Cell Membrane: Structure and Function 6.2-6.3 The cell membrane is the gateway into the cell.

Key words you should know: Phospholipids Solution Pinocytosis Polar Solute Hydrophilic Solvent Exocytosis Hydrophobic Partially permeable Phospholipid bilayer Fluid mosaic model Surface area Glycoproteins Turgid Plasmolysis Passive transport Cholesterol Plasmolysed Proteins Selectively permeable Transport proteins Active transport Enzymes Carrier protein Receptor molecules Bulk transport Diffusion Endocytosis Concentration gradient Phagocytosis Facilitated diffusion Phagocytes Osmosis Crenated

Cell Membrane Structure All living cells are surrounded by a membrane. A cell membrane is also known as plasma membrane. To understand the function of anything in biology, you must study the structure first! Cell Membranes from Opposing Neurons (TEM x436,740). Nerve cell Gap between cells Cell membrane { } cell membrane Nerve cell

Functions of the Plasma Membrane Protective barrier Regulate transport in & out of cell Allow cell-cell recognition Provide anchoring sites for filaments of cytoskeleton Provide a binding site for enzymes and hormones Interlocking surfaces bind cells together (junctions) Contains the cytoplasm (fluid in cell)

Structure of Cell Membrane The Plasma Membrane Structure of Cell Membrane 4/19/2017 Phospholipids Proteins (peripheral and integral) Cholesterol Carbohydrates G. Podgorski, Biol. 1010

PHOSPHOLIPID REVIEW Phospholipids are important structural components of cell membranes. Phospholipids are modified so that a phosphate group (PO4-) replaces one of the three fatty acids normally found on a lipid. The addition of this group makes a polar "head" and two nonpolar "tails".

HYDROPHILIC HEAD At one end of the phospholipid is a phosphate group and several double bonded oxygens. The electrons at this end of the molecule are not shared equally. This end of the molecule has a charge and is attracted to water. It is POLAR HYDROPHOBIC TAILS The two long chains coming off of the bottom of this molecule are made up of carbon and hydrogen. Because both of these elements share their electrons evenly these chains have no charge. They are NON POLAR. Molecules with no charge are not attracted to water; as a result water molecules tend to push them out of the way as they are attracted to each other. This causes molecules with no charge not to dissolve in water.

Phospholipids can form: BILAYERS -2 layers of phospholipids with hydrophobic tails protected inside by the hydrophilic heads. The PHOSPHOLIPID BILAYER is the basic structure of membranes.

Polar heads are hydrophilic “water loving” Nonpolar tails are hydrophobic “water fearing” Phospholipids make membranes Selectively Permeable- able to control what crosses

Nonpolar, Hydrophobic Fatty acid “tails” Polar, Hydrophilic “head” Polar, Hydrophilic “head”

FLUID MOSAIC MODEL Cell membranes also contain proteins within the phospholipid bilayer. This ‘model’ for the structure of the membrane is called the: FLUID MOSAIC MODEL FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid. MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.

Proteins Are Critical to Membrane Function The Plasma Membrane 4/19/2017 Proteins Are Critical to Membrane Function G. Podgorski, Biol. 1010

Proteins can float in the membrane or be fixed and also have hydrophobic and hydrophilic portions. Proteins may be embedded in the outer layer or in the inner layer and some span the two layers. Hydrophobic and Hydrophilic parts of the protein molecules sit next to the Hydrophobic and Hydrophilic portions of the phospholids of the membrane. This ensures the proteins stay in the membrane.

Some of the proteins have carbohydrates attached to them – GLYCOPROTEINS. Glycoproteins act as chemical id tags. Blood types are the result different glycoproteins.

The membrane also contains molecules of CHOLESTEROL regulates the fluidity of the membrane gives mechanical stability helps to prevent ions from passing through the membrane.

The fluid mosaic model was developed using Freeze Fracture Studies. The fracture occurs between the two phospholipid layers. You can clearly see the exposed proteins sticking out of the two layers. Individual phospholipids are too small to see.

Cell Membrane Permeability Hydrophobic pass easily Hydrophilic DO NOT

Semipermeable Membrane Small molecules and larger non-polar molecules move through easily. e.g. O2, CO2, H2O

Semipermeable Membrane Ions Hydrophilic molecules larger than water Large molecules such as proteins do not move through the membrane on their own.

Types of Transport Across Cell Membranes

Two Forms of Transport Across the Membrane The Plasma Membrane Two Forms of Transport Across the Membrane 4/19/2017 G. Podgorski, Biol. 1010

Passive Transport Simple Diffusion Doesn’t require cell energy The Plasma Membrane 4/19/2017 Passive Transport Simple Diffusion Doesn’t require cell energy Molecules move from high to low concentration Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out. G. Podgorski, Biol. 1010

Molecules move from area of HIGH to LOW concentration, down their concentration gradient.

