Organisms, cells and transport
The characteristics and classification of living things, the structure of cells, and how substances move into and out of cells.
📌 Key points
- The seven characteristics of living organisms are movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.
- Organisms are named by the binomial system: genus then species, e.g. Homo sapiens. A species is a group of organisms that can reproduce to produce fertile offspring. Dichotomous keys are used to identify organisms.
- Animal and plant cells both have a cell membrane, cytoplasm, nucleus, ribosomes and mitochondria. Plant cells also have a cellulose cell wall, chloroplasts and a large permanent vacuole containing cell sap.
- Bacterial cells have a cell wall, cell membrane, cytoplasm, ribosomes, a circular DNA molecule and plasmids, but no nucleus.
- Mitochondria are the site of aerobic respiration; ribosomes are the site of protein synthesis.
- Cells are specialised for their functions, e.g. ciliated cells move mucus, root hair cells absorb water and ions, palisade mesophyll cells photosynthesise, red blood cells carry oxygen. Cells form tissues, tissues form organs, organs form organ systems.
- Magnification = image size ÷ actual size. Both sizes must be in the same units; 1 mm = 1000 µm.
- Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement. It needs no energy from respiration.
- Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
- Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration and carrier proteins.
📖 Key terms
- Species
- A group of organisms that can reproduce to produce fertile offspring.
- Tissue
- A group of cells with similar structures, working together to perform a shared function.
- Organ
- A structure made of a group of tissues, working together to perform specific functions.
- Diffusion
- Net movement of particles from higher to lower concentration, down a concentration gradient.
- Osmosis
- Net movement of water molecules from higher to lower water potential through a partially permeable membrane.
- Active transport
- Movement of particles against a concentration gradient, using energy from respiration.
- Partially permeable membrane
- A membrane that lets some molecules (such as water) through but not others.
✏️ Worked example
A drawing of a cell is 45 mm long. The magnification is ×1500. Calculate the actual length of the cell in µm.
- Rearrange the formula: actual size = image size ÷ magnification.
- Actual size = 45 mm ÷ 1500 = 0.03 mm.
- Convert to µm: 0.03 × 1000 = 30 µm.
- Actual length = 30 µm.
In osmosis answers, use the words "water potential" and "partially permeable membrane", and say that water molecules move, not "the solution" or "sugar".
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