Absorption and Secretion

Learning Outcomes Identify the structure and properties of the cell wall. Identify the structure of the plasma membrane with reference to the fluid mosaic model.

Absorption and Secretion Absorption means the uptake of materials by a cell from its external environment. Secretion means the discharge of useful intracellular molecules into the surrounding medium by a cell. All living cells are surrounded by a cell membrane (plasma membrane). In addition, plant cells have a cell wall.

The cell wall The cell wall is a non-living layer composed mainly of cellulose. Cellulose is a complex carbohydrate that is composed of unbranched chains of glucose molecules grouped together as fibers. Tip- Always describe cellulose as fibers.

The cell wall The layers of cellulose fibers make the cell wall: –Strong –Elastic –Hydrophilic – water is attracted to cellulose –Non-living –Freely permeable – presents no barrier to the movement of molecules. –Provides a continuous water conducting channel. (allows water to move easily from tissue to tissue without having to enter and leave every living cell on the way)

The plasma membrane The plasma membrane is composed of protein and phospholipid in an arrangement called a fluid mosaic. The plasma membrane is composed of a fluid bilayer of constantly moving phospholipid molecules. The term ‘mosaic’ describes the proteins which are embedded into the lipids in a random fashion.

Plasma Membrane The plasma membrane is made of: A phospholipid bilayer With proteins embedded in it Phospholipid bilayer Protein The Fluid Mosaic Model

The Plasma Membrane One end of a phospholipid molecule is hydrophilic (water loving), this is called the tail. The other end is hydrophobic (water hating), this is called the head. The hydrophillic heads are water soluble and make up the 2 outer surfaces, where they form bonds with hydrogen molecules. The hydrophobic tails are water-insoluble and point inwards to the centre of the bilayer since they are attracted to those in the opposite layer.

Plasma Membrane video

The Plasma Membrane The arrangement of phospholipid molecules is fluid but at the same time forms a stable and effective boundary around the cell. This allows tiny molecules like water to pass through it rapidly. Larger molecules such as glucose depend upon the protein molecules for entry or exit to the cell.

Proteins The protein molecules in the plasma membrane vary in size and structure. Some of the proteins have narrow channels which make the membrane porous.

Proteins The protein molecules vary in function: –Provides structural support ( strength ) –Enzymes – to speed up reaction –Receptors – for cell signalling –Channels – for movement of larger molecules such as glucose –Antigenic markers for cell recognition –Carriers – by active transport

Summary - The Fluid Mosaic Model Channel forming protein Carrier Antigen markerReceptor Structural protein

Beetroot Experiment Aim: To investigate the chemical nature of the cell membrane. Results: Test tube A is a control. In test tube B and D there was bleeding as low pH and high temperature denatured the proteins in the plasma membrane. In test tube C there was bleeding because lipids are soluble in alcohol, so the phospholipids in the plasma membrane dissolved.

Questions What is the main component of a plant cell wall? How does the fibrous nature of this substance make the wall suited to its function? Name 2 types of molecule present in the plasma membrane. Which of these would be denatured at 70 0 C? State 3 functions carried out by proteins in the cell membrane.

Learning Outcomes Explain the mechanisms of osmosis and diffusion as basic cell processes. Define the terms hypertonic, hypotonic and isotonic in relation to water content of cell contents and their surroundings. Describe the fate of both animal and plant cells in hypertonic, hypotonic and isotonic solutions.

Diffusion Diffusion is the movement of molecules or ions from a region of high concentration to a region of low concentration. The difference in concentration that exists before diffusion occurs is called the concentration gradient. During diffusion, molecules or ions always move down a concentration gradient from high to low. high concentration low concentration

Diffusion Diffusion is important as it is the way that useful substances like oxygen enter a cell and waste substances like carbon dioxide leave a cell. Diffusion is a passive process and doesn’t require any energy. The cell wall is freely permeable to all molecules in solution, so it does not act as a barrier to diffusion.

The plasma membrane + diffusion The plasma membrane is freely permeable to tiny molecules like oxygen that are small enough to diffuse rapidly through the bilayer. Molecules like urea are able to diffuse slowly through the bilayer. Larger molecules like glucose depend upon protein molecules to help them across. Even larger molecules like starch are unable to pass through the membrane. The plasma membrane is selectively permeable.

Osmosis Osmosis is the movement of water from a region of high water concentration to low water concentration through a selectively permeable membrane. Osmosis is a passive process and does not require energy.

