Transport in Cells

Molecules can pass through cell membranes by diffusion, osmosis, or active transport. Diffusion and osmosis are passive but active transport requires energy to move molecules against their gradient.

Diffusion

Diffusion is the movement of molecules down a concentration gradient — from an area of high concentration to an area of lower concentration. Some substances can move into cells by diffusion through the cell surface membrane. For example, oxygen and carbon dioxide diffuse between cells and the bloodstream by passing directly through the membrane.

There are several factors which increase the speed of diffusion:

• The size of the concentration gradient – the larger the difference in concentration, the faster the rate of diffusion.

• Temperature – the higher the temperature, the more kinetic energy molecules have so they diffuse faster.

• Surface area – the larger the surface area, the more space there is for diffusion to take place.

The effect of surface area: volume ratio in diffusion

Single-celled organisms such as bacteria, have a rod-like shape which gives them a large surface area to volume ratio. This means that they can use diffusion to obtain oxygen and glucose for respiration and for the removal of waste products out of the cell.

Smaller objects will always have a bigger surface area to volume ratio. To wrap your head around this, just picture a balloon. A deflated balloon is mostly rubber (surface area) and hardly any air (volume). As you inflate the balloon, the ratio between the surface area and the volume decreases as the amount of air increases. If we apply this idea to organisms, animals like worms and mice will have a large surface area to volume ratio whereas humans and elephants have a much smaller surface area to volume ratio.

Organisms with small surface area to volume ratios need specialised gas exchange systems to deliver the substances needed for respiration and remove the waste products. This is why larger mammals, such as humans, have lungs and a blood transport system to deliver oxygen and glucose to cells. If we relied on diffusion, the cells in the middle of our bodies would not receive much glucose and oxygen because of the large distance for diffusion. In contrast, smaller organisms such as worms do not have a gas exchange system as there is a much shorter diffusion pathway to obtain the substances they need.

Features of exchange surfaces

Exchange surfaces have the following features which increase the speed of diffusion:

• Large surface area

• Thin walls to provide a short diffusion pathway

• Rich blood supply to maintain a steep concentration gradient

In the lungs, structures called alveoli provide a large surface area. They are high in number (around 700 million) and provide a total surface area of 70 square meters. They have a rich blood supply as lots of capillaries surround each alveolus. There is a short diffusion distance for oxygen and carbon dioxide since the alveolar walls are just a single cell thick.

Fish are also adapted for efficient gas exchange. Their gills consist of lots of thin branches called gill filaments which provide a large surface area for gas exchange. Each gill filament is covered in structures called gill plates which further increase surface area. The gill plates are enriched with blood capillaries and have a thin surface layer of cells to ensure a short diffusion distance.

The leaves of plants are adapted for gas exchange by having structures called stomata which open and close to allow gases into and out of the leaf. A layer within the leaf called the spongy mesophyll layer contains air spaces to allow the gases to circulate between the cells.

Plant roots are adapted for the diffusion of mineral ions. Roots are made up of tiny hairs which increase the surface area of the root. The root hair is made up of root hair cells which have a long, thin shape to further increase surface area.

Diffusion also occurs in the intestine. After a meal, our bodies digest large food molecules into smaller food molecules, which pass through cells of the small intestine and enter the bloodstream. The cells on the surface of the small intestine contain finger-like projections called villi on their surface which increases the surface area for diffusion. Each villus consists of many smaller microvilli which further increases surface area. The walls of the intestinal cells are just a single cell thick, providing a short diffusion pathway. They also have a rich blood supply to maintain a steep diffusion gradient.

Osmosis

Osmosis is the movement of water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane.

If plant cells are placed in dilute solution, the water potential is higher in the solution compared to the cell cytoplasm. Water moves into the cell by osmosis, causing the cytoplasm to push up against the cell wall. The cell becomes turgid. The strong cell wall protects the plant cell from bursting.

When plant cells are placed in concentrated solution, the water potential is higher inside the cell compared to the solution. Water moves by osmosis from the cell cytoplasm to the solution, causing the cytoplasm to lose volume and pull away from the cell wall (plasmolysis). The cell becomes flaccid and the plant will wilt.

If animal cells (like red blood cells) are placed in a dilute solution, water moves into the cell by osmosis and the cell will burst. Animal cells have no cell wall to protect them and the cell membrane is not strong enough to withstand the high pressure from the extra water inside the cell.

If animal cells are placed in concentrated solution, they lose water by osmosis and become crenated (wrinkled).

Investigating osmosis

A common method of investigating osmosis is by placing pieces of potato or other plant tissue into solutions with different concentrations. If we place the potato in a concentrated solution, the potato should lose mass because water will move out of the plant by osmosis. Remember that if a solution has a high solute concentration, it will have a low water potential. If we place the potato into a dilute solution, we’d expect the potato to gain mass because water will move into the potato by osmosis.

Instead of simply recording the mass gained or lost by the potato, it is better to convert this into a percentage (i.e. the mass gained/lost relative to the original mass). This means that our results can be compared with experiments using potato of different starting masses. To calculate the percentage change in mass of plant tissue, you use the formula:

Active transport

Active transport is the movement of molecules from low to high concentration, against a concentration gradient. This process requires energy. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.

Examples of active transport include:

• The uptake of glucose by epithelial cells in the ileum of the small intestine – this is important for the health of organisms because glucose is used for respiration. If we relied on diffusion for glucose uptake, absorption would stop once an equilibrium was reached and not all the glucose would be absorbed.

• The uptake of mineral ions from soil water by the root hair cells of plants. Plants require mineral ions for healthy growth — for example, magnesium is used to make chlorophyll for photosynthesis. Concentrations of mineral ions are usually higher inside cells than the soil, so they cannot be absorbed by diffusion.