Tissues

• A tissue is a cluster of cells with similar structures working together to perform a specific function.

6.1 Plants vs Animals Tissues

• Plants remain stationary, requiring supportive tissue, made mostly of dead cells, for uprightness.
• Animals move in search of resources and thus consume more energy, leading to most of their tissues being living.
• Plant growth is limited to specific regions, containing tissues that divide throughout their lives and are classified as meristematic or permanent.
• Animal cell growth is more uniform, lacking defined dividing and non-dividing regions.
• Complex animals have a more specialized structural organization of organs/systems compared to complex plants.
• Differences in organ system design arise from different modes of life, feeding methods, and adaptations to sedentary (plants) vs. active (animals) lifestyles.

6.2 Plant Tissues

• Plant tissues are divided into meristematic and permanent tissues.

6.2.1 Meristematic Tissue

• Plant growth is restricted to specific regions with dividing meristematic tissue.
• Meristematic tissues are categorized as apical, lateral, and intercalary based on location.
• Apical meristem is at the tips of stems/roots to increase length.
• Lateral meristem (cambium) increases stem/root girth.
• Intercalary meristem, found in some plants, is located near nodes.
• New cells from meristem resemble the meristem itself initially, then differentiate as they mature.
• Meristematic cells are very active and have dense cytoplasm, thin cellulose walls, and prominent nuclei, but lack vacuoles.

6.2.2 Permanent Tissue

• Permanent tissue is formed when cells from meristematic tissue take on a specific role and lose the ability to divide.
• Differentiation is the process of cells taking on a permanent shape, size, and function.
• Differentiation leads to the development of various types of permanent tissues.

6.2.2 (i) Simple Permanent Tissue

• Parenchyma, the most common simple permanent tissue, is made of unspecialized living cells with thin walls.
• Parenchyma cells are loosely arranged, resulting in large intercellular spaces, and generally store food.
• Chlorenchyma is a type of parenchyma that contains chlorophyll and performs photosynthesis.
• Aerenchyma is a type of parenchyma in aquatic plants that contains large air cavities to help them float.
• Collenchyma, another permanent tissue, provides flexibility and mechanical support to plants.
• Collenchyma is found in leaf stalks below the epidermis and has living, elongated, irregularly thickened cells with minimal intercellular space.
• Sclerenchyma makes plants hard and stiff (ex: coconut husk).
• Sclerenchyma consists of dead, long, and narrow cells with walls thickened by lignin, often lacking internal space.
• Sclerenchyma is found in stems, around vascular bundles, in leaf veins, and in seed/nut coverings, providing strength.
• Epidermis is the outermost cell layer.
• Epidermis is usually a single layer of cells.
• Plants in dry habitats may have thicker epidermis.
• Epidermis protects all plant parts.
• Epidermal cells on aerial parts secrete a waxy, water-resistant layer.
• This layer protects from water loss, injury, and parasitic fungi.
• Epidermal tissue forms a continuous layer without intercellular spaces.
• Most epidermal cells are relatively flat with thicker outer and side walls.
• Stomata are small pores in the epidermis of leaves, enclosed by kidney-shaped guard cells.
• Stomata facilitates gas exchange with the atmosphere, and transpiration occurs through stomata.
• Epidermal cells of roots have long hair-like parts that increases water absorption surface area.
• Desert plants have a thick waxy cutin coating on the epidermis which is waterproof.
• Cork, formed from secondary meristem, constitute a protective tissue in older plants.
• Cork cells are dead, compactly arranged without intercellular spaces, and contain suberin.
• Suberin makes cork impervious to gases and water.

6.2.2 (ii) Complex Permanent Tissue

• Simple permanent tissue: made of one type of cells.
• Complex tissue: made of more than one type of cells coordinating to perform a common function.
• Xylem and phloem are examples of complex tissues.
• They are both conducting tissues that constitute a vascular bundle.
• Xylem has tracheids, vessels, xylem parenchyma, & xylem fibres.
• Tracheids and vessels have thick walls, many being dead cells when mature.
• Tracheids and vessels are tubular structures that transports water/minerals vertically.
• The parenchyma stores food, while xylem fibres provide support.
• Phloem has sieve cells, sieve tubes, companion cells, phloem fibres, & phloem parenchyma.
• Sieve tubes are tubular cells with perforated walls.
• Phloem transports food from leaves to other parts of the plant.
• Except for phloem fibres, other phloem cells are living.

6.3 Animal Tissues

• Animal tissues include epithelial, connective, muscular, and nervous tissue.

6.3.1 Epithelial Tissue

• Epithelial tissue covers organs and cavities within the body.
• It forms a barrier to keep body systems separate.
• Epithelial covers the the skin, lining of the mouth, the lining of blood vessels, lung alveoli and kidney tubules.
• Epithelial tissue cells are tightly packed with little cementing material/intercellular spaces.
• Exchange of materials in the body must cross the epthelium at least one layer of epthelium.
• Epithelia is usually separated from the underlying tissue by an extracellular fibrous basement membrane.
• Differing structures of epithelia correlate with unique functions.
• Example: cells lining blood vessels/lung alveoli has simple flat epithelium to transport substances.
• Simple squamous epithelium is thin, flat cells that form a delicate lining.
• Squamous epithelium covers the esoph and the mouth lining.
• Skin (squamous epithelium) is made of many layers to to prevent wear and tear.
• Arranged skin pattern of layers → epithelium is called stratified squamous epithelium.
• Columnar epithelium is present in the inner lining of the intestine, tall epithelial cells are present to provide absorption and secretion.
• Cilia is present in the epithelium of the respiratory tract, to move mucus forward to clear it.
• Cuboidal epithelium cells forms the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support.
• Gland cells, is when epithelial cells acquire further specialisation and secrete substances at the epithelial surface.
• Multicellular gland: a portion of the epithelial tissue folds inward.

6.3.2 Connective Tissue

• Blood is a type of connective tissue.
• Cells of connective tissue are loosely spaced within the intercellular matrix.

6.3.3 Muscular Tissue

• Muscular tissue has elongated cells called muscle fibers and are responsible for movement.
• Muscles contain contractile proteins that contracts to cause movement.
• Muscles can also be called skeletal muscles as they are mostly attached to bones and help in body movement.
• Striated muscles shows striations (light and dark bands).
• Striated muscales includes long, cylindrical, unbranched and multinucleate cells.
• Smooth muscles/involuntary muscles controls movement (e.g. movement of food in the alimentary canal or the contraction and relaxation of blood vessels).
• Smooth muscles are are found in the iris of the eye, in ureters and in the bronchi of the lungs.
• Smooth includes long cells with pointed ends, and has a single nucleus.
• The muscles of the heart show rhythmic contraction and relaxation throughout life → involuntary cardiac muscles.
• Cardiac muscles cells: cylindrical, branched and uninucleate.
• Voluntary muscles are when muscles can be moved by conscious decision.

6.3.4 Nervous Tissue

• Nervous tissue's cells are specialised for being stimulated and then transmitting rapidly.
• Brain, spinal cord and nerves are composed of the nervous tissue, called nerve cells or neurons.
• A neuron has a cell body with a nucleus and cytoplasm.
• Each neuron has a long part, called the axon, and many short, branched parts (processes) called dendrites.
• Nerve fibres: nerve fibres bound together by connective tissue.
• Nerve impulse: signal that passes along the nerve fibre.
• Nerve impulses transmits to the dendrites of the next nerve cell.
• Functional combination of nerve and muscle tissue → animals to move rapidly in response to stimuli.