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Biology - Cells and Organisms

Grade 7Cambridge (IGCSE)

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Cell Theory states that all living organisms are made of cells, and the cell is the basic functional unit of life.

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Eukaryotic cells (found in plants and animals) contain membrane-bound organelles such as the nucleus, which stores genetic material in the form of DNADNA.

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Mitochondria are the site of aerobic respiration, where glucose reacts with oxygen to produce energy in the form of ATPATP.

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Plant cells possess unique structures: a cell wall made of cellulose for support, a large permanent vacuole for turgor pressure, and chloroplasts containing chlorophyll for photosynthesis: 6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2.

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Specialised cells are adapted for specific functions; for example, Red Blood Cells lack a nucleus to maximize space for hemoglobin, and Root Hair Cells have a large surface area for the absorption of H2OH_2O and mineral ions.

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The levels of biological organization are: Cell →\rightarrow Tissue →\rightarrow Organ →\rightarrow Organ System →\rightarrow Organism.

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Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient.

📐Formulae

Magnification=Size of ImageActual Size of ObjectMagnification = \frac{\text{Size of Image}}{\text{Actual Size of Object}}

ActualSize=Size of ImageMagnificationActual Size = \frac{\text{Size of Image}}{\text{Magnification}}

1 mm=1000 μm1\text{ mm} = 1000\text{ }\mu\text{m}

💡Examples

Problem 1:

A bacterial cell is observed under a microscope. The image of the cell measures 30 mm30\text{ mm} in length. If the actual size of the cell is 2 μm2\text{ }\mu\text{m}, calculate the magnification used.

Solution:

First, convert units so they are the same: 30 mm=30,000 μm30\text{ mm} = 30,000\text{ }\mu\text{m}. Using the formula M=IAM = \frac{I}{A}, we get M=30,000 μm2 μm=15,000M = \frac{30,000\text{ }\mu\text{m}}{2\text{ }\mu\text{m}} = 15,000.

Explanation:

Magnification is calculated by dividing the image size by the actual size. Units must be consistent (both in mm\text{mm} or both in μm\mu\text{m}) before performing the division.

Problem 2:

Identify the organelle responsible for the following reaction: C6H12O6+6O2→6CO2+6H2O+EnergyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}.

Solution:

The organelle is the Mitochondrion.

Explanation:

The formula represents aerobic respiration, which occurs in the mitochondria of both plant and animal cells to release energy for cellular processes.

Cells and Organisms Grade 7 Notes & Examples