Cells, Tissues and the Microscope

A cell is the smallest unit that carries out life's processes. Cells take in materials, release energy, grow and respond to their surroundings. An organism is a living thing. Every organism consists of one or more cells, including plants, animals and fungi.

Small, but not flat

Many plant and animal cells are about 0.01–0.1 millimeter across. That is 10–100 micrometers, written µm. One millimeter contains 1,000 micrometers. A human red blood cell is about 7.5 µm wide. More than 130 could fit side by side across one millimeter.

Cells have depth as well as length and width. Some resemble boxes; others are rounded or threadlike. A thin slice shows an outline through part of this three-dimensional shape. A drawing of that outline does not mean the real cell is flat.

How microscopes changed the picture

In 1665, Robert Hooke examined a thin slice of cork. He saw many tiny spaces surrounded by walls. He named them "cells," from cella, meaning a small room. The cork cells were dead. Their walls remained, but their living contents had gone.

In the 1670s, Antonie van Leeuwenhoek used tiny single-lens microscopes. Some magnified more than 200 times. He described living single-celled organisms in water, calling them "animalcules," or little animals. He also described blood cells, sperm cells and bacteria. Microscopes revealed both cells within large bodies and whole organisms made of one cell.

Three ideas in cell theory

Observations from many scientists led to cell theory:

1. All living things consist of one or more cells.
2. Cells are the basic units of living structure and function.
3. New cells come from existing cells.

Schleiden concluded that plants contain cells in 1838. Schwann reached that conclusion for animals in 1839. In 1855, Virchow made the third idea widely known. When a cell divides, it produces new cells. Increasing cell numbers helps a seedling or child grow.

A unicellular organism consists of one cell, such as an amoeba or yeast. A multicellular organism consists of many cells, such as a mushroom or oak tree. An adult human has roughly 30 trillion cells and more than 200 cell types. An elephant generally has more cells than a mouse; its body is not built from giant versions of mouse cells.

Read the evidence

Imagine examining a thin wood shaving beside some salt crystals. Repeated cell walls in the wood show that it came from living tissue. The wood cells need not still be alive. Salt crystals have regular shapes but contain no cells. A neat pattern alone is not enough to identify living material.

Viruses are not cells. They cannot carry out life's processes independently and multiply only inside living cells.

A light microscope sends light through a thin specimen and lenses. The lenses make its image appear larger. This increase in apparent size is magnification. An image at ×100 looks 100 times larger than the real specimen.

Know the controls

The arm supports the upper parts. The base supports the whole microscope. Carry it with one hand on the arm and one under the base. Handle glass by its edges, clean lenses with lens paper, and keep a mirror away from direct sunlight to protect your eyes.

Find the image on low power

1. Select the lowest-power objective.
2. Place the specimen over the stage opening and secure the slide.
3. Watch from the side. Use coarse focus to bring the objective close to the slide without touching it.
4. Look through the eyepiece. Slowly move the objective and slide apart with coarse focus until the image appears.
5. Sharpen with fine focus. Adjust the diaphragm for a clear view.

Watching from the side lets you check the gap. Looking through the eyepiece while bringing the lens down risks hitting the glass.

Increase power after finding the cells

First move an interesting cell into the center of the view. Turn the next objective into place. Use fine focus to sharpen it. The high-power objective is longer and lies close to the slide. At high power, use fine focus only.

Low power shows a wide area, making cells easier to find. Higher power enlarges a smaller area. You see fewer cells, with more visible detail, and the view often becomes dimmer. Adjusting the light can help. If the cell was near the edge, it may disappear when the view narrows.

The two lens powers multiply. With a 10× eyepiece:

• A 4× objective gives ×40 overall.
• A 10× objective gives ×100 overall.
• A 40× objective gives ×400 overall.

For example, selecting the 10× objective does not make the total ×10. Light passes through both lenses, so the total is ×100.

Follow movement and visible detail

The image is upside down and reversed left to right. If you move the slide left, its image moves right. Make small movements while finding a cell, then center it before changing objectives.

