Cell Membranes & Transport

விளையாடிக் கற்றுக்கொள்ளுங்கள்

ஆற்றல் சம்பாதிக்க இந்த கேள்விகளுக்குப் பதிலளியுங்கள், பின்னர் மீன் பிடித்து ஆராயுங்கள். கணக்கு தேவையில்லை.

கல்வியாளர்களுக்கு: Cell Membranes & Transport (Biology, AQA)-க்கான தயாரான பாட ஸ்லைடுகள், திருப்புதல் குறிப்புகள் — உங்கள் பாடத்தில் அவற்றைப் பயன்படுத்தவும், அல்லது கற்பவர்கள் நேரலை விளையாட்டாக விளையாடும் ஊடாடும் வகுப்பு செயல்பாடாக தலைப்பை இயக்கவும்.

பாட குறிப்புகள்

The Fluid Mosaic Model

All cell membranes share the same basic structure, including the cell surface membrane and the membranes surrounding eukaryotic organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, chloroplasts and lysosomes.
Membranes are composed of a phospholipid bilayer and contain intrinsic and extrinsic proteins; they may also include cholesterol (in animal cells), glycoproteins and glycolipids.
The term fluid refers to the lateral movement of phospholipids and some proteins, giving the membrane flexibility.
The term mosaic reflects the scattered arrangement of proteins within the bilayer.
The model explains how membranes are partially permeable, act as sites for cell signalling, recognition and communication, and control the exchange of substances across compartments.

Structural Components of Cell Membranes

Each phospholipid has a polar (hydrophilic) phosphate head that is soluble in water and two non-polar (hydrophobic) fatty acid tails that are insoluble in water.
In the bilayer, the hydrophobic tails face inward forming a hydrophobic core, while the hydrophilic heads face outward towards aqueous environments.
This structure forms a selectively permeable barrier, preventing most polar or water-soluble substances (e.g. ions, glucose, amino acids) from freely crossing the membrane.
Phospholipids can be chemically modified to act as signalling molecules by moving within the bilayer to activate other molecules (e.g. enzymes) or by being hydrolysed to release smaller water-soluble molecules that bind to specific receptors in the cytoplasm.
Cholesterol regulates fluidity: it sits between phospholipids, preventing them from packing too closely at low temperatures (preventing freezing and fracturing) and stabilising the membrane at higher temperatures by stopping it becoming too fluid.
Cholesterol binds to the hydrophobic tails of phospholipids, stabilising them and causing them to pack more closely together; this makes the membrane less permeable to small charged particles (like ions) and strengthens the membrane so the cell doesn't burst.
Glycolipids and glycoproteins contain carbohydrate chains on the surface that act as receptor molecules; they include signalling receptors for hormones and neurotransmitters, receptors for endocytosis, and receptors for cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell).
Some glycolipids and glycoproteins act as cell markers or antigens for cell-to-cell recognition (e.g. the ABO blood group antigens differ slightly in their carbohydrate chains).
Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane; there are two types: channel (pore) proteins and carrier proteins.
Each transport protein is specific to a particular ion or molecule, allowing the cell to control which substances enter or leave.

Factors Affecting Membrane Fluidity

Membranes become less fluid with an increased proportion of saturated fatty acid chains because the chains pack together tightly, resulting in a high number of intermolecular forces between the chains.
Membranes become less fluid at a lower temperature because the molecules have less energy and are not moving as freely, causing the structure to be more closely packed.
Membranes become more fluid with an increased proportion of unsaturated fatty acid chains because these chains are bent, meaning they are less tightly packed together and there are fewer intermolecular forces.
Membranes become more fluid at higher temperatures because the molecules have more energy and move more freely.

Diffusion

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration; they move down a concentration gradient.
Movement is random and is caused by the natural kinetic energy of the molecules or ions.
As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space.
The rate of diffusion is affected by the steepness of the concentration gradient: a greater difference in concentration means more molecules move from high to low concentration, increasing the net rate of diffusion.
Temperature affects the rate: higher temperatures give molecules more kinetic energy, so they move faster, resulting in a higher rate of diffusion.
Surface area affects the rate: a larger surface area allows more molecules to diffuse at once; folding (e.g. microvilli, cristae) increases surface area, while in larger cells a lower surface area to volume ratio slows diffusion.
Properties of molecules or ions affect the rate: large molecules diffuse more slowly as they need more energy; uncharged, non-polar molecules diffuse directly through the bilayer and diffuse faster than polar ones.

Facilitated Diffusion

Certain substances cannot diffuse through the phospholipid bilayer, including large polar molecules such as glucose and amino acids, and ions such as sodium ions (Na⁺) and chloride ions (Cl⁻).
These substances can only cross the phospholipid bilayer with the help of certain proteins in a process known as facilitated diffusion.
There are two types of proteins that enable facilitated diffusion: channel proteins and carrier proteins; they are highly specific (they only allow one type of molecule or ion to pass through).
Channel proteins are water-filled pores that allow charged substances (e.g. ions) to diffuse through the cell membrane; most are 'gated', meaning part of the channel protein on the inside surface can move to close or open the pore, controlling the exchange of ions.
Carrier proteins can switch between two shapes, causing the binding site to be open to one side of the membrane first, and then open to the other side when the carrier protein switches shape.
Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule).

