Cell Membranes

Cell Membranes

• Cells are surrounded by a membrane: the cell surface membrane or plasma membrane

• On an EM it appears as a double line• The distance

across the

membrane is

7-8nm

WHAT DOES A MEMBRANE LOOK LIKE?

The Fluid Mosaic Model

TRANSPORT ACROSS MEMBRANES

There are four main methods by which substances can move across a cell membrane:

• 1. Diffusion

Osmosis• 2. Active Transport• 3. Endocytosis and Exocytosis

Passive Transport

1: Diffusion

• Diffusion is the movement of molecules from a region where they are in high concentration to a region of low concentration.

• The difference in concentration is the concentration gradient

molecule

membrane

• Diffusion is a PASSIVE process

• There is a net movement of molecules down the concentration gradient until equilibrium is reached

• Ions move by diffusion along electrochemical gradients

Rate of diffusion

• The rate of diffusion depends on:– Concentration gradient– Temperature– Size of molecule– Lipid solubility

Extension: Look up Fick’s Law…

LipidBilayer

concentrationgrad ient

concentrationgradient

Intrinsic globular proteinswithin the membrane function

as carriers for the transportof certain molecules

Carriers are specific for the molecules that they transport them across the

membrane in the direction of the concentration gradient at a faster rate

than occurs for simple passive diffusion

Glucose molecules enter body cells from thebloodstream by facilitated diffusion

Facilitated Diffusion

LipidBilayer

concentrationgrad ient

concentrationgradient

LipidBilayer

concentrationgrad ient

concentrationgradient

LipidBilayer

concentrationgrad ient

concentrationgradient

LipidBilayer

concentrationgrad ient

concentrationgradient

LipidBilayer

concentrationgrad ient

concentrationgradient

LipidBilayer

concentrationgrad ient

concentrationgradient

LipidBilayer

concentrationgrad ient

concentrationgradient

LipidBilayer

concentrationgrad ient

concentrationgradient

LipidBilayer

concentrationgrad ient

concentrationgradient

Increasing external concentration of glucose

Rate ofuptake

A

Rate of uptakeincreases as the

glucose concentration

increases(i.e. the steepness ofthe concentration gradient increases)

Rate of uptake reachesa maximum at glucose

concentration A

No further increase inthe uptake rate despite the

increasing glucose concentration

When glucose concentrations exceed a certain value, there are

insufficient glucose carriers within the membrane for

the rate of uptake to increase any further

LIMITING FACTORS AND FACILITATED DIFFUSION

The rate at which molecules like glucose enter cells by facilitated diffusion is affected by factors other than the concentration gradient

The availability ofcarriers in the membrane

is the limiting factor

Summary of Passive Transport

2: Osmosis• Osmosis is the diffusion of water molecules from an area

of high concentration (of water) to an area of lower concentration across a semi-permeable membrane

watermolecules solute

molecules

low concentration of solute

membrane

high concentration of solute

high concentration of water low concentration of water

high water potential ( ) low water potential ( )

low osmotic pressure (OP) high osmotic pressure (OP)

dilute solution concentrated solution

net movement of water

Water Potential

Osmosis can be quantified using water potential, so we can calculate which way water will move, and how fast. Water potential (ψ, the Greek letter psi, pronounced "sy") is simply the effective concentration of water. It is measured in units of pressure (Pa, or usually kPa)

water always "falls" from a high to a low water potential

100% pure water has ψ  = 0, which is the highest possible water potential, so all solutions have ψ  < 0

you cannot get ψ  > 0.

pure water = 0 kPa

water diffusesfrom 0 to -200 kPa

dilute solution = -200 kPa

concentrated solution = -500 kPa

water diffusesfrom -200 to -500 kPa

ψ ψψ

Cells and Osmosis.

The concentration of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three possible concentrations of solution to consider:

• Isotonic solution a solution of equal concentration to a cell

• Hypertonic solution a solution of higher (salt) concentration than a cell

• Hypotonic solution a solution of lower (salt) concentration than a cell

Hypotonic solution Isotonic solution Hypertonic solution

AnimalCell

PlantCell

water enters, cell swellsand may burst (lysis).

no net movement ofwater, cell normal size

water leaves, cellshrinks and crenates

water enters, cell swellsa bit and becomes turgid.

no net movement ofwater, cell normal size

water leaves, cytoplasmshrinks and plasmolyses

3: Active Transport

• Pumping of molecules across cell membrane using a protein pump

UP a concentration Gradient

protein pump

ATP ADP + Pi

active site

• Most Active Transport systems are driven by metabolic energy derived from ATP

• Active Transport allows cells to uptake necessary ions and molecules and remove waste products

• These Mechanisms often referred to as Pumps

• Most common is Sodium Pump

Na+ / K+ Pump

• Present in all animal cells, most abundant and important of membrane pumps

outside cell3 Na+

2 K+

inside cell

-

+

ATP ADP + Pi

Na+K+

pump

• Sodium-Potassium Pump important in controlling cell volume, reducing Na thereby reducing water uptake by osmosis

• The accumulation of K ions are used in cell metabolic processes eg protein synthesis

• Na-K pump can be linked to active uptake of organic molecules such as glucose and amino acids

4: Endocytosis & Exocytosis

• Endocytosis

• Phagocytosis

Eg. Amoeba

White Blood Cells (neutrophils & monocytes)

• Pinocytosis

Eg. Protozoans, white blood cells, cells in embryos, liver and kidneys.