Membrane Structure & Function BIOLOGY 11 Membrane Structure & Function

Life At The Edge The plasma membrane is the boundary that separates the cell from it’s surroundings. Remarkably enough, it’s only 8nm thick, and yet controls all the traffic into and out of the cell. The plasma membrane is selectively permeable – which means that is allows some substances to cross through more easily than others. The most abundant lipids in most membranes are phospholipids –molecules that have both a hydrophilic and hydrophobic region.

The Fluid Mosaic Model The membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer (bilayer) of phospholipids. Phospholipids move laterally in a membrane, but flip-flopping across the membrane is quite rare. The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells, and helps keep the membrane impermeable to water soluble substances. It also helps to stabilize the membrane.

Integral Proteins Integral Proteins – Penetrate the hydrophobic core of the lipid bilayer. Many are trans membrane proteins, which completely span the membrane.

Peripheral Proteins Peripheral Proteins – Are not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to the exposed parts of integral proteins.

Glycoproteins Glycoproteins – Carbohydrates that are bonded to protein. Note: glyco refers to the presence of carbohydrate.

Six Major Functions Transport A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.

Six Major Functions 2. Enzymatic Activity A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution.

Six Major Functions 3. Signal Transduction A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The signal may cause a conformational change in the protein that relays the message to the inside of the cell.

Six Major Functions 4. Cell-cell recognition Some glycoproteins serve as identification tags that are specifically recognized by other cells.

Six Major Functions 5. Intercellular Joining Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. .

Six Major Functions 6. Attachment to the cytoskeleton and extracellular matrix (ECM) Microfilaments may be bonded to membrane proteins, a function that helps maintain shape and stabilizes the location of certain membrane proteins.

Review The cell membrane is: This means that: The cell membrane is composed of a: Some of the proteins associated with the cell membrane are: Where are they found? The proteins of the cell membrane serve several functions, such as:

Passive Transport Process that moves materials across the plasma membrane. Does not require energy from the cell. Materials move with the concentration gradient: high concentration  low concentration Three types Diffusion Osmosis Facilitated Diffusion

Passive Transport - Diffusion The movement of molecules across a membrane, with a net movement from high to low concentration Molecules tend to “spread out” to reach equilibrium.

Passive Transport - Osmosis Diffusion of water across a selectively permeable membrane. Does not require energy from the cell.

Passive Transport - Osmosis The rule for osmosis: If the area outside the cell has more salt, then water will be sucked out of the cell. (Salt sucks!)

Passive Transport – Solutions Isotonic: Two solutions have equal concentrations of solutes. Hypotonic: When comparing two solutions, the solution with a lesser concentration (hypo = under). Hypertonic: When comparing two solutions, the solution with a greater concentration (hyper = over). Cells will shrink in a hypertonic solution, and will swell and burst in a hypotonic solution.

Passive Transport – Facilitated Diffusion Carrier proteins help small molecules move across the plasma membrane. Does not require energy from the cell.

Housekeeping Quiz – Thursday, October 23rd (Biochem handout, Cell Membrane & Function, Transport) Egg project – Due October 27th. Please sign up, even if you are working alone. Outline also on website. My name is now on the CHS staff list! Library – Wednesday to work on your lab report write ups.

Housekeeping Cont’d Homework – If it is a homework check, it’s simply done or not done. If it is homework handed in, or an in class assignment, you may hand it in up until I mark and hand it back – however, if it is not passed in when it is due, it will be marked late. Reminder: No school this Friday, October 24th

Housekeeping Cont’d Please get out a piece of paper and write down the following: Parents/Guardians names Phone number Email Allergies I want to make sure the information on PowerSchool is correct, as I’ve been noticing a lot of discrepancies.

Active Transport Active transport is the transport of molecules against the cell membrane against the concentration gradient. The molecules go from an area of low concentration to an area of high concentration. To do this, the cell must expend energy in the form of ATP, because it requires work. Cells can secrete macromolecules (What are these?) via vesicles to the plasma membrane. This is called exocytosis. Cells can also take in macromolecules during a process called endocytosis. Pinocytosis is known as “cellular drinking” Phagocytosis is known as “cellular eating”

Pinocytosis

Phagocytosis