Osmosis

Osmosis is 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 partially permeable membrane.
A dilute solution has a high water potential; a concentrated solution has a low water potential.
The water potential of pure water (without any solutes) at atmospheric pressure is 0 kPa; any solution that has solutes will have a water potential lower than 0 kPa (a negative value).
The more negative a water potential value, the lower the water potential is said to be.
In plant cells, a hypertonic solution causes water to leave by osmosis, the protoplast shrinks and pulls away from the cell wall — this is called plasmolysis; without enough water, cells lose turgor and the plant wilts.
In a hypotonic solution, water enters the plant cell, the vacuole expands, and the cell becomes turgid — the cell wall prevents bursting; turgidity supports the plant, helping it stay upright and catch sunlight.
In an isotonic solution, water moves in and out equally, so there is no net change, and the cell is neither turgid nor plasmolysed.
Animal cells lack a cell wall, so the effects of osmosis are more severe: in a hypertonic solution water leaves causing the cell to shrink and shrivel; in a hypotonic solution water enters and the cell may swell and burst (cytolysis); in an isotonic solution there is no net change.
Maintaining a stable water potential in animal tissue fluid is essential to prevent cell damage.

Required Practical: Investigating Water Potential

Plant tissue can be immersed in solutions of different water potentials to estimate the water potential of the tissue itself; the most common osmosis practical involves cutting cylinders of potato and placing them into solutions with a range of different water potentials (usually sucrose solutions of increasing concentration – at least 5 different concentrations are usually required).
Method: cut the required number of potato cylinders (one for each solution, or more than one per solution if repeats are required); cut to the same length, blot dry to remove excess moisture, then measure and record their initial mass; place the potato cylinders into the solutions; leave in the solutions for a set amount of time (e.g. 30 minutes), usually in a water bath (set at around 30°C); remove the cylinders and dry to remove excess liquid; measure and record the final length and mass of each potato cylinder.
The percentage change in mass for each potato cylinder is calculated by dividing the change in mass by the initial mass and then multiplying by 100.
A positive % change in mass means the potato gained water by osmosis, so the solution had a higher water potential; the cells became turgid, and the potato felt firm.
A negative % change means the potato lost water, so the solution had a lower water potential; the cells became flaccid, and the potato felt floppy; the largest mass decrease occurs in the most concentrated sucrose solution, where the water potential gradient is greatest; cells may be plasmolysed.
If there is no change in mass, the solution was isotonic with the potato, meaning that water moved in and out equally and there was no net movement.
The concentration of sucrose inside the potato cylinders can be found if a graph is drawn showing how the percentage change in mass changes with the concentration of sucrose solution – the point at which the line of best fit crosses the x-axis is the concentration of sucrose inside the potato cylinders.
When plotting data: for qualitative and discrete data, bar charts or pie charts are most suitable; for continuous data, line graphs or scatter graphs are most suitable; scatter graphs are especially useful for showing how two variables are correlated.
Tips for plotting data: plot the independent variable on the x-axis and the dependent variable on the y-axis; plot data points accurately; use appropriate linear scales on axes; choose scales that enable all data points to be plotted within the graph area; label axes, with units included; make graphs that fill the space the exam paper gives you; draw a line of best fit (straight or curved depending on the trend), with a balance of data points above and below the line; in some cases, the line or curve of best fit should be drawn through the origin (but only if the data and trend allow it).

Active Transport & Co-transport

Active transport is the movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.
Active transport requires carrier proteins (each carrier protein being specific for a particular type of molecule or ion).
The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane; energy is provided by the hydrolysis of ATP (adenosine triphosphate) into ADP and inorganic phosphate.
Active transport is important in: reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules; absorption of some products of digestion from the digestive tract; loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant; loading inorganic ions from the soil into root hairs.
Co-transport is the coupled movement of two substances across a membrane via a carrier protein; one moves down its concentration gradient, allowing the other to move against its gradient.
In the mammalian ileum, co-transport absorbs glucose and sodium ions: (1) active transport moves Na⁺ from the epithelial cell into the blood, creating a Na⁺ gradient; (2) Na⁺ then diffuses in from the ileum, carrying glucose via a co-transporter; (3) glucose moves into the blood by facilitated diffusion.

