Why living things need transport
A cell stays alive only if it keeps getting food (for energy and building), oxygen (to release energy) and water, and keeps removing waste. In a tiny single-celled creature, everything simply drifts in and out. In a big body, many cells are far from the outside, so the body needs a delivery system. This is called mass transport: moving substances in a flowing liquid, pushed by a pump or pulled by evaporation.
Types of nutrition
Nutrition means getting the food a body needs.
- Autotrophic (self-feeding): the organism makes its own food from simple things. Green plants and algae use light (photosynthesis). Some bacteria use energy from chemicals (chemosynthesis).
- Heterotrophic (other-feeding): the organism takes ready-made food from others. Animals eat plants or other animals. Fungi and many bacteria feed on dead matter (saprotrophs); parasites live on or in another living body.
- Animals are also grouped as herbivores (eat plants), carnivores (eat animals) and omnivores (both).
- Feeding styles: holozoic (take in food and digest it inside the body, like us), saprotrophic (digest outside, then absorb), parasitic.
Membrane transport
The cell membrane lets some substances pass and blocks others. It is selectively permeable.
- Diffusion: particles spread from where they are many to where they are few. No energy needed. Oxygen and carbon dioxide cross this way.
- Facilitated diffusion: same direction, but through a protein channel (for glucose and ions).
- Osmosis: diffusion of water through a membrane, from the weaker solution to the stronger solution.
- Active transport: a protein pump moves particles from few to many, against the slope. It needs energy (ATP).
- Bulk transport: big things are wrapped in a bubble: endocytosis (in) and exocytosis (out).
Gas exchange in animals and plants
Gases cross by diffusion, so a good exchange surface is thin, wet, large and close to blood (or to cells).
- Earthworm, frog skin: moist skin.
- Fish: gills with many thin plates; water flows over them.
- Insects: tiny tubes (tracheae) carry air straight to the cells.
- Humans: lungs. Millions of tiny air sacs (alveoli) give a huge surface; oxygen goes into the blood and carbon dioxide comes out.
- Plants: air enters and leaves through tiny pores called stomata in leaves (and lenticels in stems). In daylight a leaf takes in CO₂ and gives out O₂. Day and night it also respires.
Circulatory systems and blood transport
A circulatory system has a fluid, vessels and a pump.
- Open system (insects, snails): the fluid leaves the vessels and bathes the organs.
- Closed system (earthworm, humans): blood stays inside vessels. Faster and more controlled.
- In humans the heart has four chambers. Arteries carry blood away from the heart, veins bring it back, and thin capillaries swap food, oxygen and waste with the cells.
- Blood carries oxygen (red blood cells with haemoglobin), food (in the liquid plasma), waste (CO₂, urea), heat and hormones. White blood cells fight germs; platelets close cuts.
- Double circulation: the right side sends blood to the lungs to pick up oxygen; the left side sends it to the body.
Transport in plants: xylem and phloem
Plants have no heart, but they have two sets of tubes.
- Xylem: dead, hollow, strong tubes. They carry water and minerals upward from the roots. Water evaporates from the leaves (transpiration) and pulls the water column up, like a drink through a straw. Roots also push a little (root pressure).
- Phloem: living tubes. They carry sugar made in the leaves to roots, fruits and growing tips, up or down (translocation). This needs energy.
- Roots absorb water by osmosis from the soil, helped by thin root hairs.
Try it
In the 3D, tap each button in the last step and move the speed slider. Predict first: what will happen to the blue particles if the speed is zero? Then check. At home: put a celery stalk in water with a few drops of food colour for a few hours. Cut it and look for the coloured dots. These are the xylem tubes.
Key formulas and definitions
- Autotroph = makes own food; Heterotroph = eats ready-made food
- Diffusion: high concentration to low concentration, no energy
- Osmosis: water from weak solution to strong solution through a membrane
- Active transport: low to high concentration, needs ATP
- Photosynthesis: carbon dioxide + water → glucose + oxygen (light, chlorophyll)
- Xylem: water UP (pulled by transpiration); Phloem: food both ways (translocation)
Worked examples
1. A mushroom grows on a dead log. Which type of nutrition is this?
It takes ready-made food from dead matter, so it is heterotrophic, and more exactly saprotrophic (it digests outside and then absorbs).
2. Oxygen is high in the air sacs of the lung and low in the blood arriving there. In which direction does oxygen move, and does it need energy?
Oxygen moves from high (air sacs) to low (blood). It moves by diffusion, so no energy is needed.
3. A root cell has more mineral ions inside than the soil outside, yet it takes in more. How?
It moves ions from low to high concentration. That is active transport, using pumps and ATP energy.
4. A thin layer of grease is smeared on the leaf surface and blocks the stomata. What happens to the water pull in the xylem?
Less water evaporates (less transpiration), so the pull on the xylem is smaller and water moves up slowly. The leaf also cannot exchange gases, so the plant will suffer if this lasts long.
Common mistakes
- Saying 'plants do not respire'. Plants respire day and night; in daylight photosynthesis is just faster.
- Mixing up xylem and phloem. Xylem = water up; phloem = food (sugar) both ways.
- Thinking diffusion needs energy. Only active transport needs ATP.
- Saying all veins carry blue, low-oxygen blood. The pulmonary vein carries oxygen-rich blood.