Cell walls, stained nuclei and chloroplasts can show clearly. Mitochondria are too small for most school microscopes to show clearly. A diagram can therefore include structures missing from your view. Finish by returning to the lowest-power objective.

A microscope needs a specimen thin enough for light to pass through. A wet mount holds a thin specimen in liquid on a glass slide. A cover slip rests over it, keeping the specimen flat and slowing drying.

Prepare onion skin

1. Put a drop of water or iodine solution on a clean slide.
2. Use forceps to peel the thin, see-through skin from inside a fleshy onion layer.
3. Spread the skin flat in the drop. Smooth out folds.
4. Touch one cover-slip edge to the liquid. Lower the other edge slowly with a mounting needle.
5. Draw away extra liquid with a paper towel. Observe on low power first.

Lowering the cover slip at an angle pushes air out ahead of it. Dropping it flat often traps bubbles. Thick or folded skin blocks light and makes overlapping cell layers hard to follow.

Add contrast with a stain

Many cell parts are almost colorless. A stain adds color so their outlines and contents stand out. Iodine colors onion cells yellow-brown and helps reveal their walls and nuclei. It also turns starch blue-black. Methylene blue makes animal-cell nuclei dark blue.

To stain an existing wet mount, add stain at one cover-slip edge. Touch a paper towel to the opposite edge. The towel draws the stain under the glass. Too much stain can make the view dark.

A cheek-cell mount uses a clean cotton swab. Gently rub inside the cheek, then smear the sample onto a slide. Used swabs go into the school's disinfectant container; prepared animal-cell slides are another option. Wear eye protection when handling stains, keep them away from eyes, and wash hands afterward.

Recognize what is in view

Onion skin cells form rows of long, bricklike shapes. You can often see cell walls, cytoplasm and stained nuclei. Onion bulb cells lack chloroplasts. A pondweed leaf, such as Elodea, shows many green chloroplasts near cell edges instead. Cheek cells have irregular, thin outlines and dark stained nuclei. They may overlap or fold.

An air bubble usually has a round shape, thick dark edge and bright empty center. It lacks the contents seen inside cells. Inspect both the outline and the inside before deciding what to draw.

Draw an observation

Use a sharp pencil and large, clear outlines. Choose a few cells you can follow accurately. Use single, unbroken lines without shading or color. Add ruled label lines that touch the structures, stay separate and do not cross. Include a title and the magnification used.

Suppose you see four neighboring onion cells at ×100. Draw their actual shapes and relative sizes. If one nucleus is visible, draw it where it appears. Label the cell walls and that nucleus. The membrane lies against the wall and may be impossible to see separately; leave it unlabeled in this observation. A biological drawing records the specimen, so adding every part from a memorized diagram would make it less accurate.

Plant and animal cells share a basic set of structures: a nucleus, cell membrane, cytoplasm and mitochondria. Parts such as nuclei and mitochondria are called organelles. Each has a particular job inside the cell. Specialized cells can differ from this model; mature human red blood cells have no nucleus.

Instructions, boundary and reactions

The nucleus contains genetic material called DNA. DNA carries instructions for growth, cell activities and division. The nucleus controls the cell through these instructions. A stain often makes it easier to see.

The cell membrane forms the thin, flexible boundary around the cytoplasm. It controls which substances enter and leave. Water, oxygen and glucose are useful supplies; carbon dioxide is a waste of respiration. The membrane lets some substances through more easily than others, so it is partially permeable.

The cytoplasm is jelly-like and mostly water. It contains dissolved substances and surrounds the organelles. Many chemical reactions happen there.

Release energy from glucose

Mitochondria are sites of aerobic respiration. A single one is a mitochondrion. In aerobic respiration, glucose reacts with oxygen. Energy is released, and carbon dioxide and water form:

glucose + oxygen → carbon dioxide + water

The released energy supports activities such as growth and muscle contraction. Cells with high energy needs, including muscle cells, have many mitochondria. Mitochondria release stored energy from glucose; they do not create energy from nothing.

