Membranes & Membrane Transport

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शिक्षकों के लिए: Membranes & Membrane Transport (Biology, HL) के लिए इस्तेमाल के लिए तैयार लेसन स्लाइड्स, रिवीज़न नोट्स — इन्हें अपने लेसन में इस्तेमाल करें, या टॉपिक को एक इंटरैक्टिव क्लास एक्टिविटी की तरह चलाएं जिसे आपके शिक्षार्थी लाइव गेम की तरह खेलें।

लेसन नोट्स

Lipid Bilayers: Structure and Barrier Function

Phospholipids are the main component of cell membranes and are amphipathic, meaning they have both hydrophilic and hydrophobic regions.
A phospholipid consists of a hydrophilic phosphate head (polar, soluble in water) bonded to two hydrophobic hydrocarbon tails (nonpolar, insoluble in water).
In water, phospholipids arrange themselves so that hydrophilic heads face the water and hydrophobic tails face away, forming a monolayer at low concentrations and a bilayer at higher concentrations.
A phospholipid bilayer has a hydrophobic core and hydrophilic outer layers; this arrangement forms a barrier that controls what enters and leaves the cell.
The hydrophobic core is tightly packed and has low permeability to large molecules, while polar molecules and ions cannot pass through because they do not interact with the hydrophobic tails.

Lipids and proteins in the cell membrane

Membrane Proteins: Types and Functions

Integral proteins are partially hydrophobic (amphipathic) and are embedded within the phospholipid bilayer, either across both layers or just one.
Peripheral proteins are hydrophilic and attach to the surface of integral proteins or to the membrane via a hydrocarbon chain; they can be inside or outside the cell.
The protein content of membranes varies with cell function; for example, mitochondrial and chloroplast membranes have high protein content due to many electron carriers.
Membrane proteins carry out functions including transport, receptors, cell adhesion, cell-to-cell recognition, and immobilised enzymes.
Transport proteins allow ions and polar molecules to cross the membrane; they are specific to a particular ion or molecule and include channel proteins (pores) and carrier proteins (change shape).
Receptors bind peptide hormones (e.g., insulin), neurotransmitters, or antibodies, generating a signal that triggers reactions inside the cell.
Immobilised enzymes are integral proteins with their active site exposed on the membrane surface, either inside or outside the cell.
Cell adhesion allows cells to attach to neighbouring cells, and glycoproteins act as cell markers (antigens) for cell-to-cell recognition, such as ABO blood group antigens.

Membrane Transport: Diffusion and Osmosis

Simple diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, due to random motion caused by kinetic energy.
Molecules move down a concentration gradient; if diffusion continues long enough, equilibrium is reached with even distribution on both sides of the membrane.
Oxygen diffuses into cells (used in respiration, creating a low internal concentration) and carbon dioxide diffuses out (produced by respiration, creating a high internal concentration).
The rate of diffusion depends on the steepness of the concentration gradient, temperature, surface area, and properties of the molecules or ions (size, charge, polarity).
Large molecules diffuse more slowly; uncharged and non-polar molecules diffuse faster as they can move directly through the phospholipid bilayer; small polar molecules like urea can diffuse at low rates.
Osmosis is the diffusion of water molecules from a dilute solution to a solution with a higher solute concentration, across a partially permeable membrane.
Osmosis can also be described as the net movement of water from a region of higher water potential to a region of lower water potential; water potential describes the tendency of water to move.
Water can move directly between phospholipids, but aquaporins (channel proteins) allow water to pass through membranes more freely; water is unusual as a polar molecule that can cross membranes directly.

Diffusion across the cell membrane

Facilitated Diffusion and Active Transport

Facilitated diffusion is a passive form of transport that uses transport proteins to move large molecules, polar molecules, and ions down a concentration gradient.
Channel proteins form pores that allow specific substances (e.g., ions) to diffuse; some are gated and can open or close to control ion exchange.
Carrier proteins change shape to transport a substance across the membrane; the substance binds to a specific site, causing a shape change that opens to the other side.
Active transport is the movement of molecules and ions across a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.
Active transport occurs against the concentration gradient and requires carrier proteins (often called pumps) and ATP; ATP is hydrolysed to release energy for the shape change.
Selective permeability is the ability of the membrane to differentiate between molecules, allowing some through while blocking others; facilitated diffusion and active transport enable this.

