Think of a cell like a highly organized factory, where every department has a specific physical structure designed perfectly for its daily biological role. Animal cells are classified as , meaning they are complex and contain membrane-bound structures called organelles.
are rod-shaped organelles and are the site of . They release energy for the cell in the form of ATP. They are highly concentrated in active cells, such as muscle or sperm cells.
To understand how they work, you must link their structure to their function. are enclosed by a double membrane. The inner membrane is highly folded into structures called . This folding provides a very large surface area for the enzymes involved in respiration, significantly increasing the efficiency of energy release.
The chemical reaction taking place on these folded membranes is:
Have you ever wondered why a plant can stand upright without a bony skeleton? Plant cells possess all the sub-cellular structures found in animal cells, but they have three additional features specifically adapted for structure and making food.
The interaction between these structures is crucial. When water enters a plant cell via osmosis, the fills with and expands. This exerts outward turgor pressure, pushing the against the rigid . The wall resists this pressure, keeping the cell (firm) and supporting the whole plant.
Bacteria are ancient survivors with a stripped-down, simpler cellular toolkit. They are , meaning they lack a and completely lack membrane-bound organelles. Prokaryotes do not have or .
Instead of a , bacterial genetic material is split into two types:
Bacteria are enclosed by a and a . However, the bacterial is made of (murein), not . They also contain for , though bacterial are physically smaller than eukaryotic ones.
Many bacteria feature (singular: flagellum). These are long, thin, whip-like protein structures. They function by rotating like a propeller or motor, allowing the bacterium to move through liquids towards nutrients or away from toxins.
Bacteria reproduce asexually through a process called . During this process, the circular DNA and plasmids replicate, the cell elongates, and a new forms down the middle to divide the , resulting in two identical daughter cells.
When comparing the two cell types, you must look at both scale and structure. are typically in length, whereas are roughly 10 to 100 times smaller, usually measuring just .
| Feature | Eukaryotic Cells (Plant/Animal) | Prokaryotic Cells (Bacteria) |
|---|---|---|
| Genetic Material | Linear DNA enclosed within a . | Circular DNA loop free-floating in the . |
| Extra DNA | Plasmids are not generally present. | Often contain small rings of . |
| Organelles | Contain membrane-bound organelles (e.g., ). | Do NOT contain membrane-bound organelles. |
| (plants only), none in animals. | Made of . |
Never describe the nucleus as the 'brain' of the cell. Examiners require you to use the phrases 'controls the cell's activities' or 'contains genetic material'.
Always state that mitochondria 'release energy' or 'release ATP'. Never write that they 'make', 'create', or 'produce' energy, as energy cannot be created.
In 6-mark 'Compare' questions, make sure to explicitly state what BOTH cells have to get the mark (e.g., 'Eukaryotes have linear DNA in a nucleus, WHEREAS prokaryotes have circular DNA free-floating in the cytoplasm').
In Edexcel mark schemes, you must clearly distinguish that the cell membrane is 'partially permeable', whereas the plant cell wall is 'freely permeable'.
Eukaryotic cell
A complex cell that contains a nucleus and membrane-bound organelles, such as animal and plant cells.
Prokaryotic cell
A simple, unicellular organism that lacks a nucleus and membrane-bound organelles, such as a bacterium.
Nucleus
A sub-cellular structure that contains genetic material (DNA) as linear chromosomes and controls the activities of the cell.
Chromosomes
Linear structures made of DNA, found in the nucleus of eukaryotic cells, which carry genetic information.
Cell membrane
A partially permeable barrier that controls the movement of substances into and out of the cell.
Partially permeable
Describes a membrane that allows certain substances to pass through but restricts others based on size and chemical properties.
Cytoplasm
A jelly-like substance mostly made of water and dissolved solutes, where most chemical (metabolic) reactions occur.
Ribosomes
Tiny structures found in the cytoplasm where the translation of genetic material takes place to produce proteins.
Protein synthesis
The cellular process (translation) where ribosomes assemble amino acids to create proteins.
Mitochondria
The sub-cellular structures where aerobic respiration takes place, releasing energy (ATP) for the cell.
Aerobic respiration
The chemical process occurring in mitochondria that uses oxygen to break down glucose, releasing energy (ATP) for the cell.
Cristae
The highly folded inner membrane of mitochondria that provides a large surface area for the enzymes involved in respiration.
Chloroplasts
Organelles found in plant cells that contain chlorophyll and are the site of photosynthesis.
Chlorophyll
A green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Photosynthesis
The process by which plants use light energy to convert carbon dioxide and water into glucose.