Adaptations for Rapid Transport

The rate at which substances are transported across cell membranes varies depending on the type of transport involved (e.g. diffusion, facilitated diffusion, or active transport).
Some cells are specialised to allow rapid transport of molecules across their internal or external membranes to support key functions such as absorption, secretion, or exchange of gases.
The rate of transport depends on several factors, including: temperature; surface area of the exchange surface; concentration gradient across the membrane; thickness (or diffusion distance) of the exchange surface; number of protein channels or carrier proteins; availability of ATP (for active transport).
Increased surface area provides more membrane surface for substances to cross simultaneously (e.g. microvilli on epithelial cells in the small intestine).
More channel proteins allow faster facilitated diffusion of specific ions or polar molecules (e.g. Na⁺/K⁺ channels in neurones).
More carrier proteins speed up facilitated diffusion and active transport of larger molecules (e.g. glucose carriers in kidney tubules and intestinal epithelium).
Thin exchange surface reduces diffusion distance, speeding up the rate of diffusion (e.g. alveolar and capillary walls are one cell thick).
Rich blood supply maintains a steep concentration gradient by constantly removing or supplying substances (e.g. capillary networks in alveoli and villi).
Ventilation or flow of the surrounding medium replaces substances to maintain high/low external concentrations, sustaining a gradient (e.g. ventilation in lungs maintains O₂/CO₂ gradients).
Many mitochondria provide more ATP for active transport, supporting uptake against a concentration gradient (e.g. root hair cells for ion uptake from the soil).
Root hair cells are adapted for the absorption of water and mineral ions from the soil: they have long 'hair-like' projections that increase the surface area, boosting the rate of osmosis and active transport; a thin cell wall gives a shorter diffusion distance for water; the permanent vacuole stores water and mineral ions as they enter the cell, helping to maintain a steep water potential gradient.
Epithelial cells of the small intestine are adapted for the absorption of digested food molecules: they have microvilli on the surface providing a large surface area for increased diffusion; a rich capillary network continually transports the products of digestion away from the epithelial cells, ensuring a steep concentration gradient; many co-transport proteins facilitate active uptake of glucose and amino acids.
Cells in the collecting duct of the kidney are adapted for the uptake of water: their membranes contain a very high number of aquaporins, which are special channel proteins that allow the facilitated diffusion of water through cell membranes, allowing these kidney cells to reabsorb water.
Neurones and muscle cells are adapted for the transport of sodium, potassium and calcium across the membrane necessary for the transmission of electrical impulses around the body: cell membranes in these cells have channel proteins for sodium, potassium and calcium ions; the opening and closing of ion channel proteins, and the number of channels present, affect how quickly ions move by facilitated diffusion; this directly influences the speed of electrical transmission along neurone membranes during nerve impulses and across muscle cell membranes during muscle contraction.

Required Practical: Factors Affecting Membrane Fluidity

The permeability of cell membranes is affected by different factors or conditions, such as temperature and solvent concentration.
Permeability can be investigated using beetroot: beetroot cells contain a dark purple-red pigment, and the higher the permeability of the beetroot cell membrane, the more of this pigment leaks out of the cell.
Apparatus: scalpel, cork borer (optional), cutting board, ruler, test tubes, water baths, stopwatch, colorimeter (a machine that passes light through a liquid sample and measures how much of that light is absorbed).
Method: using a scalpel, cut five equal-sized cubes of beetroot (the pieces must have the same dimensions so that they all have equal surface areas and volumes, as these factors could affect the rate at which the pigment leaks out; a cork borer can also be used, as long as the cores are cut to the same length); rinse the beetroot pieces to remove any pigment released during cutting; add the beetroot pieces to five different test tubes, each containing the same volume of water (e.g. 5 cm³); put each test tube in a water bath at a different temperature (e.g. 10°C, 20°C, 30°C, 40°C, 50°C) for the same length of time (e.g. around 30 minutes); remove the beetroot pieces, leaving just the coloured liquid in the five test tubes; use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid.
The higher the absorbance, the more pigment must have been released, due to a greater membrane permeability.
The general pattern expected is that as temperature increases, membrane permeability also increases.
As temperature increases, the phospholipids within the cell membrane move more because they have more energy; this means the phospholipids are not as tightly packed together, increasing the permeability of the membrane.
At high temperatures, the phospholipid bilayer may even start to melt and break down, further increasing the permeability of the membrane.
In addition, the volume of water inside the cells expands, putting pressure on the membrane, causing channel and carrier proteins to deform so they can no longer control what enters and leaves the cell; these factors also increase the permeability of the membrane.
Temperature also affects the conformation (3D shape) of proteins as at high temperatures the intermolecular forces between amino acids are broken, which affects the protein's specificity and function.
If experimenting with temperatures below 0°C, membrane permeability may also be increased (once the cells have thawed again); this can be caused by channel or carrier proteins deforming at these low temperatures, and ice crystals that form can pierce the cell membrane, making it highly permeable.
Limitations: cuvettes may differ in thickness (very slightly) — a thicker (or scratched) cuvette will absorb slightly more light than a thinner unscratched cuvette; this can be overcome by using the same cuvette for every reading, or repeating the investigation many times and finding a mean.
The beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others; this can be overcome by cutting the discs as accurately as possible using a scalpel and ruler, and by repeating each investigation several times to find a mean.
Some parts of beetroot tissue have more pigment in their cells than others; this can be overcome by several repeats, using different parts of the beetroot and finding a mean.
You could also investigate how solvent concentration affects cell membrane permeability by placing beetroot pieces in test tubes containing increasing concentrations of solvents (such as alcohol or acetone); solvents can increase cell membrane permeability as they dissolve the lipids in the membrane, causing the membrane to lose its structure.

