Cell Membrane Structure and Function

Phospholipid molecules form a bilayer - phospholipids are fluid and move laterally

Peripheral proteins are bound to either the inner or outer surface of the membrane

Integral proteins - permeate the surface of the membrane

The membrane is a fluid mosaic of phospholipids and proteins

Proteins can move laterally along membrane

Diffusion is the movement of particles from a region of high concentration to a region of low concentration, and is the result of the random motion of particles.

Facilitated diffusion is similar to simple diffusion, except that it requires channel proteins or carrier proteins, which are specific to the molecules being transported across the plasma membrane from high concentration to low concentration.

Osmosis is the passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration across a partially permeable membrane.

Active transport is the movement of particles across membranes, requiring energy in the form of ATP (mnemonic: Active TransPort requires ATP) and a carrier protein. The energy is used to move substances against a concentration gradient, from a region of low concentration to one of a higher concentration.

How does the sodium/potassium pump work?

When the pump is open to the inside of the axon, three sodium ions (Na+) enter the pump and attach to their binding sites.

ATP donates a phosphate group to the pump.

The previous stage causes the protein to change shape expelling Na+ to the outside.

Two potassium ions (K+) from outside then enter and attach to their binding sites.

The binding of the K+ leads to the release of the phosphate which causes the pump to change shape again so that it is only open to the inside of the axon.

K+ is released inside.

Na+ can now enter and bind to the pump again.

Photosynthesis

Photosynthesis is the production of carbon compounds in cells using light energy.

Oxygen is produced in photosynthesis from the photolysis of water.

Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.

The light dependent reactions take place in the thylakoid membrane.

Thylakoid membranes of the chloroplast provide a large surface area.

Chlorophyll is located in the membrane in groups of molecules called photosystems.

Photolysis of water occurs in thylakoid space and generates electrons for Photosystem II.

Absorption of light in the photosystems gives “excited electrons” (photoactivation).

Excited electrons from photosystem II are passed to electron carriers (electron transport chain).

Electron carriers are embedded in the thylakoid membrane.

Excited electrons from photosystem I are used to reduce NADP+.

NADP+ accepts two high energy electrons and an H+ ion (proton) to form NADPH.

Electron flow through electron carriers causes H+ to be pumped into the thylakoid space

A proton concentration gradient is formed in the space between thylakoids.

ATP synthase is embedded in the thylakoid membrane;

H+ ions flow back through ATP synthase channels to produces ATP by chemiosmosis.

ATP (APT Synthesis) and NADPH (reduced NADP) are produced in the light dependent reactions.

ATP and NADPH produced in the light dependent reactions are used in the light independent reactions.

The light dependent reactions occur in stroma of chloroplast.

The enzyme ribulose bisphosphate carboxylase (Rubisco) catalyses attachment of CO2 to ribulose bisphosphate (RuBP).

This briefly forms an unstable six-carbon intermediate compound.

The six carbon compound splits to form two glycerate-3-phosphate (G-3-P) molecules.

Each (of the two) glycerate-3-phosphate then receives one phosphate from ATP;

Each (of two) phosphorylated glycerate-3-phosphate is reduced by NADPH + H;

The result is two molecules of triose phosphate (TP).

For every six molecules of triose phosphate one goes to form glucose.

The five remaining TP molecules are reorganised to reform RuBP.

The reorganisation of TP into RuBP requires ATP.

Calvin cycle

Xerophytes

Xerophytes will have high rates of transpiration due to the high temperatures and low humidity of desert environments

• Reduced leaves
• Rolled leaves –
• Thick, waxy cuticle
• Stomata in pits
• Low growth
• CAM physiology

Halophytes

Halophytes will lose water as the high intake of salt from the surrounding soils will draw water from plant tissue via osmosis

• Cellular sequestration
• Tissue partitioning
• Root level exclusion
• Salt excretion
• Altered flowering schedule

Transpiration

Water and mineral is transported through xylem. Light energy concert water in the leaves to vapour. Water is absorbed from the soil by the roots, creating pressure. (from roots to leaves(evaportation via stomata)

When the stomata are open, gases can be exchanged, but water vapour can also escape from the leaves. This water must be replaced by water taken into the roots and carried through the plants to the leaves. This process is called transpiration and is the result of gas exchange in the leaves.

Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata

New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)

Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream)

Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata.

Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll.

This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf.

This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull).

The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls.

The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant

It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the free movement of water

Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only

The cell wall contains numerous pores (called pits), which enables water to be transferred between cells

Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension

Xylems can be composed of tracheids (all vascular plants) and vessel elements (certain vascular plants only)

Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer

In vessel elements, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer

All xylem vessels are reinforced by lignin, which may be deposited in different ways:

In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other

In spiral vessels, the lignin is present in the form of a helix or coil

Organic compounds are transported from sources to sinks via a vascular tube system called the phloem Active transport is used to load organic compounds into phloem sieve tubes at the source.

