life processes at the cellular level · level 2 biology | life processes at the cellular level . 3...

NCEA | Walkthrough GuideLevel 2BIOLOGY

TT TtTt

Tt

Tt

TT

t

t

tt

LIFE PROCESSES AT THE CELLULAR LEVEL

Introduction 3

Cells 4Organelles 5Cell Membrane 6Cytoplasm 8Nucleus 8Mitochondria 9Chloroplasts 10Cell Wall 12

Enzyme Activity 13Enzymes: Biological Catalysts 13How Enzymes Work 14Factors Affecting Enzyme Activity 15Temperature 17pH 19Inhibitors 19

Movement of Materials 21Passive Transport 21Active Transport 23

Photosynthesis 25Defining Photosynthesis 26Photosynthesis Equation 27Types of Reactions 27Photosynthesis Reaction Summary 28The Chloroplast: Maximising Photosynthesis 28

Respiration 30Defining Aerobic Respiration 30Aerobic Respiration Equation 31Aerobic Respiration Summary 32Anaerobic Respiration 32The Mitochondrion: Maximising Aerobic Respiration 33

Cell Division 34Cell Cycle 35DNA Replication 35Mitosis 37

Factors Affecting Life Processes 38Similarities and Differences between Cells 39

Key Terms 40

Level 2 Biology | Life Processes at the Cellular Level

33

This standard is all about the processes occurring within the cell.

Cells are the teensy tiny building blocks of all life – so, they’re pretty important to study if you are a biologist! All the millions of different animals, plants and bacteria most probably originated from a humble cell roaming the desolate Earth.

Every cell in your body is a busy little factory, working away to be a skin cell, a heart cell, an eye cell - or whatever else it is assigned to be. The processes these cells carry out are what keeps us and all other living things alive – that’s why we call them life processes!

What will you learn in this walkthrough guide?

In this guide, we’re going to go on a bit of a journey. We’ll introduce you to all of the things needed for a cell to carry out an important life process, then get into exactly what each process involves.

We’ll kick off our journey of cellular exploration inside these little cells. In order to get the job done they use pieces of machinery, called organelles, which are like mini organs inside the cell (just like how the body has organs doing different jobs). We’ll have a look at the most important organelles, including the control centre (the cell nucleus), the powerhouse of the cell (the mitochondria) and the border wall (cell membrane).

Once we’ve got the machinery down, we’ll talk about important proteins called ‘enzymes’ which are just as important in making sure these processes take place.

Once we’ve built our infrastructure, we’ll tackle the other ‘ingredients’ we need in our processes - and talk about the transport systems used to bring them in and out of the cell.

Finally, we’ll zoom out of the cell and have a look at a handful of very important life processes:

1 Photosynthesis2 Respiration 3 DNA replication and cell division

For each one we’ll have a look at how it occurs and why it’s important, rounding off our journey by discussing how the rate of each process is effected by the enzymes, transport and cellular conditions themselves.

INTRODUCTION

Level 2: Life Processes at the Cellular Level Cram Guide| © Inspiration Education Limited 2017. All rights reserved.

A word on exam strategy.

In Level 2 Biology there are a lot of different concepts being covered. It is important to be able to link all of these processes together. You can’t talk about respiration without thinking about the enzymes that make it possible or the organelles that carry it out. Similarly, you can’t think about what affects the rate of respiration without considering the factors that affect enzymes or the structural features of the mitochondria. As we go through this guide we’ll help you make these important links.

Here at StudyTime, we’re pretty much GCs (good citizens), so to help you out, we’ve made this guide in plain English as much as we can. We’ve also included a glossary for some of the key terms that you’ll need to master for your exam.

If learning key words first off scares you (or bores you), then focus on understanding the concepts the first time around, and then memorise the definitions.

In fact, in this guide, we focus on helping you to understand the concepts first. We use examples and analogies to help you understand Biology in a way that is fun, and makes sense in the real world.

However, the language we use isn’t always something you can directly write in your exam. When this is the case, we offer a more scientific definition or explanation (in a handy blue box) underneath. These boxes are trickier to understand on your first read through, but contain language you are allowed to write in your exam. Look out for them to make sure you stay on target!

CELLSIt’s in the title of the standard - so I guess we should probably explain to you what it means!

There are a whole bunch of different cliches thrown around when it comes to cells. If your teacher is poetic, you may have even heard them referred to as the ‘building blocks of life’. But what does this really mean?

We define cells as the smallest unit in a living organism. This means that every piece of a living organism (whether it is an eye, an arm, or a muscle) is made up of hundreds of cells. Cells are very small, and have a number of different functions, depending on whereabouts they are in the body. Every second that you are alive, your cells are carrying out thousands of processes which give you the ability to move, grow, and perform all sorts of subconscious tasks.

4 Level 2: Life Processes at the Cellular Level| © Inspiration Education Limited 2017. All rights reserved.


5

A cell is the smallest structural unit of an organism.

In order to carry out each of these tasks, cells have all sorts of structures inside of them that help them carry out their duties - which we’ll cover in the next section. For now, think of cells as the tiny workers all throughout the body which carry out the life processes living things need to survive. Conveniently, it’s these very life processes we will be discussing throughout this external!

In this section, we’ll be learning more about what cells do, by peeling back their membranes and learning more about the structures inside them that help carry out their jobs! To sum it up, we’ll be looking into:

What organelles are, and how their structure helps them carry out the responsibilities of the cell.Some of our most important organelles, including:

• The cell membrane• The cytoplasm• Chloroplasts• The cell wall

Organelles

If we consider that cells are the building blocks of life, it makes sense that they are pretty darn complicated. They have heaps of different structures, each with their own special roles which work together to run a more complex organism.

Like the different settings and attachments you can fit onto your Nutribullet, the cell uses these structures at different times in order to perform specific tasks when the organism requires it.

Each of these structures inside a cell are called ‘organelles’. If we zoom out a bit, we can work out exactly where this name comes from. In fact, ‘organelle’ simply means ‘small organ’. In the same way that your body needs a heart, lungs and other organs to fulfill its tasks, the cell needs organelles to carry out its everyday processes.

As you move further through Biology, you will be exposed to all sorts of weird and wonderful pieces of the cell - and slowly build your knowledge of how each organelle makes the cell tick.