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பதிவு செய்க — இலவசம்உள்நுழைக

பயிற்சி கேள்விகள்

இலவச முன்னோட்டம் — 62-இல் 8 கேள்விகள். அனைத்தையும் பார்க்க பதிவு செய்யவும்.

1.Which term describes the model of cell membrane structure in which phospholipids and some proteins can move laterally within the bilayer?
Easy
- AFluid mosaic model
- BLock and key model
- CInduced fit model
- DUnit membrane model
2.What is the main role of cholesterol in animal cell membranes?
Easy
- ATo regulate membrane fluidity
- BTo act as a channel for ions
- CTo provide energy for active transport
- DTo form the hydrophobic core
3.Which part of a phospholipid molecule is hydrophobic?
Easy
- AThe fatty acid tails
- BThe phosphate head
- CThe glycerol backbone
- DThe carbohydrate chain
4.Which of the following molecules would diffuse most rapidly through the phospholipid bilayer of a cell membrane?
Easy
- AOxygen
- BGlucose
- CSodium ions
- DAmino acids
5.Which of the following is the correct definition of osmosis?
Easy
- AThe net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
- BThe net movement of water molecules from a region of lower water potential to a region of higher water potential through a partially permeable membrane
- CThe movement of any solvent molecules from high to low concentration through a fully permeable membrane
- DThe active transport of water molecules against a water potential gradient using ATP
6.A plant cell is placed in a hypertonic solution. What is the expected effect?
Medium
- AWater leaves the cell, the protoplast shrinks and pulls away from the cell wall
- BWater enters the cell, the vacuole expands and the cell becomes turgid
- CWater moves in and out equally, so there is no net change
- DThe cell swells and bursts due to the lack of a cell wall
7.Which of the following is NOT a factor that increases the rate of diffusion across a membrane?
Medium
- AA shallower concentration gradient
- BA larger surface area
- CA higher temperature
- DA shorter diffusion distance
8.Which of the following statements about active transport is correct?
Medium
- AIt moves substances from lower to higher concentration using energy from ATP hydrolysis
- BIt moves substances from higher to lower concentration using energy from ATP hydrolysis
- CIt requires channel proteins and occurs down a concentration gradient
- DIt does not require any membrane proteins

