1.1 Cell structure and function
All living things are made of cells. A cell is the basic unit of life.
Animal and plant cells
| Part | Found in | Function |
|---|---|---|
| cell membrane | animal, plant and bacterial cells | controls what enters and leaves the cell |
| cytoplasm | all cells | where most chemical reactions happen; a jelly-like liquid |
| nucleus | animal and plant cells | contains the DNA (genetic material) and controls the cell's activities |
| ribosomes | all cells | make proteins (protein synthesis) |
| mitochondria | animal and plant cells | release energy from glucose by aerobic respiration |
| cell wall | plant and bacterial cells | made of cellulose in plants; a strong layer that supports the cell and keeps its shape |
| chloroplasts | plant cells (the green parts) | contain chlorophyll; carry out photosynthesis |
| sap vacuole | plant cells | a large space filled with cell sap (a solution of sugars and salts); keeps the cell firm |
Differences between plant and animal cells. A plant cell has a cellulose cell wall, chloroplasts and a large sap vacuole. An animal cell has none of these, and is usually rounder and smaller. Both have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes.
Bacterial cells
A bacterium is a single cell with a simpler structure. It has:
- a cell wall (not made of cellulose) and a cell membrane,
- cytoplasm containing ribosomes,
- circular DNA lying free in the cytoplasm (there is no nucleus),
- plasmids: small rings of DNA that carry extra genes.
Using a microscope
Cells can be examined using a light microscope and a temporary stain that makes the parts easier to see.
- Animal cells (for example cheek cells scraped from the inside of the mouth) are stained with methylene blue.
- Plant cells (for example a thin layer of onion epidermis peeled from a bulb) are stained with iodine solution.
Method: place the thin sample on a slide, add a drop of stain, lower a cover slip at an angle (to avoid trapping air bubbles) and look under low power first. When you draw what you see: use a sharp pencil, draw clear single lines, make the drawing large, do not shade, and label the parts with straight, ruled label lines.
1.2 Specialised cells, tissues and organs
| Cell | Adaptation | Function |
|---|---|---|
| root hair cell | a long, thin extension that increases the surface area | absorbs water and ions from the soil |
| palisade mesophyll cell | many chloroplasts, packed at the top of the leaf | photosynthesis |
| xylem vessel | no cell contents, thick walls strengthened with lignin | carries water and supports the plant |
| red blood cell | no nucleus, biconcave disc shape, contains haemoglobin | carries oxygen |
| sperm cell | a tail (flagellum), many mitochondria, enzymes in the acrosome | swims to the egg and fertilises it |
| egg cell | large, with food stores and a jelly coat | gets fertilised, and provides nutrients for the early embryo |
| ciliated cell | hair-like cilia on the surface | moves mucus along the airways |
Levels of organisation
Example (the digestive system). Muscle cells and gland cells (cells) form muscle tissue and glandular tissue (tissues). Together with other tissues they form the stomach, an organ. The stomach, small intestine, liver and pancreas are part of the digestive system (an organ system), and all the organ systems together make the organism.
Example (the leaf). Palisade cells form palisade tissue, and with xylem, phloem and epidermis they form the leaf (an organ).
Magnification
Use the same units for both sizes. Rearranged: and .
(.)
Practise this chapter
Every past-paper question on these topics, with the mark scheme.