GCSE Biology Organization: Cells to Organ Systems Explained

[Music] hi everyone this is going to be the first lesson on the topic of

organization for GCSE biology students in this topic we're going to be expanding upon some of the ideas that we

encountered in the previous topic on cell biology and looking at the ways in which cells are organized into tissues

organs and organ systems and taking a look at specific types of organ systems organs and tissues in different types of

organisms plants animals and so on in this lesson we're going to outline the fundamental ideas in organization in

biology and we're going to start by recapping some of the important ideas from that previous topic on cell biology

so in this lesson we're going to be looking specifically at what constitutes a tissue an organ and an organ system

how our cells organized in biology and more importantly perhaps why are they organized in this way so let's take a

look at as I say some of the fundamental ideas from the previous lesson now as always I'm joined by a number of

students via the zoom application many of you have asked a number of important and relevant and insightful questions

concerning the topic for today's lesson I will be addressing those as always in the Q&A discussion session that will

follow the recorded segments of this lesson so those of you in attendance with me via Xue please hold fire on

those questions and if you if any other questions occur to you during the course of this lesson then you can type those

into the Q&A box in your zoom client and you can vote on those questions as the lesson progresses and then at the end of

the recorded segments of this lesson I will address those questions in order now some of those questions I'm going to

be addressing throughout the recorded segments of this lesson but as always if you're watching this as a recording and

you have questions about the contents of this lesson you can always send me a private message via the this lessons

page on the website so you can simply log in go to this lessons page on the GCSE biology website and you can send me

those questions as private messages via the student notes funk in which case I'll respond to your

questions within 24 hours typically via a private message if your if you prefer you can type your

questions into the live chat on this lessons page and perhaps starts a discussion with other students on that

page where I will respond alternatively you can always email me and I'll be happy to answer any of your questions

that way if you're watching this as a recording elsewhere such as on YouTube then by all means type your questions

into the comments and I will do my best to get back to you there as well with some answers so before we go any further

then let's take a look at the learning objectives for today's lesson so once you've watched this lesson video

if you're watching it as a recording and if you're watching it on my website once you've completed the various questions

that will pop up at intervals throughout this video when the video pauses together with the other learning

activities on this lessons page on the website such as the end of lesson quiz and so on you should be able to do the

following things so we first need to start by defining our terms what do we mean by the terms tissue organ and organ

system now we're also going to look at something that we encountered in a previous lesson in that last topic on

cell biology the idea that single-celled organisms and single cells in general are limited in their size and their

growth by the surface area to volume ratio that they have and the necessity for them to obtain the necessary

substances to support their metabolism things like oxygen for example by diffusion through their cell membranes

the idea that surface area to volume ratio will restrict the maximum size that a cell can grow to because beyond

that size its surface area to volume ratio is no longer feasible for diffusion to satisfy the needs for

transport of substances so we will then go on to look at the way in which cells are organized into the most important

types of animal tissues and also plant tissues we're going to be looking at the main types of animal tissue the specific

as specialized cells that make up those tissues and relating the structure of those tissues to their function

we're going to do the same for the most important types of plant tissues as well and then we're going to go on to look at

the structures of some human organs and also the structure of a leaf as an example of a plant organ and the whole

idea behind this is to lay the groundwork for the remainder of this topic where we're going to be looking at

various organ systems in the way in which they work now in particular we're going to restrict ourselves to looking

at a couple of organ systems in humans so the digestive system the circulatory system and the respiratory system and

looking at the ways in which cells and tissues and organs in those systems work relating their structures to their

functions but we're also going to be looking at exchange and transport systems in plants as well so obviously

the groundwork for all of that is going to be laid in this lesson where we look at these fundamental concepts of the

ideas of tissues organs organ systems and the need for this level of organization in biology so as you can

tell there's a lawful lot to get through so let's make a start then so let's quickly recap some basic concepts now so

the idea of a cell is something that we covered in a previous lesson where we defined the cell as the smallest living

unit of matter and this begs the question how does a biologist distinguish from the living and the

nonliving material universe what constitutes a living thing well by almost universal agreement

biologists have chosen to define living things as those which can perform seven characteristic functions now these

