Membranes & Membrane Transport

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Notas de aula

Lipid Bilayers: Structure and Properties

Phospholipids form the basic structure of cell membranes, arranged as a phospholipid bilayer.
A phospholipid is made of a hydrophilic phosphate head bonded to two hydrophobic hydrocarbon (fatty acid) tails.
Because they have both hydrophilic and hydrophobic parts, phospholipids are amphipathic.
The phosphate head is polar and therefore soluble in water; the fatty acid tails are nonpolar and insoluble in water.
In water, phospholipids orient with heads towards the water and tails away, forming a monolayer; at sufficient concentration, two layers form a bilayer with tails facing inwards and heads outwards.
The bilayer has a hydrophobic core and hydrophilic outer layers, creating a barrier that controls which molecules pass through.

Lipids and proteins in the cell membrane

Membrane Proteins: Types and Functions

Integral proteins are partially hydrophobic (amphipathic), embedded in the phospholipid bilayer, and may span one or both layers.
Peripheral proteins are hydrophilic, attached to the surface of integral proteins or to the membrane via a hydrocarbon chain, and can be inside or outside the cell.
Membrane protein content varies with cell function; mitochondria and chloroplast membranes have the highest protein content due to many electron carriers.
Transport proteins allow ions and polar molecules to cross; they include channel proteins (pores) and carrier proteins (change shape).
Each transport protein is specific to a particular ion or molecule, allowing the cell to control what enters or leaves.
Other functions include receptors for hormones/neurotransmitters, immobilised enzymes with active sites on the membrane surface, cell adhesion, and cell-to-cell recognition via glycoproteins.

Simple Diffusion

Simple diffusion is the net movement of molecules or ions from a region of higher concentration to lower concentration due to random motion (kinetic energy).
Molecules move down a concentration gradient directly between the phospholipids; eventually equilibrium is reached.
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).
Rate of diffusion increases with a steeper concentration gradient, higher temperature, and greater surface area.
Small, nonpolar molecules diffuse fastest as they are soluble in the nonpolar bilayer; large molecules diffuse more slowly; uncharged molecules diffuse faster than charged ones.
Small polar molecules (e.g. urea) can diffuse at low rates, but most polar molecules and ions cannot pass through the hydrophobic core.

Diffusion across the cell membrane

Osmosis

Osmosis is the diffusion of water molecules from a dilute solution (high water concentration) to a solution with a higher solute concentration (low water concentration) across a partially permeable membrane.
Water moves down its concentration gradient, so osmosis is a type of diffusion.
Osmosis can also be defined as the net movement of water from higher water potential to lower water potential.
Water can move directly between phospholipids, but aquaporins (channel proteins) allow water to pass more freely.
Water is unusual as a polar molecule that can pass directly across cell membranes.

Osmosis through a partially permeable membrane

Facilitated Diffusion

Facilitated diffusion is the passive movement of substances down a concentration gradient with the help of transport proteins.
It is needed for large molecules, polar molecules, and ions that cannot cross the phospholipid bilayer directly.
Channel proteins form pores and allow charged 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: the substance binds to a specific site, causing a shape change that moves it across the membrane.
Transport proteins are highly specific, and the direction of movement depends on the relative concentrations on each side of the membrane.
Facilitated diffusion is passive and does not require energy.

Active Transport

Active transport is the movement of molecules and ions across a membrane from a region of lower concentration to higher concentration, using energy from respiration.
It occurs against (up) a concentration gradient and requires carrier proteins (sometimes called pumps).
Energy is needed to allow the carrier protein to change shape and transfer the substance across the membrane.
The energy is provided by ATP produced during respiration; ATP is hydrolysed to release energy.
Unlike facilitated diffusion, active transport is active and requires energy, although both use carrier proteins.

Active transport by carrier proteins

Selective Permeability

Selective permeability is the ability of the membrane to differentiate between different types of molecules, allowing some through while blocking others.
Facilitated diffusion and active transport are mechanisms that allow selective permeability.
Simple diffusion provides less control because it depends only on size and hydrophobic/hydrophilic nature of molecules.
Simple diffusion is not selective for small, nonpolar molecules (they diffuse easily) but is selective for large or polar molecules, which require transport proteins.