High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.

Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.

Translocation is the movement of organic compounds (e.g. sugars, amino acids) from sources to sinks

1.Sugars produced by photosynthesising tissues or other sources are actively loaded (i.e. ATP is used for this process) into sieve tubes by companion cells. This causes the concentration of solute to build up in the sieve tubes.

2.Water then enters the sieve tubes by osmosis from neighbouring xylem vessels.

3.As water is incompressible and sieves elements have a rigid cell wall, this inflow of water creates a great deal of internal pressure. The pressure causes movement of water and carbohydrates through the pores of the sieve plates, down the tube towards the sink. The pressure that drives this mass flow is called hydrostatic pressure.

4.At the sink, companion cells actively unload the sieve tube. some of the carbohydrates are converted into starch and stored and some are used by the respiring cells. As sugars leave the sieve tube, the concentration of solute decreases, which in turn leads to water moving to the neighbouring vessel by osmosis.

5.The loss of water from the sieve tube will lead to a drop in hydrostatic pressure. This is important as it allows transport along hydrostatic pressure gradients in sieve tubes. As phloem sap flows from source to sink , it is transported from a region of high hydrostatic pressure to one of lower hydrostatic pressure. This process is also referred to as the pressure-flow mechanism.

The nutrient-rich, viscous fluid of the phloem is called plant sap

Sieve elements or sieve tube elements are the elongated living cells that form the phloem tissue. Several sieve elements are connected end to end to form a sieve tube. The cross walls within the sieve tubes become perforated during development to give rise to sieve plates.

Sieve elements are long and narrow cells that are connected together to form the sieve tube

Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells

Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials

The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow

Companion Cells Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink

Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange

Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink

Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube

Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell

This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)

Plasmodesmata exist between sieve elements and companion cells in relatively large numbers

These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites

Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells

Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)

Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)

Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure

Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)

The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient

Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement

This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source

At the Sink

§The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)

§This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)

§Consequently, water is drawn out of the phloem and back into the xylem by osmosis

§This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source

§Hence, phloem sap will always move from the source towards the sink

§When organic molecules are transported into the sink, they are either metabolised or stored within the tonoplast of vacuoles

Xylem and Phloem

§The vascular bundle functions to connect tissues in the roots, stem and leaves as well as providing structural support

Xylem

§Moves materials via the process of transpiration

§Transports water and minerals from the roots to aerial parts of the plant (unidirectional transport)

§Xylem occupy the inner portion or centre of the vascular bundle and is composed of vessel elements and tracheids

§Vessel wall consists of fused cells that create a continuous tube for the unimpeded flow of materials

§Vessels are composed of dead tissue at maturity, such that vessels are hollow with no cell contents

Phloem

§Moves materials via the process of active translocation

§Transports food and nutrients to storage organs and growing parts of the plant (bidirectional transport)

§Phloem occupy the outer portion of the vascular bundle and are composed of sieve tube elements and companion cells

§Vessel wall consists of cells that are connected at their transverse ends to form porous sieve plates (function as cross walls)

§Vessels are composed of living tissue, however sieve tube elements lack nuclei and have few organelles

Meristems

§They are analagous to totipotent stem cells in animals, except that they have specific regions of growth and development

§Meristematic tissue can allow plants to regrow structures or even form entirely new plants (vegetative propagation)

§Meristematic tissue can be divided into apical meristems and lateral meristems:

§Apical meristems occur at shoot and root tips and are responsible for primary growth (i.e. plant lengthening)

§Lateral meristems occur at the cambium and are responsible for secondary growth (i.e. plant widening / thickening)

§Apical meristems give rise to new leaves and flowers, while lateral meristems are responsible for the production of bark

The growth of the stem and the formation of new nodes is controlled by plant hormones released from the shoot apex

§One of the main groups of plant hormones involved in shoot and root growth are auxins (e.g. indole-3-acetic acid / IAA)
When auxins are produced by the shoot apical meristem, it promotes growth in the shoot apex via cell elongation and division

§The production of auxins additionally prevents growth in lateral (axillary) buds, a condition known as apical dominance

§Apical dominance ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants

§As the distance between the terminal bud and axillary bud increases, the inhibition of the axillary bud by auxin diminishes

§Different species of plants will show different levels of apical dominance

§Cytokine promotes axillary bud to grow .

