Cell Biology and Organisation Study Notes

Cellular Components and Functions

• Mitochondrion (Plural: Mitochondria):

  • Shape: Rod-shaped or spherical organelle.
  • Structure: Consists of two layers of membranes: a smooth outer membrane and a highly folded inner membrane.
  • Enzymatic Content: Contains specialized respiratory enzymes that play an essential role in cellular respiration.
  • Function: Serves as the primary site for energy generation via the glucose oxidation process during cellular respiration. Energy is released in the form of $\text{ATP}$ molecules ($\text{adenosine triphosphate}$) to be utilized by cells for metabolic work.

• Centriole:

  • Structure & Appearance: Small cylindrical cellular components existing in pairs within animal cells. Made up of a complex arrangement of microtubules.
  • Occurrence: Absent in plant cells; present exclusively in animal cells.
  • Function: Forms spindle fibres during cell division in animal cells.

• Golgi Apparatus:

  • Structure: Consists of a stack of parallel flattened sacs enclosed by a single membrane. Membrane dynamics involve the continuous addition of new membrane at one end and the budding off of membrane vesicles from the opposite end.
  • Function: Processes, modifies, packages, and transports chemical substances, including proteins, carbohydrates, and glycoproteins (complexes formed by the combination of carbohydrate and protein).

• Plasma Membrane:

  • Structure: The outer membrane enclosing the entire contents of the cell, composed of a phospholipid bilayer and embedded proteins forming a thin, elastic film.
  • Permeability: Partially permeable (selectively permeable), allowing specific molecules to pass through while restricting others.
  • Function:
  • Separates internal cell components from the external environment.
  • Controls the movement of substances entering and exiting the cell.
  • Facilitates the exchange of nutrients, respiratory gases, and metabolic waste products between the cell and its surroundings.

• Lysosome:

  • Structure: Small spherical sac enclosed within a single membrane.
  • Enzymatic Content: Rich in hydrolytic enzymes (such as lysozyme).
  • Function:
  • Hydrolyses complex organic macromolecules, including proteins, nucleic acids, and lipids.
  • Breaks down ingested bacteria and worn-out or damaged cell components.

• Nucleus (Plural: Nuclei):

  • Structure: The largest cellular component; spherical, compressed structure enclosed by a double-layered nuclear membrane containing nuclear pores. Internally composed of chromosomes, the nucleolus, and nucleoplasm.
  • Function:
  • Regulates and controls all cellular activities.
  • Houses chromosomes containing deoxyribonucleic acid ($\text{DNA}$). $\text{DNA}$ carries hereditary genetic codes that determine cell characteristics and metabolic functions.

• Ribosome:

  • Structure: The smallest organelle; small, compact, spherical granules consisting of ribosomal proteins and ribonucleic acid ($\text{RNA}$). Present attached to the membrane surface of the rough endoplasmic reticulum or existing freely in the cytoplasm.
  • Function: Acts as the dedicated site for protein synthesis.

• Endoplasmic Reticulum (ER):

  • Structure: An extensive network of interconnected, folded, flattened sacs whose membrane is continuous with the nuclear membrane.
  • Types:
  • Rough Endoplasmic Reticulum (RER): Characterized by ribosomes attached to its outer surface. Function: Transports proteins synthesised by the attached ribosomes.
  • Smooth Endoplasmic Reticulum (SER): Lacks surface ribosomes. Function: Synthesises and transports lipids and glycerol, and conducts cellular detoxification of drugs, poisons, and metabolic by-products.
  • Overall Function: Serves as an internal transport system within the cell and provides an expanded surface area for enzyme attachment and biochemical reactions.

• Vacuole:

  • Structure: Fluid-filled sac filled with cell sap, surrounded by a specialized membrane termed the tonoplast.
  • Cell Sap Composition: Contains water, organic acids, sugars, amino acids, enzymes, mineral salts, oxygen, carbon dioxide, and metabolic by-products.
  • Occurrence: Young plant cells contain numerous small vacuoles, whereas mature plant cells feature a single large central vacuole. Animal cells contain small, temporary vacuoles (or none).
  • Function: Water absorption into the plant vacuole generates internal turgor pressure, keeping cells turgid and structurally supported. In unicellular organisms, contractile vacuoles contract periodically to perform osmoregulation and excretion.

