Topic 4.2: Cell Transport

Liquids inside and outside cells are solutions

Remember solutions have two components:

Solutes

Solvent

Solutes

the substance within the solvent

Solvent

the substance dissolving the solutes

Concentrations

The amount of solute dissolved in a given amount of solvent

(Amount solute)/(Amount solvent)

Actions that increase concentrations

Increase the amount of solute

Decrease the amount of solvent

Actions that decrease concentrations

Decrease the amount of solute

Increase the amount of solvent

Concentration gradients

Uneven distribution of solutes in the solvent (or solution)

Equilibrium

There is no net change in the system

Solutes movement

Naturally move down their concentration gradients (from high to low concentration)

Concentration gradients exist across cell membranes

Membranes create natural barriers between inside and outside the cell

Different solutions exist
on both sides

Concentration gradients influence movement across cell membranes

Gradients impact the direction of movement with respect to the cell

Cell membranes are selectively permeable

Only certain substances can move freely through the phospholipid bilayer

Small

Non-polar

Selectively Permeable (AKA semipermeable)

Refers to phospholipids allowing the movement of some substances across the membrane but restricting the movement of others.

Concentration gradients have potential energy

Input of energy is needed to generate a concentration gradient

Energy is released when molecules move down their concentration gradient

The released energy as molecules moves down their concentration gradient can be used to do work

Diffusion

Movement of molecules from a high to a low concentration

Does not require the input of energy

2 types of Diffusion

Passive (simple)

Facilitated

Passive (Simple) Diffusion

Movement of solute down a concentration gradient without the use of a transport protein

Does not require energy

Molecules that can passively diffuse (Passive Diffusion)

Small

Uncharged

Non-polar

Facilitated Diffusion

The movement of solutes from a high to a low concentration using a transport protein (channel or carrier protein)

Does not require energy

(Allows for large or charged / polar molecules to diffuse across membranes)

Osmosis

Passive movement of water (solvent) down its concentration gradient, until equilibrium is reached

Aquaporins

Protein channels in the cellular membrane that specifically allow for water molecules to pass through

Tonicity

Extracellular solution to make water move into or out of a cell by osmosis (sum of all non-permeable solutes)

3 types of Tonicities

Hypotonic

Isotonic

Hypertonic

Hypotonic

Solution is less concentrated than the cell

Isotonic

Solution matches the concentration of the cell

Hypertonic

Solution is more concentrated than the cell

Active Transport

Movement of molecules from a low to high concentration

Requires an input of energy

2 types of active transport

Primary active transport

Secondary active transport

Primary active transport

Energy (ATP) is used to move molecules against their concentration gradient

Molecules move from less to more concentrated

Secondary active transport

Energy in the concentration gradient of X molecules is used to move Y molecules against their concentration gradient

molecules move down a conc. gradient release a lot of energy

Other molecules move against their gradient can co-opt the energy to help them move.

Bulk Transport

Movement of large amounts of molecules across the membrane at once

Molecules are brought over on mass instead of individually

Defined by direction relative to cell, not direction of concentration gradient

Utilizes vesicles

small membrane bound pockets for sorting and transporting molecules

Endocytosis

brings things into the cell

3 types of Endocytosis

•Phagocytosis

•Pinocytosis

•Receptor mediated endocytosis 

Exocytosis

brings things outside of the cell