Cell structure — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 20Animal cells

Nucleus

• Nucleus has a distinct membrane and contains the DNA that controls the activities of the cell.

No cell wall

• No cell wall animal cells have no cellulose cell wall.

No chloroplasts

• No chloroplasts animal cells cannot photosynthesise.

Glycogen

• Glycogen animal cells store carbohydrate as glycogen.

Section 2 of 20Plant cells

Nucleus

• Nucleus has a distinct membrane and contains the DNA.

Cellulose cell wall

• Cellulose cell wall surrounds the cell.

Chloroplasts

• Chloroplasts allow plant cells to photosynthesise.

Storage

• Storage plant cells store carbohydrate as starch or sucrose.

Section 3 of 20Functions of cell structures

Structure	Function
Nucleus	Contains DNA (genetic material), which controls the activities of the cell
Cytoplasm	Gel of water and dissolved solutes that supports the structures and is the site of many reactions, including anaerobic respiration
Cell membrane	Separates the inside from the outside, holds the cell together and controls what enters and leaves
Ribosomes	Site of protein synthesis
Mitochondria	Site of aerobic respiration
Cell wall (plants)	Cellulose wall that gives extra support and defines the shape of the cell
Chloroplast (plants)	Contains chlorophyll, which absorbs light energy for photosynthesis
Permanent vacuole (plants)	Holds cell sap, a solution of sugar and salt, used for storage and to support the shape of the cell

Section 4 of 20Bacterial cells

Bacterial cells

Size

• Size bacteria are microscopic single-celled organisms in a wide variety of shapes.

Structures they have

• Structures they have a cell wall made of peptidoglycan (not cellulose), cell membrane, cytoplasm and ribosomes.

No nucleus

• No nucleus the DNA is a circular chromosome floating in the cytoplasm.
• Plasmids are small rings of DNA that carry extra genes.

Missing structures

• Missing structures bacteria have no mitochondria, chloroplasts or other membrane-bound organelles.

Flagella

• Flagella some bacteria have long, whip-like tails that allow them to move.

Section 5 of 20Examples of bacteria

Lactobacillus

• Lactobacillus a rod-shaped bacterium used to make yoghurt from milk.

Pneumococcus

• Pneumococcus a spherical bacterium that causes pneumonia.

Section 6 of 20Structures seen under the microscope

Structures seen under the microscope

Light microscope

• Light microscope shows the nucleus, cytoplasm, and in plants the cell wall, chloroplasts and vacuole.

Electron microscope

• Electron microscope shows mitochondria, ribosomes, vesicles and the rough endoplasmic reticulum.
• Ribosomes can be free in the cytoplasm or attached to the rough endoplasmic reticulum, which gives it the name rough.
• Vesicles are small circular structures that move through the cytoplasm.

Section 7 of 20New cells

• Cell division
  • new cells are made by the division of existing cells.
  • This lets the body grow and repair itself.

Section 8 of 20Specialised animal cells

Cell	Function	Adaptations
Ciliated cell	Moves mucus up the trachea and bronchi	Cilia beat to move mucus and trapped particles to the throat
Nerve cell	Conducts impulses	Long with branches, and a fatty sheath around the axon that speeds up the impulse
Red blood cell	Carries oxygen	Biconcave disc shape, haemoglobin, and no nucleus
Sperm cell	Fertilises the egg	Haploid nucleus, acrosome with enzymes, mitochondria in the mid-piece and a tail
Egg cell	Is fertilised and grows into an embryo	Large store of cytoplasm and a haploid nucleus

Section 9 of 20Specialised plant cell: root hair cell

Specialised plant cell: root hair cell

Function

• Function absorbs water and mineral ions from the soil.

Root hair

• Root hair increases the surface area for absorption.

Thin wall

• Thin wall lets water move through quickly.

No chloroplasts

• No chloroplasts the cell is underground and does not photosynthesise.

Section 10 of 20Differentiation

Specialised cell

Specialised cell a cell with features that suit it to a particular function.

Differentiation

Differentiation the process in which cells develop the structure they need for their function, and genes in the nucleus control it.

Section 11 of 20More from the specification

More from the specification

Endomembrane compartmentalization system

• Endomembrane compartmentalization system the endoplasmic reticulum and Golgi apparatus form an integrated intracellular transport system coordinating protein trafficking.

Magnification Equation Triangle

• Magnification equals image size divided by actual biological size (M = I / A), necessitating identical dimensional units before executing calculations.

Unit Conversions Between Millimetres and Micrometres

• Because microscopic cellular structures are measured in micrometres (µm), linear millimetre measurements must be multiplied by 1000 prior to magnification computation.

Structural Adaptations of Xylem Vessels

• Dead, hollow xylem vessels feature thick secondary cell walls reinforced with lignified rings or spirals, providing mechanical support and uninterrupted capillary transport of water and mineral ions under negative pressure.

Formula triangle

• A triangle used to rearrange the magnification formula.

Magnification

• Image size divided by actual size.

Formula

• Symbols and numbers that show the atoms in a substance.

Mesophyll

• The leaf tissue made of palisade and spongy cells.

Muscle

• Tissue that contracts to cause movement.

Section 12 of 20Xylem vessels

Xylem vessels

Function

• Function xylem vessels conduct water through the plant and support it.

No end walls

• No end walls there are no top and bottom walls between vessels, so a continuous column of water can run through.

Dead cells

• Dead cells they lack organelles and cytoplasm, which gives free passage for water.

Lignin

• Lignin the walls are thickened with lignin, which gives extra structural support.

Section 13 of 20Palisade mesophyll cells

Palisade mesophyll cells

Function

• Function palisade cells carry out photosynthesis.

