Tissues in Action — Notes

Why are Plant and Animal Tissues Different?

Plants and animals exhibit fundamental differences in their structure, function, and lifestyle, which are reflected in the types of tissues they possess. These differences arise from their distinct modes of nutrition, habitat, and growth patterns.

Key Differences Driven by Lifestyle and Nutrition

Sessile vs. Motile Lifestyle

Plants are fixed to the ground, meaning they don't need to move to find food or escape predators. This immobility is supported by rigid cell walls in plant cells, providing essential structural strength and enabling them to stand upright. In contrast, animals are generally motile, requiring a body structure that facilitates movement. Animal cells lack rigid cell walls, giving them flexibility which is crucial for locomotion and varied activities.

Autotrophic vs. Heterotrophic Nutrition

Plants are autotrophs, meaning they produce their own food through photosynthesis. This process requires specialized tissues that capture sunlight and convert carbon dioxide and water into energy-rich sugars. Animals are heterotrophs, obtaining nutrients by consuming other organisms. This necessitates the development of tissues dedicated to the ingestion, digestion, and absorption of food, as well as tissues for transporting these nutrients throughout the body.

Growth Patterns

A significant difference lies in their growth. Plants exhibit indeterminate growth, meaning they can continue to grow throughout their lifespan. This is facilitated by specific regions of actively dividing cells, known as meristems (discussed further in Section 2 and 3). Animals, on the other hand, generally have determinate growth, where growth ceases once they reach a mature size.

Transport Systems

Both plants and animals have specialized vascular tissues for transport, but their roles are adapted to their needs. In plants, xylem transports water and minerals from roots to leaves, and phloem transports manufactured food (sugars) from leaves to other parts. In animals, the circulatory system, composed of blood vessels and specialized tissues like blood and muscle (in the heart), transports oxygen, nutrients, hormones, and waste products.

Remember

The fundamental differences in plant and animal tissues are a direct consequence of their distinct lifestyles, modes of nutrition, and growth strategies. Plants' need for rigidity and photosynthesis drives their tissue development, while animals' mobility and consumption of external food sources necessitate different tissue specializations.

Exam-smart

In a case study, you might be given a description of an organism's characteristics (e.g., fixed in place, produces its own food, has a rigid outer layer). You would need to infer that this organism likely possesses plant-like tissues, emphasizing the role of cell walls for support and photosynthetic tissues for energy. Conversely, an organism described as mobile, consuming other organisms, and having flexible body parts would suggest animal-like tissues.

Tissues for Growth in Plants

Plants exhibit growth throughout their lives, a process driven by specific types of tissues. This growth manifests in various ways: increasing length (height of the stem and depth of roots), increasing girth (thickness of the stem), and regeneration after damage, such as cut branches or grazed leaves.

Meristematic Tissue: The Basis of Plant Growth

The ability of plants to grow, repair, and increase in size is due to meristematic tissues. These are tissues composed of actively dividing cells. The meristem (plural: meristems) is a region in the plant where cell division occurs, leading to new cells that differentiate to form permanent tissues.

The key characteristics of meristematic cells are:

• Active Cell Division: They are perpetually dividing to produce new cells.
• Small and Isodiametric: Cells are generally small and roughly spherical or polygonal in shape.
• Thin Cell Walls: Their cell walls are thin and made of cellulose.
• Dense Cytoplasm: They possess abundant, dense cytoplasm.
• Large, Prominent Nucleus: Each cell has a large and distinct nucleus.
• Small or Absent Vacuoles: Vacuoles are typically small or may not be present at all.
• No Intercellular Spaces: Cells are compactly arranged without any gaps between them.

Meristematic tissues are classified based on their location in the plant body and their role in growth. The primary types of meristems are:

• Apical Meristem: Located at the tips of roots and shoots. Responsible for primary growth (increase in length).
• Lateral Meristem: Found along the sides of roots and stems. Responsible for secondary growth (increase in girth or thickness).
• Intercalary Meristem: Located at the nodes or at the base of leaf blades. Responsible for growth in length at specific points, allowing for regrowth after cutting or grazing.

Remember

Meristematic tissues are responsible for all plant growth. Their constantly dividing cells provide the raw material for the development of new plant parts.

