Transport of Substances Into and Out of Cells (OCR A-Level Biology A): Revision Notes

Transport of Substances Into and Out of Cells

Introduction to membrane transport

Cells continuously exchange materials with their surroundings through the plasma membrane. The direction and mechanism of transport depend on whether a concentration gradient exists and whether it favours the movement.

When substances move from higher to lower concentration (down a gradient), passive transport occurs without energy expenditure. Three types exist:

• Simple diffusion
• Facilitated diffusion
• Osmosis

When substances must move from lower to higher concentration (against a gradient), cells must expend energy through active transport. Additionally, cells use bulk transport mechanisms (endocytosis and exocytosis) to move large quantities of materials.

Diffusion

Diffusion describes the net movement of particles (atoms or molecules) down a concentration gradient, from regions where they are more concentrated to regions where they are less concentrated.

Mechanism of diffusion

Particles in liquids and gases move randomly and continuously. Although individual particles move in all directions, when more particles exist in one region than another, random movement results in net particle flow towards the less concentrated region. This occurs because:

• More particles move away from the concentrated region
• Fewer particles move towards the concentrated region
• The difference creates net movement down the gradient

No metabolic energy input is needed, making diffusion a passive process.

Diffusion rates in different states

• Solids: Particles vibrate but remain fixed in position, so very little diffusion occurs
• Liquids: Particles move freely, allowing diffusion to proceed readily
• Gases: Particles move most freely, enabling rapid diffusion

Diffusion and membranes

The plasma membrane forms a partially permeable barrier. Even with a favourable concentration gradient, some particles cannot cross the lipid bilayer. These molecules require facilitated diffusion.

Facilitated diffusion

Facilitated diffusion enables larger molecules, polar molecules, and ions to cross the membrane using membrane proteins. Two protein types mediate this process:

Channel proteins

Channel proteins function as pores through the membrane, providing a hydrophilic pathway for ions and small polar molecules.

Key features:

• Different channels transport different substances
• Many channels can open and close to regulate flow
• Transport remains passive (no metabolic energy required)
• Movement still follows concentration gradients

Carrier proteins

Carrier proteins transport larger molecules through a conformational change mechanism:

1. The specific molecule binds to the carrier protein
2. The protein changes shape in response
3. This shape change transfers the molecule across the membrane
4. The protein returns to its original conformation

Key features:

• Each carrier shows specificity for particular substances
• Shape changes occur spontaneously (no metabolic energy needed)
• Process remains passive
• Movement follows concentration gradients

Comparison with simple diffusion

Feature	Simple diffusion	Facilitated diffusion
Molecules transported	Small, non-polar molecules	Large molecules, polar molecules, ions
Membrane components	Lipid bilayer only	Channel or carrier proteins
Energy requirement	None (passive)	None (passive)
Specificity	Non-specific	Specific to transported substance
Regulation	Cannot be regulated	Can be regulated (especially channels)

Active transport

Active transport moves substances against their concentration gradient, from regions of lower concentration to regions of higher concentration.

Energy requirement

Because particles cannot move against concentration gradients passively, cells must provide energy:

• Adenosine triphosphate (ATP) provides this energy directly
• The process is therefore active (energy-dependent)
• Respiratory inhibitors (e.g., cyanide) stop active transport by preventing ATP formation

Mechanism of active transport

Active transport uses carrier proteins but not channel proteins:

1. The transported particle binds to a specific carrier protein
2. ATP provides energy for the carrier to change shape
3. The shape change moves the particle across the membrane against its gradient
4. The protein returns to its original conformation

Characteristics of active transport

• Each carrier protein is specific to one substance or a restricted range of substances
• Some carriers perform two-way active transport, simultaneously pumping one substance in and another out
• Active transport can accumulate substances to high concentrations inside cells
• The process continues regardless of concentration gradient direction

Evidence for active transport

Experiments comparing aerobic and anaerobic conditions demonstrate active transport:

Osmosis

Osmosis is the net movement of water molecules from a more dilute solution to a more concentrated solution across a partially permeable membrane.