include the following respiration movement sensitivity nutrition excretion growth reproduction now all of these are

characteristic of living things that is to say that any living thing of any size or scale will perform these seven

characteristic functions now this includes unicellular organisms individual free living single-celled

organisms they are defined as being living because they've performed all seven of these functions now according

to some biologists homeostasis the maintenance of a constant internal in Ireland despite fluctuations in the

external environment this is also a characteristic function of living things but for now we'll restrict ourselves to

these things so a cell is defined as the smallest piece of matter the smallest material object that will perform all

seven of these characteristic functions of life now in more complex multicellular organisms cells are

organized specialized cells are organized into tissues organs and organ systems in order to maximize the

efficiency with which these processes occur so we encountered the idea that there are such things as free living

unicellular organisms which is each made up of a single cell so things like the amoeba for example or a bacterium these

are single-celled organisms where the living thing is a single cell but in multicellular organisms we have

specialization of cells now again the idea of specialization is something that we encountered in that previous topic on

cell biology so if you need to recap what we mean by the term specialization I would suggest going back to the second

lesson in that series and also taking a look at the fifth lesson where we talked about the process by which cells become

specialized which we call differentiation now in multicellular organisms specialized cells are

organized into tissues and we used the analogy that just as a wall is made up of smaller units called bricks animals

and plants multicellular animals and plants and multicellular organisms in general are made up of smaller living

units that we call cells so here we can see a section of a leaf viewed under an optical microscope and we can see that

it is indeed made up of smaller building blocks just as the brick wall is made up of individual bricks now we talked in

the previous topic where we looked at transport of substances across membranes about the fact that as cells are limited

in their size by this principle of surface area to volume ratio now a single-celled organism such as

amoeba has to obtain all of its nutrients all of its oxygen and also eliminate all of its waste products

across its cell surface membrane and so single-celled organisms typically have to have a large surface area to volume

ratio and this means that dissolved substances don't have very far to travel to get into or out of the organism

because in these cases diffusion and other processes across the membrane are entirely responsible for supporting the

metabolism of that organism so the word metabolism means the sum of all chemical reactions happening inside an organism

that support its life so in a single-celled organism all of the chemical reactions and processes that

occur inside that organism that sustain its life have to be powered have to be supported by the transport of substances

into and out of the cell and this all has to occur across the cell surface membrane and so the bigger the volume of

the cell gets the more of those processes have to be supported but as we saw in a previous topic in the previous

topic where we looked at the transport of substances as the size of a cell increases its surface area does not

increase in proportion to its volume and so there comes a point where the surface area of the organism is no longer

sufficient to support the metabolic activities occurring within its volume so in cases while we're talking about

single celled organisms the process of diffusion typically for a single-celled organism is adequate to supply the

organism with all of the materials that it needs such as oxygen and nutrients and also the removal of waste products

such as carbon dioxide and again this is because the distance over which the materials have to diffuse is relatively

short so let's take a look again at the principle of surface area to volume ratio just to illustrate this because

this is essentially the reason why you can't have single celled organisms such as amoebic grow to the size of say blue

whales because eventually the cell will reach a size where its surface area is not sufficiently large to service the

needs of its volume so let's take a look at a cube let's imagine we have a cube with sides of

length one millimeter and so if we calculate its surface area and its volume and compare those to each other

as a ratio we see that because the cube has six faces and each of those six faces has an area of one square

millimeter the total surface area of this cube is six millimeters squared it's volume is simply going to be its

height multiplied by its width multiplied by its depth and that's going to give us one cubic millimeter and so

we see that it has a surface area to volume ratio of six to one so let's imagine now that this cuboidal cell this

cube which we're using to represent a single cell doubles in size in terms of the length of its sides going from one

millimeter to two millimeters let's see what that does to its surface area to volume ratio well when we calculate the

surface area and the volume of this cube we find that its surface area is 24 square millimeters and it's volume has

now gone up to 8 cubic millimeters now if we represent this as the lowest whole number ratio we see that the surface

area to volume ratio is now three to one so by doubling the length of the sides of the cube we have halved the surface

area to volume ratio if we then increase the size of the cube further to let edges of length three millimeters we can