Exam-smart

A question might describe a plant growing taller or a root growing deeper. You would identify this as primary growth and associate it with the apical meristem, located at the shoot and root tips, respectively. If the question describes a tree trunk becoming thicker over years, you would link this to lateral meristem and secondary growth.

Apical Meristem — How Do Plants Grow in Length?

Plants grow in length through the activity of apical meristems. These are actively dividing cells found at the tips of roots and shoots.

Why Apical Meristems Cause Lengthwise Growth:

• The cells in the apical meristems undergo continuous mitosis, producing new cells.
• These new cells elongate and differentiate, contributing to the primary growth of the plant body, extending the roots further into the soil for water and nutrient absorption, and the shoots towards sunlight for photosynthesis.
• The experiment with onion root tips demonstrates this: when the root tips (containing the apical meristem) are cut, root growth in length stops. This confirms that the growth in length specifically occurs at the tips.

Location of Apical Meristem:

• Found at the apex (tip) of the root system.
• Found at the apex (tip) of the shoot system (stems and leaves).

Characteristics of Meristematic Cells:

Meristematic cells are specialized for rapid division and possess distinct features:

• Shape: Small and roughly isodiametric (similar in all dimensions).
• Cell Wall: Thin.
• Nucleus: Large and prominent.
• Cytoplasm: Dense, with abundant organelles.
• Vacuoles: Generally absent or very small, allowing maximum space for the nucleus and cytoplasm for metabolic activities related to division.
• Intercellular Spaces: Little to none; cells are tightly packed.

Remember

The absence of large vacuoles in meristematic cells is crucial because it maximizes the space for the nucleus and cytoplasm, which are vital for the high metabolic rate required for cell division.

Exam-smart

A question might present data from an experiment similar to the onion root tip activity, showing root lengths over several days with and without root tip excision. You'll need to infer that the root tip is the region of growth and that it contains actively dividing cells, identifying it as the apical meristem.

Lateral Meristem — How Do Plants Grow in Girth?

Plants also increase in thickness or girth through the activity of lateral meristems. These meristems are found along the sides of stems and roots.

How Lateral Meristem Works:

• The cells of the lateral meristem divide perpendicular to the circumference of the stem or root.
• This division produces new cells that are added both inward and outward.
• The outward-adding cells contribute to the formation of secondary tissues like bark and cork, while the inward-adding cells form secondary xylem and phloem, increasing the diameter.
• This secondary growth is particularly prominent in dicotyledonous plants and gymnosperms.
• The annual growth rings seen in the trunk of a tree are a result of the cyclical activity of lateral meristems over seasons, with wider rings in favourable conditions and narrower rings in unfavourable ones.

Location of Lateral Meristem:

• Found as a ring around the circumference of stems and roots.
• Key examples include the vascular cambium (between xylem and phloem) and the cork cambium (in the outer bark).

Remember

Lateral meristems are responsible for secondary growth, leading to an increase in girth, whereas apical meristems are responsible for primary growth, leading to an increase in length.

Exam-smart

You might be shown a cross-section of a tree trunk with visible annual rings. A question could ask you to explain how these rings are formed and what information they provide about the plant's history, pointing to the role of the lateral meristem.

Intercalary Meristem — How Do Plants Grow After Branch Being Cut?

Intercalary meristems are responsible for regrowth and branching, particularly in plants like grasses and some monocots, allowing them to recover after being cut or grazed.

How Intercalary Meristem Works:

• These meristems are located at the base of nodes or internodes.
• When the upper parts of the plant (like the tip of a stem) are removed, the intercalary meristems become active.
• They produce new cells that lead to the development of new branches or the regrowth of lost parts.
• This is why grass can regrow after mowing, and why hedges can become bushy again after trimming.

Location of Intercalary Meristem:

• Found at the nodes or base of internodes in many monocots (like grasses).
• Also found at the base of leaf blades or petioles in some plants.

Example:

• In grasses, intercalary meristems at the nodes allow for rapid regrowth after the lawn is mowed or after an animal grazes on it.

Remember

Intercalary meristems are crucial for the regenerative capacity of certain plants, especially those that experience frequent damage to their aerial parts. They allow for elongation of older parts or formation of new ones.