Active transport by carrier proteins

Glycolipids and Glycoproteins

Glycoproteins are membrane proteins with a carbohydrate chain attached on the extracellular side; glycolipids are lipids with carbohydrate chains attached, also on the outer surface.
The carbohydrate chain enables them to act as receptor molecules, binding substances at the cell surface.
Receptor types include signalling receptors (bind hormones and neurotransmitters), receptors for endocytosis, and receptors for cell adhesion and stabilisation.
Some glycoproteins and glycolipids act as cell markers (antigens) for cell identification, allowing the immune system to distinguish body cells from pathogens.

The Fluid Mosaic Model

The fluid mosaic model was first outlined in 1972 by Singer and Nicolson and explains how biological molecules are arranged in cell membranes.
Membranes are described as fluid because phospholipids and proteins can move around within their own layers.
Membranes are described as mosaic because the scattered pattern of proteins within the phospholipid bilayer looks like a mosaic when viewed from above.
The model includes four main components: phospholipids, cholesterol, glycoproteins and glycolipids, and integral and peripheral proteins.
Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles, and within organelles; they also play a role in cell signalling.
When drawing the fluid mosaic model, label the phospholipid bilayer (showing phosphate heads and hydrocarbon tails), integral proteins (e.g., channel/carrier), peripheral proteins, glycoproteins, and cholesterol (with OH group near phosphate heads).

Membrane Fluidity

Saturated fatty acids have no double bonds, are straight, pack tightly, and have higher melting points, helping membranes maintain stability at higher temperatures.
Unsaturated fatty acids have one or more double bonds, causing kinks that prevent tight packing, resulting in lower melting points and allowing membranes to be fluid and flexible.
Bacteria produce fatty acid desaturases to increase double bonds in fatty acids, maintaining membrane fluidity in cold temperatures.
Cholesterol is an important membrane lipid with hydrophobic and hydrophilic regions; it affects fluidity and permeability.
Cholesterol maintains membrane fluidity at low temperatures by disrupting close packing of phospholipids, and at high temperatures by holding fatty acid tails together, providing stability.
Cholesterol acts as a barrier by fitting in spaces between phospholipids, preventing water-soluble substances from diffusing across the membrane.

Bulk Transport: Endocytosis and Exocytosis

Bulk transport moves larger quantities of materials into or out of cells and requires energy, making it a form of active transport.
Bulk transport requires the formation of vesicles, which are small spherical sacs of plasma membrane; vesicle formation depends on membrane fluidity.
Endocytosis transports material into cells by the plasma membrane engulfing material and forming a sac; it includes phagocytosis (solids) and pinocytosis (liquids).
Phagocytosis is the bulk intake of solid material by cells called phagocytes, forming phagocytic vacuoles; an example is the engulfing of bacteria by white blood cells.
Exocytosis is the process by which materials are removed from cells; substances are packaged into secretory vesicles that fuse with the cell membrane and release contents outside.
An example of exocytosis is the secretion of digestive enzymes from pancreatic cells.

Gated Ion Channels and Pumps

Gated ion channels are specialised ion channels that operate in response to chemical or electrical stimuli.
Nicotinic acetylcholine receptors are neurotransmitter-gated ion channels; binding of acetylcholine triggers opening, allowing ions such as calcium (Ca²⁺) or sodium (Na⁺) to pass through.
The influx of ions changes the membrane potential, which can generate an action potential in neurones; these receptors are found at the neuromuscular junction.
Sodium-potassium pumps are integral proteins that generate an electrochemical gradient across nerve cell membranes; they are an example of an exchange transporter.
The pump moves three sodium ions out and two potassium ions in using one ATP molecule, always against their concentration gradients via active transport.
The pumping process involves phosphorylation (ATP transfers a phosphate) causing shape changes that release sodium outside and potassium inside.
This process is essential for nerve cell function: more positive sodium ions are moved out than potassium ions in, making the inside of the cell negatively charged relative to the outside.
When nerve cells are stimulated, sodium ion channels open and sodium ions rush in down the electrochemical gradient, reversing the charge and generating a nerve impulse.