see now that this cube has a total surface area of 54 square millimeters and a total volume of 27 cubic

millimeters now this equates to a surface area to volume ratio of just two to one so as we can see here as the size

of the ink of the cube increases as the lengths of the sides of the cube increase we can then go on to calculate

the surface areas and the volumes of these cubes and then have a look at how the surface area to volume ratio changes

in relation to the sides of the length the lengths of the sides of the cube and what we see is that as the length of the

sides of a structure increase if we're talking about a cube the surface area and the volume will both increase

exponentially but the volume will increase at a greater rate then the surface area does and this

makes sense because to calculate a volume you're multiplying three dimensions you're raising something to

the power of three whereas if you're trying to calculate an area you're simply raising something to the power of

two and so we see that as the size of the object increases its surface area and volume both increase exponentially

but the volume increases at a faster rate than the surface area does so if we look at this then we see that the

surface area to volume ratio if we plot the surface area to volume ratio and how that changes with the cubes length of

edge we see that the surface area to volume ratio decreases exponentially as the cubes length of edge increases so

what this means is that as the size of as Aldus of a structure or an organism increases the surface area does not

increase in proportion to its volume so this means that once a single cell reaches a particular size there comes a

point where its surface area is not sufficient in terms of diffusion active transport facilitated diffusion in terms

of transport of substances across that surface area it's no longer sufficiently large to for those processes of

transport to be sufficient to support the internal chemical reactions the metabolism happening inside the volume

of that cell so multicellular organisms have to rely upon processes other than simple diffusion in order to maintain

the life of that organism so the process of cells dividing is is triggered essentially at least in part by the cell

reaching a size where it has to divide in order to increase its surface area to volume ratio so that the daughter cells

can continue to live but also the development of multicellular multicellularity the evolution of

multicellular organisms is it self partly a consequence of the increased efficiency of multicellular

structures for in terms of their surface area to volume ratio so in larger multicellular organisms diffusion

becomes less efficient because the surface area does not increase in line with the size so multicellular organisms

have evolved various transport systems to carry dissolved substances that they need to the cells where they're needed

so the exchange surfaces in these multicellular organisms themselves are also highly modified to maximize the

surface area to volume ratio now this entire topic is going to be looking at various transport systems mass transport

systems and exchange systems such as in the lungs of the of a mammal and we're going to be looking at the ways in which

the tissues and organs within those systems are adapted for those functions but all of this goes back to the the

significance of surface area to volume ratios so in multicellular organisms the cells and tissues do not all perform the

same function there is a division of labour so an analogy that you can use to describe this is that at school you'll

talk different subjects by different teachers who are specialized in one or perhaps two subjects which they know

very well and this ensures that you're effectively and efficiently educated to a high standard in those subjects now

there's an old saying which I'm sure you've come across the idea that a jack-of-all-trades is a master of none

so to make the organism even more efficient at survival at achieving the seven characteristics of living things

that tissues are further organized into organs and ultimately into body systems so let's take a look now one that one

final look at how this works in terms of surface area to volume ratio how the jump from a single cell structure to a

multi cellular structure improves surface area to volume ratio so consider the following tubular and

cubic structures these both have a total volume of 8 cubic millimeters so let's look at the cube again which has sides

of length 2 millimeters again as we saw before the surface area to volume ratio of this cube is 3 to 1

now let's instead of this structure which has a total volume of 8 cubic millimeters let's instead imagine that

we take 8 single cells single cubes each of which have sides of one millimeter and stack them into one long cubic

tubular structure like this so if we look at what this does to the surface area to volume ratio we now see that the

total surface area of this structure is 34 squares square millimeters we have the ends so one square millimeter here

one square millimeter at that end and then we have eight times four for the sides of this this structure here so

this gives us a total surface area of 34 square millimeters we still have a total volume of eight so we can see here that

we've gone from a surface area to volume ratio of three to one for a cuboidal structure with this volume to a surface

area to volume ratio of four point two five to one for this lengthened elongated tubular structure so the

elongated structure has a greater surface area to volume ratio than a cube shaped structure and we'll see the

importance of this later on where we look at the digestive system and the the importance of the villi and microvilli