Glycolipids & Glycoproteins

Glycoproteins are membrane proteins with a carbohydrate chain attached on the extracellular side.
Glycolipids are lipids with carbohydrate chains attached, also located on the outer surface of cell membranes.
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 act as cell markers (antigens) for cell identification, e.g. 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.
They 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.
The model helps explain passive and active movement, cell-to-cell interactions, and cell signalling.
Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles, and within organelles; they also compartmentalise different regions of the cell.
For a two-dimensional diagram, label the phospholipid bilayer (heads and tails), integral proteins (channel/carrier), peripheral proteins, glycoproteins, and cholesterol (OH group next to phosphate heads).

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CADASTRE-SE — GRÁTIS ENTRAR

Questões de prática

Prévia grátis — 8 de 60 perguntas. Cadastre-se para ver todas.

1.Which term describes a molecule that has both hydrophilic and hydrophobic regions?
Easy
- AAmphipathic
- BHydrophilic
- CHydrophobic
- DNonpolar
2.Which part of a phospholipid is hydrophobic?
Easy
- APhosphate head
- BHydrocarbon tails
- CGlycerol backbone
- DCarbohydrate chain
3.Which of the following molecules would diffuse most rapidly directly through the phospholipid bilayer?
Easy
- AOxygen
- BGlucose
- CSodium ions
- DUrea
4.What is the main difference between facilitated diffusion and active transport?
Medium
- AFacilitated diffusion requires energy, active transport does not
- BActive transport requires energy, facilitated diffusion does not
- CFacilitated diffusion uses channel proteins, active transport uses carrier proteins
- DActive transport moves substances down the concentration gradient, facilitated diffusion moves them against it
5.Which of the following statements about the fluid mosaic model is correct?
Medium
- AThe phospholipids are fixed in position and cannot move
- BThe proteins are arranged in a regular, repeating pattern
- CThe phospholipids and proteins can move laterally within the membrane
- DThe membrane is a rigid, solid structure
6.Osmosis is a type of diffusion that requires energy from ATP.
Easy
True or false?
7.Glycolipids and glycoproteins are located on the inner surface of the cell membrane.
Easy
True or false?
8.A red blood cell is placed in a solution with a higher solute concentration than its cytoplasm. What will happen to the cell?
Medium
- AIt will swell and burst
- BIt will shrink as water moves out by osmosis
- CIt will remain unchanged
- DIt will actively pump water out