The good news is, for this external, you only need to know the important ones. We’ll

5 Level 2: Life Processes at the Cellular Level | © Inspiration Education Limited 2017. All rights reserved.


guide you through the key players in the cell - and how they contribute to making a cell, and when working together, an entire organism do its job.

An organelle is a specialised structure within a cell, which aids the cell in completing a specific life process.

Cell Membrane

If you were creating a farm, what is the first thing you would build?

You’d need something to make sure the important animals didn’t escape - and that dangerous ones didn’t get in.

For this very reason, we are going to start our journey of the cell by introducing you to the cell membrane. The cell membrane is simply a special ‘fence’ which surrounds the very outside of the cell.

The cell membrane surrounds the entire cell and decides what can get in or out of the cell

The fact that the membrane can decide what can get in and out of the cell is super important - as the cell needs important materials to do its job - but must be kept safe from harmful bacteria and toxins. Therefore, it can’t just let anything roam in and out as it pleases.

The cell membrane is responsible for preventing dangerous substances from entering a cell, as well as maintaining the structure and contents of a healthy cell.

We’ll talk about how the cell membrane manages to fulfill its duties as ‘bouncer’ of the cell later on, but for now, let’s get a little more comfortable with its structure.

Cell membranes are phospholipid bilayers

The name ‘phospholipid bilayer’ sounds complicated, but it makes a little more sense as we break it down.



7

‘Lipid’ is just the scientific name for ‘fats’ and ‘phospho’ is our fancy way of referring to a phosphate molecule.

So, a ‘phospholipid’ is simply a phosphate molecule attached to a pair of fatty acids:

Head

Tail

phosphate head

fatty acid chain

If we take a whole bunch of these phospholipids, and line them up in two lines, we can see what a cell membrane would look like under a microscope.

In fact the two rows is what makes it a ‘bilayer’.

extracellular fluid

intracellular fluid

phospholipidphosphate “head”

lipid “tail”

cholesterol

You don’t need to understand too much about the chemistry behind phosphates and lipids for this standard. You just need to know that for this fence, we can’t buy our materials from Mitre 10 - and that the specific structure of the membrane is what helps it carry out its job.

STOP AND CHECK:

Turn your book over and see if you can remember:

The basic structure of the cell membrane – what are phospholipids? The function of the cell membrane.

Try to explain it in your own words.

7 Level 2: Life Processes at the Cellular Level | © Inspiration Education Limited 2017. All rights reserved.


Cytoplasm

Now that we’ve constructed our fence, let’s see what we can fill it with. A regular farm would use grass, but we have a special material called ‘cytoplasm’.

The cytoplasm is the jelly-like stuff that fills all cells

Think of the cytoplasm as a jelly that allows all of our other organelles to float around and do their jobs. Like a paddock, the cytoplasm is full of nutrients for the organelles, and provides the environment for all of the processes we will learn about soon to take place.

The cytoplasm is responsible for suspending the organelles of the cell, as well as providing the necessary nutrients to the cell.

STOP AND CHECK:


What the cytoplasm is. The purpose of the cytoplasm.


Nucleus

Now that we’ve got our cell all fenced off and with the right base inside of it, it’s time to fill it with some pretty important pieces.

The first organelle we’ll learn about is the nucleus.

The nucleus is the ‘control centre’ of the cell

It contains the DNA, and therefore all of the instructions which tell the cell how to do its job. Because the DNA is so important, it is safely stowed away in the nucleus, with a special membrane, called the ‘double nuclear membrane’ separating it from the rest of the cell.

The nucleus is a double-membraned organelle, which protects the DNA within the cell.



9

ChromosomeCell

Nucleus of cell

STOP AND CHECK:


What is contained inside the nucleus.


Mitochondria

Now that we’ve got the instructions, we’re going to need somewhere for the cell to create the energy to put them to use.

Mitochondria are known as the ‘powerhouses’ of the cell

They are the factories that turn the sugar from our food into energy so that the cell can go about its business. This process is called respiration. Respiration is a major part of this topic and we will be talking about it a great deal later on. So, it is really important to remember the mitochondria!

Ribosomes

MatrixCristae

Inner MitochondriaMembrane

Space betweenInner and OuterMembranes

Outer MitochondrialMembranes

Mitochondria have two membranes

The outer one is nice and smooth and the inner one is wiggly. The wiggles are called cristae.

What’s the point of a wiggly membrane? Well, if we think about how important energy is to a cell, we know that we want to be able to make as much of it as possible. The wiggles increase the surface area of the inner membrane, which is where lots of reactions happen. This means that the mitochondria can convert more sugar into energy



in the same amount of time – and therefore is more efficient.

The space between the inner and outer membrane is called the intermembrane space and the liquid-filled space inside the inner membrane is called the matrix. These words are important to be familiar with - but you don’t need to be able to discuss them in depth. The most important thing to remember is that mitochondria are the cell’s generators for making energy efficiently to use in their other processes.

Mitochondria are where cellular respiration takes place. They have inner and outer membranes which fulfill different roles in respiration.

Different types of cells have different numbers of mitochondria, depending on how much energy they need

For example, a muscle cell - which is constantly having to live up to expectations of creating lots of energy, will have heeeeeaps of mitochondria, whilst a skin cell will have much less.

STOP AND CHECK:


Why the mitochondria are sometimes called the “powerhouses of the cell”. The structure of each of the mitochondrion’s two membranes. What is the function of the cristae. Why some cells have different numbers of mitochondria.


Chloroplasts

We’ll be talking about chloroplasts just as much as we’ll be talking about mitochondria, because…

…chloroplasts are where photosynthesis happens

Photosynthesis is the process of using light energy (from the sun) to make sugar out of carbon dioxide and water. But don’t worry - we’ll get into this more later on!

Like mitochondria, chloroplasts have two membranes (inner and outer).

Boringly, neither of these two membranes are interestingly wiggly like the inner membrane of the mitochondria.



11

Chloroplasts are where photosynthesis takes place. They have inner and outer membranes which fulfill different roles in photosynthesis.

Inside the inner membrane of chloroplasts is a liquid-filled space called the stroma. Some of the photosynthesis reactions take place here.

Within the stroma are important stacks of sacs

The sacs are called thylakoids and, together, the stacks of thylakoids are called grana.

Each thylakoid is covered with pigment called chlorophyll.

