Biology Essay Finally

7/31/2019 Biology Essay Finally

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Biology Essay: Transport in plants

Transport systems play an essential role in the survival of flowering plants. Largemulticellular organisms such as flowering plants require specialised transportsystems to provide sufficient exchange of needed materials and waste products. The

movement of substances, in larger organisms, through body fluids between theexternal surface and internal cells by diffusion alone would be insufficient in meetingcellular requirements. Tiny organisms have a high surface area to volume ratio andcan therefore directly exchange materials with the environment to adequately meetrequirements of all cells. Larger organisms have a lower surface area to volume ratioand as a result the distances to be covered through the external surface by diffusionwould be too far to meet the requirements of all cells. Hence, flowering plants(vascular plants) have developed specialised transport systems and effectiveexchange organs (leaves and roots) to efficiently transport required substances toand from all cells.

Materials that are moved throughout the transport systems of flowering plantsinclude water, carbohydrates (sugars), dissolved mineral ions and organicsubstances (amino acids) In flowering plants water and mineral ions (macro andmicronutrients) are absorbed by root hairs from the soil and passed into xylem tissuewhich transports the water and dissolved nutrients upwards throughout the plant tothe leaves. Sugars created by photosynthesis are actively transported from theleaves of the plant through to the phloem tissue usually in the form of sucrose (cellsap) to other parts of the plant either for storage or to be used for energy. Sucrose isalso transported down phloem tissue from sources to sinks which store the sucroseas starch for future use.

Water enters a plant through the hairs on the root, and moves across the root cells into the

xylem, which transports it up and around the plant. That, and solutes are moved around by

the xylem and the phloem, using the root, stem and plant. Water enters the root through the

root hairs, and then takes one of two paths (apoplast and symplast) to the xylem vessel. A

root hair is a simple extension of the epidermis of a root cell, and reaches into the soil to

absorb water. It exists to increase the surface area and therefore the rate at which water can

be absorbed. Water moves into the root hair cells because it is moving down a water

potential gradient, since a root cell has relatively low water potential due to its inorganic ions

and organic substances. Water will enter through the membrane and into the cytoplasm and

vacuole.

From the root hair cells, water again moves down a concentration gradient toward the xylem,

and can take one of two paths - apoplast and symplast. The apoplast pathway is where

water takes a route going from cell wall to cell wall, not entering the cytoplasm at any point.

The symplast pathway is where water moves between cytoplasm/vacuoles of adjacent cells.

However, the apoplast pathway can only take water a certain way, near the xylem, the

Casparian strip forms an impenetrable barrier to water in the cell walls, and it must move into


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the cytoplasm to continue. This gives the plant control over the ions that enter its xylem

vessels, since water must cross a plasma membrane to get there.

The xylem is constructed of three main elements;

Vessel elements, including tracheids - cells involved in water transport

Fibres - elongated cells with lignified walls that support the plant

Parenchyma cells - normal plant cells, except no chloroplasts.

These vessel elements make up the xylem - and are many cells laid end to end, and are

normal plant cells with their walls strengthened by lignin, a hard strong substance that is

impermeable to water, and is designed to provide structure and strength to the plant. When

these plant cells are strengthened by lignin, the cell inside dies, leaving a space inside.

However, in some plasmodesmata, there was no lignin laid down and these appear as gaps

in the xylem vessel, know as pits. These have permeable un-thickened cellulose cell wall.

Thus, a continuous tube is formed, known as the xylem vessel.Tracheids are dead cells with

lignified walls, but do not have open ends and thus do not form vessels - their ends are

tapered. All plants have them, but some plants use them as main conducing tissue.

The transport of soluble organic substances (assimilates) within a plant is known as

translocation. The solutes are transported in sieve elements, found in the phloem tissues,

along with other as companion cells, parenchyma and fibres. Sieve Elements are specialised

cells, with few mitochondrion and endoplasmic reticulum, no nucleus or ribosomes. Where

two ends of sieve elements meet, a sieve plate is formed, made from walls of both elements,

with large pores allowing free flow of liquids between them.

