Science notes, chapter 2

Cell as the Basic Unit of Life

Textbook Science Form 1 (DLP), pages 44–69

By the end of this chapter you can

Explain that living things are made of cells; prepare slides of cheek and onion cells; state the function of each cell structure; compare animal and plant cells; describe unicellular and multicellular organisms, the types of animal and plant cells, and the order cell → tissue → organ → system → organism (2.1)
Describe cell respiration and photosynthesis, test a leaf for starch, compare the two processes and explain how they complement each other (2.2)

2.1Cell – Structure, Function and Organisation

What is a cell?

A cell is the basic unit of life. All living things are made up of cells, just as a house is built from bricks.

In 1665, Robert Hooke looked at a thin piece of cork through a simple microscope he made. He saw many small box-shaped spaces and called them cells.
In 1674, Antonie van Leeuwenhoek used a more powerful microscope and saw tiny moving microorganisms in a drop of rain water.

A cell is like a tiny factory where thousands of chemical reactions happen. Cells carry out all of life's functions, such as growth, respiration, reproduction and excretion.

Cell division: one cell divides to become two cells, then four, and so on. This makes new cells for growth and replaces old or damaged cells.

Cancer happens when normal cells keep dividing without control. The extra cells can form a lump called a tumour.

Preparing slides of animal and plant cells

	Animal cells (Activity 2.1)	Plant cells (Activity 2.2)
Cells used	Cheek cells, scraped gently from inside the cheek with a toothpick	Onion epidermis, a thin layer peeled from the inside of an onion scale
Stain	Methylene blue solution	Iodine solution
Why stain?	Cells are almost colourless. A stain dyes the cell so its parts (especially the nucleus) can be seen clearly under the microscope.

Six steps to prepare a cheek cell slide — Preparing a cheek cell slide. (Textbook Figure 2.4, p. 48)

Steps (both slides)

1. Put a drop of distilled water on a clean glass slide.
2. Place the cells (cheek cells or onion epidermis) in the water.
3. Add a drop of stain (methylene blue for cheek cells, iodine for onion cells).
4. Lower the cover glass slowly at an angle (about 45°) using a mounting needle, so that no air bubbles are trapped.
5. Use filter paper to absorb any excess stain.
6. Look at the slide under the low power objective lens first, then the high power lens. Draw and label what you see.

Dropping the cover glass flat traps air bubbles, which look like round empty circles and hide the cells. Lower it slowly from one side.

Onion cells under a microscope: brick-shaped cells in rows — Left: onion cells (fixed, box-like shape). Right: cheek cells (irregular shape). (Textbook Photograph 2.4, p. 49)

Cheek cells under a microscope: rounded blue cells — Left: onion cells (fixed, box-like shape). Right: cheek cells (irregular shape). (Textbook Photograph 2.4, p. 49)

Structures in a cell and their functions

Labelled plant cell with the function of each structure — The function of structures in a plant cell. (Textbook Figure 2.6, p. 50)

Labelled animal cell with the function of each structure — The function of structures in an animal cell. (Textbook Figure 2.7, p. 51)

Structure	Function	Found in
Nucleus	Controls all activities in the cell. Contains chromosomes made of DNA (deoxyribonucleic acid), which carries genetic information.	Animal and plant cells
Cell membrane	Controls the flow of materials in and out of the cell.	Animal and plant cells
Cytoplasm	A jelly-like medium where chemical reactions occur.	Animal and plant cells
Mitochondria	Produce energy for the cell's reactions (cell respiration happens here).	Animal and plant cells
Cell wall	Gives support, protection and a fixed shape to the cell.	Plant cells only
Chloroplast	Contains chlorophyll that absorbs light energy for photosynthesis.	Plant cells only
Vacuole	Gives support to the cell when it is full of cell sap.	Plant cells (large, central)

Memory trick for the plant-only parts: W-C-V — cell Wall, Chloroplast, large Vacuole. Plant cells also have a fixed shape; animal cells do not.

Comparing animal cells and plant cells

Double bubble map comparing animal and plant cells — Similarities (middle) and differences (outside) between animal and plant cells. (Textbook Figure 2.8, p. 52)

	Animal cell	Plant cell
Same in both	Nucleus, cell membrane, cytoplasm, mitochondria
Cell wall	No	Yes
Chloroplast	No	Yes
Vacuole	No (some have small vacuoles)	Yes, one large vacuole
Shape	No fixed shape	Fixed shape

"Plant cells have a cell membrane, animal cells have a cell wall" is wrong. Both have a cell membrane; only plant cells have a cell wall outside the membrane.

You see a cell under the microscope that has a cell wall and chloroplasts. Is it an animal cell or a plant cell? Give one more feature you would expect to see.

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A plant cell. It would also have a large vacuole and a fixed shape.

