Movement into & out of Cells — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

Section 1 of 33Diffusion

• Diffusion = the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient,
  • as a result of their random movement.
  • It is passive, so no ATP is needed.
  • Particles move because of their natural kinetic energy, and at equilibrium movement continues but there is no net change.

Factor	Effect on rate of diffusion
Steeper concentration gradient	More particles cross in one direction than the other, so the rate increases
Higher temperature	Particles have more kinetic energy and move faster
Larger surface area	More particles can cross at any moment; folding (microvilli, cristae) increases area
Shorter distance / thinner membrane	Particles have less far to travel
Smaller, non-polar molecules	They move faster and dissolve in the lipid core, so they cross the bilayer more quickly than large or polar ones

• Fick's law (idea)
  • rate of diffusion is proportional to ÷ diffusion distance.
    • (surface area × concentration difference)

Section 2 of 33Facilitated diffusion

Why it is needed

• Why it is needed ions and polar molecules such as glucose and amino acids cannot cross the hydrophobic core of the bilayer.

Proteins used

• Proteins used specific channel or carrier proteins provide the route, and the process is passive and goes down the concentration gradient.

Channel proteins

• Channel proteins water-filled pores with a fixed shape that let ions through, and most are gated so that they can open and close.

Carrier proteins

• Carrier proteins they bind a specific molecule and switch between two shapes, so the binding site opens first to one side and then to the other.

Rate limit

• Rate limit the rate levels off when all the proteins are in use, so it depends on the number of channel and carrier proteins.

Section 3 of 33Osmosis and water potential

Osmosis

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.

Water potential (ψ)

• Water potential (ψ) the tendency of water molecules to move from one place to another, measured in kPa.
• Pure water has ψ = 0 kPa and all solutions have a negative ψ.
• Adding solute lowers ψ because solute molecules attract water molecules, so fewer are free to move.

Equation

• Equation water potential = solute potential + pressure potential, ψ = ψ_s + ψ_p.

Tonicity

• Tonicity a hypotonic solution has a lower solute concentration (higher ψ) than the cell, a hypertonic solution has a higher solute concentration (lower ψ),
  • and an isotonic solution is equal.

Section 4 of 33Osmosis in animal and plant cells

External solution	Animal cell (no wall)	Plant cell (cellulose wall)
Higher ψ (more dilute)	Gains water and may burst (haemolysis in red blood cells)	Gains water; the protoplast pushes on the wall, ψp rises until ψ inside equals ψ outside, and the cell becomes turgid but does not burst
Equal ψ	No net water movement	No net movement; incipient plasmolysis if ψp = 0
Lower ψ (more concentrated)	Loses water, shrinks and becomes crenated	Loses water, becomes flaccid and then plasmolysed

1Plasmolysis

• Plasmolysis the protoplast shrinks and the cell surface membrane pulls away from the cell wall, and it occurs only in plant cells.

2Gap in a plasmolysed cell

• Gap in a plasmolysed cell the freely permeable cellulose wall lets the external solution fill the gap, so it is not empty or air-filled.

Section 5 of 33Active transport

Active transport

Active transport movement of molecules or ions across a membrane against their concentration gradient, using energy from ATP hydrolysis and carrier proteins (pumps).

Mechanism

• Mechanism a carrier protein binds the substance on one side, ATP hydrolysis changes its shape, and the substance is released on the other side.
  • (often phosphorylation of the carrier)

Sodium–potassium pump

• Sodium–potassium pump each ATP hydrolysed moves 3 Na⁺ out and 2 K⁺ in.

Inhibition

• Inhibition respiratory poisons such as cyanide and low oxygen reduce the rate because less ATP is made.

Example

• Example uptake of mineral ions by root hair cells.

Section 6 of 33Endocytosis and exocytosis

Bulk transport

• Bulk transport larger materials move in vesicles, and both processes need ATP to move vesicles and reshape membranes.

Endocytosis

• Endocytosis the cell surface membrane folds inwards around material and forms a vesicle, taking bulk material into the cell.
• Phagocytosis takes in solids, for example engulfing of bacteria by neutrophils.
• Pinocytosis takes in liquid droplets.

