Substances enter and leave cells through the cell membrane WJEC Module 2

Questions you’ll be answer by the end! How do substances enter and leave cells? What is diffusion? What is osmosis? Why does some transport need energy?

Learning Outcomes know that diffusion is the movement of substances down a concentration gradient. The process does not require energy and only certain substances pass through the cell membrane in this way

Diffusion Diffusion is The movement of particles in a gas or any dissolved substance from a region of higher concentration to a region of lower concentration In cells, diffusion occurs across the cell membrane. The cell membrane can be described as partially permeable. It will only allow some substances to pass through it.

Learning outcome know that osmosis is the diffusion of water through a selectively permeable membrane, from a region of high-water concentration to region of low-water concentration.

Osmosis Osmosis is the diffusion of water, through a partially permeable membrane, from a region of high water concentration to a region of low water concentration.

Learning outcome carry out experimental work using living plant material and visking tubing as non-living material and be able to interpret experimental results in terms of membrane pore and particle size: the pore size is large enough to allow water molecules through but restricts the movement of solute molecules.

Osmosis Practical Activity 1 Changing the rate of osmosis Two students carried out an investigation using cylinders of potato. They cut out 5 potato cylinders and measured the mass of each one. They made up five different glucose solutions with a different concentration and left a potato cylinder in each solution for 24 hours. After 24 hours they dried each potato cylinder and measured its mass again. Look at the table of results Answer the questions that follow

Osmosis in potato Solution Mass before (g) Mass after (g) Change in mass (g) %age change in mass Water 0.1 0.2 0.5 1.0

Osmosis practical A model Cell Visking tubing is partially permeable. Cut 2 pieces of Visking tubing to 12cm long Tie a knot in one end Fill “cell A” with sugar solution Fill “cell B” with water Tie the other end of each piece of visking tubing Weigh each “cell” Put “cell A” into a boiling tube of water Put “cell B” into a boiling tube of sugar solution After 30 minutes reweigh each “cell”

Conclusions Describe and explain your results for each cell Cell A increases in mass because water has moved into the cell by osmosis, from a high water concentration in the boiling tube to a low water concentration inside the visking tubing. Cell B decreases in mass because water has moved by osmosis.......

Osmosis Practical Osmosis in potato cells Cut three potato cylinders to the same length Measure their mass and write it down Set up the following test tubes Test tube A – distilled water Test tube B – weak sugar solution Test tube C – strong sugar solution Place one potato cylinder in each test-tube and leave it for 30 minutes Re-weigh each chip

conclusions Calculate the %age increase in mass for each potato chip Which chip has increased in size? Why? Which chip has decreased in mass? Why? Describe the “feel” of each of the potato cylinders Explain why the potato cylinder in A feels different to the potato cylinder in C.

Learning outcome understand active transport as an energy requiring process whereby substances can enter cells against a concentration gradient.

Active Transport Active transport is the movement of particles across a membrane up a concentration gradient. Particles move from where they are in low concentration to where they are in high concentration. This requires energy from respiration.

Comparing diffusion and active transport Substances move from a region of higher concentration to a region of lower concentration This relies on the random movement of molecules Diffusion is like freewheeling down a steep hill Substances are moved from a region of lower concentration to a region of higher concentration This process required energy Active transport is like cycling up a steep hill, you use lots of energy