Co-transport and Cell Adhesion

Co-transport is the coupled movement of substances across a cell membrane via a carrier protein, combining facilitated diffusion and indirect active transport.
Indirect active transport uses the energy released by one molecule moving down its concentration gradient to move another against its concentration gradient; ATP sets up the initial gradient.
In the sodium-dependent glucose co-transporter in the mammalian ileum, sodium-potassium pumps create a sodium gradient; sodium moves into the cell down its gradient, drawing glucose in with it against its gradient.
Glucose then moves down its concentration gradient into the blood; the active part is the generation of the sodium gradient, so glucose transport itself does not directly require energy.
This process also occurs in the kidney, where glucose is reabsorbed back into the blood via sodium-dependent glucose cotransporter proteins.
Cell adhesion is required for multicellular organisms; cells stick together to form tissues, and this can be permanent or temporary.
Cell adhesion molecules (CAMs) are cell surface proteins that bind cells to other cells or to the extracellular matrix (which contains supporting structures like collagen).
Different CAMs are present in different cell-cell junctions, such as tight junctions, adherens junctions, desmosomes, and gap junctions.

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प्रैक्टिस सवाल

फ्री प्रीव्यू — 61 में से 8 सवाल। सभी देखने के लिए साइन अप करें।

1.Which part of a phospholipid molecule is hydrophobic?
Easy
- AThe phosphate head
- BThe glycerol backbone
- CThe fatty acid tails
- DThe carbohydrate chain
2.Which property of phospholipids causes them to form bilayers when placed in water?
Easy
- AThey are amphipathic
- BThey are completely hydrophobic
- CThey are completely hydrophilic
- DThey are saturated
3.What is the function of cholesterol in animal cell membranes?
Medium
- ATo act as a channel for ions
- BTo regulate membrane fluidity
- CTo provide energy for active transport
- DTo form the hydrophilic head of phospholipids
4.Which of the following is the correct definition of active transport?
Medium
- AThe net movement of molecules from high to low concentration without energy
- BThe movement of water across a partially permeable membrane
- CThe movement of molecules and ions across a membrane from low to high concentration, using energy from respiration
- DThe bulk intake of solid material by a cell
5.Which of the following is NOT a factor that affects the rate of diffusion across a membrane?
Medium
- AThe steepness of the concentration gradient
- BThe temperature
- CThe surface area
- DThe number of ribosomes in the cell
6.In the sodium-potassium pump, how many sodium ions are moved out of the cell and how many potassium ions are moved into the cell per ATP molecule used?
Medium
- A2 sodium out, 3 potassium in
- B3 sodium out, 2 potassium in
- C3 sodium out, 3 potassium in
- D2 sodium out, 2 potassium in
7.Which of the following is an example of indirect active transport?
Hard
- ASimple diffusion of oxygen
- BFacilitated diffusion of glucose via a channel protein
- CSodium-dependent glucose co-transport in the ileum
- DOsmosis of water through aquaporins
8.Which of the following are functions of membrane proteins? (select all that apply)
Medium
- ATransport of ions and polar molecules
- BActing as receptors for hormones
- CActing as immobilised enzymes
- DCell-to-cell recognition
- EStoring genetic information