Companion cells are normal plant cells with high numbers of mitochondria and ribosomes,

and have many plasmodesmata pass through to their neighbouring sieve cell walls, making

direct contact between the cytoplasms of companion cell and sieve element. Mass flow is

the theory by which we think solute transport occurs in plants. Any area where sucrose is

produced in a plant is known as a source, and any area where it is taken out (usually, used

in respiration) is known as a sink.

Sucrose is actively transported into the sieve tubes of the phloem at the source (i.e. source),

lowering the water potential inside the sieve and so water enters the tubes via osmosis,

creating a higher pressure inside the sieve tubes at the source. At the sink, sugars leave the

phloem to be used up, increasing the water potential inside the sieve tubes, so water leaves

via osmosis, lowering the pressure inside the sieve tubes. The result is a pressure gradient

between source to sink, pushing sugars to where they're needed.


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Transpiration

Transpiration is the loss of water vapour from the leaves of a plant. As water evaporates

from the leaf, a constantly occurring process, more water is taken in to replace it. The

removal of water reduces the hydrostatic pressure (pressure exerted by a liquid). Since this

pressure becomes lower at the top of the xylem vessel than at the bottom, this pressure

difference causes water to move up the xylem vessels.

This process is known as mass flow - as long with the fact that water molecules move

together as a body of water - aided by water's property of being cohesive, and attracted to

the lignin in the walls of the xylem vessels, known as adhesion.

Once water is in the leaf, it can be lost through the stomata, if there is a

concentration gradient that it can go down, which small pores are in direct

contract with the air outside. This process is known as transpiration. re are a

number of factors that determine transpiration rates:

Temperature: Transpiration rates go up as the temperature goes up. Higher

temperatures cause the plant cells which control the openings (stoma), where

water is released to the atmosphere, to open, whereas colder temperatures

cause the openings to close.,

Relative humidity: As the relative humidity of the air surrounding the plant

rises the transpiration rate falls. It is easier for water to evaporate into dryer

air than into more saturated air.

Wind and air movement: Increased movement of the air around a plant will

result in a higher transpiration rate. This is somewhat related to the

relative humidity of the air, in that as water transpires from a leaf, the water

saturates the air surrounding the leaf. If there is no wind, the air around the

leaf may not move very much, raising the humidity of the air around the leaf.

Wind will move the air around, with the result that the more saturated air

close to the leaf is replaced by drier air. Soil-moisture availability: When soil moisture is lacking, plants can begin

to senesce (premature ageing, which can result in leaf loss) and transpire

less water.

Type of plant: Plants transpire water at different rates. Some plants which

grow in arid regionsfor example, cacti and succulentsconserve

precious water by transpiring less water than other plants.
http://www.eoearth.org/article/Temperaturehttp://www.eoearth.org/article/Atmospheric_compositionhttp://www.eoearth.org/article/Atmospheric_humidityhttp://www.eoearth.org/article/Physical_properties_of_waterhttp://www.eoearth.org/article/Evaporationhttp://www.eoearth.org/article/Atmospheric_humidityhttp://www.eoearth.org/article/Physical_properties_of_waterhttp://www.eoearth.org/article/Windhttp://www.eoearth.org/article/Soilhttp://www.eoearth.org/article/Physical_properties_of_waterhttp://www.eoearth.org/article/Physical_properties_of_waterhttp://www.eoearth.org/article/Physical_properties_of_waterhttp://www.eoearth.org/article/Physical_properties_of_waterhttp://www.eoearth.org/article/Soilhttp://www.eoearth.org/article/Windhttp://www.eoearth.org/article/Physical_properties_of_waterhttp://www.eoearth.org/article/Atmospheric_humidityhttp://www.eoearth.org/article/Evaporationhttp://www.eoearth.org/article/Physical_properties_of_waterhttp://www.eoearth.org/article/Atmospheric_humidityhttp://www.eoearth.org/article/Atmospheric_compositionhttp://www.eoearth.org/article/Temperature


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Components of the cell membrane

Components

The previous section spoke about several components that may be new to you, their

structures and roles are below.