• Chloroplast:

  • Structure: Oval-shaped organelle bounded by two layers of membrane. Contains chlorophyll pigments localized within internal granum membrane stacks (grana).
  • Function: Chlorophyll absorbs sunlight and converts light energy into chemical energy during photosynthesis.

• Cytoplasm:

  • Structure: A gel-like (jelly-like) fluid matrix that suspends all intracellular organelles. Contains organic compounds (proteins, lipids, carbohydrates) and inorganic ions (such as potassium ions, $\text{K}^+$).
  • Function: Acts as a medium for vital intracellular biochemical reactions.

• Cell Wall:

  • Structure: A rigid, strong outer layer composed of cellulose fibers. Fully permeable to water and solutes.
  • Occurrence: Present in plant cells; absent in animal cells.
  • Function: Maintains a constant, fixed shape for plant cells and provides mechanical support and protection.

Comparison Between Animal and Plant Cells

• Similarities:

  • Both animal and plant cells possess a nucleus, cytoplasm, plasma membrane, Golgi apparatus, mitochondrion, endoplasmic reticulum (rough and smooth), and ribosomes.

• Differences:

  • Plant Cells:
  • Possess a fixed, regular shape.
  • Have an outer cell wall composed of cellulose.
  • Contain chloroplasts with chlorophyll pigments.
  • Mature cells contain a single large central vacuole filled with cell sap.
  • Store energy/carbohydrate in the form of starch.
  • Lack centrioles.
  • Animal Cells:
  • Do not have a fixed shape.
  • Lack a cell wall.
  • Lack chloroplasts.
  • Vacuoles are small or absent.
  • Store energy/carbohydrate in the form of glycogen.
  • Contain centrioles in pairs.

Living Processes in Unicellular Organisms

• Unicellular organisms are composed of a single cell that functions as a complete unit of life. They execute all essential life processes: respiration, movement, nutrition, response to stimuli, growth, excretion, and reproduction.

• Protozoans represent the simplest form of unicellular organisms (e.g., Amoeba sp. and Paramecium sp.).

• Movement:

  • Amoeba sp.: Continuously changes shape upon encountering obstacles. Moves by extending cytoplasmic projections known as pseudopodia (false feet), followed by cytoplasmic streaming into the extended pseudopodium.
  • Paramecium sp.: Moves rapidly through aquatic environments using synchronized, rhythmic beats of cilia.

• Responding to Stimuli:

  • Both Amoeba sp. and Paramecium sp. respond to external stimuli (chemical changes, physical contact, or bright light) by moving away from adverse stimuli.

• Respiration:

  • Exchange of respiratory gases (oxygen, $\text{O}_2$, and carbon dioxide, $\text{CO}_2$) occurs directly across the plasma membrane via simple diffusion across the cell surface.

• Nutrition:

  1. Ingestion: Amoeba sp. moves toward food particles and extends pseudopodia to trap them via phagocytosis, forming a food vacuole. Paramecium sp. uses cilia movement to direct food particles into its oral groove.
  2. Digestion: The formed food vacuole fuses with a lysosome. Hydrolytic lysozyme enzymes digest and hydrolyse food particles into soluble nutrients.
  3. Absorption: Digested nutrients diffuse directly into the cytoplasm.
  4. Egestion: Undigested waste in Amoeba sp. is left behind and discharged as the organism moves away. In Paramecium sp., undigested material is expelled through a specialized anal opening (anus).

• Growth:

  • Amoeba sp. and Paramecium sp. achieve cellular growth by synthesising new cytoplasm.

• Excretion and Osmoregulation:

  • Waste products such as carbon dioxide ($\text{CO}_2$) and ammonia ($\text{NH}_3$) are excreted by diffusion across the plasma membrane.
  • Freshwater environments are hypotonic to cytoplasm, causing water to diffuse into cells via osmosis.
  • Water accumulates inside the contractile vacuole. When the contractile vacuole swells to its maximum volume, it contracts periodically to expel water from the cell, maintaining fluid balance via osmoregulation.

