Cell Biology: Structure, Organisation, and Transport Mechanisms

Levels of Organisation & Cell Classification

  • Life Processes (MRS GREN): Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.

  • Hierarchy of Organisation:

    • Organelle: Structure inside a cell carrying out a specific function.

    • Cell: The basic unit of life.

    • Tissue: A group of similar cells working together to perform a particular function.

    • Organ: An aggregation of different tissues working together to perform a specific function.

    • Organ System: A group of organs working together to perform a complex physiological function.

    • Organism: A living individual made up of integrated organ systems.

Levels of organisation in the human circulatory system

  • Cell Types:

    • Eukaryotic Cells: Genetic material is enclosed within a membrane-bound nucleus (e.g., animal cells, plant cells, malarial protist).

    • Prokaryotic Cells: Much smaller cells without a nucleus; genetic material forms a single circular loop of DNA, and small extra rings of DNA called plasmids are often present (e.g., Salmonella bacteria).

    • Common Features: Both cell types contain a cell membrane, cytoplasm, and DNA.

Cell Structures & Organelles

  • Universal Eukaryotic Organelles:

    • Nucleus: Contains genetic material (DNA) and controls cell activities.

    • Cytoplasm: Jelly-like fluid containing enzymes where metabolic chemical reactions take place.

    • Cell Membrane: Selectively permeable barrier that controls the movement of substances into and out of the cell.

    • Mitochondria: Sites of aerobic respiration, releasing energy for cellular processes.

    • Ribosomes: Sites of protein synthesis.

  • Plant-Specific Structures:

    • Cell Wall: Made of rigid cellulose fibers; provides structural support and prevents osmotic bursting.

    • Chloroplasts: Contain the green pigment chlorophyll, which absorbs light energy for photosynthesis.

    • Permanent Vacuole: Filled with cell sap; maintains internal turgor pressure to support plant tissues.

Microscopy & Slide Preparation

Microscope components and adjustment points

  • Standard Operating Technique:

    • Lower the stage before mounting slides.

    • Always begin viewing using the lowest magnification objective lens.

    • Use the coarse focus knob to bring the specimen into view, then sharpen with the fine focus knob.

    • Switch to higher power objective lenses to view cellular details.

  • Specimen Slide Procedures:

    • Cheek Cells (Animal): Scraped with a sterile cotton bud, smeared onto a clean glass slide, stained with dilute methylene blue, and sealed with a coverslip without trapping air bubbles.

    • Onion Epidermis (Plant): A thin transparent peel of inner onion skin is cut and mounted flat in two drops of water, covered using a mounted needle to gently lower the coverslip, and viewed under varying magnifications.

Onion epidermal peel preparation steps

Diffusion & Factors Affecting Exchange

  • Definition: The passive net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient until dynamic equilibrium is reached.

  • Biological Role: Supplies necessary materials (e.g., oxygen, glucose, water) and removes cellular waste products (e.g., carbon dioxide, urea) across the cell membrane without expenditure of metabolic energy.

Concentration gradient and particle net movement across a membrane

  • Factors Influencing Rate of Diffusion:

    • Surface Area to Volume Ratio (SA:VSA:V): Smaller cells or surfaces with folded membranes have a much larger SA:VSA:V, accelerating the rate of diffusion.

    • Concentration Gradient: A steeper difference in particle numbers between two regions increases net movement speed.

    • Diffusion Distance: A shorter diffusion path allows faster substance transfer.

    • Temperature: Higher temperatures increase the kinetic energy of particles, causing faster movement and higher diffusion rates.

Agar cube model demonstrating diffusion penetration depth

Osmosis & Cell Water Balance

  • Definition: The net movement of water molecules from a dilute solution (higher water potential) to a concentrated solution (lower water potential) across a selectively (partially) permeable membrane.

  • Solution Tonicity:

    • Dilute / Hypotonic: High proportion of water molecules, low solute concentration.

    • Isotonic: Equal concentration of solutes inside and outside the cell.

    • Concentrated / Hypertonic: Low proportion of water molecules, high solute concentration.

Red blood cell behavior in hypertonic, isotonic, and hypotonic media

  • Effects on Animal Cells:

    • Hypotonic: Water enters rapidly; lacking a rigid cell wall, the cell swells and bursts (lysis).

    • Hypertonic: Water leaves the cell; cell shrinks and wrinkles (crenation).

  • Effects on Plant Cells:

    • Hypotonic: Water moves in; vacuole expands, creating turgor pressure against the cell wall (turgid state providing tissue support).

    • Hypertonic: Water leaves the cell; cell becomes flaccid, and the cell membrane pulls away from the cell wall (plasmolysis).

Active Transport & Transport Comparison

  • Definition: The movement of substances against a concentration gradient (from low concentration to high concentration) across a semi-permeable membrane using carrier proteins and requiring metabolic energy (ATPATP) released by cellular respiration in mitochondria.

Active transport across a membrane via carrier proteins using ATP

  • Biological Examples:

    • Root Hair Cells: Absorb mineral ions (such as Mg2+\text{Mg}^{2+}) from dilute concentrations in the soil against the higher internal cellular concentration.

    • Small Intestine: Epithelial cells actively absorb glucose into the blood even when intestinal lumen concentrations are lower.

  • Comparison of Cellular Transport Mechanisms:

    • Diffusion: Movement down a concentration gradient; requires no energy; does not strictly require a membrane (e.g., O2O_2 diffusion from alveoli into blood; CO2CO_2 into leaves via stomata).

    • Osmosis: Movement of water down a water potential gradient (dilute to concentrated solution); requires no energy; requires a partially permeable membrane (e.g., water absorption by plant roots; water reabsorption in kidneys).

    • Active Transport: Movement against a concentration gradient; requires energy (ATPATP) from respiration; requires carrier proteins in a membrane (e.g., mineral ion uptake in roots; glucose absorption in the small intestine).

Questions & Discussion

  • Question: What is the correct order of organisation in a multicellular organism?

    • Response: Cell→Tissue→Organ→Organ System→Organism\text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{Organ System} \rightarrow \text{Organism}.

  • Question: Which level of organisation is made from a group of similar cells?

    • Response: Tissue.

  • Question: Why can eukaryotic cells form complex multicellular organisms?

    • Response: They possess a nucleus and can specialize into differentiated cells adapted to work cooperatively.

  • Question: Why does active transport require energy from respiration?

    • Response: Because particles must be transported against (up) the concentration gradient via membrane carrier proteins.

  • Question: What happens to an animal red blood cell in pure distilled water versus concentrated salt solution?

    • Response: In distilled water, water enters down the water potential gradient and the cell bursts (lysis). In concentrated salt solution, water exits the cell by osmosis, causing it to shrink and shrivel (crenation).

  • Question: Why do plants become less wilted after being watered?

    • Response: Water moves into plant cells by osmosis, filling the vacuole and pushing the cell membrane firmly against the cell wall, generating turgor pressure that restores structural rigidity.