BSC 2010 Cell Membranes and Cell Signaling

Glycolipids

A carbohydrate bonded to a lipid. Cell to cell recognition protein.

Glycoproteins

A carbohydrate bonded to a protein. Cell-to-cell recognition protein

What is a cell membrane?

A cell membrane is a phospholipid bilayer with proteins embedded in one or both halves.

What are gated channels?

A gated ion channel opens and closes in response to a stimulus, selectively allowing ions to move down their concentration gradients. There are two kinds of gated channels: ligand-gated channels (which respond to a ligand, or a chemical signal) and voltage -gated channels (which respond to a change in voltage along the membrane).

When is a solution hypertonic?

A solution is hypertonic to another solution if it has a higher concentration of solutes than the other solution. Ex. tap water is hypertonic to distilled water because tap water has a higher concentration of solutes. In a cell in a hypertonic solution of water has a lower concentration of solutes than the water around it. Causing a tendency for water to flow out of the cell. This causes the cell to get smaller.

Hypotonic?

A solution is hypotonic to another solution if it has a lower concentration of solutes than the other solution. Ex. tap water is hypotonic compared to sea water bc tap water has a lover concentration of solutes. Also a cell in a hypotonic solution of water has a higher concentration of solutes than the water around it. Naturally tendency for water to flow into cell, causing the cell to get larger.

When is a solution isotonic?

A solution is isotonic to another solution if the two solutions have the same concentration of solutes. A cell in an isotonic solution of water has a concentration of solutes that is equal to the water around it. There will be an equal flow of water in and out of the cell, and the cell will remain a stable size.

Particle theory

All molecules are constantly undergoing Brownian motion. Because of this brownian motion, substances have the tendency to move from an high concentration to an area of low concentration. -Substances move down their concentration gradients until equilibrium is achieved. This causes a release of free energy and an increase in entropy.

Why does all of our body's exchanges occur across cell membranes?

Because cell membranes are what separates the living cell from its nonliving environment. All of our control is ultimately at the level of the cell membrane.

DIFFERENCE BETWEEN CHANNEL PROTEINS AND CARRIER PROTEINS

Carrier proteins change the shape to allow the molecules to enter the cell. Channel proteins do not change shape.

*NOTE* about carrier proteins

Carrier proteins do not require any energy input, as the diffusion is passive-down the substance's concentration gradient. The protein does not need to do any work to move the substances down their concentration gradients.

What are carrier proteins?

Carrier proteins take in molecules from outside the cell and then change their shape to allow the molecules to enter the cell. Ex. Glucose transporters, found in mammalian cells

Cell membrane at normal temp?

Cell membranes are fluid at normal temperature

Cell membrane at low temp?

Cell membranes turn solid at extremely low temperatures

How do phospholipids differ?

Chain length of fatty acid tails- longer chains are generally less fluid than shorter chains Cholesterol- Cholesterol stabilizes the cell membrane. At high temperatures, it stabilizes the membrane and raises its melting point. At low temps, it separates the phospholipids and prevents them from clustering together and causing the cell membrane to become to stiff. Degree of saturation- If the carbon chain is unsaturated, meaning that is has double bonds), the tails will have kinks in them, and the membrane will be more fluid. If the carbon chain is saturated (meaning that there are only single bonds), the tails will be straight, without kinks, and the membrane will be very viscous.

Two kinds of proteins that assist with facilitated diffusion...

Channel proteins and carrier proteins

What are channel proteins?

Channel proteins do not need to change their shape to allow molecules in from the outside. They simply form a hydrophilic channel through which some of these molecules pass.

ALL of these kinds of transports..

DO NOT require energy to move molecules across the cell membrane. That is, they do not use ATP, because they release free energy and increase entropy. This is the natural flow of the system.

What is diffusion?

Diffusion refers to the tendency of the molecules of any substance to spread out into the available space, in a process of random movement toward equilibrium.

2 specific kinds of channel proteins:

Gated channels, and aquaporins

EASY way to remember Tonicity

HYPO- below HYPER- above ISO- equal to

Transmembrane proteins have?

Hydrophilic and hydrophobic regions. Regions that interact with aqueous solutions, either the cytoplasm or extracellular fluid, are hydrophilic, while regions that interact with phospholipid tails are hydrophobic

3 types of Tonicity?

Hypotonic, hypertonic, and isotonic solutions **know different**

Proteoglycans

In extracellular matrix also play a role in cell adhesion.

What are Integral membrane proteins?

Intergral membrane proteins cross into the hydrophobic portion of the bilayer. (they are integral because they cannot be removed without disrupting the membrane). When an integral protein completely crosses the entire membrane, it is called a TRANSMEMBRANE PROTEIN.

NORMAL passage of a solute through a membrane...

It will move from an area of higher concentration to one of lower concentration

Where do the hydrophobic portions lie?

Lie within the plasma membrane

The fluidity of the membrane is influenced by the following factors:

Lipid composition and Temperature

When a human cell is fused with a mouse cell, membrane proteins of both cells become uniformly distributed over the surface of the hybrid cell. Why?

