Transport through cell membranes

What are the two types of transport?

Passive, meaning without the use of energy

Active, meaning with energy in the form ATP

What are the forms of transport?

Passive: diffusion, facilitated diffusion

Active: active transport, transport in vesicles (endocytosis or exocytosis)

What is diffusion?

The movement of ions or molecules from a region of high concentration to a region of low concentration down a diffusion/concentration gradient, until an equilibrium is reached

How does diffusion occur with a membrane?

Membranes are not needed, however diffusion of non-polar molecules happens through the phospholipids and polar molecules move through the hydrophilic protein channels

Factors affecting diffusion

Concentration gradient, temperature, size of molecule, lipid solubility through membranes, distance, surface area, pores in membranes

How does concentration gradient affect the rate of diffusion?

Diffusion happens faster with a steeper concentration gradient

How does temperature affect the rate of diffusion?

A faster rate of diffusion happens at higher temperatures because molecules have more kinetic energy so move faster

How does the size of molecule affect the rate of diffusion?

The smaller the molecule, the faster the rate of diffusion as they fit more easily into the phospholipid membrane and into the protein channels

How does lipid solubility affect the rate of diffusion?

A lipid soluble molecule diffuses faster than a non lipid soluble molecule as they can pass directly through the membrane without need for a carrier or channel protein as they can dissolve through the fatty acid tails

How does distance affect the rate of diffusion?

The shorter the distance, the faster the rate of diffusion

How does surface area affect the rate of diffusion?

The presence of microvilli, which increases the surface area, increases the rate of diffusion

How does the number of pores in the membrane affect the rate of diffusion?

The more pores there are, the faster the rate of diffusion as it allows larger and higher numbers of molecules to leave the cell or organelles at one time, per second

How does facilitated diffusion occur?

Carrier molecules in the membrane temporarily bind with the molecules enabling passage through protein channels in the membrane, moving molecules from high concentration to low concentration

How are membranes involved in facilitated diffusion?

Membranes must be present as it requires carrier or channel proteins which are only present in membranes

What is transported through facilitated diffusion and how quickly?

Amino acids are frequently transported this way, and it is passive but faster than simple diffusion

What is cotransport?

A type of facilitated diffusion where two substances are simultaneously transported across a membrane by a carrier protein

Example of cotransport

In the kidneys, glucose and sodium are transported together in the same direction

What is a uniport?

A protein carrying only one molecule so is not a type of cotransport

What is a symport?

A protein which carries two molecules in the same direction together

What is a antiport?

A protein carrying two molecules in opposite directions

Describe the process of osmosis

The movement of water molecules from an area of high water potential to an area of low water potential through a selectively permeable membrane, along a concentration gradient until an eqilibrium is reached

Is osmosis active or passive?

A passive process requiring no ATP and is unaffected by respiratory inhibitors because of this

What is water potential?

Depends on the solute concentration and the pressure exerted on the solution (the pressure potential)

Water potential = solute potential + pressure potential

What is the water potential for pure water?

Has a value of 0kPa (kilopascals) - any other solution has a negative value

What is a hypotonic solution?

A solution with a lower solute concentration (and a higher water potential) than the solution it is being compared to

What is an isotonic solution?

Has the same concentration of solute and the same water potential to the solution it is being compared to

What is a hypertonic solution?

Has a higher concentration of solute (and a lower water potential) than the solution it is being compared to

How do plant cells become turgid?

When they are immersed in solutions with a higher water potential than the cell contents, water enters the cell by osmosis so the cytoplasm and vacuole expand. When the cell becomes turgid and no more water can enter, the vacuole has pushed up against the cell wall, which restricts further expansion by exerting pressure on the cytoplasm inwards

What is the water potential of a turgid cell?

0kPa

What happens when cells lose water?

When cells are immersed in solutions that have a lower water potential than the cells, water leaves the cell by osmosis, which causes the vacuole and cytoplasm to shrink due to the loss of water - the cytoplasm pulls away from the cell wall

What is plasmolysis?

When the vacuole has pulled away from the cell wall

What is incipient plasmolysis?

The point at which the cell membrane just begins to pull away from the cell wall - this is judged when 50% of the cells have become plasmolysed

What is the water potential of plasmolysed cells and why?

0kPa as there is no pressure generated from each side - the vacuole and cell wall is not touching so there are no inward or outward forces. As the cell wall is fully permeable, the solution diffuses through it so the water potentials on both sides are the same

Describe the process of active transport

The movement of substances from low to high concentrations against a concentration gradient using specific protein carriers and energy from ATP

How do respiratory inhibitors impact active transport?

Aerobic respiration requires oxygen (removed when respiratory inhibitors are introduced), and the more oxygen there is, the faster the rate of respiration and thus active transport. Active transport stops when respiratory inhibitors are introduced

How does cyanide work?

Cyanide is a respiratory inhibitor that binds to cytochrome in the electron transport chain, preventing the production of ATP and preventing active transport

What is exocytosis?

The mechanism by which large particles such as enzymes and hormones are secreted from cells; the vesicle containing the particle migrates to the cell membrane and fuses with it to release the particles to the outside of the cell

What is endocytosis?

The mechanism by which particles pinch off from the plasma membrane. The membrane extends around the particles being taken into the cell, forming a vesicle in the cytoplasm

What types of endocytosis are there and what do they do?

Pinocytosis is the movement of liquids

Phagocytosis is the movement of solids, like the cell or large protein