Mais perguntas

9.Which of the following are functions of membrane proteins? (Select all that apply)
Hard
- ATransport of ions and polar molecules
- BActing as receptors for hormones
- CForming the hydrophobic core of the bilayer
- DCell-to-cell recognition
- EServing as immobilised enzymes
10.Match each membrane component with its correct description.
Medium
- Phospholipid
- Channel protein
- Carrier protein
- Glycoprotein
- Forms a pore through which ions can diffuse
- Has a carbohydrate chain attached and acts as a cell marker
- Amphipathic molecule with a hydrophilic head and hydrophobic tails
- Changes shape to transport a substance across the membrane
11.Which of the following molecules can pass directly through the phospholipid bilayer by simple diffusion? (Select all that apply)
Medium
- AOxygen
- BCarbon dioxide
- CSodium ions (Na+)
- DGlucose
- EUrea
12.Match each membrane transport process with its correct description.
Medium
- Simple diffusion
- Facilitated diffusion
- Osmosis
- Active transport
- Movement of water across a partially permeable membrane from a dilute to a concentrated solution
- Net movement of molecules from high to low concentration directly between phospholipids
- Movement of molecules from low to high concentration using energy from ATP
- Passive movement of molecules or ions through a transport protein down a concentration gradient
13.Place the steps of carrier-protein-mediated facilitated diffusion in the correct order.
Medium
- The substance binds to a specific binding site on the carrier protein.
- The carrier protein changes shape, opening to the other side of the membrane.
- The substance is released on the other side of the membrane.
- The carrier protein returns to its original shape.
14.Which statement correctly describes the behaviour of phospholipid molecules in a bilayer?
Medium
- AThey can move within their own monolayer but rarely flip between monolayers.
- BThey are fixed in position and cannot move.
- CThey frequently flip-flop between the two monolayers.
- DThey can move freely between the two monolayers and also rotate.
15.Facilitated diffusion requires energy from ATP.
Easy
True or false?
16.Which statement best explains the difference in magnesium ion uptake between Group Y and Group Z?
Medium
- AGroup Y has less ATP available for active transport of magnesium ions.
- BGroup Y has more ATP available for active transport of magnesium ions.
- CGroup Y uses facilitated diffusion, which is slower than active transport.
- DGroup Z uses simple diffusion, which is faster than active transport.
17.Which of the following are functions of glycoproteins and glycolipids in the cell membrane? (Select all that apply)
Medium
- AActing as receptor molecules for cell signalling
- BActing as cell markers or antigens for cell recognition
- CTransporting ions across the membrane
- DProviding energy for active transport
- EAllowing cells to adhere to one another
18.What is the main difference between channel proteins and carrier proteins?
Medium
- AChannel proteins form pores and do not change shape; carrier proteins change shape to transport substances.
- BChannel proteins change shape; carrier proteins form pores.
- CChannel proteins require ATP; carrier proteins do not.
- DChannel proteins are only for water; carrier proteins are only for ions.
19.What term describes a molecule that has both hydrophilic and hydrophobic regions?
Easy
- Aamphipathic
- Bhydrophilic
- Chydrophobic
- Dpolar
20.In an investigation, the uptake of magnesium ions by plant roots was measured in the presence and absence of a respiratory inhibitor. Which statement best explains the difference in magnesium ion uptake between the two groups?
Medium
- AThe inhibitor prevents simple diffusion of magnesium ions.
- BThe inhibitor reduces ATP production, so active transport of magnesium ions is reduced.
- CThe inhibitor increases the rate of osmosis, so more ions are taken up.
- DThe inhibitor blocks channel proteins, preventing facilitated diffusion.
21.In an experiment, Visking tubing bags containing 0.7 mol dm⁻³ sodium chloride or 0.7 mol dm⁻³ glucose were placed in distilled water at 23 °C. The mass of each bag was measured over time. Which prediction is correct?
Medium
- ABoth bags will lose mass as water moves out by osmosis.
- BBoth bags will gain mass as water moves in by osmosis.
- CThe sodium chloride bag will gain mass, while the glucose bag will lose mass.
- DNeither bag will change mass because the solutions are isotonic.
22.Which of the following statements about the CFTR chloride channel and cystic fibrosis are correct? (Select all that apply)
Hard
- AWhen CFTR is active, chloride ions move into the mucus.
- BCFTR inhibits sodium channels, limiting sodium ion movement into cells.
- CIn cystic fibrosis, non-functioning CFTR channels lead to thick, sticky mucus.
- DCFTR channels actively transport chloride ions against their concentration gradient.
- EA high salt concentration in mucus draws water out of cells, making mucus less sticky.
23.Match each type of membrane protein with its correct description.
Medium
- integral protein
- peripheral protein
- channel protein
- carrier protein
- embedded in the phospholipid bilayer, often amphipathic
- attached to the surface of the membrane or to integral proteins
- forms a pore through which specific ions can diffuse
- changes shape to transport a specific substance across the membrane