Auxins are a group of hormones produced by the tip of a shoot or root (i.e. apical meristems) that regulate plant growth

§Auxin efflux pumps can set up concentration gradients within tissues – changing the distribution of auxin within the plant

§These pumps can control the direction of plant growth by determining which regions of plant tissue have high auxin levels

§Auxin efflux pumps can change position within the membrane (due to fluidity) and be activated by various factors 
Auxin has different mechanism of action in the roots of plants versus the shoots of plants:

§In the shoots, auxin stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger)

§In the roots, auxin inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller)

Auxin is a plant hormone and influences cell growth rates by changing the pattern of gene expression with a plant’s cells

§Auxin’s mechanism of action is different in shoots and roots as different gene pathways are activated in each tissue
In shoots, auxin increases the flexibility of the cell wall to promote plant growth via cell elongation

§Auxin activates a proton pump in the plasma membrane which causes the secretion of H⁺ ions into the cell wall

§The resultant decrease in pH causes cellulose fibres within the cell wall to loosen (by breaking the bonds between them)

§Additionally, auxin upregulates expression of expansins, which similarly increases the elasticity of the cell wall

§With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size

Tropisms describe the growth or turning movement of an plant in response to a directional external stimulus 

§Phototropism is a growth movement in response to a unidirectional light source

§Geotropism (or gravitropism) is a growth movement in response to gravitational forces

§Other tropisms include hydrotropism (responding to a water gradient) and thigmotropism (responding to a tactile stimulus)
Both phototropism and geotropism are controlled by the distribution of auxin within the plant cells:

§In geotropism, auxin will accumulate on the lower side of the plant in response to the force of gravity

§In phototropism, light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant
In shoots, high auxin concentrations promote cell elongation, meaning that:

§The dark side of the shoot elongates and shoots grow towards the light (positive phototropism)

§The lower side of the shoot elongates and roots grow away from the ground
In roots, high auxin concentrations inhibit cell elongation, meaning that:

§The dark side of the root becomes shorter and the roots grow away from the light (negative phototropism)

§The lower side of the root becomes shorter and the roots turn downwards into the earth

Micropropagation is a technique used to produce large numbers of identical plants (clones) from a selected stock plant

§Plants can reproduce asexually from meristems because they are undifferentiated cells capable of indeterminate growth

§When a plant cutting is used to reproduce asexually in the native environment it is called vegetative propagation

§When plant tissues are cultured in the laboratory (in vitro) in order to reproduce asexually it is called micropropagation
The process of micropropagation involves a number of key steps:

§Specific plant tissue (typically the undifferentiated shoot apex) is selected from a stock plant and sterilised

§The tissue sample (called the explant) is grown on a sterile nutrient agar gel

§The explant is treated with growth hormones (e.g. auxins) to stimulate shoot and root development

§The growing shoots can be continuously divided and separated to form new samples (multiplication phase)

§Once the root and shoot are developed, the cloned plant can be transferred to soil

Plant growth is initiated at regions called meristems – undifferentiated cells capable of indeterminate divisions

§Meristems are equivalent to embyronic stem cells in animals, but are retained throughout the adult life of the plant

§This allows plants to regrow structures and even reproduce asexually (vegetative propagation)
All the differentiated tissues in a plant are derived from meristems – either apical or lateral meristems

§Apical meristems give rise to the primary tissues needed to increase a plant’s length and grow new leaves and fruits

§Lateral meristems give rise to the secondary tissues needed to support an increase in the plant’s width (e.g. bark)

Auxinaffects gene expression in shoots:

·Cells contain an auxin receptor.

·When auxin binds to receptors, transcription of specific genes is promoted.

·The expression of these genes causes secretion of hydrogen ions into cell walls.

·hydrogen ions loosen connections between cellulose fibres, allowing cell expansion.

Concentration gradients of auxinare necessary to control the direction of plant growth.

Different factors can affect transporter proteins and hence the direction in which auxin can move:

The location of transporter proteins can be changed as the plasma membrane is fluid, e.g. efflux transporters can congregate at the top of cells in roots to move auxin upwards

Transporter proteins can be activated and/or inhibited by stimuli such as light

Sexual reproduction in flowering plants involves the transfer of pollen (male gamete) to an ova (female gamete)

§This involves three distinct phases – pollination, fertilization and seed dispersal

Pollination:

§The transfer of pollen grains from an anther (male plant structure) to a stigma (female plant structure)

§Many plants possess both male and female structures (monoecious) and can potentially self-pollinate

§From an evolutionary perspective, cross-pollination is preferable as it improves genetic diversity

Fertilisation:

§Fusion of a male gamete nuclei with a female gamete nuclei to form a zygote

§In plants, the male gamete is stored in the pollen grain and the female gamete is found in the ovule

Seed dispersal:

§Fertilisation of gametes results in the formation of a seed, which moves away from the parental plant

§This seed dispersal reduces competition for resources between the germinating seed and the parental plant