so cells within the body of a multicellular organism are organized as follows so you have the fundamental

building block of life itself the smallest living unit of matter the cell cells then differentiate into

specialized cells as we as we saw in that previous lesson on differentiation and specialization in the last topic so

cells can specialized to perform specific functions they develop specific structural features through the process

of differentiation that allow them to perform specific functions but then if you take specialized cells

of a similar type and group them together you form a tissue and the tissue will then carry out one or more

definite functions within that multicellular organism different tissues can then be organized into functional

structures called organs and those organs can then carry out one or more definite functions within that

multicellular organism but then different tissues and organs can all work together in a specific way to form

a system an organ system that carries out one or more definite larger-scale functions within that organism and

finally all of these different tissues organs and organ systems work together to form an organism and multicellular

living thing that carries out all seven of the characteristic functions of life so tissues organs and organ systems are

levels of organization which allow for the more efficient carrying out of those seven characteristic functions of life

so let's now take a look at some specialized animal tissues to exemplify what we've been talking about so there

are about 20 different types of specialized animal cells and many subtypes of each of these and they all

contain the following basic structures that we saw in that previous topic on cell biology there's a nucleus there's a

cytoplasm there's a cell membrane there's mitochondria ribosomes and so on so all of these structures are common to

all of these specialized animal cells they're necessary for the functions of all of those cells now I say all there

are some exceptions so for example red blood cells will lack a nucleus but in general almost all specialized animal

cells will contain these structures but their overall structure is going to be different depending on the specific

functions of that particular specialized type of cell so when we organize specialized cells of

a particular type all together in one place we formed a tissue and there are six

principal kinds of animal tissues so if we look in humans for example we will find epithelial tissue connective tissue

skeletal tissue blood tissue muscle tissue and nervous tissue these can be thought of as the six main

types of tissues and again a tissue is a group of similar specialized cells that work together to perform a particular

set of functions so these six principal kinds of animal tissues with subtle differences are found in many different

kinds of animal we're going to be looking at them in humans in particular but they're found in all kinds of

different animals so epithelial tissue is perhaps the simplest type of animal tissue it's made up of a sheet of cells

that fit closely together kind of like the slabs in a pavement so in fact what type of epithelium is actually referred

to as pavement epithelium pavement epithelium is found covering the body surface and various organs now pavement

epithelium also forms the lining of some of the tubes in the body such as the mouth and also the air sacs in the lungs

and it also forms the innermost layer of cells in the walls of arteries and veins as well so here we can see a micrograph

with stained epithelial cells from a cheek swab now there are many different kinds of epithelium not just pavement

epithelium all epithelial cells surround the edges of organs or the inner linings of tubes and cavities in the body but

there are several different variants each of which performs one or more slightly different functions so we have

cuboidal epithelium we have a squamous or pavement epithelium we have columnar epithelium we have

stratified epithelium we have glandular epithelium and we also have ciliated columnar epithelium these are all

different kinds of epithelial tissues that we're going to be encountering all the way through this topic when we look

at these structures of various tissues organs in the various body systems in humans so where for example we're going

to encounter glandular epithelium where we look at the digestive system we're also going to encounter ciliated

columnar epithelium for example when we look at the respiratory system and the type of epithelium that lines the

Airways of the respiratory system and so on now connective tissue is literally just

tissue that connects other tissues together so the types of connective tissue are things like ligaments tendons

meninges which are the fibrous layers of tissue surrounding the brain collagen bone marrow fat cells or adipose tissue

these are all technically different types of connective tissue so if we those of you who have perhaps

vegetarians or vegans you might want to look away now so this picture is of a beef steak and what we see here is the

connective tissue in this white color here under that we see the more reddish color of the muscle tissue scope so

skeletal tissue supports and protects the body's delicate organs and also allows for movement because of the

attachment of muscles to the ends of the bones of the skeleton so very often we refer to the muscles and the skeleton

and also various connective tissues that connect them together as the musculoskeletal system so here we can

see a thin section of bone when observed under an optical and optical microscope at a magnification of about a thousand

times here we can see the bone cells and here we can see deposits of calcium phosphate which is the mineral which

