BIS 1A Handout 7 - Lipids and Membranes

Life at the Edge

The plasma membrane separates the living cell from its nonliving surroundings.

This thin barrier, 8 nm thick, controls traffic into and out of the cell.

Like all biological membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

Cellular membranes are fluid mosaics of lipids and proteins

The main macromolecules in membranes are lipids and proteins, but carbohydrates are also important.

The most abundant lipids are phospholipids.

Phospholipids and most other membrane constituents are amphipathic molecules.

Amphipathic molecules have both hydrophobic regions and hydrophilic regions.

The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic model.

In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water, and the hydrophobic regions are in a nonaqueous environment within the membrane.

A specialized preparation technique, freeze-fracture, splits a membrane along the middle of the phospholipid bilayer.

When a freeze-fracture preparation is viewed with an electron microscope, protein particles are interspersed in a smooth matrix, supporting the fluid mosaic model.

Membranes are fluid.

Membrane molecules are held in place by relatively weak hydrophobic interactions.

Most of the lipids and some proteins drift laterally in the plane of the membrane, but rarely flip-flop from one phospholipid layer to the other.

Membrane fluidity is influenced by temperature. As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids pack more closely.

Membrane fluidity is also influenced by its components. Membranes rich in unsaturated fatty acids are more fluid that those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails at the locations of the double bonds prevent tight packing.

The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells.

At warm temperatures (such as 37°C), cholesterol restrains the movement of phospholipids and reduces fluidity.

At cool temperatures, it maintains fluidity by preventing tight packing.

Membranes are mosaics of structure and function.

The plasma membrane and the membranes of the various organelles each have unique collections of proteins.

There are two major populations of membrane proteins.

Peripheral proteins are loosely bound to the surface of the protein, often connected to integral proteins.

Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (as transmembrane proteins).

The hydrophobic regions embedded in the membrane's core consist of stretches of nonpolar amino acids, often coiled into alpha helices.

Where integral proteins are in contact with the aqueous environment, they have hydrophilic regions of amino acids.

The proteins of the plasma membrane have six major functions:

Transport of specific solutes into or out of cells.

Enzymatic activity, sometimes catalyzing one of a number of steps of a metabolic pathway.

Signal transduction, relaying hormonal messages to the cell.

Cell-cell recognition, allowing other proteins to attach two adjacent cells together.

Intercellular joining of adjacent cells with gap or tight junctions.

Attachment to the cytoskeleton and extracellular matrix, maintaining cell shape and stabilizing the location of certain membrane proteins.

Membrane carbohydrates are important for cell-cell recognition.

The plasma membrane plays the key role in cell-cell recognition.

Cell-cell recognition, the ability of a cell to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism.

Cells recognize other cells by binding to surface molecules, often carbohydrates, on the plasma membrane.

Membrane carbohydrates are usually branched oligosaccharides with fewer than 15 sugar units.

They may be covalently bonded to lipids, forming glycolipids, or more commonly to proteins, forming glycoproteins.

The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

Membranes have distinctive inside and outside faces.

Membranes have distinct inside and outside faces. The two layers may differ in lipid composition. Each protein in the membrane has a directional orientation in the membrane.

Membrane lipids and proteins are synthesized in the endoplasmic reticulum. Carbohydrates are added to proteins in the ER, and the resulting glycoproteins are further modified in the Golgi apparatus. Glycolipids are also produced in the Golgi apparatus.

When a vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes continuous with the inside layer of the plasma membrane. In that way, molecules that originate on the inside face of the ER end up on the outside face of the plasma membrane.

Membrane structure results in selective permeability

The plasma membrane allows the cell to take up many varieties of small molecules and ions and exclude others. Substances that move through the membrane do so at different rates.

Movement of a molecule through a membrane depends on the interaction of the molecule with the hydrophobic core of the membrane.

Hydrophobic molecules, such as hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily.

The hydrophobic core of the membrane impedes the direct passage of ions and polar molecules, which cross the membrane with difficulty.

This includes small molecules, such as water, and larger molecules, such as glucose and other sugars.

An ion, whether a charged atom or molecule, and its surrounding shell of water also has difficulty penetrating the hydrophobic core.

Proteins assist and regulate the transport of ions and polar molecules.

Specific ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane.

Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane.

Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane.

Each transport protein is specific as to the substances that it will translocate.

For example, the glucose transport protein in the liver will carry glucose into the cell but will not transport fructose, its structural isomer.

Passive transport is diffusion of a substance across a membrane with no energy investment

Diffusion is the tendency of molecules of any substance to spread out in the available space.

Diffusion is driven by the intrinsic kinetic energy (thermal motion or heat) of molecules.

In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient.

The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen.

The concentration gradient itself represents potential energy and drives diffusion.

Diffusion of molecules of limited permeability through the lipid bilayer may be assisted by transport proteins.

Osmosis is the passive transport of water.

Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other.

The solution with the higher concentration of solutes is hypertonic relative to the other solution. More of the water molecules in the hypertonic solution are bound up in hydration shells.

