Transport Across Membranes

Active Transport: The movement of substances from a low to high concentration across a cell membrane and requires energy.

• Particles can't pass passively up a concentration gradient so the cell has to provide eneryg to move them
• This process is carried out by carrier proteins in the membrane. ATP provides the energy needed.
• Each carrier protein is specific and will only transport a certain substance or a restricted range of substances.
• Some protein carriers are involved in two-way transport - they pump one substance into a cell and another out at the same time.

Exam Tip

• Respiratory inhibitors such as cyanide stop active transport - results in the formation of ATP needed for respiration being prevented.
• If a question describes an experiment in which a respiratory inhibitor is used, active transport is likely to be involved as the answer

Diffusion: The net movement of atoms or molecules from an area of high to low concentration. It requires no energy - passive process.

Particles are constantly moving - however, the state of a substance may restrict the extent of this movement.

Solids: movement is confined to vibration & particles stay in fixed position - little diffusion takes place.

Liquids & Gases: particles are free to move from place to place so diffusion can occur easily. This movement is random but it naturally results in the particles spreading out from a highly concentrated area.

Facilitated Diffusion assists larger molecules, polar molecules and ions to pass through the cell membrane.

• Uses channel and carrier proteins in the membrane
• The two routes have different mechanisms, but in both cases the process is passive and no metabolic energy is required
• Ions and small polar molecules are transported through the membrane by channel proteins, which function like pores in the membrane
• Larger molecules use carrier proteins specific to the substance being transported. The molecule attaches to the protein, which then changes shape, transferring the molecule to the other side of the membrane - passive, requires no metabolic energy

Osmosis: the net movement of water molecules from a more dilute solution to a more concentrated solution across a partially permeable membrane.

The concentration of water in a solution is called water potential - a measure of relative tendency of water to move from one area to another. Water moves form an area of higher water potential to an area of lower water potential. The lower the water potential, the greater the tendency for water to move to the area.

Animal cells

• If water potential is higher surrounding the cell, the cell bursts
• If water potential is equal to the cell, there is equilibrium
• If water potential is lower surrounding the cell, the cell becomes crenated

Plant cells

• Water potential higher surrounding cell = cell is turgid
• Water potential equal to cell = equilibrium
• Water potential lower surrounding cell = cell becomes plasmolysed

In animal cells, the bursting of the cells in a hyoptonic solution (higher water potential) and crenation in a hypertonic solution (lower water potential) is fatal. Isotonic is the same water potential.

For plant cells, being turgid is a good thing - it is essential for the provision of support, the cells become rigid preventing the plant from wilting. Plasmolysis however can result in cell death because the cytoplasm pulls away from the cell wall. When a cell is not turgid nor plasmolysed it is referred to as flaccid.

Substances are secreted and egested from a cell by the process of exocytosis.

1. A vesicle containing the products to be exported from the cell is produced by the Golgi complex.

2. The vesicle fuses with the plasma membrane

3. The contents of the vesicle are released from the cell into the extracellular environment.

• Endocytosis is the process by which cells absorb molecules by engulfing them. There are two forms of endocytosis: phagoscytosis and pinocytosis.
• In phagocytosis, protrusions called pseudopodia extend from the cell and wrap themselves around a larger particle. The membrane then fuses to seal the particle into a vesicle. In both pinocytosis and phagocytosis, the particles end up inside the cell but not in the cytoplasm - they are separated by being in a vesicle.

1. The cell membrane surrounds the particle.

2. The cell membrane pinches in - when the membrane folds in on itself it is called invagination.

3. Phagosome ferments

4. Phagosome fuses with a lysosome with hydrolytic enzymes

1. Cell membrane surrounds drops of fluid with dissolved solutes

2. Cell membrane pinches in

3. Vesicle forms.