Movement into and out of Cells

The One Thing

Movement across cell membranes occurs by diffusion, osmosis, or active transport. Diffusion and osmosis are passive processes down concentration or water-potential gradients, whereas active transport uses energy from respiration to move substances against a concentration gradient.

Reactions, Processes and Experiments

Key Terms

• Cell membrane: A partially permeable boundary that controls the movement of substances into and out of a cell.
• Partially permeable: Allows some substances to pass through but prevents or limits the passage of others.
• Diffusion: The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
• Concentration gradient: The difference in concentration of a substance between two regions.
• Net movement: The overall movement of particles in one direction when particles move randomly in both directions.
• Osmosis: The net movement of water molecules from a region of higher water potential, usually a dilute solution, to a region of lower water potential, usually a more concentrated solution, through a partially permeable membrane.
• Water potential: A measure of the tendency of water molecules to move; pure water has the highest water potential.
• Active transport: The movement of particles from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy released by respiration.
• Carrier protein: A membrane protein that binds to a specific substance and changes shape to move it across the membrane.
• Turgid: A plant cell that has gained water and has firm cell contents pressing against the cell wall.
• Flaccid: A plant cell that has lost water and is no longer firm.
• Plasmolysis: The condition in which a plant cell loses enough water for the cell membrane to pull away from the cell wall.
• Crenation: The shrinking or wrinkling of an animal cell after water leaves it by osmosis.
• Lysis: The bursting of an animal cell when too much water enters by osmosis.
• Surface area to volume ratio: The amount of membrane surface available for exchange compared with the volume of the cell; a larger ratio generally allows faster exchange.

Easily Confused

• Diffusion and osmosis: Diffusion involves particles generally moving down a concentration gradient; osmosis specifically involves water molecules moving through a partially permeable membrane.
• Osmosis and active transport: Osmosis is passive and moves water down a water-potential gradient; active transport requires energy from respiration and moves particles against a concentration gradient.
• Turgid and flaccid: A turgid plant cell has gained water and is firm; a flaccid plant cell has lost water and is no longer firm.
• Plasmolysis and crenation: Plasmolysis occurs when the membrane pulls away from a plant cell wall; crenation is the shrinking or wrinkling of an animal cell.
• Crenation and lysis: Crenation results from water leaving an animal cell; lysis results from too much water entering and bursting the cell.
• Concentration gradient and water potential gradient: Diffusion is described using a concentration gradient, whereas osmosis is described using movement from higher to lower water potential.

What Gets Asked

• Explain how diffusion produces a net movement despite particles moving randomly, and state that the movement is down a concentration gradient. A common error is to describe diffusion as requiring energy from respiration.
• Compare diffusion, osmosis, and active transport. Marks are lost by omitting that osmosis involves water molecules and a partially permeable membrane, or that active transport requires energy and can move substances against a concentration gradient.
• Predict the effect of dilute and concentrated solutions on plant and animal cells. The specific outcomes must be distinguished: plant cells become turgid, flaccid, or plasmolysed, while animal cells may undergo lysis or crenation.
• Explain why diffusion is faster under particular conditions. Relevant factors are a steeper concentration gradient, higher temperature, larger surface area, and shorter diffusion distance.
• Describe an osmosis investigation using plant tissue, including equal sample size, blotting samples dry before weighing, consistent conditions, and identification of independent, dependent, and control variables.
• Interpret a graph of percentage change in mass against solution concentration to estimate the concentration at which there is no net movement of water.