Cells: the building blocks of life
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Lesson notes
Big idea: cells as systems
- Big idea. Key concept: Systems. A cell is a tiny system, with many small parts working together to keep the cell alive.
- Related concept: Structure. The shape and make-up of each part suits the job it does.
- Global context: Scientific and technical innovation. We only know about cells because people invented microscopes and kept improving them.
- Cells are the building blocks of life. Every living thing is made of one or more cells, and new cells are made from cells that already exist.
- Most cells are far too small to see without a microscope. A single leaf contains millions of them.
- The small working parts inside a cell are called organelles, which means “little organs”.
Animal cells and their parts
- The nucleus contains the cell’s DNA, its instructions. It controls what the cell does.
- The cell membrane is a thin, flexible layer around the cell. It controls which substances go in and out.
- The cytoplasm is a jelly-like liquid. Many of the cell’s chemical reactions happen here.
- Mitochondria release energy from glucose in a process called respiration. Cells that use a lot of energy, such as muscle cells, have many of them.
- Ribosomes are tiny structures that make proteins.
A typical animal cell
Plant cells: three extra parts
- A plant cell has a nucleus, cell membrane, cytoplasm, mitochondria and ribosomes, just like an animal cell. It also has three extra structures.
- The cell wall is a tough outer layer made of cellulose. It supports the cell and helps the whole plant stay upright. It lies outside the cell membrane.
- Chloroplasts contain green chlorophyll. They absorb light for photosynthesis, which makes glucose. Cells in parts of the plant that get no light, such as roots, usually have no chloroplasts.
- The permanent vacuole is a large space filled with cell sap, which is water with sugars and salts dissolved in it. It keeps the cell firm.
A typical plant cell
Plant cells and animal cells compared
- Both have a nucleus, cell membrane, cytoplasm, mitochondria and ribosomes.
- Only plant cells have a cell wall, chloroplasts and a large permanent vacuole.
- Plant cells usually have a fixed, box-like shape because of the cell wall. Animal cells often have a rounder or more irregular shape.
- Plant cells can make their own food using chloroplasts. Animal cells cannot, so animals must eat.
- A plant cell has a cell wall and a cell membrane. The wall does not replace the membrane.
Plant and animal cells compared
Single-celled organisms
- An organism made of one cell is unicellular. An organism made of many cells is multicellular.
- A unicellular organism must carry out all seven life processes inside its one cell: it moves, respires, senses, grows, reproduces, excretes and feeds.
- Amoeba changes shape and wraps around its food. Paramecium swims by beating tiny hairs called cilia. Euglena has chloroplasts and a whip-like tail called a flagellum.
- Yeast is a single-celled fungus. Bacteria are single cells too, but they are simpler, with no nucleus.
- Multicellular organisms such as humans and oak trees are built from many cells, which do different jobs.
Single-celled and multicellular organisms
Think like a scientist: chloroplasts in leaves and roots
- Question: do leaf cells contain more chloroplasts than root cells?
- The independent variable is the type of cell (leaf or root). The dependent variable is the number of chloroplasts counted in each cell.
- Control variables include using the same microscope and magnification, the same type of plant and equally thin pieces of tissue.
- Count several cells from each part, for example 10, and work out an average. One cell may not be typical.
- A good prediction comes with a reason. Here, leaf cells should have more chloroplasts because leaves receive light.
- Inquiry task: plan your own investigation comparing cells from two parts of a plant. Write your question, name the independent, dependent and control variables, say how many cells you will count, and explain one thing that could make your results unreliable.
Slides
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Practice questions
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1.What is the basic unit of all living things?
Easy- AThe atom
- BThe organ
- CThe cell
- DThe tissue
2.Which part of a cell contains the genetic material, DNA?
Easy- AThe nucleus
- BThe cytoplasm
- CThe cell membrane
- DThe vacuole
3.What does the cell membrane do?
Easy- AIt makes sugar from carbon dioxide and water
- BIt stores the cell’s genetic information
- CIt gives the whole cell a stiff, fixed shape and outline
- DIt controls which substances enter and leave the cell
4.Where do most of a cell’s chemical reactions take place?
Easy- AIn the nucleus
- BIn the cytoplasm
- CIn the cell wall
- DIn the chloroplast
5.Which structure releases energy from glucose during respiration?
Easy- AThe ribosomes
- BThe nucleus
- CThe cell membrane
- DThe mitochondria
6.Which part of a plant cell carries out photosynthesis?
Easy- AThe mitochondrion
- BThe chloroplast
- CThe permanent vacuole
- DThe nucleus
7.Which structure is found in plant cells but not in animal cells?
Easy- AThe cell wall
- BThe nucleus
- CThe cytoplasm
- DThe cell membrane
8.What is the plant cell wall made of?
Easy- AProtein
- BFat
- CCellulose
- DChlorophyll
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