gives bones their density and hardness blood is also characterized as a foot as a tissue because again it is a

collection of similar specialized cells in one place now the fact that it's a liquid doesn't change the fact that it

actually does conform to the definition of a tissue that we gave earlier so here we can see how a blood smear appears

when observed at a Mac if occation of about a thousand times under a microscope so blood is in fact a

complex tissue containing a number of different types of specialized cells suspended in a fluid matrix so the

fluids that we find the cells suspended in is referred to as the blood plasma here we can see the platelets which are

fragments of cells involved in blood clotting these cells here are white blood cells and of course the rest of

them are the biconcave disk shaped red blood cells that you see here now red blood cells which are technically

referred to as erythrocytes these contain a respiratory pigment called hemoglobin this carries oxygen around

the body and delivers it to the tissues mammalian red blood cells don't have a nucleus when they mature and they have

this biconcave disc shape that you see here now the biconcave disc shape of the red blood cells and also the fact that

they don't contain the nucleus together these features allow them to maximize their surface area to volume ratio which

in turn allows them to carry the maximum amount of of oxygen from the lungs to the tissues where that oxygen is needed

now the other feature that these have is that they're highly flexible they're able to squeeze and fold and pass

through the narrowest capillaries and this in turn allows them to deliver oxygen to the tissues more efficiently

now as you can see in this this photo the diameter of red blood cells is almost the same as the diameter of the

capillaries through which they passed red blood cells typically passed through capillaries one cell at a time in single

file now this means that there's less distance and a larger surface area for gas exchange to occur between the walls

of the red blood cell and the capillary wall and this makes the process of diffusion of oxygen from the red blood

cell outwards through the wall of the capillary and into the surrounding tissues more efficient so muscle tissue

is made up of specialized muscle cells here we can see a micrograph of individual muscle cells

or muscle fibers you can see that they have this striped or striated appearance so these types of cells these skeletal

muscle cells that you see here are specialized for contraction for getting shorter and generating a tension of

force which can then be used to move the muscles and move the bones so the individual muscle cells here again are

specialized to perform that function their structure contains specific features which allow them to contract

and generate forces and this is important in the function of movement again one of the characteristic

functions of living things now the type of cells that make up heart muscle tissue are rather different to those

making up skeletal muscle tissue heart muscle tissue doesn't ever get tired it's constantly contracting and relaxing

and unlike skeletal tissue it will never stop doing so and so obviously it needs to be structurally quite different to

skeletal muscle tissue so cardiac muscle is the type of muscle tissue only found in the heart it's it rather than having

linear fibers as you saw in the skeletal muscle tissues it has branched fibers which contract rhythmically and the

branched arrangement allows for the whole heart to squeeze to contract around the volume of blood inside it to

allow the heart to fulfill its function of pumping blood around the body so if we zoom in on a section of heart muscle

tissue we will see that the heart the cardiac muscle fibers have these branch points between them as you see here

which are again quite different from the type of structure that you see in skeletal muscle tissue if we were to

look at cardiac muscle under a microscope we might see something like this so here again we're looking under a

optical microscope at a magnification of around 1,000 times and again you can see here the muscle cells the nuclei of

those muscle cells and you can also see how they're branched and linked to each other in a branched arranged

our nerve tissue is made up of long thin cells which transmit electrochemical impulses nerve impulses around the body

and these nerve impulses can travel very long distances such as between the brain and the muscles of the big toe so here

is a diagram of a type of nerve cell so you can see here the cell bodies can have these many branching points through

which they can contact and synapse with other nerve cells and again we're going to be looking at the nervous system in a

later topic where we talk about homeostasis and the response to stimuli this is another type of nerve cell here

called an intermediate or relay neurone so there are many different kinds of nerve cells or neurons they have many

different types of structures but they all have in common the ability to pass these electrochemical impulses if we

were to look at some neurons under an electron microscope we could actually see the branched nature of these cells

quite clearly so we've been taking a look at various types of specialized cells and also types of tissues that you

find in animals exemplified by human cells and tissues let's take a look at some organs now again we're going to be

revisiting many of these organs in later lessons both in this topic and in subsequent topics but let's just take an

