BIOLOGY UNIT 2

What is the definition of levels of organisation?

The hierarchical structure of living organisms, from smallest chemical components to complete body systems

List the 6 levels of organisation from smallest to largest.

Organelles → Cells → Tissues → Organs → Organ systems → Organism

Define organelle.

Tiny structures inside a cell that carry out specific functions. Cannot be seen with a light microscope at standard school magnification (except nucleus) – require electron microscope for detailed view

Define cell.

The basic structural and functional unit of all living organisms. The smallest unit that can carry out all life processes (MRS GREN)

Define tissue.

A group of similar cells that work together to perform a particular function. Held together by intercellular substances

Define organ.

A group of different tissues working together to perform a specific function. Made of multiple tissue types (e.g., heart = muscle + nervous + connective + epithelial tissue)

Define organ system.

A group of organs that work together to perform a major body function (e.g., digestion, circulation, gas exchange)

Define organism.

A complete living individual made up of organ systems. Can be unicellular or multicellular

Give an example of each level using the human digestive system.

Organelle: mitochondrion in epithelial cell; Cell: epithelial cell lining stomach; Tissue: epithelial tissue; Organ: stomach; Organ system: digestive system; Organism: human

Give an example of each level using a plant leaf.

Organelle: chloroplast in palisade cell; Cell: palisade mesophyll cell; Tissue: palisade mesophyll tissue; Organ: leaf; Organ system: shoot system; Organism: oak tree

Common exam mistake: listing "atoms" or "molecules" as a level of organisation.

IGCSE starts at organelles – do not include atoms/molecules unless asked for "chemical level" (which is not on 4BI1)

Common exam mistake: putting "tissues" before "cells".

Cells make up tissues – order is cells → tissues

Common exam mistake: saying "organ systems" come before "organs".

Organs make up organ systems – order is organs → organ systems

Common exam mistake: confusing "tissue" with "organ".

Tissue = group of similar cells; Organ = group of different tissues

[DIAGRAM NEEDED]

levels of organisation in organisms hierarchy pyramid IGCSE

[DIAGRAM NEEDED]

digestive system showing cells tissues organs diagram

What is the key fact for Section 2.2?

You need to be able to identify and label cell structures on a diagram (plant cell and animal cell)

Describe the nucleus as seen in a diagram.

Large, round, dark circle (often with a smaller dark dot inside – the nucleolus). Usually near centre in animal cells; pushed to side by vacuole in plant cells

Describe the cytoplasm as seen in a diagram.

Granular or dotted filling inside the cell (between nucleus and cell membrane)

Describe the cell membrane as seen in a diagram.

Thin continuous line just inside the cell wall (in plants) OR the outermost boundary (in animals). Drawn as a single solid line

Describe the cell wall as seen in a diagram.

Thick outer layer surrounding the cell membrane. Drawn as a continuous, slightly thicker line. In plants: rigid, straight edges (rectangular/polygonal shape)

Describe mitochondria as seen in a diagram.

Small oval or sausage-shaped structures with inner lines (cristae). Drawn as ovals with a squiggly line inside OR a smaller oval inside

Describe chloroplasts as seen in a diagram.

Oval discs with internal lines (grana). Drawn as green ovals with lines across the inside. Only in photosynthetic cells (e.g., palisade mesophyll)

Describe ribosomes as seen in a diagram.

Tiny dots scattered in the cytoplasm. Too small to see with light microscope – drawn as small black dots

Describe the vacuole as seen in a plant cell diagram.

Large empty-looking circle in plant cells (filled with cell sap). Leave completely empty/white

Which structures are found in plant cells but not animal cells?

Cell wall (cellulose), chloroplasts (in green parts), large permanent vacuole

Which structures are found in both plant and animal cells?

Nucleus, cytoplasm, cell membrane, mitochondria, ribosomes

[DIAGRAM NEEDED]

plant cell labelled diagram IGCSE

[DIAGRAM NEEDED]

animal cell labelled diagram IGCSE

How should you draw a plant cell shape for full marks?

Rectangular or polygonal (because of rigid cell wall)

How should you draw an animal cell shape for full marks?

Irregular, rounded (no fixed shape – draw as a blob)

How should you draw the nucleus for full marks?

Draw a circle. Shade lightly or outline dark. Add a smaller dark circle inside = nucleolus

How should you draw the cell wall (plant) for full marks?

Draw a thick outer line. Use a ruler for straight edges if possible

How should you draw mitochondria for full marks?

Draw as ovals. Draw a squiggly line inside OR draw an inner oval. Label clearly

How should you draw chloroplasts for A* depth?

Draw as ovals. Draw 2-3 lines across the inside. Add small dots (starch grains) inside

How should you draw the vacuole (plant) for full marks?

