Cell Structure and Organisation

Cell Structure

Learning Objectives

  • Describe and compare the structure of plant and animal cells, including:

    • Cell wall

    • Cell membrane

    • Nucleus

    • Cytoplasm

    • Chloroplasts

    • Ribosomes

    • Mitochondria

    • Vacuoles

  • Describe the structure of a bacterial cell, including:

    • Cell wall

    • Cell membrane

    • Cytoplasm

    • Ribosomes

    • Circular DNA

    • Plasmids

  • Identify cell structures in diagrams and images of plant, animal, and bacterial cells.

  • State that specialized cells have specific functions, including:

    • Ciliated cells: movement of mucus in the trachea and bronchi.

    • Root hair cells: absorption.

    • Palisade mesophyll cells: photosynthesis.

    • Neurons: conduction of electrical impulses.

    • Red blood cells: transport of oxygen.

    • Sperm and egg cells (gametes): reproduction.

Animal Cells

  • Main features:

    • Contain a nucleus with a distinct membrane.

    • Lack cellulose cell walls.

    • Lack chloroplasts (cannot perform photosynthesis).

    • Store carbohydrates as glycogen.

Plant Cells

  • Main features:

    • Contain a nucleus with a distinct membrane.

    • Have cell walls made of cellulose.

    • Contain chloroplasts (can perform photosynthesis).

    • Store carbohydrates as starch or sucrose.

Structures and Functions

Nucleus

  • Function: Contains DNA (genetic material) that controls cell activities.

Cytoplasm

  • Function:

    • Gel-like substance composed of water and dissolved solutes.

    • Supports internal cell structures.

    • Site of many chemical reactions, including anaerobic respiration.

Cell Membrane

  • Function:

    • Holds the cell together, separating the inside from the outside.

    • Controls which substances enter or leave the cell.

Ribosomes

  • Function: Site of protein synthesis.

Mitochondria

  • Function: Site of aerobic respiration.

Cell Wall (Plant Cells)

  • Made of cellulose (a polymer of glucose).

  • Function: Provides extra support and defines cell shape.

Chloroplast (Plant Cells)

  • Contains chlorophyll, a green pigment that absorbs light energy for photosynthesis.

Permanent Vacuole (Plant Cells)

  • Contains cell sap, a solution of sugar and salt.

  • Function: Storage of materials and support of cell shape.

Bacterial Cells

  • Characteristics:

    • Microscopic, single-celled organisms.

    • Possess a cell wall (made of peptidoglycan, not cellulose), cell membrane, cytoplasm, and ribosomes.

    • Lack a nucleus but contain a circular chromosome of DNA floating in the cytoplasm.

    • Plasmids (small rings of DNA) may be present, containing extra genes.

    • Lack mitochondria, chloroplasts, and other membrane-bound organelles.

Flagella

  • Some bacteria have flagella (singular) or flagella (plural), which are long, thin, whip-like tails for movement.

Examples of Bacteria

  • Lactobacillus: A rod-shaped bacterium used in yoghurt production.

  • Pneumococcus: Spherical bacterium causing pneumonia.

Identifying Cell Structures

  • Using electron microscopes, organelles visible in most cells (except prokaryotes) include:

    • Mitochondria

    • Ribosomes (free or attached to the endoplasmic reticulum)

    • Rough Endoplasmic Reticulum (R.E.R.): Endoplasmic reticulum studded with ribosomes.

    • Vesicles: Small circular structures moving throughout the cytoplasm.

Specialised Cells

  • Cells with specific functions due to developed characteristics.

  • Differentiation: The process by which cells develop the structure and characteristics needed to perform their functions.

Red Blood Cells

  • Function: Transport of oxygen.

  • Adaptations:

    • Biconcave disc shape increases surface area for efficient oxygen diffusion.

    • Contains haemoglobin, which binds to oxygen.

    • Lacks a nucleus to maximize space for hemoglobin.

Sperm Cells

  • Function: Reproduction.

  • Adaptations:

    • Head contains genetic material in a haploid nucleus.

    • Acrosome in the head contains digestive enzymes to penetrate an egg.

    • Mid-piece is packed with mitochondria for energy to swim.

    • Tail enables swimming.

Ciliated Cells

  • Function: Movement of mucus in the trachea and bronchi.

  • Adaptations:

    • Cilia (hair-like structures) beat to move mucus and trapped particles up to the throat.

Nerve Cells (Neurons)

  • Function: Conduction of impulses.

  • Adaptations:

    • Long to run to and from different body parts to the central nervous system.

    • Extensions and branches to communicate with other nerve cells, muscles, and glands.

    • Axon (extension of cytoplasm) is covered with a fatty sheath for insulation and speed.

Egg Cell (Ovum)

  • Function: Reproduction.

  • Adaptations:

    • Contains a lot of cytoplasm with nutrients for early embryo growth.

    • Haploid nucleus contains genetic material.

    • Cell membrane changes after fertilization to prevent more sperm from entering.

Root Hair Cells

  • Function: Absorption of water and mineral ions from soil.

  • Adaptations:

    • Root hair increases surface area for maximum absorption.

    • Thin walls ensure quick water movement.

    • No chloroplasts present.

Xylem Vessels


  • Function:

    • Conduction of water through the plant.

    • Support of the plant.

  • Adaptations:

    • No top and bottom walls for a continuous water column.

    • Cells are dead without organelles or cytoplasm to allow free passage of water.

    • Walls thickened with lignin for support.

Palisade Mesophyll Cells

  • Function: Photosynthesis.

  • Adaptations:

    • Column-shaped to maximize sunlight absorption.

    • Many chloroplasts for maximum photosynthesis.

Levels of Organisation in an Organism

  • Cells: Basic functional and structural units.

  • Tissues: Groups of similar cells performing the same function.

  • Organs: Made from different tissues working together for specific functions.

  • Organ Systems: Groups of organs with related functions working together.