Biological Membranes (OCR A Level Biology): Flashcards

It increases the mechanical strength and stability of membranes (without it membranes would break down and cells would burst).

Their carbohydrate chains allow them to act as receptor molecules and as cell markers/antigens for cell-to-cell recognition.

Receptor types include:

• Signalling receptors for hormones and neurotransmitters

• Receptors involved in endocytosis

• Receptors involved in cell adhesion and stabilisation

The two types are:

• Channel (pore) proteins

• Carrier proteins

Each is specific to a particular ion or molecule.

They create hydrophilic channels that allow ions and polar molecules to pass through.

• Channel (pore) proteins form a hydrophilic channel

• Carrier proteins change shape to move a substance across

False — although many proteins move within the bilayer, some are fixed in position.

False — cholesterol decreases the membrane's permeability to ions.

• Temperature

• Solvent concentration

As temperature increases, the lipids become more fluid.

This reduces the membrane's effectiveness as a barrier to polar molecules, so they can pass through more easily.

Molecules have more kinetic energy, so any diffusion taking place through the membrane occurs at a higher speed.

Reversible — if the temperature decreases again, the lipids return to their normal levels of fluidity.

Many proteins begin to denature.

This disrupts the membrane structure so it no longer forms an effective barrier, and substances can pass freely through the disrupted membrane. This change is irreversible.

They dissolve the lipids in the membrane, causing it to lose its structure.

As temperature increases, the phospholipids in the cell membrane move more because they have more energy, so they are not as tightly packed together, increasing membrane permeability.

Beetroot cells contain a dark purple-red pigment.

The higher the permeability of the beetroot cell membrane, the more of this pigment leaks out of the cell.

Equal sizes ensure they have equal surface areas and volumes, which could otherwise affect the rate at which the pigment leaks out.

A colorimeter measures how much light is absorbed as it passes through each sample of coloured liquid.

The higher the absorbance, the more pigment was released, indicating a greater membrane permeability.

As temperature increases, membrane permeability also increases.

• Channel or carrier proteins can deform at these low temperatures

• Ice crystals that form can pierce the cell membrane, making it highly permeable (permeability is seen once the cells have thawed)

Cuvettes may differ slightly in thickness, or be scratched, so a thicker or scratched cuvette absorbs slightly more light.

Solution: use the same cuvette for every reading, or repeat the investigation and find a mean.

Rinsing removes any pigment released during cutting.

The phospholipids gain more energy and move more, so they are less tightly packed.

At high temperatures the bilayer may begin to melt and break down, and channel/carrier proteins deform, further increasing permeability.

False — increased fluidity is reversible, but once proteins denature at high temperatures the change is irreversible.

False — organic solvents increase permeability by dissolving the membrane lipids.

A machine that passes light through a liquid sample and measures how much of that light is absorbed.

The small cuboid container that holds the liquid sample to be measured in a colorimeter.

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.

The diffusion of substances across a membrane with the help of specific transport proteins (channel or carrier proteins), used by substances that cannot pass directly through the phospholipid bilayer.

Down a concentration gradient (from a region of higher concentration to a region of lower concentration).

The natural kinetic energy of the molecules or ions. Diffusion is a passive process and does not require metabolic energy (ATP).

Given sufficient time, the molecules or ions reach an equilibrium, where they are evenly spread within a given volume of space.

• The steepness of the concentration gradient

• The temperature

• The surface area of the membrane

• The diffusion distance (membrane thickness)

• The size of the diffusing molecules or ions

• Large polar molecules, such as glucose and amino acids

• Ions, such as sodium ions (Na⁺) and chloride ions (Cl⁻)

• Channel proteins

• Carrier proteins

Channel proteins are water-filled pores that allow charged substances (e.g. ions) to diffuse across the membrane.

Most are 'gated' — part of the protein on the inside surface of the membrane can move to open or close the pore, allowing the protein to control the exchange of ions.

Unlike channel proteins, carrier proteins can switch between two shapes.

