2.3 Living Processes in Multicellular Organisms
In multicellular organisms, cells undergo cell specialisation and differentiation to perform specific functions efficiently. Cells vary in size, shape, and internal structure to correlate directly with their specialised roles.
Learning Objectives
- Describe cell specialisation in humans and plants and relate cell structure to its specific function.
- Identify and classify the main types of tissues in humans: epithelial tissue, muscle tissue, nerve tissue, and connective tissue.
- Classify and state the characteristics and functions of plant tissues: meristematic tissues and permanent tissues (dermal, ground, and vascular).
- Relate the abundance or density of specific cell components (mitochondria, chloroplasts, rough endoplasmic reticulum, and Golgi apparatus) to specialised cell functions.
- Explain the consequences of cell component deficiency, absence, or dysfunction on living processes.
Specialised Human and Plant Tissues & Organelle Densities
A side-by-side comparative summary linking specialised cell structures to their specific functions and showing the structural differences among human connective tissues and plant ground tissues.
Human Specialised Cells
Muscle, white blood, red blood, nerve, epithelial, and sperm cells.
Plant Specialised Cells
Sieve tube element, xylem vessel, palisade mesophyll, spongy mesophyll, guard cell, and root hair cell.
Human Tissues
Epithelial, muscle, nerve, and connective tissue.
Plant Tissues
Meristematic tissues and permanent tissues (dermal, ground, vascular).
Organelle Density
Mitochondria, chloroplasts, rough endoplasmic reticulum, and Golgi apparatus.
Short Explanation
Cell Specialisation in Multicellular Organisms
Cell specialisation is the process in which cells develop distinctive structures to adapt and carry out specific physiological functions. Cells differ in size, shape, and arrangement to correlate with their functions.
Specialised Human Cells
Muscle cell: Arranged as multinuclear striated fibres; contracts and relaxes to generate movement.
White blood cell: Can change shape; functions in destroying pathogens.
Red blood cell: Lacks a nucleus, shaped as a biconcave disc; functions to optimise the transportation of oxygen.
Nerve cell: Long and thin in shape; functions in sending nerve impulses.
Epithelial cell: Thin and flat cells; coats the surface of organs such as the digestive tract.
Sperm cell: Has a long tail to enable swimming towards the ovum in the Fallopian tube; the head carries a set of chromosomes from the male.
Specialised Plant Cells
Sieve tube element: Long cylindrical tubes arranged from end to end; transports organic materials from leaves to storage organs like fruits.
Xylem vessel: Long, continuous hollow tube; transports water and mineral salts from roots to other plant parts.
Palisade mesophyll cell: Consists of long cylindrical cells arranged vertically and closely packed; contains high chlorophyll density to allow maximum absorption of sunlight for photosynthesis.
Spongy mesophyll cell: Loosely arranged with large intercellular air spaces; allows gaseous exchange from inside the leaf to the palisade mesophyll cells.
Guard cell: Modified lower epidermal cells with thicker cell walls on the inner side; controls the opening and closing of the stoma (the pore allowing exchange of \(\text{O}_2\) and \(\text{CO}_2\)).
Root hair cell: Has a long projection to increase surface area for the absorption of water and mineral salts.
Cell Organisation in Humans
A tissue is a group of cells with the same structure and function arranged together to perform a specific function. Four main types:
Epithelial tissue: Covers outer body surfaces (skin) and lines hollow internal surfaces (digestive tract and respiratory tract).
Muscle tissue: Smooth muscle, skeletal muscle, and cardiac muscle.
Nerve tissue: Composed of neurons (nerve cells). Each neuron consists of a cell body and nerve fibres (dendrites and axon).
Connective tissue: Distributed throughout the body; links organs and provides support. Includes loose connective tissue, fibrous connective tissue, adipose tissue, bone, cartilage, and blood tissue.
Tissue Organisation in Plants
Plant tissues are divided into meristematic tissues and permanent tissues:
Meristem tissue: Actively dividing cells; divided into apical meristem tissue and lateral meristem tissue.
Permanent tissue: Mature tissues that have differentiated or are differentiating; divided into dermal tissue, ground tissue (parenchyma, collenchyma, sclerenchyma), and vascular tissue (xylem, phloem).
Density of Cell Components and Specialised Cell Functions
The abundance or density of specific organelles directly corresponds to the metabolic requirements of specialised cells:
Mitochondrion: Abundant in sperm cells, flight muscle cells in insects and birds, and plant meristem cells to generate large amounts of energy (ATP).
