1. Cell membrane structure and function. 2. Movement across membrane. Lecture 3 Cell Biology 1. Cell membrane structure and function. 2. Movement across membrane. Prepared by Mayssa Ghannoum

Overview The plasma membrane is the edge of life. The boundary that separates the living cell from it surroundings. It controls traffic into and out of the cell it surrounds. It exhibits selective permeability; (allows some substances to cross it more easily than others.) The plasma membrane and its proteins not only acts as an outer boundary but also enable the cell to carry out its functions. To better understand how membranes work, we’ll examine their architecture.

Membrane structure and function Cellular membranes are fluid mosaics of lipids and proteins The major ingredients in cellular membranes are: lipids and proteins. Less are carbohydrates. The most abundant lipids in membranes are phospholipids. A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region. Hydrophilic: have affinity for water (soluble in water) Hydrophobic : don’t have affinity for water (soluble in lipids)

This structure of the plasma membrane is called: Fluid mosaic model The plasma membrane consist of a double layer (bilayer) of phospholipids with a “mosaic” of various proteins attached to or embedded in it. In the interior of the membrane, the phospholipids tails are hydrophobic. In contact with the aqueous solution on either side of the membrane, the phospholipid heads are hydrophilic. This structure of the plasma membrane is called: Fluid mosaic model

The Fluidity of Membranes Membranes are not static sheets of molecules locked rigidly in place. Most of the lipids and some of the proteins can shift about laterally. The lateral movement of phospholipids within the membrane is rapid. Proteins are much larger than lipids and move more slowly.

Membrane proteins and their functions In the membrane, different proteins are embedded in the fluid matrix of the lipid bilayer. Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane’s functions. there are two major populations of membrane proteins: Integral proteins and peripheral proteins. Integral proteins penetrate the hydrophobic core of the lipid bilayer. Peripheral proteins are not embedded in the lipid bilayer at all; they are attached to the surface of the membrane.

Membrane proteins functions There are six major functions performed by proteins of the plasma membrane: 1. Transport 2. Enzymatic activity 3. Signal transduction 4. Cell-cell recognition 5. Intercellular joining 6. Attachment to the cytoskeleton and extracellular matrix (ECM).

The Role of Membrane carbohydrates in cell-cell Recognition Cell-cell recognition is the cell’s ability to distinguish one type of neighboring cell from another. Cells recognize other cells by binding to surface molecules, often to carbohydrates, on the plasma membrane. Membrane carbohydrates are usually short, some are bonded to lipids forming molecules called glycolipids. Most are bounded to proteins forming glycoproteins.

Four steps of Synthesis of Membranes The synthesis of membrane proteins and lipids is in the endoplasmic reticulum. Carbohydrates are added to proteins making them glycoproteins. Inside the Golgi apparatus, the glycoproteins undergo further carbohydrate modification and lipids acquire carbohydrates and become glycolipids. The transmembrane proteins, glycolipids and secretory proteins are transported in vesicle to the plasma membrane. The vesicles fuse with the membrane, releasing secretory proteins from the cell.

Membrane structure results in selective permeability The fluid mosaic model of the plasma membrane helps regulate the cell’s molecular traffic. A steady traffic of small molecules and ions moves across the plasma membrane in both directions. sugars, amino acids, and other nutrients enter the cell, and metabolic waste products leave it. The cell takes in oxygen for use in cellular respiration and expels carbon dioxide. Also, the cell regulates its concentrations of inorganic ions, such as Na+, K+, Ca²+ and Cl¯.

The permeability of the lipid bilayer Nonpolar molecules, such as hydrocarbons, carbon dioxide and oxygen, are hydrophobic and can dissolve in the lipid bilayer of the membrane and cross it easily. The hydrophobic core of the membrane makes difficult the direct passage of ions and polar molecules, which are hydrophilic, through the membrane. Water, an extremely small polar molecule does not cross rapidly the membrane. The lipid bilayer is then only one aspect of the gatekeeper system responsible for the selective permeability of cell. Proteins built into the membrane play key roles in regulating transport.

Transport Proteins Hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane. Some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules use as a tunnel through the membrane. The water channel proteins are known as aquaporins. Other transport proteins, called carrier proteins, change shape in a way that shuttles the passengers across the membrane. A transport protein is specific for the substance it moves, allowing only a certain substances to cross the membrane.

The selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane. At a given time, what determines whether a particular substance will enter the cell or leave? To answer this question, We will explore the modes of membrane traffic.

Movement Across Membranes The different ways of transporting molecules across the membrane are: Passive transport (Diffusion/Osmosis) Facilitated Diffusion Active transport Exocytosis and Endocytosis

Passive Transport Passive transport is diffusion of a substance across a membrane with no energy investment. The rule of diffusion is: In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Each substance will diffuse down its concentration gradient. Diffusion is a spontaneous process, needing no input of energy. Note that each substance diffuses down its own concentration gradient unaffected by the concentration differences of other substances.

The diffusion of solutes across a membrane

Osmosis The diffusion of water across a selective permeable membrane is called: Osmosis Water diffuses across the membrane from an area of higher to lower free water concentration; water moves from the area of lower solute concentration to that of higher solute concentration until the solute concentrations on both sides of the membrane are equal.

Facilitated Diffusion: Passive transport Aided by Proteins Many Polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. This phenomenon is called facilitated diffusion. Channel protein: has a channel through which water or other solute can pass Carrier protein: alternate between two shapes, moving a solute across the membrane during the shape change.

Active Transport Active transport uses energy to move solutes against their gradients. The transport proteins involved in active transport are all carriers proteins, rather than channel proteins. Active transport enables a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment. An animal cell has a much higher concentration of potassium ions and a much lower concentration of sodium ions. The plasma membrane helps maintain these gradients by pumping sodium out of the cell and potassium into the cell. ATP supplies the energy for most active transport.

An example of one transport system that works in active transport is the sodium-potassium pump. which exchange sodium (Na+) for potassium (K+) across the plasma membrane on animal cells.

Bulk Transport across the membrane occurs by exocytosis and endocytosis. Water and small molecules enter and leave the cell by passive or active transport. Large molecules, such as proteins and polysaccharides, generally cross the membrane in bulk by mechanisms that involve packaging in vesicles. These mechanisms require energy.

Exocytosis Definition of exocytosis: Transport vesicles migrates to the plasma membrane, fuse with it, and release their contents. Mechanism? 1. A transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane. 2. When the vesicle membrane and plasma membrane come into contact, the lipid molecules of the two bilayers rearrange themselves so that the two membranes fuse. 3. The contents of the vesicle then spill outside of the cell. 4. The vesicle membrane becomes part of the plasma membrane.

Endocytosis In endocytosis, molecules enter cells within vesicles that pinch inward from the plasma membrane. The three types of endocytosis are: Phagocytosis Pinocytosis Receptor-mediated endocytosis

Phagocytosis A cell engulfs a particle by wrapping around it and packaging it within a membrane enclosed sac that can be large enough to be classified as vacuole.

Pinocytosis The cell “gulps” droplets of extracellular fluid into tiny vesicles. It’s not the fluid itself that is needed by the cell, but the molecules dissolved in the droplets. Pinocytosis is nonspecific in the substances it transports.

Receptor-Mediated Endocytosis This type of endocytosis enables the cell to acquire a bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptors sites exposed to the extracellular fluid. The receptors proteins are clustered in regions of the membrane called coated pits. The specific substances bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the specific molecules. After this ingested material is liberated from vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

Receptor-Mediated Endocytosis