Unit 1 Biology

Chapter 2 Structure and function of cells

Key knowledge

This chapter is designed to enable students to:

Investigate the defining characteristics of prokaryotic and eukaryotic cells

Identify cell structure and organisation Identify cell organelles and understand their function Investigate the different modes of transport of

materials across plasma membranes Understand and apply the principal of surface area to

volume ratio

Looking at cells

All cells are similar in some ways:• Each cell is a small compartment surrounded by a cell

membrane, which holds in the fluid (cytosol) that the organelles float around in.

• All cells also have DNA in some form inside of them, that carries their genetic material.

But cells can also differ greatly. Living cells can be classified into two groups based on differences of their internal structure:

• Prokaryotic cells – very basic internal structure with no membrane-bound nucleus (e.g. bacterial cells)

• Eukaryotic cells – more complex internally, with membrane-bound organelles, including a nucleus (e.g. animal cells)

Cells vary in size

Prokaryotes vs eukaryotes

Characteristics of living things

Animal cells

Plant cells

Biological drawings

A biological drawing of an animal cell

A biological drawing of a plant cell

Draw up a table to show which organelles are:

a) Only present in plant cells b) only present in animal cells and c) present in both animal and plant cells

The plasma membrane

• Present in all living cells both eukaryotic and prokaryotic.

• Controls the movement of dissolved substances in and out of the cell, therefore maintaining the internal environment of the cell.

• It is made up of protein and lipid (fat) molecules.• A model designed to describe the appearance of the

plasma membrane is shown below

Proteins of the plasma membrane

Movement in and out of cells

• All cells need to be able to take in and expel various substances across their cell membrane in order to survive, grow and reproduce.

• Because the membrane (m/b) only allows some substances to pass through, it is called partially permeable

• Dissolved substances can cross the plasma m/b in a number of ways:

- Diffusion require no energy- Osmosis

- Active transport requires energy- Bulk transport

Diffusion

• A way that substances move into and out of cells, that requires no energy by the cell.

• Movement is from a region of high concentration of that solute to a region of low concentration for that solute, through a semi-permeable membrane.

• Refer to Fig 2.7• Movement will continue until the concentration of the

solute on both the inside and the outside are equal.

Osmosis

• Osmosis is a special type of diffusion – it involves water

• So, osmosis is the movement of water across a semi permeable membrane from a region where it is at a high concentration, to a region where is at low concentration.

• Osmosis can result in cells lysing (bursting) or shrinking depending on the solution they are in.

• Refer to the blue box of information on p28.

• E.g. Freshwater fish do not need to drink, they rely on osmosis to get their water. Over what surface might the fish obtain their water?

Active transport• AT is another way in which substances can move into

and out of cells• Different to diffusion and osmosis in that it requires

energy from the cell in the form of adenosine triphosphate (ATP

• Movement of molecules is the opposite to that in diffusion – molecules move from a region of low concentration to a region of high concentration

• Refer to figure 2.9 b

• In comparing diffusion to active transport we use the slippery slide model- going up the slippery slide (concentration gradient) requires energy- going down the slippery slide does not require energy

Facilitated diffusion

• This special type of diffusion that requires energy.• This occurs as some substances cannot dissolve

through the plasma membrane to cross it.• These molecules are carried through the protein

channels by special molecules called carrier proteins.

• The inherited disorder cystic fibrosis results from a defective carrier protein that is meant to transport chloride ions into the cell

Bulk transport - Exocytosis

• Particles can also be moved into and out of cells via bulk transport.

• Exosytosis is the bulk transport of substances out of a cell.

It involves:1. a vesicle forming around the particles that need to

be moved out, 2. the vesicle moves up to and fuses with the cell

membrane, 3. the lipid bilayer opens up and moves the contents of

the vesicle out of the cell• Refer to Fig 2.13

Bulk transport - Endocytosis• Endocytosis is the bulk transport of substances into a

cell.• Endocytosis can involve the bulk transport of:a) Solids – called phagocytosisb) Liquids – called pinocytosisIt involves:1. A cleavage forming in the lipid bilayer and the

particles moving into that,.2. the cell membrane coming together to form a lisosome3. The lisosome breaking off from the membrane and

then dissolving the particles inside it with digestive enzymes

Only some cells (white blood cells and single celled organisms) are capable of phagocytosis, whereas most cells are capable of pinocytosis.