Breathing and respiration have different meanings. Breathing moves air into and out of lungs. Respiration is a chemical process inside cells. Plants respire in daylight and darkness, so their cells need mitochondria too.

Move substances by diffusion

Diffusion is the overall movement of particles from higher to lower concentration. Concentration means how much of a substance is present in a given space. Diffusion does not use energy supplied by the cell.

Consider oxygen around a working muscle cell:

1. The cell uses oxygen in respiration.
2. Its oxygen concentration becomes lower than in the surrounding blood.
3. Oxygen diffuses from the blood into the cell through its membrane.

Carbon dioxide forms inside the cell. When its concentration is higher inside, it diffuses outward. Individual particles move in different directions. "Overall" describes the balance of those movements.

Connect the jobs

A muscle cell needs its membrane to allow supplies in. It needs mitochondria to release energy, cytoplasm for many reactions, and DNA instructions for its activities. Damage to the membrane can upset the whole cell by changing what enters and leaves. The parts depend on one another.

Plant cells have a strong outer wall and usually a large, permanent vacuole. Cells that carry out photosynthesis also contain chloroplasts. These structures help a plant make food and hold up its leaves and stems.

A wall outside the membrane

The cell wall is a strong layer made mainly of cellulose, a tough fiber. It supports the cell and helps maintain its shape. The wall lies outside the cell membrane. Water and dissolved substances can pass through it. The membrane inside controls entry and exit.

Keeping their positions clear helps explain their jobs. A wall provides support; a membrane controls exchange. Plant cells need both. Cellulose walls also form much of the material in wood and cotton.

Chloroplasts capture light

Chloroplasts contain chlorophyll, a green pigment that absorbs light energy. They are the sites of photosynthesis. This process uses light energy to make glucose from carbon dioxide and water, releasing oxygen:

carbon dioxide + water → glucose + oxygen

Glucose is food the plant makes. Roots absorb water and mineral ions from soil, but they do not take in ready-made plant food. Chloroplasts connect the plant's food supply to light.

Root cells and onion bulb cells lack chloroplasts. Most cells in a leaf's outer skin also lack them, even though light reaches the leaf. A plant cell does not have to be green. Different cell types carry out different jobs.

Water keeps cells firm

The large permanent vacuole is a space surrounded by its own membrane. It contains cell sap, a solution of water, sugars and salts. It stores these materials and helps keep the cell firm.

As water fills the vacuole, it pushes the cytoplasm outward against the wall. The strong wall resists this push. Together, the filled vacuole and wall support the cell. Many firm cells help support a whole leaf or stem.

Follow what happens to a lettuce leaf that has become limp:

1. Its cells have lost water, so their vacuoles are smaller.
2. The cell contents press less firmly against the walls.
3. Placing the leaf in water lets its cells take water in.
4. The vacuoles swell, and the leaf becomes crisp again.

The cell walls did not disappear when the leaf wilted. The change was in the water inside its cells. A wall alone does not provide the same firmness as a wall with full, water-filled cells pressing against it.

Compare the three jobs

Animal cells lack cellulose walls and chloroplasts. Some contain small, temporary vacuoles, so "animal cells have no vacuoles" is too broad. They lack the large, permanent vacuole shown in a typical plant-cell diagram.

A cell's structures give better evidence than its outline alone. Plant cells often look boxlike, while animal cells often look rounded or irregular. These shapes help, but several types of cell can look alike.

Compare the usual structures

The strong wall holds a plant cell in a fairly fixed shape. Its filled vacuole helps support the plant. Animal cells have flexible membranes without walls. Their varied shapes suit jobs such as moving, carrying signals and covering surfaces.

Plants make glucose using light, so their photosynthetic cells need chloroplasts. Animals eat other organisms for food. Both still need respiration to release energy from food, and both have mitochondria.

