Plant and Animal Cells

Introduction to Eukaryotic Cells

A Cell consists of three basic components: a Plasma Membrane, Cytoplasm containing various organelles, and a Nucleus with chromatin material. Cells that possess a distinct, membrane-bound nucleus are called Eukaryotic cells. Both plant and animal cells belong to this category. In contrast, Prokaryotic cells (such as bacteria) lack a definite nucleus — their nuclear material is directly submerged in the cytoplasm without a separating membrane.
Three Basic Components: Every cell has a plasma membrane, cytoplasm with organelles, and a nucleus with genetic material
Eukaryotic vs Prokaryotic: Eukaryotic cells have a membrane-bound nucleus and organelles; prokaryotic cells lack both
Cell Fractionation: Modern techniques allow isolation of cellular components by Cell Fractionation — tissues are homogenized and components are separated by density gradient centrifugation or ultracentrifugation
Eukaryotic cells vary greatly in size. Most are microscopic and measured in micrometres (μm). One micrometre equals metres (0.000001 m). Despite their small size, cells contain highly organised internal structures that carry out all the functions of life.
Conversion factor for the standard unit of cell measurement
=micrometre(micrometres)
=one-millionth(dimensionless)
=metre(metres)
Unit of Measurement: Cell sizes are expressed in micrometres (μm), where 1 μm = 10⁻⁶ m
Typical Size Range: Most eukaryotic cells range from 10 to 100 μm in diameter
Visibility: Cells are invisible to the naked eye and require light or electron microscopy for observation

Plasma Membrane and Outer Boundaries

The Plasma Membrane (also called the cell membrane) is the outermost boundary of the cell. In plant cells it lies beneath the Cell Wall, while in animal cells it is the outermost layer. Chemically, it is composed of 60–80% proteins and 20–40% lipids, with a small quantity of carbohydrates. The currently accepted model of membrane structure is the Fluid Mosaic Model, which describes proteins embedded in a lipid bilayer in a mosaic pattern rather than sandwiched between two protein layers as in the older unit membrane model.
Chemical Composition: 60–80% proteins, 20–40% lipids, and a small amount of carbohydrates
Fluid Mosaic Model: Proteins are embedded within the phospholipid bilayer in a mosaic pattern; this is the most widely accepted model
Peripheral and Integral Proteins: Peripheral proteins sit on the surface, while integral proteins span across the lipid bilayer
Cholesterol and Carbohydrates: Cholesterol is embedded in the bilayer; carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface

Components of the Fluid Mosaic Model

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Phospholipid bilayer — forms the basic structural framework
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Integral proteins — span the bilayer and function as channels or transporters
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Peripheral proteins — loosely attached to the membrane surface
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Glycoproteins — proteins with carbohydrate chains on the outer surface
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Glycolipids — lipids with carbohydrate chains on the outer surface
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Cholesterol — stabilises membrane fluidity
The plasma membrane is Selectively Permeable — it allows only certain substances to pass through. Lipid-soluble substances cross it more easily. Small neutral molecules (water, glucose, small gas molecules) pass readily, while ions face difficulty due to their charge. Movement can occur by Passive Transport (down the concentration gradient, no energy required) or Active Transport (against the concentration gradient, requiring ATP). In animal cells, the membrane can also take in materials by Endocytosis, which includes Phagocytosis (engulfing solid particles) and Pinocytosis (taking in liquid material).
Passive Transport: Movement of substances down the concentration gradient without energy expenditure
Active Transport: Uphill movement from low to high concentration, powered by ATP
Phagocytosis: Engulfing of solid particles by membrane infolding — common in white blood cells and amoeba
Pinocytosis: Uptake of liquid material through membrane invagination
Selective Permeability: Lipid-soluble substances and small neutral molecules cross more easily; charged ions face resistance
The Cell Wall is the outermost boundary in most plant cells. It is secreted by the protoplasm and its thickness varies among different plant cells. It is composed of three layers: the Middle Lamella (formed first, between adjacent cells), the Primary Wall (composed of cellulose, pectin, and hemicellulose with criss-cross cellulose arrangement), and the Secondary Wall (thick and rigid, containing inorganic salts, silica, waxes, cutin, and lignin). The cell wall provides a definite shape and rigidity to the cell but does not act as a barrier to material passage. Unlike plant cell walls, prokaryotic cell walls lack cellulose and are made of Peptidoglycan (murein), while fungal cell walls contain chitin.
Middle Lamella: First layer formed between primary walls of neighbouring cells; cements adjacent cells together
Primary Wall: True wall of growing cells; composed of cellulose, pectin, and hemicellulose in a criss-cross arrangement
Secondary Wall: Thick, rigid layer on the inner surface of the primary wall; contains lignin, waxes, cutin, silica, and inorganic salts
Shape and Rigidity: The cell wall gives plant cells their characteristic rectangular shape and provides structural support
Permeability: Unlike the plasma membrane, the cell wall is fully permeable and does not regulate material passage

