5 Announcements

• Chapter 4 On-line Quiz – Deadline Tonight

• Reminder: Access each on-line quiz only once. Review the Policies & Guidelines on the course syllabus.

5Cellular Membranes

Stephen Hawking, an exceptional physicist currently at Cambridge, has contributed significantly to our understanding of black holes and the origin of the universe. His condition reflects degenerating motor neurons.

5 Stephen Hawking’s Condition

• “Muscle cells respond to stimulation by nerve cells by opening protein-lined channels in their plasma membranes. Because his nerves cannot stimulate them, the channels of Hawking’s muscle cells do not open, and his muscles do not contract.” [Source: pg. 87, Purves et al.]

5 Neuromuscular Junction

Source: http://www.biol.andrews.edu/fb/spring/Chap.49-Sesnory/4930.jpg

5 Neuromuscular Junction and Protein Channels

5 Cellular Membranes

• Membrane Composition and Structure

• Cell Recognition and Adhesion

• Passive Processes of Membrane Transport

• Active Transport

• Endocytosis and Exocytosis

• Membranes Are Not Simply Barriers

• Membranes Are Dynamic

Figure 5.1 The Fluid Mosaic Model

5 Membrane Composition and Structure

• Cell membranes are bilayered, dynamic structures that:

Form boundaries between cells and their environments (Don’t forget the importance of homeostasis!)

Regulate movement of molecules into and out of cells

• Cell membranes are composed of lipids, proteins, and carbohydrates in various combinations depending on the type of cell.

5 The Fluid Mosaic Model

Fluid Mosaic Model

“A molecular model for the structure of biological membranes consisting of a fluid phospholipid bilayer in which suspended proteins are free to move in the plane of the bilayer.”

(Source: Purves et al. glossary)

5 Layered Figures

Layered Figure: The Fluid Mosaic Model

(link on Chapter 5 webpage)

5 Membrane Composition and Structure

• Most of the lipid molecules found in biological membranes are phospholipids.

• Each has a hydrophilic region, where the phosphate groups are located, and a hydrophobic region, the fatty acid “tails.”

• The phospholipids organize themselves into a bilayer.

• The interior of the membrane is fluid (consistency of lightweight machine oil), which allows some molecules to move laterally in the membrane.

Figure 5.2 A Phospholipid Bilayer Separates Two Aqueous Regions

5 Membrane Composition and Structure

• All biological membranes contain proteins.

• The ratio of protein to phospholipid molecules varies depending on membrane function. Typically, there is approximately one protein for every 25 phospholipid molecules

• Many membrane proteins have hydrophilic and hydrophobic regions.

• The association of protein molecules with lipid molecules is not covalent; both are free to move around laterally, according to the fluid mosaic model.

5 Membrane Composition and Structure

• Integral membrane proteins have hydrophobic regions of amino acids that penetrate or entirely cross the phospholipid bilayer.

Transmembrane proteins have a specific orientation, showing different “faces” on the two sides of the membrane.

• Peripheral membrane proteins lack hydrophobic regions and are not embedded in the bilayer. They exist on one side or the other of the membrane.

Figure 5.4 Interactions of Integral Membrane Proteins

5 Membrane Composition and Structure

• Some proteins are restricted in movement because they are anchored to components of the cytoskeleton or are trapped within regions of lipid rafts.

• This causes an unequal distribution of proteins, allowing for specialization of certain regions of the cell membrane.

5 Membrane Composition and Structure

• Some cells have carbohydrates associated with their external surfaces.

• Carbohydrate-bound lipid is called glycolipid.

• Most of the carbohydrate in the membrane is covalently bonded to proteins, forming glycoproteins.

• Plasma membrane glycoproteins enable cells to be recognized by other cells and proteins.

• Tissue-specific and species-specific aggregation occur because of plasma membrane recognition proteins.

Figure 5.5 Cell Recognition and Adhesion

5 Cell Recognition and Adhesion

• There are two general ways that cell adhesion molecules work:

Homotypic binding occurs when both cells possess the same type of cell surface receptor and their interaction causes them to stick together.

Heterotypic binding occurs between two different but complementary proteins and resembles a plug and socket.

5 Cell Recognition and Adhesion

• Specialized cell junctions form between cells in a tissue.

• Animals have three types of cell junctions: tight junctions, desmosomes, and gap junctions.