The chlorophyll is the stuff that traps the sunlight so that it can be used in photosynthesis.

Outer membrane

Inner membraneStroma

Thylakoid

The important thing to remember here is that chloroplasts use a special pigment called chlorophyll to absorb sunlight to turn into energy. In order to be as efficient as possible, they stack up their bundles of this pigment so that they can absorb as much sunlight as possible.

Not all organisms have chloroplasts

For example, we don’t! This is because we don’t carry out photosynthesis - so they would be a waste of space in our cells!

It’s mostly just plants who have chloroplasts, and, interestingly, the more photosynthesis a plant carries out, the more chloroplasts it tends to have!

STOP AND CHECK:


How the chloroplast’s two membranes compare to the two membranes of each mitochondrion.

What is carried out by the chloroplast. What is found inside the chloroplast.



Cell Wall

I know we already constructed our fence, but sometimes we need a little more support. Because plants often have to stand up nice and tall - and aren’t as flexible as other organisms, they need a second wall which gives them a rigid structure.

The cell wall is found around plant cells, but not around animal cells.

In plant cells, the cell wall is a rigid structure that wraps around the outside of the cell membrane.

It has no control over what goes in and out of the cell – that’s the cell membrane’s job. Its function is to give the cell a rigid shape and structure.

Cell wall

ChloroplastCytoplasm

Nucleus

Cell membrane

Animal cell Plant cell

STOP AND CHECK:


What kinds of cells have cell walls – do all cells have a cell wall? How the cell wall differs in function to the cell membrane.


Quick Questions

What are the parts and organelles found in a cell?• What is the structure of each one?• What is the purpose of each one?

Discuss the similarities and differences between the animal and plant cell. What do the differences tell us about the functions of these cells, and the processes they carry out?


?


13

ENZYME ACTIVITYNow that we’ve covered the main ingredients that make up a cell, we should be ready to do things with them.

Not so fast. There’s one thing remaining that is necessary for every life process to occur. We call them enzymes.

The rule of thumb is that, if enzymes aren’t working, nothing else is.

Let’s take a look about what you need to know (and hopefully appreciate) about these guys:

What exactly are enzymes?How do enzymes do what they do bestWhy enzymes are ‘fussy’ - and how we can help them do their job as efficiently as possible.

Enzymes: Biological Catalysts

Let’s start by drawing your mind back to level one science.

You may have heard of the word ‘catalyst’?

A catalyst is something that makes a reaction happen without getting used up itself. They are very important, as they allow reactions to happen that would take far too long naturally.

Enzymes are just like this, except they exist inside living things. They are present in organisms, and act by helping reactions to occur without actually being used up in the process.

An enzyme is a protein that allows a biological reaction to occur, without being used up in the reaction itself.

The active site of enzymes is really, really important

In a reaction, reactants (known as substrates) bind to an important location on the enzyme - which we call the active site – and the reaction is catalysed to produce the products.


Enzyme

Substrate

Active site

Enzyme-substrate complex

Enzymes are specific, meaning that they will only catalyse the particular reaction they are made for, because nothing else will fit into its active site.

Turns out, enzymes aren’t living things.

Enzymes are made up entirely of protein and so are therefore not alive (and since they were never alive they aren’t dead). They’re lifeless beings floating in the cell, with everything they do down to pure chemistry. Think of them as non-living bundles of protein which facilitate everything our body needs to do.

STOP AND CHECK:


The effect that enzymes have on a biological reaction. Why enzymes aren’t considered living (or dead). The definition of enzymes.


How Enzymes WorkRemember from the previous section: substrates bind to the active site of enzymes, and enzymes are specific for certain substrates because those substrates fit into the active site.

The old-school model of how enzymes work is called the “Lock and Key” model

Unsurprisingly, it suggests that enzymes work like a lock and key.

The active site of the enzyme is like the lock, while the substrate it like the key.

The substrate fits into the active site because their physical shapes fit together nicely.



15

Enzyme Enzyme

SubstrateProducts

Enzyme - SubstratecomplexGood Substrate

Bad Substrate

Turns out that an identical match is a bit too much to ask – it’s a biologist’s fantasy.

Instead, enzymes work by an “Induced Fit” model

In this model, both the active site and the substrate change shape ever so slightly to get that nice cosy fit.

You could think of it as smashing two jigsaw pieces together so that they end up fitting, except the shape change isn’t so dramatic with enzymes.

Enzyme Enzyme - substrate complex Enzyme

ProductSubstrates

* * * ** * * *

STOP AND CHECK:


The function of the active site of an enzyme. How the “Lock and Key” model explains how enzymes work. How the “Induced Fit” model explains how enzymes work.


Factors Affecting Enzyme ActivitySo, what’s on an enzyme’s “hot and not” list?

Well, to sum it up, enzymes have an optimum temperature and an optimum pH in which they work

Any changes there means they’ll get lazy and reduce their activity, or they might stop working all together.



The optimum temperature and pH of an ezyme define the conditions that the enzyme works most efficiently at. Different enzymes often have different optimum conditions.

As well as this, the enzyme activity depends on a number of factors.

These include the concentration of enzymes and/or substrates and whether there is any heavy metals present.

Heavy metals are like an enzyme’s kryptonite and only have a negative effect.

When talking about the factors that affect enzymes we like to talk about enzyme activity

This activity basically just means their ability to catalyse the biological reaction: the more activity, the faster the reactions occur.

Enzyme or Substrate Concentration

Substrate concentration

Rat

e of

rea

ctio

n

active sites not all occupied

saturation of active sites

If you remember back to Level 1 Science: Acids and Bases, there was something about concentration affecting reaction rates:

This had to do with the idea that, as concentration increases, there are more chances that reactants (and the necessary enzymes) will run into each other.

More concentration means more crowding in the cell. This means that, as the particles cruise around, there’s less free space - and it becomes more likely that they’ll crash and collide with another particle. This causes more reactions to occur, and therefore a faster reaction rate.

In contrast, as the concentration decreases, the cell becomes less crowded. With more room to move, particles are less likely to crash and collide into one another. Less successful reactions will occur and therefore the reaction rate will decrease.

This applies to enzymes and their substrates as well.