Chloroplast: High densities in palisade mesophyll cells and spongy mesophyll cells to absorb sunlight for photosynthesis.
Rough Endoplasmic Reticulum (RER) and Golgi Apparatus: Abundant in pancreatic cells, goblet cells, and liver cells to increase the synthesis and secretion of digestive enzymes, hormones, and mucus.
Effects of Deficiency, Absence, or Failure of Cell Components
Mitochondrial dysfunction / disjunction: Results in deficient energy production, causing stunted growth, weak muscles, hearing problems, and vision problems.
Lysosomal failure (Tay-Sachs disease): A hereditary disease caused by the failure or absence of specific hydrolytic enzymes produced in the lysosomes, leading to lipid accumulation in the brain, stunted growth, and mental retardation.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Why do palisade mesophyll cells have a high density of chloroplasts compared to other cells in the leaf?
Differentiate between the functions of tendons and ligaments in fibrous connective tissue.
Explain the physiological effect on an individual if their mitochondria fail to function normally.
Common Mistakes
- Confusing the functional support roles of collenchyma and sclerenchyma: students often forget that collenchyma supports young, non-woody (herbaceous) stems, while sclerenchyma provides mechanical support and rigidity to all mature parts of the plant.
- Swapping the connections of tendons and ligaments: mistaking tendons as bone-to-bone connectors and ligaments as muscle-to-bone connectors.
- Forgetting that red blood cells lack a nucleus at maturity to provide a biconcave disc shape and maximise surface area-to-volume ratio for oxygen transport.
- Mistaking smooth muscle for voluntary muscle: smooth muscle actions (like peristalsis along the digestive tract) are involuntary, whereas only skeletal muscle is under controlled/voluntary movement.
Concept Misunderstandings
All plant cells contain chloroplasts and carry out photosynthesis.
Only specific green plant cells contain chloroplasts, such as palisade and spongy mesophyll cells and guard cells. Underground root hair cells and internal vascular cells (xylem and phloem) do not contain chloroplasts.
The rough endoplasmic reticulum itself secretes enzymes directly out of the cell.
The rough endoplasmic reticulum synthesises proteins, but it collaborates with the Golgi apparatus, which modifies, packages, and transports the secretory proteins (such as digestive enzymes) for secretion.
Blood is classified as a fluid or independent substance rather than a tissue.
Blood is classified anatomically as a connective tissue because it consists of distinct cells suspended in an extracellular fluid matrix (plasma) and links various systems together.
Summary
- Multicellular organisms are composed of specialised cells that correlate in structure and size to perform specific functions.
- Animal cells include muscle, nerve, epithelial, white blood, red blood, and sperm cells; plant cells include sieve tube elements, xylem vessels, palisade and spongy mesophyll, guard cells, and root hair cells.
- Animal tissues are grouped into four types: epithelial, muscle (smooth, skeletal, cardiac), nerve (neurons), and connective (loose, fibrous, adipose, bone, cartilage, blood).
- Plant tissues consist of meristematic tissues (apical and lateral) and permanent tissues (dermal, ground [parenchyma, collenchyma, sclerenchyma], and vascular [xylem, phloem]).
- The abundance of organelles matches functional demands: high mitochondria in sperm, flight muscles, and meristems; high chloroplasts in mesophyll; and high RER and Golgi apparatus in pancreatic, goblet, and liver cells.
- Cellular component defects disrupt metabolism, as seen in mitochondrial disorders and Tay-Sachs disease.
Short Activity
Objective: identify specialised cells, tissues, and organelle density relationships.
A. Objective Quiz
Which type of ground tissue in plants is specialised to provide mechanical support and elasticity specifically to young, herbaceous stems?
Which of the following cells possesses a high density of rough endoplasmic reticulum and Golgi apparatus to support its specialised function?
B. Fill in the Blanks
Fibrous connective tissue forms , which connect bones to other bones.
In human nerve tissue, each neuron consists of a cell body, dendrites, and an .
C. Matching / Drag and Drop
Drag each specialised cell or tissue to its correct textbook function. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Cell / Tissue to Its Function
Write the correct cell or tissue next to each function.
| Function | Cell / Tissue |
|---|---|
| 1. Transports organic materials such as sucrose from leaves to storage organs | |
| 2. Has a long projection to add surface area for the absorption of water and mineral salts | |
| 3. Stores fat under the skin dermis and on the surface of main internal organs | |
| 4. Contracts and relaxes involuntarily to enable peristalsis along the digestive tract |
Answer choices: Sieve tube element • Root hair cell • Adipose tissue • Smooth muscle