24.Place the following steps of an experiment to investigate osmosis in potato tissue in the correct order.
Medium
- Cut potato cylinders of equal size and blot them dry.
- Measure the initial mass of each potato cylinder.
- Place each cylinder into a different concentration of salt solution.
- After a set time, remove the cylinders, blot them dry and measure their final mass.
- Calculate the percentage change in mass for each cylinder.
25.A student places a plant cell in a solution with a water potential higher than the cell's cytoplasm. Which of the following will occur?
Hard
- AWater will move out of the cell by osmosis, causing plasmolysis.
- BWater will move into the cell by osmosis, but the cell wall prevents bursting.
- CWater will move into the cell by active transport, requiring ATP.
- DThere will be no net movement of water because the cell wall is impermeable.
26.A student places equal-sized potato cylinders into six salt solutions of different concentrations and measures the percentage change in mass after 30 minutes. Why is it important to use equal-sized cylinders?
Medium
- ASo that the surface area available for osmosis is the same in each tube
- BSo that each cylinder contains the same number of cells
- CSo that the temperature of each solution stays constant
- DSo that the salt concentration inside each cylinder is identical
27.In an experiment, Visking tubing bags containing 0.7 mol dm⁻³ sodium chloride or 0.7 mol dm⁻³ glucose were placed in distilled water at 23 °C. The mass of each bag was measured every 5 minutes. Which result would you predict?
Medium
- AThe glucose bag gains mass faster than the sodium chloride bag
- BThe sodium chloride bag gains mass faster than the glucose bag
- CBoth bags gain mass at exactly the same rate
- DBoth bags lose mass because water moves out of the tubing
28.A student investigates osmosis using potato tissue. Which of the following are valid precautions to improve the reliability of the results? (Select all that apply)
Hard
- AUse the same balance for all mass measurements
- BBlot each potato cylinder dry before weighing
- CUse a different volume of solution for each concentration
- DRepeat the experiment at each concentration and calculate a mean
- ELeave the potato cylinders in the solutions for different lengths of time
29.In an experiment to investigate osmosis, the potato tissue should be blotted dry before weighing to remove excess surface water.
Easy
True or false?
30.Match each experimental observation with the correct conclusion about membrane transport.
Medium
- Uptake of magnesium ions stops when a respiratory inhibitor is added
- Glucose enters a cell faster when its external concentration is increased
- Water moves into a cell placed in a dilute solution
- A nonpolar dye spreads evenly through a membrane over time
- Active transport requires energy from respiration
- Facilitated diffusion occurs down a concentration gradient
- Osmosis moves water down a water potential gradient
- Simple diffusion results from random molecular motion
31.Which statement best explains why the uptake of magnesium ions is much lower in Group Y than in Group Z?
Medium
- AThe respiratory inhibitor stops ATP production, so active transport of magnesium ions cannot occur
- BThe respiratory inhibitor blocks channel proteins, preventing facilitated diffusion of magnesium ions
- CThe respiratory inhibitor increases the rate of simple diffusion of magnesium ions out of the roots
- DThe respiratory inhibitor lowers the temperature, reducing the kinetic energy of magnesium ions
32.Which part of a phospholipid molecule is hydrophilic?
Easy
- APhosphate head
- BFatty acid tail
- CGlycerol backbone
- DHydrocarbon chain
33.Which of the following is the best definition of active transport?
Easy
- AThe movement of molecules from high to low concentration without energy
- BThe movement of molecules from low to high concentration using energy from respiration
- CThe movement of water across a partially permeable membrane
- DThe movement of molecules through channel proteins down a concentration gradient
34.Which of the following is a correct function of glycoproteins and glycolipids in the cell membrane?
Easy
- AActing as receptor molecules for cell signalling
- BProviding energy for active transport
- CForming the hydrophobic core of the bilayer
- DActing as channel proteins for ion transport
35.Which of the following statements about the fluid mosaic model are correct? (Select all that apply)
Medium
- APhospholipids and proteins can move laterally within their own layers.
- BThe model was first outlined by Singer and Nicolson in 1972.
- CThe 'mosaic' appearance is due to the scattered pattern of proteins.
- DCholesterol is completely absent from the model.
- EThe model explains passive and active movement across membranes.
36.Place the following steps of carrier-protein-mediated facilitated diffusion in the correct order.
Medium
- Substance binds to a specific binding site on the carrier protein.
- The carrier protein changes shape.
- The substance is released on the other side of the membrane.
- The carrier protein returns to its original shape.
37.Which of the following are examples of molecules that move across cell membranes by simple diffusion? (Select all that apply)
Medium
- AOxygen
- BCarbon dioxide
- CSodium ions
- DGlucose
- EUrea
38.Which of the following factors increase the rate of simple diffusion of a substance across a membrane? (Select all that apply)
Medium
- AA steeper concentration gradient