17

It’s important to realise that increasing substrate concentration only increases enzyme activity to a certain point before stopping

This is because a single enzyme can only deal with one reaction at a time. If there’s too many substrates present, there’s no available enzymes to deal with them. We call this special situation, where there are too many reactants for the enzymes to deal with, the point of saturation.

At this point, the only way to increase enzyme activity is to increase the number of enzymes able to work through the reactants.

STOP AND CHECK:


The effect of substrate and/or enzyme concentration on enzyme activity. What is meant by “enzyme saturation” – why is there a limit to the increase in

enzyme activity with increasing substrate concentration.


Temperature

Temperature / °C20 40 60

Rat

e of

rea

ctio

n

We’ve already established that enzymes are the Goldilocks of the cellular world: they don’t like when it’s too cold or too hot; they like the temperature to be just right.

This optimum temperature is different for different enzymes

Most enzymes in the human body have an optimum temperature around 37°C, which is the normal body temperature.

Again, the effect of temperature can be explained using Level 1 Science:

More heat energy available means that the particles are able to move at faster speeds.



The faster they’re moving, the sooner they’ll meet another reactant, and the more likely a collision will occur.

In contrast, if the temperature is decreased, the particles will have less energy to move around with. Rather than zooming around in various directions, they enjoy their peaceful walk around the block. Eventually they’ll collide with another particle, but, because it takes a lot longer, the reaction rate drops when the temperature decreases.

From this explanation, we can see that, the hotter things get, the more likely our reactants are to meet each other (and the necessary catalyst), and the more reactions will occur.

Unfortunately, this only applies up to the optimum temperature.

Beyond the optimum temperature things become somewhat strange

Rather than the enzyme activity increasing or decreasing, when the temperature becomes higher than the optimum temperature, enzyme activity completely stops.

It’s as if the enzymes just disappeared!

The enzymes have actually denatured

An enzyme denatures when its active site loses its shape – think of the temperature melting the active site (though, technically, the high temperature breaks the hydrogen bonds holding the shape together).

Denaturing refers to what happens when an enzyme becomes too hot. The bonds holding the enzyme together begin to break down, which means the enzyme can no longer hold its structure.

Remember, the shape of the active site is super duper important for its role as a biological catalyst.

As soon as the active site loses its specific shape, it can no longer fit the specific substrate and catalyse the biological reaction.

STOP AND CHECK:


How temperature affects the enzyme activity – at what temperature do enzymes work best?

Why the enzyme activity completely stops shortly after the optimum temperature. What denaturation (enzymes denaturing) means.




19

pH

pH7 14

Rat

e of

rea

ctio

n

1

Thankfully we can keep this section short and sweet! Essentially, enzymes have a favourite pH where they will work their best – a happy worker is a good worker. Above or below the optimum and the enzyme activity decreases.

The optimum pH depends on where the enzyme is.

For example, amylase in the saliva works best at a neutral pH, while many of the stomach enzymes work best in acidic conditions.

STOP AND CHECK:


The effect that pH has on enzyme activity. What determines an enzyme’s optimum pH.


Inhibitors

Heavy metals, such as mercury and lead, are bullies of the cellular world. They are a type of enzyme inhibitor that prevents enzymes from carrying out their job.

Enzyme inhibitors can compete with the substrates for the active site of the enzyme

Once they get there they simply block the active site and stop the substrate and enzyme from getting together.

These types of inhibitors are called competitive inhibitors. But don’t worry, it’s not compulsory to call them ‘competitive’ as long as you know how they work.



Enzyme

Substrate Competitiveinhibitor

Enzyme

Other inhibitors bind to the enzyme and alter the shape

Often this shape change affects the shape of the active site. We already know how important the active site shape is, and therefore these inhibitors prevent the enzyme from functioning.

These types of inhibitors are called non-competitive inhibitors. But don’t worry, it’s not compulsory to call them ‘non-competitive’ as long as you know how they work.

Enzyme

Substrate

Noncompetitiveinhibitor

These inhibitors may have reversible or irreversible effects.

STOP AND CHECK:


The main ways in which enzyme inhibitors may stop enzyme activity.


Quick Questions What are enzymes and what are they doing to keep themselves busy? How do enzymes do what they do? Compare both models of enzyme functioning. Discuss the different ways in which enzyme activity may be altered.


?


21

MOVEMENT OF MATERIALSLet’s review where we’re up to in our journey. We’ve learnt about the pieces of the cell that help it to do its job, and we’ve discussed the enzymes responsible for making the reactions happen.

Surely we have everything we need now? Not quite.

Remember how we said that the cell membrane is responsible for monitoring what moves in and out of the cell? For important life processes such as making energy, we need special substances and ingredients from the environment and our food to make things happen.

For example, in photosynthesis, carbon dioxide and water need to get into the cell and oxygen (the waste product) needs to get out.

It turns out cells have a couple of different ways of allowing these additional compounds in and out.

By the end of this section you should be able to:

Discuss the different types of passive transport that occur in a cell: diffusion, osmosis and facilitated diffusion. Think about the similarities between all of these processes and their subtle differences. Discuss how cells achieve active transport of molecules, including an explanation of exocytosis and endocytosis. Explain why passive transport does not require energy but active transport does.

Passive Transport

It turns out there are actually two different ways of transporting materials in and out of the cell. Materials can either move on their own, without any energy from the cell - or they can use the cell’s energy to bring in material that can’t move on its own.

Transport which doesn’t require any use of energy is called passive transport, whilst transport which uses energy is called active transport.

There are three main types of passive transport:

Diffusion involves a substance moving from an area of high concentration to an area of low concentration


Think about someone spraying a can of Lynx in the changing rooms after PE. When they spray it, there is a high concentration of the smell where it was sprayed. However, over time, the smell moves through the room to the areas where it wasn’t sprayed. Eventually the whole room smells like boys.

The difference between the areas with a high concentration of the particles and the areas that the particles haven’t reached yet is called the ‘concentration gradient’.

We say that the substance moves down its concentration gradient (from high concentration to low concentration) until the concentration is the same everywhere.

This idea isn’t limited to gym changing rooms. If the substance is small enough, it may be able to diffuse across the cell membrane. By the laws of diffusion, this movement happens when the inside of the cell has a lower concentration of particles compared to the outside.

Diffusion involves particles moving from an area where they are in high concentration to an area where they are in low concentration, without the expenditure of energy.