மேலும் கேள்விகள்

9.Which of the following statements about facilitated diffusion are correct? (Select all that apply.)
Medium
- AIt requires transport proteins.
- BIt occurs down a concentration gradient.
- CIt requires energy from ATP.
- DIt can transport ions such as sodium and chloride.
- EIt uses carrier proteins that change shape.
10.Which of the following are functions of glycolipids and glycoproteins in the cell membrane? (Select all that apply.)
Medium
- AActing as receptor molecules for hormones
- BActing as cell markers or antigens for cell recognition
- CProviding energy for active transport
- DForming hydrogen bonds with water to stabilise the cell
- ETransporting ions across the membrane
11.Which of the following adaptations increase the rate of transport across a cell membrane? (Select all that apply.)
Medium
- AIncreased surface area of the membrane
- BA thin exchange surface
- CA rich blood supply
- DA lower temperature
- EMany mitochondria
12.The fluid mosaic model states that phospholipids and some proteins can move laterally within the membrane.
Easy
True or false?
13.Cholesterol increases membrane fluidity at low temperatures by preventing phospholipids from packing closely together.
Easy
True or false?
14.Osmosis is the movement of water molecules from a region of lower water potential to a region of higher water potential.
Easy
True or false?
15.Facilitated diffusion requires energy from ATP hydrolysis.
Easy
True or false?
16.Active transport uses carrier proteins that change shape to move molecules or ions against their concentration gradient.
Easy
True or false?
17.Match each membrane component with its correct function.
Medium
- Phospholipid bilayer
- Cholesterol
- Channel protein
- Glycoprotein
- Regulates membrane fluidity
- Provides a selectively permeable barrier
- Allows facilitated diffusion of ions
- Acts as a receptor for cell signalling
18.Match each transport process with its correct description.
Medium
- Simple diffusion
- Facilitated diffusion
- Osmosis
- Active transport
- Movement of water from high to low water potential through a partially permeable membrane
- Movement of molecules from high to low concentration directly through the phospholipid bilayer
- Movement of molecules from low to high concentration using ATP
- Movement of molecules from high to low concentration via channel or carrier proteins
19.Place the steps of glucose absorption by co-transport in the mammalian ileum in the correct order.
Medium
- Active transport moves Na⁺ from the epithelial cell into the blood, creating a Na⁺ gradient.
- Na⁺ diffuses into the epithelial cell from the ileum, carrying glucose via a co-transporter protein.
- Glucose moves into the blood by facilitated diffusion.
20.Place the steps of the beetroot practical to investigate membrane permeability in the correct order.
Medium
- Cut five equal-sized cubes of beetroot.
- Rinse the beetroot pieces to remove pigment released during cutting.
- Place each beetroot piece into a test tube containing the same volume of water.
- Put each test tube into a water bath at a different temperature for the same length of time.
- Remove the beetroot pieces and measure the absorbance of the coloured liquid using a colorimeter.
21.The appearance of cell membranes is often described as a 'fluid mosaic'. Which statement best explains why the mosaic is described as 'fluid'?
Easy
- APhospholipids and some proteins can move laterally within the bilayer
- BThe membrane is made only of liquid phospholipids with no proteins
- CWater can pass freely through the membrane at all times
- DThe carbohydrate chains on glycolipids are constantly broken down
22.Which of the following statements about channel proteins are correct? (Select all that apply.)
Medium
- AThey are water-filled pores that allow charged substances such as ions to diffuse through the membrane
- BThey have a fixed shape and do not change shape during transport
- CThey require ATP hydrolysis to move ions across the membrane
- DMany are 'gated', meaning part of the protein can move to open or close the pore
- EThey are highly specific and only allow one type of ion or molecule to pass through
23.Which of the following statements about carrier proteins are correct? (Select all that apply.)
Medium
- AThey can switch between two shapes during transport
- BTheir binding site opens to one side of the membrane and then the other
- CThey are water-filled pores that allow ions to diffuse freely through the membrane
- DThey are involved in facilitated diffusion and active transport
- EThey are not specific and can transport any molecule or ion
24.Which of the following factors increase the rate of diffusion across a cell membrane? (Select all that apply.)
Medium
- AA steeper concentration gradient
- BA larger surface area of the membrane
- CA higher temperature
- DA greater diffusion distance (thicker exchange surface)
- EA lower temperature
25.The water potential of pure water at atmospheric pressure is 0 kPa.
Easy
True or false?
26.The more negative the water potential value, the higher the water potential is said to be.
Easy
True or false?
27.Match each membrane transport process with its correct description.
Medium
- Simple diffusion
- Facilitated diffusion
- Osmosis
- Active transport
- Net movement of molecules or ions from higher to lower concentration directly through the bilayer
- Movement of molecules or ions down a concentration gradient through a channel or carrier protein
- Net movement of water from higher water potential to lower water potential through a partially permeable membrane
- Movement of molecules or ions from lower to higher concentration using energy from respiration
28.Match each membrane protein or molecule with its correct role.
Medium
- Aquaporin
- Channel protein
- Carrier protein
- Glycolipid
- Special channel protein that allows facilitated diffusion of water
- Water-filled pore that allows charged substances such as ions to diffuse through
- Protein that changes shape to transport molecules or ions, including in active transport
- Molecule with a carbohydrate chain that acts as a receptor or cell marker
29.Place the stages of the method to investigate the effect of temperature on membrane permeability in beetroot in the correct order.
Medium
- Cut five equal-sized cubes or cores of beetroot
- Rinse the beetroot pieces to remove pigment released during cutting
- Place the beetroot pieces into five test tubes each containing the same volume of water
- Put each test tube into a water bath at a different temperature for the same length of time
- Remove the beetroot pieces and measure the absorbance of the coloured liquid using a colorimeter
30.A student placed equal-sized cylinders of potato tissue into five sucrose solutions of increasing concentration. The graph of percentage change in mass against sucrose concentration gave a line of best fit that crossed the x-axis at 0.30 mol dm⁻³. What does this value represent?
Medium
- AThe concentration of sucrose inside the potato cells
- BThe water potential of pure water
- CThe concentration of sucrose in the most concentrated solution used
- DThe rate of osmosis in the potato tissue
31.In an investigation using beetroot, the absorbance of the solution at 10°C was 0.15 and at 50°C was 0.27. What is the percentage increase in absorbance between 10°C and 50°C?