और सवाल

9.Osmosis is the diffusion of water molecules from a dilute solution to a solution with a higher solute concentration across a partially permeable membrane.
Easy
True or false?
10.Facilitated diffusion requires energy from ATP.
Easy
True or false?
11.Integral proteins are hydrophilic and are attached to the surface of the membrane.
Medium
True or false?
12.Match each membrane component with its correct function.
Medium
- Glycoprotein
- Cholesterol
- Channel protein
- Peripheral protein
- Cell-to-cell recognition
- Regulates membrane fluidity
- Allows ions to diffuse through a pore
- Attached to the membrane surface
13.Place the steps of the sodium-potassium pump cycle in the correct order.
Medium
- Three sodium ions bind to the pump
- ATP phosphorylates the pump, causing a shape change
- Three sodium ions are released outside the cell
- Two potassium ions bind to the pump
- Phosphate is released, causing another shape change
- Two potassium ions are released inside the cell
14.Which of the following correctly describes the arrangement of phospholipids in a bilayer?
Medium
- AHydrophilic heads facing inwards, hydrophobic tails facing outwards
- BHydrophobic heads facing outwards, hydrophilic tails facing inwards
- CHydrophilic heads facing outwards, hydrophobic tails facing inwards
- DBoth heads and tails facing outwards
15.A cell is placed in a solution with a higher solute concentration than its cytoplasm. What will happen to the cell?
Medium
- AWater will move into the cell by osmosis, causing it to swell
- BWater will move out of the cell by osmosis, causing it to shrink
- CSolutes will move into the cell by active transport
- DNothing will happen because the membrane is impermeable
16.Which of the following statements about co-transport in the ileum is correct?
Hard
- AGlucose is actively transported directly using ATP
- BSodium ions are actively transported out of the epithelial cell by the sodium-potassium pump
- CGlucose moves into the epithelial cell by simple diffusion
- DSodium ions move out of the epithelial cell by facilitated diffusion
17.Which of the following are characteristics of active transport? (select all that apply)
Medium
- ARequires energy from ATP
- BMoves substances against a concentration gradient
- CRequires carrier proteins
- DMoves substances from high to low concentration
- EIs a passive process
18.Glycolipids are located on the outer surface of cell membranes and have carbohydrate chains attached to lipids.
Easy
True or false?
19.Which of the following molecules would diffuse most rapidly across a cell membrane?
Medium
- AA large polar molecule
- BA small nonpolar molecule
- CA charged ion
- DA large polar ion
20.In the fluid mosaic model, proteins are arranged in a fixed, rigid pattern within the membrane.
Medium
True or false?
21.Which of the following statements about simple diffusion across a cell membrane are correct? (select all that apply)
Medium
- AIt is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration.
- BIt requires ATP hydrolysis to occur.
- CIt is driven by the kinetic energy of the molecules or ions.
- DIt occurs directly between the phospholipids of the membrane.
- EIt can move substances against their concentration gradient.
22.Which of the following are functions of glycoproteins and glycolipids in the cell membrane? (select all that apply)
Medium
- AActing as receptor molecules that bind substances at the cell surface.
- BActing as cell markers or antigens for cell identification.
- CProviding ATP for active transport.
- DForming the hydrophobic core of the bilayer.
- EAllowing cells to attach to each other to form tissues.
23.Match each type of transport protein with its correct description.
Medium
- Channel protein
- Carrier protein
- Sodium-potassium pump
- Aquaporin
- Forms a pore through which specific ions can diffuse
- Changes shape to transport a substance across the membrane
- Uses ATP to move three Na+ out and two K+ into the cell
- Allows water to pass through the membrane more freely
24.Place the stages of sodium-dependent glucose co-transport in the ileum into the correct sequence.
Medium
- Sodium-potassium pumps actively transport sodium ions into the blood, reducing sodium concentration in the epithelial cell.
- Sodium ions move down their concentration gradient into the epithelial cell via a co-transporter protein.