However, way back when we talked about cell membranes, we said they had a pretty important structure. This meant that only some particles were able to diffuse through them.

We say cell membranes are ‘semi-permeable’. This simply means that only some particles, are able to pass through, or ‘permeate’ them.

semipermeable membrane

time

Osmosis is simply diffusion involving water, and the solutes dissolved in it

A solute is a substance that is dissolved in water. In osmosis, water moves from an area of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration). This can easily happen across cell membranes.

Low High EquallsedConcentrationConcentration

OsmosisH

2O

H2O



23

Facilitated diffusion helps move substances that can’t diffuse on their own

Sometimes a substance may be too big or charged to simply slip through the gaps in the cell membrane, so the cell may insert special channels into its membrane to let them through. This is still a type of passive transport because it doesn’t take any energy and involves particles moving from high concentration to low concentration (along that concentration gradient).

Facilitated diffusion involves the use of specific channels and proteins to enable proteins to move from an area where they are in high concentration to an area where they are in low concentration, without the expenditure of energy.

Outside of cell

highconcentration

lowconcentrationInside of cell

STOP AND CHECK:


The definition of diffusion. How osmosis differs from diffusion. What is similar between these two processes. The difference between diffusion and facilitated diffusion.

Try to explain it in your words.

Active Transport

Now that we’ve got the easy stuff out of the way, it’s time to learn what happens when our substances simply can’t be moved through passive transport. The cell actually has three different ways of actively moving substances in and out of the cell when passive transport simply isn’t going to cut it.

Special pumps can use energy to move substances against the concentration gradient

Moving a substance from an area of low concentration to an area of high concentration



(against its concentration gradient) takes energy (in a special form called ATP), because the substance is being forced to go ‘the wrong way’. If a cell needs to do this, it puts active pumps into its cell membrane.

These pumps work to move substances into an area where they are already highly concentrated.

Solute

Transport protein

high concentrationEnergy

low concentration

Endocytosis and exocytosis involve a cell ‘engulfing’ or ‘spitting out’ substances.

Some cells can ‘eat’ large particles by stretching their cell membranes around the big molecule and joining the ends of the membrane to engulf the molecule. Endocytosis takes energy (ATP).

A cell can also ‘spit out’ large particles by creating special sacs around the substances called ‘vesicles’ and combining them with the cell membrane. This is the opposite of endocytosis and it takes energy (ATP).

(a) Endocytosis

Extracellularenvironment

Cytoplasm

(b) Exocytosis



25

STOP AND CHECK:


Why active transport requires energy in the form of ATP. Hint: think about concentration gradients.

How cells achieve active transport. What endocytosis and exocytosis are. What are their similarities and differences?


Quick Questions When red blood cells are placed in a solution containing a higher solute

concentration (a lower water concentration) they shrink, but when red blood cells are placed in a solution containing a lower solute concentration (a higher water concentration) they expand and eventually burst. Explain these observations using your understanding of osmosis.

Discuss the differences between active and passive transport, giving examples of each type of transport. Why does active transport require energy to occur, while passive transport does not?

PhotosynthesisWe’re finally ready to roll! We know what’s in our cell, the enzymes we need to make our reactions happen, and we know how to transport everything else in and out of them.

So, let’s get into the actual life processes needed to make living things work - starting with photosynthesis!

With a name like photosynthesis you’d think there’s something about taking photos, or maybe creating light. Well, it’s none of these! We’ll leave the juicy details out of this introduction, but it’s basically the way that plants get food from the sun. Imagine if humans could carry out photosynthesis – endless amount of food for free!! Here’s what we’ll be covering:

How to define ‘Photosynthesis.’Next, we’ll bring back any traumatic memories from chemistry and introduce you to the chemical equations of photosynthesis. These make it easy to see what the starting materials and products of photosynthesis are. Finally, we’ll see how plants get their money’s worth of light for photosynthesis. We’ll see how chloroplasts are well-adapted for light absorption, and how the cells inside the leaf work together to maximise the rate of photosynthesis.


?


Defining Photosynthesis

The word photosynthesis is a bit daunting for anyone who encounters it. As with a lot of scientific words, it’s not so bad once you break it down.

Photo: This is a word for light. Think of the word ‘photograph’. Photography uses light to take pictures.

Synthesis: This means ‘making’.

Photosynthesis is making something using light.

Annoyingly, the name doesn’t give us a clue about what it is we’re making. In fact, the product of photosynthesis is a type of sugar called glucose.

Just like us, plants need sugar to be able to have the energy to carry out their life processes so that they can survive.

We get our food by eating it. Plants get their food by making it themselves using photosynthesis.

Here’s a concise definition for you:

Photosynthesis is the process of using light energy to convert water and carbon dioxide into glucose for the plant to use as fuel to carry out its life processes.

Oxygen is also produced as a waste product.

This occurs in the chloroplasts of plant cells. Remember, chlorophyll needs to be present to trap the sunlight for photosynthesis to occur. Chlorophyll is found inside chloroplasts which is why photosynthesis occurs here.

STOP AND CHECK:


Where photosynthesis occurs inside the plant cell. What is needed for photosynthesis – in other words, what are the starting

materials, or the reactants? What the cell gets out of photosynthesis – in other words, what are the end

materials, or the products? The definition of photosynthesis.




27

Photosynthesis EquationWhenever you need to talk about photosynthesis in an exam question, write down that definition before you do anything else! Next, it always gets you some marks to add in an equation. This isn’t too tricky, as the equation says exactly the same thing as the definition.

Word equation: Water + Carbon Dioxide + Light Energy Glucose + Oxygen

Symbol equation: H2O + CO2 + Light Energy C6H12O6 + O2

Balanced symbol equation: 6H2O + 6CO2 + Light Energy C6H12O6 + 6O2

There’s no need to bother with the symbol equation or the balanced symbol equation if you don’t want to. The word equation gets just as much points. But if you like your Chemistry, it might help you to remember what’s going on.

STOP AND CHECK:


The word equation for photosynthesis. The balanced symbol equation for photosynthesis.

Types of Reactions

There are two main stages in photosynthesis:

1. The Light-Dependent Phase 2. The Light-Independent Phase.

In the Light-Dependent Phase, light is required.

In this stage, sunlight hits the leaves of the plant, which are packed with chloroplasts. The chlorophyll (which we introduced right at the beginning as covering our thylakoid sacs) then traps the sunlight.

