2.1 Cell Structure and Function
Cells are the basic structural and functional units of all living organisms. Plant and animal cells consist of living components called protoplasm (cytoplasm and nucleus) enclosed by membranes.
Structures and Functions of Cellular Components
- Plasma Membrane: Semi-permeable membrane made of phospholipids and proteins. Regulates the movement of substances into and out of the cell.
- Cell Wall: Rigid outer layer found only in plant cells, made of cellulose. Provides shape, support, and mechanical strength.
- Nucleus: Surrounded by a nuclear membrane; contains nucleolus and chromatin (DNA). Controls all cellular activities.
- Cytoplasm: Jelly-like medium containing organelles; acts as the site for metabolic reactions.
- Mitochondrion: Rod-shaped organelle with folded inner membrane (cristae). Site of cellular respiration to generate ATP energy.
- Ribosome: Small dense granules, free in cytoplasm or attached to RER. Site of protein synthesis.
- Endoplasmic Reticulum (ER):
- Rough Endoplasmic Reticulum (RER): Studded with ribosomes; transports proteins synthesized by ribosomes.
- Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids and steroids, performs detoxification.
- Golgi Apparatus: Stack of flattened membrane-bound sacs. Modifies, sorts, packages, and transports proteins/lipids in vesicles.
- Lysosome: Membrane-bound sac containing hydrolytic enzymes (found mostly in animal cells). Digests complex molecules, worn-out organelles, and foreign substances.
- Centriole: Small cylindrical structures made of microtubules (found in animal cells). Forms spindle fibers during cell division.
- Chloroplast: Oval-shaped organelle containing chlorophyll (found in plant cells). Site of photosynthesis.
- Vacuole: Fluid-filled sac surrounded by the tonoplast membrane. Stores cell sap (water, mineral salts, sugars), maintaining cell turgidity in plants.
Comparison Between Animal Cells and Plant Cells
- Plant Cells: Have a fixed shape, cell wall, chloroplasts, and a large central vacuole; lack centrioles.
- Animal Cells: Have no fixed shape, no cell wall, no chloroplasts, and small temporary vacuoles (if any); possess centrioles.
2.2 Living Processes in Unicellular Organisms
Unicellular organisms, such as Amoeba sp. and Paramecium sp., carry out all living processes within a single cell.
- Feeding / Ingestion: Amoeba sp. extends pseudopodia (false feet) to engulf food particles by phagocytosis, forming a food vacuole. Lysosomes release hydrolytic enzymes to digest food.
- Locomotion: Amoeba sp. moves by extending pseudopodia via cytoplasmic streaming. Paramecium sp. moves using rhythmic beating of cilia.
- Osmoregulation: Water enters cells by osmosis. Excess water accumulates in the contractile vacuole, which expands, moves to the cell surface, and contracts to expel water.
- Respiration & Excretion: Exchange of gases ($O_2$ and $CO_2$) and excretion of waste products (e.g., ammonia) occur via simple diffusion across the plasma membrane.
- Reproduction:
- Favorable conditions: Asexual reproduction via binary fission.
- Unfavorable conditions: Amoeba sp. forms cysts; Paramecium sp. undergoes conjugation (sexual reproduction).
2.3 Living Processes in Multicellular Organisms
Cell Specialization in Animals
- Red Blood Cells (Erythrocytes): Biconcave disc shape without a nucleus to optimize oxygen transport via hemoglobin.
- White Blood Cells (Leukocytes): Irregular shape, change form to engulf pathogens (phagocytosis) or produce antibodies.
- Sperm Cells: Long tail (flagellum) for motility; packed with mitochondria in the midpiece to generate energy for swimming.
- Nerve Cells (Neurons): Long elongated fibers to transmit electrical impulses throughout the body.
- Muscle Cells: Contain contractile proteins (actin and myosin) that contract and relax to produce movement.
Cell Specialization in Plants
- Root Hair Cells: Long projection to increase surface area for maximum absorption of water and mineral salts.
- Xylem Vessels: Hollow, continuous tubes lignified on walls to transport water/minerals and provide mechanical support.
- Phloem Sieve Tubes: Columns of sieve tube elements with perforated sieve plates to transport organic substances (sucrose).
- Guard Cells: Bean-shaped cells surrounding stomata; control opening and closing of stomatal pores.
- Palisade Mesophyll Cells: Closely packed vertically with a high density of chloroplasts for maximum light absorption during photosynthesis.
- Spongy Mesophyll Cells: Loosely arranged with large intercellular air spaces for efficient gas exchange.
Levels of Organisation in Multicellular Organisms
$$\text{Cells} \longrightarrow \text{Tissues} \longrightarrow \text{Organs} \longrightarrow \text{Systems} \longrightarrow \text{Multicellular Organism}$$
- Main Animal Tissues: Epithelial tissue, Muscle tissue, Nerve tissue, Connective tissue (blood, bone, cartilage, adipose).
- Main Plant Tissues: Meristematic tissue (apical/lateral) and Permanent tissue (dermal, ground, vascular).
- Major Human Organ Systems: Digestive, Circulatory, Respiratory, Excretory, Skeletal, Muscular, Nervous, Endocrine, Reproductive, Lymphatic, Integumentary systems.
2.4 Orderliness in Cellular Components
The harmonious functioning of cellular components ensures cell survival. Abnormality in cell organelles can lead to diseases (e.g., failure of mitochondria leads to metabolic energy disorders; uncontrolled cell division causes cancer).