Unicellular and multicellular organisms

Unicellular organism ("uni" = one) — made of only one cell. That single cell carries out all the processes of life, such as growth and reproduction.
Multicellular organism ("multi" = many) — made of more than one cell.

Amoeba, Paramecium, Euglena and Chlamydomonas — Examples of unicellular organisms. (Textbook Figure 2.9, p. 53)

Unicellular	Multicellular
Animal-like: Amoeba, Paramecium Plant-like: Chlamydomonas Both: Euglena (makes food with sunlight like a plant and moves like an animal) Also: yeast	Spirogyra, Hydra, Mucor, humans, cats, hibiscus, sunflowers

Mucor, Spirogyra, Hydra, a girl, a cat and a sunflower — Examples of multicellular organisms. (Textbook Figure 2.10, p. 53)

Spirogyra and Mucor are tiny, but they are multicellular (made of many cells joined together). Small does not always mean unicellular.

Types and functions of animal cells

Types of cells in the human body — The types of cells in our bodies. (Textbook Figure 2.11, p. 54)

Cell	Function / special feature
Nerve cell	Long fibres that carry information as impulses to all parts of the body
Epithelium cell	Forms a layer that protects organs; also secretes mucus
Muscle cell	Contracts and relaxes to make movement possible
Red blood cell	No nucleus; biconcave disc shape gives a larger surface area; contains haemoglobin to carry oxygen; also carries carbon dioxide to the lungs
White blood cell	Changes its shape to surround and destroy foreign particles (germs)
Reproductive cells	Sperm carries male genetic material; ovum carries female genetic material

Types and functions of plant cells

Cross-section of a leaf, guard cells and root hair cells — The types of cells in plants. (Textbook Figure 2.12, p. 55)

Cell	Function
Epidermal cell	Reduces water loss; allows gas exchange and absorption of water and nutrients
Palisade cell	Has many chloroplasts with chlorophyll to absorb sunlight for photosynthesis
Guard cell	A pair of guard cells opens and closes the stoma (a small pore). Stomata are usually open in the day and closed at night or on a very hot day to save water.
Root hair cell	Long and thin to increase surface area to absorb more water and nutrients from the soil

Why does a root hair cell have a long, thin "hair"?

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To increase its surface area so it can absorb more water and nutrients from the soil.

Cell organisation: from cell to organism

Cell → tissue → organ → system → organism
Tissue — a group of cells with the same function.
Organ — different tissues working together for a specific function.
System — a few organs with related functions working together.
Organism — all the systems working together to support life.

Cell organisation in an animal and a plant — Cell organisation in animals (epithelium cell → epithelium tissue → stomach → digestive system → human) and plants (epidermal cell → epidermal tissue → leaf → transport system → plant). (Textbook Figure 2.13, p. 56)

Example: why is the stomach an organ?

The stomach is made of several different tissues (muscle tissue, epithelium tissue and nerve tissue) that work together to digest food. One type of tissue alone is not an organ. In the same way, the skin (the largest organ in the body) is an organ, not a tissue.

Systems in the human body

System	Main organs	Function
Digestive	Mouth, oesophagus, stomach, small and large intestines	Breaks down complex food into simpler forms that the body can absorb
Respiratory	Nose, lungs	Takes in oxygen and gives out carbon dioxide
Blood circulatory	Heart, blood vessels, blood	Carries oxygen, nutrients and hormones to all parts of the body
Excretory	Lungs, kidneys, skin	Removes waste products from the body
Nervous	Brain, spinal cord, nerves	Carries information as impulses between the brain and the whole body
Skeletal	Bones, skull	Supports the body and protects inner organs such as the heart and lungs
Muscular	Muscles	Moves the body and the inner organs
Integumentary	Skin	Protects the body from drying out and controls body temperature
Lymphatic	Lymph nodes, lymph vessels	Drains lymph into the blood and protects the body from infection
Endocrine	Pituitary, thyroid, adrenal glands, pancreas	Produces hormones that coordinate reactions in the body
Reproductive	Testes, ovaries	Produces sperm and ova to make offspring

The systems must work together (be coordinated) so the body can carry out all life processes well. We should be thankful to God for the many kinds of organisms around us.

Put these in order from simplest to most complex: kidney, kidney cell, human, excretory system, kidney tissue.

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Kidney cell → kidney tissue → kidney → excretory system → human.

2.2Cell Respiration and Photosynthesis

Respiration

Respiration has two parts:

External respiration (breathing) — exchange of gases between the organism and its surroundings.
Internal respiration (cell respiration) — glucose is broken down (oxidised) inside living cells to release energy.

Cell respiration is the process of breaking down food (glucose) to release energy. It happens in the mitochondria.
Glucose + Oxygen → Carbon dioxide + Water + Energy

The energy released is used for all life processes in the body, such as growth, movement and keeping warm.