Water Concentration A solution is made when a solute is dissolved in a solvent. A hypertonic solution is one that has a lower concentration of water (solvent). A hypotonic solution is one that has a higher concentration of water (solvent). An isotonic solution is one where the concentration of solvent and solute are equal.

Water Concentration If a cell is said to be in a hypertonic solution, this means that the water concentration of the cell contents is higher that the water concentration of the cell surroundings. If a cell is placed in a hypertonic solution, water will move out of the cell, from high to low. If water moves out of an animal cell, it will shrink.

Water Concentration If a cell is said to be in a hypotonic solution, this means that the water concentration of the cell contents is lower that the water concentration of the cell surroundings. If a cell is placed in a hypotonic solution, water will move into the cell, from high to low. If an animal cell is placed in a hypotonic solution, it will take on water and burst (haemolysis).

Osmosis in Animal cells How does water move in these 3 cases? What are the relative water concentrations?

Osmosis in Plant cells Osmosis in plant cells has slightly different results because plant cells have a cell wall. If a plant cell is placed in a hypertonic solution, it will lose water by osmosis and the cell contents will shrink. The cell becomes plasmolysed. If a plant cell is placed in a hypotonic solution, it will gain water by osmosis but the cell wall prevents it from bursting. Instead it becomes turgid.

Osmosis in Plant Cells

Learning Outcomes Explain the basic cell process of active transport. State the function of the plasma membrane in relation to selective ion uptake (active transport) and absorption and release of chemicals.

Active Transport Active transport is the movement of molecules and ions across the plasma membrane from a low to a high concentration (against a concentration gradient). Active transport requires energy. This energy is provided by ATP from respiration. video

Role of the plasma membrane Certain protein molecules present in the plasma membrane act as carrier molecules. These proteins ‘recognise’ specific ions and transfer them across the plasma membrane.

Sodium Potassium Pump Active transport carriers are often called pumps. Some carriers have two roles: they exchange one type of ion for another. An example of this is the sodium potassium pump. The same carrier molecule actively pumps sodium ions out of the cell and potassium ions into the cell. video

Sodium Potassium Pump

Conditions Required Factors such as temperature, availability of oxygen and concentration of respiratory substrate (e.g. Glucose) affect the rate of active transport. Increase in temperature causes an increase in rate of ion uptake, until at high temperatures the enzymes needed for respiration becomes denatured.

Conditions Required An increase in oxygen concentration results in increased rate of ion uptake until some other factor affecting the process becomes limiting (e.g. temperature).

10 Mark Question Write notes on: a)Structure of the plasma membrane (4) b)Function of the plasma membrane in active transport (3) c)Structure and function of the cell wall (3)

Structure of the plasma membrane 1.Contains protein 2.Contains phospholipid 3.Bilayer/double layer/two layers of phospholipids 4.Phospholipids are fluid/constantly moving 5.Protein arranged as a mosaic/scattered 6.Fluid mosaic pattern/model (only awarded if 4 or 5 not scored) 7.Has channel forming proteins/pores

Function in active transport Proteins/carriers pick up/bind ions/molecules AND move/carry/assist them across the membrane/into/out of the cell Ion/molecule/substance uptake is selective Low to high concentration OR against concentration gradient Requires energy/ATP

Structure + function of cell wall Made of cellulose fibers Fully/freely permeable Provides support/rigidity for cells/plants OR gives cells shapes (not strength or structure) Stops cells bursting after water uptake/when turgid/when placed in hypotonic solution OR allows cells to become turgid.

Learning Outcomes Describe the process of endocytosis.

Endocytosis Endocytosis is the process by which a cell engulfs and takes in relatively large particles or quantities of material. This involves the plasma membrane folding inwards to form a ‘pouch’. The pouch then becomes closed off and detached from the cell membrane. It is now called an intracellular vesicle.

Endocytosis There are 2 types of endocytosis: –Phagocytosis (cell eating) –Pinocytosis (cell drinking)

Phagocytosis Phagocytosis is the engulfing of large solid particles (eg bacteria by white blood cells). The contents of the vesicle are then digested.

Pinocytosis Pinocytosis involves the formation of small fluid-filled vesicles by the cell membrane. This is how cells take in smaller particles. These are then digested using enzymes.

Exocytosis Exocytosis is the reverse of endocytosis. Vesicles formed inside the cell fuse with the plasma membrane, allowing their contents to be expelled from the cell. In this way, waste and intracellular products like enzymes and hormones can be secreted by the cell. video