Movement into and out of cells

· Movement across cell membranes depends on membrane permeability, concentration gradients, water potential gradients, ATP availability and surface area to volume ratio.
· Key processes: simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.
· Exam focus: be able to describe, explain, compare, interpret graphs/data and evaluate practical investigations.

Simple diffusion

· Simple diffusion = net movement of particles from higher concentration to lower concentration down a concentration gradient.
· It is passive: no ATP required.
· Particles move due to random kinetic movement.
· Occurs directly through the phospholipid bilayer for small non-polar molecules, e.g. oxygen and carbon dioxide.
· Rate increases with: steeper concentration gradient, shorter diffusion distance, larger surface area and higher temperature.
· Diffusion reaches dynamic equilibrium when there is no net movement, but particles still move randomly.

This diagram compares passive transport with active transport. Use it to distinguish movement down a concentration gradient from movement against a concentration gradient. Source

Facilitated diffusion

· Facilitated diffusion = net movement of particles from higher concentration to lower concentration through membrane transport proteins.
· It is passive: no ATP required.
· Used by substances that cannot pass easily through the hydrophobic core of the phospholipid bilayer.
· Channel proteins allow specific ions or polar molecules to pass through a hydrophilic pore.
· Carrier proteins bind to a specific molecule, change shape and release it on the other side.
· Facilitated diffusion is specific because transport proteins have specific shapes.
· Rate may plateau when all channel proteins or carrier proteins are occupied.

This image shows facilitated diffusion through membrane proteins. It is useful for showing why polar or charged substances need channel proteins or carrier proteins to cross membranes. Source

Osmosis and water potential

· Osmosis = net movement of water molecules through a partially permeable membrane from higher water potential to lower water potential.
· Water potential is measured in kPa.
· Pure water has the highest water potential, usually 0 kPa.
· Adding solute makes water potential more negative.
· Water moves from less negative water potential to more negative water potential.
· CIE does not require detailed knowledge of solute potential or pressure potential for this topic.
· A partially permeable membrane allows water through but restricts some solutes.

Osmosis in animal and plant cells

· In animal cells, water entering by osmosis may cause the cell to swell and burst because there is no cell wall.
· In animal cells, water leaving by osmosis may cause the cell to shrink.
· In plant cells, water entering by osmosis makes the vacuole expand, pressing the cytoplasm against the cell wall: the cell becomes turgid.
· Turgid plant cells help support plant tissues.
· In plant cells, water leaving by osmosis causes the vacuole and cytoplasm to shrink: the cell becomes flaccid.
· Severe water loss causes plasmolysis, where the cell surface membrane pulls away from the cell wall.
· Plant cells do not usually burst because the cell wall resists expansion.

This diagram shows how osmosis changes the condition of plant cells. It helps compare turgid, flaccid and plasmolysed cells. Source

Active transport

· Active transport = movement of substances across a membrane against a concentration gradient using ATP.
· Requires carrier proteins or pumps in the membrane.
· ATP provides energy for the protein to change shape and move the substance across the membrane.
· Active transport allows cells to absorb substances even when their concentration is already higher inside the cell.
· Examples include uptake of mineral ions by root hair cells and movement of ions across animal cell membranes.
· Active transport is specific because each carrier protein transports a particular substance.

Endocytosis and exocytosis

· Endocytosis = bulk movement of material into a cell using vesicles formed from the cell surface membrane.
· The membrane folds around material, pinches off and forms a vesicle inside the cytoplasm.
· Phagocytosis is endocytosis of solid material.
· Pinocytosis is endocytosis of liquid material.
· Exocytosis = bulk movement of material out of a cell when vesicles fuse with the cell surface membrane.
· Exocytosis is used to secrete substances such as enzymes, hormones and mucus.
· Both processes require ATP because they involve membrane movement and vesicle formation.

This diagram shows how cells take in material by endocytosis. It is useful for comparing bulk transport with diffusion and active transport. Source

Surface area to volume ratio

· Surface area to volume ratio (SA:V) affects how efficiently substances enter and leave cells.
· As cell size increases, volume increases faster than surface area.
· Therefore, larger cells have a smaller SA:V ratio.
· A smaller SA:V ratio means less membrane surface is available per unit volume, so exchange by diffusion becomes less efficient.
· Small cells exchange substances faster because they have a larger SA:V ratio and shorter diffusion distances.
· Formulae for cubes: surface area = 6l², volume = l³, SA:V = surface area ÷ volume.
· Multicellular organisms often need specialised exchange surfaces because diffusion alone is too slow for large bodies.

This activity models how surface area to volume ratio affects diffusion. Smaller agar blocks allow diffusion to reach the centre faster than larger blocks. Source

Investigating diffusion using non-living materials

· Agar blocks can model cells when investigating diffusion.
· Blocks of different sizes are placed in a solution, often acid or alkali with an indicator.
· Measure the depth of colour change or calculate the percentage volume diffused.
· Independent variable: block size or surface area to volume ratio.
· Dependent variable: diffusion distance, time taken, or percentage diffusion.
· Control variables: temperature, concentration of solution, time immersed, agar composition and shape of blocks.
· Conclusion: blocks with a larger SA:V ratio show faster or more complete diffusion.

Investigating osmosis using plant tissue

· Plant tissue, often potato cylinders, is placed in solutions of different water potentials or sucrose concentrations.
· Measure initial mass and final mass, then calculate percentage change in mass.
· Percentage change in mass = change in mass ÷ initial mass × 100.
· If mass increases, water has entered by osmosis because the tissue had lower water potential than the solution.
· If mass decreases, water has left by osmosis because the tissue had higher water potential than the solution.
· If there is no change in mass, the solution and tissue have the same water potential.
· Estimate tissue water potential by plotting percentage change in mass against solution water potential and finding where the graph crosses 0% change.
· Control variables: same tissue type, same cylinder diameter/length, same immersion time, same temperature, same blotting method.

Investigating osmosis using dialysis / Visking tubing

· Dialysis tubing is a partially permeable membrane used to model cell membranes.
· It allows small molecules such as water to pass through but restricts larger molecules.
· Tubing containing sucrose solution can be placed in water or solutions of different concentrations.
· A change in mass shows net movement of water by osmosis.
· It can also model selective permeability by showing that some solutes cannot pass through the membrane.

Comparing transport processes

· Simple diffusion: passive; down a concentration gradient; through phospholipid bilayer; no proteins required.
· Facilitated diffusion: passive; down a concentration gradient; uses channel proteins or carrier proteins.
· Osmosis: passive; water moves down a water potential gradient through a partially permeable membrane.
· Active transport: active; moves substances against a concentration gradient; uses ATP and carrier proteins.
· Endocytosis: active; bulk movement into cells using vesicles.
· Exocytosis: active; bulk movement out of cells using vesicles.

Common exam traps

· Do not say diffusion “stops” at equilibrium; say there is no net movement.
· Do not confuse concentration gradient with water potential gradient.
· Do not say osmosis is movement of “solution”; osmosis is movement of water molecules.
· Do not say active transport is diffusion; active transport moves substances against a gradient using ATP.
· Do not say plant cells burst in pure water; they become turgid because of the cell wall.
· Always use percentage change in mass rather than raw mass change when comparing different plant tissue samples.
· When estimating water potential, identify the value at 0% change in mass.