• Reproduction:

  • Favourable Conditions (Abundant food, optimal environment): Reproduce asexually through binary fission via mitotic nuclear and cellular division.
  • Unfavourable Conditions (Drought, extreme low temperatures, food scarcity):
  • Amoeba sp. forms protective spores that remain dormant and germinate only when environmental conditions improve.
  • Paramecium sp. undergoes sexual reproduction through conjugation.

Living Processes in Multicellular Organisms: Specialised Cells

• Cells in multicellular organisms undergo structural differentiation and specialization to correlate directly with specific functions.

• Specialised Human Cells:

  • Muscle Cell: Arranged as elongated, multinucleated striated fibers. Function: Contracts and relaxes to generate mechanical movement.
  • Nerve Cell (Neuron): Possesses a long, thin, branched structure with dendrites and an axon. Function: Transmits nerve impulses (electrical signals) throughout the nervous system.
  • White Blood Cell: Has an flexible, variable shape that can change. Function: Destroys invading pathogens through phagocytosis and antibody production.
  • Red Blood Cell: Shaped as a biconcave disc devoid of a nucleus. Function: Optimizes surface area and hemoglobin volume for efficient oxygen transport.
  • Epithelial Cell: Thin, flattened cells arranged in continuous layers. Function: Coats outer and inner surfaces of organs such as the digestive tract.
  • Sperm Cell: Equipped with a long flagellum tail to enable swimming toward the ovum in the Fallopian tube. Head contains a haploid nucleus carrying male chromosomes.

• Specialised Plant Cells:
  • Sieve Tube Element: Long cylindrical tube structures arranged end-to-end. Function: Transports organic nutrient materials (phloem sap such as sucrose) from photosynthetic leaves to storage organs (e.g., fruits).
  • Xylem Vessel: Continuous, elongated, hollow conducting tubes with lignified walls. Function: Transports water and inorganic mineral salts from roots to all upper plant structures.
  • Palisade Mesophyll Cell: Long cylindrical cells arranged vertically and tightly packed beneath the upper leaf epidermis. Contains a high density of chloroplasts to maximize sunlight absorption for photosynthesis.
  • Spongy Mesophyll Cell: Irregularly shaped, loosely arranged cells with large intercellular air spaces. Function: Facilitates internal gas exchange between the interior leaf environment and palisade cells.
  • Guard Cell: Modified lower epidermal cells featuring thicker inner cell walls surrounding a stomatal pore. Function: Controls the opening and closing of stomata for exchange of oxygen ($\text{O}_2$) and carbon dioxide ($\text{CO}_2$).
  • Root Hair Cell: Features an elongated tubular projection that significantly expands the cell surface area for the absorption of water and mineral salts from soil.

Tissue Organisation in Humans

• Definition of Tissue: A group of similar cells possessing identical structures that are organized together to perform a specific function.

• Human tissues are categorized into four major types: epithelial tissue, muscle tissue, nerve tissue, and connective tissue.

• 1. Epithelial Tissue:

  • Covers external body surfaces (skin) and lines internal hollow organs, tracts, and cavities (digestive tract, respiratory tract, kidney ducts, blood vessels).
  • Skin Epithelium: Protects against physical injuries, pathogen infections, chemical exposure, and dehydration.
  • Tracheal and Bronchial Epithelium: Contains microscopic hair-like projections called cilia (singular: cilium) that sweep mucus and trapped particles.
  • Intestinal Epithelium: Covers the internal surface of the small intestine to absorb digested nutrients.
  • Tubular Epithelium: Lines exocrine and endocrine glands, kidney tubules, and ducts.

• 2. Muscle Tissue:

  • Smooth Muscle: Non-striated, spindle-shaped cells located in walls of the digestive tract, blood vessels, urinary tract, and reproductive tract. Executes involuntary contractions, driving activities like peristalsis.
  • Skeletal Muscle: Striated fibers attached to bones and limbs. Executes voluntary, controlled contraction and relaxation to generate body locomotion and limb movement.
  • Cardiac Muscle: Interconnected, striated muscle fibers constructing the wall of the heart. Executes involuntary rhythmic contractions to pump blood throughout the entire body.