The binding site is first open to one side of the membrane, then open to the other side once the protein changes shape, moving the molecule across.

The direction of net movement depends on the relative concentrations on each side — it occurs down the concentration gradient.

Both involve net movement of substances down a concentration gradient.

Facilitated diffusion requires the aid of a transport protein (e.g. because the substance is charged or too large to cross the phospholipid bilayer directly), whereas simple diffusion passes straight through the bilayer.

The two types of transport protein involved in facilitated diffusion are channel proteins and carrier proteins.

Channel proteins are water-filled pores that are often 'gated' to control the exchange of ions.

They are highly specific — each protein only allows one type of molecule or ion to pass through.

False — diffusion is a passive process, driven by the random kinetic energy of the molecules or ions.

True — carrier proteins change shape to move the molecule across, whereas channel proteins are fixed water-filled pores.

A non-living, partially permeable membrane made from cellulose whose pores are small enough to stop large molecules passing but allow smaller molecules through by diffusion.

The pores are small enough to stop large molecules (starch and sucrose) passing through, but allow smaller molecules like glucose to diffuse out.

• Fill a section of Visking tubing with a mixture of starch and glucose solutions

• Suspend the tubing in a boiling tube of water for a set period of time

• Test the water outside the tubing at regular intervals for starch and glucose

• The results should show that glucose, but not starch, diffuses out

• Estimate the concentration of glucose that has diffused into the surrounding water at each time interval (separate boiling tubes per interval) using the semi-quantitative Benedict's test

• Compare glucose concentrations between intervals using colour standards (from known glucose concentrations) or a colorimeter

• Draw a graph of how the rate of diffusion changes with the concentration gradient between the inside and outside of the tubing

The effect of surface area to volume ratio, investigated by timing the diffusion of ions (acid) through different sized agar cubes.

• Cut coloured agar (made with Universal Indicator, sometimes with dilute sodium hydroxide) into cubes of different sizes

• Place the cubes into boiling tubes containing a diffusion solution such as dilute hydrochloric acid

• The acid should have a higher molarity than the sodium hydroxide so its diffusion shows as a colour change

• Measure the time taken for the acid to change the indicator colour, or the distance travelled into the block in a given time

So that the acid's diffusion into the agar can be monitored by a colour change of the indicator (the acid overwhelms the alkali).

Rate = 1 ÷ time taken. A graph can then be drawn showing how the rate of diffusion changes with the surface area to volume ratio of the agar cubes.

As a cube gets bigger, its volume increases faster than its surface area (volume is cubed, surface area is squared).

With proportionately less surface area per unit volume, diffusion to the centre takes longer.

The greater the surface area to volume ratio, the faster the rate of diffusion.

Universal Indicator can be added to make purple agar, allowing the diffusion of acid into the cube to be monitored by a colour change.

• Visking tubing (to investigate the effect of concentration gradient)

• Cubes of agar (to investigate the effect of surface area to volume ratio)

False — starch molecules are too large to fit through the pores, so only glucose diffuses out.

True

The movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.

From a region of lower concentration to a region of higher concentration (i.e. against the concentration gradient).

Carrier proteins.\n\nEach carrier protein is specific for a particular type of molecule or ion.

Active transport requires energy, whereas facilitated diffusion does not.\n\nThis allows active transport to move substances against the concentration gradient.

The energy is used to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane.

The energy is provided by ATP (adenosine triphosphate), produced during respiration.

The energy required for active transport is provided by ATP, which is hydrolysed to release energy.

Carrier proteins undergo a conformational (shape) change to move substances across the membrane.\n\nChannel proteins do not change shape and are not used in active transport.

• The reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules\n\n- The absorption of some products of digestion from the digestive tract

• The loading of sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant\n\n- The loading of inorganic ions from the soil into root hairs

Each carrier protein used in active transport is specific for a particular type of molecule or ion.

The ATP is hydrolysed to release the energy.