Check the source and structures

Start with the specimen's source, if known. Next look for a wall, chloroplasts and a large central vacuole. Keep shared structures in mind: a nucleus alone fits either plant or animal cells.

Work through this observation: cells come from a flower stem. They have firm outer walls and large central vacuoles, but no green chloroplasts are visible.

1. The stem source identifies the cells as plant material.
2. Their walls and vacuoles fit the plant-cell model.
3. Missing chloroplasts fit a plant cell whose job is not photosynthesis.

The last observation does not turn them into animal cells. It also leaves the exact cell type open; many plant cells lack chloroplasts.

Fungal cells share some features

Fungi, including yeast and mushrooms, have cells with nuclei, membranes, cytoplasm and mitochondria. They also have walls and can have large vacuoles. Their walls contain chitin and other materials. Plant walls are mainly cellulose. Fungi have no chloroplasts and obtain food from other material.

A wall and vacuole can separate a typical plant cell from an animal cell. When fungi are also possible, those two features are shared evidence. Chloroplasts help distinguish a photosynthetic plant cell from a fungal cell. A specimen known to be yeast is fungal even though it has a wall.

Separate a diagram from an observation

Diagrams make parts easy to distinguish. They may show a membrane just inside the wall and several large mitochondria. In a real onion mount, the membrane often lies too close to the wall to see separately, and mitochondria may be unclear.

Use a diagram to learn positions and jobs. Use microscope observations to decide what you can label in an actual specimen. A structure missing from the view may simply be too small or too faint to see.

A unicellular organism is a whole living thing made of one cell. An amoeba or yeast cell must meet its own needs. It has no heart, lungs or stomach because it has no tissues or organs. Its cell structures carry out the work.

Recognize the life processes

MRS GREN helps recall seven life processes:

Amoeba shows all seven. It pushes out parts of its cytoplasm to move. It surrounds smaller organisms for food. Its mitochondria release energy through respiration. It responds to light or harmful chemicals, grows as it takes in materials, and reproduces by splitting into two. Waste carbon dioxide leaves through its membrane.

Notice the difference between nutrition and respiration. Taking in food supplies materials. Respiration releases energy from food. Removing the carbon dioxide that respiration produces is excretion. These are connected processes with different jobs.

Small size makes exchange possible

Oxygen and many dissolved substances cross a single-celled organism's membrane by diffusion. Because the cell is tiny, they travel only a short distance to reach its contents. Wastes can leave across the same boundary.

Imagine the same method supplying a large animal. Cells deep inside its body are far from the outside. Diffusion from the body surface alone cannot supply them fast enough. A transport system, such as blood circulation, carries materials close to these cells.

A large amoeba can reach about half a millimeter across. It may appear as a tiny speck, but a microscope is needed to see its parts. "Single-celled" describes its cell number, not whether a speck is visible.

Compare independent and shared work

In a multicellular organism, different cell types share jobs. Some carry substances, some make movements and others send signals. This division of work supports a much larger body. Most of these cells depend on other cells and cannot live independently.

A single muscle cell and a yeast cell may both respire. The yeast cell is a complete organism. The muscle cell is one working part of an animal, supplied and supported by the animal's systems.

Microorganisms are not all harmful germs. Many are harmless or useful. Yeast helps bread rise, and photosynthetic single-celled organisms in oceans release oxygen. Being small says nothing by itself about whether an organism causes disease.

Single-celled organisms share life's needs, but meet them in different ways. Some surround food. Others sweep it inward or make glucose using light. An adaptation is a feature that helps an organism survive in its surroundings.

Moving and feeding

Amoeba has no fixed shape. Its cytoplasm pushes outward into pseudopodia, meaning "false feet." These extensions help it creep along. They can also flow around a smaller organism, enclosing it in a food vacuole where digestion happens. Amoeba reproduces by splitting into two cells.

Paramecium keeps a slipper-like shape. Thousands of short hairs called cilia beat over its surface. Their waves move it through water and sweep bacteria toward a feeding groove. Food then enters food vacuoles.