• 3. Nerve Tissue:

  • Composed of neurons (nerve cells). Each neuron consists of a cell body, dendrites, and an axon.
  • Functions by detecting external and internal stimuli, processing signals, and transmitting electrical nerve impulses to effectors (muscles and glands), thereby regulating and coordinating body activities.

• 4. Connective Tissue:
  • Distributed throughout the body to bind, support, and link organs and tissues together.
  • Loose Connective Tissue: Binds epithelial tissue to underlying tissues and anchors internal organs in position.
  • Fibrous Connective Tissue: Dense collagenous fibers forming tendons (connecting muscles to bones) and ligaments (connecting bones to bones).
  • Blood Tissue: Composed of blood plasma, red blood cells (erythrocytes), and white blood cells (leukocytes). Functions in homeostatic regulation, transport of materials, and body defense.
  • Bone Tissue: Rigid mineralized connective tissue forming the internal skeleton, providing structural support and organ protection.
  • Adipose Tissue: Specialized tissue storing fat droplets beneath the dermis of the skin and around primary visceral organs for insulation, energy storage, and cushioning.
  • Cartilage Tissue: Flexible, tough tissue capping bone tips at joint articulation sites to prevent bone friction and wear.

Tissue Organisation in Plants

• Plant tissues are classified into Meristematic Tissue and Permanent Tissue.

• 1. Meristematic Tissue:

  • Composed of young, actively dividing undifferentiated cells.
  • Apical Meristem Tissue: Located at root tips and shoot tips, responsible for primary growth (lengthening).
  • Lateral Meristem Tissue: Located along lateral plant axes (cambium), responsible for secondary growth (girth expansion).

• 2. Permanent Tissue:

  • Differentiated tissues that have completed specialization.
  • Dermal Tissue: Outer protective cell layer covering stems, leaves, and roots.
  • Ground Tissue:
  • Parenchyma Tissue: Thin-walled cells that function to store starch, protein, and water, and carry out photosynthesis.
  • Collenchyma Tissue: Cells with unevenly thickened walls (cellulose and pectin); provides flexible mechanical support to young, non-woody stems (herbaceous plants).
  • Sclerenchyma Tissue: Thickened, heavily lignified secondary cell walls (dead at maturity); provides structural support and mechanical strength to mature plant parts.
  • Vascular Tissue:
  • Xylem Tissue: Transports water and dissolved mineral salts from roots up to leaves. Lignified walls provide rigid mechanical support.
  • Phloem Tissue: Transports organic nutrients (sucrose) from leaves to all non-photosynthetic parts and storage sinks.

Density of Cell Components and Specialised Cell Functions

• The intracellular density of specific organelles correlates directly with the specialized functional demands of particular cell types.

• Relationship Between Organelle Density and Cellular Function:

  • Sperm Cell: High density of Mitochondria. Function: Supplies large amounts of $\text{ATP}$ energy required for flagellar propulsion through the uterus and Fallopian tube to fertilize secondary oocytes.
  • Flight Muscle Cells (Insects and Birds): High density of Mitochondria. Function: Generates abundant $\text{ATP}$ energy required for rapid contraction and relaxation cycles during flight.
  • Plant Meristem Cell: High density of Mitochondria. Function: Provides cellular energy needed to drive rapid mitotic cell division.
  • Palisade Mesophyll Cell & Spongy Mesophyll Cell: High density of Chloroplasts. Function: Maximizes solar light absorption to drive photosynthesis.
  • Pancreatic Cell: High density of Rough Endoplasmic Reticulum (RER) and Golgi Apparatus. Function: Enhances synthesis, modification, packaging, and secretion of digestive enzymes.
  • Goblet Cell (Intestinal and Respiratory Epithelium): High density of Rough Endoplasmic Reticulum (RER) and Golgi Apparatus. Function: Synthesises and secretes mucus for protection and lubrication.
  • Liver Cell (Hepatocyte): High density of Smooth Endoplasmic Reticulum (SER) and Golgi Apparatus. Function: Metabolises carbohydrates and executes enzymatic detoxification of drugs, alcohol, and metabolic poisons.