Cell Wall Composition Across Organisms

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Plant cells — cellulose, pectin, hemicellulose, lignin, cutin, waxes, silica, inorganic salts
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Prokaryotic cells — peptidoglycan (murein)
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Fungal cells — chitin

Cytoplasm and Shared Organelles

The Cytoplasm is the living contents of the cell excluding the nucleus. Together with the nucleus, it forms the Protoplasm. Cytoplasm consists of an aqueous ground substance called Cytosol (about 90% water) containing dissolved molecules and ions, plus various cell organelles and inclusions. Small molecules and ions form true solutions, while large molecules form colloidal solutions that can be a sol (non-viscous) or a gel (viscous). Peripheral parts of the cell are often gel-like. Cytoplasmic Streaming — an active mass movement of cytoplasm — causes free-floating organelles like mitochondria to move within the cell.
Cytosol: The soluble, aqueous portion (about 90% water) forming the ground substance of cytoplasm; contains fundamental molecules of life
Sol and Gel States: Cytoplasm exists as sol (non-viscous) or gel (viscous); peripheral regions tend to be gel-like
Metabolic Functions: Cytosol is the site of glycolysis and other metabolic processes
Cytoplasmic Streaming: Active mass movement that distributes organelles and materials throughout the cell
The Endoplasmic Reticulum (ER) is a network of membrane channels extending throughout the cytoplasm. These channels are continuous with the Plasma Membrane at some points and appear in contact with the nuclear membrane. The material inside the channels is separated from the cytoplasm by spherical or tubular membranes called Cisternae. ER exists in two forms: Rough Endoplasmic Reticulum (RER) with attached Ribosomes for protein synthesis, and Smooth Endoplasmic Reticulum (SER) without ribosomes, involved in lipid metabolism, drug detoxification, and impulse transmission in muscle and nerve cells. ER also provides mechanical support to maintain cell shape.
Cisternae: Spherical or tubular membranes that form the channels of ER, separating their contents from cytoplasm
RER Functions: Synthesises proteins that are either stored in the cytoplasm or exported out of the cell through ER channels
SER Functions: Metabolises lipids, detoxifies harmful drugs, transmits impulses (in muscle and nerve cells), and transports materials within the cell
Structural Support: ER provides mechanical support to maintain the shape of the cell
Ribosomes are tiny granular structures composed of roughly equal amounts of RNA (Ribosomal RNA) and protein, making them ribonucleoprotein particles. They exist in two forms: free in the cytoplasm or attached to RER. Each Eukaryotic Ribosome consists of a large 60S subunit and a small 40S subunit, which combine (controlled by Mg²⁺ ions) to form an 80S particle. Ribosomes attach to mRNA through their small subunit, and a group of ribosomes bound to a single mRNA strand is called a Polysome. Ribosomes are assembled in the Nucleolus and exported to the cytoplasm via nuclear pores.
Eukaryotic ribosome assembly from its two subunits
=small ribosomal subunit(Svedberg units)
=large ribosomal subunit(Svedberg units)
=magnesium ions required for subunit attachment(ions)
=complete eukaryotic ribosome(Svedberg units)
Composition: Equal amounts of ribosomal RNA and protein (ribonucleoprotein particles)
Free vs Bound: Free ribosomes synthesise proteins for use within the cell; bound ribosomes (on RER) synthesise proteins for export