overview here just to see how cells org or rather how tissues are organized into organs so the stomach is an example of

an organ it mostly consists of epithelial and muscle tissues here is the location of the stomach in this

model and the tissues in the stomach wall produce both acid hydrochloric acid and digestive enzymes in particular

protease enzymes such as pepsin and the whole organ has a muscular wall which contracts to churn the food into a

liquid called chyme the heart is another example of an organ it mostly consists of a specialized type of muscle tissue

called cardiac muscle tissue and again the structure of the heart is related to its function the tissues in the heart

all contract rhythmically and the whole organ pumps blood to the lungs and all the cells the tissues and organs of the

body the brain is another example of an organ it's composed mostly of nerve and blood tissues so here we can see on the

left a photograph of a brain and on the right we can see a diagrammatic representation of this so here we see

the cerebrum we can also see other parts of the brain this is the part of the brain called the cerebellum which is

responsible for coordinating movement down here we can see the spinal cord where it meets the brain and at the

bottom of the brain we have a part of the brain referred to as the medulla oblongata so the brain the function of

the brain is to control movement and other body functions now again the structure and function of the various

parts of the brain is somewhat beyond the scope of GCSE biology but it is important to know that there are main

functional areas of the brain so here we can see some of those areas highlighted so at the back of the brain we have an

area of the brain on the surface called the occipital cortex which is responsible for the the assembling the

data the information coming in through the optic nerve from the eyes into our perception of the visual field we also

have areas of the brain involved in hearing and speech such as Broca's area we also have the sensory cortex which is

the part of the brain responsible for interpreting the signals received from sensory organs in the skin for example

and we also have next to that an area of the brain referred to as the motor cortex which is responsible for the

sending of signals to the muscles to cause contraction and movement of the limbs here these two halves of the brain

are referred to as the left and right cerebral hemispheres which make up the cerebrum at the back we have the

cerebellum and at the bottom we have the spinal cord here now here again is the medulla oblongata this is involved in

regulating involuntary activities such as heartbeat and blood pressure for example now this again seems quite

complex but this in fact ism is a is simplification all we're looking at here are some of the externally external

structures within the brain there are various internal structures and even within these areas there are

subdivisions which deal with specific functions of sensation of speech of cognition or thinking and so on so we've

looked at tissues we've looked at organs let's now take a look at the idea of an organ system so once again an organ

system is made up of specific organs and tissues which work together to perform a particular set of specific functions

within that multicellular organism so we have the digestive system we have the muscular or musculoskeletal system we

have the circulatory system we have the nervous system and we have the reproductive system but we also have

other systems such as the respiratory system and the endocrine system now all of these are collections of tissues and

specific organs which have specific jobs to perform within the body so we've taken a look at animals let's now move

on to look at plants so in the exact same type of division of labour exists in multicellular plants these are made

up of thousands of specialised cells grouped into tissues organs and organ systems now there are many different

specialized types of plant cells but we can broadly say that they all contain the following basic structures they

contain a nucleus a cytoplasm a cell membrane mitochondria ribosomes and there are specific types of cells

which as we'll see form photosynthetic tissues which also contain organelles referred to as chloroplasts which are

the site of cellular photosynthesis so this is a diagram of a plant cell here we see the cell membrane we see

mitochondria we see the cytoplasm the nucleus and we also see the ribosomes here now as in animals the overall

structure of plant cells is specialised in two particular types of specialized cells and these

specialized cells allowed these multicellular plants to carry out one or more particular functions which relates

to the characteristic functions of life much more efficiently than they could if you are dealing with single individual

cells so let's take a look at the main types of plant tissues now so again like animals plants have a level of

organization based upon cells being organized into tissues tissues being organized into organs and then organs

being organized into organ systems there are six principle types of plant tissues now again this is a bit of a

simplification but for GCSE purposes this is sufficient so there is epidermis tissue now this is the type of tissue

which typically forms an outer protective layer there are photosynthetic tissues which are

composed of cells which contain chloroplasts for photosynthesis there are also vascular tissues now the term

vascular is used in both animals and plants to refer to in animals for example to refer to the circulatory