Effects of Cell Component Dysfunction and Absence

• Normal cellular functioning requires precise, coordinated interaction among all internal components, operating like an efficient factory.

• Mitochondrial Dysfunction or Disjunction:

  • Impairs cellular respiration and energy production.
  • Manifests clinically as stunted growth, muscular weakness, hearing deficits, and visual impairment.

• Lysosomal Enzyme Deficiency (Tay-Sachs Disease):

  • A hereditary genetic disorder caused by the failure to synthesize specific lysosomal hydrolytic enzymes.
  • Results in toxic accumulation of undigested lipid substances within brain cells, causing progressive mental retardation, severe physical decline, and stunted growth.

Levels of Organisation in Multicellular Organisms

• Multicellular organisms maintain an organized structural hierarchy:   $\text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{System} \rightarrow \text{Multicellular Organism}$

• Organ Structure Example: The Heart

  • Composed of multiple tissue types working synergistically:
  • Epithelial Tissue: Covers outer surfaces and lines internal heart chambers.
  • Cardiac Muscle Tissue: Formulates thick muscular walls that contract rhythmically to pump blood.
  • Connective Tissue (Blood): Connects cardiovascular structures and transports oxygen, nutrients, and waste.
  • Nerve Tissue: Transmits electrical signals to regulate and coordinate heartbeat rhythm.

• 11 Main Human Organ Systems:
  1. Respiratory System: Comprises trachea, nose, lungs, and diaphragm. Function: Facilitates gaseous exchange of oxygen and carbon dioxide between the body and external environment.
  2. Muscular System: Comprises skeletal, smooth, and cardiac muscles. Function: Contracts and relaxes to generate movement across body parts.
  3. Male Reproductive System: Comprises testes, prostate gland, and penis. Function: Produces sperm cells and male sex hormones (testosterone).
  4. Female Reproductive System: Comprises ovaries, Fallopian tubes, uterus, vagina, and cervix. Function: Produces ova and female sex hormones (estrogen, progesterone).
  5. Lymphatic System: Comprises spleen, lymph nodes, and lymphatic vessels. Function: Maintains bodily fluid balance and protects against infectious diseases.
  6. Endocrine System: Comprises endocrine glands secreting chemical hormones. Function: Coordinates physiological body processes alongside the nervous system.
  7. Nervous System: Comprises brain, spinal cord, and peripheral nerves. Function: Detects environmental stimuli, transmits sensory/motor information, and coordinates bodily responses.
  8. Blood Circulatory System: Comprises heart, arteries, veins, and capillaries. Function: Transports oxygen, nutrients, hormones, and waste products.
  9. Digestive System: Comprises mouth, oesophagus, stomach, liver, pancreas, small intestine, and large intestine. Function: Digests food into absorbable nutrients.
  10. Urinary System: Comprises kidneys, ureters, bladder, and urethra. Function: Eliminates metabolic wastes such as urea and uric acid.
  11. Skeletal System: Comprises bones, cartilage, ligaments, and tendons. Function: Supports body structure, protects internal organs, and provides leverage for muscle action.
  12. Integumentary System: Comprises skin. Function: Protects against physical trauma, chemical damage, infection, and dehydration.

• Plant Organ Systems:
  • Shoot System: Comprises stems, shoots, leaves, flowers, and fruits.
  • Stems and twigs hold leaves vertically to maximize sunlight absorption during photosynthesis.
  • Flowers mediate pollination and reproduction.
  • Root System: Comprises all primary, lateral, and fibrous roots. Functions to absorb water and mineral salts while anchoring the plant in soil.