Diffusion is a PASSIVE process which means no cell energy is used to make the molecules move, they have a natural KINETIC ENERGY

Diffusion of Liquids

Diffusion through a Membrane Cell membrane Solute moves DOWN the concentration gradient from HIGH to LOW concentration

Osmosis Diffusion of water across a membrane Moves from HIGH water potential (low solute) to LOW water potential (high solute)

Osmosis-Diffusion of H2O Across A Membrane Low solute concentration High solute concentration

CELL 10% NaCL 90% H2O ENVIRONMENT NO NET MOVEMENT 10% NaCL 90% H2O What is the direction of water movement? equilibrium The cell s at _______________.

CELL 10% NaCL 90% H2O 20% NaCL 80% H2O What is the direction of water movement?

CELL 15% NaCL 85% H2O ENVIRONMENT 5% NaCL 95% H2O What is the direction of water movement?

Cells in Solutions

Isotonic Solution Hypotonic Solution Hypertonic Solution NO NET MOVEMENT OF H2O (equal amounts entering & leaving) CYTOLYSIS (water diffuses into the cell) CRENATION (water diffuses out of the cell)

Cytolysis & Crenation Cytolysis Crenation

Osmosis in Red Blood Cells In Isotonic Solution In Hypertonic Solution In Hypotonic Solution

hypotonic hypertonic hypertonic isotonic hypotonic (Crenated cell) Indicate the diffusion gradient of water with an arrow. hypotonic hypertonic hypertonic isotonic hypotonic (Crenated cell) hypertonic isotonic hypotonic (Plasmolyzed cell) (Turgid cell)

Transport Proteins Channel proteins are embedded in the cell membrane & have a pore for materials to cross Carrier proteins can change shape to move material from one side of the membrane to the other

Facilitated Diffusion The Plasma Membrane 4/19/2017 Passive Transport Facilitated Diffusion Doesn’t require energy Uses transport proteins to move high to low concentration Moves materials with the concentration gradient. Examples: Glucose or amino acids moving from blood into a cell. G. Podgorski, Biol. 1010

Facilitated Diffusion Molecules will randomly move through the pores in Channel Proteins.

Facilitated Diffusion Some Carrier proteins do not extend through the membrane. They bond and drag molecules through the lipid bilayer and release them on the opposite side.

Carrier Proteins Other carrier proteins change shape to move materials across the cell membrane

Active Transport Requires energy or ATP The Plasma Membrane 4/19/2017 Active Transport Requires energy or ATP Moves materials from LOW to HIGH concentration AGAINST concentration gradient G. Podgorski, Biol. 1010

Active Transport Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients. Called Na+-K+ Pump

Sodium-Potassium Pump (+) (-) 3 Na+ pumped out for every 2 K+ pumped in; creates a membrane potential

Moving the “Big Stuff” Out Exocytosis- moving things out. Molecules are moved out of the cell by vesicles that fuse with the plasma membrane. This is how many hormones are secreted and how nerve cells communicate with one another.

Exocytosis Large molecules that are manufactured in the cell are released through the cell membrane. Inside Cell Cell environment

The Plasma Membrane Exocytosis 4/19/2017 Exocytic vesicle immediately after fusion with plasma membrane. G. Podgorski, Biol. 1010

Moving the “Big Stuff” In The Plasma Membrane Moving the “Big Stuff” In 4/19/2017 Large molecules move materials into the cell by one of three forms of endocytosis. Pinnocytosis Receptor-mediated EndoCytosis Phagocytosis G. Podgorski, Biol. 1010

1. Pinocytosis Most common form of endocytosis. The Plasma Membrane 1. Pinocytosis 4/19/2017 Most common form of endocytosis. Takes in dissolved molecules as a vesicle. G. Podgorski, Biol. 1010

Pinocytosis Cell forms an invagination Materials dissolve in water to be brought into cell Called “Cell Drinking”

Example of Pinocytosis The Plasma Membrane Example of Pinocytosis 4/19/2017 mature transport vesicle pinocytic vesicles forming Transport across a capillary cell (blue). G. Podgorski, Biol. 1010

2. Receptor-Mediated Endocytosis The Plasma Membrane 4/19/2017 2. Receptor-Mediated Endocytosis Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc. G. Podgorski, Biol. 1010

Receptor-Mediated Endocytosis The Plasma Membrane Receptor-Mediated Endocytosis 4/19/2017 Coated pit Vesicle G. Podgorski, Biol. 1010

The Plasma Membrane 4/19/2017 3. Phagocytosis Used to engulf large particles such as food, bacteria, etc. into vesicles Called “Cell Eating” G. Podgorski, Biol. 1010