Cell wall
A rigid outer layer that provides structural support; made of cellulose in plants and peptidoglycan in bacteria.
Cellulose
A complex carbohydrate that makes up the plant cell wall, providing it with high tensile strength.
Freely permeable
Describes a barrier, like the plant cell wall, that allows all substances to pass through without restriction.
Permanent vacuole
A large central sac in plant cells that contains cell sap, helping to keep the cell turgid.
Cell sap
A liquid solution of water, dissolved sugars, mineral ions, and salts found inside the permanent vacuole of a plant cell.
Chromosomal DNA
A single, long, circular loop of DNA found free-floating in the cytoplasm of bacteria, carrying essential genetic information.
Nucleoid
The irregular region within the cytoplasm of a prokaryote where the chromosomal DNA is located.
Plasmid DNA
Small, circular rings of extra, double-stranded DNA found in bacteria that can carry advantageous genes such as antibiotic resistance.
Antibiotic resistance
The ability of a bacterium to survive exposure to antibiotics, often conferred by genes carried on plasmid DNA.
Peptidoglycan
A polymer that makes up the cell wall of bacteria, providing structural support and protection.
Flagella
Long, whip-like protein structures on some bacteria that rotate to allow the cell to move.
Turgid
The state of a plant cell when it is swollen and firm due to high internal water pressure pushing the cell membrane against the cell wall.
Binary fission
A type of asexual reproduction used by bacteria where the cell divides into two identical daughter cells after replicating its DNA.
Put your knowledge into practice — try past paper questions for Biology
Eukaryotic cell
A complex cell that contains a nucleus and membrane-bound organelles, such as animal and plant cells.
Prokaryotic cell
A simple, unicellular organism that lacks a nucleus and membrane-bound organelles, such as a bacterium.
Nucleus
A sub-cellular structure that contains genetic material (DNA) as linear chromosomes and controls the activities of the cell.
Chromosomes
Linear structures made of DNA, found in the nucleus of eukaryotic cells, which carry genetic information.
Cell membrane
A partially permeable barrier that controls the movement of substances into and out of the cell.
Partially permeable
Describes a membrane that allows certain substances to pass through but restricts others based on size and chemical properties.
Cytoplasm
A jelly-like substance mostly made of water and dissolved solutes, where most chemical (metabolic) reactions occur.
Ribosomes
Tiny structures found in the cytoplasm where the translation of genetic material takes place to produce proteins.
Protein synthesis
The cellular process (translation) where ribosomes assemble amino acids to create proteins.
Mitochondria
The sub-cellular structures where aerobic respiration takes place, releasing energy (ATP) for the cell.
Aerobic respiration
The chemical process occurring in mitochondria that uses oxygen to break down glucose, releasing energy (ATP) for the cell.
Cristae
The highly folded inner membrane of mitochondria that provides a large surface area for the enzymes involved in respiration.
Chloroplasts
Organelles found in plant cells that contain chlorophyll and are the site of photosynthesis.
Chlorophyll
A green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Photosynthesis
The process by which plants use light energy to convert carbon dioxide and water into glucose.
Cell wall
A rigid outer layer that provides structural support; made of cellulose in plants and peptidoglycan in bacteria.
Cellulose
A complex carbohydrate that makes up the plant cell wall, providing it with high tensile strength.
Freely permeable
Describes a barrier, like the plant cell wall, that allows all substances to pass through without restriction.
Permanent vacuole
A large central sac in plant cells that contains cell sap, helping to keep the cell turgid.
Cell sap
A liquid solution of water, dissolved sugars, mineral ions, and salts found inside the permanent vacuole of a plant cell.
Chromosomal DNA
A single, long, circular loop of DNA found free-floating in the cytoplasm of bacteria, carrying essential genetic information.
Nucleoid
The irregular region within the cytoplasm of a prokaryote where the chromosomal DNA is located.
Plasmid DNA
Small, circular rings of extra, double-stranded DNA found in bacteria that can carry advantageous genes such as antibiotic resistance.
Antibiotic resistance
The ability of a bacterium to survive exposure to antibiotics, often conferred by genes carried on plasmid DNA.
Peptidoglycan
A polymer that makes up the cell wall of bacteria, providing structural support and protection.
Flagella
Long, whip-like protein structures on some bacteria that rotate to allow the cell to move.
Turgid
The state of a plant cell when it is swollen and firm due to high internal water pressure pushing the cell membrane against the cell wall.
Binary fission
A type of asexual reproduction used by bacteria where the cell divides into two identical daughter cells after replicating its DNA.