Gas Exchange, Cell Membranes & Transport (Edexcel A Level Biology (A) SNAB): Flashcards

Aerobic respiration requires oxygen and produces carbon dioxide as waste.

Photosynthesis requires carbon dioxide and produces oxygen as waste.

It is solved by a mass transport system, such as a circulatory system.

This is a separate problem from the exchange surface itself, which addresses the surface area available for diffusion.

As an organism increases in size, the diffusion distance to its innermost cells becomes longer.

The surface area.

The concentration difference.

The thickness of the exchange surface.

rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of membrane

The rate of diffusion also doubles, because rate is directly proportional to surface area.

The rate of diffusion doubles, because rate is inversely proportional to the thickness of the exchange surface.

Rate = P × A × ((C₁ − C₂) ÷ T)

The terms are:

• P — the permeability constant, a measure of how readily a molecule crosses that membrane

• A — the surface area

• C₁ − C₂ — the difference in concentration between the two areas

• T — the thickness of the exchange surface

In Fick's law, the rate of diffusion is inversely proportional to the thickness of the exchange surface.

False.

The equation is given to you in the exam, but you must be able to interpret it and use it in calculations.

Their huge number gives an enormous surface area (A).

The single layer of squamous epithelium gives a very small thickness (T).

Ventilation and blood flow maintain a large concentration difference (C₁ − C₂).

Rate = P × A × ((C₁ − C₂) ÷ T)

Rate = 0.012 × 3 × (1.05 × 10⁻¹⁶ ÷ 1.5)

Rate = 2.52 × 10⁻¹⁸ molecules µm⁻² s⁻¹

It is a quantitative measure of the rate at which a particular molecule can cross a particular membrane.

If the concentration difference across a membrane doubles, the rate of diffusion will also double.

To maximise gas exchange while minimising water loss across the exchange surface.

They keep the tube open at all times so it does not collapse.

The C-shape prevents friction with the oesophagus behind it and allows flexibility when food is swallowed.

Mucus traps dust and pathogens, preventing them from reaching the lungs.

Cilia waft the mucus up towards the top of the trachea, removing the trapped particles.

Bronchi have thinner walls and a smaller diameter.

Their cartilage rings are full circles rather than C-shaped.

They allow the size of the airway to be adjusted, increasing or decreasing airflow.

They have no smooth muscle, although they do still contain elastic fibres.

It is a single layer of flattened, or squamous, epithelium.

This makes it very thin and permeable, giving a short diffusion distance for the easy diffusion of gases.

They allow the alveoli to stretch during inhalation and recoil during exhalation.

Oxygen and carbon dioxide can dissolve in it.

This means gas exchange occurs in solution rather than with the air itself, which speeds up diffusion.

Oxygen diffuses from the alveoli into the capillaries to be carried around the body.

Carbon dioxide diffuses out of the capillaries into the alveoli to be exhaled.

The alveoli are surrounded by an extensive capillary network, which maintains the concentration gradient for gas exchange.

False.

They have an alveolar wall made of squamous epithelium — cell walls are found only in plants.

The lungs are located in the chest cavity, also known as the thorax.

A bilayer of phospholipids, with proteins and other components embedded within it.

A phospholipid is a molecule made of a glycerol backbone, a phosphate head and two fatty acid tails, which forms the bilayer of cell membranes.

The phosphate head is polar, so it is attracted to water and is hydrophilic.

The fatty acid tails are non-polar, so they are repelled by water and are hydrophobic.

The hydrophilic heads face outwards towards the watery environments inside and outside the cell.

The hydrophobic tails point inwards, away from water, forming the centre of the bilayer.

Cholesterol sits between the phospholipids and regulates membrane fluidity.

It stops the membrane becoming too fluid at high temperatures and too rigid at low temperatures.

Intrinsic (integral) proteins are embedded within the bilayer.

Extrinsic (peripheral) proteins are found on the outer or inner surface of the membrane.

They aid cell-to-cell communication and recognition.

They bind to substances such as hormones and can act as antigens or cell markers.

It is fluid because the phospholipids and proteins can move around within the bilayer.

It is a mosaic because the scattered proteins give a patterned appearance when viewed from above.

Small, non-polar molecules can pass directly through the membrane, but large, polar molecules must pass through transport proteins.

False.

The membrane is partially permeable — it only allows some substances to cross freely, controlling the exchange of others.

It was proposed by Singer and Nicolson in 1972.

It stated that the membrane is fluid, with globular proteins that are both peripheral and integral within the bilayer.

As technology advances, new discoveries are made.

If new evidence does not fit the current model, the model is altered or replaced to better account for the data.

Freeze-etched electron micrographs showed proteins extending into the centre of the membrane.