Medium
- A80%
- B12%
- C44%
- D180%
32.A scientist investigated the rate of diffusion of potassium ions out of a cell. The graph of rate of diffusion against internal potassium ion concentration showed a steep increase initially, then levelled off. Which factor is limiting the rate of diffusion between the points where the graph plateaus?
Medium
- AThe number of channel proteins in the membrane
- BThe concentration gradient of potassium ions
- CThe temperature of the cell
- DThe surface area of the cell membrane
33.In a co-transport experiment using rice plants, group Y had a respiratory inhibitor added and group Z did not. The graph showed that group Z absorbed magnesium ions at a steady rate, while group Y absorbed very little. What does this demonstrate about magnesium ion uptake?
Medium
- AIt requires energy from respiration and is therefore active transport
- BIt occurs by simple diffusion down a concentration gradient
- CIt is unaffected by respiratory inhibitors
- DIt occurs by facilitated diffusion through channel proteins
34.A student investigated osmosis in potato cylinders. They calculated the percentage change in mass for each cylinder. Which of the following statements about their results are correct? (Select all that apply.)
Hard
- AA positive percentage change in mass means the potato gained water by osmosis.
- BA negative percentage change means the solution had a lower water potential than the potato.
- CIf there is no change in mass, the solution was isotonic with the potato.
- DThe largest mass decrease occurs in the most dilute sucrose solution.
- EA positive percentage change means the potato cells became flaccid.
35.In an experiment on beetroot membrane permeability, a student measured absorbance of the surrounding water at different temperatures. Which of the following would increase the absorbance reading? (Select all that apply.)
Hard
- AIncreasing the temperature of the water bath
- BUsing a thicker cuvette for the colorimeter reading
- CPlacing the beetroot in a solvent such as alcohol
- DUsing a larger volume of water in each test tube
- ECutting the beetroot into smaller pieces of the same total mass
36.In the beetroot practical, rinsing the beetroot pieces before placing them in water removes any pigment released during cutting.
Easy
True or false?
37.In an investigation of osmosis using potato cylinders, blotting the cylinders dry before weighing removes excess water from the surface, making the mass reading more accurate.
Medium
True or false?
38.Match each adaptation with the way it increases the rate of transport across a membrane.
Medium
- Microvilli on epithelial cells
- Rich blood supply
- Many mitochondria in root hair cells
- Thin exchange surface
- Increases surface area for diffusion
- Maintains a steep concentration gradient
- Provides more ATP for active transport
- Reduces diffusion distance
39.Name the process taking place at A in Figure 1.
Medium
- ACo-transport
- BSimple diffusion
- COsmosis
- DActive transport
40.Name the process taking place at B in Figure 1.
Medium
- AFacilitated diffusion
- BActive transport
- CCo-transport
- DSimple diffusion
41.A beetroot sample has an absorbance of 0.15 at 10°C and 0.27 at 50°C. Calculate the percentage increase in absorbance between 10°C and 50°C. Give your answer to the nearest whole number.
Medium
42.A potato cube loses 5 g in mass over 20 minutes. Calculate the rate of loss in mass per minute. Give your answer in g/min.
Medium
43.Complete the sentence about the fluid mosaic model.
Easy
The fluid mosaic model describes the arrangement of molecules in cell membranes. The term 'fluid' refers to the lateral movement of ____ and some proteins, while 'mosaic' reflects the scattered arrangement of ____ within the bilayer.
44.Complete the definitions of transport processes.
Easy
____ is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration. ____ is 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.
45.Name the process by which water moves across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
Medium
46.Identify the type of membrane protein that is water-filled and allows charged substances such as ions to diffuse through the membrane.
Medium
47.Complete the sentences about the fluid mosaic model. Use one word or short phrase per blank.
Medium
The term 'fluid' refers to the ____ movement of phospholipids and some proteins within the bilayer, while the term 'mosaic' reflects the ____ arrangement of proteins within the bilayer.
48.Name the three states in which the effects of osmosis on plant cells can be described, depending on the solution the cell is placed in.
Easy
A plant cell can be described as ____, ____, or ____, depending on whether it is in a hypotonic, hypertonic or isotonic solution.
49.At 20 minutes, the total mass of the eight small cubes was 50 g and the total mass of the single large cube was 55 g. Calculate the rate of loss in mass per minute for the single large cube during the first 20 minutes. Give your answer in g/min.
Hard
50.In the beetroot practical, what piece of apparatus is used to measure the absorbance of the coloured liquid?
Medium
51.A potato cylinder lost 5 g in mass over 20 minutes. Calculate the rate of loss in mass per minute. Give your answer in g/min.
Medium
52.In an osmosis experiment, the total mass of eight small potato cubes decreased from 55 g to 50 g over 20 minutes. Calculate the rate of loss in mass per minute for the eight cubes combined. Give your answer in g/min.
Hard
53.Explain how the structure of the phospholipid bilayer allows it to act as a selectively permeable barrier.
Hard
54.Describe how glucose is absorbed from the lumen of the small intestine into the blood by co-transport.
Hard
55.Explain why an increase in temperature increases the permeability of cell membranes.
Hard
56.State two functions of membranes in living cells.
Medium
57.Explain why the phosphate heads of phospholipids form the outer surfaces of a cell membrane in the aqueous environment of the cytoplasm.
Medium
58.A beetroot sample has an absorbance of 0.15 at 10°C and 0.27 at 50°C. Calculate the percentage increase in absorbance between 10°C and 50°C. Give your answer to the nearest whole number.
Medium
- A88 %
- B80 %
- C96 %
- D72 %
59.A potato cube loses 5 g in mass over 20 minutes. Calculate the rate of loss in mass per minute. Give your answer in g/min.
Medium
- A1.25 g/min
- B0.25 g/min
- C-0.75 g/min
- D2.25 g/min
60.At 20 minutes, the total mass of the eight small cubes was 50 g and the total mass of the single large cube was 55 g. Calculate the rate of loss in mass per minute for the single large cube during the first 20 minutes. Give your answer in g/min.
Hard
- A-0.75 g/min
- B0.25 g/min
- C2.25 g/min
- D1.25 g/min
61.A potato cylinder lost 5 g in mass over 20 minutes. Calculate the rate of loss in mass per minute. Give your answer in g/min.
Medium
- A0.25 g/min
- B2.25 g/min
- C1.25 g/min
- D-0.75 g/min
62.In an osmosis experiment, the total mass of eight small potato cubes decreased from 55 g to 50 g over 20 minutes. Calculate the rate of loss in mass per minute for the eight cubes combined. Give your answer in g/min.
Hard
- A1.25 g/min
- B0.25 g/min
- C2.25 g/min
- D-0.75 g/min

