Cell structure Revision Notes

Both plant and animal cells have a cell membrane, a flexible layer that controls the movement of substances in and out of the cell. It acts as a selective barrier, allowing nutrients to enter and waste products to leave.

Plant cells have an additional outer layer called the cell wall, which animal cells lack. The cell wall is made of cellulose, a strong carbohydrate, providing rigidity and support to the plant cell. This helps plants maintain their shape and resist mechanical stress.

For example, the cell wall enables a tree's stem to stand upright against wind, whereas animal cells rely on other structures like the skeleton for support.

• Cell membrane: present in both plant and animal cells; controls substance movement.
• Cell wall: present only in plant cells; made of cellulose; provides structural support.

The nucleus is a membrane-bound organelle found in both plant and animal cells. It contains the cell’s genetic material (DNA) and controls cell activities such as growth and reproduction.

Surrounding the nucleus is the cytoplasm, a jelly-like substance where many cellular processes occur. It contains enzymes and organelles necessary for metabolism.

For instance, in both cell types, the cytoplasm facilitates chemical reactions like respiration and protein synthesis by providing a medium for molecules to interact.

• Nucleus: contains DNA; controls cell functions; present in both cell types.
• Cytoplasm: gel-like fluid; site of metabolic reactions; present in both cell types.

Chloroplasts are organelles found only in plant cells. They contain chlorophyll, a green pigment that captures light energy to carry out photosynthesis, producing glucose and oxygen from carbon dioxide and water.

Animal cells do not have chloroplasts because they obtain energy by consuming organic matter rather than making their own food.

For example, chloroplasts enable a leaf cell to convert sunlight into chemical energy, which supports the plant’s growth and development.

• Chloroplasts: present only in plant cells; site of photosynthesis; contain chlorophyll.
• Absent in animal cells.

Ribosomes are small structures found in both plant and animal cells. They are the sites where proteins are made by assembling amino acids according to instructions from the nucleus.

Ribosomes can be free-floating in the cytoplasm or attached to the endoplasmic reticulum (not required to be named here).

For example, ribosomes produce enzymes that catalyse reactions essential for cell survival.

• Ribosomes: present in both cell types; responsible for protein synthesis.

Mitochondria are organelles present in both plant and animal cells. They are often called the 'powerhouses' of the cell because they generate energy through aerobic respiration, converting glucose and oxygen into ATP (adenosine triphosphate).

This energy is vital for all cellular activities, such as movement, growth, and repair.

For instance, muscle cells in animals have many mitochondria to meet high energy demands during movement.

• Mitochondria: present in both cell types; site of aerobic respiration; produce energy (ATP).

Vacuoles are membrane-bound sacs used for storage. Plant cells typically have a large central vacuole filled with cell sap, which helps maintain turgor pressure, keeping the cell firm and supporting the plant structure.

Animal cells may have small, temporary vacuoles used for storing nutrients or waste, but these are much smaller and less prominent than in plant cells.

For example, the large vacuole in a plant cell helps a sunflower stem remain upright by exerting pressure against the cell wall.

• Vacuoles: large central vacuole in plant cells for storage and turgor pressure.
• Small or absent vacuoles in animal cells.

The bacterial cell wall is a rigid outer layer that maintains the cell’s shape and protects it from mechanical damage and osmotic pressure changes. Unlike plant cell walls made of cellulose, bacterial cell walls are primarily composed of peptidoglycan, a polymer that provides strength and rigidity. This structure prevents the cell from bursting in hypotonic environments, where water tends to enter the cell by osmosis.

• Provides structural support and shape.
• Protects against physical damage and osmotic lysis.
• Made mainly of peptidoglycan in bacteria.

Beneath the cell wall lies the cell membrane, a phospholipid bilayer embedded with proteins. It controls the movement of substances into and out of the cell, allowing nutrients to enter and waste products to leave. The membrane’s selective permeability is essential for maintaining the internal environment of the bacterial cell, enabling processes such as respiration and nutrient uptake.