Biochemical analysis showed the membrane proteins are free to move within the bilayer.

Beetroot cells contain a dark purple-red pigment.

The more permeable the membrane, the more pigment leaks out, which can be measured.

A colorimeter is a machine that passes light through a coloured liquid sample and measures how much light is absorbed or transmitted.

It is zeroed using a cuvette of distilled water.

This ensures the readings measure only the pigment released, not the water.

That more pigment has been released.

This means the beetroot cell membrane was more permeable.

So they have equal surface areas and volumes.

These factors affect the rate at which pigment leaks out, so they must be controlled.

To remove any pigment released from cells damaged during cutting.

This pigment would otherwise increase the absorbance and give inaccurate results.

As temperature increases, permeability increases.

Phospholipids gain kinetic energy and move apart, and at high temperatures membrane proteins denature, both increasing permeability.

As temperature increases, the phospholipids gain more kinetic energy and move apart, increasing membrane permeability.

Permeability increases.

Alcohol dissolves the lipids in the cell membrane, so it loses its structure and pigment leaks out.

Cuvettes can differ in thickness or be scratched.

This would alter how much light is absorbed, so using the same one removes this variable.

False.

Below 0 °C, ice crystals can form and pierce the membrane, making it highly permeable once thawed.

To select the correct wavelength of light for the pigment being measured.

This gives an accurate measure of the optical density of that specific pigment.

Diffusion is the net movement of a substance from a region of its higher concentration to a region of its lower concentration.

It does not require energy from the cell.

The molecules move using their own kinetic energy, down a concentration gradient.

The overall direction of movement.

Molecules move randomly in all directions, but most move from high to low concentration.

Small and non-polar molecules.

They are small enough to fit between the phospholipids and can interact with the non-polar tails.

Facilitated diffusion is the diffusion of substances across a membrane with the help of transport proteins.

They cannot pass directly through the hydrophobic centre of the phospholipid bilayer.

They need transport proteins to help them cross.

Channel proteins and carrier proteins.

It forms a pore through the membrane that allows charged substances such as ions to pass.

Most are gated, so they can open and close to control which ions cross.

It changes shape between two forms.

This moves the binding site, and the molecule, from one side of the membrane to the other.

Factors include:

• The concentration gradient

• The temperature

• The surface area of the membrane

Diffusion continues until the concentration on each side of the membrane is equal, a state known as equilibrium.

False.

Facilitated diffusion is still passive — substances move down a concentration gradient, just with the help of a protein.

Active transport is the movement of molecules or ions across a membrane from a region of lower concentration to a region of higher concentration, using energy.

It moves substances against their concentration gradient.

This cannot happen passively, so energy from ATP is needed.

ATP, produced during respiration.

The ATP is hydrolysed to release the energy.

Carrier proteins, each specific to a particular molecule or ion.

Energy from ATP causes the carrier protein to change shape.

This moves the bound molecule or ion across the membrane against its concentration gradient.

Examples include:

• Reabsorption of useful molecules into the blood in the kidney tubules

• Loading inorganic ions from the soil into root hair cells

Endocytosis is the active process in which a cell engulfs a substance using its membrane, forming a temporary vacuole inside the cell.

Some substances, such as proteins and lipids, are too large to pass through membrane proteins.

The cell surrounds them with membrane instead.

Exocytosis is the active process in which vesicles fuse with the cell surface membrane to secrete their contents out of the cell.

They pinch off from the sacs of the Golgi apparatus.

They then move to the cell surface membrane and fuse with it.

Active transport, endocytosis and exocytosis all require energy in the form of ATP.

False.

Active transport uses carrier proteins, which change shape — not channel proteins.

Osmosis is the net movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, through a partially permeable membrane.

Water molecules move down their own concentration gradient.

They move from where there are more free water molecules to where there are fewer.

One that is not surrounding a dissolved substance.

When a substance dissolves, the water molecules around it are no longer free and cannot move through a membrane readily.

Water molecules are very small.

This lets them pass between the phospholipids despite being polar.

Water moves into the cell by osmosis and it swells.

Without a cell wall, the animal cell may burst.

Water enters and the cell swells, but the cell wall is strong and inelastic.

It resists the pressure and prevents the cell from bursting.

Water moves out of the cell by osmosis.

The cell shrinks and shrivels.

Water moves out by osmosis, so the vacuole and cytoplasm shrink.

The cell wall keeps the overall shape of the cell.

From a dilute solution (lower solute concentration) to a concentrated solution (higher solute concentration).

Osmosis is the net movement of water across a partially permeable membrane.

False.

Water moves the other way — from a dilute solution to a concentrated solution.

It lets small water molecules pass through.

But it does not let larger solute molecules cross, so a water concentration gradient is maintained.