Cilia are short and numerous. A flagellum is a long, whip-like structure; a cell has one or a few. Both can move cells, but they look and work differently.

Reaching light to make food

Euglena has chloroplasts but no cell wall. Its light-sensitive eyespot helps detect light. A flagellum moves the cell toward light, where its chloroplasts can photosynthesize. In darkness it can also take in food from the surrounding water. It is a single-celled organism with some features found in plants and some found in animals.

Chlamydomonas is a single-celled alga. It has a wall, one large cup-shaped chloroplast, an eyespot and two flagella. The flagella help it swim toward light. Movement does not make it an animal, just as chloroplasts do not make Euglena a plant.

Keep extra water under control

Freshwater cells take in water from their surroundings. Amoeba, Paramecium and Chlamydomonas have contractile vacuoles. These collect extra water and squeeze it out. Removing water stops the cell swelling until it bursts.

A contractile vacuole has a different job from a food vacuole. One removes extra water; the other holds food during digestion. Their names help keep these jobs separate.

Yeast feeds without chloroplasts

Yeast is a single-celled fungus. Its oval cells have nuclei, cytoplasm, membranes, walls, vacuoles and mitochondria. They have no chloroplasts, so they take in sugars from their surroundings.

Yeast reproduces by budding. A small outgrowth appears on a parent cell. It grows and separates as a new cell. People use yeast to make bread rise.

Explain how a feature helps

For Paramecium, build an explanation in three steps:

1. Name the feature: cilia cover its surface.
2. State what it does: they beat and move the cell.
3. Link it to a need: this helps the organism move through its watery habitat.

"It has cilia" names a structure. Adding the movement and its use explains the adaptation.

A specialized cell has features suited to a particular job. Its shape, size and parts help it do that job well. Most specialized animal cells use the same basic structures as other animal cells, but in different amounts or arrangements.

Carry oxygen and signals

Red blood cells carry oxygen. They contain hemoglobin, a protein that picks up oxygen in the lungs and releases it in tissues. Mature human red blood cells lack a nucleus, leaving more room for hemoglobin.

Their flattened discs are dented on both sides. This shape gives a large surface for oxygen to diffuse across. They are small and flexible, helping them squeeze through narrow blood vessels.

Nerve cells, or neurons, carry electrical signals called impulses. A long extension carries a signal over a long distance. Some nerve cells reach about 1 meter, from the spinal cord toward a foot. Branched endings connect them with other cells.

Produce movement and clear airways

Muscle cells contract, meaning they become shorter. Long cells arranged in bundles pull together to move bones or squeeze organs. Contraction needs energy, so these cells have many mitochondria to support respiration.

Ciliated cells line airways such as the trachea. Their cilia beat in waves, moving mucus toward the throat to be swallowed. Other cells make the sticky mucus, which traps dust and microorganisms. Keep the jobs separate: mucus traps particles; cilia move the mucus.

Reach and support an egg

A sperm cell has a long tail for swimming. Mitochondria in its middle section release energy for movement. Its head contains a nucleus with half the genetic material of a body cell. Enzymes at the tip help it pass through the egg's outer layer.

An egg cell has a large amount of cytoplasm containing nutrients for early development. A human egg is about 0.1 millimeter across. Its nucleus also holds half the genetic material of a body cell. Once a sperm enters, the egg's outer layer changes and blocks further sperm.

Turn a description into an explanation

Start with the cell's job. Choose a feature and explain its effect. For a red blood cell:

"It has no nucleus, leaving more room for hemoglobin. More hemoglobin lets it carry more oxygen."

This answer joins structure to function. A list saying "no nucleus, red, flexible" leaves those links unexplained. Each feature needs its own link: flexibility helps the cell pass along narrow vessels.

Plant cells are specialized too. Their structures fit the materials they handle and their positions in the plant. A cell underground has different needs from one near a leaf's sunny upper surface.