Draw a large central circle. Leave it completely empty/white. Label "vacuole" or "cell sap"

Common exam mistake: drawing a nucleus in a bacterial cell.

Bacteria are prokaryotic – NO nucleus

Common exam mistake: drawing a cell wall on an animal cell.

Animal cells have NO cell wall

Common exam mistake: drawing a large central vacuole in an animal cell.

Animal cells have NO large permanent vacuole

Common exam mistake: forgetting to draw the cell membrane inside the plant cell wall.

Plant cells have BOTH cell wall AND cell membrane

Common exam mistake: drawing chloroplasts in a root cell.

Root cells have NO chloroplasts (no photosynthesis underground)

What is the function of the nucleus?

Contains the cell's genetic material (DNA). Controls the cell's activities (e.g., cell division, protein synthesis)

What is the function of the cytoplasm?

A jelly-like substance where most chemical reactions (metabolic reactions) take place. Contains enzymes. Suspends organelles

What is the function of the cell membrane?

Controls the movement of substances into and out of the cell (selectively permeable / partially permeable)

What is the function of the cell wall?

Provides structural support and strength to the cell. Prevents the cell from bursting when water enters (in plants). Made of cellulose

What is the function of mitochondria?

The site of aerobic respiration. Releases energy

What is the function of chloroplasts?

The site of photosynthesis. Contains chlorophyll (green pigment) which absorbs light energy

What is the function of ribosomes?

The site of protein synthesis (where proteins are made)

What is the function of the vacuole (plant)?

Contains cell sap (a solution of sugars, salts, and pigments). Helps maintain turgor pressure (keeps the cell firm/rigid)

Define selective permeability.

The cell membrane allows some substances to pass through but not others

What can pass easily through the cell membrane?

Water (small molecule), oxygen, carbon dioxide

What cannot pass easily through the cell membrane?

Large molecules (e.g., starch, proteins), ions

Common exam mistake: saying the cell membrane is "fully permeable".

Wrong – it is selectively permeable (or partially permeable)

Define turgid (plant cell).

When a plant cell is full of water, vacuole pushes against cell wall → cell is firm (healthy plant)

Define plasmolysed (plant cell).

When a plant cell loses water, vacuole shrinks, cell membrane pulls away from cell wall (wilting plant)

Which cell type has the most mitochondria? Why?

Muscle cell – needs lots of energy for contraction

Which cell type has NO mitochondria?

Red blood cell (in mammals) – cannot respire aerobically, gets energy anaerobically

Which cell type has the most ribosomes? Why?

Pancreatic cell (makes digestive enzymes) and antibody-producing white blood cells – need to synthesise lots of protein

Common exam mistake: "The nucleus makes proteins"

Wrong – nucleus contains DNA which codes for proteins; ribosomes make proteins

Common exam mistake: "Mitochondria are for photosynthesis"

Wrong – mitochondria are for respiration; chloroplasts are for photosynthesis

Common exam mistake: "The cell wall controls what enters/leaves"

Wrong – cell wall is fully permeable; cell membrane controls movement

List the similarities between plant and animal cells.

Both have: nucleus, cytoplasm, cell membrane, ribosomes, mitochondria

List the differences between plant and animal cells.

Plant: cellulose cell wall, chloroplasts (in green parts), large permanent vacuole, regular/rectangular shape, stores starch. Animal: no cell wall, no chloroplasts, no large vacuole, irregular/rounded shape, stores glycogen

Common exam mistake: "Plant cells have a nucleus but animal cells don't"

Both have a nucleus (eukaryotes)

Common exam mistake: "Plant cells don't have mitochondria"

Wrong – plant cells DO have mitochondria

[DIAGRAM NEEDED]

plant cell vs animal cell comparison diagram IGCSE

Define cell differentiation.

The process by which a cell becomes specialised to perform a specific function. During differentiation, the cell develops specific structures and may lose others (e.g., nucleus in red blood cells)

Why is differentiation important? (4 reasons)

1) Division of labour – different cells perform different functions efficiently; 2) Increased efficiency – specialised cells have structures adapted to their role; 3) Formation of tissues and organs; 4) Survival of the organism

What happens to most animal cells after differentiation?

Most lose the ability to divide (except stem cells)

What happens to many plant differentiated cells?

Many retain the ability to divide (meristem cells)

Describe the red blood cell: specialisation, function, and adaptations.

No nucleus – bi-concave disc shape – contains haemoglobin. Function: transport oxygen. Adaptations: no nucleus = more space for haemoglobin; bi-concave shape = large SA:V ratio; flexible = squeeze through capillaries

Describe the nerve cell (neuron): specialisation, function, and adaptations.

Long axon – branched dendrites – myelin sheath. Function: transmit electrical impulses. Adaptations: long axon = carry impulses over distance; branched dendrites = connect to many neurons; myelin sheath = speeds up impulse transmission

Describe the muscle cell: specialisation, function, and adaptations.