Figure 5.6 Junctions Link Animal Cells Together (Part 1)

Figure 5.6 Junctions Link Animal Cells Together (Part 4)

•Connexons are made of proteins called connexins, which snap together to generate a pore. The pore allows movement of dissolved molecules and electric signals from one cell to another.

5 Passive Processes of Membrane Transport

• Biological membranes are selectively permeable. They allow some substances to pass, while others are restricted.

• Passive transport across the membrane does not require an input of energy, whereas active transport does.

• Some substances can move by simple diffusion (passive transport) through the phospholipid bilayer.

• Some must travel through proteins to get in, but the driving force is still diffusion. This process is called facilitated diffusion (also passive transport).

Transport Across a Membrane

5 Passive Processes of Membrane Transport

• Diffusion is the process of random movement toward the state of equilibrium.

• Although individual particles move randomly, in diffusion the net movement is directional, from regions of greater concentrations to regions of lesser concentrations, until equilibrium is reached.

Source: http://www.furniture2yourdoor.com/images/pooltable.jpg

What is equilibrium on the pool table?

Figure 5.7 Diffusion Leads to Uniform Distribution of Solutes

5 Until next time…

5 Announcements

• Check your email for a note entitled bio lec. The email includes your unique password and instructions on how to access your BIO 121 grades.

• If you have problems accessing your grades or your grades do not look correct, let me know.

• The meaning of “Dynamic Curve”

Figure 5.7 Diffusion Leads to Uniform Distribution of Solutes

5 Passive Processes of Membrane Transport

• With all diffusion, it is the relative concentrations that determine the net direction of movement. Substances move from high concentration to low concentration.

• The rate of diffusion is a function of: Size of molecule (smaller molecules tend to

diffuse faster) Temperature (higher the temperature, faster

the diffusion) Electric charge (impact is variable) Concentration gradient (the steeper the

gradient, the faster the diffusion)

5 Passive Processes of Membrane Transport

BUT,• The presence of a biological membrane affects

the movement of chemicals in solution according to the membrane’s properties. It may be permeable to some molecules and impermeable to others. The chemistry of the membrane (e.g., hydrophobic and hydrophilic regions) is very important!

5 Passive Processes of Membrane Transport

• Many small molecules can move across the lipid bilayer by simple diffusion.

• The more lipid-soluble the molecule, the more rapidly it diffuses.

• An exception to this is water, which can pass through the lipid bilayer more readily than its lipid solubility would predict (hydrated ions & ion channels; water channels (aquaporins)).

• Polar and charged molecules such as amino acids, sugars, and ions do not pass readily across the lipid bilayer.

5 Passive Processes of Membrane Transport

• Osmosis is the diffusion of water across membranes.

• Osmosis is a completely passive process and requires no metabolic energy.

• Water will diffuse from a region of its higher concentration (low concentration of solutes) to a region of its lower concentration (higher concentration of solutes).

5 Don’t Forget the Meaning of “Concentration”

concentration ((chemistry) the strength of a solution; number of molecules of a substance in a given volume (expressed as moles/cubic meter))

Source:http://wordnet.princeton.edu/perl/webwn?s=concentration

Figure 5.8 Osmosis Modifies the Shapes of Cells

5 Passive Processes of Membrane Transport

• Isotonic solutions have equal solute concentrations.

• A hypertonic solution has a greater total solute concentration than the solution to which it is being compared.

• A hypotonic solution has a lower total solute concentration than the solution to which it is compared.

5 Passive Processes of Membrane Transport

• Polar and charged substances do not diffuse across lipid bilayers.

• One way for these important raw materials to enter cells is through the process of facilitated diffusion.

• Facilitated diffusion depends on two types of membrane proteins: channel proteins (e.g., ion channels) and carrier proteins (e.g., glucose transporter).

5 Passive Processes of Membrane Transport

• Channel proteins are integral membrane proteins that form channels lined with polar amino acids.

• Nonpolar (hydrophobic) amino acids face the outside of the channel, toward the fatty acid tails of the lipid molecules.

Figure 5.9 A Gate Channel Protein Opens in Response to a Stimulus

5 Passive Processes of Membrane Transport

• The best-studied protein channels are the ion channels.

• Ion channels can be open or closed (i.e., they are “gated”).

• Ion channels are specific for one type of ion.