Exam-smart

A scenario might describe a plant that has been repeatedly cut or grazed, yet continues to grow and remain healthy. You would need to identify the intercalary meristem as the tissue responsible for this regrowth and resilience.

Permanent Tissues

Permanent tissues in plants are derived from meristematic tissues. Meristematic tissues are actively dividing cells. When these cells lose their ability to divide, they differentiate to perform specific functions, forming permanent tissues. This differentiation can lead to cells that are simple (all similar) or complex (different types of cells working together). Permanent tissues can be classified based on their function: protective, supporting, or conducting.

Protective tissue — Epidermis

The epidermis forms the outermost layer of the plant body, including stems, leaves, flowers, and roots. Its primary role is protection.

• Structure: It typically consists of a single layer of tightly packed, flat, and rectangular cells. These cells are often thin-walled, but their outer walls can be thicker.
• Protection: The epidermis shields the plant from mechanical injury, invasion by harmful microorganisms, and extreme environmental conditions.
• Cuticle: Most epidermal cells, especially on aerial parts (stems and leaves), are covered by a waxy layer called the cuticle. This layer is made of a substance called cutin.
• Function of Cuticle: The cuticle significantly reduces water loss through transpiration and prevents excessive water uptake. In desert plants, the cuticle is often very thick.
• Stomata: In leaves and stems, the epidermis contains small pores called stomata (singular: stoma). Each stoma is surrounded by a pair of specialized cells called guard cells.
• Functions of Stomata: Stomata are crucial for gaseous exchange (taking in $C{O}_2$ for photosynthesis and releasing $O_2$) and transpiration (evaporation of water vapour). Transpiration creates a "pull" that helps in the upward movement of water and minerals from the roots.
• Root Hairs: In roots, epidermal cells often form root hairs, which are tiny, hair-like projections.
• Function of Root Hairs: Root hairs greatly increase the surface area of the root, leading to more efficient absorption of water and minerals from the soil.
• Epidermal Outgrowths: In some plants, epidermal cells can develop into various hair-like structures or other outgrowths, contributing to functions like reducing water loss or defense.

Remember

While stomata are essential for gas exchange and transpiration, they also represent a potential pathway for water loss. Plants have evolved mechanisms to regulate stomatal opening and closing.

Exam-smart

A question might describe a plant in a hot, dry environment and ask why it has a thick cuticle or show a diagram of a leaf and ask about the function of stomata in relation to photosynthesis and transpiration.

Supporting tissue — Simple permanent tissues

Simple permanent tissues provide structural support to the plant. They are composed of only one type of cell. There are three main types: parenchyma, collenchyma, and sclerenchyma.

a. Parenchyma

• Structure: Consists of living cells that are usually rounded or oval and loosely packed, leaving large intercellular spaces. Their cell walls are thin and flexible.
• Functions:
• Food Storage: Their primary role is storing food reserves.
• Photosynthesis: In green parts of the plant (like young stems and leaves), parenchyma cells contain chloroplasts and perform photosynthesis.
• Buoyancy: In aquatic plants, some parenchyma cells are modified to form large air cavities (aerenchyma), which help the plant float.

b. Collenchyma

• Structure: Composed of living cells that are elongated and often polygonal. Their most distinctive feature is the uneven thickening of their cell walls, especially at the corners, due to the deposition of pectin and cellulose. They have very few or no intercellular spaces.
• Functions:
• Flexible Support: Provides mechanical support and flexibility to growing plant parts, allowing them to bend without breaking. This is why young stems and petioles (leaf stalks) can bend. It is often found below the epidermis in stems and leaf stalks.

c. Sclerenchyma

• Structure: Composed of cells that are typically dead at maturity. They have very thick, rigid cell walls due to the deposition of lignin, a hard and strong substance. They usually lack intercellular spaces.
• Functions:
• Strength and Rigidity: Provides the plant with considerable strength and rigidity.
• Protection: Forms the hard coverings of seeds and nuts (like coconut husk and walnut shells).
• Location: Found in stems, vascular bundles, veins of leaves, and hard parts of plants.

Remember

Collenchyma provides flexible support to growing parts, while sclerenchyma provides rigid support to mature parts. The presence of lignin is characteristic of sclerenchyma.