Defining characteristics

Two factors define osmosis specifically:

• It involves only water molecules (not other substances)
• Movement occurs through a partially permeable membrane

Water potential

Scientists describe the concentration of water in solutions using water potential (symbol: $\Psi$, unit: kPa):

Water potential measures the relative tendency of water to move from one area to another. Water always moves from regions of higher water potential to regions of lower water potential.

Key points:

• Pure water has the highest water potential (defined as $0$ kPa)
• Adding solutes lowers water potential (values become negative)
• The more concentrated a solution, the lower (more negative) its water potential
• Lower water potential = greater tendency for water to move to that region

Mechanism of osmosis

Osmosis operates as a two-way process:

• Water molecules move in both directions through the membrane
• More water molecules move from higher to lower water potential
• Fewer water molecules move in the opposite direction
• The difference creates net movement from higher to lower water potential

The process never stops (while water molecules remain on both sides), but may reach equilibrium when water movement in each direction becomes equal.

Effects of osmosis on cells

Different cell types respond differently to osmotic changes because of structural differences.

Effects on animal cells

Animal cells lack rigid cell walls, making them vulnerable to osmotic damage:

Water potential of surrounding solution	Effect on cell	Term
Higher than cell (hypotonic)	Net water entry causes swelling and bursting	Lysis
Equal to cell (isotonic)	No net water movement	Equilibrium
Lower than cell (hypertonic)	Net water loss causes shrivelling	Crenation

Both bursting and crenation are typically fatal to animal cells.

Effects on plant cells

Plant cells possess rigid cell walls, which modify osmotic effects:

Water potential of surrounding solution	Effect on cell	Term
Higher than cell (hypotonic)	Net water entry; cell becomes rigid	Turgid
Equal to cell (isotonic)	No net water movement; cell loses rigidity	Flaccid
Lower to cell (hypertonic)	Net water loss; cytoplasm pulls away from cell wall	Plasmolysed

Turgid cells provide essential support in non-woody plants, preventing wilting. The rigid cell wall prevents bursting even when considerable water enters.

Plasmolysis can lead to cell death as the cytoplasm separates from the cell wall, creating gaps between the membrane and wall.

Important terminology

When using osmotic terms, always define them using water potential:

• Hypotonic = higher water potential than the cell
• Isotonic = equal water potential to the cell
• Hypertonic = lower water potential than the cell

Investigating osmosis

Plasmolysis in plant tissue

Plasmolysis can be observed microscopically in plant epidermal cells:

Experimental approach:

1. Plant tissue is mounted in solutions of known water potential
2. Cells are observed for plasmolysis (cytoplasm pulling away from cell wall)
3. The percentage of plasmolysed cells is counted at each water potential
4. Results show the relationship between external water potential and plasmolysis

Endocytosis and exocytosis

For large molecules or bulk quantities of materials, cells use membrane-based mechanisms that engulf or expel substances.

Endocytosis

Endocytosis is the process by which cells absorb molecules or particles by engulfing them. Two forms exist:

Pinocytosis

Pinocytosis (cell drinking) takes in small particles and fluid:

• The plasma membrane invaginates (folds inward)
• The invagination deepens to form a pocket
• Membranes fuse around the fluid, forming a small vesicle
• The vesicle separates into the cytoplasm

Phagocytosis

Phagocytosis (cell eating) takes in larger particles such as bacteria:

• Membrane extensions called pseudopodia (singular: pseudopodium) extend from the cell
• Pseudopodia wrap around the particle
• Membranes fuse to seal the particle in a vesicle
• The vesicle separates into the cytoplasm

In both processes, engulfed materials remain inside a membrane-bound vesicle, not free in the cytoplasm.

Exocytosis

Exocytosis secretes or expels substances from cells, essentially reversing pinocytosis:

1. A vesicle containing materials to be secreted moves to the plasma membrane
2. The vesicle membrane fuses with the plasma membrane
3. The vesicle opens to the cell exterior
4. Contents are released outside the cell

Energy requirements

Both endocytosis and exocytosis are active processes requiring ATP as an energy source. Membrane fusion and vesicle formation demand energy input.