• Selectively permeable barrier.
• Regulates transport of molecules.
• Site of metabolic activities like respiration.

The cytoplasm is a gel-like substance filling the interior of the bacterial cell. It contains water, enzymes, nutrients, wastes, and other molecules necessary for cellular processes. Most metabolic reactions, including glycolysis and protein synthesis, occur here. The cytoplasm also houses ribosomes and genetic material.

• Medium for metabolic reactions.
• Contains enzymes and molecules for cell function.
• Houses ribosomes and genetic material.

Ribosomes in bacteria are smaller than those in eukaryotic cells (70S compared to 80S). They are the sites of protein synthesis, translating messenger RNA into polypeptide chains. Proteins produced are vital for cell structure, enzymes, and other functions.

• Smaller 70S ribosomes.
• Synthesize proteins from mRNA.
• Essential for cell growth and function.

Bacterial genetic material is typically a single circular DNA molecule located in the nucleoid region, not enclosed by a membrane. This DNA contains all the essential genes required for the cell’s survival and reproduction. Unlike eukaryotic cells, bacteria lack a nucleus, so their DNA is free in the cytoplasm.

• Single circular chromosome.
• Contains essential genetic information.
• Located in nucleoid region without membrane.

Plasmids are small, circular DNA molecules separate from the main chromosome. They often carry genes that provide advantages, such as antibiotic resistance or the ability to metabolise unusual substances. Plasmids can be transferred between bacteria, facilitating genetic variation and adaptation.

• Extra-chromosomal circular DNA.
• Carry beneficial genes like antibiotic resistance.
• Can be exchanged between bacteria.

Cells are the basic units of life and come in different types with distinct structures. The three main types studied here are plant cells, animal cells, and bacterial cells. Each type has characteristic components that can be identified in diagrams or microscope images. Recognising these structures helps understand cell function and classification.

• Plant and animal cells are eukaryotic, meaning they have a nucleus.
• Bacterial cells are prokaryotic and lack a nucleus.

Plant cells have several structures that are visible in diagrams and images. The cell wall is a rigid outer layer that provides support and shape. Inside, the cell membrane controls movement of substances in and out. The nucleus contains genetic material and controls cell activities. Cytoplasm is the gel-like substance where chemical reactions occur. Chloroplasts are green structures where photosynthesis happens. Vacuoles are large fluid-filled sacs that maintain cell pressure and store substances. Ribosomes and mitochondria are also present but usually too small to see clearly in basic diagrams.

• Cell wall: rigid, made of cellulose, surrounds the cell membrane.
• Chloroplasts: contain chlorophyll, site of photosynthesis.
• Large central vacuole: maintains turgor pressure.

Animal cells lack a cell wall and chloroplasts, which distinguishes them from plant cells. They have a flexible cell membrane that controls substance movement. The nucleus contains DNA and regulates cell functions. Cytoplasm fills the cell interior. Vacuoles are smaller and less prominent than in plant cells. Animal cells contain mitochondria for energy release and ribosomes for protein synthesis. Centrioles, involved in cell division, are present but not always shown in diagrams.

• No cell wall or chloroplasts.
• Cell membrane is flexible.
• Smaller vacuoles compared to plant cells.
• Presence of centrioles (not in plant cells).

Bacterial cells are prokaryotic and structurally simpler. They have a cell wall that provides shape and protection but differs chemically from plant cell walls. The cell membrane lies beneath the cell wall. Cytoplasm contains ribosomes but no membrane-bound organelles like mitochondria or chloroplasts. Instead of a nucleus, bacteria have circular DNA free in the cytoplasm. Plasmids are small rings of DNA that carry extra genetic information. These features can be identified in diagrams to distinguish bacteria from eukaryotic cells.

• Cell wall present but chemically different from plants.
• No nucleus; DNA is circular and free in cytoplasm.
• Plasmids: small DNA rings.
• No membrane-bound organelles.