Exam-smart

You might be given descriptions of plant parts (e.g., a flexible tendril, a hard nut shell, a young stem) and asked to identify the primary supporting tissue involved.

Conducting tissues — Complex permanent tissues

Complex permanent tissues are made up of more than one type of cell working together to perform a common function. In plants, these are the vascular tissues: xylem and phloem. They are responsible for the long-distance transport of substances throughout the plant.

a. Xylem

• Function: Primarily responsible for the transport of water and dissolved minerals from the roots to the rest of the plant, and also provides mechanical strength.
• Components: Xylem is composed of four types of cells:

1. Tracheids: Elongated, dead cells with tapering ends. Their walls are lignified and often have pits. Water passes from one tracheid to another through these pits.
2. Vessels: Long, tubular structures formed by the joining of many dead cells placed end to end. The end walls of these cells are perforated or absent, forming continuous channels for water transport. Vessels are the main water-conducting elements in angiosperms.
3. Xylem Parenchyma: The only living component of xylem. They store food and help in the lateral conduction of water.
4. Xylem Fibres: Dead, lignified cells that provide mechanical support to the xylem.

• Transport Direction: Unidirectional (from roots upwards).

b. Phloem

• Function: Responsible for the transport of food (sugars produced during photosynthesis) from the leaves to other parts of the plant where it is needed for growth or storage.
• Components: Phloem is composed of four types of cells:

1. Sieve Tubes: Long, slender, tubular structures formed by elongated cells called sieve elements, placed end to end. Their end walls are sieve plates, which are perforated, allowing the passage of food. Sieve tubes are the main food-conducting channels.
2. Companion Cells: These are specialized parenchyma cells closely associated with sieve tube elements. They are living cells and regulate the cellular functions of the sieve tube elements, including the loading and unloading of sugars.
3. Phloem Parenchyma: Living cells that store food, oils, and starch. They also help in the lateral transport of food.
4. Phloem Fibres: The only dead component of phloem. They are elongated and lignified, providing structural support. (Note: This is an exception; most phloem cells are living).

• Transport Direction: Bidirectional (from source to sink, can be upwards or downwards).

Remember

Xylem is primarily composed of dead cells and transports water upwards, providing rigidity. Phloem is mostly living cells and transports food in any direction.

Exam-smart

A case study might describe a plant suffering from wilting and ask which vascular tissue's function is compromised. Or, it might provide a diagram showing the arrangement of xylem and phloem and ask about their roles in transport.

Animal Tissues

Like plants, animal cells also group together and specialize in performing different functions. These groups of similar cells form animal tissues. These tissues are adapted to perform specific functions, enabling the body to carry out complex activities.

Epithelial tissues — Structure and functions

Epithelial tissue, also known as epithelium, forms the outer covering of the body (skin) and also lines internal organs such as the mouth, lungs, blood vessels, and intestine.

• Structure: Epithelial tissue is composed of closely packed cells with very little intercellular space. This tightly packed arrangement is crucial for its protective and absorptive functions. The shape and arrangement of these cells vary depending on their location and function.
• Functions:
• Protection: It protects the underlying tissues from mechanical injury, friction, and the entry of microbes.
• Absorption: In organs like the intestine, epithelial cells are specialized for absorbing nutrients.
• Secretion: Glands, which are made of epithelial tissue, secrete substances like mucus, enzymes, and hormones.
• Excretion: Epithelial tissues in the kidneys help in filtering waste products.
• Sensory Reception: Specialized epithelial cells in sensory organs detect stimuli like touch, taste, and smell.
• Diffusion and Filtration: In the lungs and blood vessels, thin epithelial layers facilitate the exchange of gases and nutrients.

Types of Epithelial Tissues:

Remember

The structure of epithelial tissue is directly related to its function. For example, the thin, flat cells of squamous epithelium are ideal for diffusion, while the tall columnar cells of the intestine are specialized for absorption.

Exam-smart

A diagram of different epithelial tissues might be provided, and you'll be asked to identify the tissue type based on its structure and associate it with a specific function and location in the body. For instance, identifying columnar epithelium with microvilli in the small intestine for absorption.

How are various parts connected in our body?

