Structure and Functions in Living Organisms
Cell Structure and Specialisation
Pearson Edexcel International GCSE Biology
Levels of organisation
| Level | Meaning |
|---|---|
| Organelle | a structure inside a cell with its own function |
| Cell | the smallest unit of an organism |
| Tissue | a group of similar cells working together to carry out a function |
| Organ | several different tissues, carrying out a particular function |
| System | a group of organs working together to carry out a function |
Cell structures and functions
| Structure | Animal | Plant | Function |
|---|---|---|---|
| Nucleus | yes | yes | contains DNA; controls the cell |
| Cytoplasm | yes | yes | where chemical reactions occur |
| Cell membrane | yes | yes | controls what enters and leaves the cell |
| Mitochondria | yes | yes | site of respiration |
| Ribosomes | yes | yes | protein synthesis |
| Chloroplasts | yes | absorb light energy to make carbohydrate in photosynthesis | |
| Vacuole | small | large permanent | contains cell sap |
| Cell wall | yes | made of cellulose; provides support |
Plant and animal cells compared
- Both: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes.
- Plant cells only: cellulose cell wall, chloroplasts, a large permanent vacuole.
- Plant cells store carbohydrate as starch; animal cells store glycogen.
Magnification
magnification = image size ÷ actual size
- Measure the image with a ruler, in mm.
- Convert both sizes to the same unit first: 1 mm = 1000 µm; 1 cm = 10 000 µm.
- Actual size = image size ÷ magnification, given with its unit.
- A magnification is written × and a number, with no unit.
Differentiation and stem cells
Differentiation
- Differentiation: unspecialised cells develop into specialised cells, with specific functions, to produce tissues and organs.
- Plant cells can differentiate into all types of cells throughout the life of the plant, and can regenerate a whole new plant.
Stem cells
- Stem cells are undifferentiated cells that divide by mitosis and differentiate into specialised cells.
- Found in the embryo, bone marrow and umbilical cord; adult stem cells are found in some tissues and organs.
- Cells from embryos can make any cell type; adult stem cells can become fewer cell types.
- Most adult tissue cells are specialised: they can only form one cell type, and the tissue has fewer stem cells.
| Advantage of stem cells in medicine | Disadvantage |
|---|---|
| made to differentiate into the cells needed to replace damaged tissue | ethical issues about the use of embryonic cells |
| the patient’s own cells have the same DNA and same antigens: less risk of rejection, and no need to take immunosuppressants | embryonic stem cells can form tumours |
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