Cell Transport and Diffusion

Membranes and Transport

What are the main ways substances are transported across cell membranes?

Simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.

What factors affect how a substance crosses a cell membrane?

Its size, lipid solubility, water solubility and whether it has an electrical charge.

Why can concentrations of substances be very different on either side of a cell membrane?

The membrane selectively controls which substances can cross it.

Why is transport across cell membranes important?

Cells need substances to enter and leave for processes such as respiration, metabolism and responses to stimuli.

Which substances can pass directly through the phospholipid bilayer relatively easily?

Small non-polar or lipid-soluble molecules.

Give two examples of molecules that can diffuse directly through the phospholipid bilayer.

Oxygen and carbon dioxide.

Why can oxygen and carbon dioxide diffuse directly through the membrane?

They are small, non-polar molecules that can dissolve in the hydrophobic region of the phospholipid bilayer.

Can some large lipid-soluble molecules pass directly through membranes?

Yes. Some lipid-soluble molecules such as steroid hormones can pass through the phospholipid bilayer.

Why can charged particles not easily pass directly through the phospholipid bilayer?

The hydrophobic centre of the membrane acts as a barrier to charged particles.

How do ions such as sodium ions cross cell membranes?

Through specific membrane proteins such as channels or carriers.

What are the two broad categories of membrane transport?

Passive transport and active transport.

Define passive transport.

The movement of substances as a result of concentration, pressure or electrochemical gradients without requiring energy from ATP.

Does passive transport require ATP?

No.

What are the three main types of passive transport?

Simple diffusion, facilitated diffusion and osmosis.

Define active transport.

The movement of substances across a membrane using ATP as an immediate source of energy.

Where does the ATP used in active transport come from?

Cellular respiration.

What is ATP?

Adenosine triphosphate, the immediate source of energy for cellular processes.

What is the key difference between passive and active transport?

Passive transport does not use ATP, whereas active transport uses ATP.

What membrane protein is always involved in active transport?

A carrier protein.

What provides the energy for active transport?

The breakdown, or hydrolysis, of ATP.

What is endocytosis?

The movement of large substances into a cell by formation of membrane-bound vesicles.

Does endocytosis require energy?

Yes, it requires ATP.

What property of the cell membrane allows vesicles to form during endocytosis?

The fluid nature of the cell membrane.

What is exocytosis?

The movement of large substances out of a cell when vesicles fuse with the cell surface membrane and release their contents.

Does exocytosis require energy?

Yes, it requires ATP.

What is the main difference between endocytosis and exocytosis?

Endocytosis moves substances into a cell, whereas exocytosis moves substances out of a cell.

Define diffusion.

The net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient.

What is a concentration gradient?

A difference in the concentration of a substance between two areas.

What does moving down a concentration gradient mean?

Moving from an area of higher concentration to an area of lower concentration.

Why does diffusion occur?

Particles have kinetic energy and move randomly.

What determines the kinetic energy of particles?

Temperature.

What happens to the rate of particle movement when temperature increases?

Particles gain kinetic energy and move faster.

What happens during diffusion when particles initially have an uneven distribution?

Random movement causes a net movement from the area of higher concentration to the area of lower concentration.

What eventually happens to particles during diffusion in a closed system?

They become evenly distributed.

Do particles stop moving when they become evenly distributed?

No. They continue moving randomly.

Why is there no net movement once particles are evenly distributed?

Equal numbers of particles move in each direction.

What is dynamic equilibrium in diffusion?

A state where particles continue to move randomly but there is no net movement because movement occurs equally in both directions.

What type of transport is simple diffusion?

Passive transport.

Does simple diffusion require ATP?

No.

What provides the energy for diffusion?

The kinetic energy of the particles themselves.

Why is it incorrect to say diffusion requires no energy?

The particles require kinetic energy to move, but no metabolic energy from ATP is required.

What is simple diffusion across a cell membrane?

The net movement of molecules directly through the phospholipid bilayer from high to low concentration.

Give examples of molecules transported by simple diffusion.

Oxygen and carbon dioxide.

Why can hydrophilic molecules not usually cross the membrane by simple diffusion?

They cannot pass easily through the hydrophobic centre of the phospholipid bilayer.

Why can ions not cross the membrane by simple diffusion?

Their electrical charge prevents them from passing through the hydrophobic centre of the phospholipid bilayer.

What is facilitated diffusion?

The passive movement of substances down their concentration gradient through specific channel or carrier proteins in a membrane.

Does facilitated diffusion require ATP?

No.

Why is facilitated diffusion described as passive?

Substances move down their concentration gradient and no ATP is used.

Why is facilitated diffusion needed?

Large, polar or charged substances cannot easily pass directly through the phospholipid bilayer.

What does "facilitated" mean in facilitated diffusion?

Helped; membrane proteins provide a pathway that allows diffusion to occur.

What two types of membrane protein are involved in facilitated diffusion?

