Lecture 7 (cell walls)

Question 1

Why is there a difference between plants and animals on a cellular and sub cellular level?

Answer 1

Plants and animals have different life strategies e.g. plants aren’t mobile compared to animals and are effectively ‘stuck’ in the environment that they are in

Question 2

Plant cell components

Answer 2

nucleus 
Golgi apparatus 
Central vacuole 
Mitochondria 
Cell wall 
Chloroplasts 
Plasmodesmata

Question 3

Protoplast

Answer 3

Plasma membrane and everything that is inside it excluding the cell wall

Question 4

Cell wall structure

Answer 4

cellulose is a major component of the cell wall. It is the most abundant organic macromolecule on earth. It is a glucose polymer, highly ordered, and it forms long ribbon like structures

Cellulose forms microfibrils. These highly organised structures are strong and form a major component of both primary and secondary cell walls

In cellulose there is a 1,4 linkage/bonding between each glucose molecule and because they are so ordered you get bonding between the glucose molecules in the different chains. These then go together to make up a bigger structure known as a microfibril. Cellulose microfibrils are made of glucose bonded in such a way that it makes it very strong

Question 5

Two phases in cell wall structures

Answer 5

Crystalline microfibrillar phase - cellulose (this is a very ordered phase)

Non-crystalline matrix - Pectic polysaccharides and hemicellulosic polysaccharides plus a network of extensin (a protein)

The plant cell wall can be separated into two distinct phases; a highly crystalline phase consisting of the microfibrils and an ‘amorphous’ phase which is the matrix in which the microfibrils are embedded.

Question 6

Non-crystalline matrix - hemicellulose and pectin

Answer 6

Hemicellulose - a heterogeneous group of polysaccharides. Long chain of one type of sugar and short side chains (of different sugars) form a rigid structure (not as rigid as cellulose as it is not as ordered as cellulose)

Pectin - Branched polysaccharides, found in plant cell walls, where they form a matrix in which cellulose microfibrils are added. Pectin is branched, negatively charged polysaccharides. Bond water and have gel like properties.

Question 7

Hemicellulose

Answer 7

Heterogeneous group of polysaccharides (mixture of polysaccharides) deposited along with cellulose in the cell walls of plants and fungi to provide added strength.

Question 8

Pectin

Answer 8

Branched polysaccharides, found in plant cell walls, where they form a matrix in which cellulose microfibrils are added.

Question 9

Protein - extensin

Answer 9

Extensin cross-linking of pectin and cellulose dehydrates the cell call, reduces extensibility and increases strength. It controls the expansion of cells.When it loses water it becomes stronger and more rigid

The extensin protein can cross link between the cellulose and the pectin and this can dehydrate the cell wall and change the properties of the cell wall so that the cell wall becomes even more rigid

Question 10

Primary cell wall

Answer 10

In plants, a relatively thin and flexible layer that surrounds the plasma membrane of a young cell.

Question 11

Synthesis of primary cell wall

Answer 11

Coordinated (sense and respond to environment) synthesis and delivery of (all the components to the cell wall) :

1) Cellulose microfibrils at plasma membrane (the glucose polymer cellulose is assembled into long microfibrils. The rigidity and orientation of these microfibrils controls cell expansion) (this layer becomes the cell wall)
2) Polysaccharides (pectin and hemicellulose) in the Golgi apparatus are transported to the wall in vesicles (these vesicles are filled with polysaccharides and are extruded out to where the cellulose microfibrils are)
3) Cell wall proteins (extensins) from the rough ER (in vesicles, they travel through the cell to the plasma membrane, fuse with the plasma membrane and are extruded outside)

The vesicles fuse to the plasma membrane. (exocytosis) (now cellulose microfibrils, hemicellulose, pectin and extensins are all outside.) (cell wall can now be considered the ECM)

Question 12

Exocytosis

Answer 12

Transports material out of the cell or delivers it to the cell surface

Constitutive exocytosis releases ECM proteins, we can think of the cell wall as the ECM. In plant cells, instead of ECM proteins, the hemicellulose, pectin and extensin proteins that make up the cell wall are released

Question 13

Cytoplasmic streaming

Answer 13

Cytoplasmic streaming involves active movement of the cytoplasmic contents due to the action of microfilaments. Microfilaments make up part of the cell’s cytoskeleton.

Chloroplasts are the site of photosynthesis therefore cytoplasmic streaming is important in gaining the maximum amount of sunlight to use in this process

Question 14

Synthesis of primary cell wall - rosettes and microtubules

Answer 14

The cellulose-producing rosettes move parallel to the cortical microtubules

This is a highly regulated process with lots of components that give us the cell wall

Question 15

Cellulose producing rosettes

Answer 15

produce cellulose and they sit across the cell membrane and they extrude the cellulose microfibrils to the outside (they sit inside the membrane)

Also associated with microtubules on the inside of the cell so they run down the length of the microtubule and push the plasma membrane out of it way as it runs down the length of the microtubule, it is extruding cellulose microfibrils outside of the cell

Position of microtubules determines the location of where the cellulose is laid down

Question 16

Cortical microtubules

Answer 16

The position of microtubules determines the location of where the cellulose is laid down

Inside the cell

Question 17

Middle lamella

Answer 17

The middle lamella is a pectin layer which cements the cell of walls of two adjoining plant cells together.

