Revision Notes — Cell: Structure and Functions

What is a cell?

A cell is the smallest structural and functional unit of life. Every living organism is made up of one or more cells. In unicellular organisms (like Amoeba), a single cell performs all life functions. In multicellular organisms (like humans and plants), cells are specialised and organised into tissues, organs and systems.

Discovery and cell theory (short)

The invention of the microscope led scientists to discover cells. The modern cell theory states that all living organisms are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. This provides the foundation for understanding life at the microscopic level.

Overview of cell types

Cells are broadly classified into prokaryotic and eukaryotic. Prokaryotic cells (bacteria) lack a true nucleus and membrane-bound organelles. Eukaryotic cells (plant and animal) have a well-defined nucleus and membrane-bound organelles. For Class 8, focus on eukaryotic plant and animal cells and their organelles.

Major cell organelles and their functions

• Cell membrane: A thin layer surrounding the cell, made of lipid and protein. It controls movement of materials in and out of the cell and maintains internal balance.
• Cell wall (plant cells): A rigid outer layer made of cellulose that provides shape, support and protection to plant cells.
• Cytoplasm: Jelly-like substance where organelles are suspended and many metabolic reactions occur.
• Nucleus: Controls the activities of the cell; contains genetic material (DNA). It acts as the control centre, coordinating growth, metabolism and reproduction.
• Mitochondria: Known as the powerhouse of the cell; site of cellular respiration where energy (ATP) is released from food.
• Chloroplasts (plant cells): Contain chlorophyll and carry out photosynthesis — converting light energy into chemical energy (glucose) in plants.
• Vacuole: A storage organelle; plant cells usually have a large central vacuole that stores water, nutrients and waste, helping maintain turgor pressure.
• Ribosomes: Sites of protein synthesis where amino acids are assembled into proteins.
• Endoplasmic reticulum (ER): Network of membranes; rough ER has ribosomes and helps in protein synthesis and transport; smooth ER is involved in lipid synthesis and detoxification.
• Golgi apparatus: Processes, packages and ships materials (like proteins) to different parts of the cell or outside the cell.
• Lysosomes: Vesicles containing digestive enzymes that break down waste material and worn-out organelles (more prominent in animal cells).

Comparing plant and animal cells (quick table)

Key differences to remember:

• Cell wall: Present in plant cells, absent in animal cells.
• Chloroplasts: Present in plant cells (for photosynthesis), absent in animal cells.
• Vacuole: Large central vacuole in plant cells; smaller and more numerous in animal cells.
• Shape: Plant cells usually have a fixed rectangular shape, animal cells are more rounded and irregular.

Specialised cells and adaptation

Different cells are adapted to perform specific functions. Examples include:

• Root hair cells: Have long extensions to increase surface area for water and mineral absorption.
• Muscle cells: Elongated and packed with mitochondria to supply energy for contraction.
• Nerve cells (neurons): Long processes (axons and dendrites) to transmit signals across distances.
• Guard cells (leaf): Control opening/closing of stomata for gas exchange and transpiration.

Microscopes and observing cells

Microscopes magnify tiny objects. Light microscopes are used in school labs to view cell walls, nucleus and large organelles. For slide preparation, follow safety and cleanliness: thin sample, stain if necessary, and place cover slip gently. Practice drawing observations: a neat labelled diagram with magnification and staining details scores marks in exams.

Cell processes (simple explanations)

• Photosynthesis (plants): Process where chloroplasts convert light energy into chemical energy (glucose), releasing oxygen. Basic equation: Carbon dioxide + Water —(light & chlorophyll)—> Glucose + Oxygen.
• Respiration (all cells): Food is broken down in mitochondria to release energy (ATP) used for cellular activities — this is cellular respiration.
• Diffusion: Passive movement of molecules from a region of higher concentration to lower concentration (e.g., oxygen entering cells).
• Transport across membrane (basic): Some substances pass through the cell membrane by simple diffusion, others by specialised transport (not required in-depth at Class 8 but mention selective permeability).

Linking structure to function — short examples

Understanding how structure supports function helps answer higher-mark questions. Examples:

• Mitochondria have many folds (cristae) to increase surface area for respiration enzymes — producing more ATP.
• Cell membrane is semi-permeable to control internal environment, permitting nutrients in and wastes out.
• Large vacuole in plant cells maintains turgidity, keeping plants upright.

Diagrams to practise

Students must practise clear labelled diagrams of:

1. Typical plant cell (label cell wall, membrane, nucleus, chloroplasts, vacuole, mitochondria, ER, Golgi).
2. Typical animal cell (label membrane, nucleus, mitochondria, lysosome, ER, Golgi).
3. Simple diagram of a leaf showing stomata and guard cells (link to gas exchange).

Common exam questions & model answers (brief)

Practical tips for students (exam-focused)

• Make flashcards for organelles and functions — quick recall in MCQs and short questions.
• Always write neat labelled diagrams — one or two well-labelled diagrams can fetch easy marks.
• Use comparison tables (plant vs animal cell) in answers — concise and examiner-friendly.
• Relate structure to function in answers — this shows deeper understanding and helps score in long answers.

Practice Questions — Short & Long

1. Short: What is the function of ribosomes?
2. Short: Name two differences between plant and animal cells.
3. Long: Explain the structure and function of chloroplasts and their importance to plants.
4. Long: Describe how the cell membrane maintains homeostasis in a cell.
5. Application: Suggest a simple classroom activity to observe onion epidermal cells and draw the observed cell with labels.

Suggested answers (brief)

• Ribosomes synthesise proteins by assembling amino acids according to mRNA instructions.
• Plant vs animal differences: cell wall and chloroplasts present in plants; animal cells lack them and often have lysosomes.
• Chloroplasts contain chlorophyll and membranes (thylakoids) to capture light and make food; they are vital for photosynthesis and oxygen production.
• Cell membrane controls entry/exit of substances, enabling nutrient uptake and waste removal, keeping internal conditions stable.
• Activity: Peel thin onion epidermis, place on slide with a drop of water, stain with iodine (if allowed), add cover slip and view under microscope. Draw cells, label nucleus and cell wall.