§There are a variety of seed dispersal mechanisms, including wind, water, fruits and animals

§Seed structure will vary depending on the mechanism of dispersal employed by the plant

Flower Structures

The male part of the flower is called the stamen and is composed of:

§Anther – pollen producing organ of the flower (pollen is the male gamete of a flowering plant)

§Filament – slender stalk supporting the anther (makes the anther accessible to pollinators)

The female part of the flower is called the pistil (or carpel) and is composed of:

§Stigma – the sticky, receptive tip of the pistil that is responsible for catching the pollen

§Style – the tube-shaped connection between the stigma and ovule (it elevates the stigma to help catch pollen)

§Ovule – the structure that contains the female reproductive cells (after fertilisation, it will develop into a seed)

In addition to these reproductive structures, flowers possess a number of other support structures:

§Petals – brightly coloured modified leaves, which function to attract pollinators

§Sepal – Outer covering which protects the flower when in bud

§Peduncle – Stalk of the flower

The purpose of flowering is to enable the plant to sexually reproduce via pollination, fertilisation and seed dispersal

§Consequently, flowers need to bloom when pollinators are most active and abundant – this is dependent on seasons

§Some plants bloom in long day conditions (summer), whereas other plants bloom in short day conditions (autumn / winter)

The critical factor responsible for flowering is the length of light and dark periods, which is detected by phytochromes

Phytochromes

Phytochromes are leaf pigments which are used by the plant to detect periods of light and darkness

§The response of the plant to the relative lengths of light and darkness is called photoperiodism 

Phytochromes exist in two forms – an active form and an inactive form:

§The inactive form of phytochrome (P_r) is converted into the active form when it absorbs red light (~660 nm)

§The active form of phytochrome (P_fr) is broken down into the inactive form when it absorbs far red light (~725 nm)

§Additionally, the active form will gradually revert to the inactive form in the absence of light (darkness reversion)

Because sunlight contains more red light than moonlight, the active form is predominant during the day

§Similarly, as the active form is reverted in darkness, the inactive form is predominant during the night

Photoperiodism

Only the active form of phytochrome (P_fr) is capable of causing flowering, however its action differs in certain types of plants

§Plants can be classed as short-day or long-day plants, however the critical factor in determining their activity is night length

Short-day plants flower when the days are short – hence require the night period to exceed a critical length

§In short-day plants, P_fr inhibits flowering and hence flowering requires low levels of P_fr (i.e. resulting from long nights)

Long-day plants flower when the days are long – hence require the night period to be less than a critical length

§In long-day plants, P_fr activates flowering and hence flowering requires high levels of P_fr (i.e. resulting from short nights)

Horticulturalists can manipulate the flowering of short-day and long-day plants by controlling the exposure of light

§The critical night length required for a flowering response must be uninterrupted in order to be effective

Long-day plants require periods of darkness to be less than an uninterrupted critical length

§These plants will traditionally not flower during the winter and autumn months when night lengths are long

§Horticulturalists can trigger flowering in these plants by exposing the plant to a light source during the night

§Carnations are an example of a long-day plant

Short-day plants require periods of darkness to be greater than an uninterrupted critical length

§These plants will traditionally not flower during the summer months when night lengths are short

§Horticulturalists can trigger flowering in these plants by covering the plant with an opaque black cloth for ~12 hours a day

§Crysanthemums are an example of a short-day plant

A typical seed will possess the following features:

§Testa – an outer seed coat that protects the embryonic plant

§Micropyle – a small pore in the outer covering of the seed, that allows for the passage of water

§Cotyledon – contains the food stores for the seed and forms the embryonic leaves

§Plumule – the embryonic shoot (also called the epicotyl)

§Radicle – the embryonic root

For germination to occur, a seed requires a combination of:

§Oxygen – for aerobic respiration (the seed requires large amounts of ATP in order to develop)

§Water – to metabolically activate the seed (triggers the synthesis of gibberellin)

§Temperature – seeds require certain temperature conditions in order to sprout (for optimal function of enzymes)

§pH – seeds require a suitable soil pH in order to sprout (for optimal function of enzymes)

Additionally, certain plant species may require additional conditions for germination:

§Fire – some seeds will only sprout after exposure to intense heat (e.g. after bushfires remove established flora)

§Freezing – some seeds will only sprout after periods of intense cold (e.g. in spring, following the winter snows)

§Digestion – some seeds require prior animal digestion to erode the seed coat before the seed will sprout

§Washing – some seeds may be covered with inhibitors and will only sprout after being washed to remove the inhibitors

§Scarification – seeds are more likely to germinate if the seed coat is weakened from physical damage

Experiments can be developed using any of these factors as an independent variable

§Germination can be measured by the rate of seed growth over a set period of time