Life Process

We eat food to survive. After it is digested in the stomach and intestines, how do the nutrients reach the cells of your toes and your brain? And when you swallow a tablet for a cut finger, the medicine goes to your stomach: so how does it reach your finger to help it heal?

Every living cell needs oxygen, nutrients and water to survive and stay healthy. Every cell also makes substances the body doesn’t need: wastes such as carbon dioxide. These must be carried away before they harm the cell. The process of carrying the substances the cells need to them, and carrying unwanted substances from the cells to the organs that remove them, is called transportation. Different living things do it in different ways.

5.1.1 Transportation in plants

A plant has two kinds of “pipes”, called vascular tissues, running from its roots through the stem to every leaf:

• Xylem carries water and minerals (salts) absorbed by the roots from the soil up to the stem, the leaves, the flowers and the fruits. Xylem is made of long, hollow, dead cells joined end to end like a pipe.
• Phloem carries the food made in the leaves (and other green parts) during photosynthesis to all parts of the plant: up to the buds, flowers and fruits, and down to the roots and stores, such as a potato or a radish. Phloem is made of living cells.

Gases move in and out too. Air enters a leaf through tiny pores called stomata (one is a stoma), mostly on the underside of the leaf. Inside the leaf, oxygen and carbon dioxide spread through the air spaces between the cells and dissolve in the wet cell walls, so each cell gets the gas it needs. Water also leaves through the stomata as vapor, and that loss is what pulls more water up the xylem from the roots, like drinking through a straw.

5.1.2 Transportation in animals

How substances are carried depends on how big and complex the animal is. Very small and simple animals have no special organs for transportation, while larger, more developed animals have a whole transport system. Invertebrates and vertebrates do it in different ways.

Unicellular animals. The body of Amoeba is a single cell. Oxygen and dissolved food enter it, and carbon dioxide and other wastes leave it, straight through its cell membrane, the thin skin around the cell. Inside, the cytoplasm keeps flowing, and carries these substances from one part of the cell to another.

Simple many-celled animals. Sponges, Hydra and jellyfish have no blood and no heart. Every cell of their thin bodies is close to the water around them or to a water-filled space inside. A sponge has many small pores, the ostia (one is an ostium), all over its body, and a large hole, the osculum, at the top. Water carrying food and oxygen flows in through the ostia, through channels to all the cells, and out through the osculum, taking wastes with it. This system of water channels is the sponge’s canal system.

Hydra has a single opening, its mouth, ringed by tentacles. Food goes in, and undigested waste comes out, through the mouth. The food is digested in the hollow inside its body, the coelenteron, which carries the dissolved food to the cells lining it. Gases are exchanged through the whole surface of its body.

Developed animals. Animals like us are far too big for this. Oxygen spreads very slowly through living things by diffusion (the spreading of a substance from where there is a lot of it to where there is less). Diffusion is fast over the width of a cell, but terribly slow over the width of a body. Try it:

So humans and other large animals have a blood circulatory system, made of the blood, the blood vessels (tubes) and the heart (a pump). Oxygen, digested food and other substances the cells need dissolve in the blood. The heart pumps the blood through the blood vessels to every part of the body. The thinnest vessels, the capillaries, pass close to every cell. There the cells take what they need from the blood, and pass their wastes into it, by diffusion. The blood carries the wastes to the organs that remove them: carbon dioxide to the lungs, other wastes to the kidneys.

While a living thing carries out its life processes, its body makes some substances it doesn’t need, such as carbon dioxide and urea. If they stayed in the body for long, they would harm it. Some useful substances, like water and salt, can also be present in more than the needed amount. The process of removing such wastes and extra substances from the body is called excretion. Both animals and plants excrete, in different ways and to different extents.

5.2.1 Excretion in plants

Plants excrete oxygen, carbon dioxide and water:

• Oxygen. Green plants make oxygen during photosynthesis. They use some of it for their own respiration, and give out the rest.
• Carbon dioxide. It is made during respiration, and leaves mostly through the stomata of the leaves (by day, a green plant uses it again for photosynthesis).
• Water. Plants give out extra water as water vapor, mostly through the stomata of the leaves. This is called transpiration. Water is also given out as liquid drops through hydathodes, openings at the ends of fine veins at the edges and tips of leaves. This is called guttation.

Plants have no special excretory organs like ours. Some of their wastes are stored in parts that are later shed, such as old leaves and bark; others come out as sticky resin (pine trees), milky latex (rubber trees, papaya, aank), gum (acacia trees such as babul) and oils.

5.2.2 Excretion in animals

Animals have excretory systems to get rid of wastes, but different animals use very different organs.

• Unicellular animals such as Amoeba excrete through their cell membrane: wastes such as carbon dioxide and ammonia simply diffuse out into the water. Amoeba also has a contractile vacuole, a bubble that fills with extra water and bursts it out of the cell again and again (Figure 5.4).
• Sponges and Hydra excrete through their body surface into the water around them.
• Insects such as the grasshopper have thin tubes called Malpighian tubules, which collect wastes from the blood and empty them into the gut, to leave with the droppings.
• Humans and other developed animals have several excretory organs: the skin, the lungs and the kidneys.