- Glucose is drawn into the epithelial cell along with sodium ions via the same co-transporter protein.
- Glucose moves down its concentration gradient into the blood.
25.Phospholipids are described as amphipathic because they have both a hydrophilic phosphate head and two hydrophobic hydrocarbon tails.
Easy
True or false?
26.Integral proteins are partially hydrophobic and are embedded within the phospholipid bilayer.
Easy
True or false?
27.Saturated fatty acids in a membrane contain double bonds that create kinks, making the membrane more fluid.
Medium
True or false?
28.Which statement best describes the relationship shown in the table?
Medium
- AAs oxygen concentration increases, the rate of respiration and the rate of sodium ion uptake both increase.
- BAs oxygen concentration increases, the rate of respiration increases but sodium ion uptake decreases.
- CThere is no relationship between oxygen concentration and sodium ion uptake.
- DSodium ion uptake increases only until oxygen concentration reaches 15.6% and then stops.
29.A student examines a diagram of a phospholipid monolayer at an air–water interface. Which part of each phospholipid molecule is oriented towards the water?
Medium
- AThe phosphate head, because it is polar and soluble in water
- BThe fatty acid tails, because they are nonpolar and insoluble in water
- CThe glycerol backbone, because it is hydrophobic
- DBoth the phosphate head and the fatty acid tails, because phospholipids are amphipathic
30.A cell is placed in a solution with a water potential of −150 kPa. The cell's cytoplasm has a water potential of −300 kPa. Which statement describes the net movement of water?
Medium
- AWater moves into the cell because the cytoplasm has a lower water potential.
- BWater moves out of the cell because the solution has a higher water potential.
- CWater moves into the cell because the solution has a higher solute concentration.
- DWater moves out of the cell because the cytoplasm has a higher water potential.
31.The sodium-potassium pump moves three Na⁺ out of a cell and two K⁺ into the cell per ATP hydrolysed. What is the net charge movement per ATP molecule?
Medium
- AOne positive charge moves out of the cell.
- BOne positive charge moves into the cell.
- CFive positive charges move out of the cell.
- DNo net charge movement occurs.
32.Which of the following are functions of the carbohydrate chains of glycoproteins and glycolipids? (select all that apply)
Medium
- AActing as receptor molecules that bind hormones
- BActing as cell markers or antigens for cell identification
- CProviding a hydrophobic barrier to water-soluble substances
- DAllowing cells to adhere to one another to form tissues
- EActing as ATP-powered pumps for ion transport
33.Unsaturated fatty acids in a membrane contain double bonds that create kinks, making the membrane more fluid.
Easy
True or false?
34.In an investigation into the uptake of magnesium ions by rice plant roots, group Y had a respiratory inhibitor added to the solution while group Z did not. After 40 minutes, the total mass of magnesium ions absorbed by group Y was 0.6 mg and by group Z was 1.8 mg. What is the ratio of the mass of magnesium ions absorbed by group Y to that absorbed by group Z?
Hard
- A1:3
- B3:1
- C1:1.2
- D1.2:1
35.Which of the following is the best explanation for why a respiratory inhibitor reduces the uptake of magnesium ions by plant roots?
Medium
- AIt prevents the synthesis of ATP, so active transport of magnesium ions cannot occur.
- BIt blocks the magnesium ion channels in the root hair cell membrane.
- CIt increases the rate of diffusion of magnesium ions into the root hair cells.
- DIt causes the root hair cells to lose water by osmosis, reducing the surface area for uptake.
36.Which of the following statements about the fluid mosaic model of cell membranes are correct? (select all that apply)
Hard
- APhospholipids and proteins can move within their own layers, giving the membrane a fluid nature.
- BThe scattered arrangement of proteins in the bilayer gives a mosaic appearance.
- CThe model was first outlined by Singer and Nicolson in 1972.
- DThe hydrophilic phosphate heads of phospholipids face the interior of the bilayer.
- ECholesterol is found only on the outer surface of the membrane.