Absorb water or capture light

A root hair cell absorbs water and mineral ions from soil. Its long, thin extension fits between soil particles. The extension gives a large surface for absorption. A root hair is part of one cell, not a tiny root containing many cells. Root hair cells lack chloroplasts because they are underground without light.

A palisade cell carries out photosynthesis. Tall, column-shaped cells are packed beneath a leaf's upper surface, where light is strongest. Their many chloroplasts absorb light for making glucose.

Compare them by linking each difference to a job:

Both types still have basic plant-cell structures, including a membrane, wall, cytoplasm, nucleus, vacuole and mitochondria.

Carry water and sugars

Xylem transports water and mineral ions from roots up to stems and leaves. Long cells join end to end. As the tubes mature, these cells die and lose their contents and end walls. The resulting hollow route lets water flow through. Lignin, a tough material in the walls, strengthens the tubes and supports the plant.

Phloem transports dissolved sugars from leaves to places that need or store them, including roots, growing tips and fruits. Its transport cells are living. They join through end walls with small holes, allowing dissolved sugars to pass between them.

The materials distinguish the routes: xylem supplies water upward; phloem distributes sugars from food-making leaves. Roots need sugars even though they are underground and cannot photosynthesize.

Control openings in a leaf

Guard cells occur in pairs around a tiny pore called a stoma. More than one pore is called stomata. Changing guard-cell shape opens or closes the pore. This controls carbon dioxide entering the leaf and water vapor leaving it.

An open pore helps carbon dioxide reach photosynthetic cells. It also provides a route for water loss. Closing the pore reduces both movements. The guard cells form the moving edges; the stoma is the opening between them.

To explain xylem's structure, connect its hollow tube to water flow. Its thickened walls have another job: strength. Giving these separate links explains two features without mixing water transport with sugar transport.

A multicellular organism has levels of organization:

cell → tissue → organ → organ system → organism

The arrows show how smaller working parts combine into larger ones. An organ is part of an organism; the two words describe different levels.

Cells working together make a tissue

A tissue is a group of cells working together on a particular job. Its cells are mostly of one or a few similar types. Some tissues include several types. Blood contains red blood cells, white blood cells and platelets, all contributing to its jobs.

Animal tissues include muscle tissue, which contracts, and nervous tissue, which carries signals. Epithelial tissue covers surfaces and lines organs and tubes. In plants, epidermis covers and protects surfaces. Palisade tissue carries out photosynthesis. Xylem transports water, and phloem transports sugars.

A palisade cell is one cell. A layer of cooperating palisade cells forms palisade tissue. Having many cells does not by itself make something an organ.

Several tissues make an organ

An organ contains different tissues working together on a larger job. The stomach is an organ:

• Muscle tissue churns food.
• Glandular tissue makes digestive juice.
• Epithelial tissue lines the stomach.

These jobs contribute to digestion in the same structure. A group of muscle cells alone forms muscle tissue, not a whole stomach.

The heart also contains several tissues. Muscle tissue pumps blood. Nervous tissue helps control its rate, and other tissues hold the organ together. Skin is an organ too. It includes epithelial, nervous and fatty tissues, and is the body's largest organ.

Plants have organs

Roots anchor a plant and absorb water and mineral ions. Stems support leaves and carry substances between plant parts. Leaves carry out photosynthesis, and flowers take part in sexual reproduction.

A leaf contains several tissues with different jobs. Epidermis protects its surface. Palisade tissue makes glucose. Xylem and phloem in its veins carry water and sugars. Because these different tissues work together, the leaf is an organ.

Place a structure at its level

For an animal, follow: muscle cell → muscle tissue → heart → circulatory system → dog. For a plant, follow: palisade cell → palisade tissue → leaf → shoot system → sunflower.

To place an unfamiliar structure, ask what it contains and how its parts work. One cell is at the cell level. Cooperating cells form tissue. Several different tissues working in one structure identify an organ. A large size alone cannot decide the level: a small leaf is still an organ, and a large sheet of one tissue is still tissue.