Phospholipids

You may remember phospholipids from chapter two. It was stated that they have hydrophilic

heads and hydrophobic tails, the importance of which should be becoming clear. A

phospholipids bi-layer forms the body of the membrane, creating a hydrophobic interior and

isolating the cell from the outside environment. Do not forget that organelles within a cell are

also often surrounded by a membrane, and this too is a phospholipid membrane.

Proteins

Proteins have a variety of functions within membranes. Some membrane proteins are

enzymes, catalysing reactions such as the ones in the surface of the small intestine, that

hydrolyse disaccharide molecules, among others. However, most are transport proteins,

providing hydrophilic channels through the bi-layer for ions and polar molecules to pass

through. They are usually specific to a certain ion or molecule. They can control the types of

substances that can leave or enter the cell. The membranes of organelles also hold proteins

- for example mitochondria and chloroplasts require them for processes of respiration and

photosynthesis.

Glycoproteins and Glycolipids

Lipid and proteins on the cell membrane surface often have short carbohydrate chains

protruding out from the cell surface, known as glycolipids and glycoproteins. They form

hydrogen bonds with the water molecules surrounding the cell and thus help to stabilise

membrane structure. However, more importantly, they are used as receptor molecules,

binding with hormones or neurotransmitters to trigger a series of chemical reactions within

the cell itself. Using insulin as an example, only some cells within the body (liver, muscles),

have receptors for insulin and as such, insulin can be released to the entire body without

upsetting anything as any cell without an insulin receptor will not be affected.

Cholesterol

Cholesterol helps to regulate fluidity of the membrane and also to provide mechanical

stability of the membranes - without it cells will burst open as their membranes break. Their

hydrophobic regions help to prevent ions or polar molecules inadvertently passing through

the membrane.


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Fluid Mosaic Model

A diagram of the fluid mosaic model can be seen below.

Features of the fluid mosaic model;

The membrane primarily consists of a bilayer of phospholipid

molecules. These molecules can move about by diffusion in their own layer.

Width is about 7nm on average

Some of the phospholipids are saturated and some are unsaturated. This affects the

fluidity of the membrane; an unsaturated membrane means a more fluid membrane. This

is due to the kink in unsaturated tails causing the molecules to not sit closely together.

Phospholipid tails point inwards, facing each other, meaning that inside the membrane itis non-polar hydrophobic.

The protein molecules within the structure can move around although some are fixed to

structures inside the cell and do not move. Also, some of them span the width of the

membrane; some are only on the inner layer and some on the outer layer.

Many proteins and lipids have short carbohydrate chains attached to them, forming

glycoprotein and glycolipids.

Contains cholesterol.
http://en.wikibooks.org/wiki/File:Cell_membrane_detailed_diagram_en.svg


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The blood circulation system we have is made up of three maincomponents: , the and the . It is made up oftwodifferent blood

systems a . The diagram shows that one transports blood from the

heart to the lungs and back again, the other takes blood around the rest of the body.

Having a double circulation is vital in animals like ourselves because we are constantly

active and in need of a rich blood supply and with this system, we are constantly

receiving oxygenated blood from the lungs which is sent around the body in one cycle.

There are three main blood vessels in the system, which have all adapted to carry out

specific functions. The diagram below shows each of them


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The (left diagram)carry blood away from the heart to the organs in the body.

This is usually oxygenated blood, explaining the redtubes. When you feel your pulse,

that is the arteries stretching as blood is forced through them and returning back into

their original shape.

The carry blood towards the heart, usually low in oxygen and hence are deep

purple-red in colour. No pulse in veins, but they do contain valves usually which

prevent the backflow of blood.

The are found in junctions between the arteries and veins. These are found in

huge networks. The walls are a single cell thick so that substances which need to get

out of the blood and into body cells can easily via diffusion.

Our hearts are made of two pumps, for the double circulation. These together beat

around seventy times a minute. The walls of the heart are made pretty much entirely

from muscle, which gets oxygen from the coronary blood vessels.

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Biology Essay Finally