Plant and Animal Tissues
Plant and Animal Tissues
Overview
Living organisms are composed of cells arranged into tissues. A tissue is a group of cells that are similar in structure and work together to perform a specific function. Tissues interact with one another and with the organism's environment; they develop, differentiate and change to maintain life.
Tissues - definition and organisation
Tissues may be simple (made of similar cells) or complex (made of different types of cells). In both plants and animals, tissues combine to form organs that perform specialised functions required for the organism's survival.
Plant tissues
Plant bodies consist of two broad classes of tissue:
- Meristematic (embryonic) tissue - regions of dividing, immature cells that produce new cells for growth.
- Permanent (mature) tissue - cells that have stopped dividing and have become specialised for particular functions.
Meristematic tissue
Locations:
- Apical meristems - at root and shoot tips; responsible for primary growth (increase in length).
- Lateral meristems - in vascular cambium and cork cambium; responsible for secondary growth (increase in thickness or girth).
- Intercalary meristems (in some plants) - located at internodes or leaf bases enabling regrowth.
| Structure | Function |
|---|---|
| Thin-walled, small immature cells | Divide frequently to produce new cells for growth |
| Cells tightly packed, no intercellular spaces | Provide a continuous zone of division |
| Large nucleus, dense cytoplasm, small or absent vacuole | High metabolic activity for cell division and differentiation |
| Some daughter cells remain meristematic; others differentiate | Form permanent tissues (dermal, ground, vascular) |
Permanent tissues
Permanent tissues are classified into three functional groups:
- Dermal tissue - the outer protective layer (epidermis).
- Ground tissue - fills internal spaces; includes parenchyma, collenchyma and sclerenchyma.
- Vascular tissue - transports water, minerals and food; includes xylem and phloem.
Dermal (epidermal) tissue
The epidermis forms a continuous outer layer covering leaves, stems and roots.
| Structure | Function |
|---|---|
| Single layer of tightly packed, thin-walled cells | Protects internal tissues; forms a barrier |
| Cuticle (waxy layer) on aerial parts | Reduces water loss |
| Stomata with guard cells on leaves | Control gaseous exchange and transpiration |
| Root epidermis with root hairs | Increase surface area for water and mineral absorption |
Ground tissues
Ground tissues include parenchyma, chlorenchyma (a chloroplast-containing parenchyma), collenchyma and sclerenchyma.
| Tissue | Structure | Function |
|---|---|---|
| Parenchyma | Many-sided, thin-walled cells, large vacuoles, loosely packed with intercellular spaces | Storage of food and water; allows movement of gases and solutes; forms bulk of soft tissues |
| Chlorenchyma | Parenchyma with chloroplasts; palisade and spongy mesophyll in leaves | Performs photosynthesis; stores starch; facilitates gas exchange in leaf |
| Collenchyma | Cells with cell walls thickened at corners; cells often elongated and closely packed | Provides flexible mechanical support to growing parts (e.g., young stems, petioles) |
| Sclerenchyma | Cells with uniformly thickened, lignified secondary walls; occurs as fibres and sclereids (stone cells) | Provides rigid support and strength; forms hard structures (seed coats, nutshells) |
Vascular tissues: Xylem and Phloem
Xylem and phloem form continuous vascular bundles (veins) in plants and function as transport systems.
| Xylem - structure | Xylem - function |
|---|---|
| Non-living conducting cells: vessel elements and tracheids; also xylem parenchyma and lignified fibres; cell walls contain lignin; patterned secondary thickenings (annular, spiral, pitted) | Transport water and mineral salts (ions) from roots to shoots and leaves; provide mechanical strength and support |
| Phloem - structure | Phloem - function |
|---|---|
| Living conducting elements: sieve tubes (lack nuclei), companion cells (nucleated), phloem parenchyma; sclerenchyma fibres provide support; sieve plates between sieve tube elements | Translocate manufactured organic food (sugars and other solutes) from leaves to other plant parts (sources to sinks) |
Animal tissues
Animal tissues are groups of cells specialised for particular functions. Vertebrates typically have embryonic tissues (stem cells) during development and four principal types of adult permanent tissue: epithelial, muscle, connective and nerve tissue.
Embryonic tissue - stem cells
Embryonic stem cells are present in embryos and can give rise to all cell types (pluripotent). Adult stem cells (also called somatic stem cells) are present in tissues and replace old or damaged cells. Characteristic features include a relatively large nucleus, capacity for self-renewal and the ability to differentiate into specialised cells.