திருப்புதல் குறிப்புகள்

## The Fluid Mosaic Model - All cell membranes share the same basic structure: a **phospholipid bilayer** with **intrinsic and extrinsic proteins**, plus **cholesterol** (animal cells), **glycoproteins** and **glycolipids**. - **Fluid**: phospholipids and some proteins move laterally, giving flexibility. - **Mosaic**: proteins are scattered within the bilayer. - Membranes are **partially permeable**, sites for **cell signalling, recognition and communication**, and control exchange of substances. ### Phospholipid bilayer - Each phospholipid: **polar hydrophilic phosphate head** + **two non-polar hydrophobic fatty acid tails**. - Tails face inward (hydrophobic core); heads face outward (aqueous environments). - Forms a **selectively permeable barrier**; most polar/water-soluble substances (ions, glucose, amino acids) cannot freely cross. - Phospholipids can act as signalling molecules: move within bilayer to activate other molecules (e.g. enzymes) or be hydrolysed to release water-soluble molecules that bind receptors in the cytoplasm. ### Cholesterol - Regulates fluidity: sits between phospholipids, preventing tight packing at low temperatures (prevents freezing/fracturing). - At higher temperatures, stabilises membrane by stopping it becoming too fluid. - Binds hydrophobic tails, stabilising them and packing phospholipids closer. - Makes membrane **less permeable to small charged particles (ions)** and strengthens it so the cell doesn't burst. ### Glycolipids & glycoproteins - Carbohydrate chains on the surface act as **receptor molecules**. - Three main receptor types: **signalling receptors** (hormones, neurotransmitters), **endocytosis receptors**, **cell adhesion/stabilisation receptors** (H-bonds with water). - Some act as **cell markers/antigens** for cell-to-cell recognition (e.g. ABO blood group antigens). ### Proteins - **Transport proteins** create hydrophilic channels for ions and polar molecules. - Two types: **channel (pore) proteins** and **carrier proteins**. - Each transport protein is **specific** to a particular ion or molecule. - Allow cell to control which substances enter or leave. ### Membrane fluidity - **Less fluid**: more saturated fatty acid chains (pack tightly, more intermolecular forces); lower temperature (less energy, molecules move less freely). - **More fluid**: more unsaturated fatty acid chains (bent, less tightly packed, fewer intermolecular forces); higher temperatures (more energy, move more freely). ## Diffusion - **Definition**: The net movement of molecules or ions from a region of higher concentration to a region of lower concentration. - Movement is **random**, caused by natural **kinetic energy**; down a concentration gradient. - Reaches **equilibrium** given sufficient time. ### Factors affecting rate of diffusion - **Steepness of concentration gradient**: greater difference → more molecules move → higher net rate. - **Temperature**: higher → more kinetic energy → faster movement → higher rate. - **Surface area**: larger → more molecules diffuse at once; folding (microvilli, cristae) increases SA; larger cells have lower SA:V slowing diffusion. - **Properties of molecules/ions**: large molecules diffuse more slowly; uncharged, non-polar molecules diffuse directly through bilayer; non-polar diffuse faster than polar. ### Facilitated diffusion - Substances that cannot diffuse through bilayer: **large polar molecules** (glucose, amino acids) and **ions** (Na⁺, Cl⁻). - Cross with help of proteins: **channel proteins** and **carrier proteins**. - Highly specific (only one type of molecule/ion). - **Channel proteins**: water-filled pores; allow charged substances (ions) to diffuse; most are **'gated'** (part of protein moves to open/close pore) controlling ion exchange. - **Carrier proteins**: can switch between two shapes; binding site opens to one side then the other; net diffusion down concentration gradient. ## Osmosis - **Definition**: Osmosis is 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 partially permeable membrane. - **Water potential**: pure water at atmospheric pressure = **0 kPa**; any solution has lower (negative) water potential. More negative = lower water potential. ### Osmosis in plant cells - **Hypertonic**: water leaves, protoplast shrinks and pulls away from cell wall → **plasmolysis**; plant wilts. - **Hypotonic**: water enters, vacuole expands, cell becomes **turgid**; cell wall prevents bursting; supports plant. - **Isotonic**: no net change; neither turgid nor plasmolysed. ### Osmosis in animal cells - **Hypertonic**: water leaves, cell shrinks and shrivels. - **Hypotonic**: water enters, cell may swell and burst (**cytolysis**). - **Isotonic**: no net change. - Maintaining stable water potential in tissue fluid is essential to prevent damage. ## Required Practical: Investigating Water Potential - Use potato cylinders in sucrose solutions of different water potentials (at least 5 concentrations). - Method: cut cylinders, blot dry, measure initial mass; place in solutions; leave set time (e.g. 30 min) in water bath at ~30°C; remove, dry, measure final length and mass. - **Percentage change in mass** = (change in mass / initial mass) × 100. - **Positive % change**: potato gained water; solution had higher water potential; cells turgid, firm. - **Negative % change**: potato lost water; solution had lower water potential; cells flaccid, floppy; may be plasmolysed. - **No change**: solution isotonic; no net movement. - **Graph intercept**: point where line of best fit crosses x-axis = concentration of sucrose inside potato cylinders. ## Active Transport & Co-transport - **Active transport**: The movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration. - Requires **carrier proteins** (specific); energy from **hydrolysis of ATP** into ADP and inorganic phosphate; energy makes carrier protein change shape to transfer molecules/ions. - Important in: reabsorption in kidney tubules, absorption from digestive tract, loading sugar into phloem, loading inorganic ions into root hairs. - **Co-transport**: the coupled movement of two substances across a membrane via a carrier protein; one moves down its concentration gradient, allowing the other to move against its gradient. - In mammalian ileum: (1) Active transport moves Na⁺ from epithelial cell into blood, creating Na⁺ gradient. (2) Na⁺ diffuses in from ileum, carrying glucose via co-transporter. (3) Glucose moves into blood by facilitated diffusion. ## Adaptations for Rapid Transport - Rate depends on: temperature, surface area, concentration gradient, thickness (diffusion distance), number of protein channels/carriers, availability of ATP. - **Increased surface area** (microvilli) → more substances cross simultaneously. - **More channel proteins** → faster facilitated diffusion. - **More carrier proteins** → faster facilitated diffusion and active transport. - **Thin exchange surface** → reduces diffusion distance. - **Rich blood supply** → maintains steep concentration gradient. - **Ventilation/flow** → maintains gradients. - **Many mitochondria** → more ATP for active transport. ### Examples - **Root hair cells**: long hair-like projections increase SA; thin cell wall shortens diffusion distance; permanent vacuole stores water/ions maintaining steep water potential gradient. - **Epithelial cells of small intestine**: microvilli increase SA; rich capillary network maintains steep concentration gradient; many co-transport proteins for active uptake of glucose and amino acids. - **Collecting duct cells**: high number of **aquaporins** (channel proteins for facilitated diffusion of water) to reabsorb water. - **Neurones and muscle cells**: channel proteins for Na⁺, K⁺, Ca²⁺; opening/closing and number affect speed of facilitated diffusion, influencing electrical transmission. ## Required Practical: Factors Affecting Membrane Fluidity - Beetroot cells contain dark purple-red pigment; higher permeability → more pigment leaks out. - **Method**: cut five equal-sized beetroot cubes (same dimensions for equal SA and volume); rinse to remove pigment from cutting; add to test tubes with same volume of water; place in water baths at different temperatures (e.g. 10, 20, 30, 40, 50°C) for same time (~30 min); remove beetroot; use colorimeter to measure absorbance. - **Results**: as temperature increases, membrane permeability increases. - **Explanation**: higher temperature → phospholipids move more (more energy) → less tightly packed → increased permeability; at high temperatures bilayer may melt/breakdown; water inside cells expands putting pressure on membrane, deforming channel/carrier proteins; high temperatures break intermolecular forces in proteins affecting conformation, specificity and function. - Below 0°C: permeability may increase after thawing due to protein deformation and ice crystals piercing membrane. - **Limitations**: cuvettes may differ in thickness/scratches (use same cuvette or repeats); beetroot pieces may not be identical (cut accurately, repeats); pigment varies in different parts (repeats, different parts, mean). - **Solvent concentration**: solvents (alcohol, acetone) dissolve lipids, increasing permeability.