- BA higher temperature
- CA larger surface area
- DA larger molecular size of the diffusing substance
- EA more non-polar nature of the diffusing molecule
39.Which of the following statements about channel proteins and carrier proteins are correct? (Select all that apply)
Medium
- AChannel proteins form pores through which specific substances can pass.
- BCarrier proteins change shape to transport a substance across the membrane.
- CBoth channel and carrier proteins are involved in facilitated diffusion.
- DChannel proteins are only found in active transport.
- ECarrier proteins are involved in both facilitated diffusion and active transport.
40.Match each type of membrane transport with its correct description.
Medium
- Simple diffusion
- Facilitated diffusion
- Osmosis
- Active transport
- Movement of water across a partially permeable membrane from a dilute to a concentrated solution
- Movement of molecules directly between phospholipids down a concentration gradient
- Movement of molecules or ions across a membrane using energy from respiration, against a concentration gradient
- Movement of molecules or ions across a membrane via a transport protein, down a concentration gradient
41.Complete the definition of simple diffusion.
Medium
Simple diffusion is the net movement of a substance from a region of ____ concentration to a region of ____ concentration, due to the random motion of molecules.
42.A potato cube has an initial mass of 10.0 g. After being placed in a salt solution, its mass decreases to 8.5 g. Calculate the percentage change in mass.
Medium
43.Complete the definition of osmosis by filling in the blanks.
Easy
Osmosis is the diffusion of water molecules from a ____ solution to a solution with a higher ____ concentration, across a ____ permeable membrane.
44.Name the property of phospholipids that causes them to form bilayers when placed in water.
Medium
45.In Group Z, the uptake of magnesium ions is 8 arbitrary units in the first 20 minutes and 4 arbitrary units in the second 20 minutes. Calculate the ratio of the mean rate of uptake in the first 20 minutes to the mean rate of uptake in the second 20 minutes.
Hard
46.A celery sample had an initial mass of 9.6 g and a final mass of 9.3 g after being placed in a 0.4 mol dm⁻³ salt solution. Calculate the percentage change in mass.
Medium
47.Name the process by which water molecules cross a partially permeable membrane from a dilute solution to a concentrated solution.
Medium
48.Complete the sentence about the fluid mosaic model.
Medium
The fluid mosaic model describes membranes as '____' because the phospholipids and proteins can move around within their own layers, and as '____' because the scattered pattern of proteins looks like a mosaic.
49.State the name of the property of phospholipids that causes them to form bilayers when placed in water.
Easy
50.Complete the definition of active transport by filling in the blanks.
Easy
Active transport is the movement of molecules and ions across a cell membrane, from a region of ____ concentration to a region of ____ concentration, using energy from ____.
51.A potato cube has an initial mass of 10.0 g. After being placed in a salt solution, its mass decreases to 8.5 g. Calculate the percentage change in mass. Give your answer to one decimal place.
Medium
52.In an investigation, the uptake of magnesium ions by plant roots was measured. In Group Z, the uptake was 8 arbitrary units in the first 20 minutes and 4 arbitrary units in the second 20 minutes. Calculate the ratio of the mean rate of uptake in the first 20 minutes to the mean rate of uptake in the second 20 minutes. Give your answer as a whole number or a decimal.
Medium
53.Compare the passive transport of substances across membranes, using named examples.
Hard
54.Explain how the structure of the phospholipid bilayer allows it to act as a barrier to some substances but not others.
Hard
55.Explain why phospholipid molecules rarely flip-flop between the two layers of a bilayer, but can move laterally within a layer.
Hard
56.A potato cube has an initial mass of 10.0 g. After being placed in a salt solution, its mass decreases to 8.5 g. Calculate the percentage change in mass.
Medium
- A-14 %
- B-15 %
- C-16 %
- D-13 %
57.In Group Z, the uptake of magnesium ions is 8 arbitrary units in the first 20 minutes and 4 arbitrary units in the second 20 minutes. Calculate the ratio of the mean rate of uptake in the first 20 minutes to the mean rate of uptake in the second 20 minutes.
Hard
- A2 :1
- B3 :1
- C4 :1
- D1 :1
58.A celery sample had an initial mass of 9.6 g and a final mass of 9.3 g after being placed in a 0.4 mol dm⁻³ salt solution. Calculate the percentage change in mass.
Medium
- A-2.125 %
- B-1.125 %
- C-4.125 %
- D-3.125 %
59.A potato cube has an initial mass of 10.0 g. After being placed in a salt solution, its mass decreases to 8.5 g. Calculate the percentage change in mass. Give your answer to one decimal place.
Medium
- A-14 %
- B-15 %
- C-16 %
- D-13 %
60.In an investigation, the uptake of magnesium ions by plant roots was measured. In Group Z, the uptake was 8 arbitrary units in the first 20 minutes and 4 arbitrary units in the second 20 minutes. Calculate the ratio of the mean rate of uptake in the first 20 minutes to the mean rate of uptake in the second 20 minutes. Give your answer as a whole number or a decimal.
Medium
- A1
- B4
- C3
- D2

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