Transport in Cells (AQA GCSE Biology)

Surface area to volume ratio is the surface area of an object in relation to its volume.

As cell size increases the surface area to volume ratio decreases.

Multicellular organisms need exchange surfaces because their small surface area to volume ratio means that exchange of substances at their body surface is too slow to meet their needs.

Multicellular organisms need transport systems because of their large volume; their bodies contain many layers of cells, meaning that internal transport by diffusion alone is too slow.

Features of an exchange surface include:

• A large surface area

• A thin barrier / short diffusion distance

• A steep concentration gradient, maintained by having a good blood supply and a good supply of required substances

True.

The alveoli provide an exchange surface for gas exchange between the lungs and the blood.

Alveoli facilitate gas exchange as follows:

• They have a large surface area

• Their walls are thin

• They have a good blood supply

• They are well ventilated

These are all features of an efficient exchange surface.

Stomata are tiny openings that allow air to move in and out of leaves. This decreases the diffusion distance for gas exchange inside the leaf.

Gas exchange occurs between the leaf air spaces and the interior of the spongy mesophyll cells.

Fish gills are adapted for gas exchange as follows:

• Many filaments covered in lamellae increase the surface area

• A dense capillary network maintains concentration gradients

False.

The villi in the small intestine do have a large surface area, but they also have a good blood supply to maintain a steep concentration gradient between the intestine and the blood.

Osmosis is the diffusion of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution), through a partially permeable membrane.

True.

Only water can be transported by osmosis.

The type of concentration difference that leads to osmosis is a difference in water concentration between two regions. This can also be referred to as water potential.

True.

Osmosis must take place across a partially permeable membrane.

If water moves in any other way, e.g. water vapour moving out of a leaf via the stomata, this is diffusion.

The dependent variable is the variable that is measured during an investigation.

The independent variable in the osmosis and potato practical is the solute concentration of the solution in which the potato cylinders are placed.

The potato cylinders are blotted with paper towels before weighing to remove any surface water that could affect the mass measurement.

It is important that all pieces of potato have the same surface area when measuring the effect of concentration of a solution on the mass of plant tissue because surface area affects osmosis, so differences in the surface area of the potato pieces would affect the results.

True.

Potato cylinders placed in distilled water will gain mass due to the movement of water into the cells by osmosis. This will occur because distilled water has a higher water concentration than potato cell cytoplasm.

Examples of control variables in the osmosis and potato practical include:

• Volume of (e.g. sucrose) solution

• Surface area of potato cylinders

• Potato cylinders are all thoroughly dried before weighing

• Potato cylinders are left in the solutions for the same length of time

True.

If a potato cylinder does not change mass this indicates that the solution has the same water concentration as the cell cytoplasm, so there is no overall movement of water by osmosis.

Repeating the experiment with multiple potato cylinders allows for the identification and exclusion of anomalous results when calculating the mean.

The equation used to calculate the percentage change in mass for each potato cylinder is:

The concentration of sugar/salt inside the potato cells can be estimated by identifying where the line of best fit crosses the x-axis.

Active transport is the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration; this requires energy.

False.

Active transport moves particles against a concentration gradient.

Energy for active transport is released during the cellular process of respiration.

Root hair cells use active transport to move mineral ions from a region of lower concentration in the soil to a region of higher concentration inside the cell cytoplasm.

Active transport allows glucose to be transported into the bloodstream from the lumen of the small intestine when the concentration of glucose in the blood is higher than that in the intestine.

The energy requirements of diffusion, active transport and osmosis can be compared as follows:

• Diffusion and osmosis are both passive processes that do not require energy

• Active transport does requires energy

Diffusion, active transport and osmosis differ in terms of concentration gradients as follows:

• Diffusion and osmosis move substances down a concentration gradient, from higher to lower concentration

• Active transport moves substances against a concentration gradient, from lower to higher concentration

Cell transport mechanisms that can only take place across a cell membrane are:

• Osmosis

• Active transport

Diffusion can occur across a cell membrane, but can also occur in the absence of a membrane.