Exocytosis

• Exocytosis vesicles from the Golgi body move to the cell surface membrane, fuse with it and release their contents.
• Examples are secretion of digestive enzymes, hormones and neurotransmitters.

Section 7 of 33Surface area to volume ratio

1Size and ratio

• Size and ratio as a cell or organism grows, volume increases faster than surface area (volume ∝ side³, area ∝ side²), so SA:V falls.

2Consequences

• Consequences diffusion distances to the centre increase,
• which limits cell size and explains the need for specialised exchange surfaces and transport systems in large organisms.

3Importance of a high ratio

• Importance of a high ratio it helps obtain resources, remove wastes and exchange heat and chemicals with the surroundings.

Shape	Surface area	Volume
Cube	6 × side2, so SA:V = 6 ÷ side	side3
Cuboid	2(lw + lh + wh)	l × w × h
Cylinder	2πrh + 2πr2	πr2h

Section 8 of 33Practical: agar cubes and SA:V

1

Make agar blocks containing phenolphthalein and dilute sodium hydroxide (pink) and cut cubes of side 0.5, 1, 2 and 3 cm.

2

Calculate surface area, volume and SA:V for each cube.

3

Immerse all cubes at the same time in equal volumes of dilute hydrochloric acid of the same concentration at constant temperature.

4

Time how long each cube takes to turn completely colourless as acid diffuses in and neutralises the alkali.

5

Repeat, calculate means, and plot rate of colour change (1 ÷ time) against SA:V.

• Conclusion
  • smaller cubes with a higher SA:V decolourise faster, showing diffusion alone is adequate only over short distances.

Section 9 of 33Practicals: Visking tubing, plasmolysis and water potential

Visking tubing

• Visking tubing a non-living, partially permeable cellulose membrane.
• Fill it with starch and glucose solution and stand it in water.
• Test the water at intervalsglucose passes out, but starch is too large to pass through the pores.

Plasmolysis

• Plasmolysis mount epidermis strips from coloured tissue such as red onion in a range of sucrose or salt solutions and view under a light microscope.
• Count the proportion of cells whose protoplast has pulled away from the wall.

1

To estimate the water potential of plant tissue, cut equal cylinders of tissue, blot and weigh each.

2

Place them in a range of sucrose concentrations for a set time at constant temperature.

3

Blot and reweigh, then calculate percentage change in mass = (final mass − initial mass) ÷ initial mass × 100.

4

Plot percentage change against sucrose concentration.

5

Read the concentration where the line crosses 0%, because the tissue has the same ψ as that solution.

Section 10 of 33Comparing the transport mechanisms

Mechanism	Gradient	ATP?	Proteins?	Example
Diffusion	Down	No	No	O2 and CO2 across alveoli
Facilitated diffusion	Down	No	Channel or carrier	Glucose through carrier proteins
Osmosis	Down the ψ gradient (water)	No	Aquaporins can help	Water uptake by root hair cells
Active transport	Against	Yes	Carrier (pump)	Na+/K+ pump
Endocytosis / exocytosis	Bulk	Yes	Vesicles	Phagocytosis of bacteria; secretion of insulin

Section 11 of 33More from the specification

Osmosis

• Osmosis the net movement of water across a partially permeable membrane from higher to lower water potential.

Selectively permeable membrane

• Selectively permeable membrane a phospholipid bilayer with proteins lets water pass but restricts many solutes.

Potato cylinder investigation

• Potato cylinder investigation place cylinders in different sucrose solutions and measure mass or length change to find the isotonic point.

Plant cells

• Plant cells they become turgid in dilute solutions and plasmolysed in concentrated solutions.

Investigating diffusion

• Investigating diffusion use agar blocks with indicator to compare rates for different surface area to volume ratios.

Indicator

• A chemical that changes colour depending on pH.

Selectively permeable

• Allowing only some substances through.

