Basic 9 · Science · Living Cells
Specialised Cells in Plants and Humans
In this topic you will discuss specialised cells and how they are formed in dicotyledonous plants and humans, and examine the functions of specialised plant cells.
By the end of these notes you should be able to:
- Explain the meaning of cell specialisation
- Describe how specialised cells form
- Relate plant cell structure to function
What you already know
- Plant and animal cells
Copy everything that has a blue line on its left into your notebook. The other parts are for reading.
Lesson 1
- Strand
- Diversity Of Matter
- Sub-strand
- Living Cells
- Indicator
- B9.1.2.1.1 – Discuss the concepts of specialized cells and how they are formed in dicotyledonous plants and humans
Introduction
You began as a single cell. Today you have trillions of them: some carry messages, some carry oxygen and some fight disease.
1. Specialisation
Specialised cell: A cell that has developed a particular shape and structure for a particular job.
Single-celled organisms such as Amoeba do everything within one cell. In larger organisms, it would be inefficient for each cell to do all the jobs. So cells are specialised, just as people in a community have different jobs. A nurse, a farmer and a teacher each do different work, and the community benefits from all of them.
2. Levels of Organisation
| Level | Meaning | Example |
|---|---|---|
| cell | basic unit | muscle cell |
| tissue | group of similar cells | muscle tissue |
| organ | group of tissues | heart |
| system | group of organs | circulatory system |
| organism | whole living thing | human being |
3. Examples
| Plant cells | Function | Animal cells | Function |
|---|---|---|---|
| guard cell | controls gas exchange | red blood cell | carries oxygen |
| root hair | absorbs water | nerve cell | carries messages |
| xylem | carries water | muscle cell | contracts |
| phloem | carries food | sperm cell | fertilises egg |
4. Division of Labour
Division of labour means sharing work among specialised parts. It makes an organism more efficient. However, it also means that cells depend on each other. A muscle cell cannot live without oxygen from red blood cells. If one system fails, others suffer. This explains why damage to one organ can make the whole body ill. Plants show the same pattern, with roots, stems and leaves working together.
5. Dicotyledonous Plants
Flowering plants are divided into monocots and dicots. A dicot has two seed leaves, net-veined leaves and a ring of transport tissue in the stem. Beans, cocoa, mango and groundnuts are dicots. Maize and rice are monocots. In this topic, we use dicots as examples, because their tissues are clearly arranged.
New words
- specialised cell
- A cell adapted to a particular job.
- tissue
- A group of similar cells.
- organ
- A group of tissues working together.
- division of labour
- Sharing work among parts.
Summary
- Large organisms have specialised cells.
- Cells form tissues, organs and systems.
- Cells depend upon one another.
Class activity: Level Cards
- Arrange cards from cell to organism.
- Give an example of each.
- Explain the links.
You need: Printed onion-root photomicrograph, Printed blood-cell photomicrograph, Modelling clay
Exercises
Answer in your exercise book first, then check your answers.
1. A group of similar cells is a…
- A. tissue
- B. organ
- C. system
- D. organism
Show answer
Answer: tissueLevel.2. Which is a dicot?
- A. beans
- B. maize
- C. rice
- D. millet
Show answer
Answer: beansTwo seed leaves.3. Red blood cells carry oxygen.
- A. True
- B. False
Show answer
Answer: TrueFunction.4. Sharing work among parts is division of ______.
Show answer
Answer: labourTerm.5. Put in order: organ, cell, system, tissue.
Show answer
Answer: Cell, tissue, organ, system.Levels.6. Why is specialisation useful?
Show answer
Answer: It makes the organism more efficient, as each cell does a particular job well.Function.
Lesson 2
- Strand
- Diversity Of Matter
- Sub-strand
- Living Cells
- Indicator
- B9.1.2.1.1 – Discuss the concepts of specialized cells and how they are formed in dicotyledonous plants and humans
Introduction
How can one fertilised egg produce both bone and brain? The answer lies in cell division and differentiation.
1. Cell Division
Organisms grow by increasing their number of cells. A parent cell divides to form two daughter cells. This process is known as mitosis. Each daughter cell receives a complete copy of the genetic information, or DNA. Mitosis takes place in growth and repair: when you cut your finger, new skin cells form in this way.
2. Differentiation
Differentiation: The process by which a cell becomes specialised.
All your cells carry the same DNA, but each cell uses only part of it. Genes are switched on or off, and this makes a cell become a nerve, muscle or skin cell. Once specialised, most cells cannot change into another type. Stem cells are cells that have not yet become specialised. Doctors are studying stem cells, as they may one day be used to repair damaged tissues.
3. In Humans
| Stage | Description |
|---|---|
| zygote | a fertilised egg |
| embryo | a ball of dividing cells |
| fetus | organs form |
| baby | specialised tissues working together |
4. In Plants
Plants have special growing zones called meristems, found at the tips of roots and shoots. They keep producing new cells throughout the plant's life. Cambium is a thin layer of dividing cells in dicot stems and roots, and it produces new xylem and phloem. This is why trees grow thicker each year. In many trees, growth rings can show the age of a tree.
