The Cell The Cell MembraneMembrane

Purpose of the Purpose of the membranemembrane

1) Transport raw materials into the cell. 2) Transport manufactured products and

wastes out of the cell. 3) Prevent the entry of unwanted matter

into the cell. 4) Prevent the escape of matter needed

for cellular functions.

CompositionComposition The cell membrane is composed of

phospholipid molecules.A Phosphate group and two fatty acids

bonded to a glycerol “backbone”.

The phosphate group (head) is ‘hydrophillic’ (likes water) the two fatty acid “tails” are ‘hydrophobic’ (water hating).

(Pg 51 Fig 2.22)

Importance of their dual Importance of their dual naturenature

When phospholipids are spread throughout water, a curious thing happens. The attractions

and repulsions of the heads and tails cause them to form a cage-like spherical layer. (Pg 51, fig 2.23).

-The water attracting heads face in and out of the sphere.

-The non-polar tails face each other. -This phospholipid bilayer forms the basis of

the cell membrane.

Fluid-Mosaic ModelFluid-Mosaic Model

There are numerous proteins, carbohydrates and other lipids spread and moving throughout the membrane creating a fluid-like consistency.

Benefits of the Fluid Mosaic Benefits of the Fluid Mosaic ModelModel

Cholesterols found in animal cell membranes. These keep the membrane fluid at lower temperatures. They also prevent some other molecules from passing through.

Protein and carbohydrate arrangements in the cell membrane that allow the cell to be “recognized” by other cells. (Pg 52, fig 2.24)

Outside the cellOutside the cell

In multicellular organisms, the cells are bathed in a thin layer of extracellular fluid, which is made of water, mineral and some cell wastes that have been discarded.

Transport Transport Through the Through the MembraneMembrane

Passive and ActivePassive and Active

Crossing the MembraneCrossing the Membrane

The membrane is a cell’s primary means The membrane is a cell’s primary means for it to maintain for it to maintain homeostasis homeostasis (the (the constant steady state of conditions constant steady state of conditions achieved to remain healthy).achieved to remain healthy).

The plasma membrane is The plasma membrane is selectively selectively permeable. permeable. It allows some materials to It allows some materials to pass.pass.

TransportTransport

• Diffusion- Diffusion- a a passive passive way that molecules can way that molecules can move across the membrane from an area of move across the membrane from an area of [high] to [low] ([ ] means concentration).[high] to [low] ([ ] means concentration).- materials that pass easily are:- materials that pass easily are:

gases, small uncharged molecules, and fat gases, small uncharged molecules, and fat soluble molecules.soluble molecules.

MotionMotion

Any atom above absolute zero (-273 Any atom above absolute zero (-273 ooC) is in C) is in motion. The random motion within a liquid is motion. The random motion within a liquid is called called Brownian motionBrownian motion. (Think of dye . (Think of dye spreading through a liquid).spreading through a liquid).

The difference in concentrations of a substance The difference in concentrations of a substance across a membrane is called across a membrane is called the the concentration gradient. concentration gradient.

OSMOSISOSMOSIS

The diffusion of water is called Osmosis. The diffusion of water is called Osmosis. Where water moves from a [high] to [low]. Where water moves from a [high] to [low]. There are different types of environments that There are different types of environments that will lead to a net direction of movement:will lead to a net direction of movement:

Isotonic condition – Isotonic condition – when the concentrations when the concentrations of solutes on both sides of the membrane are of solutes on both sides of the membrane are the same causing no net movement of water.the same causing no net movement of water.

Hypotonic condition – Hypotonic condition – when the [water] is higher when the [water] is higher ([lower solute]) outside the cell. Water will move ([lower solute]) outside the cell. Water will move into the cell to balance out concentrations, causing into the cell to balance out concentrations, causing the cell to swell and possibly burst.the cell to swell and possibly burst.

Hypertonic condition – Hypertonic condition – when the [water] is higher when the [water] is higher ([lower solute]) inside the cell. Water will move out ([lower solute]) inside the cell. Water will move out of the cell, causing the cell to shrivel.of the cell, causing the cell to shrivel.

No energy is required to diffuse water, runs on No energy is required to diffuse water, runs on concentration gradient.concentration gradient.

Facilitated DiffusionFacilitated Diffusion

Facilitated DiffusionFacilitated Diffusion – molecules that do not – molecules that do not readily pass, due to size or charge etc.., may readily pass, due to size or charge etc.., may need assistance to cross the membrane.need assistance to cross the membrane.Specialized transport proteins help substances Specialized transport proteins help substances move in this case.move in this case.

Transport proteins are selective and will only Transport proteins are selective and will only move the molecule/ion it was designed to move the molecule/ion it was designed to move. move.