Phospholipid bilayer

• Two layers of phospholipids with hydrophobic tails inwards

Section 12 of 33More on this topic

Explanation

• Explanation as a cube gets larger, volume (cubed) increases faster than surface area (squared), so diffusion takes longer and is less effective.

Trend

• Trend the rate of diffusion increases as SA:V increases, because a larger relative surface lets more acid enter per unit volume of agar.

Meaning

• Meaning small organisms can rely on diffusion but large ones cannot.

Weigh the cylinders before and after, drying the surface first.

Process	ATP?	Direction	Proteins	Example
Simple diffusion	No	Down gradient	None	Gas exchange in alveoli
Facilitated diffusion	No	Down gradient	Channel or carrier	Glucose into red blood cells
Osmosis	No	High ψ to low ψ	Aquaporins (some)	Water into root hair cells
Active transport	Yes	Against gradient	Carrier (pump)	Na+/K+ pump
Endocytosis	Yes	Into cell	Vesicles	Phagocytosis
Exocytosis	Yes	Out of cell	Vesicles fuse with membrane	Insulin secretion

Section 13 of 33Bulk transport

Endocytosis

• Endocytosis large materials enter a cell by vesicle formation, for example phagocytosis of a bacterium by a white blood cell.

Exocytosis

• Exocytosis the reverse process, in which substances such as enzymes or hormones are packaged into secretory vesicles from the Golgi body.
• The vesicles travel to the cell surface membrane and fuse with it.

Energy

• Energy active transport, endocytosis and exocytosis all need energy from hydrolysis of ATP made in respiration.

Terminology

• Terminology bulk transport in is endocytosis and bulk transport out is exocytosis, and the general term bulk transport is not specific enough.

Section 14 of 33Investigating transport in plant tissue

Beetroot

• Beetroot membrane permeability changes with chemicals and temperature, and pigment leakage from beetroot strips shows this.

Plasmolysis

• Plasmolysis epidermal strips such as red onion are viewed under a light microscope, and plasmolysis may take several minutes.

Visking tubing

• Visking tubing a non-living, selectively permeable cellulose membrane whose pores stop large molecules such as starch and sucrose but let small ones such as glucose diffuse.
• It is filled with starch and glucose solutions and suspended in water to test this.

Section 15 of 33Surface area to volume ratio and diffusion

Agar cubes

• Agar cubes cubes of different sizes coloured with an indicator show how size affects diffusion.
  • (for example 0.5, 1 and 2 cm sides)

Size and ratio

• Size and ratio as size increases, volume (cubed) increases faster than surface area (squared), so the ratio decreases and diffusion takes longer.

Rule

• Rule the greater the SA:V, the faster the rate of diffusion.

Shape	Surface area	Volume
Cube	6s2	s3
Cuboid	2lh + 2lw + 2wh	l × w × h
Cylinder	2πrh + 2πr2	πr2h

Section 16 of 33Why size matters: exchange surfaces

Why size matters: exchange surfaces

Single-celled organisms

• Single-celled organisms they have a large SA:V, so oxygen diffuses in fast enough to supply all the cytoplasm.

Large multicellular organisms

• Large multicellular organisms they have a small ratio, so they need specialised exchange surfaces and transport systems.
  • (lungs, gills, villi)

Efficient exchange surface features

• Efficient exchange surface features a large surface area, a thin barrier giving a short diffusion distance,
  • and a maintained concentration gradient from blood flow or ventilation.

Fick's law

• Fick's law rate of diffusion is proportional to ÷ distance.
  • (surface area × concentration difference)

Section 17 of 33More from the specification

Water potential and osmosis

• Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane down a water potential gradient.
  • (dilute solution)

Potato cylinder investigation

• When potato tissue cylinders are placed into sucrose solutions of varying concentrations, cells gain mass in dilute solutions and lose mass in concentrated solutions,
  • (due to endosmosis)
• allowing determination of cell sap water potential at the zero-change intercept.

