Cells at work

Diffusion is the spreading of particles from where there is a high concentration to where there is a low concentration. It happens in gases and liquids, because the particles are always moving about at random. The cell does not use energy for it. When the particles are evenly spread they keep moving, but there is no net movement.

Substances diffuse across the cell membrane: oxygen and glucose into respiring cells, carbon dioxide out. In the lungs, oxygen diffuses from the air into the blood. In a leaf in the light, carbon dioxide diffuses in and oxygen diffuses out.

Specialised cell	Job	Adapted by
Root hair cell	Absorbs water and minerals from the soil	A long, thin extension that gives a large surface area
Palisade cell	Carries out most of the leaf’s photosynthesis	Tall, thin and packed with chloroplasts near the light
Red blood cell	Carries oxygen from the lungs round the body	No nucleus, leaving more room for the haemoglobin that carries oxygen
Nerve cell	Carries electrical impulses round the body	A very long fibre, with branched endings that connect to other cells
Sperm cell	Swims to the egg cell and fertilises it	A tail to swim with, and many mitochondria to release the energy
Egg cell	Joins with a sperm cell, then feeds the early embryo	A large store of food in its cytoplasm
Ciliated cell	Sweeps mucus and trapped dirt up out of the airways	Tiny hairs (cilia) on its surface that beat together

Single-celled organism	What it is and how it is adapted
Amoeba	A single-celled organism that lives in ponds and damp soil. It moves by pushing out part of its cytoplasm (a pseudopod), and flows round its food to take it in.
Euglena	A single-celled organism that lives in ponds. It has chloroplasts to make its own food, an eyespot that detects light, and a flagellum (a whip-like tail) to swim towards it.
Yeast	A single-celled fungus. It has a cell wall, a nucleus and mitochondria, and reproduces by budding: a new cell grows out of the old one.
Bacterium	A single-celled organism, much smaller than an animal cell. It has no nucleus: its genetic material is a loop in the cytoplasm. Some have a flagellum to swim.

From cells to organisms. In a many-celled organism, similar cells work together as a tissue; several tissues make an organ; organs work together in an organ system; and the systems together make the organism. Muscle cell → muscle tissue → the heart → the circulatory system → you. Palisade cell → palisade tissue → a leaf → the shoot system → the plant. Organs and organ systems are covered in full in the Body systems & health unit.