Transport across the Cell Membrane

Unit C Cycling of Matter in Living Systems

Cell Membrane (pg 271-273) Plasma membrane, semi permeable

membrane Protective layer between environment &

cell’s fragile contents (“biological barrier”) Maintains equilibrium (balance) inside cell Structure

Phospholipid bi-layer (head and tail) Contains embedded proteins for transport and

chemical reactions “Fluid Mosiac Model”

Transport across cell membrane ( pg 274-283)

Semi-permeable/Selectively permeable= only certain solutes are SELECTED to cross membrane

Cells use the following to help transport materials: Energy (ATP) Theory of Brownian motion Natural concentration gradient

End result always equilibrium

Background

Solute Substance that is dissolved in a solution

Eg. Glucose, salt, Potassium, Iron, oxygen, carbon dioxide

Background

Brownian Motion Particles are constantly moving in

random motion

Background

Concentration Gradient Difference of concentrations of

solutes/water between 2 areas Determines direction water or solutes

will move Brownian motion (random movement)

means particles (solutes/water) will naturally flow to an area where they are less concentrated

Two categories of Transport

Passive Transport Active Transport

Passive Transport

Natural movement of particles/water from an area of high concentration to area of low concentration (DOWN the concentration gradient)

Does NOT require energy 3 types of Passive Transport

1. Diffusion2. Osmosis3. Facilitated Diffusion

#1 Diffusion

Movement (high to low) of SOLUTES (fatty acids, glycerol, CO2, O2)

Rate of diffusion how long it takes for diffusion to occur Can be altered by adding energy (eg

stirring or heating) to speed up movement of particles

#2 Osmosis

Movement (high to low) of WATER If a solute is too big, has a charge or not

soluble then it won’t pass by diffusion so water moves instead

Can predict water movement based on the solute concentrations inside and outside of cell 3 conditions that are relative to each other:▪ Isotonic▪ Hypertonic▪ Hypotonic

Isotonic

When two solutions have same concentrations of solutes

Solutes/Water will move between solutions but no net change in amount of either will occur

Hypertonic vs Hypotonic

Two solutions differ in concentration of solutes/water The one with more solutes is hypertonic to

one with less The solution with less solute is hypotonic to

one with more solutes Water concentration of the hypertonic

solution is less than water concentration of hypotonic solution so net movement of water is from hypotonic to hypertonic solution

#3 Facilitated Diffusion

Movement (high to low) of larger SOLUTES (glucose) that need the help of PROTEIN FACILITATOR Channel Protein▪ Creates channels or pores for small water

particles to move Carrier Protein▪ Attach to larger molecules, changes shape

and physically moves molecules across membrane

ActiveTransport

Movement of particles from an area of low concentration to area of high concentration (AGAINST the concentration gradient)

Requires ATP (adenosine triphosphate) energy

Use carrier proteins as a “pump” 3 types of Active Transport

1. Endocytosis2. Exocytosis3. Protein Carriers

#1 Endocytosis

Bringing large particles INTO cell Engulfs large particle using a vesicle

sac to surround, contain then “pinch” off

E.g. Ameoba getting food

#2 Exocytosis

Getting large particles OUT of cell Vesicle surrounds particle and fuses

with cell membrane then ruptures and contents leave cell

#3 Protein Carriers

Movement (low to high) of larger SOLUTES (glucose) that need the help of PROTEIN Carrier Carrier Protein▪ Attach to larger molecules, changes shape

and physically moves molecules across membrane

REVIEW TRANSPORT

Passive Transport Does not require energy Movement DOWN/WITH concentration gradient Transports smaller, water soluble particles (CO2,

O2, H2O, glucose) Includes diffusion, osmosis, facilitated diffusion

Active Transport Requires energy Movement UP/AGAINST concentration gradient Transport of larger particles & ions (Al, Fe, Ca) Includes protein carriers, endocytosis, exocytosis

Why are cells so small?

Transport of materials in/out of cells critical

Ability to transport materials must be at maximum

Larger the cell the more: Volume it has Molecules will be needed to carry out life

functions Distance molecules has to travel from cell

to surface increases

Surface area to volume

As a cell size increases its surface to volume ratio decreases (SA/V)

Meaning that there is less cell membrane to do transport but more cell to get materials to

Think about it

Size of cell is limiting factor

The greater the surface to volume ratio the more efficient the cell is at transportation

Size is the limiting factor of cells Smaller cell with greater SA/V ratio

require less molecules to diffuse across the membrane with more membrane to do it

Example

Oxygen exchange Oxygen is obtained form the surrounding

environment such as water or blood (depends on the cell) and DIFFUSES across the cell membrane.

More membrane more diffusion (Surface area= increases by the 2).

A big cell needs more oxygen than a little cell (volume= increase by the 3)

Big cell has relatively small surface area compared to its volume i.e. the surface area: volume ratio is small.

Larger cells become limited by the rate of gas exchange.

Explain

Obtaining nutrient (glucose) Excretion of waste molecules ( urea,

ammonia, carbon dioxide).