Long – many mitochondria – protein filaments (actin and myosin). Function: contract to cause movement. Adaptations: many mitochondria = lots of energy; long shape = span length of muscle; protein filaments = slide to contract

Describe the sperm cell: specialisation, function, and adaptations.

Flagellum (tail) – many mitochondria in midpiece – acrosome – haploid nucleus. Function: fertilise egg cell. Adaptations: flagellum = swimming; many mitochondria = energy; acrosome = digest egg coat; haploid nucleus = correct chromosome number after fertilisation

Describe the egg cell (ovum): specialisation, function, and adaptations.

Large – food reserves (yolk) – haploid nucleus – jelly coat. Function: fertilisation – provides nutrients for early embryo. Adaptations: large = stores nutrients; haploid nucleus = correct chromosome number; jelly coat = allows sperm binding and prevents polyspermy

Describe the root hair cell: specialisation, function, and adaptations.

Long thin projection (root hair) – large vacuole – no chloroplasts – thin cell wall. Function: absorb water and mineral ions from soil. Adaptations: long root hair = increases SA; large vacuole = stores water; thin cell wall = short diffusion distance; no chloroplasts = no photosynthesis needed (underground)

Describe the palisade mesophyll cell: specialisation, function, and adaptations.

Columnar shape – many chloroplasts – large vacuole – thin cell wall. Function: carry out photosynthesis. Adaptations: many chloroplasts = more photosynthesis; columnar shape = packed tightly; large vacuole = pushes chloroplasts to edge (short diffusion distance for CO2)

Describe the xylem vessel: specialisation, function, and adaptations.

Dead at maturity – no cell contents – hollow lumen – lignified cell walls. Function: transport water and mineral ions upwards. Adaptations: hollow lumen = no obstruction; lignin = prevents collapse; no cell contents = no resistance; pits = allow sideways water movement

Describe the phloem sieve tube element: specialisation, function, and adaptations.

Living but no nucleus – sieve plates – companion cell (with nucleus). Function: transport sucrose and amino acids (translocation). Adaptations: sieve plates = allow flow; no nucleus = reduces resistance; companion cell = provides proteins and energy

[DIAGRAM NEEDED]

root hair cell diagram labelled IGCSE

[DIAGRAM NEEDED]

palisade cell diagram labelled IGCSE

[DIAGRAM NEEDED]

red blood cell diagram IGCSE

[DIAGRAM NEEDED]

nerve cell diagram IGCSE

[DIAGRAM NEEDED]

sperm cell diagram IGCSE

Common exam mistake: "Differentiation happens only in plants"

Wrong – occurs in animals AND plants

Common exam mistake: "Red blood cells have a nucleus"

In mammals – NO nucleus

Common exam mistake: "Xylem cells are alive"

Wrong – xylem vessels are dead at maturity

Common exam mistake: "Phloem tubes have nuclei"

Wrong – sieve tube elements have no nucleus (companion cell has the nucleus)

Common exam mistake: "Root hair cells have chloroplasts"

Wrong – underground, no light, so no chloroplasts

Define stem cell.

An unspecialised cell that can: 1) Divide (by mitosis) to produce more stem cells (self-renewal), and 2) Differentiate into different types of specialised cells

What are the two key abilities of a stem cell?

Divide (self-renewal) and differentiate

Where are embryonic stem cells sourced from?

Embryo (blastocyst – early stage, ~5 days after fertilisation)

What is the potency of embryonic stem cells?

Pluripotent – can become almost any cell type (over 200 types)

Where are adult stem cells found?

Bone marrow, skin, brain (in adults and children)

What is the potency of adult stem cells?

Multipotent – can become a limited range of cell types (e.g., bone marrow stem cells → red blood cells, white blood cells, platelets)

Where are plant stem cells found?

Meristems (tips of roots and shoots)

Give 5 advantages of using stem cells in medicine.

1) Regenerative medicine (replace damaged cells); 2) Drug testing (reduces animal testing); 3) Treat blood disorders (leukaemia, sickle cell anaemia); 4) Potential for personalised medicine (reduces immune rejection); 5) Unlimited source (can divide many times in lab)

Give 7 disadvantages/ethical concerns of using stem cells in medicine.

1) Ethical concerns (embryo destroyed); 2) Religious objections; 3) Immune rejection; 4) Tumour formation (teratomas); 5) High cost; 6) Unproven treatments (experimental); 7) Limited differentiation (adult stem cells – multipotent only)

Compare embryonic vs adult stem cells: potency.

Embryonic: pluripotent (almost any cell). Adult: multipotent (limited range)

Compare embryonic vs adult stem cells: ethical issues.

Embryonic: high (embryo destroyed). Adult: low (no embryo involved)