When identifying cell structures, look for shape, size, and presence or absence of key components. Plant cells often appear rectangular due to the cell wall, while animal cells are more rounded. Bacterial cells are smaller and simpler. Labelled diagrams help confirm identification. Understanding the function of each structure supports correct recognition. For example, chloroplasts are only in plant cells and appear as green oval shapes in images. Recognising the absence of structures, such as no nucleus in bacteria, is equally important.

• Plant cells: rectangular shape, cell wall, chloroplasts visible.
• Animal cells: rounded shape, no cell wall or chloroplasts.
• Bacterial cells: smaller, no nucleus, presence of plasmids.

The nucleus is a membrane-bound structure that contains the cell's genetic material in the form of DNA. It controls all the activities of the cell by regulating gene expression and directing protein synthesis. For example, in animal cells, the nucleus signals when to divide or produce enzymes needed for metabolism. Without a functioning nucleus, the cell cannot properly regulate its functions or reproduce.

• Contains DNA which stores genetic information.
• Controls cell activities by regulating gene expression.
• Directs protein synthesis essential for cell function.

Mitochondria are often called the 'powerhouses' of the cell because they generate energy through aerobic respiration. They convert glucose and oxygen into ATP (adenosine triphosphate), the energy currency of the cell. For instance, muscle cells have many mitochondria to meet high energy demands. The process involves glucose reacting with oxygen to produce carbon dioxide, water, and energy:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP) (in mitochondria)

• Site of aerobic respiration producing ATP.
• Convert glucose and oxygen into usable energy.
• More mitochondria present in energy-demanding cells.

The cell membrane surrounds both plant and animal cells, controlling the movement of substances in and out. It is selectively permeable, allowing essential nutrients to enter and waste products to leave. For example, oxygen and glucose enter the cell through the membrane, while carbon dioxide exits. This regulation maintains the internal environment of the cell, essential for homeostasis.

• Controls entry and exit of substances.
• Maintains internal cell environment.
• Allows selective permeability for nutrients and waste.

Found only in plant cells, the cell wall is a rigid layer outside the cell membrane made mainly of cellulose. It provides structural support and protection, helping the plant maintain its shape and resist mechanical stress. For example, the cell wall prevents plant cells from bursting when water enters by osmosis, maintaining turgor pressure essential for plant rigidity.

• Made of cellulose, providing rigidity.
• Protects and supports plant cells.
• Prevents excessive water uptake by maintaining turgor pressure.

Chloroplasts are present only in plant cells and some algae. They contain chlorophyll, a green pigment that captures light energy to convert carbon dioxide and water into glucose and oxygen during photosynthesis. For example, leaves have many chloroplasts to maximise energy capture. This process provides the energy source for the plant and oxygen for other organisms.

Photosynthesis equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2

• Contain chlorophyll for capturing light energy.
• Convert CO2 and water into glucose and oxygen.
• Essential for autotrophic nutrition in plants.

Vacuoles are fluid-filled sacs within cells. In plant cells, a large central vacuole stores water, nutrients, and waste products, helping maintain cell turgor pressure which supports the plant structure. Animal cells may have small vacuoles for storage but they are less prominent. For example, when a plant cell loses water, the vacuole shrinks, causing wilting.

• Store water, nutrients, and waste products.
• Maintain turgor pressure in plant cells.
• Support cell structure and regulate internal conditions.

A cell is the smallest unit capable of carrying out all life processes. It forms the foundation of biological structure and function. For example, a red blood cell in humans carries oxygen, while a leaf cell in plants performs photosynthesis. Cells vary in shape and function but share common features such as a cell membrane and cytoplasm.

• Cells perform essential life functions individually.
• Examples: red blood cell (animal), palisade mesophyll cell (plant).

A tissue consists of many similar cells that perform a shared function. Grouping cells into tissues allows organisms to carry out more complex tasks efficiently. For instance, muscle tissue in animals is made of muscle cells that contract to produce movement. In plants, xylem tissue transports water from roots to leaves.

• Tissues are collections of similar cells.
• They specialise in particular functions.
• Examples: muscle tissue (animal), xylem tissue (plant).