Polysome: Multiple ribosomes attached to a single mRNA molecule, enabling simultaneous translation
Assembly Site: Ribosomes are assembled in the nucleolus and exported to the cytoplasm through nuclear pores
The Golgi Apparatus (discovered by Golgi in 1898) consists of stacks of flattened, membrane-bound sacs called cisternae, along with associated vesicles — collectively forming the Golgi complex. It has a forming face (outer convex surface) where vesicles from SER fuse, and a maturing face (inner concave surface) where secretory vesicles bud off. The Golgi apparatus modifies proteins and lipids by adding carbohydrates, converting them into Glycoproteins or glycolipids. It packages and processes cell secretions for export — for example, pancreatic digestive enzymes are packaged into granules by the Golgi complex.
Forming Face: Outer convex surface where transport vesicles from SER fuse with Golgi cisternae
Maturing Face: Inner concave surface where processed materials bud off as secretory vesicles
Protein Modification: Adds carbohydrates to proteins and lipids, forming glycoproteins and glycolipids
Packaging and Secretion: Converts raw products into finished products and packages them in membrane-bound vesicles for export
Mitochondria are double-membrane-bound organelles known as the powerhouses of the cell. The outer membrane is smooth, while the inner membrane forms infoldings called Cristae that project into the Mitochondrial Matrix. The inner surface of cristae bears small knob-like structures called particles. Mitochondria contain their own DNA and ribosomes, making them self-replicating organelles. The matrix contains enzymes, coenzymes, and organic and inorganic salts for vital metabolic processes including Kreb's cycle, aerobic respiration, and fatty acid metabolism. Energy from organic food is extracted and converted into ATP, which supplies energy to the cell on demand.
Double Membrane: Outer membrane is smooth; inner membrane forms cristae that increase surface area for metabolic reactions
F1 Particles: Small knob-like structures on the inner surface of cristae involved in ATP synthesis
Own DNA and Ribosomes: Mitochondria can synthesise some of their own proteins and replicate independently
ATP Production: Energy from food is transformed into ATP through aerobic respiration and Kreb's cycle in the matrix
Variable Number: The number of mitochondria depends on the cell's physiological activity — highly active cells have more
The Cytoskeleton is a network of protein fibres in the cytosol, made up of three components: Microtubules, Microfilaments, and Intermediate Filaments. Microtubules are long, unbranched tubulin structures that assemble and disassemble during spindle formation in mitosis. Microfilaments are slender cylinders of contractile actin protein linked to the inner face of the plasma membrane, responsible for cyclosis and amoeboid movement. Intermediate filaments have diameters between microtubules and microfilaments and are involved in maintaining cell shape and integrating cellular compartments.
Microtubules: Long, unbranched tubulin structures; essential for spindle formation during mitosis and for forming cilia, flagella, basal bodies, and centrioles
Microfilaments: Slender actin cylinders attached to the inner plasma membrane; drive cyclosis and amoeboid movement
Intermediate Filaments: Diameter between microtubules and microfilaments; determine cell shape and integrate cellular compartments
Derived Organelles: Cilia, flagella, basal bodies, and centrioles are special assemblies of microtubules