The light energy is then used to split water into hydrogen and oxygen.

Now that we have our hydrogen and oxygen kindly gifted to us from the work of sunlight and water, a special cycle, called the Calvin cycle combines these ingredients with Carbon Dioxide to create glucose.

The creation of glucose is called the light-independent phase.



Energy

Food

Oxygen (O2)

Released

Carbon Dioxide (CO

2)

Water from soil

STOP AND CHECK:


Why the light-dependent reactions require sunlight – what time of day would these reactions occur?

The result of the light-dependent phase of photosynthesis. The result of the light-independent phase.


Photosynthesis Reaction SummaryThat was a lot of information to throw at you in one go, so make sure you go through it several times and figure out ways to learn it. To help you, here is a summary table of the vital points:

PhotosynthesisPurpose: To produce glucose as a fuel to enable the plant to

carry out its life processes in order to survive.Where it happens: In the chloroplasts.Reactants (things needed):

Water, carbon dioxide and light energy (usually sunlight).

Products (things produced):

Glucose.Oxygen (a waste product).

The Chloroplast: Maximising Photosynthesis

Chloroplasts have two membranes (inner and outer). Inside the inner membrane of chloroplasts is a liquid-filled space called the stroma. Some of the photosynthesis reactions take place here. Within the stroma are stacks of thylakoids are called grana.



29

The thylakoids are covered with pigment called chlorophyll. The chlorophyll is the stuff that traps the sunlight so that it can be used in photosynthesis.

Evolution is nature’s greatest designer, with natural selection leading to the survival of individuals with traits most advantageous to their environment.

Even the teensy-tiny structures inside the chloroplast are moulded by evolution

As a result, the structures inside the chloroplast maximise the efficiency of photosynthesis and lead to better chances of survival for the plant.

The most important features are the clear appearance of the stroma and the thylakoids being stacked upon each other:

By being clear, the stroma allows the maximum amount of light to reach the chlorophyll in the thylakoid sacs

If the liquid was dark and murky, much of the light would be lost. More light = more energy = more photosynthetic reactions can take place.

Stacking up the thylakoid sacs increases the surface area, leading to more chlorophyll being available to absorb light

More light = more energy = more photosynthetic reactions can take place.

STOP AND CHECK:


The internal structure of the chloroplast. The features of the chloroplast which maximise the rate of photosynthesis.


Quick Questions What is photosynthesis, and why is it used by plants? Include either a word or

chemical equation to show the reactants and products. How is the process of photosynthesis carried out in both the light-dependent and

light-independent phases?


?


RESPIRATIONSo far, we’ve seen how plants get their fuel (and we know that animals get their fuel simply by eating), now it’s time to see how they use this fuel to get energy. If you feel in need of a study snack, now might be a good time. Consuming some sugar will be very relevant to what you’re about to learn. Tell your family it’s in the name of science!

In this section get ready for:

Another sciency-sounding definition for a sciency-sounding term: “aerobic respiration”.

Chemistry coming back again to bring us a chemical equation for aerobic respiration. But, if you made sure you learnt the equation for photosynthesis you’ll already know the aerobic respiration equation! It’s just the reverse of the photosynthesis equation.

The evil sibling of aerobic respiration: anaerobic respiration. What does it do? When does it occur? Why’s it so evil?

How do the mitochondria get the most out of the food you eat?

Defining Aerobic Respiration

What we need to know now is how organisms use their fuel to get the energy they need to carry out their life processes and survive. When we put petrol in a car, it gets burned to produce the energy to make the car move. ‘Burning’ something is simply reacting it with oxygen. Surprisingly, we, and all other organisms, do a very similar thing with our fuel.

We take the glucose we’ve eaten or photosynthesised and we react it with oxygen

As a result we make ATP, the form of energy that is used by the cells that make up our bodies.

This is the process of respiration.

Now, one of the most common mistakes students make is confusing this form of respiration with the act of breathing. From now on, as level 2 Biologists, you must never think of respiration as breathing.

Instead, respiration is the creation of energy - and when it requires oxygen, we must call it ‘aerobic respiration’.


31

If you took our advice and grabbed a sugary snack, then your cells will be working away to make ATP. We’ll bet that a lot of that ATP is going into keeping your brain cells whirring away!

Again, here’s a nice definition for you:

Aerobic respiration is the process of reacting glucose with oxygen to produce ATP as energy for the cell, to enable it to carry out its life processes.

Carbon dioxide and water are waste products.

Aerobic respiration occurs in the mitochondria of cells.

As with photosynthesis, whenever you need to talk about respiration in an exam question, write down the definition above before you do anything else!

STOP AND CHECK:


What is needed for aerobic respiration – in other words, what are the starting materials, or the reactants?

What the cell gets out of aerobic respiration – in other words, what are the end materials, or the products?

The definition of aerobic respiration.


Aerobic Respiration Equation

Adding in an equation always gets you marks too.

Word equation: Glucose + Oxygen ATP + Water + Carbon DioxideSymbol equation: C6H12O6 + O2 ATP + H2O + CO2

Balanced symbol equation: C6H12O6 + 6O2 ATP + 6H2O + 6CO2

Notice anything funny? If you compare the respiration equation with the photosynthesis equation, you’ll see that they are pretty much identical, just the other way around. This can make them easier to remember, but it can also make them easy to muddle up, so make sure you learn which is which!

Photosynthesis: Water + Carbon Dioxide + Light Energy → Glucose + OxygenRespiration: Glucose + Oxygen → ATP + Water + Carbon Dioxide



Again, there’s no need to bother with the symbol equation or the balanced symbol equation if you don’t want to. The word equation gets just as much points. But if you like your Chemistry, it might help you to remember what’s going on.

STOP AND CHECK:


The word equation for aerobic respiration. The balanced symbol equation for aerobic respiration.

Aerobic Respiration SummaryAerobic respiration has three main steps required to create ATP. The good news is that, for this topic, you only need to know that they exist. For future record, they are:

1. Glycolysis2. The Krebs Cycle3. The Electron Transport Chain

What happens in each of these steps isn’t important - what is important is that you understand the purpose of respiration, and why it is an important life process.

To help you with the vital information, here is a summary table:

Aerobic RespirationPurpose: To break down glucose to produce ATP as energy to

allow the cell to carry out its life processes.Where it happens: In the mitochondria.Reactants (things needed):

Glucose and oxygen.