Facilitated diffusion via transport proteins

• Polar and charged molecules cannot easily penetrate the hydrophobic core of the bilayer and require transmembrane proteins, using channel proteins (pore-forming) or carrier proteins for passive transport.
  • (conformational changers)

Turgor pressure in plants

• In plants, water uptake into the vacuole pushes the cytoplasm against the rigid cellulose cell wall, generating turgor pressure that provides mechanical support to non-woody tissues.

Facilitated diffusion

• Passive movement through transport proteins.

Channel protein

• Hydrophilic pore for specific ions or water.

Dilute solution

• Solution with a high water concentration and low solute concentration.

Animal cell

• A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.

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.

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Carrier

• A female with one recessive sex-linked allele who is unaffected.

Bilayer

• A double layer of phospholipids with tails facing inwards.

Water potential

• A measure of the tendency of water to move; pure water has the highest.

Section 18 of 33More on this topic

• White blood cells
  • they engulf pathogens by phagocytosis.

Section 19 of 33Calculating SA:V ratios

1Cube

• Cube surface area = 6 × side² and volume = side³.

2Cylinder

• Cylinder surface area = 2πrh + 2πr² and volume = πr²h.

3Ratio

• Ratio divide surface area by volume, so the volume term becomes 1.

4Convert units first

• Convert units first for example nanometres to micrometres before calculating.

Section 20 of 33Agar block investigation

1

Cut coloured agar into cubes of known sizes, for example 0.5, 1 and 2 cm.

2

Calculate and record surface area, volume and SA:V for each cube.

3

Place each cube in a boiling tube of diffusion solution such as dilute hydrochloric acid.

4

Time how long the acid takes to change the indicator colour completely.

5

Convert times to rates (1 ÷ time).

6

Plot rate against SA:V.

Section 21 of 33Estimating the water potential of potato

1

Cut identical cylinders of potato.

2

Place them in solutions of different water potential, such as a range of sucrose concentrations, measuring volumes accurately.

3

Leave for a set time at constant temperature, then blot dry and reweigh.

4

Calculate the percentage change in mass for each cylinder.

5

Plot percentage change against concentration and find where the line crosses zero.

Section 22 of 33Interpreting the potato results

No change in mass

• No change in mass no net movement of water, so the solution has the same water potential as the potato cells.

Positive change

• Positive change the potato gained water by osmosis, the solution had a higher water potential, and the cells became turgid and firm.

Negative change

• Negative change the solution had a lower water potential than the potato.

Zero crossing point

• Zero crossing point the solution at this point has the same water potential as the tissue.

Section 23 of 33Data handling and sources of error

Agar end point

• Agar end point measure from the moment of immersion until the last trace of the original colour disappears.
• The end point is a judgement, so repeat and use means.

Agar cubes

• Agar cubes cut with a sharp blade and measure to the nearest millimetre, because percentage uncertainty is greater for small cubes.

Potato cylinders

• Potato cylinders use a cork borer for equal diameter, blot each cylinder dry before weighing, and keep time and temperature the same.

Estimate

• Estimate the point where the calibration line crosses the x-axis gives the estimated water potential of the tissue.

Section 24 of 33More from the specification

Selectively permeable cell membranes

• The phospholipid bilayer acts as a selectively permeable barrier, permitting small nonpolar molecules to diffuse freely while restricting large, polar,
• or charged solutes.

Facilitated transport via channel and carrier proteins

• Transmembrane transport proteins assist the diffusion of polar molecules down their concentration gradient without metabolic energy expenditure.

Active transport mechanism

• Active transport uses specialized carrier proteins to move solute molecules across the membrane against their concentration gradient, requiring metabolic energy derived from ATP hydrolysis.

Temperature and kinetic energy effects

• As temperature increases, the kinetic energy of molecules increases, fluidizing membrane phospholipids and accelerating transport rates until transport proteins denature at excessive temperatures.

Selectively permeable cell membranes

• The phospholipid bilayer acts as a selectively permeable barrier, permitting small nonpolar molecules to diffuse freely while restricting large, polar,
• or charged solutes.

Facilitated transport via channel and carrier proteins

• Transmembrane transport proteins assist the diffusion of polar molecules down their concentration gradient without metabolic energy expenditure.