Nucleus

The Nucleus controls all life activities of the cell. In animal cells it generally occupies the central space, while in plant cells it is pushed towards the periphery by the large central vacuole. The nucleus is visible only during the non-dividing stage and contains Chromatin network and soluble sap called Nucleoplasm. During cell division, chromatin condenses into Chromosomes. Nucleus consists of the nuclear envelope, nucleoli, nucleoplasm, and chromosomes. Cells may be Mononucleate (one nucleus), binucleate (two nuclei), or multinucleate (more than two).
Position: Central in animal cells, peripheral in plant cells (due to the large vacuole)
Nucleoplasm: Soluble sap inside the nucleus containing dissolved molecules
Nucleus Types: Mononucleate (one nucleus), binucleate (two), or multinucleate (more than two)
Composition: DNA, RNA, proteins, and enzymes

Components of the Nucleus

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Nuclear envelope (double membrane with pores)
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Nucleolus (rRNA synthesis and ribosome assembly)
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Nucleoplasm (soluble nuclear sap)
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Chromatin / Chromosomes (genetic material)
The Nuclear Membrane (nuclear envelope) is a double membrane that separates nuclear material from the cytoplasm. The outer membrane is continuous with the Endoplasmic Reticulum, while the inner membrane encloses the nuclear contents. The two membranes are fused at certain points to form Nuclear Pores, which allow exchange of materials between the nucleus and cytoplasm. The number of pores varies greatly — undifferentiated cells like eggs have about 30,000 pores per nucleus, while differentiated cells like erythrocytes have only 3–4. Each pore has a definite structure that controls the traffic of substances.
Double Membrane: Outer membrane continuous with ER; inner membrane encloses nuclear contents
Nuclear Pores: Fusion points of outer and inner membranes; regulate material exchange between nucleus and cytoplasm
Variable Pore Numbers: Undifferentiated cells (e.g., eggs) have about 30,000 pores; differentiated cells (e.g., erythrocytes) have only 3–4
Selective Transport: Each pore has a definite structure controlling which molecules pass through
The Nucleolus is a darkly stained body within the nucleus without a membranous boundary. It synthesises and stores Ribosomal RNA. The nucleolus has two regions: a peripheral granular area (precursors of ribosomal subunits) and a central fibrillar area (high molecular weight rRNA and rDNA). Ribosomes are assembled in the nucleolus and exported to the cytoplasm via nuclear pores. During cell division, chromatin condenses into Chromosomes — thread-like structures made of DNA and proteins. Each chromosome has a Centromere where spindle fibres attach and consists of two identical Chromatids held together at the centromere.
Nucleolus Composition: Peripheral granular area (ribosomal subunit precursors) and central fibrillar area (rRNA and rDNA)
Ribosome Factory: rRNA is synthesised in the nucleolus; ribosomes are assembled here and exported via nuclear pores
Chromatids: Each chromosome consists of two identical chromatids held together at the centromere at the start of cell division
Centromere: The attachment point for spindle fibres during cell division

Chromosome Numbers in Various Organisms

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Human — 46 (diploid), 23 (haploid)
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Chimpanzee — 48
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Frog — 26
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Fruit fly (Drosophila melanogaster) — 8 (diploid), 4 (haploid)
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Onion — 16
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Potato — 48
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Garden pea — 14

Plant Cell Specific Features

Plastids are membrane-bound, mostly pigment-containing bodies found exclusively in plant cells. There are three main types: Chloroplasts, Chromoplasts, and Leucoplasts. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy for photosynthesis. Chlorophyll resembles the haem group of haemoglobin but has Mg²⁺ as the central atom instead of Fe²⁺. Chloroplasts are 4–6 μm in diameter and contain three components: the double-membrane envelope, the fluid Stroma (with ribosomes, proteins, and circular DNA — the site of fixation), and Thylakoids (flattened vesicles stacked into Grana with about 50 thylakoids per granum). Chloroplasts are self-replicating. Chromoplasts provide non-green colours to petals and ripened fruits, aiding pollination and seed dispersal. Leucoplasts are colourless, found in underground parts, and store food.
Chloroplasts: Contain chlorophyll (Mg²⁺ centre); site of photosynthesis; have envelope, stroma (site of fixation), and thylakoids stacked as grana (site of light reactions and ATP formation)
Chromoplasts: Impart yellow, orange, and red colours to flowers and fruits; assist in pollination and seed dispersal
Leucoplasts: Colourless plastids in underground parts (roots, tubers); store starch, lipids, or proteins
Self-Replicating: Like mitochondria, chloroplasts contain their own DNA and ribosomes