Products (things produced):

ATP.Water and carbon dioxide (waste products).

Anaerobic Respiration

While aerobic respiration occurs in the presence of oxygen, there are other forms that occur in the absence of oxygen: anaerobic respiration, which we can also call fermentation.

Fermentation is a common process in yeast and bacteria, and also occurs in muscle cells of animals



33

Imagine you are doing intense exercise – like sprinting down the field or maybe frantically chasing to catch up with your workload at school.

Because the exercise is so intense, sometimes the amount of oxygen available can’t keep up with the rate of aerobic respiration needed for ATP generation.

When oxygen becomes limited, this less efficient form of respiration creates a small amount of ATP, as well as lactic acid as a bi-product. Interestingly, lactic acid is what can cause cramp in your legs when you are exercising too much.

Fermentation is a process which occurs when there is a limited amount of oxygen available within a cell. It produces a small amount of ATP, as well as various bi-products.

STOP AND CHECK:


When anaerobic metabolism occurs. The product of fermentation in muscle cells.


The Mitochondrion: Maximising Aerobic Respiration

This section is a recap on what you should remember from the first section:

Mitochondria have two membranes. The outer one is nice and smooth and the inner one is wiggly. The wiggles are called cristae. The space between the inner and outer membrane is the intermembrane space.The liquid-filled space inside the inner membrane is called the matrix.

The wiggly cristae maximise the surface area of the membrane involved in the electron transport chain

This increases the rate of these reactions and means we can get the most energy out of each glucose molecule. In other words, it maximises the rate of aerobic respiration.

Different types of cells have different numbers of mitochondria

This depends on how much energy they need.



A higher energy demand means there will be more mitochondria in that cell. Liver cells which are constantly making and releasing stuff, and muscle cells which are constantly contracting, needs lots of ATP and so have lots and lots of mitochondria. A lower energy demand means there will be less mitochondria in that cell. Skin cells, for example, aren’t doing a whole lot compared to other cells, and so they only need a handful of mitochondria.

STOP AND CHECK:


What are the features which maximise the rate of aerobic respiration in cells?


Quick Questions What is the purpose of aerobic respiration? How is energy in the form of ATP produced in both aerobic respiration and in

fermentation. When would fermentation occur in the human body and why? How is surface area maximised inside the mitochondria, and what effect does

this have on aerobic respiration?

CELL DIVISIONHow many science fiction books and films feature cloning? Too many to count! But cloning isn’t limited to science fiction. Mitosis is the process of splitting one cell into two copies of the original cell. It is essentially cloning and it is how the body grows, repairs and replaces old cells.

Before you can all become mad scientists, cloning yourself or creating hybrid monstrosities, there are a few concepts you need to understand first:

Cells are a lot more exciting than you might think. Rather than just sitting around, they are constantly going through what is called the “cell cycle”.

Before cells start dividing willy-nilly they need to first replicate their DNA. Otherwise DNA will get lost and all the cells will be weird and non-functional. So, how is DNA replicated?

Finally, how does one single cell become two?


?


35

Cell Cycle

Cells live out their lives by going through something called the Cell Cycle, over and over again. Here are the four stages of the cell cycle:

Mitosis

S

G2

G1

Interphase

The cell “double checks” the duplicated chromosomes for error, making any needed repairs

Each of the 46 chromosomes is duplicated by the cell

Cellular contents, excluding the chromosomes, are duplicated

G1 Phase: This is where the cell grows and makes all of the machinery it will need for S phase.

S Phase: In S (‘synthesis’) phase, the DNA in the nucleus of the cell is replicated, so that the cell then has two copies of everything (so humans go from having 46 chromosomes to having 92 chromosomes).

G2 Phase: Again, the cell grows and makes all of the machinery it will need for M phase.

M Phase: This is mitosis – the process of splitting into two identical cells.

While G1 and G2 are interesting, I think you’ll agree that S phase and M phase are where the dramatic action happens. Let’s look at each of them in more detail.

STOP AND CHECK:


The four stages of the cell cycle. What happens in each of the four stages.

Try to explain it in your own words, and draw a diagram if it helps!

DNA Replication

Why does the DNA need to replicate in S phase? This is the cell preparing for mitosis.



When the cell divides, both of the new cells need to have the complete set of DNA.So, first the DNA needs to be copied.

DNA replication involves a cell copying all of the genetic material within it to create two identical copies. This is necessary for other life processes such as Mitosis to occur.

For those who need a refresher from Level 1 Science:

DNA is in the shape of a ladder, wrapped into a double helix. The sides of the ladder are made of a sugar and phosphate backbone. The rungs of the ladder are made of base pairs (A pairs with T and C pairs with G).

To copy the DNA, the ladder is split in two by a special enzyme which ‘unzips’ the two strands

More enzymes then add new nucleotides (sets of a sugar, a phosphate and a base) to each original strand to build up two identical chromosomes.

This is possible through complementary base pairing

Does that sound familiar? It means that if the next pair-less base is an A, the enzymes will add a nucleotide with a T base to it. If the next pair-less base is a G, the enzymes will add a nucleotide with a C base to it.

The strand that the bases are added to - and therefore guides the addition of new bases is called the ‘template’ strand.

This goes on until all of the DNA has been copied.

Two stands of DNA Two exact copies

NucleotidesStrands separate

ATCGCTATGACTG

TAGCGATACTGAC

ATCGCTATG

A

A

C

C C

C

A

C

T

G

TG

TAGCGATACT

T

GAC

ATCGCTATGACTG

TAGCGATACTGAC

ATCGCTATGACTG

TAGCGATACTGAC

We call the process of DNA replication ‘semi-conservative’

This is because when the new copies of the DNA have been made, one strand of each chromosome is from the original chromosome, while the other is completely new. So, half (‘semi’) of the original chromosome has been conserved in both copies.



37

STOP AND CHECK:


Why DNA needs to be replicated. From the last section, what stage of the cell cycle DNA is replicated in. Which direction new nucleotides are added to the new strand. How the leading and lagging strand are produced – why are they replicated differently. Why DNA replication is called semi-conservative.


MitosisOnce the cell has two copies of its DNA, it is ready to undergo Mitosis. This simply means it is ready to replicate itself for the good of the organism.

