Biology - Movement into and out of cells

Define diffusion.*

• Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement.

State where the energy for diffusion comes from.

• The energy for diffusion comes from the kinetic energy of random movement of molecules and ions.

State how some substances move into and out of cells by diffusion.

• Some substances move into and out of cells by diffusion through the cell membrane.

Explain why diffusion is important in living organisms.

• Diffusion allows substances to move down concentration gradients without energy from respiration.

• Oxygen diffuses into cells for aerobic respiration and carbon dioxide can diffuse out as a waste product.

• At gas-exchange surfaces, oxygen and carbon dioxide diffuse between the air or water and the blood.

• Dissolved solutes can also diffuse across cell membranes where a concentration gradient exists.

Describe how surface area, temperature, concentration gradient and diffusion distance affect diffusion rate.*

• Increasing surface area increases diffusion rate because more particles can cross at the same time.

• Increasing temperature increases the kinetic energy and random movement of particles, so diffusion is faster.

• A steeper concentration gradient increases the net movement of particles and therefore increases diffusion rate.

• A shorter diffusion distance increases diffusion rate; a longer distance decreases it.

Explain why water is important as a solvent in digestion, excretion and transport.

• Water is a solvent, so many substances can dissolve in it and be moved or react in solution.

• In digestion, soluble products can dissolve before absorption.

• In transport, substances such as glucose, ions and urea can be carried in blood plasma or other body fluids.

• In excretion, soluble waste products can be transported to excretory organs and removed.

State the process by which water moves through a partially permeable membrane.*

• Water diffuses through partially permeable membranes by osmosis.

State how water moves into and out of cells.*

• Water moves into and out of cells by osmosis through the cell membrane.

How can osmosis be investigated using dialysis tubing?

• Put a measured solution inside dialysis tubing and tie the tubing securely.

• Record its initial mass or volume, then immerse it in a solution of a different concentration for a fixed time.

• Remove and dry the outside of the tubing, then measure the final mass or volume.

• A gain or loss shows net movement of water by osmosis through the partially permeable tubing.

Describe how to investigate the effect of solution concentration on plant tissue.

• Measure the mass or length of equal pieces of plant tissue before immersion.

• Place them in solutions of different concentrations for the same time while keeping other variables constant.

• Remove surface liquid, re-measure the tissue and compare the change in mass or length.

• Tissue gains water in more dilute solutions and loses water in more concentrated solutions by osmosis.

Explain how water inside cells supports a plant.*

• Water entering plant cells by osmosis makes the vacuole larger and pushes the cell contents against the cell wall.

• This produces turgor pressure. Turgid cells support non-woody plant tissues and help leaves and stems remain firm.

Define osmosis using water potential.

• Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.

Explain what happens to plant cells in dilute and concentrated solutions.*

• In a dilute solution, water enters the plant cell, the vacuole increases in volume and the cell becomes turgid because turgor pressure increases.

• In a concentrated solution, water leaves the plant cell, turgor pressure decreases and the cell becomes flaccid; if the cell membrane pulls away from the cell wall, the cell is plasmolysed.

Explain why water potential and osmosis are important to organisms.*

• Water moves by osmosis from higher water potential to lower water potential through a partially permeable membrane.

• This is important for water uptake by root cells and for movement of water into and out of cells.

• Maintaining the correct water content keeps cells at a suitable volume; in plants it also maintains turgor and support.

Define active transport.*

• Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient), using energy from respiration.

Explain why active transport is important in living organisms.*

• Active transport moves molecules or ions across membranes from lower to higher concentration, against a concentration gradient, using energy from respiration.

• It allows root hair cells to take up mineral ions even when the ion concentration is lower in the soil than inside the cell.

• Carrier proteins in the cell membrane move specific molecules or ions across the membrane during active transport.