Various parts of the body are connected and supported by connective tissues. These tissues bind other tissues and organs together, providing support, protection, and transport.

• Blood: A fluid connective tissue that transports nutrients, gases, hormones, and waste products throughout the body. Its fluid matrix is called plasma. Red blood cells, white blood cells, and platelets are suspended in the plasma.
• Bones: A type of connective tissue with a rigid matrix rich in calcium and phosphorus compounds. This provides strength, support, and protection to the body. Bones form the skeleton.
• Cartilage: A flexible connective tissue with a soft, jelly-like matrix. It provides cushioning and shock absorption at joints and also supports structures like the ear and nose.
• Tendons: Fibrous connective tissue that connects muscles to bones, enabling movement.
• Ligaments: Fibrous connective tissue that connects bones to bones, providing stability to joints and limiting excessive movement.

Types of Connective Tissues and their Roles:

Remember

Blood is a unique connective tissue because it is fluid. Bones are hard due to mineral deposition, while cartilage is flexible.

Exam-smart

Case studies might involve scenarios like a sports injury. You could be asked to identify whether a torn tendon or ligament is involved, and explain its role in connecting muscles to bones or bones to bones respectively, and how its damage affects movement.

Can we control movement in our body?

Movement in our body is primarily controlled by muscular tissues. There are three types of muscles:

• Skeletal Muscles:

• Structure: Long, cylindrical, unbranched cells (fibres) with multiple nuclei and striations (light and dark bands).

• Function: Responsible for voluntary movements, such as walking, running, lifting objects, and writing. They are attached to bones.

• Control: Under conscious control.

• Smooth Muscles (Visceral Muscles):

• Structure: Spindle-shaped cells with a single nucleus and no striations.

• Function: Responsible for involuntary movements, such as the movement of food in the digestive tract, constriction of blood vessels, and contraction of the bladder.

• Control: Not under conscious control.

• Cardiac Muscles:

• Structure: Cylindrical, branched cells with single nuclei and faint striations. They are connected by intercalated discs, which allow for coordinated contraction.

• Function: Found only in the heart. Responsible for the rhythmic and tireless beating of the heart, pumping blood.

• Control: Involuntary.

Comparison of Muscle Types:

Remember

Skeletal muscles are the muscles we consciously control, while smooth and cardiac muscles work automatically.

Exam-smart

Questions might describe a specific bodily action (e.g., the digestion of food, the beating of the heart, or lifting a heavy object) and ask you to identify the type of muscle responsible and explain its characteristics.

How does the body sense, communicate and respond?

The nervous tissue is responsible for sensing stimuli, communicating information, and coordinating responses in the body. It forms the body's control and coordination network.

• Nervous Tissue: Composed of specialized cells called neurons (nerve cells) and glial cells (supporting cells).

• Neurons: The basic functional units of the nervous system. They are specialized to receive, process, and transmit messages in the form of electrical and chemical signals.

• Cell Body (Soma): Contains the nucleus and other organelles, controls cell activities.

• Dendrites: Branched extensions that receive signals from other neurons or sensory receptors.

• Axon: A long fibre that carries signals away from the cell body to other neurons, muscles, or glands.

• Axon Terminals: The end of the axon, where signals are transmitted to other cells.

• Glial Cells: Provide support, nutrition, and protection to neurons.

• Brain: The central control centre of the nervous system, responsible for processing information, memory, thought, and coordinating all bodily activities.

• Nerves: Bundles of axons that transmit signals throughout the body.

How it works: When a stimulus is detected (e.g., touching a hot object), sensory receptors generate nerve impulses. These impulses travel along sensory neurons to the spinal cord and brain for processing. The brain then sends signals along motor neurons to effector organs (like muscles or glands) to produce an appropriate response (e.g., pulling the hand away).

Remember

Neurons are specialized for rapid transmission of information throughout the body. The brain acts as the central processing unit.

Exam-smart

You might be presented with a scenario describing a reflex action (like blinking when something approaches your eye) and asked to trace the pathway of the nerve impulse from the sensory receptor to the effector, identifying the role of different parts of the neuron and the nervous system.