Sources of stem cells include:
- In vitro fertilised embryos (embryonic stem cells).
- Umbilical cord blood and placenta.
- Bone marrow and particular adult tissues (adult stem cells).
Permanent (adult) animal tissues
Epithelial tissue
Epithelia cover body surfaces and line cavities and ducts; they are involved in protection, absorption, secretion and transport.
| Type | Structure | Location / Function |
|---|---|---|
| Simple squamous | Single thin layer of flat cells | Allows rapid diffusion (e.g., alveoli of lungs, lining of blood vessels) |
| Simple cuboidal | Single layer of cube-shaped cells | Secretion and absorption (e.g., kidney tubules, gland ducts) |
| Simple columnar | Single layer of tall cells; may have microvilli or goblet cells | Absorption and secretion (e.g., intestine) |
| Stratified squamous | Multiple layers; outer layers flattened | Protection against abrasion (e.g., skin, oesophagus) |
| Pseudostratified ciliated columnar | Appears layered but all cells contact the basement membrane; often ciliated | Traps and moves particles (e.g., respiratory tract) |
| Transitional | Specialised stratified epithelium that stretches | Found in urinary bladder |
Muscle tissue
Muscle tissue is specialised for contraction. There are three types:
| Type | Structure | Control / Function |
|---|---|---|
| Skeletal (striated) | Long multinucleated fibres with visible striations (myofibrils) | Voluntary movement of bones; locomotion, posture |
| Cardiac | Branched striated cells with single nucleus and specialised intercalated discs | Involuntary rhythmic contraction of heart to pump blood |
| Smooth (unstriated) | Spindle-shaped cells with single central nucleus; no striations | Involuntary control of internal organs (e.g., gut peristalsis, blood vessel diameter) |
Connective tissue
Connective tissues bind, support and protect organs; they generally contain cells embedded in an extracellular matrix that may contain fibres.
| Type | Structure / Components | Function |
|---|---|---|
| Areolar (loose) | Cells (fibroblasts, macrophages) in a loose matrix with collagen and elastic fibres | Supports and cushions organs; binds tissues |
| Adipose | Large fat-filled cells (adipocytes) | Energy storage, insulation, cushioning |
| Fibrous (tendons, ligaments) | Dense collagen fibres | Attach muscle to bone (tendons) or bone to bone (ligaments); resist tension |
| Cartilage | Chondrocytes in a flexible matrix rich in collagen or elastic fibres | Flexible support and shock absorption (e.g., nose, ear, joints) |
| Bone | Osteocytes in a mineralised matrix (calcium salts, collagen) | Rigid support, protection, mineral storage |
| Blood | Plasma matrix with red blood cells, white blood cells and platelets | Transport of gases, nutrients, hormones and wastes; defence; clotting |
Nerve tissue
Nerve tissue detects changes, processes information and coordinates responses. Its functional cell is the neuron.
| Feature | Role |
|---|---|
| Neuron (cell body / soma) | Contains nucleus and organelles; integrates signals |
| Dendrites | Receive incoming signals from other neurons or receptors |
| Axon | Conducts impulses away from the cell body to other neurons, muscles or glands |
| Myelination (Schwann cells in PNS) | Insulates axon and increases conduction speed; gaps called Nodes of Ranvier enable saltatory conduction |
Neurons are classified as sensory (carry impulses from receptors to the central nervous system), motor (carry impulses to effectors) and interneurons (connect neurons within the central nervous system). Neurons can be unipolar, bipolar or multipolar depending on the arrangement of processes.
Indigenous knowledge and biotechnology
Traditional and indigenous knowledge systems have long used plant and animal products for medicinal and other purposes. Modern medical biotechnology builds on such knowledge and on laboratory research to develop diagnostics, drugs and therapies. The practice raises ethical and legal questions that societies regulate through laws and guidelines.
Traditional technology and complementary medicines
Traditional healers include diviners (who diagnose through cultural/spiritual means) and herbalists (who use plant and mineral remedies). Complementary medicines derived from plants-ointments, tonics, teas and extracts-are used alongside modern treatments in many cultures. Many pharmaceutical compounds have natural origins.
Medical biotechnology
Formal research isolates active chemical substances from plants, screens them for therapeutic action, and develops pharmaceuticals. In addition, machines such as electron microscopes and life-support devices aid diagnosis, research and treatment.
Ethics and legislation
Bioethics questions the moral implications of research and treatments involving living organisms. Governments often enact legislation to regulate research practices, the sourcing of biological materials and the application of biotechnology (for example, rules on stem cell research and cloning).