Channel proteins and carrier proteins.

What are channel proteins?

Membrane proteins that form hydrophilic pores through which specific ions or polar molecules can diffuse.

Why are channel proteins selective?

The structure of each channel allows only particular ions or molecules to pass.

Give examples of specific ion channels.

Sodium ion channels and potassium ion channels.

What are gated channels?

Channel proteins that open or close in response to particular conditions.

What can cause gated channels to open?

The presence of a specific molecule or a change in electrical potential across the membrane.

Where are electrically gated channels particularly important?

In neurones during the transmission of nerve impulses.

How do channel proteins enable facilitated diffusion?

They provide hydrophilic pathways through the membrane so specific substances can move down their concentration gradient.

What are carrier proteins?

Membrane proteins that bind specific molecules and change shape to transport them across the membrane.

Why are carrier proteins specific?

Their binding sites have shapes that are complementary to particular molecules or groups of molecules.

Where is the binding site of a carrier protein located when transporting a substance into a cell?

On the outside-facing surface of the membrane.

What happens first during carrier-mediated facilitated diffusion?

A specific molecule binds to a complementary binding site on the carrier protein.

What happens after a molecule binds to a carrier protein?

The carrier protein changes shape and carries the molecule across the membrane.

What happens after the carrier protein changes shape?

The molecule is released on the other side of the membrane.

What happens to the carrier protein after releasing the molecule?

It returns to its original shape so another molecule can bind.

Does a carrier protein use ATP during facilitated diffusion?

No.

In which direction can carrier-mediated facilitated diffusion occur?

Only down the concentration gradient, from higher to lower concentration.

Give an example of carrier-mediated facilitated diffusion.

Glucose entering red blood cells through specific carrier proteins.

What is the difference between channel proteins and carrier proteins in facilitated diffusion?

Channel proteins form hydrophilic pores, whereas carrier proteins bind substances and change shape to transport them.

What determines whether facilitated diffusion produces net movement into or out of a cell?

The direction of the concentration gradient.

Can facilitated diffusion move substances against their concentration gradient?

No.

Why can facilitated diffusion only work down a concentration gradient?

It is passive and does not use ATP to provide energy for movement against the gradient.

What is the difference between simple diffusion and facilitated diffusion?

Simple diffusion occurs directly through the phospholipid bilayer, whereas facilitated diffusion requires specific membrane proteins.

What do simple diffusion and facilitated diffusion have in common?

Both are passive, move substances down a concentration gradient and do not require ATP.

Which molecules generally use simple diffusion?

Small non-polar and lipid-soluble molecules such as oxygen and carbon dioxide.

Which substances generally use facilitated diffusion?

Ions and larger or polar molecules that cannot cross the phospholipid bilayer directly.

What is osmosis?

The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

What type of transport is osmosis?

Passive transport.

Does osmosis require ATP?

No.

What moves during osmosis?

Water molecules.

What type of membrane is required for osmosis?

A partially permeable membrane.

What is a partially permeable membrane?

A membrane that allows some substances to pass through but prevents others from crossing.

What is water potential?

A measure of the tendency of water molecules to move from one region to another.

In which direction does water move during osmosis?

Down a water potential gradient, from higher water potential to lower water potential.

What is a water potential gradient?

A difference in water potential between two regions.

How does water commonly cross cell membranes?

Through specialised protein channels in the membrane.

Why does water commonly use protein channels to cross membranes?

The hydrophobic centre of the phospholipid bilayer restricts the movement of polar water molecules.

What are aquaporins?

Channel proteins that facilitate the movement of water across cell membranes.

How is osmosis different from ordinary diffusion?

Osmosis specifically describes the movement of water through a partially permeable membrane down a water potential gradient.

What is a solute?

A substance dissolved in a solvent.

What is a solvent?

A liquid in which a solute dissolves.

Why are transport systems needed across membranes?

The hydrophobic phospholipid bilayer prevents many essential charged, polar and large molecules from crossing freely.

Why can oxygen cross the phospholipid bilayer but ions cannot?

Oxygen is small and non-polar, whereas ions are charged and cannot pass through the hydrophobic centre.

What three routes of passive transport are shown across a cell surface membrane?

Simple diffusion through phospholipids, osmosis through water channels and facilitated diffusion through channel or carrier proteins.

How does simple diffusion occur through a cell surface membrane?

Small non-polar molecules dissolve in and pass directly through the phospholipid bilayer down their concentration gradient.

How does facilitated diffusion through a channel protein occur?

A specific substance passes through a hydrophilic pore from high to low concentration.

How does facilitated diffusion through a carrier protein occur?

A substance binds to a specific carrier, the carrier changes shape, releases it on the other side and then returns to its original shape.

What determines the direction of simple diffusion?

The concentration gradient.

What determines the direction of facilitated diffusion?

The concentration gradient.

What determines the direction of osmosis?

The water potential gradient.