• Refer to Fig 2.11 and 2.12

Cell walls

• Present in prokaryotes, plants and fungi.• Semi-rigid, protective structure that lies outside the

cell m/b.• Composition varies depending on organism group:

- plants: cellulose- fungi: chitin- bacteria: complex polysaccharides

Cell organelles

We will be looking at the structure and function of the following organelles in detail:

• Nucleus (control centre)• Mitochondria (energy supply)• Ribosomes (protein makers)• Endoplasmic reticulum (transport systems)• Golgi complex• Lysosomes (controlled distruction)• Chloroplasts (sunlight trappers)

• We will also briefly look at vacuoles, cilia, peroxisomes, endosomes and vacuoles.

Nucleus

• Present in eukaryotes and encloses the DNA.• Absent in prokaryotes, DNA is dispersed throughout

cell.• The nucleus is often referred to as the control centre

of the cell, as the DNA (held in the nucleus) is responsible for all of the cells activities, including:

- reproduction, instruction ribosomes to manufacture protein, cell death.• DNA is the abbreviation for deoxyrobonucleic acid.• Some specialised cells in the body have no nucleus (red blood cells) whileothers (liver cells) have two.

Mitochondria

• Provide the cell with the energyit requires to carry out all of its functions.• Only present in eukaryotes.• The form of energy used by the cellis adenosine triphosphate (ATP)• ATP is manufactured in the mitochondria by a chemical processknown as cellular respiration, the equation for which is shown below:

Ribosomes

• Ribosomes are the site at which all of the protein required by the cell is synthesised (made).

• Ribosomes are present in both eukaryotes and prokaryotes, and are so small that they can only be seen with an electron microscope.

• The protein synthesised by the cell depends on the specialised function of that cell, for example

- Red blood cells make haemoglobin- Pancreas cell make insulin- Liver cells make protein enzymes, like catalase- Stomach cells make digestive enzymes- Muscle cells make contractile proteins, actin and

myosin

Endoplasmic reticulum and Golgi complex

• These are two different organelles, but they are often referred to together.

• This is because they work together inside the cell, to make up the cell transport system, whether that be within the cell or out of the cell as to be transported around the body.

• Endoplasmic reticulum (ER) transports substances within the cell, and includes smooth ER and rough ER (has ribosomes attached – for protein transport).

• The Golgi complex (also called Golgi body or Golgi apparatus) is a prominent feature of cells that make a lot of protein, as its role is to transport protein out of the cell.

Refer to Fig 2.16 and 2.17

Lysosomes

• Present in eukaryotes, absent in prokaryotes.• Sac-like structures, surrounded by a membrane and

filled with fluid which contains dissolved digestive enzymes.

• Can release these enzymes within the cell, resulting in cell death.

• Programmed cell death like this is called apoptosis.• Important for:- cells that have a limited life span (e.g. cells that form

the webbing between fingers of an early human embryo)

Refer to Fig 2.18 and 2.19- digesting unwanted materials within cells

Chloroplasts

• Specialised organelles only found in plant cells.• Made from a series of membranes which form layers called grana.• Filled with a fluid called stroma.• Chloroplasts are green in appearance due to the presence of light trapping pigment called

chlorophyll.• Chlorophyll is one of the key components of the

chemical process known as photosynthesis (see equation below) – the means by which plants use the energy from the Sun to make their own glucose.

Other organelles• Vacuoles – large fluid-filled sacs, mainly found in plant

cells or single celled animals. Contain water and some dissolved solutes, can have a contractile mechanism to help pump water out of some freshwater protists.

• Endosomes – involved with endocytosis in animal cells, pass newly ingested material to lysosomes for digestion.

• Peroxisomes – rich in enzymes that detoxify various toxic materials that enter the bloodstream of animals.

• Cilia – found in some single celled eukaryotes, whip like structures formed by extensions of the plasma membrane involved in synchronised movement.

• Flagellum - whip like structure found in some eukaryotic organisms and some bacteria involved in movement.

Levels of organisation• Multicellular organisms, as the name suggests are

made of many cells.• When an organisms is made up of more than one

cell, its ‘body’ has different levels of organisation.

Cells

Tissues

Organs

Organ systems

Organism

Chapter Questions

• Quick-check questions - 1-3 p31- 4-8 p35- 9-14 p42- 15-16 p46

• Chapter review questions