An organ system is a group of organs working together on a major job. Each organ contributes part of the work. The body has several systems, and some organs contribute to more than one.

Supply cells and remove wastes

Food travels along the digestive tract, but blood carries absorbed nutrients to body cells. Similarly, lungs bring oxygen into the blood; the circulatory system delivers it onward.

Excretion removes wastes made by cells. Carbon dioxide leaves through the lungs. Urea leaves through the urinary system. Passing feces is called egestion: it removes mainly undigested food and gut bacteria. Undigested food never became part of the body's cells, so passing it is not the same process as excretion.

Coordinate and move the body

The nervous system includes the brain, spinal cord, nerves and sense organs. It detects changes and coordinates responses. The skeletal system includes bones, cartilage and ligaments. It supports and protects the body, and works with muscles to move it. The muscular system produces movement when muscles contract.

The skeleton provides firm parts for muscles to pull on. The nervous system sends signals controlling those muscles. Moving an arm therefore involves several systems together.

Other body-wide jobs

The reproductive system produces sex cells. The endocrine system consists of glands that release hormones, chemical messages carried in blood. The immune system defends against disease.

The pancreas belongs to both digestive and endocrine systems. It makes digestive juice, and it releases hormones that help control blood sugar. An organ's membership follows its jobs; it does not have to fit into just one system.

Plant systems

Plants have a root system, mainly below ground, and a shoot system, mainly above ground. The shoot includes stems, leaves and flowers. The roots absorb water and minerals; leaves make sugars. Transport tissues in roots and stems link their supplies.

For a tomato plant, a leaf is one organ in the shoot system. The roots form another system. The whole plant, with both systems working together, is the organism.

Body systems interact because cells need a steady supply of oxygen, nutrients and water. Cells also produce wastes that must leave. A change in one system can therefore affect cells and other systems elsewhere.

Follow oxygen during exercise

Working muscles contract more often and need more energy. Their cells increase respiration, using more glucose and oxygen. Breathing speeds up to bring more oxygen into the lungs. The heart pumps faster to carry oxygen and glucose to the muscles and take carbon dioxide away. The nervous system helps coordinate these responses.

The lungs do not carry oxygen all the way to the muscles. Oxygen enters blood at the lungs, travels with circulation, then enters muscle cells. Each stage supplies the next.

Build a scientific argument

An argument connects a claim, evidence and reasoning. The claim is the answer or explanation being supported. Evidence consists of observations or measurements. Reasoning explains why that evidence supports the claim.

A student records a pulse of 72 beats per minute at rest and 130 after two minutes of running. Breathing rises from 15 to 30 breaths per minute.

1. Claim: "The respiratory and circulatory systems respond together during exercise."
2. Evidence: "Pulse rose by 58 beats per minute, and breathing rate doubled."
3. Reasoning: "Working muscle cells need more oxygen for respiration. Faster breathing supplies oxygen at the lungs, and faster circulation carries it toward the muscles."

The evidence reports what changed. The reasoning explains the connection to cells. Writing only "both numbers went up" leaves out why the systems are involved.

Trace food and wastes

After a meal, digestion breaks food into small molecules. Nutrients pass into blood, and circulation carries them to cells. Cells use them for growth and respiration. The respiratory system removes carbon dioxide; the urinary system removes urea and helps control water balance.

If airways narrow during an asthma attack, less oxygen can reach the blood. Muscle cells may receive less oxygen and tire quickly. If spinal cord nerves are damaged, muscles below the injury may receive no movement signals even when the muscles themselves are healthy. These examples connect a problem in one system with an effect elsewhere.

Roots and shoots supply one another

Plant systems interact too. Roots absorb water, which travels upward through xylem in the stem. Leaves use water in photosynthesis. The sugars they make travel through phloem to other parts, including roots.

Trace both directions in a young bean plant. Water moves from the root system to leaves in the shoot. Sugar moves from the leaves to root cells, supporting their respiration and growth. A root supplies the leaves and depends on food from them.