid
molecules, which are
lipids that contain a
phosphate group
 The phospholipids are
arranged in 2 layers
called a bilayer
Structure of cell membranes
 Each phospholipid
molecule has a
hydrophilic head
(water loving) and a
hydrophobic tail (water
hating)
 They are arranged so
that their heads are on
the outside and tails on
the inside
Structure of cell membranes
 A model of the cell
membrane is called the
fluid mosiac model
 The membrane is said to
be fluid because the
molecules are constantly
changing position
 Its said to be mosiac as it
is composed of many
kinds of molecules
Structure of cell membranes
 Embedded in the phospholipid bilayer are cholesterol
and protein molecules
Membrane proteins
 Many of the proteins are
involved in the movement
of substances through the
membrane
 There are 4 types of
membrane proteins, each
with a different function:
 Channel proteins
 Carrier proteins
 Receptor proteins
 Cell-identity markers
Membrane transport:
Channel proteins & carrier proteins
 Channel proteins are like
open doors
 They allow certain types of
molecules (mostly ions) to
flow freely in and out of
the cell
 The cell may be able to
close these ‘doors’ when
required
 Channel proteins allow
passive diffusion across
the membrane (osmosis)
 Carrier proteins are
involved in carrier
mediated transport
 These proteins bind to
molecules to be
transported and help their
passage across the
membrane
 They bind to the molecule,
change shape and then
release the molecule
Membrane transport:
Channel proteins & carrier proteins
Membrane transport:
Carrier mediated transport
 There are 2 main types of
carrier mediated
transport :
 Facilitated diffusion is
a passive process which
substances move along
the concentration
gradient. The protein
changes shape and the
molecule is released
through the membrane
Membrane transport:
Carrier mediated transport
 Active transport requires an energy input as the
substance is being transported across the membrane
against the concentration gradient
Membrane transport:
Carrier mediated transport
 Carrier proteins are specific -
they will only bind to a
particular molecule
 Carriers can become saturated.
Once all the available carriers
are occupied, any increase in
the concentration of molecules
cannot increase the rate of
movement
 Carrier activity is regulated by
substances ie hormones
Membrane transport:
Vesicular transport
 Vesicular transport is
the movement of
substances across the
cell membrane in
membranous bags called
vesicles
Membrane transport:
Vesicular transport
 Endocytosis is taking
liquids or solids into the
cell by vesicular transport
 Pinocytosis is taking in
liquids
 Phagocytosis is taking in
solids
 Exocytosis is when the
contents of a vessicle
inside the cell are passed to
the outside of the cell
Membrane receptors
 Membrane receptor
proteins are sensitive to
certain molecules
outside the cell, and
when those molecules
are present they bind
with the receptor protein
 The receptor and the
bound molecule then
trigger changes in the
cell.
Membrane receptors
 Receptor proteins are
specific. Each type of
receptor protein will bind
to only one specific
molecule.
 There is a limited number
of receptors proteins in the
membrane of each cell, so
when each receptor is
bound to a molecule there
can be no further increase
in the rate of the cell’s
activity
Membrane receptors
 Different cells have
different types and
numbers of receptor
proteins
 Receptor proteins also
function for
communication between
cells
 Receptor proteins are
binding sites for
hormones
You tube animations
https://www.youtube.com/watch?v=RPAZvs4hvGA
Bozeman Science
14 minutes