Cell: Structure & Function

• The cell is the smallest unit of life that can survive and carry out all life activities on its own.
• It is called the basic structural and functional unit of life – the building block and the smallest working part.
• Unicellular organisms have one cell (Amoeba, bacteria) that does everything itself.
• Multicellular organisms (humans, plants) are made of many cells with a division of labour.
• Inside a cell, tiny organelles each handle one special task (energy, waste, etc.).
• Anything smaller than a cell cannot live independently.

The cell is the smallest unit of life that can survive and carry out all life activities on its own.

It is called the basic structural and functional unit of life – the building block and the smallest working part.

Unicellular organisms have one cell (Amoeba, bacteria) that does everything itself.

Multicellular organisms (humans, plants) are made of many cells with a division of labour.

Inside a cell, tiny organelles each handle one special task (energy, waste, etc.).

• The cell was discovered only after the microscope was invented.
• Robert Brown (1831) discovered the nucleus.
• Purkinje coined the term protoplasm for the living substance of the cell.
• Robert Hooke — 1665 — Coined 'cell' (dead cork)
• Leeuwenhoek — 1674 — First saw living cells
• Robert Brown — 1831 — Discovered nucleus
• Schleiden — 1838 — All plants are cells
• Schwann — 1839 — All animals are cells
• Virchow — 1855 — Cells from pre-existing cells
• Robert Hooke (1665) saw box-like rooms in dead cork and coined the word 'cell' in his book Micrographia; he saw only empty walls.
• Anton van Leeuwenhoek (1674) was the first to see living cells – bacteria, sperm, pond creatures.

The cell was discovered only after the microscope was invented.

Robert Brown (1831) discovered the nucleus.

Purkinje coined the term protoplasm for the living substance of the cell.

Robert Hooke (1665) saw box-like rooms in dead cork and coined the word 'cell' in his book Micrographia; he saw only empty walls.

Anton van Leeuwenhoek (1674) was the first to see living cells – bacteria, sperm, pond creatures.

• Rudolf Virchow (1855) showed every cell arises from a pre-existing cell – Omnis cellula e cellula.
• Point 1: All living things are made of cells.
• Point 2: The cell is the basic unit of life.
• Point 3: Every cell comes from a pre-existing cell.
• M. J. Schleiden (1838) stated all plants are made of cells.
• Theodor Schwann (1839) stated all animals are made of cells.

Rudolf Virchow (1855) showed every cell arises from a pre-existing cell – Omnis cellula e cellula.

M. J. Schleiden (1838) stated all plants are made of cells.

Theodor Schwann (1839) stated all animals are made of cells.

• A prokaryotic cell (pro = primitive) has no membranebound nucleus; its DNA lies naked in the cytoplasm.
• Examples: bacteria and blue-green algae (cyanobacteria).
• Prokaryotes have no membrane-bound organelles – no mitochondria or chloroplast, only ribosomes.
• Their membrane folds inward to form a mesosome, found only in prokaryotes.
• A eukaryotic cell (eu = true) has a true nucleus enclosed by a nuclear membrane.
• Eukaryotes: all plants, animals, fungi and protists.
• Many bacteria carry extra DNA rings called plasmids, which give antibiotic resistance and are used in genetic engineering.
• Nucleus — No membrane (naked DNA) — True, membrane-bound
• Organelles — Absent — Present
• Size — Small (1-10 µm) — Large (10-100 µm)
• DNA — Single circular — Linear, many chromosomes
• Examples — Bacteria, cyanobacteria — Plants, animals, fungi

A prokaryotic cell (pro = primitive) has no membranebound nucleus; its DNA lies naked in the cytoplasm.

Examples: bacteria and blue-green algae (cyanobacteria).

Prokaryotes have no membrane-bound organelles – no mitochondria or chloroplast, only ribosomes.

Their membrane folds inward to form a mesosome, found only in prokaryotes.

A eukaryotic cell (eu = true) has a true nucleus enclosed by a nuclear membrane.

Many bacteria carry extra DNA rings called plasmids, which give antibiotic resistance and are used in genetic engineering.

• Both are eukaryotic and share nucleus, cytoplasm, mitochondria, ribosomes, ER, Golgi and cell membrane.
• A plant cell has a rigid cell wall of cellulose outside the membrane.
• Plant cells have green chloroplasts for photosynthesis and one large central vacuole.
• An animal cell has no cell wall, no chloroplast and only small or no vacuoles.
• Animal cells have a centriole that helps in cell division.
• Plant cells keep a fixed box-like shape; animal cells take many shapes.
• Cell wall — Present (cellulose) — Absent
• Chloroplast — Present — Absent
• Vacuole — One large central — Small or absent
• Centriole — Absent — Present
• Shape — Fixed, box-like — Variable

Both are eukaryotic and share nucleus, cytoplasm, mitochondria, ribosomes, ER, Golgi and cell membrane.

A plant cell has a rigid cell wall of cellulose outside the membrane.

Plant cells have green chloroplasts for photosynthesis and one large central vacuole.

An animal cell has no cell wall, no chloroplast and only small or no vacuoles.

Animal cells have a centriole that helps in cell division.

• The cell membrane (plasma membrane) is the thin living skin around every cell.
• It is selectively permeable – a gatekeeper letting in food and oxygen and pushing out waste.
• Chemically the membrane is made of a lipid bilayer with proteins (lipids and proteins).
• The cell wall is a non-living, rigid covering in plants, fungi and bacteria, outside the membrane.
• In plants it is made mainly of cellulose and gives shape and protection.
• The young, flexible primary wall forms first; a tougher secondary wall is added inside as the cell matures.

