Cells, Organisms and Life Processes
Cell Structure
All living things share seven life processes: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. Plant and animal cells share a cell membrane, cytoplasm and nucleus; plant cells additionally have a cellulose cell wall, chloroplasts and a permanent vacuole. Bacterial cells lack a nucleus. Under a microscope, magnification = image size ÷ actual size, so rearrange carefully and keep units consistent.
Specialised Cells and Organisation
Cells specialise: root hair cells have long extensions for absorption; red blood cells lose their nucleus to carry more haemoglobin; nerve cells are elongated for transmission; palisade cells are packed with chloroplasts. Cells group into tissues, tissues into organs, organs into organ systems.
Diffusion, Osmosis and Active Transport
Diffusion is net movement of particles from high to low concentration. Osmosis is diffusion of water through a partially permeable membrane from dilute to concentrated solution. Active transport moves substances against a concentration gradient and needs energy from respiration. Enzymes are biological catalysts with an optimum temperature and pH; above the optimum they denature.
Traps to Watch
- Saying plant cells have no mitochondria or that animal cells have no vacuoles (small, temporary ones exist).
- Confusing osmosis direction: water moves to the more concentrated solute.
- Mixing magnification with actual size.
Rule of Thumb
For osmosis, ask "where is the water more plentiful?" Water always moves from the dilute side to the concentrated side through the membrane.