37.In the sodium-potassium pump, ATP is hydrolysed to ADP and phosphate, and the phosphate group is transferred to the pump protein, causing it to change shape.
Medium
True or false?
38.Match each type of cell-cell junction with its correct description.
Medium
- Tight junctions
- Adherens junctions
- Desmosomes
- Gap junctions
- Prevent leakage of substances between cells
- Anchor cells to each other via cadherins
- Provide strong adhesion between cells, like rivets
- Allow direct communication between cells through pores
39.Place the following steps of the sodium-potassium pump cycle in the correct order.
Medium
- Three sodium ions from inside the cell bind to the pump.
- ATP transfers a phosphate group to the pump, causing a shape change.
- The three sodium ions are released to the outside of the cell.
- Two potassium ions from outside the cell bind to the pump.
- The phosphate group is released, causing another shape change.
- The two potassium ions are released inside the cell.
40.Which of the following molecules is most likely to be transported across a cell membrane by simple diffusion?
Easy
- AOxygen
- BGlucose
- CSodium ions
- DPotassium ions
41.State the name of the process by which a cell engulfs solid material such as bacteria.
Easy
42.A celery sample had an initial mass of 11.3 g and a final mass of 10.5 g. Calculate the percentage change in mass. Give your answer to one decimal place.
Medium
43.Complete the sentence about membrane structure.
Easy
The fluid mosaic model describes membranes as 'fluid' because the phospholipids and ____ can move around within their own layers.
44.Name the two types of transport proteins involved in facilitated diffusion.
Medium
The two types of transport proteins are ____ proteins and ____ proteins.
45.State the name of the model that describes the arrangement of molecules in cell membranes.
Medium
46.A potato cube has a surface area of 6 cm² and a volume of 1 cm³. Calculate the surface area to volume ratio. Give your answer to one decimal place.
Hard
47.Complete the definition of osmosis.
Medium
Osmosis is the diffusion of water molecules from a ____ solution to a solution with a higher solute concentration, across a partially permeable membrane.
48.Name the process by which a cell takes in liquid droplets in bulk.
Easy
The bulk intake of liquids by a cell is called ____.
49.State the name of the process by which materials are transported out of a cell in bulk.
Medium
50.A plant cell has a water potential of -200 kPa. It is placed in a solution with a water potential of -400 kPa. In which direction will water move? Enter 1 for into the cell, 2 for out of the cell.
Medium
51.A potato cube has a side length of 2 cm. Calculate its surface area to volume ratio.
Medium
52.Complete the sentence about the fluid mosaic model.
Easy
The fluid mosaic model describes membranes as ____ because the phospholipids and proteins can move around within their own layers, and as ____ because the scattered proteins look like a pattern when viewed from above.
53.Explain why the phospholipid bilayer acts as a barrier to large polar molecules and ions.
Hard
54.Explain how the structure of cholesterol allows it to regulate membrane fluidity at both low and high temperatures.
Hard
55.Explain how the structure of the phospholipid bilayer allows it to act as a barrier to large polar molecules and ions.
Hard
56.Suggest reasons for the relationship between oxygen concentration, rate of respiration and rate of uptake of sodium ions.
Hard
57.Compare the passive transport of substances across membranes, using named examples.
Medium
58.A celery sample had an initial mass of 11.3 g and a final mass of 10.5 g. Calculate the percentage change in mass. Give your answer to one decimal place.
Medium
- A-6.1 %
- B-8.1 %
- C-7.1 %
- D-5.1 %
59.A potato cube has a surface area of 6 cm² and a volume of 1 cm³. Calculate the surface area to volume ratio. Give your answer to one decimal place.
Hard
- A6 cm⁻¹
- B7 cm⁻¹
- C5 cm⁻¹
- D8 cm⁻¹
60.A plant cell has a water potential of -200 kPa. It is placed in a solution with a water potential of -400 kPa. In which direction will water move? Enter 1 for into the cell, 2 for out of the cell.
Medium
- A2
- B3
- C4
- D1
61.A potato cube has a side length of 2 cm. Calculate its surface area to volume ratio.
Medium
- A2 cm⁻¹
- B3 cm⁻¹
- C4 cm⁻¹
- D5 cm⁻¹

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