system of the vessels of the circulatory system such as arteries veins arterioles venules and capillaries so those are the

vascular system in animals and it is a transport system but also in plants you have a vascular system vascular tissues

which are responsible for the transport of substances and in plants this includes the xylem tissue which

transports water and dissolved minerals from the roots to the parts of the plant that require it in particular the leaves

and the shoots and you also have the phloem tissue which is responsible for transporting the products of

photosynthesis sugars and also other assimilates are the substances to the growing parts of the plant from the

leaves you also have meristem tissue which is the tissue within which rapid growth and

differentiation takes place so meristem tissue is typically found in the growing regions of the plant the tips of the

shoots and the tips of the roots has resaw in that previous lesson in the last topic on cellular differentiation

in plants you also tissue which bulks up the stem and the roots called packing tissue and finally

you have a type of tissue called strengthening tissue in which you have cells with thickened cell walls to

provide strength and support for the plant so that it doesn't fall over as it grows upwards so let's now take a look

at each of these tissues in turn and relate their structures to their functions so the plant equivalent of the

epithelial tissue that we find in animals is referred to as epidermis tissue and again this protects the

structures that lie beneath it in the plant so if we take a section through a leaf such as as being shown here we take

a leaf we take a thin section through it and review it under a microscope we're going to see that the outer region the

outer layer of cells which protect the underlying regions of the tissue of the of the leaf this layer is referred to as

the epidermis so in this micrograph here from a optical microscope we can see the

epidermis of the onion here and you can see that the cells are linked together very similar in a very similar way to

the type of the type of organization that you find in the epithelium of an animal now the roots of plants are

responsible for absorbing water and dissolved mineral ions from the soil so here we can see that the roots have

these this fine covering of root hairs so roots are often covered in thousands of final extensions like this called

root hairs which are modified epidermal cells now again the reason for this structure is to maximize the surface

area of the root so that more efficient absorption of water and dissolved minerals can take place and so for this

reason the epidermis of a root that you can see here on the right has these fine root hair cells cells which have

specialized to produce these long projections and this in turn maximizes the total surface area of the root for

more efficient absorption of water and dissolved ions from the surrounding soil water so let's

take a look now at photosynthetic tissue so the two types of photosynthetic tissues that you typically find in the

leaves are referred to as the palisade and spongy mesophyll tissues so these are the layers within a leaf so again if

we take a section through relief and view it under an optical microscope we would see that we have two layers of

cells here which are photosynthetic so here we see the palisade mesophyll and here we can see the spongy mesophyll

both of these layers of cells contain chloroplasts and large concentration of chloroplasts within them to maximize the

efficiency of photosynthesis to capture the maximum amount of sunlight energy passing through the leaf now in fact the

epidermis layer and the upper and lower epidermis layers and also the guard cells themselves also contain relatively

smaller numbers of chloroplasts but it's really these two layers of photosynthetic tissue where you find the

largest amount of chloroplasts per cell now the leaf is actually an organ it is a collection of tissues which are

specialized to perform a particular function specifically the function of photosynthesis the leaf is the

photosynthesizing organ of the plant and for this reason you have photosynthetic tissue but you also have transport

tissue vascular tissue xylem and phloem the xylem to carry water and the dissolved minerals necessary for these

cells in the leaf to photosynthesize so the xylem tissue delivers that water from the roots to the leaves but you

also have phloem tissue in these vascular bundles which make up the veins of the leaf and the phloem tissue is

responsible for transporting the sugars which are being made by the photosynthetic tissues back to the

growing regions of the plant to the tips of the shoots and the tips of the roots so again you have spongy mesophyll you

have a lower epidermis and you have these guard cells the guard cells are specialized cells which are responsible

for regulating the entry and exit of water vapor and also open and closed to allow carbon dioxide

to diffuse in to the photosynthesizing cells that need it and also oxygen to dissolve out to diffuse outwards so here

we can see the xylem and the phloem tissue within this vascular bundle and here again we see the palisade mesophyll

and the upper epidermis so all of the tissues named in this plant section work towards one goal maximizing the rate of

photosynthesis as the tissues work together to perform one or more definite functions the leaf therefore is referred