The solution with the lower concentration of solutes is hypotonic relative to the other solution.

Solutions with equal solute concentrations are isotonic.

The diffusion of water across a selectively permeable membrane is called osmosis.

Unbound water molecules will move from the hypotonic solution, where they are abundant, to the hypertonic solution, where they are rarer. Net movement of water continues until the solutions are isotonic.

The direction of osmosis is determined only by a difference in total solute concentration.

When two solutions are isotonic, water molecules move at equal rates from one to the other, with no net osmosis.

Cell survival depends on balancing water uptake and loss.

An animal cell (or other cell without a cell wall) immersed in a hypertonic environment will lose water, shrivel, and probably die.

A cell in a hypotonic solution will gain water, swell, and burst.

For organisms living in an isotonic environment (for example, many marine invertebrates), osmosis is not a problem.

The cells of most land animals are bathed in extracellular fluid that is isotonic to the cells.

Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for osmoregulation, the control of water balance, to maintain their internal environment.

For example, Paramecium, a protist, is hypertonic to the pond water in which it lives.

To solve this problem, Paramecium cells have a specialized organelle, the contractile vacuole, which functions as a pump to force water out of the cell.

The cells of plants, prokaryotes, fungi, and some protists have walls that contribute to the cell's water balance.

A plant cell in a hypotonic solution will swell until the elastic cell wall opposes further uptake.

At this point the cell is turgid (very firm), a healthy state for most plant cells.

If a plant cell and its surroundings are isotonic, there is no movement of water into the cell. The cell becomes flaccid (limp), and the plant may wilt.

A plant cell in a hypertonic solution will lose water, and its volume shrinks. Eventually, the plasma membrane pulls away from the wall. This plasmolysis is usually lethal.

Specific proteins facilitate passive transport of water and selected solutes.

The passive movement of molecules down their concentration gradient via transport proteins is called facilitated diffusion.

Two types of transport proteins facilitate the movement of molecules or ions across membranes: channel proteins and carrier proteins.

Some channel proteins simply provide hydrophilic corridors for the passage of specific molecules or ions.

For example, water channel proteins, aquaporins, greatly facilitate the diffusion of water.

Many ion channels function as gated channels. These channels open or close depending on the presence or absence of a chemical or physical stimulus.

If chemical, the stimulus is a substance other than the one to be transported.

For example, stimulation of a receiving neuron by specific neurotransmitters opens gated channels to allow sodium ions into the cell.

When the neurotransmitters are not present, the channels are closed.

Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the transport protein changes shape.

These shape changes may be triggered by the binding and release of the transported molecule.

Active transport uses energy to move solutes against their gradients

Some transport proteins can move solutes across membranes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated.

This active transport requires the cell to expend metabolic energy.

Active transport enables a cell to maintain its internal concentrations of small molecules that would otherwise diffuse across the membrane.

Active transport is performed by specific proteins embedded in the membranes.

ATP supplies the energy for most active transport.

The sodium-potassium pump actively maintains the gradient of sodium ions (Na+) and potassium ions (K+) across the plasma membrane of animal cells.

Typically, K+ concentration is low outside an animal cell and high inside the cell, while Na+ concentration is high outside an animal cell and low inside the cell.

The sodium-potassium pump maintains these concentration gradients, using the energy of one ATP to pump three Na+ out and two K+ in.

Some ion pumps generate voltage across membranes.

All cells maintain a voltage across their plasma membranes.

Voltage is electrical potential energy due to the separation of opposite charges.

The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of cations and anions on opposite sides of the membrane.

The voltage across a membrane is called a membrane potential, and ranges from "50 to "200 millivolts (mV). The inside of the cell is negative compared to the outside.

The membrane potential favors the passive transport of cations into the cell and anions out of the cell.

Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane.

One is a chemical force based on an ion's concentration gradient.

The other is an electrical force based on the effect of the membrane potential on the ion's movement.

Special transport proteins, electrogenic pumps, generate the voltage gradient across a membrane.

The sodium-potassium pump in animals restores the electrochemical gradient not only by the active transport of Na+ and K+, setting up a concentration gradient, but because it pumps two K+ inside for every three Na+ that it moves out, setting up a voltage across the membrane.

The sodium-potassium pump is the major electrogenic pump of animal cells.

In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of the cell.

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles.

During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane.

When the two membranes come in contact, the bilayers fuse and spill the contents to the outside.

Many secretory cells use exocytosis to export their products.

During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane.

There are three types of endocytosis: phagocytosis ("cellular eating"), pinocytosis ("cellular drinking"), and receptor-mediated endocytosis.

In phagocytosis, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large vacuole.

In pinocytosis, a cell creates a vesicle around a droplet of extracellular fluid. All included solutes are taken into the cell in this nonspecific process.

Receptor-mediated endocytosis allows greater specificity, transporting only certain substances.

This process is triggered when extracellular substances, or ligands,' Z - ^

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