The Musculoskeletal System

The musculoskeletal system is the body's structural framework, providing support, enabling movement, and protecting vital organs. It is a complex network comprising bones, muscles, joints, cartilage, tendons, and ligaments. This system works in close coordination with the nervous system, which controls and directs its actions.

Function of the Musculoskeletal System:

• Support: It maintains upright posture.
• Movement: It facilitates locomotion and other body movements.
• Protection: It safeguards delicate internal organs.

Components and Their Roles:

• Bones: Provide a rigid framework, support, and protection.
• Muscles: Contract to pull on bones, generating movement.
• Tendons: Tough, flexible connective tissues that attach muscles to bones. When a muscle contracts, it pulls the tendon, which in turn pulls the bone, causing movement at a joint.
• Ligaments: Strong, fibrous connective tissues that connect bones to other bones at joints, providing stability.
• Cartilage: A smooth, flexible connective tissue found at the ends of bones in joints. It acts as a cushion, reducing friction and absorbing shock during movement. It is also found between vertebrae to allow flexibility of the spine.

The human skeleton constitutes approximately 12-15% of total body weight in adults.

The musculoskeletal system in action

Movement in the body is a coordinated effort between bones and muscles, directed by the nervous system. Muscles contract and relax in a specific sequence, pulling on the bones to which they are attached via tendons. This coordinated action allows for a wide range of movements, from simple gestures to complex athletic feats. The type and range of movement at a particular body part are determined by the type of joint present.

Table: Types of Movements and Joints

This table illustrates that different parts of the body have varying degrees of mobility, which is a direct consequence of the types of joints they possess.

Types of Joints

A joint is the point where two or more bones meet. Joints are crucial for enabling movement, but they cannot initiate movement on their own; they are acted upon by muscles. The different types of joints allow for varying degrees of motion.

Ball and socket joint

This type of joint allows for the greatest range of movement, including forward, backward, sideways, and circular motions. It is formed when the rounded end of one bone fits into a shallow, cup-like depression (socket) of another bone.

• Example: The shoulder joint, where the head of the humerus (upper arm bone) fits into the glenoid cavity of the scapula (shoulder bone). This allows the arm to move in multiple directions. The hip joint is another example.

Hinge joint

A hinge joint allows movement in only one plane, similar to the hinge of a door. It permits bending and straightening.

• Example: The elbow joint (between the humerus, radius, and ulna) and the knee joint (between the femur, tibia, and patella). The kneecap (patella) protects the knee joint.

Pivot joint

A pivot joint allows for rotational movement where one bone rotates around a central axis formed by another bone. This joint permits side-to-side movement.

• Example: The joint between the atlas (the first vertebra) and the axis (the second vertebra) in the neck. This joint allows us to turn our head from side to side.

Fixed joints

Fixed joints, also known as immovable joints, are where bones are joined together in a way that prevents any significant movement. This structural arrangement provides strong protection to vital organs.

• Example: The joints between the bones of the skull. These flat bones interlock tightly to form a protective casing for the brain.

Skeletal System

The skeletal system is the body's internal framework composed of bones. It provides structural support, allows for movement, and protects internal organs. Key components include:

• Skull: Protects the brain, eyes, and ears. It is formed by fixed joints.
• Vertebral Column (Backbone): A flexible column extending from the base of the skull, composed of numerous small bones called vertebrae. It supports the body and allows us to stand upright. Cartilage discs between vertebrae act as cushions and enable bending and twisting.
• Rib Cage: Formed by 12 pairs of ribs attached to the spine and the sternum (breastbone). It protects vital organs like the heart and lungs. The flexible nature of the rib cage, due to its attachment to the spine and sternum with cartilage, allows it to expand and contract during breathing, facilitating the movement of air into and out of the lungs.

Remember

Tendons connect muscle to bone, while ligaments connect bone to bone. Both are crucial for joint stability and movement, but their functions are distinct.

Exam-smart

Questions may present scenarios involving movement difficulties or injuries. You'll need to identify the type of joint involved (e.g., a "no" head shake suggests a pivot joint, while bending an arm points to a hinge joint) and relate it to the musculoskeletal system's components like bones, muscles, tendons, and ligaments. For instance, an inability to move an arm might be related to a tendon rupture or a problem with the shoulder joint's ball-and-socket mechanism.