Chloroplast Structure

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Envelope — double membrane outer covering
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Stroma — fluid matrix containing ribosomes, circular DNA, enzymes; site of Calvin cycle ( fixation)
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Thylakoids — flattened vesicles with chlorophyll molecules; stacked to form grana
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Grana — piles of ~50 thylakoids; site of light-dependent reactions and ATP synthesis
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Intergrana — non-green connections between grana
Plant cells contain a large Central Vacuole that can occupy a major portion of the cell volume, pushing other intracellular structures into a thin peripheral layer. Vacuoles are bounded by a single membrane and form by coalescence of smaller vacuoles during growth. The plant vacuole stores water, cell products, and metabolic intermediates. It also generates Turgor pressure that provides support for the cell and contributes to the rigidity of leaves and younger plant parts.
Central Vacuole: Extremely large, occupying most of the cell volume; pushes cytoplasm and organelles to the periphery
Formation: Develops by coalescence of smaller vacuoles during the plant's growth
Storage: Stores water, metabolic intermediates, cell products, and sometimes pigments or waste products
Turgor Pressure: The vacuole is the major contributor to turgor, providing structural support and rigidity to the cell
Glyoxysomes are specialised organelles found only in plant cells, particularly abundant in germinating seedlings that rely on stored fatty acids for energy. In addition to glycolic acid oxidase and catalase, they contain enzymes for the Glyoxylate Cycle, which converts stored fatty acids into carbohydrates (succinate). Glyoxysomes are present only during a short period of germination in lipid-rich seeds (e.g., castor bean, soybean) and are absent in lipid-poor seeds (e.g., pea).
Plant-Exclusive: Glyoxysomes are found only in plant cells
Glyoxylate Cycle: Converts stored fatty acids into carbohydrates (succinate) during seed germination
Temporary Presence: Present only during germination of lipid-rich seeds; absent in mature plants and lipid-poor seeds
Key Enzymes: Contain glycolic acid oxidase, catalase, and enzymes of the glyoxylate cycle

Animal Cell Specific Features

Centrioles are present in animal cells and cells of some microorganisms and lower plants, but absent in higher plants. Two centrioles are located near the exterior surface of the nucleus, typically placed at right angles to each other. In cross-section, each centriole consists of a cylindrical array of nine Microtubule Triplets — each of the nine microtubules is itself composed of three tubules. Before cell division, centrioles duplicate and one pair migrates to the opposite side of the nucleus to form the spindle. Centrioles also play a role in the location of furrowing during cell division and in the formation of cilia.
9 + 0 Arrangement: Each centriole has nine microtubule triplets arranged in a cylinder — no central microtubule
Perinuclear Position: Located near the exterior surface of the nucleus, perpendicular to each other
Role in Mitosis: Duplicate before cell division; one pair migrates to form spindle poles
Absent in Higher Plants: Higher plant cells lack centrioles entirely — spindle formation occurs without them
Lysosomes are single-membrane-bound sacs rich in acid phosphatase and other hydrolytic enzymes, synthesised on RER and processed in the Golgi apparatus. They are most abundant in animal cells that exhibit phagocytic activity. Lysosomes engulf foreign particles through Phagocytosis and digest them using hydrolytic enzymes. They also perform Autophagy — digesting old or worn-out organelles (such as old mitochondria) to recycle cell materials. When a lysosome fuses with a phagocytic vacuole, it forms a digestive vacuole (secondary lysosome). Lysosomes that digest parts of their own cell are called autophagosomes. Defective lysosomal enzymes cause storage diseases such as glycogenosis type II and Tay-Sachs disease.
Enzyme Origin: Hydrolytic enzymes are synthesised on RER, processed in Golgi apparatus, and budded off as primary lysosomes
Phagocytosis: Engulfing and digesting foreign particles — lysosomes are most abundant in phagocytic animal cells
Autophagy: Digestion of old or worn-out cellular organelles (e.g., old mitochondria) to recycle materials
Primary vs Secondary Lysosomes: Primary lysosomes contain inactive enzymes; secondary lysosomes form when primary lysosomes fuse with a phagocytic vacuole
Storage Diseases: Caused by mutations affecting lysosomal enzymes — e.g., Tay-Sachs disease (lipid accumulation in brain cells) and glycogenosis type II (glycogen accumulation)
Peroxisomes are single-membrane-enclosed organelles found in both animal and plant cells, approximately 0.5 μm in diameter. They contain -producing oxidases and catalase, and are specifically involved in the formation and decomposition of hydrogen peroxide. Peroxisomes also play important roles in both catabolic and anabolic pathways in plant cells.
Present in Both Types: Found in animal and plant cells, as well as in protozoa and yeast
Key Enzymes: Contain peroxidase, catalase, glycolic acid oxidase, and other oxidative enzymes
Hydrogen Peroxide Metabolism: Specifically involved in the formation and breakdown of
Size: Approximately 0.5 μm in diameter

