Plant cell ultrastructure: shared organelles with animal cells
eukaryotes
Golgi apparatus, mitochondria, ribosomes, cell membrane, vacuole, nucleus, cell wall, chloroplast, cytoplasm
rough and smooth endoplasmic reticulum spread throughout the cytoplasm
Cell Wall
Plant cells are mostly rectangular => shaped by cell wall which gives the cell strength and support
made of insoluble cellulose
freely permeable to everything that dissolves in water
Suberin is added to the cell wall in cork tissues
Lignin is a part of cell wood structure in wood
=> those compounds reduce the permeability of the cell wall so that water and dissolved substances cannot pass through it
Primary Cell Wall
Primary Cell Wall: Middle lamella => made when a plant cell divides into two new cells (made of pectin which holds the cell walls of adjacent plant cells together - has lots of negatively charged carboxyl(-COOH) groups and these combine with positive Ca2+ = calcium pectate ⇒ binds to the cellulose on either side - cellulose microfibrils and the matrix build up on both sides of the middle lamella - all arranged in a similar direction
Secondary Cell Wall
Secondary cell wall builds up, with the cellulose microfibrils laid densely at different angles to each other => more rigid + hemicelluloses harden it further
=> In WOOD = lignin is then added
Draw beta-glucose
check
Cellulose: structure, bonding and structure, function, digesting cellulose
CELLULOSE = made of beta-glucose
units are inverted so bonding can take place
hydroxyl(-OH) groups stick out on both sides of the molecule
⇒ hydrogen bonds can be made between the partially positively charged hydrogen atoms of the hydroxyl groups and the partially negatively charged oxygen atoms = CROSS-LINKING and it holds neighbouring chains closely
Join in forms microfibrils -> deposited in layers which are held together by a matrix of hemicelluloses and other short-chain carbohydrates => binding to each other and to the cellulose molecules
=> increases its strength
=> don’t coil, remain long, straight chains
=> most animals do not possess the enzymes needed to break the 1,4-glycosidic bonds between the molecules of β-glucose, so they cannot digest cellulose
=> ruminant animals use the cellulose-digesting enzymes from bacteria living in their gut to digest their food
Plasmadesmata
PLASMODESMATA
special cytoplasmic bridges via which the substances are exchanged in primary cell wall
Is produced as the cells divide - two cells do not separate completely, and threads of cytoplasm remain between them => pass through gaps in the newly formed cell walls and signalling substances can pass from one cell to another through the cytoplasm
cell walls are thinner in the region of the plasmodesmata
SYMPLAST - interconnected cytoplasm of the cells
Pits
PITS - when secondary thickening takes place, area around the plasmodesmata is unaffected, leaving it thin
-> no cytoplasm in the xylem cells but the pits allow water to move between the xylem vessels => maintaining a flow of water at even pressure through the plant.
Permanent Vacuoles
any fluid-filled space inside the cytoplasm which is surrounded by a membrane
can be found in animal cells, but are TEMPORARY
surrounded by a specialised membrane = TONOPLAST -> contains many different protein channels and carrier systems; controls the movements of substances into and out of the vacuole + water potential of the cell
is filled with cell sap, a solution of various substances in water => causes water to move into the cell by osmosis ⇒cytoplasm is kept pressed against the cell wall (cell stays turgid)
used to store several different substances => can store pigments or proteins in the cells of seeds and fruits + contain lytic enzymes
often store waste products
Chloroplasts
almost all plant cells contain the genetic information to make chloroplasts (exc parasitic plants
Cells in flowers, seeds and roots contain no chloroplasts and neither do the internal cells of stems or the transport tissues
Chloroplast vs mitochondria
are large organelles - they have a biconvex shape
contain their own DNA
are surrounded by an outer membrane
have an enormously folded inner membrane that gives increased SA where enzyme-controlled reactions take place
are thought to have been free-living prokaryotic organisms that were engulfed by and became part of other cells
are the site of photosynthesis vs site of respiration
contain chlorophyll vs don’t
Amyloplast
specialised plant organelle
Are colourless and store starch => starch can be converted to glucose and used to provide energy when the cell needs it
Large numbers of amyloplasts are found in areas of plants that store starch
Draw chloroplast structure
check
Epidermis
Epidermis - outer layer, protects the cells beneath it
-> cutin - waxy substance secreted from the cells to reduce water loss
-> hairs - to trap the warm air and reduce heat loss
Collenchyma
Collenchyma - below epidermis, have thick cellulose primary cell walls => strength + support, but remain living so they stretch as the plant grows and provide flexibility
Sclerenchyma
Modified parenchyma
strong and flexible
fibres are associated with the vascular bundles and provide support to a stem
consist of bundles of dead cells which form long, hollow tubes, but they do have end walls present
lignification of cell walls occur in spiral/ring structure, which provides structural support to allow the plant to bear the load of its own mass
have more cellulose in their secondary cell walls positioned at right angle at each other compared to other plant cells
Parenchyma
Parenchyma cells - most common plant cells (unspecialised = can be modified to develop into other cells), act as packing for the other tissues, protecting them, can be used for storage and photosynthesis
Draw a stem structure
check
Xylem
transports water and minerals
cell walls contain lignin, which enables the vessels to withstand the pressure created by the moving column of water
form long, hollow straw-like structures that are formed by dead cells (due to lignification of cell walls)
do not contain any cytoplasm or organelles that could slow down the flow of water
small regions in the walls that are not lignified, known as pits, which allows for lateral movement of water and minerals between xylem vessels
water moves up against the gravity
Water moves out of the xylem into the surrounding cells through the specialised pits in the walls of the xylem vessels
unidirectional
Protoxylem
xylem starts off as living tissue => protoxylem - it can stretch and grow because the walls are not fully lignified (cellulose microfibrils are arranged vertically in the stem) = increases the strength of the tube and allows it to resist the compression forces from the weight of the plant pressing down on it
Metaxylem
Increasing amounts of lignin are incorporated into the cell walls as the stem ages and the cells stop growing => cells become impermeable to water and other substances. = becomes stronger and more supportive => metaxylem, endwalls between the cells mostly break down so the xylem forms hollow tubes
Development of xylem diagram
check
Structures and functions of the xylem
Lignified cell wall - strength to withstand hydrostatic pressure so that the vessels don’t collapse, impermeable to water
No end plates - allow mass flow, cohesive and adhesive forces are not impeded
No protoplasm - doesn’t impede with the mass flow
Pits in the wall - lateral movement of water, allows for continuous flow in case an air bubble forms
Small diameter of vessel - prevents water columns from forming, assist with capillary function
Phloem
transports sugars (in form of sucrose) and proteins (in form of amino acids) => organic compounds are dissolved in water to form sap => TRANSLOCATION
composed of living cells which forms a non-hollow tube => its bulk is made up of sieve tube (walls between the cells) elements which are the main conducting cells and companion cells
Phloem sap moves through the sieve pores in the end walls of each sieve tube cell
bidirectional