Fluid mosaic model: Model describing membrane structure: 'fluid' refers to lateral movement of phospholipids and some proteins; 'mosaic' reflects scattered arrangement of proteins within the bilayer.
Phospholipid bilayer: Two layers of phospholipids; hydrophobic tails face inward, hydrophilic heads face outward, forming a selectively permeable barrier.
Cholesterol: Regulates membrane fluidity; prevents tight packing at low temperatures and stabilises membrane at high temperatures; makes membrane less permeable to small charged particles and strengthens it.
Glycolipids & glycoproteins: Molecules with carbohydrate chains on the surface; act as receptor molecules for signalling, endocytosis, cell adhesion; some act as cell markers/antigens for cell-to-cell recognition.
Channel proteins: Water-filled pores that allow charged substances (e.g. ions) to diffuse through the membrane; most are 'gated' to control ion exchange.
Carrier proteins: Proteins that switch between two shapes to transport specific molecules/ions across the membrane; used in facilitated diffusion and active transport.
Diffusion: The net movement of molecules or ions from a region of higher concentration to a region of lower concentration.
Facilitated diffusion: Diffusion of substances across a membrane with the help of channel or carrier proteins, down a concentration gradient.
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 partially permeable membrane.
Water potential: The potential of water to move; pure water at atmospheric pressure has 0 kPa; solutions have negative water potential; more negative = lower water potential.
Plasmolysis: In a hypertonic solution, water leaves the plant cell, the protoplast shrinks and pulls away from the cell wall.
Turgid: In a hypotonic solution, water enters the plant cell, the vacuole expands, and the cell becomes turgid; the cell wall prevents bursting.
Cytolysis: In a hypotonic solution, water enters an animal cell, which may swell and burst.
Active transport: The movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.
Co-transport: The coupled movement of two substances across a membrane via a carrier protein; one moves down its concentration gradient, allowing the other to move against its gradient.
Aquaporins: Special channel proteins that allow the facilitated diffusion of water through cell membranes.

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