Cell Membranes

Recall that the smallest unit of life is the cell.

All cells are surrounded by a plasma membrane that is vital in various aspects of the cell’s biology.

General functions:

1. Selective isolation of cell’s contents from the external environment.

2. Regulation of material that moves in and out of the cell.

3. Communication with other cells.

Cell Membranesmembrane structure

Cellular membranes are composed of phospholipids that have hydrophilic heads and hydrophobic tails.

How do you think these phospholipidswould line up in a water environment?And Why?

They form a bilayer.

Note how the hydrophobic tails are allLined up together towards the inside, Whereas the hydrophilic heads are linedUp towards the outside watery environment.

Most biological molecules are water soluble(i.e. polar) and can not pass this membrane.The cell has effectively isolated the inside fromthe outside!!

Cell Membranesmembrane structure

The cellular membrane (or plasma membrane) is more complex than that…

Membranes contain varying amounts of cholesterol… They regulate the fluidity and flexibility of the membrane… (remember that the membrane is “fluid”)

There are also a lot of proteins and glycoproteins (proteins with sugar moieties) throughout the membrane. We will discuss their function separately.

Cell Membranesmembrane proteins

Membrane protein types and functions:

1. Transport proteins - these proteins are involved in regulating transport ofvarious substances in and out of the cell.

2. Receptor proteins – these proteins recognize “signals” or “messages” fromtheir environment and can induce biological responses within the cells.

3. Recognition proteins - these proteins allow the cell to be “recognized” byother cells. In multicellular organisms, these proteins are involvedin self recognition.Recognition proteins are often glycoproteins.

Cell MembranesTransport or material across plasma membranes

1. Simple diffusion:

The net movement of molecules down a concentration gradient. The greaterthe gradient, the faster the movement (until the gradient is eliminated).

Note that diffusion is a very slow process.

Simple diffusion is involved in transport of water, dissolved gases, and small,Lipid soluble substances (why do you think that is?)

Cell MembranesTransport or material across plasma membranes

2. Facilitated diffusion:

Diffusion of molecules through a channel or carrier protein. Why wouldyou think there would be a need for “channels” or a “carrier” proteins?

Cell MembranesTransport or material across plasma membranes

2. Facilitated diffusion: (cont.)

Diffusion of molecules through a channel or carrier protein. Why wouldyou think there would be a need for “channels” or a “carrier” proteins?

Cell MembranesTransport or material across plasma membranes

3. Osmosis:Diffusion of water across a differentially permeable membrane - this means

a membrane that allows water to pass through freely, but not variousdissolved molecules. (e. g. human blood cells)

Cell MembranesTransport or material across plasma membranes

Vocabulary review for osmosis:

Isotonic solution - concentrations of solutes is same inside and outside the membrane. Which direction do you think water will flow?

Hypertonic solution – concentration of solutes is higher outside the membrane. Which direction will water flow?

Hypotonic solution – concentration of solutes is lower outside the membrane. Which direction will water flow?

Cell MembranesTransport or material across plasma membranes

Cells however sometimes need to transfer molecules from areas of low concentrationto areas of higher concentration. This processes are energy-requiring transport systems (i.e. active transport).

In case of simple diffusion, facilitated diffusion, and osmosis, substances were movingfrom areas of high concentration to low concentration. There was no need for energy(i.e. transport was passive).

Cell MembranesTransport or material across plasma membranes

Active transport – uses ATP as a source of energy to transfer material (in this exampleCalcium ions) up a concentration gradient. In this process, the ATP is brokendown to ADP.

Cell MembranesTransport or material across plasma membranes

Cells can also use energy to actively engulf extracellular fluid or small particles. This process is referred to as endocytosis, and there are three general forms:

Cell MembranesTransport or material across plasma membranes

Review of three types of endocytosis:

Pinocytosis – pinching off of a small amount of the fluid around the cell.

Receptor-mediated endocytosis – this is the process where the cell selectively bind to and engulf material found outside the cell.

Phagocytosis – process by which cells pick up large particles, includingwhole microorganisms.

Cell MembranesTransport or material across plasma membranes

Review of different forms of transport:

Cell MembranesTransport or material across plasma membranes

Note that both active and passive transport can be for material being transferred bothinto as well as out of the cells. In case of endocytotic processes, the release of materialout of the cell is referred to as exocytosis.

Cell Membranes

In case of plants, cells secrete one or two additional primarily cellulose cell walls aroundtheir plasma membrane.

Cell Structure and Function

Next lecture…