Example: potassium ion channel

5 Passive Processes of Membrane Transport

• Facilitated diffusion using carrier proteins involves not just opening a channel but also binding the transported substance.

• Carrier proteins allow diffusion in both directions.

• The concentration gradient can be kept by metabolizing the transported substance once it enters the cell.

• If the limited number of carrier protein molecules are loaded with solute molecules, the carrier proteins are said to be saturated.

Figure 5.11 A Carrier Protein Facilitates Diffusion (Part 1)

5 Layered Figure – Glucose Carrier Protein

• Glucose Carrier Protein – Layered Figure 05-11

Figure 5.11 A Carrier Protein Facilitates Diffusion (Part 2)

5 Active Transport

• In contrast to diffusion, active transport requires the expenditure of energy.

• Ions or molecules are moved across the membrane against the concentration gradient.

• ATP is the energy currency used either directly or indirectly to achieve active transport.

“But a hallmark of living things is that they can have a composition quite different from that of their environment.

One way that they achieve this is by not relying solely on concentration gradients, but instead by moving substances

against their natural tendencies to diffuse.” (Purves et al. pg. 98)

5 Active Transport

• Three different protein-driven systems are involved in active transport:

Uniport transporters move a single type of solute, such as calcium ions, in one direction.

Symport transporters move two solutes in the same direction.

Antiport transporters move two solutes in opposite directions, one into the cell, and the other out of the cell (e.g., sodium/potassium pump)

Figure 5.12 Three Types of Proteins for Active Transport

Figure 5.13 Primary Active Transport: The Sodium–Potassium Pump

5 Layered Figure – Sodium/Potassium Pump

• Sodium/Potassium Pump – Layered Figure 05-13

5 Endocytosis and Exocytosis

• The group of processes called endocytosis brings macromolecules, large particles, small molecules, and even other cells into the eukaryotic cell.

• There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

• In all three, the plasma membrane invaginates toward the cell interior while surrounding the materials on the outside.

Figure 5.15 Endocytosis and Exocytosis

5 Endocytosis and Exocytosis

• During phagocytosis, which involves the largest vesicles, entire cells can be engulfed.

• Phagocytosis is common among unicellular protists.

• White blood cells in humans and other animals also use phagocytosis to defend the body against invading foreign cells.

5 Endocytosis and Exocytosis

• Pinocytosis, which means “cellular drinking,” involves vesicle formation as well, but the vesicles are far smaller.

• Dissolved substances and fluids are brought into the cell.

• In humans, the single layer of cells separating blood capillaries from surrounding tissue uses pinocytotic vesicles to acquire fluids from the blood.

5 Endocytosis and Exocytosis

• Receptor-mediated endocytosis is similar to pinocytosis, but it is highly specific.

• Receptor proteins are exposed on the outside of the cell in regions called coated pits. Clathrin molecules form the “coat” of the pits.

• Coated vesicles form with the macromolecules trapped inside.

Figure 5.16 Formation of a Coated Vesicle (Part 1)

Figure 5.16 Formation of a Coated Vesicle (Part 2)

5 Endocytosis and Exocytosis

• Exocytosis is the process by which materials packaged in vesicles are secreted from the cell.

• The vesicle membranes fuse with the plasma membrane and release vesicle contents (wastes, enzymes, hormones, etc.) into the environment.

5 Membranes Are Not Simply Barriers

• Membranes have many functions, including: Information processing Energy transformation

The inner mitochondrial membrane helps convert the energy of fuel molecules to the energy in ATP.

The thylakoid membranes of chloroplasts are involved in the conversion of light energy in photosynthesis.

• Membranes are involved in organizing chemical reactions, allowing them to proceed rapidly and efficiently.

Figure 5.17 More Membrane Functions (Part 1)

Figure 5.17 More Membrane Functions (Part 2)

5 Membranes Are Dynamic

• Membranes actively participate in numerous cellular processes.

• Membranes continually form, move, and fuse.

• Eukaryotic cells form their membranes through a series of activities.

• Within cells, segments of membrane move about, change their structures, and fuse with other membranes.

• Each organelle modifies its membranes to carry out specific functions.

5 Membranes Are Dynamic

• Despite the similar appearance and interconvertibility of membranes, they show major chemical differences depending on their location in the cell and the functions they serve.

• Dynamic in both structure and activity, membranes are central to life.