An organ is a structure made up of several types of tissues working together to perform specific functions. Different tissues contribute distinct roles within the organ. For example, the heart contains muscle tissue to pump blood, connective tissue for support, and nervous tissue to regulate heartbeat. In plants, a leaf is an organ containing tissues for photosynthesis, transport, and protection.

• Organs combine multiple tissue types.
• Each tissue contributes to the organ's overall function.
• Examples: heart (animal), leaf (plant).

An organ system is a collection of organs that work together to carry out broad physiological functions essential for survival. For example, the circulatory system includes the heart, blood vessels, and blood, collaborating to transport nutrients and oxygen throughout the body. In plants, the root and shoot systems coordinate to absorb water and nutrients and perform photosynthesis.

• Organ systems integrate multiple organs.
• They perform complex, vital functions.
• Examples: circulatory system (animal), root and shoot systems (plant).

An organism is a complete living individual made up of multiple organ systems functioning together. It can carry out all life processes independently, such as growth, reproduction, and response to stimuli. Examples include a human, a dog, or a flowering plant. The organism represents the highest level of biological organization in this hierarchy.

• Organisms are whole living beings.
• They consist of integrated organ systems.
• Examples: human, oak tree, bacterium.

In multicellular organisms, cells are not all identical; instead, many become specialized to perform specific tasks efficiently. This specialization involves structural changes that suit the cell’s role, allowing the organism to function effectively as a whole. Understanding these specialized cells helps explain how complex biological processes are carried out at the cellular level.

• Specialized cells have structures adapted to their functions.
• They contribute to the organism’s survival by performing specific roles.
• Examples include root hair cells, palisade mesophyll cells, and red blood cells.

Root hair cells are found in the roots of plants and are responsible for absorbing water and mineral ions from the soil. Their main adaptation is the presence of a long, thin extension called a root hair, which increases the surface area available for absorption. This large surface area allows more water and minerals to enter the plant efficiently.

The cell membrane of root hair cells is selectively permeable, controlling the uptake of substances. Additionally, these cells have many mitochondria to provide energy for active transport of mineral ions against concentration gradients when necessary.

• Long, thin root hair increases surface area for absorption.
• Selective cell membrane controls uptake of water and minerals.
• Many mitochondria supply energy for active transport.

Palisade mesophyll cells are located in the upper part of plant leaves and are the primary site of photosynthesis. They contain numerous chloroplasts, which house chlorophyll pigments that capture light energy. The cells are elongated and closely packed to maximise light absorption.

The arrangement of palisade cells near the leaf surface allows them to receive maximum sunlight. Their thin cell walls facilitate the diffusion of carbon dioxide into the cells, which is essential for photosynthesis.

• Contain many chloroplasts with chlorophyll for light absorption.
• Elongated shape and dense packing maximise light capture.
• Thin walls allow efficient carbon dioxide diffusion.

Red blood cells (erythrocytes) are specialized animal cells that transport oxygen from the lungs to tissues throughout the body. They have a distinctive biconcave disc shape, which increases their surface area relative to volume, enhancing oxygen absorption and release.

Unlike most cells, mature red blood cells lack a nucleus, providing more internal space for haemoglobin, the oxygen-carrying protein. Their flexible membranes allow them to squeeze through narrow capillaries, ensuring oxygen delivery even in small blood vessels.

• Biconcave shape increases surface area for oxygen exchange.
• No nucleus allows more haemoglobin storage.
• Flexible membrane enables passage through narrow capillaries.

Each specialized cell type has structural features that directly support its function. Root hair cells maximise absorption through increased surface area and energy supply for active transport. Palisade mesophyll cells optimise photosynthesis by containing many chloroplasts and positioning near the leaf surface. Red blood cells are shaped and structured to carry and deliver oxygen efficiently throughout the body.

• Structure of specialized cells matches their specific function.
• Adaptations improve efficiency of absorption, photosynthesis, or oxygen transport.
• Understanding these cells aids comprehension of organism biology.