|
|
LESSON 3:
THE CHARACTERISTICS OF BIOLOGICAL MEMBRANES
|
Lessons:
1 | 2 | 3 |
4 |
Overview
|
<Previous
Next> |
|
 The
development of a cell membrane around 3.8 billion years ago had
profound ramifications for the development of life. The complex
mosaic of
macromolecule that make up cell and
organelle membranes provides protection for the
cell’s interior while regulating the cell’s interaction with its
surroundings. In this module we will explore the structure of
cell membranes using the
fluid-mosaic model. We will also see how materials
are transported across cell membranes by both
passive and
active transport mechanisms.
Learning Objectives
-
Understand the importance of
compartmentalization
-
Know the structure of a typical
phospholipid bilayer membrane
-
Understand the various
transport mechanisms and how the structure of the
membrane helps make them possible
-
Think about how membranes work
in your own bodyUnderstand the importance of
compartmentalization
Topics covered in this Lesson
|
|
|
 Biological
membranes are essential to life, whether they surround
individual cells, or the organelles inside of eukaryotic cells.
Membranes are active structures, composed of lipids, proteins
and carbohydrates, that help maintain an internal environment
distinct from the external environment of the cell or organelle.
Learning Objectives
-
Understand the
fluid-mosaic model of membrane structure.
-
Understand that membranes
encompass eukaryotic and prokaryotic cells, and many
organelles of eukaryotic cells.
-
Know that membranes are
primarily composed of phospholipids, but proteins and
carbohydrates also play essential roles in membranes.
-
Understand the basic structure
of a
phospholipid, and how its two regions affect
the structure of biological membranes.
-
Understand the difference
between
integral and
peripheral membrane proteins.
-
Know some of the basic roles
that carbohydrates and proteins play as a part of
membranes.
|
|
|
 One
of the most essential roles of membranes is controlling what
substances are able to cross it. Some substance, such as ions,
are able to diffuse across membranes through protein channels
when the channels are open. Other, larger substances, require
slightly different membrane proteins to move across a membrane.
When substances move across a membrane toward either chemical or
electrical equilibrium, the movement typically requires no net
input of energy. Both diffusion and passive transport are
instances of such movement.
Learning Objectives
-
Understand that substances tend
to diffuse toward
equilibrium.
-
Be able to describe how membranes
are
selectively permeable.
-
Know why small, non-polar molecules
may move directly across membranes without any aid.
-
Know why larger or polar molecules
only move across membranes with the aid of integral membrane
proteins.
-
Understand the process of
osmosis as the
diffusion of water from areas of high concentration (low solute
concentration) to areas of low concentration (high solute
concentration)
-
Understand the difference between
hypotonic,
isotonic, and
hypertonic solutions.
-
Understand the characteristics of
ion channels and how they work to allow the diffusion of ions
across a membrane.
-
Understand how
carrier proteins work to transport molecules across membranes,
and how they differ from ion channels.
|
|
|
 Cells
cannot rely solely on passive movement of substances across
their membranes. In many instances, it is necessary to move
substances against their electrical or chemical gradient to
maintain the appropriate concentrations inside of the cell or
organelle. Moving substances against their gradient requires
energy, because they are being moved away from equilibrium.
Cells use two different types of active transport, which
directly or indirectly require ATP, to move substances in this
way. Both types of active transport require integral membrane
proteins.
Cells must occasionally move very large particles, such as food
particles or volumes of water, across their membranes. Cells do
this by processes called endocytosis and exocytosis, where the
substance to be transported is surrounded by an infolding of the
cell membrane.
Learning Objectives
|
|
|
 In multicellular organisms, directly adjacent cells are often
physically connected to each other. Some of these connections
are channels from cell to cell, through which small substances,
such as ATP or signal molecules, may pass. Plants (and protists),
fungi and animals all have specific types of channels connecting
the cytoplasm of neighboring cells.
Learning Objectives
-
Understand the benefits of
having cytoplasmic connections between the cells of a
multicellular organism.
-
Know the basic characteristics
of
gap junctions,
plasmodesmata and
pores, and in which types of organisms each is found.
|
|
| |
<Previous Next> |
|