to as an organ now I'm not going to fill in these gaps for you if you're watching this video as a recording on my website

the video is about supports and you can attempt this exercise for yourself so see if you can fill in the gaps in this

passage with the correct words from the bottom here so vascular tissue consisting of xylem and phloem in plants

is the main tissue responsible for transport of substances in flowering plants a system of tubes called silent

vessels which in the diagram here are colored blue are responsible for the transport of water and minerals upwards

from the roots where they're being absorbed up the stem to the leaves where that water is used for photosynthesis so

in the xylem tissue the direction of the transport of water is always upwards now the water and carbon dioxide from the

air absorbed through the tiny holes called stomata in between the guard cells on the undersides of the leaves

these are all used in photosynthesis to produce glucose as sugar and oxygen now another system of tubes called phloem

tissue which is colored red here in this diagram is responsible for transporting the sugars made by photosynthesis from

the leaves to the growing tips of the roots but also to the growing tips of the shoots so the phloem tissue actually

transports sugars in both directions upwards and downwards through the stem of the plant to where they're needed

again we're going to be looking at transport systems in plants in more detail in a later lesson of this topic

so here we can see the vascular system made up of the xylem and the phloem tissue so you find this vasculature this

vascular system throughout the plants you find it in the veins of the leaves you find it in the stem everywhere

because again these substances need to be transported throughout the structure of this multicellular plant the meristem

tissue is the are the regions of tissue typically found in the tips of the shoots and also the tips of the roots

plants will grow in height due to the activity of these zones of meristem tissue which have found a few

millimeters behind the chute tips so this in the chute tips this region is referred to as apical meristem apical

comes from the word apex which means the tip of something so if we take a vertical section through a root a chute

tip here we can see the developing leaves and in the bottom we can see the developing bud and at this region here

that the tip of the chute this is where if we look under a microscope we will find the meristem cells these are a

layer of rapidly dividing and differentiating cells which are responsible for the rapid growth and

differentiation of the tip of the chute into these various structures the developing leaves and the developing

buds now it the roots at the other end of the plant also grow in length and this is also due to the activity of the

zones of meristem tissue which are again found a few millimeters behind the tips of the roots now once again this is also

referred to as apical meristem because it's found behind the tips of the roots so this is a diagram of a section taken

through a root here we can see the phloem tissue we can also see the xylem tissue here but we can also see around

the root we see these root hairs which emerge from the epidermis of the root in this region here we have the meristem

this is the rapid this is the region where the cells are undergoing rapid division by

mitosis and differentiation to form these various tissues as the root grows downwards into the soil now to protect

this very delicate region of meristem tissue we have a root cap which protects that region of cells as the root pushes

downwards through the soil and again if we look at this region under a microscope we can identify within the

nuclei of these cells the stages of mitosis occurring here packing tissue is found in various parts of the plant good

examples of this are the cortex and pith of the stem the the purpose of this is essentially to provide rigidity and also

to divide the various types of tissue found in various parts of the plant so here we can see a diagrammatic

representation of this in the center of this section of stem we have the central pith packing tissue and if we zoom in on

this we can see the cells they're very nondescript cells they're essentially there to pack this central region so

packing tissue can be compressed and will recoil upon release of pressure and so this gives the stem office of a plant

the ability to bend in the wind without breaking so once again I'm not going to spend time filling this in for you the

video will pause and the question will pop up to allow you to attempt this activity for yourself so have a read of

this passage and see if you can drag the appropriate words to fill in the spaces in the passage for yourself now the

final type of our plant tissue that we're going to look at is strengthening tissue strengthening tissue which is

composed of cells with thickened cell walls is a good example of a specialized plant tissue and this is typically found

on the outer rim of the vascular bundles of xylem and phloem tissue so here we can see a vascular bundle and on the

outer rim of that vascular bundle we can see these strengthen cells with thickened cell walls that make up the

packing to shoot here we can see the phloem here we can see the cambium or meristem tissue

and here we can see the xylem tissue okay everyone that's going to do it for this lesson I hope that was useful to

you I'm now going to go into the Q&A discussion with the students that I have in attendance with me thanks once again

for watching and I'll see you in the next one take care