The Plasma Membrane Gateway to the Cell The

The Plasma Membrane – Gateway to the Cell

The Plasma Membrane is Semipermeable The physical properties of phospholipids account for membrane assembly and many of its properties. Small molecules and larger hydrophobic molecules move through. Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.

Plasma Membrane Functions Maintain a high concentration of materials in the cell. Keep harmful materials out. Control the movement of materials into and out of the cell. Let the cell sense its environment.

Membrane Components Phospholipids Proteins (peripheral and integral) Cholesterol Carbohydrates

Proteins Are Critical to Membrane Function

Transport Processes - Diffusion Solutes move down a concentration gradient until they are evenly distributed. This is diffusion. Another way of saying this is that solutes move from a region of higher concentration to a region of lower concentration until there is no difference in concentration.

Three Forms of Transport Across the Membrane

Three Forms of Transport Across the Membrane Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out.

Three Forms of Transport Across the Membrane Examples: Glucose or amino acids moving from blood into a cell. An nerve electrical impulse results from opening protein channels for ions that move by facilitated diffusion.

Three Forms of Transport Across the Membrane Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients.

Moving the “Big Stuff” Large molecules move in via one of three forms of endocytosis.

Pinocytosis This is the most common form of endocytosis. Pinocytosis takes up most proteins and other large molecules.

Pinocytosis pinocytic vesicles forming mature transport vesicle Transport into a capillary cell (blue).

Receptor-Mediated Endocytosis Receptor proteins make this a highly specific form of transport. Cholesterol is taken-up this way.

Receptor-Mediated Endocytosis

Endocytosis – Phagocytosis Transports Large Particles

In Preparation for Phagocytosis

The Threshold of Phagocytosis - Capture of a Yeast Cell (yellow) by Membrane Extensions of an Immune System Cell (blue)

Moving the “Big Stuff” 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 Exocytic vesicle immediately after fusion with plasma membrane.

Exocytosis and Nervous System Function A nerve cell communicates to another cell by releasing chemicals via exocytosis at the synaptic terminal.

Exocytosis and Chemical Communication at the Synapse The synapse is the region where a nerve cell and its target cell are closely apposed.

Black Widow Spider Venom and Exocytosis Black widow spider venom causes massive exocytosis of neurotransmitter into the synapse.

Botulism and Exocytosis Botulism is caused by botulinim toxin – a protein produced by a bacterium that sometimes contaminates foods. Botulinim toxin blocks exocytosis at the synapse and causes paralysis. Mechanism of botulinum toxin web site.

ENZYMES • Definition: A protein that speeds up chemical reactions WITHOUT being changed. (catalyst)

Enzymes Speed Biochemical Reactions Enzymes are biological catalysts – substances that speed a reaction without being altered in the reaction. Most enzymes are proteins, some are RNA. Enzymes are essential for life. Model of the surface of an enzyme.

Enzymes Lower a Reaction’s Activation Energy

Enzyme Action

The Fit Between Enzyme and Substrate is Critical and Precise Hexokinase, an enzyme (blue), binding its substrate, glucose (yellow).

ENZYMES • Substrate: Substance acted upon by the enzyme.

ENZYMES • Products: substances resulting from enzyme action.

• Substrate(s) • + Enzymes • Product(s)

Enzymes • Lock and Key – Enzymes are like a key that can ONLY fit a certain lock or substrate.

Enzymes • Active site – site on an enzyme molecule directly responsible for the chemical change.

Many Enzymes Work by Altering the Shape of Their Substrates The active site of an enzyme is where substrate is bound.

The “Nature of Life” – Coupling Favorable to Unfavorable Reactions

ATP - Life’s Energy Currency Energy is released when ATP is hydrolyzed (broken down) to ADP. ATP is restored from ADP and an input of energy. ATP’s energy is used to drive endergonic (energy -requiring) reactions.