The cell membrane (plasma membrane) is the thin living skin around every cell.

It is selectively permeable – a gatekeeper letting in food and oxygen and pushing out waste.

Chemically the membrane is made of a lipid bilayer with proteins (lipids and proteins).

The cell wall is a non-living, rigid covering in plants, fungi and bacteria, outside the membrane.

In plants it is made mainly of cellulose and gives shape and protection.

The young, flexible primary wall forms first; a tougher secondary wall is added inside as the cell matures.

• The structure of the membrane is explained by the Fluid Mosaic Model (Singer & Nicolson, 1972).
• It has a phospholipid bilayer with hydrophilic heads outside and hydrophobic tails inside.
• Proteins float in or across this layer like a mosaic of tiles.
• 'Fluid' means the lipids and proteins can move sideways, giving the membrane flexibility.
• Cholesterol in animal membranes keeps them stable.

The structure of the membrane is explained by the Fluid Mosaic Model (Singer & Nicolson, 1972).

It has a phospholipid bilayer with hydrophilic heads outside and hydrophobic tails inside.

Proteins float in or across this layer like a mosaic of tiles.

'Fluid' means the lipids and proteins can move sideways, giving the membrane flexibility.

Cholesterol in animal membranes keeps them stable.

• The nucleus is the brain or control centre directing all cell activities.
• It stores the instruction manual of life – the genes (DNA).
• It is wrapped in a double-layered nuclear envelope dotted with nuclear pores for transport.
• Inside is the nucleolus, which makes ribosomes.
• DNA winds with protein into thread-like chromatin, which condenses into chromosomes during division.
• Humans have 46 chromosomes (23 pairs).

The nucleus is the brain or control centre directing all cell activities.

It stores the instruction manual of life – the genes (DNA).

It is wrapped in a double-layered nuclear envelope dotted with nuclear pores for transport.

Inside is the nucleolus, which makes ribosomes.

DNA winds with protein into thread-like chromatin, which condenses into chromosomes during division.

• Cytoplasm is the jelly-like fluid filling the cell where organelles float and reactions occur.
• Mitochondria are the powerhouse of the cell, making energy as ATP through respiration.
• Mitochondria have their own DNA, can self-replicate, and have a folded inner membrane (cristae).
• Chloroplasts contain chlorophyll and convert solar energy into food (and ATP) by photosynthesis.
• Chloroplasts also have their own DNA, which qualifies them for self-replication.
• Mitochondria and chloroplasts are called semiautonomous organelles.

Cytoplasm is the jelly-like fluid filling the cell where organelles float and reactions occur.

Mitochondria are the powerhouse of the cell, making energy as ATP through respiration.

Mitochondria have their own DNA, can self-replicate, and have a folded inner membrane (cristae).

Chloroplasts contain chlorophyll and convert solar energy into food (and ATP) by photosynthesis.

Chloroplasts also have their own DNA, which qualifies them for self-replication.

Mitochondria and chloroplasts are called semiautonomous organelles.

• Ribosomes are the site of protein synthesis; they are the only organelle in prokaryotes.
• The Endoplasmic Reticulum (ER) is a network of tubes; rough ER (with ribosomes) makes proteins, smooth ER makes fats.
• The Golgi apparatus packages, modifies and dispatches materials, like a post office.
• Lysosomes are the suicide bags with digestive enzymes that clear waste and worn-out parts.
• Vacuoles store water, food and waste; large in plant cells.
• Centriole/centrosome (animal cells) organises fibres during cell division.
• Ribosome — Protein synthesis
• Rough ER — Makes proteins
• Smooth ER — Makes fats/lipids
• Golgi body — Packaging & dispatch
• Lysosome — Digestion (suicide bag)
• Vacuole — Storage

Ribosomes are the site of protein synthesis; they are the only organelle in prokaryotes.

The Endoplasmic Reticulum (ER) is a network of tubes; rough ER (with ribosomes) makes proteins, smooth ER makes fats.

The Golgi apparatus packages, modifies and dispatches materials, like a post office.

Lysosomes are the suicide bags with digestive enzymes that clear waste and worn-out parts.

Vacuoles store water, food and waste; large in plant cells.

Centriole/centrosome (animal cells) organises fibres during cell division.

• A virus is on the borderline of living and non-living – it is acellular (not a true cell).
• It is made of a nucleic acid core (DNA or RNA) inside a protein coat (capsid).
• Viruses have no enzymes and no cell machinery of their own.
• Outside a host they behave like non-living crystals; inside a host cell they reproduce.
• They are an obligate intracellular parasite, depending fully on a host cell.
• Examples of viral diseases: COVID-19, influenza, AIDS, polio.

A virus is on the borderline of living and non-living – it is acellular (not a true cell).

Outside a host they behave like non-living crystals; inside a host cell they reproduce.

They are an obligate intracellular parasite, depending fully on a host cell.

• The cell membrane is composed of lipids and proteins (biological membranes).
• Solar energy is converted into ATP in the chloroplast.
• Viruses have no enzymes in them.
• The largest cell is the ostrich egg; the longest cell is the nerve cell (neuron).
• The smallest cell is the bacterium Mycoplasma (PPLO).
• Human RBCs and sieve tube cells lack a nucleus when mature.

The largest cell is the ostrich egg; the longest cell is the nerve cell (neuron).

The smallest cell is the bacterium Mycoplasma (PPLO).