5. Cuttings and Grafting
Plants can often regrow complete organisms from part of a stem, as meristem cells can produce all the kinds of cell needed. This is why cassava and sugar cane are planted from stem cuttings. Farmers also graft, joining a stem of one plant to the root of another. In Ghana, this is used for mangoes and citrus to produce better fruit more quickly. The Crops Research Institute and others have developed such methods.
6. When Things Go Wrong
Cancer is a disease in which cells divide out of control. Many things can raise the risk, including smoking and exposure to some chemicals. Early screening and treatment can save lives. A healthy lifestyle helps to protect the body.
New words
- mitosis
- Cell division that makes identical cells.
- differentiation
- Cells becoming specialised.
- stem cell
- An unspecialised cell.
- meristem
- A plant's growing zone.
Summary
- Cells multiply by mitosis.
- Differentiation produces specialised cells.
- Plants grow from meristems and cambium.
Class activity: Cuttings
- Plant cuttings of cassava or another plant.
- Observe them for two weeks.
- Record what you see.
You need: Prepared leaf-epidermis slide, Light microscope, Printed guard-cell, Cambium, Xylem chart
Exercises
Answer in your exercise book first, then check your answers.
1. Cell division for growth is…
- A. mitosis
- B. digestion
- C. breathing
- D. photosynthesis
Show answer
Answer: mitosisTerm.2. Cambium produces…
- A. xylem and phloem
- B. flowers only
- C. seeds
- D. roots only
Show answer
Answer: xylem and phloemPlants.3. All body cells carry the same DNA.
- A. True
- B. False
Show answer
Answer: TrueGenetics.4. An unspecialised cell is a ______ cell.
Show answer
Answer: stemTerm.5. What is differentiation?
Show answer
Answer: The process by which a cell becomes specialised.Definition.6. Why can cassava grow from stem cuttings?
Show answer
Answer: The stem has cells that can form new roots and shoots.Application.
Lesson 3
- Strand
- Diversity Of Matter
- Sub-strand
- Living Cells
- Indicator
- B9.1.2.1.2 – Examine the functions of specialized cells in dicotyledonous plants such as epidermal guard cells cambium xylem in relation to the existence of the plants
Introduction
A leaf must breathe, a root must drink and a stem must carry. Each job has its own cell.
1. Plant Cells and Functions
| Cell | Structure | Function |
|---|---|---|
| epidermal cell | flat, tightly packed | protects the plant |
| guard cell | curved pairs around a pore | opens and closes stomata |
| palisade cell | long, many chloroplasts | photosynthesis |
| root hair cell | long extension | absorbs water and minerals |
| xylem vessel | hollow, dead, strong walls | carries water upward |
| phloem | sieve tubes | carries food |
| cambium | thin, dividing | produces new xylem and phloem |
2. Guard Cells
Stomata are tiny pores, mostly on the underside of leaves. Each is surrounded by two guard cells. When water enters the guard cells, they swell and curve, and the pore opens. Carbon dioxide can then enter for photosynthesis, and water vapour leaves. When the plant is short of water, the guard cells become limp and the pore closes, which prevents wilting.
3. Root Hairs
Root hair cells have long, thin extensions which increase the surface area for absorption. Water passes in by osmosis, and minerals are also taken in. This is why you should take care when transplanting seedlings. Damaged root hairs reduce water uptake, and the plant may wilt.
4. Transport Tissues
| Feature | Xylem | Phloem |
|---|---|---|
| carries | water, minerals | food (sugars) |
| direction | upward | up and down |
| cells | dead, hollow | living |
| wall | thickened with lignin | thin |
Xylem vessels are hollow tubes, with strong walls that also support the plant. Wood is mainly xylem. Phloem carries food made in the leaves to other parts. If a ring of bark is removed from a tree, the phloem is cut, and the tree may die, as the roots receive no food. This is called ringing or girdling.
5. Form and Function
The key idea is that structure suits function. A long root hair suits absorbing, and a hollow tube suits carrying water. When you study any cell, ask how its shape helps it do its job. This is one of the most useful ideas in biology.
New words
- stoma
- A tiny pore in a leaf.
- xylem
- Tissue that carries water.
- phloem
- Tissue that carries food.
- osmosis
- Movement of water across a membrane.
Summary
- Plant cells are adapted to their jobs.
- Guard cells control stomata.
- Xylem carries water, and phloem carries food.
Class activity: Leaf Peel
- Peel the skin from a leaf.
- View it with a microscope or lens.
- Draw what you see.
You need: Prepared leaf-epidermis slide, Light microscope, Printed guard-cell, Cambium, Xylem chart
Exercises
Answer in your exercise book first, then check your answers.
1. Water is carried by…
- A. xylem
- B. phloem
- C. stomata
- D. petals
Show answer
Answer: xylemTransport.2. Guard cells control the…
- A. stomata
- B. roots
- C. seeds
- D. flowers
Show answer
Answer: stomataFunction.3. Root hairs increase the surface area.
- A. True
- B. False
Show answer
Answer: TrueAbsorption.4. Phloem carries ______.
Show answer
Answer: foodTransport.5. Name three specialised plant cells.
Show answer
Answer: Guard cell, root hair and palisade cell.Any three.6. Why does ring-barking kill a tree?
Show answer
Answer: The phloem is cut, so the roots get no food.Application.
Homework
- Draw and label a leaf, stem and root, and mention one specialised cell in each.