Shape

• Shape they are column-shaped, which maximises absorption of sunlight.
• The shape lets as many cells as possible fit in a layer under the upper epidermis of the leaf.

Chloroplasts

• Chloroplasts they are packed with chloroplasts for maximum photosynthesis.

Section 14 of 20Levels of organisation

Level	Description
Cell	The basic functional and structural unit of all living organisms
Tissue	A group of cells of similar structure working together to perform the same function
Organ	Different tissues working together to perform a specific function
Organ system	A group of organs with related functions working together to perform body functions

Section 15 of 20Tissues

Tissues

One cell type

• One cell type a tissue is always made of one type of cell.
• A group of different tissues is an organ, not a tissue.

Identifying

• Identifying a leaf is an organ, a palisade mesophyll cell is a cell, and a layer of spongy mesophyll cells is a tissue.

Section 16 of 20Animal organ systems

Organ system	Example organs	Example tissues
Digestive	Oesophagus, stomach, small intestine, large intestine	Muscle, connective, nerve, epithelial
Circulatory	Heart, veins, arteries	Muscle, connective, nerve
Respiratory	Trachea, bronchi, lungs	Muscle, connective, epithelial
Immune	Thymus, bone marrow, spleen	Connective, blood-forming tissue
Excretory	Kidney, skin, lungs	Epithelial, muscle
Nervous	Brain, spinal cord	Nerve, connective

Section 17 of 20Plant organ systems

Organ system	Example organs	Example tissues
Shoot system	Leaf, stem, flower, fruit	Epidermis, mesophyll, xylem, phloem
Root system	Root, tuber	Xylem, phloem

Section 18 of 20More from the specification

More from the specification

Magnification Formula and Unit Conversion

• Under a light microscope, image dimensions are calculated using the magnification formula: Magnification = Image size / Actual size (M = I / A); measurement units must always be converted to identical units.
  • (e.g. 1 mm = 1000 micrometres)

Eukaryotic Animal Cell Structure

• An animal cell contains a membrane-bound nucleus housing linear chromosomes, cytoplasm for metabolic reactions, mitochondria for aerobic respiration,
• and a partially permeable cell surface membrane.

Prokaryotic Bacterial Cell Ultrastructure

• Unlike animal cells, bacteria are prokaryotes that lack a true nucleus, containing circular chromosomal DNA free in the cytoplasm, plasmids,
• and a peptidoglycan cell wall.

Organelle Specialisation and Cell Functions

• Plant cells contain additional structures absent in animal cells,
• such as chloroplasts for photosynthesis, a large permanent vacuole with cell sap,
• and a rigid cellulose cell wall.

Magnification Formula and Unit Conversion

• Under a light microscope, image dimensions are calculated using the magnification formula: Magnification = Image size / Actual size (M = I / A); measurement units must always be converted to identical units.
  • (e.g. 1 mm = 1000 micrometres)

Eukaryotic Animal Cell Structure

• An animal cell contains a membrane-bound nucleus housing linear chromosomes, cytoplasm for metabolic reactions, mitochondria for aerobic respiration,
• and a partially permeable cell surface membrane.

Prokaryotic Bacterial Cell Ultrastructure

• Unlike animal cells, bacteria are prokaryotes that lack a true nucleus, containing circular chromosomal DNA free in the cytoplasm, plasmids,
• and a peptidoglycan cell wall.

Organelle Specialisation and Cell Functions

• Plant cells contain additional structures absent in animal cells,
• such as chloroplasts for photosynthesis, a large permanent vacuole with cell sap,
• and a rigid cellulose cell wall.

Cellulose

• Tough material that makes the plant cell wall.

Ribosome

• The site of protein synthesis.

Mitochondria

• Structures where respiration releases energy from food.

Formula triangle

• A triangle used to rearrange the magnification formula.

Magnification

• Image size divided by actual size.

Formula

• Symbols and numbers that show the atoms in a substance.

Nucleus

• Controls the activities of the cell and contains the genetic material.

Microscope

• An instrument that magnifies things too small to see.

Bacteria

• Single cells with no nucleus.

Section 19 of 20Key terms

Term	Meaning
Nucleus	Contains the DNA that controls the cell's activities.
Mitochondrion	The site of aerobic respiration.
Ribosome	The site of protein synthesis.
Chloroplast	Contains chlorophyll and is the site of photosynthesis.
Cell wall	A cellulose layer that supports plant cells.
Plasmid	A small ring of extra DNA in bacteria.
Differentiation	Development of a specialised structure to suit a function.
Cell sap	A solution of sugar and salt in the vacuole.

Term	Meaning
Tissue	A group of similar cells that do the same job.
Organ	Different tissues working together to do a particular job.
Organ system	A group of organs that work together.
Lignin	A strong substance that thickens xylem walls.
Xylem vessel	A dead tube that carries water and supports the plant.
Palisade mesophyll	Leaf cells packed with chloroplasts for photosynthesis.
Cell	The basic unit of structure and function of living things.

Section 20 of 20Exam technique

1

Saying plant cells have no mitochondria — plant cells have mitochondria as well as chloroplasts.

2

Saying bacteria have a nucleus — their DNA is a loop in the cytoplasm.

3

Saying the cell wall controls what enters the cell — the cell membrane does this.

4

Saying bacterial cell walls are made of cellulose — they are made of peptidoglycan.

1

Calling a group of different tissues a tissue — a tissue contains only one type of cell.

2

Saying xylem vessels contain cytoplasm — they are dead and empty.

3

Mixing up the order — cell, tissue, organ, organ system.

4

Forgetting that a leaf is an organ — it contains several tissues.