False — active transport moves substances against the concentration gradient, from lower to higher concentration.

True

The process by which the cell surface membrane engulfs material, forming a small sac (an endocytic vacuole) around it, allowing bulk transport of larger quantities of material into the cell.

The process by which materials are transported out of cells, when secretory vesicles fuse with the cell surface membrane and release their contents outside the cell. It is the reverse of endocytosis.

Diffusion, osmosis and active transport only move individual molecules or ions across the membrane.

Bulk transport allows larger quantities of material to cross the membrane, such as:

• Large molecules (e.g. proteins or polysaccharides)

• Parts of cells

• Whole cells (e.g. bacteria)

Both processes require energy.

This energy is provided by ATP produced during respiration.

A form of endocytosis involving the bulk intake of solid material by a cell. Cells specialising in this process are called phagocytes, and the vacuoles formed are called phagocytic vacuoles (e.g. the engulfing of bacteria by phagocytic white blood cells).

A form of endocytosis involving the bulk intake of liquids by a cell. If the vesicle formed is extremely small, the process is called micropinocytosis.

• Phagocytosis = bulk intake of solid material

• Pinocytosis = bulk intake of liquids

• The substance to be released is packaged into secretory vesicles formed from the Golgi body

• These vesicles travel to the cell surface membrane

• The vesicles fuse with the cell surface membrane

• Their contents are released outside the cell

• Enzymes (e.g. digestive enzymes secreted from pancreatic cells)

• Hormones

• Cell wall building materials

Bulk transport into cells is called endocytosis, whereas bulk transport out of cells is called exocytosis.

Phagocytosis is the form of endocytosis in which a cell engulfs solid material, such as a phagocytic white blood cell engulfing bacteria.

The examiner wants to know what type of bulk transport is occurring, so you must state the specific scientific name.

• For transport out of the cell, state exocytosis

• For transport into the cell, state endocytosis (or, better, phagocytosis, pinocytosis or micropinocytosis)

True — both are forms of active transport, using ATP produced during respiration.

False — pinocytosis is the bulk intake of liquids; phagocytosis is the intake of solid material.

The net movement of water from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), across a partially permeable membrane.

The tendency of water to move out of a solution. It is used to avoid confusion between water concentration and the concentration of a solution.

A membrane that allows small molecules (such as water) through but not larger molecules (such as solute molecules).

From a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

• A dilute solution has a high water potential

• A concentrated solution has a low water potential

0 kPa.

Any solution containing solutes will have a water potential lower than 0 kPa (a negative value).

Pure water has the highest water potential, defined as 0 kPa.

Adding solutes lowers the water potential below that of pure water, so any solution with solutes has a negative water potential.

The solution at -200 kPa has the lower water potential.

The more negative the water potential, the lower it is (the further it is from pure water at 0 kPa).

Water molecules are small enough to pass between the phospholipids in the membrane.

Although water molecules are polar, their small size still allows them to cross the bilayer.

Osmosis is the diffusion of water molecules across a partially permeable membrane.

Water potential describes the tendency of water to move out of a solution.

False — water moves from a region of higher water potential to a region of lower water potential.

True

Animal cells do not have a supporting cell wall (unlike plant cells), so there is nothing to withstand the pressure changes caused by water entering or leaving the cell.

Water leaves the cell through its partially permeable cell surface membrane by osmosis.

The cell shrinks and shrivels up (becomes crenated), which is usually fatal for the cell.

The shrinking and shrivelling of an animal cell when it loses water by osmosis in a hypertonic environment (a solution with a higher solute concentration than the inside of the cell).

Water enters the cell by osmosis, because the surrounding solution has a higher water potential.

The cell continues to gain water until the membrane is stretched too far and the cell bursts (cytolysis), as it has no cell wall to withstand the pressure. This is fatal for the cell.

The bursting of an animal cell when it gains too much water by osmosis in a hypotonic environment (a solution with a lower solute concentration than the inside of the cell), because it has no cell wall to withstand the increased pressure.