Many proteins are able to move around in the bilayer. Think of a the cell membrane as a lake in which proteins and other elements float. Many proteins are able to move around in the bilayer, so a hybrid cell formed by the fusion of a human cell and a mouse cell would have proteins from both animals uniformly distributed over the surface of the membrane.

Membranes are dynamic...

Meaning they constantly change, fuse, break down, and transform into other types

How does temp effect the fluidity of the membrane?

Membranes are typically more fluid at warmer temperatures and less fluid at colder temperatures

Cell wall?

NOT in animal cells, ONLY in plant cells. Plant cell wall's help maintain water balance and limit the volume of water that can be taken in.

What is passive transport?

Passive transport refers to diffusion of a substance across a biological membrane. Passive transport occurs automatically without the need for energy input.

How does the transport of substances across a cell membrane occur?

Passive transport: Simple difffusion and facilitated diffusion Active transport: Primary active transport and secondary active transport Bulk transfer via vesicles: Endocytosis and exocytosis

What are the two types of proteins embedded in the cell membrane?

Peripheral and Integral membrane proteins

What are peripheral membrane proteins?

Peripheral membrane proteins are only loosely associated with the hydrophobic groups, so they do not cross into the hydrophobic core of the lipid bilayer.

Bilayer?

Phospholipids naturally arrange in a bilayer organization, in which the fatty acid tails associate with one another and the polar heads face the aqueous environments, either inside or outside the cell.

What is a plasma membrane?

Plasma membranes are SELECTIVELY permeable, meaning some substances pass through them more easily than others.

NOTE

Proteins can also be physically attached to membranes with lipid components that anchor them in place. Some proteins can move within the bilayer. Others cannot because they are impeded by other proteins inside the cell, or because they are attached in the cytoskeleton

NOTE

Proteins can be asymmetrically distributed along the length of the membrane.

Facilitated diffusion

Refers to the passive movement of molecules down a concentration gradient using a transport protein.

What are the 2 methods of passive transport?

Simple and facilitated diffusion

Simple diffusion?

Simple diffusion is diffusion across the phospholipid bilayer. Note that molecules just pass through the cell membrane: they do not require a protein transport.

What exactly speeds up diffusion?

Smaller molecules move faster, the larger the difference in concentration between one area and another, the faster diffusion will take place. And molecules move faster at higher temperatures.

Membrane proteins #3

Some membrane proteins are able to recognize other cells, typically by making contact with polysaccharides or carbohydrates on the cell surface.

Permeable Substances

Such as oxygen and carbon dioxide, dissolve in the lipid bilayer and can cross easily through simple diffusion. Only small molecules can pass through the membrane through simple diffusion.

Tonicity

The ability of a solution to cause water to flow into or out of a cell- affects many cells, particularly animal cells *which don't have cell walls*

What is amphipathic?

The cell membrane is made up of phospholipid constituents. These constituents are amphipathic, meaning they are hydrophilic in some portions and hydrophobic in others.

Lipid Bilayer

The cells lipid bilayer is a barrier to molecules outside the cell

Important chemical different in different parts of the membrane proteins:

The chemical composition of the different faces of the membrane may differ. The phospholipids that make up the bilayer, as well as some proteins in it, are able to transpose (move laterally in the phospholipid bilayer), a face that emphasizes the fluidity of the bilayer.

What is the fluid mosaic model?

The current working theory of cell membrane structure. It states that a cell membrane is not a rigid structure, but rather a fluid "mosaic" of phospholipids and embedded proteins.

What affect the speed of diffusion?

The diameter of the molecules, difference in concentration, and temperature of the solution.

Osmosis

The diffusion of water through a semipermeable membrane.

Turgor pressure?

The name for the pressure that builds up against the cell wall as water flows into a plant cell.

Unlike simple diffusion, the rate of facilitated diffusion is affected by...

The saturation of carrier molecules.

NOTE about particle theory

The system as a whole behaves like this, but individual molecules do not. The individual molecules move randomly throughout the space. However, the movement of the population of molecules is directional.

IF the solute CANNOT move through the membrane...

The water will pass through via osmosis until the concentration of solutes and water is equal on both sides and equilibrium is reached.

Where do the hydrophilic portions lie?

They are in the cytosol or the extracellular surface

The fact that phospholipids form bilayers is important because?

They assist in the fusion of membranes during phagocytosis

Transport Proteins

Transport proteins on the cell's surface are important determinants of which molecules across the cell.

What are aquaporins?

Water is small enough to pass through the membrane, but it usually either passes through the membrane by "hitchhiking" with ions, such as sodium, when they pass through ion channels, or by moving through aquaporins. ---> aquaporins are specific channel proteins that passively transport water from areas of high concentration to areas of low concentration.

When is an animal healthiest?

When it is in an isotonic environment.

When is a plant cell healthiest?

When it is turgid-a firm state in which the cell is in a hypotonic environment and water is pushing out on the rigid cell wall.

Lipid composition?

Within the cell membrane, the fluidity is shaped by the nature of the phospholipids. Phospholipids can differ.

Non-permeable substances

such as large molecules, polar molecules, charged molecules, glucose, potassium, sodium, hydrogen ions, chloride, magnesium, and calcium- cannot diffuse across the membrane without the aid of a transporter.