Carrier ProteinsCarrier Proteins Numerous carrier proteins lie within the membrane to Numerous carrier proteins lie within the membrane to

facilitate the movement of glucose (sugar). These facilitate the movement of glucose (sugar). These carriers still operate by concentration gradients and carriers still operate by concentration gradients and do not require energy.do not require energy.

Channel ProteinsChannel Proteins Channel Proteins – Channel Proteins – are transport proteins that carry are transport proteins that carry

charged particles. They have a tunnel-like shape and charged particles. They have a tunnel-like shape and have their own charge. They attract the ion like have their own charge. They attract the ion like attracting a magnet. Size is a limiting factor for using attracting a magnet. Size is a limiting factor for using these channels.these channels.

http://www.stolaf.edu/people/gianninhttp://www.stolaf.edu/people/giannini/flashanimat/transport/channel.swfi/flashanimat/transport/channel.swf

Active transportActive transport

Certain cells require materials that may not be Certain cells require materials that may not be present in great amounts. In this case, a cell present in great amounts. In this case, a cell may need to pump these materials in or out of may need to pump these materials in or out of the cell against the normal concentration the cell against the normal concentration gradient.gradient.

This moving materials from [low] to [high] This moving materials from [low] to [high] requires the cell to spend energy, and is requires the cell to spend energy, and is therefore called Active transport.therefore called Active transport.

ExamplesExamples

4 Important examples of Active Transport4 Important examples of Active Transport- kidney cells pump glucose and Amino - kidney cells pump glucose and Amino

acids out of the urine and back into the blood.acids out of the urine and back into the blood.- Intestinal cells pump in nutrients from - Intestinal cells pump in nutrients from

the gut.the gut.- Root cells pump in nutrients from the - Root cells pump in nutrients from the

soilsoil- Gill cells in fish pump out sodium ions.- Gill cells in fish pump out sodium ions.

Active transport pumps are protein pumps in the Active transport pumps are protein pumps in the membrane that grab ions and other materials and membrane that grab ions and other materials and pump them across the membrane against the pump them across the membrane against the gradient.gradient.- one of the best understood is the Na+/K+ pump.- one of the best understood is the Na+/K+ pump.

In some cases the unequal distribution of Na+ (or In some cases the unequal distribution of Na+ (or other molecule) can allow another molecule to other molecule) can allow another molecule to ‘piggyback’ into the cell when regular diffusion ‘piggyback’ into the cell when regular diffusion continues.continues.

http://www.northland.cc.mn.us/biologhttp://www.northland.cc.mn.us/biology/Biology1111/animations/active1.swfy/Biology1111/animations/active1.swf Once you enter this site click on Once you enter this site click on

MAIN at the bottom.MAIN at the bottom.

Bulk Membrane Bulk Membrane TransportTransport

Bulk TransportBulk Transport

Material too big or material that is Material too big or material that is charged can not freely cross the cell charged can not freely cross the cell membrane.membrane.

The cell must accommodate these The cell must accommodate these substances by folding in on itself substances by folding in on itself creating a vesicle to take in or expel creating a vesicle to take in or expel these large molecules.these large molecules.

2 Types of Bulk transport2 Types of Bulk transport EndocytosisEndocytosis (Endo = into; cyto = (Endo = into; cyto =

cell) – the membrane folds inward, cell) – the membrane folds inward, trapping matter from the extracellular trapping matter from the extracellular fluid and enters the cell as a vesicle.fluid and enters the cell as a vesicle.

3 Types of Endocytosis3 Types of Endocytosis Phagocytosis (Phagocytosis (phago = eating) – the phago = eating) – the

membrane folds inward, trapping solid membrane folds inward, trapping solid matter from the extracellular fluid.matter from the extracellular fluid.

Pinocytosis Pinocytosis (pino = drinking) – the (pino = drinking) – the membrane folds inward trapping fluids membrane folds inward trapping fluids from the extracellular fluid.from the extracellular fluid.

Receptor-mediated Endocytosis – Receptor-mediated Endocytosis – the the membrane takes in particles after they membrane takes in particles after they bind to specialized receptor proteins bind to specialized receptor proteins embedded in the membrane. Cholesterol embedded in the membrane. Cholesterol is brought in this way.is brought in this way.

Exocytosis Exocytosis (exo= exit) – the reverse of (exo= exit) – the reverse of endocytosis. A vesicle moves out towards the endocytosis. A vesicle moves out towards the plasma membrane, fuses with it and opens up plasma membrane, fuses with it and opens up its’ contents into the extracellular fluid.its’ contents into the extracellular fluid.- This is important in the body secreting - This is important in the body secreting manufactured products from within the cells.manufactured products from within the cells.

Endocytosis and Endocytosis and ExocytosisExocytosis

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