AS Level Biology Notes for Topic 4 – Unit 2 ; Development, Biodiversity & Conservation
4A Plant Structure and Function —-
The Cell Wall
➢ Cell walls are formed outside of the cell membrane and offer structural
support which is provided by the polysaccharide cellulose. They’re freely
permeable, allowing substances to enter/exit the cell.
○ Cellulose provides structural support and is
○ responsible for the regular shape due to its rigid nature.
➢ The middle lamella forms the outermost layer, acting
➢ as an adhesive to stick adjacent plant cells together,
➢ providing stability – it’s mainly composed of pectin.
○ Pectin has lots of negative -COOH groups which
○ react with Ca2+ ions to form calcium pectate – it binds to either side of the
cellulose, causing the microfibrils in the matrix to build up on both sides of
the middle lamella.
➢ Primary cell wall → very flexible plant cell walls with all the cellulose microfibrils
oriented in a similar direction.
➢ Secondary cell wall → older plant cell walls with cellulose microfibrils have
densely built up at different angles to make the cell wall more rigid.
○ Hemicelluloses (polysac.) help to harden it further.
○ Lignin (wood) and suberin (cork - waterproof) impregnates the walls to
make them impermeable.
➢ Plasmodesmata are narrow cytoplasmic bridges that connect the cytoplasms
of neighbouring plant cells (allowing for communication) – this interconnected
system is the symplast.
➢ Pits are very thin regions of the cell wall; the pits of adjacent cells are lined up
in pairs, they are formed due to the absence of secondary thickening of the cell
walls in areas where plasmodesmata are present – allowing them to stay in
contact.
○ It also aids the transportation of substances between cells – in xylem
vessels, it allows for the lateral flow of water/mineral ions between
adjacent vessels.
Plant Organelles
➢ A vacuole is any fluid-filled space inside the cytoplasm which is surrounded by
a membrane (plants have a permanent vacuole) — this is the tonoplast.
○ It’s a specialised membrane with protein channels and carrier systems
controlling the water potential of the cell by monitoring the movement of
substances in and out of the cell sap.
, AS Level Biology Notes for Topic 4 – Unit 2 ; Development, Biodiversity & Conservation
➢ The vacuole is filled with cell sap, causing water to move in via osmosis – this
ensures that the cytoplasm is still pressed up against the cell wall.
➢ Chloroplasts are double membrane-bound organelles with thylakoids
containing chlorophyll that stack to form grana which are then joined by
lamellae – they also have small pieces of DNA/ribosomes for protein synthesis.
○ Due to the presence of chlorophyll and thylakoids, chloroplasts are the
site of photosynthesis (membranes or stroma).
➢ Amyloplasts contain starch granules (amylose/amylopectin) – the starch can be
converted into glucose and used to provide energy when the cell needs it.
Plant Stems
Role of Plant Stems:
➢ Support —- they have to ensure that leaves are supported and positioned in the
best way to carry out photosynthesis (maximum sunlight) / they have to
support flowers to present them in a way to maximise pollination / they also
have to ensure that the support they provide is flexible enough to withstand
the forces of nature.
➢ Transport —- they provide the route where the products of photosynthesis are
carried, this transfer is from the leaves to the rest of the plant / they also provide
the leaves with water and mineral ions.
4A Plant Structure and Function —-
The Cell Wall
➢ Cell walls are formed outside of the cell membrane and offer structural
support which is provided by the polysaccharide cellulose. They’re freely
permeable, allowing substances to enter/exit the cell.
○ Cellulose provides structural support and is
○ responsible for the regular shape due to its rigid nature.
➢ The middle lamella forms the outermost layer, acting
➢ as an adhesive to stick adjacent plant cells together,
➢ providing stability – it’s mainly composed of pectin.
○ Pectin has lots of negative -COOH groups which
○ react with Ca2+ ions to form calcium pectate – it binds to either side of the
cellulose, causing the microfibrils in the matrix to build up on both sides of
the middle lamella.
➢ Primary cell wall → very flexible plant cell walls with all the cellulose microfibrils
oriented in a similar direction.
➢ Secondary cell wall → older plant cell walls with cellulose microfibrils have
densely built up at different angles to make the cell wall more rigid.
○ Hemicelluloses (polysac.) help to harden it further.
○ Lignin (wood) and suberin (cork - waterproof) impregnates the walls to
make them impermeable.
➢ Plasmodesmata are narrow cytoplasmic bridges that connect the cytoplasms
of neighbouring plant cells (allowing for communication) – this interconnected
system is the symplast.
➢ Pits are very thin regions of the cell wall; the pits of adjacent cells are lined up
in pairs, they are formed due to the absence of secondary thickening of the cell
walls in areas where plasmodesmata are present – allowing them to stay in
contact.
○ It also aids the transportation of substances between cells – in xylem
vessels, it allows for the lateral flow of water/mineral ions between
adjacent vessels.
Plant Organelles
➢ A vacuole is any fluid-filled space inside the cytoplasm which is surrounded by
a membrane (plants have a permanent vacuole) — this is the tonoplast.
○ It’s a specialised membrane with protein channels and carrier systems
controlling the water potential of the cell by monitoring the movement of
substances in and out of the cell sap.
, AS Level Biology Notes for Topic 4 – Unit 2 ; Development, Biodiversity & Conservation
➢ The vacuole is filled with cell sap, causing water to move in via osmosis – this
ensures that the cytoplasm is still pressed up against the cell wall.
➢ Chloroplasts are double membrane-bound organelles with thylakoids
containing chlorophyll that stack to form grana which are then joined by
lamellae – they also have small pieces of DNA/ribosomes for protein synthesis.
○ Due to the presence of chlorophyll and thylakoids, chloroplasts are the
site of photosynthesis (membranes or stroma).
➢ Amyloplasts contain starch granules (amylose/amylopectin) – the starch can be
converted into glucose and used to provide energy when the cell needs it.
Plant Stems
Role of Plant Stems:
➢ Support —- they have to ensure that leaves are supported and positioned in the
best way to carry out photosynthesis (maximum sunlight) / they have to
support flowers to present them in a way to maximise pollination / they also
have to ensure that the support they provide is flexible enough to withstand
the forces of nature.
➢ Transport —- they provide the route where the products of photosynthesis are
carried, this transfer is from the leaves to the rest of the plant / they also provide
the leaves with water and mineral ions.