The solution outside has the same solute concentration as the inside of the cell.

Water moves into and out of the cell at the same rate (no net movement of water), so there is no change to the cell.

• Water leaves the cell by osmosis through the partially permeable cell surface membrane

• As water leaves the vacuole, the volume of the cell decreases

• The protoplast shrinks and pulls away from the cell wall

• The cell becomes plasmolysed

The process in which the protoplast of a plant cell shrinks and pulls away from the cell wall as water leaves the cell by osmosis in a solution with a lower water potential than the cell.

• Water enters the cell by osmosis, as the surrounding solution has a higher water potential

• Water enters the vacuole, so the cell volume increases

• The expanding protoplast pushes against the cell wall and pressure builds up

• The inelastic cell wall prevents the cell from bursting and the cell becomes turgid

The state of a plant cell that is fully inflated with water and has become rigid and firm, because the protoplast pushes against the inelastic cell wall.

Animal cells do not plasmolyse; in a solution with a lower water potential than the cell, they instead shrink (become crenated).

When all the cells are firm (turgid), they provide support and strength, keeping the plant upright with its leaves held out to catch sunlight.

If plants do not receive enough water, the cells cannot stay turgid and the plant wilts.

• The cell membrane is a phospholipid bilayer and is partially permeable (only certain molecules can cross)

• The cell wall is made of cellulose and is freely (fully) permeable

False — only plant cells plasmolyse; animal cells have no cell wall, so they shrink (become crenated) instead.

False — the inelastic cell wall prevents bursting, so the plant cell becomes turgid instead.

To investigate the effect of immersing plant tissue in solutions of different water potentials, and to use the results to estimate the water potential of the plant tissue itself.

• Cut potato cylinders to the same length (one per solution, or more for repeats)

• Blot dry, then measure and record the initial mass of each

• Place each cylinder into a sucrose solution of a different concentration (at least 5 concentrations, giving a range of water potentials)

• Leave for a set time (e.g. 30 minutes), often in a water bath

• Remove, dry off excess liquid, then measure and record the final mass and length

The change in mass is divided by the initial mass and then multiplied by 100.

The potato has gained water by osmosis, so the solution had a higher water potential than the potato.

The cells become turgid and the potato feels hard.

The potato has lost water by osmosis, so the solution had a lower water potential than the potato.

The cells become flaccid and the potato feels floppy.

There is the greatest concentration gradient between the potato cells (higher water potential) and the sucrose solution (lower water potential).

So the most water moves out of the cells by osmosis, making them flaccid and reducing the cylinder's mass the most.

There was no net movement of water into or out of the cells.

The solution had the same water potential as the cytoplasm of the potato cells, so there was no concentration gradient.

It is the concentration at which the line of best fit crosses the x-axis (where the percentage change in mass is zero).

At this point there is no net movement of water, so the water potential of the solution equals that of the potato cells.

The process in which the protoplast (the living part of a plant cell) shrinks and pulls away from the cell wall as water leaves the cell by osmosis when it is placed in a solution of lower water potential.

• Water leaves the cell through the partially permeable cell surface membrane by osmosis

• As water leaves the vacuole, the volume of the cell decreases

• The protoplast gradually shrinks and stops exerting pressure on the cell wall

• As it continues to shrink, the protoplast pulls away from the cell wall — the cell is now plasmolysed

Epidermal strips are placed in sucrose (or sodium chloride) solutions of decreasing water potential and viewed under a light microscope.

The percentage of cells that have plasmolysed is then counted.

When potato cells gain water by osmosis they become turgid, exerting turgor pressure on the cell walls so the potato feels hard.

To find the percentage change in mass, divide the change in mass by the initial mass and then multiply by 100.

Plants with coloured sap (e.g. red onion, rhubarb, red cabbage) make the plasmolysed cells easier to see under the microscope.

False — an increase in mass means the solution had a higher water potential, so water moved into the potato by osmosis.

True