Active transport mechanism

• Active transport uses specialized carrier proteins to move solute molecules across the membrane against their concentration gradient, requiring metabolic energy derived from ATP hydrolysis.

Temperature and kinetic energy effects

• As temperature increases, the kinetic energy of molecules increases, fluidizing membrane phospholipids and accelerating transport rates until transport proteins denature at excessive temperatures.

Bilayer

• A double layer of phospholipids with tails facing inwards.

Selectively permeable

• Allowing only some substances through.

Active transport

• Movement of particles against a concentration gradient using energy from respiration.

Facilitated diffusion

• Passive movement through transport proteins.

Phospholipid bilayer

• Two layers of phospholipids with hydrophobic tails inwards

Channel protein

• Hydrophilic pore for specific ions or water.

Animal cell

• A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.

Concentration gradient

• The difference in concentration of a substance between two regions.

Permeable

• Allowing substances to pass through.

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Carrier

• A female with one recessive sex-linked allele who is unaffected.

Endocytosis

• Bulk transport into a cell using a membrane vesicle.

Section 25 of 33More on this topic

Term	Meaning
Water potential	Tendency of water to move.
Turgor	Pressure from water inside a cell.
Percentage change	Change divided by the original value, times 100.
Hydrostatic pressure	Pressure exerted by a fluid.

Section 26 of 33Osmosis defined

• Osmosis the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a selectively permeable membrane.

Section 27 of 33Plant cells in different solutions

1Dilute solution or pure water

• Dilute solution or pure water it has a higher water potential than the cell, so water enters through the cell surface membrane by osmosis.
• Water enters the vacuole and the cell volume increases.

2Concentrated solution

• Concentrated solution it has a lower water potential, so water leaves the vacuole and the cell volume decreases.
• The protoplast shrinks and no longer presses on the cell wall.
• As it continues to shrink it pulls away from the wall, which is plasmolysis.

Section 28 of 33Animal cells

No wall

• No wall animal cells do not plasmolyse, because plasmolysis only occurs in plant cells.

Concentrated solution

• Concentrated solution the cell shrinks.

Dilute solution

• Dilute solution the cell may burst.

No buffering

• No buffering they have no wall and no vacuole large enough to buffer volume changes, so blood or tissue fluid must keep a stable water potential.

Section 29 of 33Plant and animal cells compared

Feature	Plant cells	Animal cells
Cell wall	Present	Absent
Osmosis into and out of the cell	Can occur	Can occur
In dilute solution	Turgid	May burst
In concentrated solution	Plasmolysed	Shrink (crenation)

Section 30 of 33States of a plant cell

Turgid

• Turgid firm because of water pressure on the wall.

Flaccid

• Flaccid enough water has been lost that the protoplast no longer presses on the wall but has not yet pulled away.

Incipient plasmolysis

• Incipient plasmolysis the point at which the protoplast just begins to separate from the wall.

Plasmolysed

• Plasmolysed the protoplast has pulled away from the wall.

Support

• Support plant tissue support depends on turgor, so lettuce goes limp in salt solution as cells lose turgor and crisps again in water.

Wall permeability

• Wall permeability the cell wall is fully permeable, so it does not control the movement of water.

Section 31 of 33More from the specification

Active Transport Mechanism

• The movement of solute particles across a cell membrane against their concentration gradient, requiring metabolic energy from ATP hydrolysis and specific carrier proteins.

Facilitated Diffusion

• The passive transport of polar or charged molecules down their concentration gradient through transmembrane channel or carrier proteins without cellular energy expenditure.

Investigating Osmosis in Potato Tissue

• Cylinders of potato tissue are placed in varying concentrations of sucrose solution to determine water potential by measuring percentage changes in mass.

Surface Area to Volume Ratio

• Cells maintain a high surface area to volume ratio to maximise the rate of nutrient transport and gas diffusion across their outer membranes.

Active Transport Mechanism

• The movement of solute particles across a cell membrane against their concentration gradient, requiring metabolic energy from ATP hydrolysis and specific carrier proteins.