Photosynthesis

Photosynthesis is the process in which green plants make their own food (glucose) using light energy. It happens in the chloroplasts.
Carbon dioxide + Water —(light energy, chlorophyll)→ Glucose + Oxygen

The four needs of photosynthesis are light energy, chlorophyll, carbon dioxide and water. The glucose made is stored in the leaf as starch, so finding starch in a leaf shows that photosynthesis has happened.

A tree taking in sunlight, carbon dioxide and water and giving out oxygen — The process of photosynthesis. (Textbook Figure 2.20, p. 64)

Testing a leaf for starch

Steps of the starch test: boiling water, ethanol, hot water, iodine solution — Steps to test a leaf for starch. (Textbook Figure 2.15, p. 60)

Step	What to do	Why
1	Dip the leaf in boiling water	Breaks the cell walls and softens the leaf
2	Put the leaf in a boiling tube of ethanol, standing in a beaker of hot water (about 5 minutes)	Ethanol removes the chlorophyll so the colour change is easy to see
3	Dip the leaf in hot water for a few seconds	Softens the leaf (it becomes brittle in ethanol)
4	Spread the leaf on a white tile and add a few drops of iodine solution	Iodine turns brown → dark blue if starch is present; stays brown if there is no starch

Never heat ethanol over a flame — it is flammable. That is why the boiling tube of ethanol is placed in a beaker of hot water instead.

What does a plant need for photosynthesis? (Experiments 2.1–2.4)

In Experiments 2.1, 2.3 and 2.4, the plants are first kept in the dark for two days to remove (destarch) the starch already in the leaves. Then any starch found must have been made during the experiment. (In Experiment 2.2 the green and non-green parts of the same leaf are compared, so a variegated leaf that has been in sunlight for a few hours is used.)

Experiment	Manipulated variable	Set-up	Result (iodine test)
2.1 Light	Presence of sunlight	One plant in the dark, one under sunlight	Only the leaf in sunlight turns dark blue
2.2 Chlorophyll	Presence of chlorophyll	A variegated leaf (green and non-green parts) kept in sunlight	Only the green part turns dark blue
2.3 Carbon dioxide	Presence of carbon dioxide	Bell jar A has potassium hydroxide solution (absorbs carbon dioxide); bell jar B does not	Only the leaf from B turns dark blue
2.4 Water	Presence of water	Plant A watered every day; plant B not watered	Only the leaf from A turns dark blue

In every experiment the responding variable is the colour change of iodine solution and the constant variable is the type of plant.

Two plants in bell jars; jar A has potassium hydroxide solution — Experiment 2.3: potassium hydroxide in jar A removes carbon dioxide from the air. The glass plates are sealed with vaseline so no air can get in. (Textbook Figure 2.18, p. 62)

In each experiment one set-up has all four needs and the other is missing only one. If the leaf without that one need has no starch, we know the plant needs it.

Cell respiration and photosynthesis compared

Cell respiration	Photosynthesis
Occurs in mitochondria	Occurs in chloroplasts
Releases energy	Absorbs (stores) energy
Uses chemical energy in food	Uses energy from light
Occurs in humans, animals, plants and microorganisms	Occurs in plants and some microorganisms
Breaks down glucose	Makes (synthesises) glucose
Uses glucose and oxygen → carbon dioxide, water and energy	Uses carbon dioxide and water → glucose and oxygen
Occurs at all times, day and night	Occurs only when there is light

Plants respire too! Plants carry out cell respiration all the time, day and night. They only photosynthesise when there is light.

How the two processes complement each other

Cycle showing photosynthesis and respiration complementing each other — Cell respiration and photosynthesis complement each other. (Textbook Figure 2.21, p. 65)

The products of photosynthesis (glucose and oxygen) are the raw materials of cell respiration.
The products of cell respiration (carbon dioxide and water) are the raw materials of photosynthesis.

So animals depend on plants for food and oxygen, and plants use the carbon dioxide that animals give out. This keeps the amounts of oxygen and carbon dioxide in the air balanced.

Which gas does a plant give out (a) during photosynthesis, and (b) during cell respiration?

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(a) Oxygen. (b) Carbon dioxide.

Figures: Science Form 1 (DLP) textbook, Chapter 2, pp. 48–65.

Summary

Remember	Key fact
Cell	The basic unit of life; first seen by Robert Hooke (1665) in cork
Stains	Methylene blue for cheek cells; iodine for onion cells
Plant cell only	Cell wall, chloroplast, large vacuole, fixed shape
In both cells	Nucleus, cell membrane, cytoplasm, mitochondria
Unicellular	One cell: Amoeba, Paramecium, Euglena, Chlamydomonas, yeast
Cell organisation	Cell → tissue → organ → system → organism
Cell respiration	Glucose + oxygen → carbon dioxide + water + energy (mitochondria, all the time)
Photosynthesis	Carbon dioxide + water → glucose + oxygen (needs light and chlorophyll; chloroplasts)
Starch test	Iodine turns brown → dark blue when starch is present