Mostly contains pectin and it is a jelly-like substance that joins two cells together

First of all, the cell excretes pectin (the middle lamella is the furthest away therefore it must be made first) to make the middle lamella and then you get laying down of the primary cell wall

Question 18

Cell wall functions in regulating cell shape

Answer 18

influences cell morphology (which is determined by how the cell wall is put down)

Provides structural support

Prevents excessive water uptake

Question 19

Cell wall function - cell morphology

Answer 19

Orientation of cellulose microfibrils influences cell morphology (the position of the cytoskeleton (microtubules) determines where the cellulose microfibrils are laid down and that determines cell structure)

Randomly orientated - the cell will expand equally in all directions (circle)

Right angles to the ultimate long axis to the cell - the cell will expand longitudinally along that axis (rectangle)

Therefore this means that plant cells can come in many different shapes as it is determined by the laying down of the cell wall which is determined by the microtubules

Question 20

Cell wall function - provides structural support

Answer 20

The protoplast pushes against the cell wall. The cells become rigid and this maintains plant structure

Wilting occurs when the protoplast is not pushing against the cell wall (there is a lack of cellular pressure)

Water loss from cells reduces the protoplast volume and the protoplast does not press on the cell wall

Question 21

Cell wall function - prevents excessive water uptake

Answer 21

Water is constantly moving around a plant in a passive process

As water enters the cell by osmosis, the protoplast expands and pushes against the cell wall

Pressure from the cell wall limits the volume of water that can be taken up

Vacuoles are important in this process because they contain water and make up such a large portion of the protoplast

Question 22

Vacuole

Answer 22

A large membrane-enclosed organelle present in many plant cells; helps maintain turgor pressure of the plant cell, plays a limited storage role, and is also capable of a lysosome-like function in intracellular digestion.

A vacuole is an organelle surrounded by a single membrane.

It is highly selective, controlling much of what enters and leaves the vacuole.

Water moves in the vacuoles by osmosis (passive transport)

Typical mature plant cell has a single large vacuole

Question 23

Osmosis

Answer 23

Osmosis is the movement of a solvent across a semipermeable membrane toward a higher concentration of solute (lower concentration of solvent)

A type of facilitated diffusion

Movement from a high water (low solute) concentration to a low water (high solute) concentration

Question 24

Vacuoles - function in regulation of cell shape

Answer 24

There is high concentration of solutes in the vacuole

This results in water uptake into the vacuole by osmosis

The plant cell wall limits water uptake and prevents the cell bursting

Plant cells build up a large internal pressure that contributes to plant structural support

Question 25

Uptake of water without or with a cell wall

Answer 25

Normal cell - Cells osmoregulate to prevent swelling or shrinking under varying conditions. Not enough water, the cell with shrink or if there is too much water, the cell could burst. In both of these cases, the cells can die

Animal cells are much more vulnerable than plant cells as they have no cell wall

Plant in hypotonic, are turgid/normal. Plant in isotonic is flaccid. Plant in hypertonic is plasmolyzed (protoplast shrinks) (a plant cell can bounce back much better from being plasmolyzed than the equivalent in an animal cell)

Question 26

Secondary cell wall

Answer 26

In plant cells, a strong and durable matrix that is often deposited in several laminated layers around the plasma membrane and provides protection and support.

Not all plants cells have a secondary cell wall

Produced only after cell growth has stopped

Thicker and stronger than primary cell walls

Provides more structural support than primary cell wall

Question 27

Secondary cell wall structure and chemical characteristics

Answer 27

Made up of multiple layers (three layered secondary wall) - layers are laid down at different times and with different orientations which gives strength

Microfibrils in each layer have different orientations

This strengthens the secondary wall

Chemical characteristics - more cellu.ose, less pectin, and lignin

Question 28

Lignin

Answer 28

A strong polymer embedded in the cellulose matrix of the secondary cell wall

Second most abundant organic macromolecule with the first being cellulose
Lignin is a complex polymer
Confers strength, rigidity to the secondary cell wall and acts to exclude water (lignin presents a very technical problem as if you can’t get water in it is very hard to break down. This is good for plants as it provides a strong inert wall meaning very few things can get to the glucose in that cell wall
Sits where the pectin would sit in a primary cell wall
Secondary cell wall is very energy rich but hard to break down which is in part due to the structure of cellulose and in part due to lignin which excludes water

Question 29

Secondary cell wall function

Answer 29

Structural support - the secondary cell wall provides structural support for specific cell types such as water transporting cells, and for the whole plant

What happens is that the secondary cell wall is laid down and then the c ell dies and then they are a hollow tube for the conduction of water and these cells will run throughout the plant (veins on a leaf) and these give strength to the leaf

Question 30

How do cells with a cell wall communicate?

Answer 30

Plasmodesmata

Question 31

Plasmodesmata

Answer 31

Open channels through the cell wall connecting the cytoplasm and plasma membrane of adjacent plant cells, allowing water, small molecules and some larger molecules to pass between cells.

Plasmodesmata are intercellular connections (equivalent to gap suctions in animal cells), that enable cell to cell communication

The plasma membrane is continuous

Small enough to prevent organelle movements - although ER is connected through plasmodesmata

Allows the free exchange of small molecules

Question 32

What is the number of plasmodesmata affected by?

Answer 32

An active cell will have lots of plasmodesmata (if It is involved with lots of transport of molecules)