Organ	What it excretes
Skin (sweat glands)	sweat: extra water, salt and a little urea
Lungs	carbon dioxide made in respiration, and some water vapor
Kidneys	urine: urea, uric acid, extra salt and water, and harmful chemicals from the blood

The urinary system plays the biggest part in excretion in the human body. It is made of two kidneys, two ureters, a urinary bladder and a urethra.

• The kidneys are the main organs. They are two bean-shaped organs, each about the size of your fist, at the back of the belly on either side of the backbone, partly sheltered by the lowest ribs. They filter the blood, taking out urea and uric acid, extra salt and water, and harmful chemicals, as urine.
• The two ureters are thin tubes that carry the urine from the kidneys down to the urinary bladder.
• The urinary bladder is a stretchy bag that stores the urine.
• The urethra is the tube through which the urine stored in the bladder is passed out of the body.

5.2.3 Differences between excretion in animals and plants

How long can you hold your breath? Most people manage less than a minute before they have to breathe. Why can’t we stay without breathing for long?

Food alone doesn’t give us energy. Digestion only breaks food down into simple nutrients, such as glucose, a sugar, that can pass into the blood. To get energy out of the nutrients, they must be broken down further, inside the cells, using oxygen. The energy released lets us walk, think, grow, keep warm, and carry out all the other life processes. The process in which food is broken down in the cells to release energy is called respiration. In short:

glucose + oxygen → carbon dioxide + water + energy

Without breathing, no fresh oxygen reaches the cells, so they can’t release energy. The brain is the first to suffer: it is damaged after only a few minutes without oxygen.

Do plants respire? Yes. Both plants and animals respire. Both use oxygen and give out carbon dioxide, day and night. (By day, a green plant’s photosynthesis takes in much more carbon dioxide than its respiration gives out, which is why, in sunlight, plants seem only to take in carbon dioxide.)

5.3.1 Respiration in plants

Plants take in oxygen and give out carbon dioxide mostly through the stomata of their leaves and through lenticels, small openings in the bark of woody stems. Roots respire too: they take oxygen from the air between the soil particles. That is why a plant in waterlogged soil, or a pot that is always too wet, can die: its roots drown for want of air.

5.3.2 Respiration in animals

Different animals use different organs to get oxygen for respiration. Which organs do unicellular animals, sponges and Hydra use? They have none: they take oxygen straight from the water through their cell membrane or body surface.

Animal	Organ for respiration	How it works
Amoeba	cell membrane	oxygen diffuses in from the water, carbon dioxide out
sponge, Hydra	body surface (pores)	every cell takes oxygen from the water around it
earthworm	moist skin	oxygen dissolves in the wet skin and passes into the blood
insects (grasshopper, cockroach)	spiracles and tracheae	air enters holes along the sides of the body and goes through branching air tubes right to the cells
fish	gills	water taken in at the mouth flows over the gills, where blood takes the dissolved oxygen
frog	lungs, moist skin, lining of the mouth	it breathes through its skin even under water
birds, mammals (humans)	lungs	air is breathed in and out through the nose

Respiration in humans

We breathe with our lungs. The human respiratory system is made of the nose, the trachea (windpipe) and the lungs. The nose is joined to the trachea, and the trachea to the lungs.

• Nose: air enters through the nostrils. Hairs and sticky mucus inside trap dust and germs, and the air is warmed and moistened.
• Trachea: the windpipe, a tube held open by rings of cartilage (you can feel them at the front of your neck). At its lower end it divides into two bronchi (one is a bronchus), one to each lung, and these branch again and again into thinner and thinner tubes, the bronchioles.
• Lungs: two soft, spongy, bag-like organs in the chest (the thoracic cavity), protected by the ribs. At the end of every bronchiole is a bunch of tiny air sacs, the alveoli (one is an alveolus). Each lung has millions of them.
• Diaphragm: a dome-shaped sheet of muscle under the lungs. As it moves down and up, air is drawn into the lungs and pushed out again (the bottle model in Unit 1 shows how).

The air breathed in through the nose passes down the trachea and the bronchi to the alveoli. Each alveolus is wrapped in a net of very thin blood vessels, the capillaries. Its wall and theirs are only one cell thick. The blood arriving in the capillaries has little oxygen and much carbon dioxide (it is called impure or deoxygenated blood). Oxygen from the air in the alveolus diffuses into the blood, and carbon dioxide diffuses out of the blood into the alveolus, to be breathed out. The blood leaving is pure (oxygenated). The heart pumps this pure blood to every cell, and in the cells the food is broken down (oxidized) using that oxygen, releasing energy for all the body’s work.

1. Choose the correct option

2. Fill in the blanks

Use these words: life process, carbon dioxide, respiration, excretion, oxygen, cell membrane. (Two words are not needed.)

3. Answer the following questions