Immunity
Immunity is the ability of the body to resist infection and to remember pathogens so that future responses are faster and stronger. Defence mechanisms include:
- Barriers: skin, mucus, cilia, tears that prevent entry of microbes.
- Innate cellular defence: phagocytic white blood cells (phagocytosis), detoxification and inflammatory responses.
- Adaptive immunity: production of specific antibodies and memory cells after exposure to pathogens.
Natural immunity may be inherited or acquired after an infection. Childhood infectious diseases such as chickenpox and measles commonly produce lasting immunity.
Vaccination
Vaccination artificially induces immunity by introducing a harmless or weakened form of a pathogen (or part of it) so the immune system mounts a response and forms memory cells without causing the full disease. Vaccines exist for many viral and bacterial diseases such as smallpox, polio, measles, mumps, rubella and influenza.
Antibiotics
Antibiotics are substances produced by microorganisms (or synthesised) that inhibit the growth of or kill bacteria. They are effective against bacterial infections but not against viral infections. Antibiotics can be administered orally, as liquids or by injection.
Blood transfusion and blood groups
A blood transfusion replaces lost blood by transfusing compatible donor blood into a recipient. Blood is typed (A, B, AB, O) and cross-matched to avoid immune rejection. Incorrect transfusion may cause a severe immune reaction.
| Recipient's blood type | Safe donor blood types |
|---|---|
| A | A, O |
| B | B, O |
| AB | A, B, AB, O |
| O | O |
Universal donor: type O (can donate to any group). Universal recipient: type AB (can receive from any group), ignoring Rh factor considerations which also matter clinically.
Cloning
Cloning produces genetically identical copies of DNA fragments, cells, tissues or whole organisms. It can be:
- Reproductive cloning - producing identical organisms.
- Molecular cloning - producing identical genes or DNA fragments for research and biotechnology.
Natural examples of cloning include identical (monozygotic) twins. Artificial cloning uses biotechnological methods and asexual propagation techniques.
Plant cloning
Plant cloning has a long history: vegetative propagation (cuttings, bulbs, tubers) produces genetically identical offspring. Modern techniques include tissue culture, where explants, isolated cells (protoplasts) or meristematic tissues are grown in sterile nutrient media to produce many identical plants. Advantages of tissue culture include rapid multiplication, production of disease-free plants and propagation of plants with desirable traits.
Animal cloning
Regeneration (e.g., some earthworms regrowing parts) is a natural form of tissue replacement. Modern animal cloning techniques include somatic cell nuclear transfer, which was used to produce the sheep Dolly. In somatic cell nuclear transfer:
- A donor somatic (body) cell is collected and its nucleus (containing DNA) identified.
- An egg cell (oocyte) from a donor is enucleated (its nucleus removed).
- The donor nucleus is transferred into the enucleated egg (fusion by electric shock or other methods).
- The fused cell is stimulated to divide and develop into an embryo.
- The embryo is implanted into a foster mother and may develop into a cloned offspring.
Cloned tissues are used for therapeutic purposes such as skin grafts, bone marrow transplants and the production of specialised tissues. Research into cloning raises significant ethical and safety questions.
Stem cell research and therapeutic cloning
Stem cell research aims to use the capacity of stem cells to self-renew and differentiate to produce specialised tissues for therapy.
Applications include:
- Treating or managing diseases such as leukaemia, diabetes, Parkinson's disease, Alzheimer's disease, osteoporosis and sickle cell anaemia.
- Replacing damaged tissues and organs (for example, skin grafts and transplants).
- Delivering gene therapies or targeted agents to diseased tissues.
Therapeutic cloning involves creating embryonic stem cells that are genetically compatible with a patient (to reduce immune rejection) and differentiating those cells into required cell types (e.g., pancreatic cells, cardiac muscle, nerve tissue, blood cells) for transplant.
Ethics and legislation concerning stem cells and cloning
Cloning and stem cell research provoke moral, religious and social debate. Many countries have laws and guidelines that specify permissible types of research and sourcing of stem cells, balancing potential medical benefits with ethical concerns.
Summary
Both plants and animals are organised into specialised tissues that perform distinct functions. Understanding tissue types, their structure and function is fundamental to biology and underpins applied fields such as agriculture, medicine and biotechnology. Advances in biotechnology-stem cell research, cloning, tissue culture and medical diagnostics-offer powerful tools for therapy and crop improvement, but also require ethical reflection and appropriate regulation.