Facilitated Diffusion

• The passive transport of polar or charged molecules down their concentration gradient through transmembrane channel or carrier proteins without cellular energy expenditure.

Investigating Osmosis in Potato Tissue

• Cylinders of potato tissue are placed in varying concentrations of sucrose solution to determine water potential by measuring percentage changes in mass.

Surface Area to Volume Ratio

• Cells maintain a high surface area to volume ratio to maximise the rate of nutrient transport and gas diffusion across their outer membranes.

Indicator

• A chemical that changes colour depending on pH.

Endocytosis

• Bulk transport into a cell using a membrane vesicle.

Bilayer

• A double layer of phospholipids with tails facing inwards.

Active transport

• Movement of particles against a concentration gradient using energy from respiration.

Facilitated diffusion

• Passive movement through transport proteins.

Phospholipid bilayer

• Two layers of phospholipids with hydrophobic tails inwards

Channel protein

• Hydrophilic pore for specific ions or water.

Diffusion

• The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient,
  • as a result of random movement.

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Carrier

• A female with one recessive sex-linked allele who is unaffected.

Sucrose

• The sugar mainly transported in phloem.

Section 32 of 33Key terms

Term	Meaning
Diffusion	Net movement of particles down a concentration gradient due to random movement; passive.
Facilitated diffusion	Diffusion of ions or polar molecules through channel or carrier proteins; passive.
Osmosis	Net movement of water from higher to lower water potential through a partially permeable membrane.
Water potential (ψ)	The tendency of water to move from one place to another; pure water is 0 kPa and solutions are negative.
Active transport	Movement against a concentration gradient using carrier proteins and ATP.
Endocytosis	Bulk transport into a cell by a vesicle formed from the cell surface membrane.
Exocytosis	Bulk transport out of a cell when vesicles fuse with the cell surface membrane.
Plasmolysis	Shrinking of the protoplast of a plant cell away from the cell wall.
Surface area to volume ratio	Surface area divided by volume; it falls as size increases.

Term	Meaning
Endocytosis	Bulk transport into a cell by vesicles.
Exocytosis	Bulk transport out of a cell by vesicle fusion.
Phagocytosis	Endocytosis of solid particles.
Visking tubing	Non-living selectively permeable cellulose tubing.
SA:V ratio	Surface area divided by volume.

Term	Meaning
Percentage change in mass	Change in mass divided by initial mass, times 100.
Turgor pressure	Pressure of the cell contents on the wall.
Rate	Amount of change per unit time.
Interpolation	Estimating a value between data points.
Water potential	Tendency of water to move from one place to another; measured in kPa.

Term	Meaning
Plasmolysis	Shrinkage of the protoplast away from the cell wall.
Turgid	Firm because of water pressure.
Vacuole	Fluid-filled organelle that gains or loses water.
Cell wall	Rigid outer layer in plant cells.
Flaccid	Limp, having lost turgor but not yet plasmolysed.

Section 33 of 33Exam technique

1

Saying active transport moves substances down a concentration gradient — it moves them against it, using ATP.

2

Using the term plasmolysis for animal cells — they shrink (crenate) because they have no wall.

3

Saying plant cells burst in dilute solutions — the wall resists, so they become turgid.

4

Confusing channel proteins (fixed pores) with carrier proteins (change shape); active transport uses carriers.

5

Writing bulk transport instead of endocytosis or exocytosis.

1

Writing only bulk transport instead of endocytosis or exocytosis.

2

Saying surface area increases faster than volume as size increases — volume increases faster.

3

Forgetting units for the surface area, volume and ratio.

4

Believing endocytosis is passive — it needs ATP.

1

Not converting to percentage change when cylinders have different starting masses.

2

Dividing the smaller number by the larger when working out the ratio.

3

Forgetting to blot the cylinders dry before weighing.

4

Using different surface areas in the agar cubes.

1

Saying animal cells plasmolyse — they shrink (crenate).

2

Saying water moves from low to high water potential — it moves from high to low.

3

Forgetting that the cell wall is fully permeable.

4

Saying cells shrink in dilute solution — they gain water.