Formative Practice Questions and Concepts

• Formative Practice 2.1:

  • Question 1: State one structural difference between the rough endoplasmic reticulum and the smooth endoplasmic reticulum.
  • Answer: The rough endoplasmic reticulum possesses ribosomes attached to its outer surface, whereas the smooth endoplasmic reticulum lacks ribosomes.
  • Question 2: Why is the use of an electron microscope important in studying cells?
  • Answer: An electron microscope provides significantly higher magnification and resolving power compared to a light microscope, allowing detailed observation of cellular ultrastructures and fine organelles such as ribosomes, ER, and mitochondrial cristae.
  • Question 3: Chloroplast is found in some plant cells only. Give one example of a plant cell that does not have chloroplast and give your reasons.
  • Answer: Root hair cells (or epidermal cells of underground roots/onion bulb scale cells) lack chloroplasts because they are located underground or unexposed to sunlight, making photosynthesis impossible in those tissues.
  • Question 4: Compare and contrast animal cells and plant cells.
  • Answer: Both cell types contain a nucleus, cytoplasm, plasma membrane, Golgi apparatus, mitochondria, endoplasmic reticulum, and ribosomes. Plant cells have a fixed shape, cell wall, chloroplasts, and a large central vacuole, but lack centrioles; animal cells lack a fixed shape, cell wall, chloroplasts, and large vacuoles, but possess centrioles and store energy as glycogen.

• Formative Practice 2.2:

  • Question 1: What is the function of the contractile vacuole in a Paramecium sp.?
  • Answer: It collects excess osmotic water that enters the cell and contracts periodically to pump water out, executing osmoregulation to prevent cell lysis.
  • Question 2: How does the Amoeba sp. move?
  • Answer: By extending a pseudopodium (false foot) forward and flowing cytoplasm into the extended pseudopodium.
  • Question 3: How does the Amoeba sp. reproduce when the environment is unfavourable?
  • Answer: It forms protective spores that remain dormant until favorable environmental conditions return.
  • Question 4: Predict what will happen to the contractile vacuole if the Paramecium sp. is placed in a concentrated salt solution.
  • Answer: In a concentrated (hypertonic) salt solution, water leaves the Paramecium sp. via osmosis. Consequently, less water accumulates internally, causing the contractile vacuole to contract less frequently or cease functioning.

• Formative Practice 2.3:

  • Question 1: State the components of cells found in high density in cells that secrete protein products.
  • Answer: Rough endoplasmic reticulum, ribosomes, Golgi apparatus, and mitochondria.
  • Question 2: Briefly describe the structure and function of the nerve tissue.
  • Answer: Nerve tissue consists of neurons composed of a cell body, dendrites, and an axon. It functions to detect stimuli and transmit electrical nerve impulses to coordinate body activities.
  • Question 3: Explain why the palisade mesophyll cell contains a lot of chloroplast.
  • Answer: Palisade mesophyll cells are situated directly under the upper leaf surface to receive maximum solar illumination; having a high density of chloroplasts maximizes sunlight capture for photosynthesis.
  • Question 4: Fat cells have three adaptive characteristics to allow the storage of fat. Describe the three characteristics.
  • Answer: Presence of a large fat droplet occupying cytoplasm volume, displacement of nucleus and thin cytoplasm layer to the cell boundary, and low organelle density relative to lipid storage volume.

• Formative Practice 2.4:

  • Question 1: State arrangement of sequence in the organisation of cells in a multicellular organism.
  • Answer: $\text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{System} \rightarrow \text{Multicellular Organism}$.
  • Question 2: The skin is the largest organ in the body. Why is skin classified as an organ?
  • Answer: Skin is classified as an organ because it is constructed from different functional tissue types—including epithelial tissue, connective tissue, nerve tissue, and smooth muscle tissue—working together to perform protective and homeostatic functions.
  • Question 3: Stems, twigs and flowers are organs in a plant. Describe how stems, twigs and flowers function in the shoot system.
  • Answer: Stems and twigs provide physical support to maintain leaves in an upright position for maximum sunlight exposure while acting as conduits for vascular transport. Flowers act as reproductive organs mediating pollination and seed formation.
  • Question 4: Predict what will happen if the organisation level of cells does not exist in an organism.
  • Answer: Individual cells would be incapable of coordinating specialized physiological tasks, preventing the formation of functional organs and systems needed to sustain life in large multicellular organisms.