Comparative Summary: Plant vs Animal Cells

Both plant and animal cells are Eukaryotic — they share a Plasma Membrane, membrane-bound Nucleus, Cytoplasm with cytosol, and many of the same organelles including Endoplasmic Reticulum (both RER and SER), Ribosomes (80S), Golgi Apparatus, Mitochondria, Peroxisomes, and a Cytoskeleton of microtubules, microfilaments, and intermediate filaments.
Shared Organelles: Plasma membrane, nucleus (with nuclear envelope, nucleolus, chromatin/chromosomes), cytoplasm, ER, ribosomes, Golgi apparatus, mitochondria, peroxisomes, cytoskeleton
Shared Properties: Both are eukaryotic, have 80S ribosomes, contain DNA in the nucleus, and carry out similar metabolic processes
Despite their shared features, plant and animal cells differ in several important ways. Plant cells have a rigid Cell Wall (cellulose-based), a large Central Vacuole for turgor and storage, Chloroplasts and other Plastids for photosynthesis and storage, and Glyoxysomes for fatty acid conversion. Animal cells have Centrioles for spindle formation, abundant Lysosomes for intracellular digestion, smaller and more numerous vacuoles, and the ability to perform Endocytosis (phagocytosis and pinocytosis). Higher plant cells completely lack centrioles and lysosomes.
Plant Cell Exclusive: Cell wall (cellulose), large central vacuole, chloroplasts, chromoplasts, leucoplasts, glyoxysomes
Animal Cell Exclusive/Prominent: Centrioles, abundant lysosomes, smaller vacuoles, endocytosis capability
Shape Difference: Plant cells are typically rectangular or rigid due to the cell wall; animal cells are irregular or round due to the flexible plasma membrane
Nucleus Position: Central in animal cells; pushed to the periphery in plant cells by the large vacuole

Plant Cell vs Animal Cell — Key Differences

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Cell wall — present in plants (cellulose), absent in animals
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Chloroplasts — present in plants, absent in animals
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Central vacuole — large and single in plants, small and multiple in animals
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Centrioles — present in animals, absent in higher plants
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Lysosomes — abundant in animals, absent in most plant cells
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Glyoxysomes — present in plants only, absent in animals
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Plastids (chromoplasts, leucoplasts) — plant cells only
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Endocytosis — common in animal cells, rare/absent in plant cells due to cell wall
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Cell shape — rectangular/rigid in plants, irregular/round in animals
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Nucleus position — peripheral in plants, central in animals