Mitosis involves a cell replicating itself to produce two identical cells. This process is used when additional cells need to be created for an organ to grow, or for a repair to be made in the body.

Mitosis is for growth and repair of cells

Therefore, it makes sense that lots of mitosis occurs in times and places of lots of growth and repair.

As an infant becomes an adult, lots of growth will be occurring, and therefore the rates of mitosis are higher in infancy and childhood. For plants, the rate of mitosis is highest during seasonal growth. Skin and hair cells are exposed to the environment and are therefore most likely to become damaged. Therefore, the rate of mitosis is higher in skin and hair cells as opposed to brain or heart cells. For plants, the roots and leaves are the most likely to be damaged, and so mitosis rates are highest here.

STOP AND CHECK:


The purpose of mitosis. The steps involved in mitosis. What mitosis looks like – in other words, get those pencils out and have a go

at recreating our diagram for mitosis.




Quick Questions What are the stages of the cell cycle and what occurs in each one? How is DNA replicated in the cell? What is the purpose of mitosis and how is it carried out? Where and when

would you expect the rates of mitosis to be highest in humans and in plants?

FACTORS AFFECTING LIFE PROCESSESNow that we’ve learnt about our life processes, let’s link them back to those enzymes we talked about earlier to see how we can make them occur as efficiently as possible.

Remember how enzymes are crucial for any life process to occur?

This means that any factor affecting enzyme activity will affect any of these life processes as well.

There are a few additional factors which have nothing to do with enzymes, such as light intensity for photosynthesis.

Factor EffectTemperature Enzymes only work at very specific temperatures.

If it gets too cold, they will slow down (so the life processes they catalyse will slow down). If it gets too hot, they will denature, meaning that their active sites get mangled and they don’t work anymore. Enzymes are so important that if this happens, the life processes stop.

Light intensity As light intensity increases, the rate of photosynthesis increases. Eventually increasing light intensity will stop increasing the rate, because another factor (like water) will become rate limiting (there will not be enough of it for that rate to get any faster).

Concentration of reactants

In photosynthesis, as the available concentration of water or carbon dioxide increases, the rate of photosynthesis increases. Eventually increasing one factor will stop increasing the rate, because another factor will become rate limiting. The same applies with oxygen and glucose in respiration.


?


39

Similarities and Differences between CellsYou will often be asked to discuss why some types of cells have more or less of a particular organelle, or why they have a particular shape.

Why might some types of cells need more mitochondria than others?

Some cells have more need for energy and so need to carry out more respiration.

Why do plant cells have chloroplasts and cell walls, but animal cells don’t?

Animals don’t carry out photosynthesis, plus animals move around and so need to be flexible, while plants need to be more rigid.

Why are root hair cells long and thin?

This gives them more surface area so that they can absorb water by osmosis more efficiently.

You see? The questions are new, but the answers are all to do with what you’ve learned about the life processes carried out by the cells.

You’ll be given unfamiliar contexts in the exam, but remember that the answers all come from what you know, so don’t be put off!



KEY TERMSTo help you out, we’ve collected the key words you need to know and put them into a glossary. The key words are in groups, rather than just arranged alphabetically, to help you to remember which words go together.

Active transport: Movement of materials across membranes by using energy.

• Endocytosis: The process of a cell’s membrane stretching around a large particle in order to engulf it.

• Exocytosis: The process of a cell ‘spitting out’ a large particle by fusing a vesicle with the cell membrane.

• Pumps: ‘Machinery’ that pumps a substance from an area of low concentration to an area of high concentration, against its concentration gradient.

Cell: The basic building block of every living thing.

DNA replication: The process of creating a copy of every chromosome in the nucleus in preparation for mitosis.

• Double helix: The spiral shape of DNA.• Nucleotide: A group of a sugar, a phosphate and a base.• Complimentary base pairing: A pairs with T and C pairs with G.

Enzymes: Biological catalysts, which speed up the reactions inside cells.

• Active site: The ‘pocket’ in the surface of an enzyme where the substrate binds.• Substrate: The reactant catalysed by the enzyme.

Mitosis: The process of splitting one cell into two identical cells (cloning).

Rate limiting factor: The reactant that is present in the smallest amount and therefore stops the rate of the reaction from increasing beyond a certain point.

Organelles: The ‘machinery’ inside cells which enables them to carry out their life processes.

• Cell membrane: A phospholipid bilayer that surrounds a cell and determines what can get in and out of the cell.

• Cell wall: A rigid structure that wraps around the outside of the cell membrane


41

to give plant cells a rigid shape and structure.• Chloroplasts: The organelle where photosynthesis is carried out (in plants).

• Chlorophyll: A pigment that traps sunlight.• Grana: Stacks of thylakoids inside the stroma.• Stroma: The liquid-filled space inside the inner membrane of chloroplasts.• Thylakoids: Sacs covered with chlorophyll.

• Cytoplasm: The jelly-like, nutrient-rich substance that fills up cells.• Mitochondria: The organelle where respiration is carried out.

• Cristae: The wiggly inner membrane of mitochondria (the wiggles increase surface area).

• Intermembrane space: The space between the inner and outer membranes in mitochondria.

• Matrix: The liquid-filled space inside the inner membrane of mitochondria.• Nucleus: The ‘control centre’ of the cell, containing the cell’s DNA.

Passive transport: Movement of materials across cell membranes without the use of energy.

• Diffusion: The movement of a substance from an area of high concentration to an area of low concentration, down its concentration gradient.

• Facilitated diffusion: Diffusion of larger or charged molecules through channels inserted in the membranes of cells.

• Osmosis: The movement of water from an area of low solute concentration to an area of high solute concentration.• Solute: A substance that is dissolved in water.

Photosynthesis: The process of using light energy to convert water and carbon dioxide into glucose for the plant to use as fuel to carry out its life processes.

• Light-Dependent Phase: The first stage of photosynthesis (the splitting of water into hydrogen and oxygen).

• Light-Independent Phase: The second stage of photosynthesis (the production of glucose from hydrogen and carbon dioxide).

Respiration: The process of reacting glucose with oxygen to produce ATP as energy for the cell, to enable it to carry out its life processes.

• ATP: A form of energy that cells can use.



life processes at the cellular level · level 2 biology | life processes at the cellular level . 3...

Documents